Timber truss trusses are considered as truss systems. Farm - what is it? Building construction. Truss with belts from wide-shelf tees with parallel edges of the shelves

16.05.2020

A farm is a system of rods interconnected at nodes and forming a geometrically unchanging structure. Under a nodal load, the rigidity of the nodes does not significantly affect the operation of the structure, and in most cases they can be considered as articulated. In this case, all truss rods experience only tensile or compressive axial forces.

Farms are more economical than beams in terms of steel consumption, but more labor-intensive to manufacture. The efficiency of trusses in comparison with solid-walled beams is the greater, the larger the span and the lower the load.

Farms are flat (all rods lie in the same plane) and spatial.

Flat trusses perceive the load applied only in their plane, and need to be fixed with their connections. Spatial trusses form a rigid spatial beam that takes the load in any direction (Fig. 9.1).

The main elements of the trusses are the belts that form the contour of the truss, and the lattice, consisting of braces and racks (Fig. 9.2). The connection of elements in the nodes is carried out by direct adjacency of some elements to others (Fig. 9.3 a) or with the help of nodal gussets (Fig. 9.3 b). Truss elements are centered along the axes of the center of gravity to reduce the nodal moments and ensure the operation of the rods for axial forces.

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

a - with direct adjacency of elements; b - on gussets

The distance between adjacent nodes of the belts is called the panel (d in is the panel of the upper belt, d n is the lower one), and the distance between the supports is called the span ( l).

Truss chords work for longitudinal forces and moment (similar to solid beam chords); the truss lattice perceives mainly the transverse force, performing the functions of the beam web.

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

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

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

according to the static scheme- beam (cut, continuous, cantilever); arched, frame, combined (Fig. 9 4);

Fig.9.4. Truss systems

a - beam split; b - continuous; in, e - console; G - arched; e - frame; w - combined

along the lines of the belts- with parallel belts, trapezoidal, triangular, polygonal, segmented (Fig. 9.5);

by lattice system– triangular, oblique, cross, rhombic

and others (Fig. 9.6);

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

by maximum effort- light - single-walled with sections from 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 truss or braces or an arch (top). Reinforcing elements reduce the bending moment in the beam and increase the rigidity of the system (Fig. 9.4, well). Combined systems are easy to manufacture (have a smaller number of elements) and are rational in heavy structures, as well as in structures with moving loads.

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

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

Rice. 9.6. Truss Lattice Systems

a - triangular; b - triangular with additional posts; in - obliquewithascending braces; G - diagonal with descending braces; e - trussed; e - cross; f - cross; and - rhombic; to - semi diagonal

9.2. Truss structure layout

Selection of the static scheme and outline of the truss - the first stage of structural design, depending on the purpose and architectural design of the structure and is made on the basis of a comparison of possible options.

Beam split systems have found application in building coverings, bridges, transport galleries and other structures. They are easy to manufacture and install, do not require complex assemblies, but are very metal-intensive. With beam spans of 40 m, split trusses are obtained oversized, and they 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 with soft soils is not recommended, since additional forces arise during the settlement of supports. In addition, continuity 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 large-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 application is dictated by architectural requirements. Cantilever trusses are used for sheds, towers, power line supports.

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

The outlines of the truss belts determine their efficiency. The most economical in terms of steel consumption is the truss, outlined by the diagram of moments. For a single-span beam system with a uniformly distributed load, it will be segment farm with a parabolic belt (see fig. 9.5, a). However, curved belts are very laborious to manufacture, so such trusses are used extremely rarely. Polygonal farms are more used (see Figure 9.5, b). In heavy large-span trusses, additional structural difficulties due to the fracture of the belts in 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 polygon outline is not rational, since the complication of nodes does not pay off with steel savings.

Farms trapezoidal ( see fig.9.5, in), although they do not quite correspond to the diagram of moments, have constructive advantages due to the simplification of nodes. In addition, the use of such trusses in the coating allows you to arrange a rigid frame assembly, which increases the rigidity of the building.

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

Farms triangular outlines (see fig.9.5, dr.,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 farms is the increased consumption of metal under a distributed load; the sharp support assembly is complex and allows only articulation with the columns. The middle braces are very long and they have to be selected according to the maximum flexibility, which leads to excessive consumption of metal. However, sometimes they are used for truss structures, when it is necessary to provide a large roof slope (over 20%) or to create one-sided uniform lighting (shed coatings).

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

The height of the 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 Figure 9.5, G), but the anchor knot should not be very sharp.

For the height of trapezoidal trusses 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 is the sum of the weight of the belts and the lattice. The weight of the belts decreases with increasing truss height, 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) allowed by the transportation condition. Therefore, taking into account the requirements of transportation, installation, unification, the height of the trusses is taken within 1/7 - 1/12 of the span (even less for light trusses).

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

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

Truss deflection is determined analytically by Mohr's formula

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

Panel dimensions must correspond to the distances between the elements that transfer the load to the truss, and correspond to the optimal angle of inclination of the braces, which is approximately 45 0 in a triangular lattice, and 35 0 in a diagonal lattice. From design considerations - the rational outline of the gusset in the knot and the convenience of attaching the braces - an angle close to 45 0 is desirable.

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

It is desirable to ensure the transfer of the load from the roof to the nodes of the truss in order to exclude the work of the belt in bending. Therefore, in pavements made of large-sized reinforced concrete or metal slabs, the distance between nodes is assumed to be equal to the width of the slab (1.5 m or 3 m), and in pavements along runs

– run spacing (from 1.5m to 4m). Sometimes, to reduce the size of the belt panel, a trussed lattice is adopted (see Fig. 9.6, d).

Unification and modulation of the geometric dimensions of trusses allows you to standardize both the trusses themselves and the elements adjacent to them (girders, 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.

At present, the geometric schemes of truss trusses of industrial buildings, bridges, radio masts, radio towers, power transmission 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 appearance design and unacceptable for operating conditions.

The sagging of the trusses is prevented by the truss lifting device, i.e.

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

The construction lift is provided by bending in the assembly units (Fig. 9.7).

Truss lattice systems and their characteristics. The truss lattice works on a transverse force, acting as a wall of a solid beam.

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

Triangular lattice system. In trapezoidal trusses or with parallel belts, a triangular lattice system is rational

(see fig.9.6, a), which gives the smallest total length of the lattice and the smallest number of nodes with the shortest path of force from the place 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 load and do not participate in the transfer of transverse force to the support.

Rice. 9.7. Construction lifting schemes with one ( a) and several(b) enlargement 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, it is used at low truss heights, as well as when large forces are transmitted along the racks (with a large nodal load).

The diagonal lattice is more laborious than the triangular one, it requires a large consumption of metal, since with an equal number of panels in the truss, the total length of the diagonal lattice is greater, and it has more nodes. The path of force from the node to the support in the diagonal lattice is longer; it goes through all lattice rods and nodes.

Special grating systems, used at high truss heights (approximately 4 - 5 m). To reduce the size of the panel, while maintaining the normal angle of inclination of the braces, a trussed lattice is used (see Fig. 9.6, d). The device of the trussed lattice is more laborious 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 estimated length of the compressed rods.

The trussed 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 a two-sided load, they arrange cross grate (see fig. 9.6, e). Such trusses include horizontal truss trusses covering 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) due to two systems of braces they have high rigidity; these systems are used in bridges, towers, masts, ties to reduce the effective length of the rods and are especially rational when structures operate on large transverse forces.

Ensuring the stability of trusses. A flat truss is unstable from its plane, therefore it must be attached to a more rigid structure or connected with ties to another truss, as a result of which a stable spatial beam is formed (Fig. 9.8, a). Because this

Rice. 9.8. Tying trusses into spatial systems

1 - diaphragm

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

In the coverings of buildings, due to the large number of flat roof trusses placed next to each other, the solution becomes more complicated, so trusses connected to each other only by girders 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 cross 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 (girders and spacers located at truss nodes). In order for the purlin to secure the truss node in the horizontal direction, it must itself be attached to

fixed point - the node of horizontal connections.

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

9.3. Types of sections of truss rods

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

In terms of steel consumption, the tubular section is the most efficient (Fig. 9.10, a). The pipe has good streamlining, so the wind pressure is less, which is important for tall structures (towers, masts, cranes). Hoarfrost and moisture do not linger on the pipes, so they are resistant to corrosion; they are easy to clean and stain. This increases the durability of tubular structures.

To prevent corrosion of the internal planes, tubular elements should be sealed. However, certain structural difficulties in mating 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 the tubular ones, allow to simplify the junctions of the elements and found wide application. However, trusses from bent closed profiles with beveled nodes require high manufacturing precision.

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

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

Often sections of truss elements are taken from different types of profiles: belts from I-beams, a lattice from bent-closed profiles or belts from tees, a lattice from 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 provide convenient pairing of elements in different planes. This requirement is best met by a tubular section.

In tetrahedral trusses with little effort, the simplest type of section of the belt is a single corner or a cross section of two corners. With great effort, I-beams are also used.

The compressed elements of the trusses should be designed 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 (the degree of aggressiveness of the environment, the nature and place of application of loads, etc.), the possibility of manufacturing, the availability of assortment and economic considerations.

Heavy truss rods differ from the lungs in more powerful and developed sections, composed of several elements. The sections of such rods are usually designed as double-walled (Fig. 9.11), and nodal conjugations are performed using gussets located in two planes. The bars of heavy trusses (braces, struts and chords) have different sections, but for the convenience of pairing at the nodes, the width of the elements “ in” should be the same.

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

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

The following types of sections of bars of heavy steel trusses are used:

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

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

channel section consists of two channels placed with shelves inside (Fig. 9.11, d); both rolled and composite channels are used. Such a section is expedient for compressed elements, especially if they are long. The disadvantage of the channel section is the presence of two branches, which have to be connected with planks or gratings (similar to centrally compressed columns).

box section consists of two vertical elements connected by a horizontal sheet from above (Fig. 9.11, e,well). Applied in

Fig.9.11. Types of sections of bars of heavy trusses

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, well) or perforated sheet.

Single wall double tee consists of a welded or wide-shelf rolled I-beam, placed vertically (Fig. 9.11, and).

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

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

9.4. Farm calculation

Determination of the design load. All load acting

is usually applied to the truss at the nodes of the truss, to which elements of the transverse structure are attached (roof purlins or dropped ceilings) transferring the load to the farm. 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 chord from the load located on it is additionally taken into account. The truss belt is considered as a continuous beam with supports at the nodes.

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

temporal- loads from suspended underground transport equipment, payload acting on an attic floor suspended from a truss, etc.

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

The calculated constant load acting on any truss node depends on the cargo area from which it is collected (fig.9.12) and is determined by the formula

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

In separate nodes, the load from the weight of the lantern is added to the load obtained by 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 design nodal load from snow is determined by the formula:

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

Meaning “ S” should be determined taking into account the possible uneven distribution of snow cover near the lantern 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, flyovers, as well as in steep triangular trusses and lanterns. Wind load is reduced to nodal. The horizontal wind load on the lantern is not taken into account when calculating the roof truss, since its effect on the operation of the truss is not significant.

Rice. 9.12. Calculation scheme of the farm

9.5. Determination of forces in truss bars

When calculating trusses with rods from angles or tees, it is assumed that there are ideal hinges in the nodes of the system, the axes of all rods are rectilinear, 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 with non-axial forces: the stresses found from these forces are the main ones.

In trusses with rods that have increased rigidity, the effect of joint stiffness in nodes is more significant. The moments that occur in the nodes lead to an earlier occurrence of plastic deformations and reduce the brittle strength of the steel. Therefore, for I-beam, tubular and H-shaped sections, the calculation of trusses on a hinged system is allowed with a ratio of section height to length of not more than for structures operated at a design temperature of at least -40 0 C. With an increase in these ratios, additional bending moments in the rods from knot stiffness.

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

transit moment in the end panel

;

span moment of intermediate panels

;

moment in knot (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 calculations, are not taken into account, since the allowable eccentricities in the trusses are small.

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

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 the design forces in the rods, taking into account the required combinations of loads, to optimize the design, i.e. find the optimal truss layout, rod material, section type, etc., get the most economical design solution.

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

If the truss works 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), the forces are determined separately for each type of combination and the bearing capacity of the rods is determined by the final design maximum force.

It is recommended that the results of the static calculation be recorded in a table, which should contain the values of the forces from a constant load, from possible combinations of live loads (for example, from one-sided loading with snow), as well as the calculated forces as a result of summing up the forces with the most unfavorable loading for all possible combinations loads.

9.6. Determination of effective length of bars

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, makes the remaining rods rotate and bend in the plane of the truss.

Adjacent bars resist bending and rotation of the assembly and

They prevent the free bending of the rod, which loses stability.

Tension rods provide the greatest resistance to the rotation of the node. Compressed rods have little resistance to bending.

Thus, the more tensioned rods adjoin the compressed rod and the more powerful they are (the greater their rigidity per unit length), the higher the degree of pinching of the rod and the smaller its estimated length; the effect of compressed rods on pinching can be neglected.

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

Thus, with a greater degree of pinching, the calculated length of the truss rod is less

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

Distance between node centers.

According to the norms, the length reduction factor “” of lattice elements from

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

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

The estimated length of the belt in a plane perpendicular to the plane of the truss is taken equal to the distance between the nodes, fixed by ties from displacement from the plane of the truss.

In non-purlin roofings, the upper chord of truss trusses is fixed in the roof plane with slabs or decking panels attached to truss chords at each node. In this case, the width of one slab is taken as the estimated length of the belt from the truss plane.

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

In tubular trusses with non-shaped nodes, the calculated 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 applied equal to 0.9.

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

9.7. Ultimate flexibility of rods

Structural elements must be designed from rigid rods. Of particular importance is the flexibility "" for compressed rods that lose stability during buckling.

Even with insignificant compressive forces, the flexibility of the compressed rods should not be too great, since the flexible rods are easily bent from random influences, sag, and vibrate under dynamic loads. Therefore, for compressed rods, the ultimate flexibility is set, 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 rods of connections………………………………………………………………200

In this case, at least 0.5 is accepted.

Stretched rods of structures also should 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 tensile truss bars subjected to dynamic loading, the following ultimate slenderness values are established:

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

other tensioned truss bars………………………………………….350

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

In structures that are not subjected to dynamic actions, the flexibility of tension rods is limited only in the vertical plane (to prevent excessive sagging), by setting the ultimate flexibility for all rods in tension.

9.8. Selection of sections of truss elements

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

From the condition of ensuring the quality of welding and increasing 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 less than 50 mm should not be used.

Profile rolled products are supplied up to 12 m long, therefore, in the manufacture of trusses with a span of 24 m (inclusive), the belt elements take on a constant section.

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

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

It is convenient to draw up the selection of sections of truss elements in tabular form (Table 9.1).

9.9. Selection of sections of compressed elements

The limit state of the compressed truss elements is determined by their stability, therefore, the bearing capacity of the elements is checked according to the formula

(9.5)

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

The coefficient “”, is a function of flexibility and the 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 section area

(9.6)

With preliminary selection, it can be taken for belts of light trusses, and for lattice . Greater flex values are applied with less effort.

According to the required area, a suitable profile is selected according to the assortment, its actual geometric characteristics A, , , are determined; . 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 an overstress or a significant (more than 5-10%) understress, then the section is corrected, taking an intermediate value between the preset and actual values of flexibility. The second approximation usually achieves the goal.

The local stability of the 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 limiting flexibility of shelves and walls is determined in accordance with the standards (see Chapter 2).

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

Estimated bar lengths l x = 2.58; l y= 5.16m. Material - steel C245; Ry= 24kN/cm2. Working conditions coefficient ? with= 0.95; gusset thickness 12mm. Insofar as l y = 2l x, we take a tee section from two unequal corners located in narrow shelves together. We ask flexibility within the limits recommended for belts: ? = 80. The accepted 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 = 38.4cm2.

We accept a section from two corners 125x80x10, put together by smaller shelves; BUT= 19.7x2 = 39.4; i x= 2.26cm; i y\u003d 6.19 cm (note that the indices of the calculated axes and axes according to the assortment for unequal angles may not match);

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

N/(?A) = 535/(39.4 = 32.6kN/cm2 > Ry? c\u003d 22.8 kN / cm 2

The cross section was selected unsuccessfully and has a large overvoltage. Accept flexibility (between preset and actual) ? = 100;

? = 0,49;

A tr = 535/(0,49

We accept two corners: 160x100x9; BUT\u003d 22.9 \u003d 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 = 21.4kN/cm2< Ry? c\u003d 22.8 kN / cm 2

We leave the accepted section of two corners with a size of 160x100x9.

9.10. Section Selection of Tensile Elements

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

Steel with normative yield strength kN/cm? have a developed yield point (see Chap. 1), so the bearing capacity of elements made of such steels is checked by the formula

(9.7)

where is the net 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 bearing capacity is checked by the formula:

where is the design resistance determined by the temporary resistance;

Reliability factor in the calculation of the temporary resistance.

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

When checking a tension member, when the load-bearing capacity is determined by the stresses that occur in the weakest 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 element in tension is determined by the formula

(9.9)

Then, according to the assortment, a profile is selected that has the closest greater value area.

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\u003d 24kN / cm 2; ? with = 0,95

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

We accept two equal-shelf corners 90x7; BUT\u003d 12.3 \u003d 24.6 cm2\u003e A tr.

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

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

Example 9.3. Select the section of the stretched lower chord under the action of an off-nodal load on it 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 nodes d=3m. Construction material - steel C245; R y \u003d 24 kN / cm 2. Working conditions factor? c = 0.95.

Rice. 9.13. For example 9.3 and 9.4

We select the section of the element from the condition of its work in tension according to the formula (9.9); A tr \u003d 800 / (24 \u003d 35.1 cm 2.

We accept a section from two corners 125x9; A \u003d 22 \u003d 44 cm 2; the moments of resistance for the butt W about x and the pen W p x are equal to:

W about x \u003d 327 / 3.4 \u003d 192.4 cm2; W p x \u003d 327 / (12.5 - 3.4) \u003d 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 bearing capacity of the belt: according to Table 5 of the application for a section of two corners n = 1, c = 1.6.

The floor of the formula (9.10) for a stretched fiber (along the butt)

800 / (44= 0,893 < 1;

for compressed fiber (per pen)

800 / (44 = 0,54 < 1

The accepted section satisfies the strength condition.

9.12. Selection of the section of the rods for ultimate flexibility

A number of light truss rods have low forces and therefore low stresses. The sections of these rods are selected according to the ultimate 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 estimated length of the rod and the value of ultimate flexibility, determine the required radius of gyration, and then select the section according to the assortment and check the bearing capacity of the selected section.

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

The rods of heavy trusses are designed, as a rule, with 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 stiffness of the nodes and select sections that are eccentrically compressed or stretched.

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

In necessary cases, gaskets are installed between the gusset and the edge of the element.

Belts of heavy trusses have different sections in different panels, related by the common type and the conditions of conjugation of rods at the nodes. Before starting

selection set the type of section (H-shaped, channel, box-shaped) and outline the places for changing the section. In welded H-shaped sections, usually

the height of the verticals changes; in extreme cases, their thickness can also change while maintaining a constant distance between the outer edges of the section. The horizontal from the condition of stability and rigidity of the section must have a thickness not less than the distance between the verticals and not less than 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 the selection of sections, they are checked. Checking the sections of compressed truss rods is carried out in the same way as for 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 rigidity of the nodes, the selection of truss sections is performed as eccentrically compressed or eccentrically tensioned elements.

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

H-shaped section (see Fig. 9.11, a or 9.11, in). Channel sections are more beneficial when working on buckling and therefore are often used for long flexible braces, but they are more laborious than H-shaped ones.

The width of the braces for ease of pairing during installation is taken 2 mm less than the distance between the edges of the gussets.

9.14. Light truss construction

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 up to 5 mm).

Angular moments are defined as the product of the normal efforts 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 between the truss elements converging at the node in proportion to their linear stiffnesses. If the rigidity of the lattice elements is small compared to the belt, then the moment

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 lattice 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 belts, overlapped by overlays.

The thickness of the gussets is chosen depending on the acting forces (Table 9.2) and the accepted thickness of the welds. With a significant difference in the forces in the lattice rods, two thicknesses can be taken within the starting element. The difference in the thickness of the gussets in adjacent nodes 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 make them easier to manufacture and reduce the amount of trimming. It is advisable to unify the dimensions of the gussets and have one or two standard sizes per farm. Roof trusses with a span of 18-24 m are divided into two sending elements with enlarged joints in the middle nodes. Joints should be designed so that the right and left half-trusses are interchangeable.

When designing trusses with rods from wide-shelf I-beams and tees, from closed bent welded profiles or from round pipes special instructions must be followed.

9.15. Farms from single corners

In light welded trusses from single corners, knots can be designed without gussets by welding the rods directly to the flange of the girdle corner with fillet welds (Fig. 9.16). 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 knots from single corners

(fig.9.16, a). If there are not enough belts to fasten the lattice rods to the shelf

places, then a bar is welded to the belt shelf (Fig. 9.16, b), which creates the necessary widening in the node.

9.16. Farms from paired corners

In trusses from paired corners made up of a brand, knots are designed on gussets that lead between the corners. The lattice rods are attached to the gusset with flank seams (Fig. 9.17). The force in the element is distributed between the seams along the butt and the feather of the corner in inverse proportion to their distances from the axis of the rod. The difference in the areas of the seams is regulated by the thickness and length of the seams. The ends of the flank seams are brought to the ends of the rod by 20 mm to reduce the stress concentration. The gussets are attached to the belt with continuous seams and

they are released behind the butt 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, in)

In the place of support on the upper belt of purlins or roofing slabs

(fig.9.17, in,G) gussets do not lead to the ends of the waist corners by 10-15 mm.

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

In order to avoid weakening the cross section of the upper chord, do not weld the lining with transverse seams.

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

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

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

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 the butt and the shelf of the corner. The decrease in its cross-sectional area is compensated by the gusset.

When installing sheet overlays, a gusset is included in the work. The center of gravity of the section at the junction does not coincide with the center of gravity of the section of the belt, and it works for eccentric tension (or compression), so the belt joint is taken out of the knot to facilitate the work of gussets.

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

Solutions for the truss extension unit when supplied from separate shipping elements are shown in Fig. 9.19.

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

With free support of trusses on the underlying structure, a possible solution for the support unit is shown in Fig. 9.20. Truss pressure through plate

a - centering of the rods; b - node with a diagonal lattice; c - attaching runs; d - attaching large-panel plates

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

(9.12)

where is the design resistance of the support material to compression.

The slab works in bending from the repulsion of the support material similarly to the column base slab (see Chapter 8).

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

The seams that weld the gusset and the support post to the slab rely on the support reaction.

Rice. 9.18. Factory joint of the belt with a change in section

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

For convenience of welding and mounting of the assembly, the distance between the lower chord and

base plate accept more than 150mm.

Similarly, we construct the support node when supporting the truss at the level of the upper belt (Fig. 9.19.b).

9.17. Truss with belts from wide-shelf tees with parallel edges of the shelves

Taurus with parallel edges of the shelves are obtained by longitudinal dissolution of wide-shelf I-beams. Tauris are used in truss belts; the grating is made of twin or single rolled or bent

corners. Farms with belts made of Taurus are more economical in terms of metal consumption for

10-12%, in terms of labor intensity by 15-20% and in terms of 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 brand (Fig. 9.21, a). With great efforts (supporting and neighboring 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 of the gusset with the wall of the tee is calculated for a shear from a force equal to the difference in forces in the adjoining belt panels.

a - in welding; b - on bolts; 1- fold line of the butt plate

a - support at the level of the lower belt; b - also, the 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, in).

Enlarged joints of shipping grades are carried out on welding or high-strength bolts.

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

a - a knot without a gusset; b - a node with an additional gusset and a change in the section of the belt end-to-end; c - a node with a change in the section of the belt using an overlay and an insert; g - truss node with a cross lattice of corners

9.18. Pipe farms

In tubular trusses, non-shaped nodes are rational with a direct adjoining of the lattice rods to the belts (Fig. 9.22, a). The nodal interfaces must ensure the sealing of the internal cavity of the truss in order to prevent corrosion there.

The rods are also centered along the geometric axes, but an eccentricity of not more than one quarter of the diameter of the belt pipe is also allowed if it is used with an incomplete bearing capacity.

The calculation of such nodal conjugation is rather complicated and belongs to the area of calculation of intersecting cylindrical shells.

The strength of the seam that attaches the tubular lattice rod can be checked for a margin of safety using the formula

where - the coefficient of the working conditions of the seam, taking into account the uneven distribution of stress along the length of the seam; - the length of the seam, 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 diameter of the pipes

are given in Table 9.3.

If the belt thickness is insufficient, it can be strengthened (Fig. 9.22, a). The plates are cut from pipes of the same diameter as the belt or bent from a sheet with a thickness of at least one and not 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 details for

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

Enlargement connection of truss trusses in the ridge assembly should be carried out with a centering gasket between the flange plugs.

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

Pipes of the same diameter are butt-joined on the remaining backing ring (Fig. 9.23, a). With a low design resistance of the deposited metal, the butt joint on the backing ring is made with an oblique seam (Fig. 9.23 b).

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

Abutting joints of pipes of different diameters, working in compression, can be made using end gaskets (Fig. 9.23, G). Often used in installation flange connections on bolts (fig.9.23, d).

Solutions of reference nodes are shown in Fig. 9.24.

9.19. Trusses from bent profiles

Trusses from bent welded closed profiles (GSP) are designed with shapeless nodes (Fig. 9.25). To simplify the design of nodes, a triangular lattice should be adopted without additional racks, in which no more than two elements adjoin the chords.

Rice. 9.22. Tubular truss nodes

a - with direct adjacency; b - with flattening of the ends of the rods;

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

The thickness of the walls of the rods should be at least 3 mm. The use of profiles of the same cross-sectional dimensions that differ in wall thickness by less than 2 mm is not allowed in one truss.

The width of the lattice rods “” (from the plane of the structure) should be taken as possibly greater. But not more from the condition of applying longitudinal welds and not less than 0.6 of the transverse dimension of the belt

AT(, - the thickness of the belt and lattice).

The angles of junction of the braces to the chord must be at least 30 0 to ensure the density of the weld area from the side of the acute corner.

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

Truss knots from open bent profiles can be made without gussets.

With a box-section femme belt and braces of two branches connected by planks, the braces are adjacent on both sides to the belt overlap and are 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 with butt welds (Fig. 9.25, b). The reference node is shown in Figure 9.25, in.

9.20. Making a working drawing of light trusses (KMD)

The detailed (working) drawing shows the facade of the sending element, plans for 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 scheme drawn on a scale of 1:20-1:30 (see Fig. 13).

The main dimensions of the node are the dimensions from the center of the node 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 assigned in multiples of 10 mm. The drawing shows the dimensions of the welds and the location of the bolt holes.

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

The notes indicate the features of the manufacture of the structure, which are not clear from the drawing.

9.21. Heavy truss nodes

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 high forces in the rods cause significant moments that must be taken into account when calculating the trusses.

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

Heavy trusses, as a rule, have a height of more than 3.85 m, so they are assembled from individual elements during installation. Mounting joints are located in the nodes or near the nodes.

When the joint is located in the 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, the assembly connections of truss elements operating on dynamic loads (bridge, crane trusses, etc.) are often performed on high-strength bolts (Fig. 9.26). With an H-shaped or channel section of the rods, the nodes on the gussets connecting all the rods suitable for the node from the outside are simple and reliable.

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

Gussets at the device of joints of the belt in the center of the node 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 knots from open bent profiles

lining, increases by 10%. The gussets should be taken thick enough, not less than the thickness of the fastened elements.

Bolts in nodes of heavy trusses should be placed according to unified risks at the distances required by the conductor and multi-spindle drilling (usually with bolts of mm, the bolt pitch is 80 mm).

In large-span trusses, the horizontal displacement of supports is very significant. To exclude additional horizontal forces, the design solution of the support units must correspond to the design scheme (one support is pivotally fixed, the other is movable). motionless

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

Fig.9.26. Bolted heavy truss knot

9.22. prestressed trusses

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

Puffs can be placed within the length of individual rods operating under a tensile load, creating a compressive prestress in them (Fig. 9.27, a). This method is effective only for heavy farms.

In trusses, the belt of which (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 farms, the most effective scheme is the type of arch with a puff (Fig. 9.27, c, g).

External puffs are possible (Fig. 9.27, d), the unloading effect of which on the truss rods can be especially significant. However, according to the conditions of the structure layout and transportation, external tightening cannot always be applied.

When placing a puff along the length of the lower belt, it is connected by diaphragms to the belt and ensures it from loss of stability during prestressing (Fig. 9.28), when the lower belt receives compressive forces.

With remote tightening and in the “arch with tightening” scheme, it is necessary to take measures to ensure the stability of the lower chord during the prestressing process. In this case, tightening should be carried out in the design position when the truss is loosened 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, of a triangular section, it is also possible to produce tension at the bottom, since the lower chord is fixed from buckling (Fig. 9.29, b).

Sections of bars in prestressed trusses can be the same as in conventional ones. When prestressing individual rods, the puffs must be placed symmetrically about the vertical axis of the rod. For structural reasons, they are often designed from two branches (see Fig. 9.28).

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

Farm called a geometrically invariable lattice structure working in bending, the elements of which are pivotally connected at the nodes and work in axial tension or compression under nodal loading.

The assumption about the ideal hinge of the nodes contradicts the actual design of the truss, but quite accurately reflects the actual operation of its elements.

The calculation of the truss according to the hinged scheme is allowed when the ratio of the height of the section to the length of the element does not exceed 1/10 in structures operated at t ≥ -40 ° C, and 1/15 at t< -40°C.

Trusses are more economical in terms of metal consumption compared to beams.

The scope of farms is very extensive. They are used in coverings of buildings and structures for supporting roofs (rafter trusses), radio and television towers, power line pylons, bridge span structures, cranes, etc.

Truss classification

The trusses consist of upper and lower chords interconnected by a lattice of braces and posts. The distance between the truss lattice nodes is called the panel; the distance between its supports is the span. gusset- a truss detail made of a sheet for connecting the truss rods in a node.

A variety of applications and design solutions for trusses allows them to be classified according to various criteria:

by appointment– trusses of bridges, coverings (truss and truss), transport overpasses, cranes, hydraulic gates and other structures.

along the lines of the belts:

With parallel belts

Trapezoidal

Arched

triangular

with triangular lattice

With triangular grille and additional uprights

With slanted grid.

The outline of the belts depends mainly on the purpose of the truss and the adopted structural scheme of the structure. according to the lattice system:

Gratings of special types:

With trussed grating

cross

Rhombic

Semi-diagonal.

The grid system depends on the load application pattern and the special requirements of the truss. The simplest is the triangular lattice. Additional racks are installed in cases where concentrated forces are applied at their location or when they want to reduce the length of the panel of the upper compressed belt.

A feature of the diagonal lattice is that all the braces have forces of the same sign, and the racks have the opposite; in the upward direction of the braces, the posts are stretched, and in the downward direction, they are compressed.

The trussed lattice is used with more frequent application of concentrated forces to the upper chord.

Trusses with a cross lattice are usually used for double-sided loading. Cross braces design their flexible elements or strands; they perceive only tensile forces, and when compressed, they are turned off. Thanks to this, cross-lattice trusses are calculated as statically determinate systems.

Rhombic and semi-diagonal gratings have increased rigidity and are used in structures with large transverse forces

- according to the type of static scheme - trusses are cut, continuous, cantilever.

By the value of the greatest effort in the elements of the farm

lungs- span l up to 50 m and with a force in the belts N max ≤ 5000 kn,

heavy- with a force in the belts N max > 5000 kN,

by design decision- conventional, combined and prestressed.

Truss layout

The task of truss layout includes determining its rational scheme, taking into account a number of requirements: metal cost-effectiveness, ease of manufacture, transportability, unification and typing requirements. These requirements often contradict each other, so it is necessary to find the optimal solution that best satisfies a set of requirements at the same time.

The mass of the truss depends on the ratio of its height to the span. The forces in the truss chords arise mainly from the bending moment, and in the lattice - from the transverse force.

The greater the height of the truss, the less effort in the belts and their mass, but with an increase in the height of the truss, the length of the lattice elements and its mass increase. Conventionally, the minimum metal consumption corresponds to the equality of the mass of the belts and the mass of the lattice together with the gussets, which is achieved at h≈1/5 L (in the beam, the mass of the belts is approximately equal to the mass of the wall).

Such a high height is inconvenient during transportation. The farm would have to be delivered to the construction site in separate elements (in bulk) and assembled at the installation site.

Additional expenses of time and means at the same time are not paid off by economy of metal.

In practice, they tend to ensure that during installation only a pre-assembly of the farm of their two halves (departure marks) is carried out. Therefore, the dimensions of the truss should not go beyond the railway gauge (vertically 3.8 m, horizontally -3.2 m). The most convenient to manufacture are trusses with parallel belts. The same lengths of the belt and lattice rods, the same solution of intermediate nodes and the minimum number of belt joints create conditions for the maximum possible unification of structural schemes and make such trusses industrial. Due to the advantages in manufacturing, trusses with parallel belts are gradually replacing trapezoidal trusses.

When assembling the truss, simultaneously with the choice of the lattice system, the dimensions of the truss panels are set, the dimensions of which must correspond to the optimal angle of inclination of the braces. From design considerations - the rational outline of the gusset in the knot and the convenience of fastening the braces - an angle close to 45 ° is desirable.

By unifying the geometric schemes of trusses and typifying the structural form, it is possible to standardize the structural details of trusses and switch to their mass production using specialized machines and fixtures.

Currently, the geometric schemes of truss trusses of industrial buildings (18, 24, 30, 36 m), bridges, radio masts, radio towers, power transmission line supports are unified.

The unification of truss trusses with roll roofing is based on the span module of industrial buildings and a panel m = 3 m, roof slope i = 1.5%, height of trusses on a support of 3150 mm along the outer edges of the chords, a triangular lattice with the possibility of adding a truss with roofing slabs wide 1.5 m

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.

The sagging of the trusses is prevented by the building hoist device, i.e. the manufacture of trusses with reverse camber, which is extinguished under the action of the load, as a result of which the truss takes the design position.

Farm calculation. Determination of loads. Determination of forces in truss rods. Estimated lengths of truss rods. Ensuring the overall stability of trusses in the coating system. Selection of the bar section type.

Farms are calculated in the following sequence:

1) determine the load on the farm;

2) calculate nodal loads;

3) determine the design forces in the truss rods by the method of structural mechanics;

4) select the sections of the rods;

5) calculate the connections of the rods, nodes and parts.

The basis of the supporting structure of the roof is trusses, which are rafter and rafter (see photo). The strength, reliability and service life of the roof depend on how well they are made. Wooden roof trusses must withstand not only the weight of the so-called roofing "pie", but also significant loads resulting from exposure to strong winds and precipitation.

What are roof trusses?

The truss truss is used for the device pitched roofs rigid construction. It is necessary to redistribute the load that the roof is subjected to on the walls of the building. Truss materials are different, but wood is most often used.

A wooden truss for the roof, as in the photo, is made of boards, timber or round timber. To connect all the elements made from timber and logs into one structure, a method such as cutting is used, and if boards are involved, then metal fasteners are nails, bolts, anchors, gear-ring dowels, and so on.

In low-rise construction, in the manufacture of wooden roof trusses, softwood lumber is usually used because of their cheapness and ease of fitting. When installing rafters, it is imperative to exclude the possibility of their sagging along the length under the weight of the roof and its own weight. This is done in one of two ways: they install the middle run - a thick supporting bar across the rafters or cross beams and spacers.

Currently, in order to avoid significant labor costs when assembling a truss, combined metal and wood structures are used, and then the installation truss system takes much less time. The option of creating a roof with open trusses is not used in the construction of residential buildings - the system is covered with ceilings. In industrial construction, on the contrary, they usually use an open structure.

Choosing a farm layout

When choosing the shape of the truss truss, the following factors are taken into account:

roof slope angle;
the type of connection that is supposed to be used when creating the structure;
roof surface covering material;
the presence / absence of a ceiling.

For example, if during the construction of a house an almost flat roof is created with a coating of bituminous roll materials, then the most optimal, according to experts, is the shape of a trapezoid or rectangle. Triangular trusses are mounted if the roof has steep slopes and heavy coatings are planned to be laid on its surface.

To determine the height of the farm, use the formulas:

if a rectangular truss - 1/6 x L;
if the design is triangular - 1/5 x L.

The letter L is the span of the truss truss.

When building a private house, as a rule, a triangular truss system is erected. This form of a truss, in combination with an inclined one, makes it possible to build single-pitched and double-pitched roofs with different angles of inclination. When cottages with gable roofs are being built, structures with hanging rafters are often used. At the same time, carved rafters can become a real decoration of the roof.

In order to ensure the reliability and strength of the trusses for their upper and lower chords, additional ligaments are mounted, which are made from boards and placed in the plane of the middle rack.

Construction of simple triangular trusses

In many ways, the layout of the rafters depends on the length of the span of the building and the presence / absence of internal bearing walls. A simple truss truss is used if it relies solely on external walls buildings (the house has no supports inside) and the span parameter does not exceed 6 meters.

The procedure for calculating roof structures

When calculating truss systems and in order to draw up a layout plan for the rafters, it is necessary to take into account the expected loads on the roof structure, which can be conditionally divided into 3 categories:

loads exerted constantly - these include the weight of the elements of the roofing "pie";
temporary - this is the mass of snow (depending on weather conditions in the region), the weight of people climbing the roof to carry out work, the wind factor, etc.;
special loads - for example, on buildings located in areas of increased seismic hazard.

The calculation of the possible snow load is performed according to the formula:

S=Sg x μ, where

Sg is the weight of the snow load based on square meter roofing. This value is conditional, and its value is determined according to special tables depending on the region.

μ is a coefficient that depends on the angle of the roof.

To determine the wind load, you need to know:

type of terrain (urban or open space);
standard value of wind load in the given region;
building height.

Manufacturing of roof trusses

AT last years in the construction of private houses, truss trusses made right on construction sites began to prefer factory-made structures. They are made on mounting and pressing equipment. In the production of wooden elements, they are pre-treated with special compounds that prevent rotting and insect damage.

Modern technologies allow the production of truss and truss trusses and elements for them for roofs of various designs and not only for residential buildings. For example, it could be a truss system gable roof baths, garages and other outbuildings (read: "").

Metal and steel truss structures

To make belts and gratings, corners for the truss system are used, and individual elements are connected by welding. The optimal solution, distinguished by reliability, experts consider the design for which the belts are made of tee wide-shelf beams. The difference between steel trusses and trusses is the presence of a parallel belt. Their dimensions correspond to the parameters of the truss structures.

For the construction of private houses, as a rule, farms are used, for the production of which profile hot-rolled or bent pipes are used, rectangular or square. This is explained simply: their weight is less than that of products made from a corner, a brand or a channel. Such a system can be easily assembled from individual prefabricated elements at the construction site before installation by welding.

Often, to create a roof, if the overlap of spans is long, reinforced concrete roof trusses are used, which are solid lattice structures. They are recommended to be mounted on the roofs of one-story buildings, the coatings of which will be subjected to increased loads.

Rafter trusses for single pitched roofs

The procedure for carrying out work when installing a truss on a pitched roof is as follows:

the value of the difference in the bearing walls is calculated according to the formula H \u003d W x tg L, where H is the desired result, W is the distance between opposite walls, and tg L is the tangent of the angle at which the roof is being erected;
depending on what wood rafters are and what are needed, they are harvested and processed with special impregnations (read: "");
then the Mauerlat is installed, the thickness of which must correspond to the thickness of the supporting walls. This beam must be rigidly fixed and qualitatively waterproofed, observing a strictly horizontal arrangement;
then markings are made on the Mauerlat, according to which the rafter legs will be installed and recesses are cut out for them;
in some cases, when assembling the structure, it is made (read: "");
finished trusses are laid in such a way that they protrude 30 centimeters beyond the surface of the beam for support, fix them using bolts and brackets;
then the supports are installed and the crate is performed. Supports are necessary when the length of the rafter legs exceeds 4.5 meters. Planks are stuffed on top of the rafters for the crate. Often, to create a truss truss, it is necessary to join the rafters along the length - it is carried out in a section where the bending moment is minimal.

"Construction Farms"

truss section rod box-shaped

Classification and scope of farms

The origin of the term "farm" originates from the Latin firmus, that is, "strong, strong."

Farms are flat (all rods lie in the same plane) and spatial.

Flat trusses perceive the load applied only in their plane, and need to be fixed with their connections. Spatial trusses form a rigid spatial beam that takes the load in any direction (Fig. 9.1).

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

The main elements of the trusses are the belts that form the contour of the truss, and the lattice, consisting of braces and racks (Fig. 9.2). The connection of elements in nodes is carried out by direct adjacency of some elements to others (Figure 9.3, a) or with the help of shch yu nodal gussets (Fig. 9.3, b). Truss elements are centered along the axes of the center of gravity to reduce the nodal moments and ensure the operation of the rods for axial forces.

Rice. 9.2. Truss elements

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

Rice. 9.3. Farm nodes: a - with direct adjacency of elements ; b - on gussets

The distance between adjacent nodes of the belts is called the panel (d in is the panel of the upper belt, d n is the lower one), and the distance between the supports is called the span (/).

Truss chords work for longitudinal forces and moment (similar to solid beam chords); the truss lattice perceives mainly the transverse force, performing the functions of the beam web.

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

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

according to the static scheme - beam (cut, continuous, cantilever);

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

Fig.9.4. Truss systems: a- beam cut; 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; in - trapezoidal; g - with parallel belts; d-i - triangular

in terms of maximum force - light - single-walled with sections from 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 truss or braces or an arch (top). 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 a smaller number of elements) and are rational in heavy structures, as well as in structures with moving loads.

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

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

Rice. 9.6. Truss Lattice Systems

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