Wooden roof trusses are considered as rod systems. Construction metal trusses design features. Selection of the section of tensile elements

Rafter truss

What is a farm

I'll try to explain it as simply as I can.

The application of a vertical force to a beam of ordinary rectangular cross-section leads to its deflection (Fig. 118). In this case, internal compressive stresses δcompressor arise in the upper part of the section, and tensile stresses δextension occur in the lower part of the section. They can be depicted in the form of a diagram which shows that the stresses reach their maximum values ​​at the upper and lower boundaries of the beam section, and in the center it is equal to zero, that is, the rectangular section of the beam works unevenly. If we remove non-working areas from it, we get an I-section. The I-beam is the main one building profile. By dividing the I-section, channels, tees and angles are obtained, which, when reassembled, can form the original I-beam, box or cross.

We will continue to remove “excess” material from the beam, reducing its weight without loss bearing capacity. Let's cut holes of the maximum possible size in the vertical partition of the I-beam. The resulting “holey” beam is a prototype of a truss, in which the upper and lower parts are called chords, and the rods connecting them are racks or hangers (depending on whether the beam is supported or suspended). It is clear that such a prototype of a truss can be made not by removing “excess” material from the body of the beam, but by more in a simple way knocking together bars and boards or welding metal profiles.

When making our truss from bars, we end up with a structure that is suitable and equal in load-bearing capacity to the original rectangular beam, but is unstable to lateral loads. After all, in essence, we got a stepladder, which can be easily destroyed if a horizontal force is applied to it. Let's eliminate this drawback by introducing diagonal connections into the design. Here they are called braces, and the racks (suspensions) are better called in one word truss (strut). The distances between truss nodes are called panels.

The main disadvantage of a conventional beam is the large deflection from the load. In building structures, the cross-section of a beam is often taken not according to its load-bearing capacity, but according to its deflection. In other words, for structures, a beam section is used that does not allow large deflections, but the beam itself is capable of carrying a much greater load than is placed on it. We have irrational use of beam material. Reducing the deflection of the beam is achieved by increasing its height. For example, if you take an ordinary student ruler, you can easily make sure that it bends well when placed flat and poorly when placed edge-on. However, as the height of the beam increases, its weight increases, and the beam begins to sag even under its own weight without external load. This is where a lightweight “leaky” beam comes to the rescue - a truss that can be made to a great height without a significant increase in weight.

Why is a beam used as a source to describe a truss, and not a hanging rafter system or some other roof structure? Because I don’t want to tie trusses only to roof structures, since they are widely used in construction and mechanical engineering, but I want to reinforce the understanding that a truss as a whole works the same way as a beam. For example, when supported on two supports and loaded from above, internal compressive stresses arise in its upper belt, and tensile stresses arise in the lower belt; it does not transmit thrust to the walls.

Trusses are loaded with a distributed load or concentrated forces (Fig. 119).

If the building structure is developed in such a way that concentrated forces are applied exclusively at the truss nodes, then bending moments will not occur in the truss elements (belts, trusses and braces).

Truss (structure)

They will only work in compression and tension, which makes it possible to reduce the cross-section of these elements to the required minimum. In this case, the trusses themselves can be made from short elements with a length from node to node, and the nodes can be made according to a hinged pattern. A truss is a geometrically unchangeable rod system with hinged joints. Such trusses are often found in metal versions. For wooden trusses, schemes are usually used in which the upper and lower chords are made not with short boards (from node to node), but with long ones, the entire available length. In this case, the truss chords are not connected by hinges at each node, but rest on them and are suspended from them. Although a wooden truss can also be assembled from short planks. The main thing you need to understand is that the load applied at the nodes in the form of concentrated forces will not bend the truss elements.

• If a uniformly distributed load acts on the truss, then a bending moment will appear in the rods of the upper chord in addition to compressive and tensile stresses. The bending moment reaches its maximum value in the middle of each truss panel chord rod with hinges embedded in the nodes, or on supports - with hinges located under/above the truss chord. Accordingly, the cross-section of the truss rods will be larger than if the truss were loaded with point forces at the nodes.

The main advantage of trusses lies in the use of a loading scheme. For the same external load, its correct distribution on the truss provides an advantage in saving material.

Trusses of the required length (span) to which the point load will be applied at the nodes can be made from short elements with a length from node to node.

Trusses that will be subject to a uniformly distributed load can also be made from short elements if the truss nodes are hinged; and from long ones if the hinges are under/above the belts.

Wooden trusses made from long planks are typically used for roofs. Since the overlapped spans are larger than the length of the boards allows, the trusses are made of two parts. Joining them at approximately 1/5 of the length of the panels, that is, where the bending moment tends to zero.

Wooden and metal-wooden trusses

These trusses are used in buildings of sawmilling and woodworking industries, as well as in auxiliary buildings and in the chemical industry. The spans of such buildings, as a rule, do not exceed 18-24 m. More common are metal-wood trusses, in which the compressed elements are made of wood, and the tensile elements are made of steel. According to their outline, trusses are divided into segmental, trapezoidal and triangular.
Segmental trusses, with spans from 12 to 24 m, are distinguished by their lightness, small number of mounting elements and ease of solving nodes (Fig. 68, a). The upper chord of these trusses is constructed from glued blocks of a curved outline, the lower - from steel strands or angles. The grating is attached to the belts with nails or bolts using steel plates.

In Fig. 68, b shows a polygonal truss made of beams two panels long, which can be used for spans from 12 to 36 m. Due to the outline of the upper chord close to the pressure curve, the forces in the lattice of these trusses are relatively small, which simplifies the design of the units.

Of the trapezoidal trusses, the truss with a girth chain has the best technical and economic indicators. The length of the trusses is 12, 18 and 24 m.

Farm under construction

The upper belt is made of beams on plate dowels or glued. The outer panels of the lower chord are wooden, hingedly connected to a metal tie. As a result, the truss is a post-truss with a spring chain consisting of supporting steel braces and a tightening of the middle panel (Fig. 68, c).

Rice. 68. Metal-wood trusses: a - segmental; b - polygonal; c - trapezoidal; g, d - triangular

Triangular trusses are recommended for use for spans from 9 to 18 m (Fig. 68, d). The upper belt can be glued or made of beams or beams on plate dowels.
More rational are triangular trusses with an upper chord made of beams or composite beams on glue or on plate dowels with round steel tightening (Fig. 68, e). Such trusses are easy to manufacture and allow the load from the suspended ceiling to be transferred to the ridge unit, which eliminates the occurrence of bending moments in the upper chord.

Frames and arches made of reinforced concrete, metal and wood are sometimes used as load-bearing structures of buildings. The pitch of the frames is taken to be 6 and 12 m. The frame consists of racks and rigidly connected crossbars of a rectilinear, broken or curved outline. The racks rest on independent foundations. Frames can be with or without lantern add-ons.

Frames and arches in most cases have good technical and economic indicators, but due to the difficulty of unification and low versatility, they are rarely used. Arches and frames are most suitable for buildings erected according to individual projects.

Related topics

Wooden and steel window panels for industrial buildings

Steel trusses and sub-trusses of industrial buildings

Reinforced concrete rafter beams and trusses of industrial buildings

Reinforced concrete trusses covering industrial buildings

Wooden beams covering industrial buildings

Farm - what is it? Building construction

The most common meaning of the word “farm” is an agricultural enterprise intended for raising livestock. But now we are not talking about the place of farming. Here is collected all the information about probably the oldest building structure, which is still relevant in modern life. It is widely used in construction, especially in the construction of bridges and sports facilities.

A truss is a system consisting of rods that remains geometrically unchanged when its rigid nodes are replaced by hinged ones. It also includes trussed beams, which are represented by a combination of a two- or three-span uncut beam and a girder.

Where is it used?

As already mentioned, a truss is an indispensable element in construction. With its help, builders facilitate the construction of structures and reduce consumption. necessary materials. The construction of bridges, stadiums, hangars, as well as decorative structures such as pavilions, stages, podiums, etc. cannot be done without the use of a truss.

When designing the hull of a ship, aircraft, or diesel locomotive, the strength is calculated in the same way as the load on the truss is calculated.

Classification

A truss is a structure consisting of rods that are connected to each other at nodes and form a statically unchanged system. Farms can be classified according to many properties.

According to the load-carrying capacity of the structure

• Lungs. They use a single-wall section. Lightweight trusses are most often used in industrial construction.
• Heavy. Heavy trusses are used in the construction of tower cranes, sports stadiums, etc. They use rods of a more complex cross-section than lungs. As a rule, they consist of two or three parts due to the large effective length and the load placed on them. Most often, a two-wall section with a two-plane nodal interface is used.

According to general characteristics

• By appointment. Depending on their purpose, trusses can be towers, bridges, cranes, roof trusses, support structures, etc.
• By type of material. Wood, steel, aluminum, reinforced concrete, etc. - from all this a construction truss can be made. This is a significant advantage of this system. You can also combine several types of material.
• According to design features. There are various types of sections, types of lattice, types supporting structures, as well as types of chords in the truss building structure.

By spatial basis

• Flat. Trusses take on the vertical load, because x rods are located in the same plane.
• Spatial. Distribute the load over its entire area. A spatial truss is formed from many flat trusses interconnected in special ways.

Type

• Virendel beam.
• Warren Farm.
• Pratt Farm.
• Bolman Farm.
• Fink's Farm.
• Triangular truss.
• Kingpost.
• Cross braced truss.
• Lattice urban structure.
• Truss under the overhead light.

Design Features

The classification of farms by design features is quite extensive. Next, each of the features will be considered in more detail.

Section types

The cross section in the construction truss is made of rolled profiles. It can be in the form:

• Corner (single or double).
• Pipes (round or square).
• Shvelera.
• Tee or I-beam.

Belt types

The outline of the belt can be presented as:

• Trapeze. Its advantage lies in the fact that this type of belt strengthens the frame assembly, and accordingly, the rigidity of the building increases along with it.
• Triangle. This type of belt is used for beam and cantilever systems. It has a lot of disadvantages, such as irrational consumption of metal when distributing the load, the complexity of the support unit, etc.
• Parabolas. This belt is the most labor-intensive. Therefore, segmented trusses are used very rarely.
• Polygon. Polygonal trusses are used more often than segmental trusses. Because in them, the fracture in the structural units is not so noticeable.
• Parallel belts. Most often used to cover industrial buildings. They have an identical layout of nodes, lattice elements of equal size, and they also have repeatability of elements and parts.

Grating types

There are six standard grille options:

• Triangular.
• Rhombic.
• Shprengelnaya.
• Cross.
• Slanting.
• Semi-oblique.

Types of support

There are 5 types of support structures. In order to select a support node, you need to know the calculation scheme. It determines whether the support unit will be hinged or rigid. Types of supports:

• Beam or cantilever.
• Arched.
• Cable-stayed.
• Frame.
• Combined.

Operating principle

The uniqueness of this design lies in its “immutability” under the influence of external factors. The load on this system can be quite large. The truss is a set of triangles combined into one structure. The load in them is concentrated at the junction of the nodes, because rods exhibit their properties better in the process of compression-tension, and not in fracture. In modern construction, a rigid rather than a hinged connection of rods is most often used. It follows from this that when one of them is separated from the whole structure, they will remain in an unchanged position in relation to each other.

The principle of calculating trusses by cutting corners

This method of calculating trusses is the simplest. This method is taught in many technical schools.

A truss is a structure whose load is concentrated in its nodes. Therefore, it is necessary to calculate all external factors that will be a load on the nodes. Then, calculate the reaction of the support and find a node in which there are 2 rods with a force applied to them. Conventionally, it is necessary to separate the rest of the farm and get a node that will have several known values ​​and 2 unknowns. Then you need to create an equation along two axes and calculate the unknown values. In the same way, the next node is selected, and so on until the truss is calculated.

Main types of farms

• Virendel beam is a system where all its parts form rectangular holes and thus connected into a rigid frame. By its design, it does not fit the strict term “trusses”, because there is no couple of forces in this beam. It was developed by the Belgian engineer Arthur Vierendel. But because This design is quite massive, it is rarely seen in modern architecture.

• Warren Farm. This is a simplified version of the Pratt-Howe design. It works on the compression-tension principle. Most often made of rolled steel.
• Pratt Farm. The patent for this structure belongs to a father and son from Boston. Caleb Pratt and Thomas Wilson were the two engineers. They used compressed parts vertically and stretched parts horizontally. Therefore, the load is equally well distributed both above and below.
• Bolman Farm It has a rather complex and inconvenient design. This structure gained its popularity in the USA due to the political merits of its creator. The inventor spoke eloquently about the farm, even if not everything corresponded to reality. Bohlman was able to promote his invention with the help of the American government, which sometimes forced city planners to use this design when designing bridges. Among the holders of construction farm patents there are many of our compatriots, but not a single “Russian” farm has ever been promoted to the masses in such an original way.
• Fink Farm is a simplified version of the Bohlman farm. He simply shortened all its elements and thereby made it more efficient. It is also similar to the Pratt truss design. It differs from it only in the absence of a lower beam.
• Triangular truss. It is also called "Belgian". This is a modern design, which is presented in the form of triangles with trusses.
• Kingpost- the simplest version of the farm. It consists of a pair of supports resting on a vertical beam.
• Lattice urban structure was created to replace huge wooden bridges. It is quite simple in its design. The usual ones are used for it. wooden boards, attached to each other at an angle, which, in turn, form a lattice.

Metal truss. Metal constructions

A metal truss is made from steel profiles; the most commonly used corner is for this purpose. If a heavier structure is to be installed, the profile should have a T-section or I-section. For hydraulic structures, a round cross-section is used, as well as a profile pipe. Metal truss trusses are widely used in structures for roofing buildings, most often the span width exceeds 24 meters.

Design features of a metal truss

Main structural elements

Types of trusses based on lattices and belts

Parts are repeated with the greatest frequency, which is due to the uniform lengths of the bars for the lattice and belts, the same patterns of nodes, as well as the smallest number of joints, which allows for the unification of structures. This makes it possible to industrialize their production. They are used most often in the construction of soft roofs.

Metal trusses, drawings of which are drawn up before installation, can be the same, that is, trapezoidal. Coupling with columns makes it possible to arrange fairly rigid frame assemblies, which increase the rigidity of the entire building. In the central part of the span, the lattice of these trusses does not have long rods. They do not imply the need for significant slopes. As for polygonal ones, they are suitable for massive buildings that use large spans. At the same time, these designs allow you to save material. Such an outline for lightweight options is irrational, since the insignificant savings cannot be compared with such design complexities.

You can also distinguish triangular ones, which are used for round roofs of a certain type. They are simple to implement, but have certain design disadvantages, which are expressed in the complexity of the support unit. Among other things, there is an excess consumption of materials in the manufacture of long rods in the central zone of the lattice. The use of triangular systems is mandatory in many cases, for example, where it is necessary to ensure a uniform and significant influx of natural light on one side.

Grid systems

Features of calculations

Work on the manufacture and connection of elements

Installation of metal trusses is carried out in stages from elements on tacks. Tying the belts is done using a corner, which is used in the amount of one or two pieces. The upper chords are made from corners that have unequal sides and also have a T-section. The pairing is carried out on the smaller sides. For the lower belts, equilateral corners are used. Metal trusses can be of considerable length, and overhead and connecting plates are used. For loads generated within the boundaries of the panels, paired channels are used.

The braces are installed at an angle of 45 degrees; as for the racks, they are installed at a right angle. To perform them, an isosceles angle is used, and the parts are fastened using plates.

If the system is completely welded, then it is performed using brands. After installation on tacks is completed semi-automatically or manually, you can begin to carry out welding work, then each seam must be cleaned. Painting is carried out at the final stage; anti-corrosion compounds should be used.

Rules for the device

To equip an attic, the bare walls must have the appropriate height; in some cases, for this purpose, the roof is provided with fractures at the supports. The dimensions of the upper and lower chord panels must be equivalent. To facilitate the process, a grid is used. If the angle of inclination should be equal to 15-22 degrees, then the height of the structure should be equal to 1/7 of the length, the nodes of the metal trusses in the lower belt should be broken, this guarantees a weight reduction of 30 percent compared to a conventional triangular one. With all this, one span should not be more than 20 meters in length. If a slope of 22-30 degrees is required, then the system must have a triangular shape; the metal structures of the truss will have a height that is equal to 1/3 of the length.

Due to the fact that the weight will be relatively small, external walls erected to a small height can be used as support. If the span length is 14-20 meters, each half should have even number panels whose length is 1.5-2.5 meters. The most suitable number of panels for this length is considered to be limited to eight.

If the span length exceeds 35 meters, then trusses should be used, which involve the use of two triangular elements connected to each other by ties. In this case, the long braces of the central panels can be eliminated, reducing the weight. A triangular metal truss in this case will have an upper chord divided into 16 panels, the length of each of which is 2-2.75 meters.

Steel profile pipes

Once you understand how a metal truss is calculated, you can think about its components. Thus, a structure made of profile pipes has less impressive weight compared to a channel or angle. Such parts are easily assembled using welding. Profile pipes can be covered with lightweight materials such as ondulin, transparent slate, and bitumen shingles. Steel pipes are made of steel and aluminum. Such materials have their own advantages; they are convenient to store, transport, and load. The material will be able to endure significant thermal and mechanical loads, and it can be easily processed.

Metal trusses are based on galvanized profile pipes because they do not corrode, have excellent performance, and also look attractive. All these factors must be taken into account when choosing a material for arranging steel trusses. Among other things, installing such systems is quite simple, which any master can handle.

Finally

Thick-walled profile pipes, which have a more impressive load-bearing capacity, are also used for this. Such structures are also used in the construction of fences, playgrounds, and partitions.

Now you know how to install metal trusses of various shapes.

Posted on http://www.Allbest.Ru/

truss section rod box-shaped

Classification and scope of farms

The origin of the term "truss" comes from the Latin firmus, that is, "strong, strong."

A truss is a system of rods connected to each other at nodes and forming a geometrically unchangeable structure. With a nodal load, the stiffness of the nodes does not significantly affect the operation of the structure, and in most cases they can be considered as hinged. In this case, all truss rods experience only tensile or compressive axial forces.

Trusses are more economical than beams in terms of steel consumption, but are more labor-intensive to manufacture. The greater the span and the lower the load, the greater the efficiency of trusses compared to solid-wall beams.

Trusses can be flat (all rods lie in the same plane) and spatial.

Flat trusses carry loads applied only in their plane and need to be secured with ties. Spatial trusses form a rigid spatial beam that absorbs load in any direction (Fig. 9.1).

Rice. 9.1. Flat (a) and spatial (b) trusses

The main elements of the trusses are the belts that form the outline of the truss, and a lattice consisting of braces and racks (Fig. 9.2). The connection of elements in nodes is carried out by directly connecting one element to another (Fig. 9.3,a) or using uh yu nodal gussets (Fig. 9.3, b). The truss elements are centered along the axes of the center of gravity to reduce nodal moments and ensure that the rods operate under axial forces.

Rice. 9.2. Truss elements

1 - upper belt; 2 - lower belt; 3 - braces; 4 – racks

Rice. 9.3. Truss nodes: A - with direct adjoining elements ; b — on gussets

The distance between adjacent nodes of the chords is called a panel (d in - the panel of the upper chord, d n - the lower), and the distance between the supports is called the span (/).

Truss chords operate on longitudinal forces and moment (similar to the chords of solid beams); the truss lattice absorbs mainly the transverse force, performing the functions of the beam wall.

The sign of the force (minus - compression, plus - tension) in the lattice elements of trusses with parallel chords can be determined if we use the “beam analogy”.

Steel trusses are widely used in many areas of construction; in coatings and ceilings of industrial and civil buildings, bridges, power line supports, communication, television and radio broadcasting facilities (towers, masts), transport overpasses, hydraulic gates, load-lifting cranes, etc.

Farms have different designs depending on the purpose, loads and are classified according to various criteria:

By static diagram— beams (split, continuous, cantilever);

according to the outline of the belts - with parallel belts, trapezoidal, triangular, polygonal, segmental (Fig. 9.5);

Fig.9.4. Truss systems: A — split beam; b — continuous; c,e — console; G — arched; d — frame;

according to the lattice system - triangular, diagonal, cross, rhombic, etc. (Fig. 9.6);

according to the method of connecting elements in nodes - welded, riveted, bolted;

Rice. 9.5. Outlines of truss belts: a - segmental; b - polygonal; c - trapezoidal; g - with parallel belts; d-i - triangular

in terms of maximum force - light - single-walled with sections made of rolled profiles (force N 300 kN).

Intermediate between the truss and the beam are combined systems consisting of a beam reinforced from below with a sprengel or braces or an arch (from above). Reinforcing elements reduce the bending moment in the beam and increase the rigidity of the system (Fig. 9.4,^). Combined systems are easy to manufacture (have fewer elements) and are efficient in heavy structures, as well as in structures with moving loads.

Farm efficiency combined systems can be increased by creating pre-stress in them.

In mobile farms crane structures and coverings of large spans, where reducing the weight of the structure provides a great economic effect, aluminum alloys are used.

Rice. 9.6. Truss lattice systems

a - triangular; b - triangular with additional racks; c - braced with ascending braces; g - braced with descending braces; d - trussed; e - cross; g - cross; and - rhombic; to - the floor is slanted

Truss (structure)

Farm(fr. ferme, from lat. firmus strong) - a rod system in structural mechanics that remains geometrically unchanged after replacing its rigid nodes with hinged ones. In the truss elements, in the absence of misalignment of the rods and extra-nodal load, only tension-compression forces arise. Trusses are formed from straight rods connected at nodes.

The truss consists of elements: a belt, a stand, a brace, a truss (support brace).

History[edit]

Classification[edit]

Farms are classified according to the following criteria:

• The nature of the outline of the external contour
  • Parallel belts
  • Broken belts
  • Polygonal belts
  • Triangular belts
• Grate type
  • Triangular
  • Diagonal
  • Semi-diagonal
  • Rhombic
• Type of support
  • Beam
  • Arched
  • Cantilevered
  • Beam-cantilever
• Purpose
  • Rafters
  1. Pratt truss (with compressed posts and stretched braces)
  2. Warren truss (with triangle lattice)
  3. Belgian (triangular) truss
  4. cross-braced truss
  5. overhead light truss
  • Rafters
  • Pavements
  • Crane
  • Tower
• Material of execution
  • Wooden
  • Metal (steel and aluminum)
  • Reinforced concrete
  • Made from polymer materials

Scope[edit]

Trusses are widely used in modern construction, mainly for covering large spans in order to reduce the consumption of materials used and lighten structures, for example, in long-span building structures such as bridges, truss systems of industrial buildings, sports facilities, as well as in the construction of small light construction and decorative structures - pavilions, stage structures, awnings and podiums;

The fuselage of an airplane, the hull of a ship, the supporting body of a car (except for open bodies that work as a simple beam), a bus or a diesel locomotive, a carriage frame with a sprengel - from the point of view of strength of materials, are trusses (even if they do not have a frame as such - a truss structure in this case form stampings and reinforcements reinforcing the casing), accordingly, appropriate methods are used in their strength calculations.

Operating principle [edit]

If you arbitrarily fasten several rods on hinges, they will randomly spin around each other, and such a structure will be, as they say in structural mechanics, “changeable,” that is, if you press on it, it will fold, just like the walls of a matchbox fold. If you make an ordinary triangle out of rods, then the structure will only come together if you break one of the rods, or tear it off from the others; such a structure is already “unchangeable.”

The truss design contains these triangles. Both the tower crane boom and the complex supports are all made up of small and large triangles. Since any rods work better in compression-tension than in fracture, the load is applied to the truss at the points of connection of the rods.

In fact, the truss rods are usually connected to each other not through hinges, but rigidly. That is, if any two rods are cut off from the rest of the structure, they will not rotate relative to each other, however, in the simplest calculations this is neglected and it is assumed that there is a hinge.

Calculation methods [edit]

There are a huge number of ways to calculate trusses, both simple and complex. One of the simplest is calculation by cutting out nodes (hinges connecting the rods). This method is universal and suitable for any statically definable trusses. To calculate a truss, all forces acting on the truss are reduced to its nodes. Below are two calculation options.

The first is to first determine the reactions of the supports using conventional statics methods (drawing up equilibrium equations), then consider any node in which only two rods converge. The node is mentally separated from the truss, replacing the action of the cut rods with their reactions directed from the node. In this case, the rule of signs applies - the stretched rod has a positive force. From the equilibrium condition of a converging system of forces (two equations in projections), the forces in the rods are determined, then the next node is considered, in which again there are only two unknown forces, and so on until the forces in all the rods are found.

Another way is not to determine the reactions of the supports, but to replace the supports with support rods, and then cut out all the nodes (number n) and for each create two equilibrium equations. Next, the system is solved 2n equations and find all 2n forces, including forces in the support rods (reactions of the supports). In statically determinate trusses the system must be closed.

The method of cutting nodes has one significant drawback - the accumulation of errors in the process of sequential consideration of the equilibrium of nodes or curse of size matrices of a system of linear equations, if a global system of equations is compiled for the entire farm. The Ritter Method does not have this drawback. There is also an archaic graphical method - the Maxwell-Cremona diagram, which is, however, useful in the learning process. Modern practice uses computer programs, most of which are based on the knot cutting method. Sometimes the calculations use the Henneberg rod replacement method.

"Construction farms"

truss section rod box-shaped

Classification and scope of farms

The origin of the term "truss" comes from the Latin firmus, that is, "strong, strong."

A truss is a system of rods connected to each other at nodes and forming a geometrically unchangeable structure. With a nodal load, the stiffness of the nodes does not significantly affect the operation of the structure, and in most cases they can be considered as hinged. In this case, all truss rods experience only tensile or compressive axial forces.

Trusses are more economical than beams in terms of steel consumption, but are more labor-intensive to manufacture. The greater the span and the lower the load, the greater the efficiency of trusses compared to solid-wall beams.

Trusses can be flat (all rods lie in the same plane) and spatial.

Flat trusses carry loads applied only in their plane and need to be secured with ties. Spatial trusses form a rigid spatial beam that absorbs load in any direction (Fig. 9.1).

Rice. 9.1. Flat (a) and spatial (b) trusses

The main elements of the trusses are the belts that form the outline of the truss, and a lattice consisting of braces and racks (Fig. 9.2). The connection of elements in nodes is carried out by directly connecting one element to another (Fig. 9.3,a) or using uh yu nodal gussets (Fig. 9.3,b). The truss elements are centered along the axes of the center of gravity to reduce nodal moments and ensure that the rods operate under axial forces.

Rice. 9.2. Truss elements

1 - upper belt; 2 - lower belt; 3 - braces; 4 – racks

Rice. 9.3. Truss nodes: A - with direct adjoining elements ; b - on gussets

The distance between adjacent nodes of the chords is called a panel (d in - the panel of the upper chord, d n - the lower), and the distance between the supports is called the span (/).

Truss chords operate on longitudinal forces and moment (similar to the chords of solid beams); the truss lattice absorbs mainly the transverse force, performing the functions of the beam wall.

The sign of the force (minus - compression, plus - tension) in the lattice elements of trusses with parallel chords can be determined if we use the “beam analogy”.

Steel trusses are widely used in many areas of construction; in coatings and ceilings of industrial and civil buildings, bridges, power line supports, communication, television and radio broadcasting facilities (towers, masts), transport overpasses, hydraulic gates, load-lifting cranes, etc.

Trusses have different designs depending on their purpose, loads and are classified according to various criteria:

according to the static scheme - beams (split, continuous, cantilever);

according to the outline of the belts - with parallel belts, trapezoidal, triangular, polygonal, segmental (Fig. 9.5);

Fig.9.4. Truss systems: A- split beam; b - continuous; c,e- console; G- arched; d- frame;

according to the lattice system - triangular, diagonal, cross, rhombic, etc. (Fig. 9.6);

according to the method of connecting elements in nodes - welded, riveted, bolted;

Rice. 9.5. Outlines of truss belts: a - segmental; b - polygonal; c - trapezoidal; g - with parallel belts; d-i - triangular

in terms of maximum force - light - single-walled with sections made of rolled profiles (force N< 300 кН) и тяжелые - двухступенчатые с элементами составного сечения (усилие N >300kN).

Intermediate between the truss and the beam are combined systems consisting of a beam reinforced from below with a sprengel or braces or an arch (from above). Reinforcing elements reduce the bending moment in the beam and increase the rigidity of the system (Fig. 9.4,^). Combined systems are easy to manufacture (have fewer elements) and are efficient in heavy structures, as well as in structures with moving loads.

The efficiency of trusses of combined systems can be increased by prestressing them.

Aluminum alloys are used in trusses of movable crane structures and coverings of large spans, where reducing the weight of the structure provides a great economic effect.

Rice. 9.6. Truss lattice systems

a - triangular; b - triangular with additional racks; c - braced with ascending braces; g - braced with descending braces; d - trussed; e - cross; g - cross; and - rhombic; to - floor slanted

Sooner or later, the owners of a private house need to build a carport or summer holiday, a gazebo, a small fence with a roof for pets, a canopy over the woodpile. In order for the roof over such a structure to be securely fastened, it is necessary to correctly design and install metal supporting structures.

We welcome our dear reader and offer him an article about what profile pipe trusses are, how to correctly calculate and install them.

A truss is a structure of rectilinear elements connected to each other at nodes into a durable system of unchangeable geometric shape. Most often, flat structures are found, but in large loaded structures, volumetric (spatial) trusses are used. Almost in private houses, farms are made of wood and metal. Small structures of rafters, canopies, and gazebos are made from wood. But durable and high-tech metal is an almost ideal material for load-bearing metal structures.

For the manufacture of complex structures, rolled solid sections and pipes are used. Profile pipes (square, rectangle) have greater resistance to crushing and bending; small structures for the house are mounted without welding, therefore, for manor buildings, a profile pipe is most often used.

Structural features of trusses

Components of the truss structure:

• Belt.
• The stand is a vertical element connecting the upper and lower belts.
• Brace (brace).
• Sprengel - support brace.
• Grills, overlays, gussets, rivets, bolts - all kinds of auxiliary and fastening materials.

The height of the truss is calculated from the lowest point of the lower chord to the highest point. Span - the distance between supports. Rise is the ratio of the height of the truss to the span. The panel is the distance between the belt nodes.

Types of trusses from professional pipes

Farms are divided according to the outline of the belts. There are two-band and three-band varieties. In small structures, simpler two-belt trusses are used. Each variety has a certain slope and height depending on the length of the span and the shape of the truss.

Types of trusses according to the outlines of the chords: beams with parallel chords (rectangular), triangular (gable and single-pitch), trapezoidal (gable and single-pitch), segmental (parabolic), polygonal (polygonal), cantilever; with a broken raised or concave lower belt and a varied shape of the upper belt; arched with horizontal and arched lower belt; complex combined forms.

Trusses are also distinguished by types of gratings - see in the figure. In private buildings, triangular and diagonal grilles are most often found - simpler and less metal-intensive. Triangular gratings are usually used in rectangular and trapezoidal structures, while diagonal gratings are used in triangular ones.

Before erecting any structure, you should decide on the choice of material. At the time of buying metal profile or pipes, you should carefully inspect the workpieces to see if there are any cracks, cavities, sagging, inconsistencies along the seam, or a large number of dented and bent workpieces. When purchasing galvanized materials, it is advisable to check the quality of the coating - whether there are any peelings or sagging.

When purchasing, you must request a copy of the certificate and a receipt. It is imperative to ensure that the pipe wall thickness corresponds to that stated in the documents. You can’t make pipes in a garage on your knees, and there are no fakes, but you can come across poor quality material, so it’s better to buy in fairly large stores.

What material to choose for the frame

In most cases, steel is chosen for the frame of manor buildings or the roof of a house. For very small structures, aluminum is sometimes used, usually in purchased products (awnings, rocking chairs). For the construction of metal structures, you can use pipes of hollow section and solid section profiles (circle, strip, square, channel, I-beam).

A huge advantage of rectangular and square pipes compared to a profile of the same weight is their high resistance to crushing and other deformations. Therefore, solid profiles can be replaced with much lighter corrugated pipes - this greatly simplifies (2 times or more) and reduces the cost of a tubular type design.

The cross-sectional dimensions of the pipes are selected depending on the span length and the distance between the supports and trusses. In private estates, sheds and other structures are not very large, and you can take the advice of experts or find ready-made drawings on the Internet.

With a distance between supports of up to 2 m, for small canopies with spans up to 4 m long, profiles of 40x20x2 mm are suitable, for spans of up to 5 m - 40x40x3, 60x30x3 mm; spans longer than 5 m – 60×40x3, 60×60x3 mm. If you are planning a carport for two cars with a width of 8-10 m, then a profile will be required from 60x60 to 100x100 with a wall thickness of 3-4 mm. The profile dimensions depend on the distance between the trusses.

Corrugated pipes are sold in lengths of 6 and 12 m. With a length of 12 m, metal is consumed more economically, but transporting such pipes requires a long length. Before purchasing materials, you should think about how you will cut the blanks and how many of them will fit in a pipe 6 m or 12 m long, and calculate how many sections of corrugated pipe you will need.

You cannot rely on the nominal weight - the weight is 1 m.p. in a particular batch will differ from the nominal one, and most likely upward (it is more profitable for sellers to produce products with a thicker wall - the price is per ton). When purchasing by weight, the material will have to be purchased and transported - and this is an extra expense.

Advantages and disadvantages of different metals

In practice, the following types of steel are used for structural profile pipes: carbon of ordinary quality and high-quality, structural, alloyed. Pipes come with a protective zinc coating. Aluminum is also used - but rarely, for small, often seasonal structures. Aluminum profiles are used for small structures.

Traditionally, for small structures on a private estate, carbon steel St3sp, St3ps, and sometimes galvanized, is used for the construction of steel structures with trusses. This steel has sufficient strength to ensure the reliability of the structure; there is practically no difference in corrosion resistance between all three types of steel.

If structures are exposed to precipitation, sooner or later both structural and alloy steel products will rust. A small amount of alloying elements do not protect against corrosion (for structures, low-alloy steels such as 30KhGSA, 30KhGSN, 38KhA can be used - the content of alloying elements in them is 2-4%, and this amount does not affect corrosion resistance).

In terms of strength, structural and alloy steels should be slightly more durable than carbon steels - they are more resistant to cyclic loads. But this quality in steels manifests itself after heat treatment - and quenching and tempering can warp pipes, and usually no one does such heat treatment on finished products. Seamless pipes can be annealed - after annealing, residual stresses in the metal are removed (hardening), but it becomes softer.

Structural steels (20A, 45, 40, 30A) have a higher quality and a higher price. Alloy steels are even more expensive (and there is a chance that they will sell you pipes made from steel 3 instead of alloy steel). Therefore, when installing structures less than 20 m wide, it does not make sense to buy professional pipes made of alloy or structural steel. It definitely makes sense to use galvanized corrugated pipe if the installation will be carried out using crab systems.

If installation is carried out by welding, the welds will rust as quickly as ordinary uncoated metal. But if you carefully monitor the seams and regularly carry out anti-corrosion treatment (cleaning, priming, painting), then a galvanized pipe is preferable. If you need a temporary shed for 10 years for building materials, and then you will demolish the shed, don’t bother, buy ordinary pipes made of carbon steel without coating.

If you are planning to build a very large canopy or hangar with a long span on the site, you should contact professional builders and make a project - they will determine what steel you should choose.

Do it yourself or order

Trusses for a carport or gazebo roof have small sizes and a simple design - most often triangular with several struts and racks. You can complete such a design yourself if you have at least basic welding skills and are not afraid to learn new jobs.

But making trusses requires accuracy, the presence of an assistant, a very flat area on the estate for laying out and welding structures, the presence of a welding machine and time. You can order ready-made structures from a factory or construction company and install them yourself.

Requirements for calculating a profile pipe for the construction of a farm

When calculating the dimensions and wall thickness of profile pipes required for the construction of your metal structures; the following conditions are taken into account:

• Dimensions of the metal structure, and in particular, length, support spacing - the distance between supports.
• Height of supports and trusses.
• Farm shape.
• Possible features of geological conditions (seismic activity, possibility of landslides).
• Coating weight.

What happens if you calculate incorrectly

If the calculations are incorrect, the following consequences are possible:

• Farm structures will be deformed under the weight of snow and wet leaves.
• In the worst case, the structures will deform under their own weight.
• The entire structure may collapse in strong winds.
• Deformation will sooner or later lead to the destruction of the truss and the entire structure, which is dangerous for humans and can damage objects located under the canopy - a car, for example.
• A fragile and movable structure will lead to the destruction of the roof laid on the truss.
• When using a profile that is too powerful and heavy, the costs of materials and work during the construction of metal structures increase unjustifiably.

We design a farm and its elements

A complete and accurate calculation of the load on a truss along with diagrams is complex, and to perform it you should contact specialists.

When designing large canopies, hangars, and garages made of metal structures, an accurate calculation of the required profile is necessary, but for the construction of not too large canopies or gazebos in a private estate, you can use the well-known recommendations of experts.

For very small structures (a canopy in an animal enclosure, a canopy over a firewood store), it is enough to use pipes measuring 40x20 mm with a wall thickness of 2 mm; for gazebos and canopies over tables, barbecues or recreation areas - 40x40 mm with a wall thickness of 3 mm; canopy over a place for a car - from 60x40 to 100x100 mm with a wall thickness of 3-4 mm.

If the canopy has several trusses and supports and the support spacing is less than 2 m, you can take a thinner pipe; if there are only 4 supports and two trusses and the span length is 6-8 m or more, you can take a thicker one.

Permissible loads on trusses are given in the table:

Drawings and diagrams

In the manufacture of metal structures, execution of a drawing with exact dimensions Necessarily! This will allow you to purchase the required amount of material, save time when assembling and preparing workpieces, and allow you to easily control the dimensions of the metal structure during installation and the finished structure. In this case, the safety of you and your household depends on the accuracy of the assembly - a structure that collapses due to snow or wind can bring a lot of trouble.

Truss Calculation Basics

The types of trusses depend on the shape of the roof, and the shape of the roof of a structure on an estate is chosen depending on the purpose and location of the metal structures. Cantilever trusses and farms adjacent to the house are usually made with single-pitched triangular ones, free-standing canopies - with polygonal, triangular, segmental structures and arches. Gazebos can have a six- or eight-slope roof or a fantasy roof with trusses of a non-standard design.

To calculate the trusses, it is necessary to calculate the load on the roof and on one truss. The calculations take into account the load of snow cover, roofing, sheathing, and the weight of the structures themselves. Accurate calculations are a task for a professional builder. The basis for the calculation is SP 20.13330.2016 “Loads and impacts. Updated edition of SNiP 2.01.07-85" and SP 16.13330.2011 "Steel structures. Updated edition of SNiP II-23-81".

For calculations, the cutting method is used: cutting out nodes (areas where the rods are hingedly connected); Ritter method; Henneberg rod replacement method. In modern computer programs, the method of cutting nodes is more often used.

It's better to use a ready-made one standard project or our recommendations for choosing profiles. Assembling a farm of a simple trapezoidal or triangular structure is not too difficult, and if you have experience in welding and installing metal structures, independent installation of canopies and gazebos is quite possible. If you want to build a large shed with a truss length of 10 m or more, you need to complete the project with specialists.

Influence of inclination angle

The design of the truss is primarily affected by the angle of inclination of the ramps (ramp). The angle of inclination is selected primarily depending on the shape of the roof and the placement of the metal structure. Sheds adjacent to buildings should have a larger roof angle to allow snow sliding off the roof to roll off more quickly and flowing water to drain away.

For single structures, the roof slope may be less. The angle of inclination also depends on the amount of precipitation that falls in your region - the more precipitation, the greater the angle of inclination of the roof should be. The steeper the roof, the less precipitation it retains.

A slight slope of the slope - up to 15° - is used on small free-standing sheds. The height of the slope is approximately equal to 1/7-1/9 of the span length. Trapezoidal trusses are used.

Slope from 15° to 22° - the height of the slope is equal to 1/7 of the span length.

Slope from 22° to 30°-35° - the height of the slope is equal to 1/5 of the span length; with this slope, triangular structures are usually used, sometimes with a broken lower chord to make the structure lighter.

Base Angle Options

To correctly calculate quantities and lengths individual elements for a truss made of corrugated pipe, it is necessary to determine the base angles between the elements. In general, the lower chord is perpendicular to the supports, the upper chord is inclined to the horizontal depending on the angle of the roof. The optimal angle of inclination of the braces to the horizontal/vertical is 45°, the racks must be strictly vertical.

The exact angle of inclination of the roof is either specified by the project, or is found according to the relationships given above ( for a slope of up to 15° - the height of the slope is approximately equal to 1/7-1/9 of the span length; for a slope from 15° to 22° - 1/7 of the span length; for a slope from 22° to 30° - 35° - the height of the slope is equal to 1/5 of the span length).

Having determined the exact angle of inclination of the roof, the lengths of the blanks for making the truss are determined - this information will be required when performing the work.

Significant factors for site selection

If you have a choice, you should choose a flat area for installing metal structures that is not prone to landslides and waterlogging. But in small garden plots there is often no choice - a carport is placed immediately behind the gate, a veranda near the house, a gazebo in the back of the plot. The area may need to be leveled and sometimes drained.

If there is a danger of soil layers sliding, or you live in an earthquake-prone area, the design of any structure above a dog kennel should be left to professionals to ensure your safety.

How to calculate the load

Snow load per 1 m² of roof is calculated according to SP 20.13330.2017 “Loads and impacts. Updated version of SNiP 2.01.07-85" depending on the region. When calculating, it is not the roof area that is taken, but the area of ​​the roof projection onto the horizontal. The weight of the sheathing and roofing is calculated in the same way. According to the drawing, the weight of one truss is calculated and multiplied by their number.

The load on one truss is calculated by dividing the sum of the total load on the roof of snow, the weight of the sheathing and covering, the weight of the structures themselves, by the number of trusses.

Entrance door and canopy

The canopies above the front door are small in size and cantilevered.

The width of the canopy should be equal to the width of the porch + 300 mm on each side. The depth of the canopy should cover the steps. The length of the canopy is equal to the sum of the length of the platform and steps. Length upper platform should be one and a half times wider than the door, that is, 0.9 × 1.5 = 1.35 m. Plus 250 mm for each step.

For example:

for a porch with two steps and a width of 1200 mm, the dimensions of the covered area (horizontal projection of the canopy) are equal to:

length (visor depth) = 1.35 + 2×0.25 = 1.85 m;

width = 1.2 + 0.3×2 = 1.8 m.

Free calculation programs

• On the site http://sopromatguru.ru/raschet-balki.php.
• On the site http://rama.sopromat.org/2009/?gmini=off.

Calculation example

Example of truss calculation separately standing canopy for a middle class car (D):

The width of the car is 1.73 m, length 4.6 m.

Minimum truss width between supports:

1.73 + 1 = 2.73 m, for ease of opening doors we take a width of 3.5 m.

Truss width including roof overhangs:

3.5 + 2×0.3 = 4.1 m.

Canopy length:

4.6 + 1 = 5.6 m, take a length of 6 m.

With this length it is possible to install supports every 2 m or less. To lighten the load-bearing structures, we take the distance between the supports to be 1.5 m.

We adopt a triangular gable roof shape - it is the easiest to manufacture and at the same time economical in terms of material consumption. We take the roof inclination angle to be 30° - at this inclination angle, snow and fallen leaves will not linger on the roof.

The height of the truss in the center (central post) will be equal to:

Total: the length of the lower chord of the truss is 4.1 m; the upper belt - two halves of 2.355 m each, total length 4.71 m, the stand in the center has a height of 1.16 m.

For such short trusses, it is quite enough to use a 40x40 mm square pipe with a wall thickness of 3 mm.

The main stages of work on the manufacture and installation of trusses with your own hands

Before installing the trusses, work is carried out on site planning, installation of supports, concreting of support foundations, welding of side braces or side trusses. Then the transverse trusses are installed.

The procedure for performing work on the manufacture and installation of trusses:

• Trusses are welded on a flat surface.
• The trusses are treated with an anti-corrosion primer and painted twice. Do not paint areas where trusses are welded to supports. This work can be done after the trusses are installed, but painting at height is inconvenient.
• They lift the trusses, install them on supports, check the angles and horizontality, and weld them to the supports. This work is performed by a team of several people.
• Paint over the welding areas.
• The sheathing is installed and the roofing is laid.

How to weld trusses

The trusses are assembled on a level area. Before assembly, the workpieces are cut, cleaned of rust, and burrs on the cuts are sanded off. The truss elements are fastened with clamps, the dimensions, angles, and flatness are checked. Weld the structure on one side, let it cool, and turn it over to the other side. Remove the clamps and boil the other side. Then the bead on the seam is sanded off. You can see the features of welding trusses in our video:

If you have limited skills as a welder and installer, you can order the manufacture of a truss from a specialized organization or team.

Conclusion

Construction of a canopy and installation of trusses is complex, skilled work. Small canopies and gazebos can be made independently with the help of family members.

It is better to entrust the installation of large metal structures to a team of professionals. But professionals also need supervision. We say goodbye to our dear reader and hope that our article will help you understand the types of trusses, the choice of design, material and the procedure for constructing canopies and gazebos on your site. Subscribe to our website's newsletter, bring friends, share interesting information with your interlocutors on social networks.

A truss is a system of rods connected to each other at nodes and forming a geometrically unchangeable structure. With a nodal load, the stiffness of the nodes does not significantly affect the operation of the structure, and in most cases they can be considered as hinged. In this case, all truss rods experience only tensile or compressive axial forces.

Trusses are more economical than beams in terms of steel consumption, but are more labor-intensive to manufacture. The greater the span and the lower the load, the greater the efficiency of trusses compared to solid-wall beams.

Trusses can be flat (all rods lie in the same plane) and spatial.

Flat trusses carry loads applied only in their plane and need to be secured with ties. Spatial trusses form a rigid spatial beam that absorbs load in any direction (Fig. 9.1).

The main elements of the trusses are the belts that form the outline of the truss, and a lattice consisting of braces and racks (Fig. 9.2). The connection of elements in nodes is carried out by directly adjoining one element to another (Fig. 9.3 a) or using nodal gussets (Fig. 9.3 b). The truss elements are centered along the axes of the center of gravity to reduce nodal moments and ensure that the rods operate under axial forces.

1 – top belt; 2 – bottom belt; 3 – braces; 4 - racks

A - with direct adjoining elements; b – on gussets

The distance between adjacent nodes of the chords is called a panel (d in - the panel of the upper chord, d n - the lower), and the distance between the supports is called the span ( l).

Truss chords operate on longitudinal forces and moment (similar to the chords of solid beams); the truss lattice absorbs mainly the transverse force, performing the functions of the beam wall.

The sign of the force (minus - compression, plus - tension) in the lattice elements of trusses with parallel chords can be determined if we use the “beam analogy”.

Steel trusses are widely used in many areas of construction; in coatings and ceilings of industrial and civil buildings, bridges, power line supports, communication, television and radio broadcasting facilities (towers, masts), transport overpasses, hydraulic gates, load-lifting cranes, etc.

Trusses have different designs depending on their purpose, loads and are classified according to various criteria:

according to a static scheme– beams (split, continuous, cantilever); arched, framed, combined (Fig. 9 4);

A - beam split; b – continuous; c, e – console; G - arched; d – frame; and - combined

according to the outline of the belts– with parallel belts, trapezoidal, triangular, polygonal, segmental (Fig. 9.5);

by grid system– triangular, diagonal, cross, rhombic

etc. (Fig. 9.6);

by the method of connecting elements in nodes– welded, riveted, bolted;

by maximum force– light – single-walled with sections made of rolled profiles (force N kN) and heavy – two-stage with elements of a composite section (N > 300 kN).

Intermediate between the truss and the beam are combined systems consisting of a beam reinforced from below with a sprengel or braces or an arch (from above). Reinforcing elements reduce the bending moment in the beam and increase the rigidity of the system (Fig. 9.4, and). Combined systems are easy to manufacture (have fewer elements) and are efficient in heavy structures, as well as in structures with moving loads.

The efficiency of trusses and combined systems can be increased by prestressing them.

Aluminum alloys are used in trusses of movable crane structures and coverings of large spans, where reducing the weight of the structure provides a great economic effect.

Rice. 9.6. Truss lattice systems

A - triangular; b – triangular with additional posts; V - diagonalWithascending braces; G - braced with downward braces; d – trussed; e – crusade; and - cross; And - rhombic; To - semi-braced

9.2. Truss structure layout

Selecting a static diagram and outline of a truss is the first stage of structural design, depending on the purpose and architectural and constructive solution of the structure and is made on the basis of a comparison of possible options.

Split beam systems have been used in building coverings, bridges, transport galleries and other structures. They are easy to manufacture and install, do not require complex components, but are very metal-intensive. With beam spans of 40m, the split trusses are oversized and are assembled during installation.

For two or more overlapped spans, continuous trusses are used. They are more economical in terms of metal consumption and have greater rigidity, which makes it possible to reduce their height. The use of continuous trusses in weak soils is not recommended, since additional forces arise when the supports settle. In addition, the continuous nature complicates installation.

Frame trusses are more economical in terms of steel consumption, have smaller dimensions, but are more difficult to install. It is rational to use them for long-span buildings. Arched systems save steel, but lead to an increase in the volume of the room and the surface of the enclosing structures. Their use is dictated by architectural requirements. Cantilever trusses are used for canopies, towers, and power line supports.

The outlines of the trusses must correspond to their static diagram and the type of loads that determine the diagram of bending moments. For roofing trusses, it is necessary to take into account the roofing material and the required slope to ensure drainage, the type of interface with the columns (rigid or hinged) and other technological requirements.

The outline of the truss belts determines their efficiency. The most economical in terms of steel consumption is a truss outlined according to a moment diagram. For a single-span beam system with a uniformly distributed load, there will be segment truss with a parabolic belt (see Fig. 9.5, A). However, curved belts are very labor-intensive to manufacture, so such trusses are used extremely rarely. Polygonal trusses are more commonly used (see Fig. 9.5, b). In heavy, long-span trusses, additional structural difficulties due to fracture of the chords at the nodes are not so noticeable, since, due to the conditions of transportation, the belts in such trusses have to be joined at each node.

For light trusses, a polygonal outline is irrational, since the complication of nodes is not compensated by the savings in steel.

Farms trapezoidal ( see Fig.9.5, V), although they do not exactly correspond to the moment diagram, they have design advantages due to the simplification of the nodes. In addition, the use of such trusses in the covering makes it possible to construct a rigid frame assembly, which increases the rigidity of the building.

Farms with parallel belts (Fig. 9 5, G) in their outline are far from the moment diagram and are uneconomical in terms of steel consumption. However, equal lengths of lattice elements, the same layout of nodes, repeatability of elements and parts, and the possibility of their unification contribute to the industrialization of their production. Therefore, trusses with parallel belts have become the main ones for covering industrial buildings.

Farms triangular outlines (see Fig. 9.5, d-j,And) are rational for cantilever systems and for beam systems with a concentrated load in the middle of the span (rafter trusses). The disadvantage of these trusses is the increased consumption of metal under distributed load; the sharp support unit is complex and allows only hinged coupling with columns. The middle braces are very long and they have to be selected for maximum flexibility, which leads to excessive consumption of metal. However, sometimes they are used for rafter structures, when it is necessary to provide a large roof slope (over 20%) or to create one-sided uniform lighting (shade coverings).

The span or length of the trusses is determined by the operational requirements and the general layout of the structure and is recommended by the designer.

Where the span is not dictated by technological requirements (for example, overpasses supporting pipelines, etc.), it is assigned on the basis of economic considerations, at the lowest total cost of trusses and supports.

Height of triangular trusses (see Fig. 9.5, d) is a function of the span and slope of the truss (25-45 0), which gives the height of the trusses h . The height is usually higher than required, so triangular trusses are not economical. The height of the truss can be reduced by giving the lower chord a raised outline (see Fig. 9.5, G), but the support node should not be very sharp.

For the height of trapezoidal and parallel chord trusses

There are no design restrictions; the height of the truss is taken from the condition of the least weight of the truss. The weight of the truss consists of the weight of the belts and lattice. The weight of the belts decreases with increasing height of the truss, since the forces in the belts are inversely proportional to the height h

The weight of the lattice, on the contrary, increases with the height of the truss, as the length of the braces and racks increases, so the optimal height of the trusses is 1/4 - 1/5 of the span. This leads to the fact that, with a span of 20 m, the height of the truss is greater than the maximum (3.85 m) permissible under transportation conditions. Therefore, taking into account the requirements of transportation, installation, and unification, the height of the trusses is taken within 1/7 - 1/12 of the span (for light trusses even less).

The smallest possible height of the truss is determined by the permissible deflection. In conventional roofing coverings, the stiffness of the trusses exceeds the required one. In structures operating on a moving load (trusses of crane trestles, overhead cranes, etc.), the rigidity requirements are so high

(f/l= 1/750 - 1/1000) that they dictate the height of the truss.

The deflection of the truss is determined analytically using Mohr's formula

Where Ni– force in the truss rod from a given load; - force in the same rod from the force, equal to one, applied at the point where the deflection is determined in the direction of the deflection.

Panel dimensions must correspond to the distances between the elements transmitting the load to the truss and correspond to the optimal angle of inclination of the braces, which in a triangular lattice is approximately 45 0, and in a braced lattice - 35 0. For design reasons - the rational outline of the gusset in the assembly and the convenience of attaching the braces - an angle close to 45 0 is desirable.

In roof trusses, panel sizes are taken depending on the roofing system.

To avoid bending of the belt, it is desirable to ensure the transfer of load from the roof to the truss nodes. Therefore, in coatings made of large reinforced concrete or metal slabs, the distance between nodes is taken equal to the width of the slab (1.5 m or 3 m), and in coatings along purlins

– purlin pitch (from 1.5m to 4m). Sometimes, to reduce the size of the belt panel, a truss lattice is used (see Fig. 9.6, d).

Unification and modulation of the geometric dimensions of trusses makes it possible to standardize both the trusses themselves and the elements adjacent to them (purlins, connections, etc.). This leads to a reduction in the number of standard sizes of parts and makes it possible to use specialized equipment for mass production of structures and switch to mass production.

Currently, the geometric diagrams of trusses of industrial buildings, bridges, radio masts, radio towers, and power line supports have been unified.

Construction lift. In trusses of large spans (more than 36 m), as well as in trusses made of aluminum alloys or high-strength steels, large deflections occur, which worsen the appearance of the structure and are unacceptable under operating conditions.

Sagging of the trusses is prevented by the rafter lifting device, i.e.

production of trusses with reverse camber, which is extinguished under the influence of load, and the truss takes its design position. The construction lift is assigned equal to the deflection from the permanent load plus half of the temporary loads. At flat roofs and for spans greater than 36 m, the construction lift should be taken, regardless of the size of the span, equal to the deflection from the total standard load plus 1/200 of the span.

Construction lifting is ensured by installing bends in the mounting units (Fig. 9.7).

Truss lattice systems and their characteristics. The truss lattice works on transverse force, performing the functions of the wall of a solid beam.

The weight of the truss, the complexity of its manufacture, and appearance depend on the lattice system. Since the load on the truss is transmitted at the nodes, the lattice must correspond to the load application pattern.

Triangular lattice system. In trusses of a trapezoidal shape or with parallel chords, a triangular lattice system is rational

(see Fig.9.6, A), giving the smallest total length of the lattice and smallest number nodes with the shortest force path from the point of application of the load to the support. In trusses supporting roof purlins or deck beams, additional posts are often added to the triangular lattice (Fig. 9.6, b), and sometimes suspensions, allowing to reduce the distance between the truss nodes. Additional racks also reduce the estimated length of the compressed belt. Additional racks work only for local loads and do not participate in the transmission of lateral force to the support.

Rice. 9.7. Construction lifting schemes with one ( A) and several(b) enlarged joints

The disadvantage of the triangular system is the presence of long compressed braces (ascending in trusses with parallel chords and descending in triangular trusses).

Diagonal grating system, used for low truss heights, as well as when large forces are transmitted through the racks (with a large nodal load).

A diagonal lattice is more labor-intensive than a triangular lattice and requires a large consumption of metal, since with an equal number of panels in the truss, the total length of the diagonal lattice is longer and there are more nodes in it. The force path from the node to the support in a braced lattice is longer; it goes through all the lattice rods and nodes.

Special grating systems, used for high truss heights (approximately 4 - 5 m). To reduce the size of the panel while maintaining the normal angle of inclination of the braces, use a truss grid (see Fig. 9.6, d). The installation of a trussed grid is more labor-intensive and requires additional metal consumption; however, such a lattice makes it possible to obtain a rational distance between the elements of the transverse structure at a rational angle of inclination of the braces and to reduce the design length of the compressed rods.

Sprengel grating is used for steep roofs and relatively large spans ( l= 20 – 24m) for a triangular truss (see Fig. 9.5, e).

In farms operating on double-sided loads, they arrange crusade grid (see Fig. 9.6, e). Such trusses include horizontal braced trusses of coatings of industrial buildings, bridges and other structures, vertical trusses of towers, masts and tall buildings.

Rhombic and semi-diagonal gratings (see Fig. 9.6, And,To) thanks to two bracing systems they have great rigidity; These systems are used in bridges, towers, masts, and connections to reduce the design length of the rods and are especially rational when structures operate under large lateral forces.

Ensuring the stability of trusses. A flat truss is unstable from its plane, so it must be attached to a more rigid structure or connected by connections to another truss, resulting in the formation of a stable spatial beam (Fig. 9.8, A). Since this

1 - diaphragm

The spatial beam is closed in cross section, it has great rigidity during torsion and bending in the transverse direction, so loss of its overall stability is impossible. Structures of bridges, cranes, towers, masts, etc. They are also spatial beams consisting of trusses (Fig. 9.8, b).

In building roofs, due to the large number of flat roof trusses placed nearby, the solution becomes more complicated, so trusses connected to each other only by purlins may lose stability.

Their stability is ensured by the fact that two adjacent trusses are fastened with ties in the plane of the upper and lower chords and vertical transverse ties (Fig. 9.9, b). Other trusses are attached to these rigid blocks

horizontal elements that prevent horizontal movement of truss chords and ensure their stability (purlins and spacers located at truss nodes). In order for the purlin to secure the truss assembly in the horizontal direction, it itself must be attached to

a fixed point - a node of horizontal connections.

1 – runs; 2 – farms; 3 – horizontal connections ; 4 – vertical connections ; 5 – spatial block

9.3. Types of truss rod sections

The most common types of sections of light truss elements are shown in Fig. 9.10.

In terms of steel consumption, the most efficient is the tubular section (Fig. 9.10, A). The pipe has good streamlining, so the wind pressure is lower, which is important for tall structures (towers, masts, cranes). The pipes retain little frost and moisture, so they are resistant to corrosion; They are easy to clean and paint. This increases the durability of tubular structures.

To prevent corrosion of internal surfaces, tubular elements should be sealed. However, certain design difficulties in pairing tubular elements and the high cost of pipes limit their use.

Rectangular bent closed sections (Fig. 9.10, b) have almost the same advantages as tubular ones, they make it possible to simplify the interfaces between elements and are widely used. However, trusses made of bent closed profiles with chamferless units require high manufacturing precision.

Technological difficulties do not allow the production of bent profiles with a thickness of more than 10-12 mm. This limits their use.

In addition, large plastic deformations at bending angles reduce the brittle strength of steel.

Often the sections of truss elements are taken from different types of profiles: belts made of I-beams, a lattice made of curved closed profiles or belts made of T-bars, a lattice made of paired or single corners. This solution turns out to be more rational.

In spatial trusses (towers, masts, crane booms, etc.), where the belt is common to two trusses, its cross-section should ensure convenient coupling of elements in different planes. This requirement is best met by a tubular section.

In tetrahedral trusses at no great effort, the simplest type of belt section is a single corner or a cross section of two corners. For greater efforts, I-beams are also used.

Compressed truss elements should be designed to be equally stable in two mutually perpendicular directions.

In each specific case, the choice of the type of section of truss elements is determined by the operating conditions of the structure (degree of aggressiveness of the environment, the nature and location of application of loads, etc.), the possibility of manufacturing, the availability of assortment and economic considerations.

Heavy Truss Rods They differ from the lungs in having more powerful and developed sections, composed of several elements. Sections of such rods are usually designed as double-walled (Fig. 9.11), and nodal connections are made using gussets located in two planes. The rods of heavy trusses (braces, racks and chords) have different sections, but for ease of mating at the nodes the width of the elements is “ V” should be the same.

For truss belts, it is desirable to use sections with two axes of symmetry, which facilitates the junction of two sections of adjacent panels of different areas at a node and does not create an additional moment due to the mismatch of the centers of gravity of these sections.

Heavy trusses that operate under dynamic loads (railroad bridges, cranes, etc.) are sometimes also designed with riveted trusses, but generally, as a rule, they are designed from welded rods with mounting units on high-strength bolts.

The following types of sections of heavy steel truss rods are used:

H-shaped(Fig.9.11, b) - two vertical sheets connected by a horizontal sheet, as well as riveted from four unequal angles connected by a horizontal sheet (Fig. 9.11, V). The development of such sections in adjacent panels is carried out by fastening additional vertical sheets (Fig. 9.11, G). Such sections are not labor-intensive. If the structure is not protected from

precipitation, then horizontal elements It is necessary to leave holes for water drainage with a diameter of 50 mm. H-shaped sections are used for belts and braces.

Channel section consists of two channels placed with shelves inside (Fig. 9.11, d); Both rolled and composite channels are used. This cross-section is suitable for compressed elements, especially when they are long. The disadvantage of the channel section is the presence of two branches that have to be connected by planks or gratings (similar to centrally compressed columns).

Box section consists of two vertical elements connected by a horizontal sheet on top (Fig. 9.11, e,and). Applicable in

Fig.9.11. Types of sections of heavy truss rods

mainly for the upper chords of heavy bridge trusses. The rigidity of the section increases if the vertical sheets are connected from below with a lattice (Fig. 9.11, and) or perforated sheet.

Single-wall I-section consists of a welded or wide-flange rolled I-beam placed vertically (Fig. 9.11, And).

Tubular rods used in heavy welded trusses, they have the same advantages as in light trusses.

Closed box section(Fig.9.11, k, l, m) has increased flexural and torsional rigidity, so it is used for long compressed elements of heavy trusses. The section can be made either from bent elements or welded elements made up of four sheets.

9.4. Truss calculation

Determination of design load. The entire load acting

it is usually applied to the truss in the truss nodes to which elements of the transverse structure are attached (roof purlins or dropped ceilings), transferring the load to the truss. If the load is applied directly to the panel, then in the main design scheme it is also distributed between the nearest nodes, but the local bending of the belt due to the load located on it is additionally taken into account. In this case, the truss belt is considered as a continuous beam with supports at the nodes.

constant, which includes the own weight of the truss and the entire supported structure (roof with insulation, lanterns, etc.).

temporal– loads from suspended underground transport equipment, payload acting on suspended to the truss attic floor, and so on.

short-term For example , atmospheric- snow, wind.

The calculated constant load acting on any rafter node depends on the load area from which it is assembled (Fig.9.12) and is determined by the formula

where is the dead weight of the truss and connections, kN/m? horizontal projection of the roof; - roof weight, kN/m?; - angle of inclination of the upper belt to the horizon; - distance between farms; and - panels adjacent to the assembly; - reliability factor for constant load.

In individual nodes, the load from the weight of the lantern is added to the load obtained from formula (9.2).

Snow is a temporary load and can only partially load the farm; Loading one half of the truss with snow may not be beneficial for medium braces.

The calculated nodal load from snow is determined by the formula:

where is the weight of snow cover per 1 m? horizontal projection of the roof; - reliability coefficient for snow load.

Meaning “ S” should be determined taking into account possible uneven distribution of snow cover near the canopy or elevation changes.

Wind pressure is taken into account only on vertical surfaces, as well as on surfaces with an angle of inclination to the horizon of more than 30 0, which happens in towers, masts, overpasses, as well as in steep triangular trusses and lanterns. The wind load is reduced to a nodal load. The horizontal load from the wind on the lantern is not taken into account when calculating the truss, since its influence on the operation of the truss is not significant.

9.5. Determination of forces in truss rods

When calculating trusses with rods made from angles or tees, it is assumed that the system nodes have ideal hinges, the axes of all rods are straight, located in the same plane and intersect at the centers of the nodes (see Fig. 9.12). The rods of such a system work only under axial forces: the stresses found from these forces are the main ones.

In trusses with rods having increased rigidity, the influence of the stiffness of the joints in the nodes is more significant. Moments arising in nodes lead to earlier occurrence of plastic deformations and reduce the brittle strength of steel. Therefore, for I-beam, tubular and H-shaped sections, the calculation of trusses using a hinged system is allowed with a ratio of section height to length of no more than for structures operated at a design temperature not lower than – 40 0 ​​C. When increasing these ratios, additional bending moments in the rods should be taken into account from knot stiffness.

In the upper chords of trusses with continuous support of the decking on them (uniform distribution of the load on the chords of the truss), it is allowed to calculate the moments using the following formulas:

passing moment in the outer panel

;

span moment of intermediate panels

;

moment at the node (reference)

,

In addition, stresses from moments arise in the rods as a result of incomplete centering of the rods in the nodes. These stresses, which are not the main ones in the calculation, are not taken into account, since the permissible eccentricities in the trusses are small.

The displacement of the truss chord axis when changing sections is not taken into account if it does not exceed 1.5% of the chord height.

The calculation of trusses should be performed on a computer, which allows you to calculate any truss scheme for static and dynamic loads.

The use of a computer makes it possible to obtain calculated forces in rods taking into account the required combinations of loads, to optimize the design, i.e. find the optimal truss design, rod material, type of sections, etc., to obtain the most economical design solution.

In the absence of a computer, the forces in the truss rods are determined graphically, i.e. by constructing Maxwell-Cremona diagrams, or analytically (by cutting out nodes). Moreover, for each type of load (load from pavement, overhead transport, etc.) they build their own diagram. For trusses with simple designs (for example, with parallel chords) and a small number of rods, analytical determination of forces is simpler.

If the truss operates on a moving load, then the maximum force in the truss rods is determined along the line of influence.

In accordance with the classification of load combinations (main and special), forces are determined separately for each type of combination and the load-bearing capacity of the rods is determined by the final calculated maximum force.

It is recommended that the results of the static calculation be recorded in a table that should show the values ​​of forces from a constant load, from possible combinations of temporary loads (for example, from one-sided loading with snow), as well as the calculated forces as a result of the summation of forces under unfavorable loading for all possible combinations of loads .

9.6. Determination of the design length of the rods

At the moment of loss of stability, the compressed rod bulges, rotates around the centers of the corresponding nodes and, due to the rigidity of the gussets, forces the remaining rods to rotate and bend in the plane of the truss.

Adjacent bars resist bending and rotation of the assembly and

Prevents free bending of the rod, which loses stability.

The greatest resistance to rotation of the assembly is provided by stretched rods. Compressed rods have little resistance to bending.

Thus, the more stretched rods are adjacent to a compressed rod and the more powerful they are (the greater their linear stiffness), the higher the degree of pinching of the rod and the shorter its design length; the influence of compressed rods on pinching can be neglected.

The compressed belt turns out to be weakly pinched at the nodes, since on each side it is adjacent to only one stretched brace, the linear stiffness of which is significantly less than the linear stiffness of the belt. Therefore, the pinching of the compressed belt in the stability margin can be neglected and its design length can be taken equal to the distance between adjacent nodes.

Thus, with a greater degree of pinch, the design length of the truss rod is shorter

where is the length reduction coefficient, depending on the degree of pinching;

Distance between node centers.

According to the standards, the coefficient of reduction of the length “” of lattice elements from

angles in the plane of the truss is 0.8. Then the design length in the plane of the truss is determined with some margin, especially for medium braces, the rigidity of which is low compared to the adjacent rods.

The exception is the supporting ascending brace, the operating conditions of which in the plane of the truss are the same as those of the upper chord, therefore the design length of the supporting brace in the plane of the truss is taken equal to the distance between the centers of the nodes.

The estimated length of the chord in a plane perpendicular to the plane of the truss is taken to be equal to the distance between the nodes secured by ties against displacement from the plane of the truss.

In non-girder roofing, the top chord of the trusses is secured in the roof plane by slabs or decking panels attached to the truss chords at each node. In this case, the width of one slab is taken as the calculated length of the chord from the plane of the truss.

The calculated length of the lattice rods when they are bent from the plane of the truss is taken to be equal to the distance between the geometric centers of the nodes, since the gussets are very flexible and are considered as sheet hinges.

In tubular trusses with formless units, the design length of the brace, both in the plane of the truss and from it, taking into account the increased torsional rigidity of closed sections, will be used equal to 0.9.

In other cases, the estimated length of the truss elements is taken along the normal.

9.7. Ultimate flexibility of rods

Structural elements must be designed from rigid rods. Flexibility is especially important for compressed rods that lose stability during longitudinal bending.

Even with minor compressive forces, the flexibility of the compressed rods should not be too great, since flexible rods easily bend from random influences, sag, and vibrate under dynamic loads. Therefore, for compressed rods, a maximum flexibility is established, depending on the purpose of the rod and the degree of its loading.

, where is the design force, is the bearing capacity of the rod:

compressed belts, as well as support posts and braces,

transmitting support reactions……………………………………………… 180-60

other compressed truss rods…………………………………………………………………… 210-60

compressed tie rods………………………………………………………200

In this case, no less than 0.5 is accepted.

The stretched rods of structures should also not be too flexible, as they can bend during transportation and installation.

The rods must have sufficient rigidity, especially in structures subject to dynamic influences.

For stretched truss rods subjected to dynamic loads, following values extreme flexibility:

stretched chords and support braces………………………………………250

other tensile truss rods………………………………………….350

stretched tie rods…………………………………………………………….400

In structures not subject to dynamic loads, the flexibility of tensile rods is limited only to vertical plane(to prevent excessive sagging) by setting all tension bars to their maximum slenderness.

9.8. Selection of sections of truss elements

In farms from rolling and bent profiles for the convenience of metal assembly, no more than 5-6 calibers of profiles are accepted.

To ensure the quality of welding and increase corrosion resistance, the thickness of profiles (pipes, bent sections) should not be taken less than 3 mm, and for corners - less than 4 mm. To prevent damage to the rods during transportation and installation, profiles smaller than 50 mm should not be used.

Profile bars are supplied in lengths of up to 12 m, therefore, when manufacturing trusses with a span of 24 m (inclusive), the chord elements take on a constant cross-section.

To reduce steel consumption, it is advisable, especially with high forces and loads, to design truss elements (belts, support braces) from high-strength steel, and the remaining elements from ordinary steel.

The choice of steel for trusses is made in accordance with the standards. Since truss rods operate under relatively favorable conditions (uniaxial stress state, low stress concentration, etc.), semi-quiet melt steels are used for them. Truss gussets operate under difficult conditions (flat field of tensile stresses, the presence of welding stresses, stress concentration near the seams), which increases the risk of brittle fracture, so a higher quality steel is required - calm.

It is convenient to select sections of truss elements in tabular form (Table 9.1).

9.9. Selection of sections of compressed elements

The limiting state of compressed truss elements is determined by their stability, therefore the load-bearing capacity of the elements is checked using the formula

(9.5)

where is the coefficient of working conditions (according to Appendix 14).

The coefficient “” is a function of flexibility and type of section (see Appendix 8).

To select a section, it is necessary to outline the type of section, set the flexibility of the rod, determine the coefficient “” according to Appendix 8 and find the required cross-sectional area

(9.6)

During preliminary selection, it can be taken for the belts of light trusses, and for the lattice . Greater slenderness values ​​are applied with less force.

Based on the required area, a suitable profile is selected according to the assortment, its actual geometric characteristics are determined A, , , are found; . With greater flexibility, the coefficient “” is specified and stability is checked using formula (9.5). If the flexibility of the rod was previously set incorrectly and the test showed overstress or significant (more than 5-10%) understress, then the section is adjusted, taking an intermediate value between the preset and actual flexibility values. The second approach usually achieves its goal.

Local stability of compressed elements can be considered ensured if the thickness of the flanges and walls of the profiles is greater than required from the stability condition.

For composite sections, the maximum flexibility of shelves and walls is determined in accordance with the standards (see Chapter 2).

Example 9.1. It is required to select the cross section of the upper chord of the truss according to the design force

Design rod lengths l x = 2.58; l y= 5.16m. Material – steel C245; Ry= 24kN/cm2. Working conditions factor ? With= 0.95; gusset thickness 12mm. Because the l y = 2l x, we take a T-section of two unequal angles located with narrow flanges together. We set flexibility within the limits recommended for belts: ? = 80. The accepted cross-section corresponds to the type of stability curve with and, therefore, at = 80 = 2,73, ? = 0,611.

Required cross-sectional area A tr = N/(?Ry? c) = 535/(0.611 = 38.4 cm2.

We take a section of two corners 125x80x10, placed together with smaller shelves; A= 19.7x2 = 39.4; i x= 2.26cm; i y= 6.19 cm (please note that the indices of the design axes and axes according to the assortment for unequal angles may not coincide);

? x= 258/2.26 = 114; ? y= 516/6,19 = 83; = 3,89; ? = 0,417;

N/(?A) = 535/(39.4 = 32.6 kN/cm2 > Ry? c= 22.8 kN/cm 2

The cross section was chosen poorly and has a large overvoltage. We accept flexibility (between pre-specified and actual) ? = 100;

? = 0,49;

A tr = 535/(0,49

We accept two corners: 160x100x9; A= 22.9 = 45.8 cm 2 ; i x= 2.85cm ( i y does not limit the cross section); ? x= 258/2.85 = 90.5;

? = 0,546;

N/(?A) = 535/(0.546 = 21.4 kN/cm 2< Ry? c= 22.8 kN/cm 2

We leave the accepted section of two corners measuring 160x100x9.

9.10. Selection of the section of tensile elements

The limiting state of tensile elements is determined by their rupture, where is the tensile strength of the steel, or the development of excessive plastic deformations, where is the yield strength of the steel.

Steel with a standard yield strength kN/cm? have a developed yield area (see Chapter 1), therefore the bearing capacity of elements made from such steels is checked using the formula

(9.7)

where is the net cross-sectional area.

For elements made of steels that do not have a yield point (conditional yield strength O 02 > 44 kN/cm?), and also if the operation of the structure is possible even after the development of plastic deformations, the load-bearing capacity is checked using the formula:

where is the design resistance determined by the temporary resistance;

Reliability factor when calculating based on temporary resistance.

In design practice, the calculation of tensile elements is carried out according to formula (9.7).

When testing a tension member, where the load-bearing capacity is determined by the stresses arising in the most weakened section (for example, bolt holes), it is necessary to take into account possible weakening and take the net area.

The required net area of ​​the tension element is determined by the formula

(9.9)

Then, according to the assortment, the profile with the nearest larger area is selected.

Example 9.2. It is required to select the section of the stretched truss brace according to the design force N=535kN. Material steel – steel C245; Ry= 24 kN/cm 2 ; ? With = 0,95

Required cross-sectional area A tr = 535/(24. The section is not weakened by holes.

We take two equal angles 90x7; A= 12.3 = 24.6 cm2 > A tr.

9.11. Selection of the cross-section of truss elements working on the action longitudinal force and bending (eccentric tension and compression)

The limit state of eccentrically stretched elements is determined by the excessive development of plastic deformations in the most loaded state. Their load-bearing capacity is determined by the formula (see Chapter 2).

Example 9.3. Select the cross-section of the stretched lower chord under the action of an off-nodal load in the middle of the panel length (Fig. 9.13, A) F=10kN. Axial force in the belt N=800kN. The distance between the centers of the nodes is d=3m. Construction material – steel C245; R y = 24 kN/cm 2. Working conditions factor? c =0.95.

Rice. 9.13. For example 9.3 and 9.4

We select the cross section of the element from the condition of its operation in tension according to formula (9.9); A tr = 800/(24 = 35.1 cm 2.

We take a section of two corners 125x9; A=22=44cm 2 ; the moments of resistance for the butt W about x and the feather W p x are equal to:

W about x = 327/3.4 = 192.4 cm2; W p x =327/(12.5 – 3.4) = 72 cm2

Moment taking into account the continuity of the belt M = (Fd / 4)0.9 = (10 /4)0.9 = 675 kN cm.

Checking the load-bearing capacity of the belt: according to Table 5 of the appendix for a section of two corners n = 1, c = 1.6.

The formula (9.10) for stretched fiber (at the butt)

800 / (44= 0,893 < 1;

for compressed fiber (per feather)

800 / (44 = 0,54 < 1

The accepted section satisfies the strength condition.

9.12. Selection of rod cross-sections for maximum flexibility

A number of light truss rods have negligible forces and, therefore, low stresses. The cross-sections of these rods are selected according to their maximum flexibility (see clause 9.4.4). Such rods usually include additional posts in a triangular lattice, braces in the middle panels of trusses, bracing elements, etc.

Knowing the design length of the rod and the value of the maximum flexibility, the required radius of gyration is determined, and then a section is selected according to the assortment and the load-bearing capacity of the selected section is checked.

9.13. Features of calculation and selection of sections of elements heavy trusses

The rods of heavy trusses are designed, as a rule, of a composite section - solid or through (see Fig. 9.11).

If the height of the section exceeds the length of the element, it is necessary to take into account the moments arising from the rigidity of the nodes and select sections that are eccentrically compressed or stretched.

The nodes of heavy trusses with great effort are made double-walled, i.e. Place the gussets along the two outer edges of the belts (Fig. 9.14). For ease of fastening the elements, the width of all rods is “ b” should be kept constant. Usually mm.

If necessary, spacers are installed between the gusset and the edge of the element.

The chords of heavy trusses have different sections in different panels, related by the common type and the conditions for pairing the rods at the nodes. Before we start

selection, establish the type of section (H-shaped, channel, box-shaped) and mark the places where the section changes. In welded H-shaped sections it is usually

the height of the verticals changes; in extreme cases, their thickness may also change while maintaining a constant distance between the outer edges of the section. For stability and rigidity of the section, the horizontal section must have a thickness of at least the distance between the verticals and at least 12 mm.

The basis of channel sections are two channels that pass through all sections (see Fig. 9.11, d).

The channel section is developed by adding vertical sheets.

After selecting the sections, they are checked. Checking the sections of compressed truss rods is carried out in the same way as centrally compressed columns (see Chapter 8). H-shaped - as solid, channel - as through, with the difference that the width “ b” of the sections here is given, and not determined from the condition of equal stability.

When taking into account the stiffness of nodes, the selection of truss sections is performed as eccentrically compressed or eccentrically tensioned elements.

Truss braces usually take channel braces (see Fig. 9.11, d) or

H-shaped section (see Fig. 9.11, A or 9.11, V). Channel sections are more advantageous when working with longitudinal bending and therefore are often used for long flexible braces, but they are more labor-intensive compared to H-shaped ones.

For ease of mating during installation, the width of the braces is taken to be 2 mm less than the distance between the edges of the gussets.

9.14. Light truss design

General design requirements. To avoid additional stresses from misalignment of the axes of the rods in the nodes, they must be centered in the nodes along the axes passing through the center of gravity (rounded to 5 mm).

Angular moments are defined as the product of the normal forces of the rods and external nodal forces on their shoulders to the point of intersection of two braces (Fig. 9.15).

Moment 1 is distributed among the truss elements converging at a node in proportion to their linear stiffnesses. If the rigidity of the lattice elements compared to the belt is small, then the moment

is perceived mainly by the truss belt. With a constant section of the belt and identical panels, the moment in the belt is .

To reduce welding stresses in the gussets, the grid rods are not

are brought to the belts at a distance of mm, but not more than 80 mm (here - the thickness of the gusset in mm). A gap of at least 50 mm is left between the ends of the joined elements of the truss chords covered with overlays.

The thickness of the gussets is selected depending on the current forces (Table 9.2) and the accepted thickness welds. If there is a significant difference in the forces in the grid rods, two thicknesses can be adopted within the sending element. The difference in gusset thicknesses in adjacent units should not exceed 2 mm.

The dimensions of the gussets are determined by the required length of the seams for fastening the elements. The gussets should be of a simple outline to simplify their production and reduce the amount of trimming. It is advisable to unify the sizes of the gussets and have one or two standard sizes per truss. Rafter trusses with a span of 18-24 m are divided into two sending elements with enlarged joints in the middle nodes. The joints should be designed so that the right and left half-trusses are interchangeable.

When designing trusses with rods made from wide-flange I-beams and T-beams, from closed bent-welded profiles or from round pipes, special guidelines must be used.

9.15. Farms from single corners

In light welded trusses from single angles, the nodes can be designed without gussets by welding the rods directly to the flange of the waist angle using fillet welds (Fig. 9.16). The corners should be attached by welding along the contour. It is allowed to weld the corner with one flank seam (at the butt) and frontal seams, as well as centering the axes of the lattice rods on the butt of the belt

Rice. 9 16. Truss nodes from single angles

(Fig.9.16, A). If there are not enough belts to attach the grille rods to the shelf

places, then a bar is welded to the belt flange (Fig. 9.16, b), creating the necessary broadening in the node.

9.16. Farms from paired corners

In trusses made of paired corners made by a brand, the nodes are designed on gussets that are inserted between the corners. The lattice rods are attached to the gusset using flank seams (Fig. 9.17). The force in the element is distributed between the seams along the butt and leg of the angle in inverse proportion to their distances to the axis of the rod. The difference in seam areas is adjusted by the thickness and length of the seams. The ends of the flank seams are brought out to the ends of the rod by 20 mm to reduce stress concentration. The gussets are attached to the belt with continuous seams and

they are released beyond the edge of the waist corners by 10-15 mm.

The seams attaching the gusset to the belt, in the absence of nodal loads, are calculated on the difference in forces in adjacent panels of the belt (Fig. 9.16, V)

Where purlins or roofing slabs rest on the upper belt

(Fig.9.17, V,G) the gussets do not reach the edges of the waist corners by 10-15mm.

To attach the purlins, a corner with holes for bolts is welded to the upper chord of the truss (Fig. 9.17, V). In places where large-panel slabs are supported, the upper chord of the truss is reinforced with mm overlays if the thickness of the chord corners is less than 10 mm with a truss pitch of 6 m and less than 14 mm with a truss pitch of 12 m.

To avoid weakening the section of the upper chord, do not weld the linings with transverse seams.

When calculating knots, they are usually set to the value “” and the required seam length is determined.

Truss gussets with a triangular lattice are designed with a rectangular cross-section, and with a diagonal lattice - in the form of a rectangular trapezoid.

To ensure smooth transfer of force and reduce stress concentration, the angle between the edge of the gusset and the grid element must be at least 15 0 (Fig. 9.17, V).

The joints of the belts must be covered with overlays made of

sheets (Fig. 9.18) or corner. To attach the corner trim

it is necessary to cut off the edge and flange of the corner. The reduction in its cross-sectional area is compensated by the gusset.

When installing sheet overlays, the gusset comes into play. The center of gravity of the section at the joint does not coincide with the center of gravity of the section of the belt, and it works in eccentric tension (or compression), so the joint of the belt is moved outside the unit to facilitate the operation of the gussets.

To ensure that the corners work together, they are connected by gaskets. The distance between gaskets should be no more than 40 i for compressed and 80 i for tension elements, where i- radius of inertia of one corner relative to the axis parallel to the gasket. In this case, at least two gaskets are placed in the compressed elements.

Solutions for the enlarged truss unit when delivered from individual sending elements are shown in Fig. 9.19.

The design of support units depends on the type of supports (metal or reinforced concrete columns, brick walls, etc.) and the method of coupling (rigid or hinged).

When the trusses are freely supported on the underlying structure, a possible solution for the support unit is shown in Fig. 9.20. Truss Pressure Through Slab

a – centering of the rods; b – unit with a diagonal lattice; c – attaching purlins; d – attaching large-panel slabs

transmitted to the support. The area of ​​the slab is determined by the bearing capacity of the support material.

(9.12)

where is the calculated compressive resistance of the support material.

The slab bends due to the resistance of the support material in the same way as the column base slab (see Chapter 8).

The pressure of the truss is transmitted to the base plate through the gusset and the support post, which form a rigid cross-section support. The axes of the belt and support brace are centered on the axis of the support post.

The seams that weld the gusset and support post to the slab are designed for support reaction.

Holes for anchors are made in the base plate. The diameter of the holes is made 2-2.5 times larger than the diameter of the anchors, and the washers of the anchor bolts are welded to the slab.

For ease of welding and installation of the unit, the distance between the lower chord and

the base plate is taken to be more than 150mm.

We similarly construct the support unit when supporting the truss at the level of the upper chord (Fig. 9.19.b).

9.17. Truss with belts made of wide-flange brands with parallel flange edges

I-beams with parallel flange edges are obtained by longitudinally unraveling wide-flange I-beams. Brands are used in truss belts; the grating is made of paired or single rolled or bent

corners. Trusses with belts made of brands are more economical in terms of metal consumption per

10-12%, in terms of labor intensity by 15-20% and in cost by 10-15% compared to

farms from paired corners. Savings are achieved by reducing the number of parts, the size of the gussets and the length of the welds.

With little effort in the braces, the seams of their fastening to the belt are placed on the wall of the tee (Fig. 9.21, A). With great forces (the support and adjacent braces), to ensure the required length of the seam, a nodal gusset of the same thickness is welded to the wall of the tee (Fig. 9.21, b). The butt seam of the connection between the gusset and the wall of the tee is calculated for a shear from a force equal to the difference in forces in the adjacent panels of the belt.

a – in welding; b – bolted; 1– fold line of the butt plate

a – support at the level of the lower belt; b – also, upper belt

Changing the section of the belt can be done end-to-end (Fig. 9.21, b) or using a sheet insert and overlay (Fig. 9.21, V).

Enlarged joints of shipping marks are made using welding or high-strength bolts.

Farms with belts made of tees and a cross lattice of single corners have high economic indicators (see Fig. 9.6, and). Brand braces without gussets (Fig. 9.21, G). At the intersection, the braces are connected by welding or bolts. An extended brace prevents the compression brace from losing stability and reduces its effective length. both in the plane and from the plane of the truss by 2 times.

a – knot without gusset; b – a unit with an additional gusset and a change in the section of the butt belt; c – a knot with a change in the section of the belt using an overlay and an insert; d – truss unit with a cross lattice of corners

9.18. Pipe trusses

In tubular trusses, non-shaped units with direct connection of the lattice rods to the chords are rational (Fig. 9.22, A). Nodal connections must ensure sealing of the internal cavity of the truss to prevent corrosion there.

The rods are also centered along the geometric axes, but an eccentricity of no more than one quarter of the diameter of the belt pipe is allowed if it is used with incomplete load-bearing capacity.

The calculation of such a nodal interface is quite complex and relates to the area of ​​calculation of intersecting cylindrical shells.

The strength of the seam attaching the tubular grating rod can be checked against the safety margin using the formula

where is the coefficient of seam operating conditions, taking into account the uneven distribution of stress along the length of the seam; - seam length, determined by the formula

l w = 0.5 ? d?[ 1.5(1 + cosec ? ) -cosec ? ] (9.15)

The value of the coefficient?, depending on the ratio of the pipe diameter

are given in Table 9.3.

If the thickness of the belt is insufficient, it can be strengthened (Fig. 9.22, A). The linings are cut from pipes of the same diameter as the belt or bent from a sheet with a thickness of at least one and no more than two wall thicknesses of the belt pipe

When transferring concentrated loads to the truss belt (from the weight of the roof, overhead transport, etc.), it is necessary to provide parts for

application of these loads symmetrically relative to the axes of the truss plane along the side sections of the wall of the belt pipe.

The enlarged connection of the rafter trusses in the ridge assembly is performed with a centering gasket between the flange plugs.

If there are no machines for shaped processing of pipe ends, tubular truss units can be flattened (Fig. 9.22, b), and in exceptional cases, perform on gussets (Fig. 9.22, V). Flattening of the ends is only permissible for pipes made of low-carbon or other ductile steel.

Pipes of the same diameter are connected end-to-end on the remaining backing ring (Fig. 9.23, A). If the design resistance of the deposited metal is low, the butt joint on the backing ring is made with an oblique weld (Fig. 9.23 b).

The butt connection can also be made using paired ring pads, bent from a sheet or cut from pipes of the same or slightly larger diameter (Fig. 9.23, V). It is recommended to take the thickness of the overlays and weld seam 20% greater than the thickness of the pipes being joined.

Butt joints of pipes of different diameters working under compression can be made using end gaskets (Fig. 9.23, G). During installation, flange connections with bolts are often used (Fig. 9.23, d).

The solutions of the support nodes are shown in Fig. 9.24.

9.19. Trusses made of bent profiles

Trusses made of bent welded closed profiles (GSP) are designed with bevelless units (Fig. 9.25). To simplify the design of the units, a triangular lattice should be adopted without additional racks, in which no more than two elements are adjacent to the belts.

a – with direct adjoining; b – with flattening of the ends of the rods;

c – on gussets; g – with inserts; 1 - plug

The wall thickness of the rods should be at least 3 mm. The use of profiles of the same cross-sectional dimensions, differing in wall thickness by less than 2 mm, is not permissible in the same truss.

The width of the grid rods (from the plane of the structure) should be taken as possibly larger. But not more from the condition of applying longitudinal welds and at least 0.6 of the transverse dimension of the belt

IN(, - thickness of the belt and grille).

The angles at which the braces join the chord must be at least 30° to ensure the tightness of the weld section on the side of the acute angle.

Weld seams attaching the lattice rods to the flanges of the belts are calculated as butt seams (see Chapter 4).

Truss joints made from open bent sections can be made without gussets.

With a box-section belt and braces made of two branches connected by planks, the braces are overlapped on both sides of the belt and welded with flank seams (Fig. 9.25, A). If the height of the belt is insufficient, then gussets are welded to it in two planes using butt seams (Fig. 9.25, b). The support unit is shown in Fig. 9.25, V.

9.20. Preparation of a working drawing of light trusses (LMG)

The detail (working) drawing shows the facade of the sending element, plans of the upper and lower chords, side view and sections. The nodes and sections of the rods are drawn on a scale of 1:10-1:15 on a truss diagram drawn on a scale of 1:20-1:30 (see Fig. 13).

The main dimensions of the assembly are the dimensions from the center of the assembly to the ends of the attached lattice rods and to the edge of the gusset (see Fig. 9.17). The length of the lattice rods and gussets is determined in multiples of 10 mm. The drawing indicates the dimensions of the welds and the location of the bolt holes.

The detail drawing contains a parts list for each shipping item and a table of factory welds or bolts.

The notes indicate the manufacturing features of the structure that are unclear from the drawing.

9.21. Heavy truss units

In heavy trusses, it is necessary to more strictly maintain the centering of the rods in the nodes along the axes passing through the center of gravity, since even small eccentricities with large forces in the rods cause significant moments that must be taken into account when calculating the trusses.

When changing the section of the chords, the centering of the elements should be carried out along the average line of the centers of gravity, while the moment from misalignment is taken into account in the calculation (if the eccentricity is more than 1.5% of the height of the chord section).

Heavy trusses usually have a height of more than 3.85 m, so they are assembled from individual elements. Assembly joints are located in nodes or near nodes.

When a joint is located in a node, the design of the node becomes more complicated.

During installation, it is not always possible to ensure the quality of the welded joint. Therefore, installation connections of truss elements operating under dynamic loads (bridges, crane trusses, etc.) are often made with high-strength bolts (Fig. 9.26). With an H-shaped or channel cross-section of the rods, the nodes on the gussets connecting from the outside all the rods suitable for the node are simple and reliable.

Only the vertical elements of the rods are attached to the gussets.

When arranging belt joints in the center of the knot, gussets serve as butt elements. To ensure the operation of the gussets, it is advisable to reinforce them at the joints with external overlays. Number of bolts attaching

Fig.9.25. Truss joints made of open bent sections

lining increases by 10%. The gussets should be sufficiently thick, no less than the thickness of the elements being fastened.

Bolts in the assemblies of heavy trusses should be placed along standardized marks at the distances required by the jig and multi-spindle drilling (usually with mm bolts, the bolt pitch is 80mm).

In long-span trusses, the horizontal displacement of supports is quite significant. To eliminate additional horizontal forces, constructive solution support nodes must correspond to the design diagram (one support is hingedly fixed, the other is movable). motionless

the support is made in the form of a tile hinge or a fixed balancer, movable on rollers like bridge trusses (see Chapter 18).

9.22. Prestressed Trusses

In trusses, prestressing is carried out by tightening, in continuous trusses - by shifting the supports. In split trusses, the ties are made of high-strength materials (steel ropes, bundles of high-strength wire, etc.). The tie rods should be placed so that, as a result of their tension, forces arise in the most loaded truss rods that are opposite in sign to the forces from the load.

Tightenings can be placed within the length of individual rods operating under tensile load, creating a preliminary compressive stress in them (Fig. 9.27, A). This method is only effective for heavy trusses.

In trusses, the belt (working in tension) has a significant specific weight in terms of metal consumption, it is possible to create a prestress with one tightening in all panels of the belt (Fig. 9.27, b).

In light trusses, the most effective scheme is the arch type with a tightening (Fig. 9.27, c, d).

Remote tightening is possible (Fig. 9.27, d), the unloading effect of which on the truss rods can be especially significant. However, due to the conditions of the structure’s layout and transportation, external tightening cannot always be used.

When the tie is placed along the length of the lower chord, it is connected by diaphragms to the belt and protects it from loss of stability during prestressing (Fig. 9.28), when the lower belt receives compressive forces.

When using external tightening and in the “arch with tightening” scheme, it is necessary to take measures to ensure the stability of the lower chord during the process of prestressing. In this case, tensioning should be carried out in the design position, when the truss is secured with ties or on the ground during installation, after which tension and lifting should be performed (Fig. 9.29, a). In spatial truss systems, for example, with a triangular cross-section, it is also possible to apply tension at the bottom, since the lower chord is secured against loss of stability (Fig. 9.29, b).

The cross-sections of the rods in prestressed trusses can be the same as in conventional ones. When prestressing individual rods, the tie rods must be placed symmetrically relative to the vertical axis of the rod. For design reasons, they are often designed from two branches (see Fig. 9.28).

The basics of calculation and design of prestressed trusses are presented in a special course (“Metal structures”).