Construction metal trusses design features. Truss in construction What are trusses in construction

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The main task when constructing a roof of any configuration and type is to ensure maximum protection of the building from negative impact external factors. This can be achieved through the use of wood or metal roof trusses.

Basic design requirements

Regardless of the specific type of building materials (wood, built-up roofing, metal tiles, ordinary tiles), before finishing work begins, a solid building structure must be erected, the main task of which should be to ensure the stability and reliability of the roof being installed.
High-quality execution of the structure, the main parts of which are rafter and sub-rafter trusses, is a guarantee that it will withstand all permanent and temporary loads placed on it.

Design features of roof trusses

In definition “rafter truss” includes a number structural elements: braces, racks, sheathings. Such a rigid structure as a result of assembly will provide the skeleton of the roof.

This definition refers to rigid structures used in the construction of pitched roofs.

The purpose of the trusses is to transfer the total load that appears on the roof to the walls of the building. The material for making trusses is mainly wood, but alternative options are also possible.

The basis for creating wooden structures are boards, round timber, timber.

The individual elements are connected using the cutting method. In the case where the parts are made of boards, bolts, nails, and toothed ring keys are used.

When constructing large buildings, when the span length is more than 16 meters, trusses with stretched metal-based posts are used during construction.

In case of using stretched wooden racks reliable fastening nodes is quite a complex task, and when using metal elements it's easy to do.

Assembling wooden trusses is a rather labor-intensive process. When using combined type trusses (with wooden and metal parts) this can be done much faster.

When constructing residential buildings, the option is rarely used roofing structure with open farms. Basically, they are closed using ceilings. In the construction of industrial buildings, open trusses are a fairly popular roofing option.

Beams can also be used as main elements. Rafter beams based on reinforced concrete are sub-rafter structures that cover the steps of columns 12, 18 meters long. Such elements serve as intermediate supports.

Types of roof trusses

When choosing a design and material for creating a roofing skeleton, it is necessary to take into account the material on which the walls are built. Supports for load-bearing structures can be reinforced concrete, metal columns, brick walls or rafter trusses.

The characteristics of farms make it possible to classify them according to the following characteristics:

the outline of the belts;
constructive design;
static scheme;
grating type.

The most significant criterion for dividing farms into individual species is the outline of the belts. It is determined by the roof structure, the size of the span to be covered, and the magnitude of the applied load. Basically, the choice of roofing material depends on the slope of the upper chord.

In the field of industrial construction, when installing flat roofs, trusses with parallel chords are used.
If asbestos-cement or steel sheets are used as roofing material, a trapezoidal type of load-bearing structure is arranged.
In the field of civil engineering, triangular frames are the most popular.
For any type of outline, different lattice systems are used. When constructing trusses with parallel chords or a trapezoidal shape, the best option is a triangular lattice, reinforced by installing additional racks.
In addition to this type, split beam designs or gratings with trusses have become widespread.

Based on design farms are divided into the following types:

lightweight, or single-walled - distinguished by the presence of one gusset in the nodes;
heavy, or double-walled - their feature is the presence of two shaped sheets in the nodes.

When constructing buildings with a significant span (assembly shops, hangars), heavy structures are used. In such industrial buildings, the skeleton serves as crossbars for frame systems.

According to the material of manufacture trusses are:

wooden;
metal;
mixed (or combined).

Shape selection

To choose the right design shape, you need to pay attention to the following important points:

material used as roofing;
roof slope angle;
the presence of ceilings;
the specifics of a particular type of connection of truss truss parts.

For example, in the case of constructing a flat roof, the coating for which is rolled bitumen materials, the best option for a roof truss is a rectangular or trapezoidal structure.

When the slope angle is more than 12 degrees and in the case of heavy coatings, triangular trusses are the optimal choice.

Calculation of the height of the truss is carried out using the formula corresponding to the selected form of the structure:

For the triangular version: 1/5 x L. The Latin letter indicates the span length.
For rectangular design: 1/6 x L.

When constructing private residential buildings, the most commonly used truss is a triangular truss. For installation of roofs with one or two slopes with different angles slope, a variety of combinations with inclined rafters are used, wooden or metal roof trusses are installed.

The stability of the trusses is increased by installing additional ties for the upper and lower chords. The production of such bundles is carried out mainly on the basis of boards.

In the case of erecting a roof with two slopes, the rational choice is a type of truss with hanging rafters.

The most important point in choosing a rafter structure is the selection building material, which can provide the required degree of strength, rigidity, and reliability. The price category to which the material belongs also matters.

Wooden trusses

Wood-based trusses are the most popular option and meet most structural requirements.

They are used in the following cases:

when installing attic roofs;
during the construction of commercial, agricultural, sports, industrial facilities;
during recovery flat roofs structures for various purposes;

The main advantages of wooden roof trusses in relation to other varieties of such structures are:

simplicity of the production process and its automation;
short deadlines for completing design work;
ease of installation (due to the presence of ready-made structural elements);
a wide perspective for the implementation of the most unusual design solutions in terms of shape, configuration, etc.;
relatively light weight;
possibility of application in case of large spans ( greatest length can be 18 m);
the possibility of insulation and the ease of its implementation;
excellent performance and technological characteristics;
belonging to the first category of fire safety (the material undergoes appropriate processing in the factory);
compliance with environmental safety requirements.

When choosing wood-based structures, it is necessary to pay attention to the high-quality treatment of the material with antiseptic and antipyretic compounds, which ensure protection of the structure from the destructive effects of rot, fungi, insects and fire.

Specifics of metal roof trusses

To ensure increased rigidity, metal-based rafter systems are used. This option is most suitable for constructing roof trusses of considerable length (more than 10 m). In such cases, not only rafters are made of steel, but also mauerlat beams and ridge supports. Laying is done not using a wooden connecting piece, but using a channel. Welded corners are used to attach the rafter legs.

The main advantages of steel roof trusses are:

increased level of strength;
durability;
resistance to destruction and rotting;
significant service life;
ease of use when it is necessary to cover structures of large area and height.

It should also be indicated disadvantages of metal trusses:

significant weight of the created structure;
the use of special equipment to lift the structure to the required height;
instability of the material and a high probability of its deformation under high temperature conditions;
high cost.

There are three types of steel roof trusses:

parallel belts;
triangular;
polygonal.

When planning soft roof Trusses with parallel belts or polygonal ones are optimal. When using sheet materials, it is better to choose triangular structures.

The industry produces roof trusses of standardized sizes, which are designed for spans of 18, 24, 30 and 36 meters in length.

For the manufacture of truss belts and grids, in most cases, corners are used, and the individual elements are fastened by welding. The most rational designs are considered to be those in which wide-flange T-beams were used to make the belts. The manufacturing process of such structures is quite simple, and it also takes less material, but the reliability and strength of the structure do not suffer from this.

A distinctive feature of a steel rafter truss from a rafter truss is the presence of a parallel belt. They are produced in similar standardized sizes.

During the construction of private houses, in many cases steel roof trusses are used, the material for which is a profile pipe. Compared to trusses based on angles, channels or tees, such structures have less weight.

Such structures can be assembled directly on the construction site. In this case, a welding machine is used.

Trusses are made on the basis of bent or hot-rolled profile pipes. In this case, steel with a thickness of one and a half to five millimeters is used. The cross-section of the pipe profile can be square or rectangular.

Currently, reinforced concrete roof trusses are also often used in construction. They are very strong lattice structures for covering long spans.

It is advisable to install such trusses on the roofs of one-story buildings, the roof of which experiences very heavy loads.

Load calculation

During the construction of a rafter system, an important point is not only right choice the shape of the structure and material of manufacture, but also the correct calculation of possible loads.

They come in three types:

permanent: total weight of the roofing pie;
temporary: the weight of the snow layer, people on the roof, the strength of the wind gusts;
special– seismic load falls into this category.

A rafter system based on trusses and beams is a load-bearing structure that includes many elements. Regardless of the specific design and shape, this is a technologically complex process, the main stage of which is the design stage. It is at this time that calculations are carried out, on the correctness of which the safety of the entire structure depends on the entire operational period. This calculation consists of complex calculations, the implementation of which requires professional knowledge.

Installation

As an example, the simplest process for installing rafter structures when constructing a pitched roof is described below.

First, in accordance with the given formula, the value of the wall difference is calculated:
W x tgL

The letter Ш indicates the distance between two supporting walls, and tgL is the tangent of the roof slope angle.

After this, the required number of wooden rafters are prepared, pre-treated with antiseptic compounds.
The next step is to install the Mauerlat. The thickness of the beam must correspond to the thickness of the walls. It must be attached as rigidly as possible and held high-quality waterproofing. During the installation of the support beam, it is necessary to ensure that it is strictly horizontal.
After the installation of the Mauerlat is completed, the installation points of the rafter legs should be marked on it and recesses for them should be cut out.
Pre-prepared trusses are laid in such a way that they protrude 30 cm onto the surface of the support beam. Fastening is carried out using bolts and brackets.
The process ends with the installation of supports and installation of the sheathing. In cases where rafter legs have a length of more than 4.5 m, the supports are mandatory element. Sheathing strips are placed on top of the mounted rafters.

CONCLUSIONS:

The use of wood or metal roof trusses protects the building from external influences.
The main requirements for the selected material and design of the truss are strength, stability and reliability.
The purpose of the trusses is to transfer the total load that appears on the roof to the walls of the building.
Trusses are classified according to the outline of the belts, design, static diagram, and lattice type.
There are trusses with parallel belts, trapezoidal, triangular.
Rafter trusses are made of wood or metal. Combination options are also possible.
The stability of the trusses is increased by installing additional ties for the upper and lower chords based on boards.
When constructing a rafter system, an important point is the correct calculation of possible loads.

For more information about the hip roof rafter system, see the video.

Today, profile pipe trusses are rightfully considered ideal solution for the construction of garages, residential buildings and outbuildings. Strong and durable, such designs are inexpensive, quick to implement, and anyone with at least a little understanding of mathematics and cutting and welding skills can handle them.

And now we will tell you in detail how to choose the right profile, calculate the truss, make jumpers in it and install it. For this we have prepared for you detailed master classes making such farms, video tutorials and valuable advice from our experts!

So what is a farm? This is a structure that ties the supports together into one cohesive whole. In other words, the truss is a simple architectural structure, among the valuable advantages of which we highlight the following: high strength, excellent performance, low cost and good resistance to deformation and external loads.

Due to the fact that such trusses have a high load-bearing capacity, they are placed under any roofing materials, regardless of their weight.

The use of metal trusses from new or rectangular closed profiles in the construction is considered one of the most rational and constructive solutions. And for good reason:

The main secret is savings due to the rational shape of the profile and the connection of all lattice elements.
Another valuable advantage of profile pipes for use in the manufacture of trusses is equal stability in two planes, remarkable streamlining and ease of use.
Despite their low weight, such trusses can withstand serious loads!

Rafter trusses differ in the outline of the belts, the type of cross-section of the rods and the types of lattice. And with the right approach, you can independently weld and install a truss from a profile pipe of any complexity! Even this one:

Stage II. We purchase a high-quality profile

So, before drawing up a project for future farms, you first need to decide on the following important points:

contours, size and shape of the future roof;
material for the manufacture of the upper and lower chords of the truss, as well as its lattice;

Remember one simple thing: a profile pipe frame has so-called balance points, which are important to determine for the stability of the entire farm. And it is very important to choose for this load quality material:

Trusses are built from profile pipes of the following types of sections: rectangular or square. These are available in different cross-sectional sizes and diameters, with different wall thicknesses:

We recommend those that are specially sold for small-sized buildings: these are up to 4.5 meters long and have a cross-section of 40x20x2 mm.
If you will make trusses longer than 5 meters, then choose a profile with parameters 40x40x2 mm.
For full-scale construction of a residential roof you will need profile pipes with the following parameters: 40x60x3 mm.

The stability of the entire structure is directly proportional to the thickness of the profile, so for the manufacture of trusses, do not use pipes that are intended only for welding racks and frames - these have different characteristics. Also pay attention to what method the product was manufactured: electric welded, hot-formed or cold-formed.

If you undertake to make such trusses yourself, then take square-section blanks - they are the easiest to work with. Purchase a square profile 3-5 mm thick, which will be strong enough and its characteristics are close to metal bars. But if you are making a truss just for a visor, then you can give preference to a more budget-friendly option.

Be sure to consider snow and wind loads in your area when designing. After all great importance when choosing a profile (in terms of the load on it), the angle of inclination of the trusses is:

You can more accurately design a truss from a profile pipe using online calculators.

Let us only note that the most simple design a profile pipe truss consists of several vertical posts and horizontal levels onto which rafters for the roof can be attached. You can purchase such a frame ready-made yourself, even to order in any city in Russia.

Stage III. We calculate the internal stress of farms

The most important and responsible task is to correctly calculate the truss from a profile pipe and select the desired format of the internal lattice. To do this, we will need a calculator or similar other software, as well as some tabular data of SNiPs, which for this:

SNiP 2.01.07-85 (impacts, loads).
SNiP p-23-81 (data on steel structures).

Please review these documents if possible.

Roof shape and angle

What specific roofing needs a truss? Single-pitch, gable, dome, arched or hipped? The simplest option, of course, is to make a standard lean-to canopy. But you can also calculate and manufacture quite complex trusses yourself:

A standard farm consists of the following important elements, such as the upper and lower chords, racks, braces and auxiliary struts, which are also called trusses. Inside the trusses there is a system of gratings; welds, rivets, special paired materials and gussets are used to connect the pipes.

And, if you are going to make a complex-shaped roof, then such trusses will become suitable for it ideal option. It is very convenient to make them according to a template directly on the ground, and only then lift them up.

Most often when building a small country house, garage or change house, so-called polonso trusses are used - a special design of triangular trusses connected by ties, and the lower chord here comes out raised.

Essentially, in this case, in order to increase the height of the structure, the lower belt is made broken, and it then amounts to 0.23 of the flight length. It is very convenient for the interior space.

So, there are three main options for making a truss, depending on the slope of the roof:

from 6 to 15°;
from 15 to 20°;
from 22 to 35°.

What's the difference you ask? For example, if the angle of the structure is small, up to only 15°, then it is rational to make the trusses trapezoidal in shape. And at the same time, it is quite possible to reduce the weight of the structure itself, taking the height from 1/7 to 1/9 of the total flight length.

Those. follow this rule: the less the weight, the greater the height of the truss should be. But if we already have a complex geometric shape, then you need to choose a different type of truss and gratings.

Types of trusses and roof shapes

Here is an example of specific trusses for each type of roof (single, gable, complex):

Let's look at the types of farms:

Triangular trusses are a classic for making the base for steep roof slopes or sheds. The cross-section of pipes for such trusses must be selected taking into account the weight of the roofing materials, as well as the operation of the building itself. Triangular trusses are good because they have simple forms, easy to calculate and implement. They are valued for providing natural light under the roof. But we also note the disadvantages: these are additional profiles and long rods in the central segments of the lattice. And here you will have to face some difficulties when welding sharp support corners.
Next view - polygonal profile pipe trusses. They are indispensable when constructing large areas. Their welding has a more complex shape, and therefore they are not designed for lightweight structures. But such trusses are distinguished by greater metal savings and strength, which is especially good for hangars with large spans.
Also considered durable parallel chord truss. This truss differs from others in that all its parts are repeating, with the same length of rods, belts and gratings. That is, there are a minimum of joints, and therefore it is easiest to calculate and weld one from a profile pipe.
A separate type is single slope trapezoidal truss supported by columns. Such a truss is ideal when rigid fixation of the structure is required. It has slopes (braces) on the sides and there are no long rods of the top sheathing. Suitable for roofs where reliability is especially important.

Here is an example of making trusses from a profile pipe: universal option, which is suitable for any garden buildings. It's about about triangular trusses, and you've probably already seen them many times:

A triangular truss with a crossbar is also quite simple, and is quite suitable for building gazebos and cabins:

And here arched farms are already much more complicated to manufacture, although they have a number of valuable advantages:

Your main task is to center the metal truss elements from the center of gravity in all directions, in simple terms, to minimize the load and distribute it correctly.

Therefore, choose the type of farm that is more suitable for this purpose. In addition to those listed above, scissor trusses, asymmetrical, U-shaped, double-hinged, trusses with parallel chords and attic trusses with and without supports are also popular. And also an attic view of the farm:

Grating types and point loads

You might be interested to know that specific design The internal grids of the trusses are selected not for aesthetic reasons, but for quite practical ones: to suit the shape of the roof, the geometry of the ceiling and the calculation of loads.

You need to design your farm in such a way that all forces are concentrated specifically at the nodes. Then there will be no bending moments in the belts, braces and trusses - they will work only in compression and tension. And then the cross-section of such elements is reduced to the required minimum, while significantly saving on material. And you can easily make the truss itself hinged.

Otherwise, the force distributed over the rods will constantly act on the truss, and a bending moment will appear, in addition to the general stress. And here it is important to correctly calculate the maximum bending values for each individual rod.

Then the cross-section of such rods should be larger than if the truss itself were loaded with point forces. To summarize: trusses on which the distributed load acts uniformly are made of short elements with hinged joints.

Let's figure out what the advantage of this or that type of grid is in terms of load distribution:

Triangular The lattice system is always used in parallel chord and trapezoidal trusses. Its main advantage is that it gives the shortest total lattice length.
Diagonal The system is good for low truss heights. But the material consumption for it is considerable, because here the entire path of effort goes through the nodes and rods of the lattice. Therefore, when designing, it is important to lay a maximum of rods so that the long elements are stretched and the racks are compressed.
Another type - trussed lattice. It is made in case of loads on the upper belt, as well as when it is necessary to reduce the length of the grating itself. The advantage here is to maintain the optimal distance between the elements of all transverse structures, which, in turn, allows you to maintain the normal distance between the purlins, which will be a practical point for installing roofing elements. But creating such a lattice with your own hands is a rather labor-intensive task with additional metal costs.
Cruciform the lattice allows you to distribute the load on the truss in both directions at once.
Another type of lattice - cross, where the braces are attached directly to the wall of the truss.
And finally semi-diagonal And rhombic gratings, the toughest of those listed. Here two systems of braces interact at once.

We have prepared an illustration for you where we have collected all types of trusses and their gratings together:

Here is an example of how a triangular lattice truss is made:

Making a truss with a diagonal lattice looks like this:

It cannot be said that one type of truss is definitely better or worse than another - each of them is valuable due to its lower consumption of materials, lighter weight, load-bearing capacity and method of fastening. The drawing is responsible for what load pattern will act on it. And the chosen type of lattice will directly determine the weight of the truss, the appearance and labor intensity of its manufacture.

Let us also note this unusual option for making a truss, when it itself becomes a part or support for another, wooden one:

Stage IV. We manufacture and install trusses

We will give you some valuable tips on how to independently weld such trusses right on your own site without much difficulty:

Option one: you can contact the factory, and they will custom-make all the necessary individual elements according to your drawing, which you just have to weld on site.
Second option: purchase a ready-made profile. Then all you have to do is cover the inside of the trusses with boards or plywood, and lay insulation in between, if necessary. But this method will, of course, cost more.

Here, for example, is a good video tutorial on how to lengthen a pipe by welding and achieve ideal geometry:

Here is also a very useful video on how to cut a pipe at a 45° angle:

So, now we come directly to the assembly of the trusses themselves. The following step-by-step instructions will help you cope with this:

Step 1: First prepare the trusses. It is better to weld them directly on the ground in advance.
Step 2. Install vertical supports for future trusses. It is vital that they are truly vertical, so test them with a plumb line.
Step 3. Now take the longitudinal pipes and weld them to the support posts.
Step 4. Raise the trusses and weld them to the longitudinal pipes. After this, it is important to clean all connection points.
Step 5. Paint the finished frame with special paint, having previously cleaned and degreased it. Pay special attention to the joints of the profile pipes.

What else do those who make such farms at home face? First, think in advance about the support tables on which you will place the truss. Far from it the best option throw it on the ground - it will be very inconvenient to work.

Therefore, it is better to install small bridge supports that will be slightly wider than the lower and upper chords of the truss. After all, you will manually measure and place jumpers between the belts, and it is important that they do not fall to the ground.

The next important point: trusses made from profile pipes are heavy in weight, and therefore you will need the help of at least one more person. In addition, it wouldn’t hurt to have help with such tedious and painstaking work as sanding metal before cooking. Also keep in mind that you will need to cut a lot of trusses for all the elements, and therefore we advise you to either purchase or build homemade machine similar to the one in our master class. Here's how it works:

So, step by step, you will draw up a drawing, calculate the truss lattice, make blanks and weld the structure on site. Moreover, you will also be using up the remains of the profile pipes, therefore, you will not need to throw anything away - all this will be needed for minor parts of the canopy or hangar!

Stage V. Clean and paint the finished trusses

After you install the trusses in their permanent place, be sure to treat them with anti-corrosion compounds and paints polymer paints. Paint that is durable and UV resistant is ideal for this purpose:

That's all, the profile pipe farm is ready! Only finishing work for covering the trusses from the inside with finishing and outside with roofing material:

Believe me, making a metal truss from a profile pipe will actually not be difficult for you. a lot of work. A huge role is played by a well-drawn drawing, high-quality welding of a truss from a profile pipe and the desire to do everything correctly and accurately.

dh f).

According to the static scheme

Depending on the outlines of belts

segmental(arched girder

More acceptable is polygonal outline with a fracture of the belt at each node (e). It corresponds fairly closely to the parabolic outline of the moment diagram and does not require the manufacture of curvilinear elements. Such trusses are sometimes used to cover large spans and in bridges, i.e. in structures delivered to the construction site in bulk (from individual elements). For coating trusses of ordinary buildings, supplied for installation, as a rule, in the form of enlarged sending elements, due to the complexity of manufacturing, these trusses are currently not used. You can only find them in old buildings built before the 50s.

Farms trapezoidal shape(V)

Farms with parallel belts in their outline they are far from the moment diagram and in terms of consumption they are not economical. However, equal lengths of lattice elements, the same layout of nodes, the highest repeatability of elements and parts and the possibility of their unification contribute to the industrialization of their production. Due to these advantages, parallel chord trusses have become a staple for roofing buildings.

Farms triangular in shape

Grid systems

Triangular system

In a braced lattice system

Truss lattice

cross lattice.

Steel trusses.

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Farms. Application area. Classification. Truss structures.

A truss is a lattice structure made of rods connected to each other at nodes and forming a geometrically unchangeable structure.

If the load is applied in the nodes, and the axes of the truss elements intersect at one point (the center of the node), then the rigidity of the nodes does not significantly affect the operation of the structure and in most cases they can be considered as hinged. Then all the truss rods experience only axial forces (tension or compression). Thanks to this, metal in trusses is used more efficiently than in beams, and they are more economical than beams in terms of material consumption, but are more labor-intensive to manufacture, since they have a large number of parts. With an increase in overlapped spans and a decrease in load, the efficiency of trusses compared to solid-wall beams increases.

Based on the material, trusses can be distinguished between steel, wooden and reinforced concrete.

Steel trusses have become widespread in many areas of construction: in coverings and floors of industrial and civil buildings, bridges, power line supports, communication facilities, television and radio broadcasting (towers, masts), conveyor galleries, hydraulic valves, lifting cranes, etc.

Farms can be flat or spatial.

Flat trusses can only support loads applied in their plane and need to be secured from their plane with ties or other elements. Spatial trusses form a rigid spatial beam capable of absorbing loads acting in any direction. Each face of such a beam is a flat truss. An example of a space beam is a tower or mast

The main elements of trusses are belts that form the outline of the truss, and a lattice consisting of braces and posts.

The distance between the belt nodes is called the panel ( d), the distance between the supports is the span (L), the distance between the axes (or outer edges) of the chords is the height of the truss ( h f).

Connections of elements in nodes are carried out by directly adjoining one element to another or using nodal gussets. In order for the truss rods to work mainly on axial forces, and the influence of moments can be neglected, the truss elements should be centered along the axes.

Depending on the purpose, architectural requirements and load application pattern, trusses can have a wide variety of structural forms. They can be classified according to the following characteristics: static diagram, outline of the belts, lattice system, method of connecting elements in nodes, amount of force in the elements.

According to the static scheme trusses are: beam (split, continuous, cantilever), arched, frame and cable-stayed.

Split beam systems are most widely used in building roofs, bridges, conveyor galleries and other similar structures. They are easy to manufacture and install and do not require complex support units.

When the number of overlapped spans is two or more, 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. But as in any externally statically indeterminate systems, in continuous trusses the installation of such structures becomes more complicated. Cantilever trusses are used for canopies, towers, and overhead power line supports. Frame systems are economical in terms of steel consumption, have smaller dimensions, but are more complex during installation. Their use is rational for long-span buildings. The use of arched systems, although it saves steel, leads to an increase in the volume of the room and the surface of the enclosing structures. Their use is dictated mainly by architectural requirements. In cable-stayed trusses, all rods work only in tension and can be made of flexible elements, such as steel cables. The tension of all elements of such trusses is achieved by choosing the outline of the chords and lattice, as well as by creating prestress. Working only in tension allows you to fully utilize the high strength properties of steel, since stability issues are eliminated. Cable-stayed trusses are rational for long-span floors and bridges.

Depending on the outlines of belts trusses are divided into triangular (a, b), arched (e), polygonal (f), trapezoidal (c), with parallel chords (d).

The outline of the truss belts largely determines their efficiency. Theoretically, 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, this will be segmental(arched girder with a parabolic belt (e). However, the curvilinear outline of the belt increases the complexity of manufacturing, so such trusses are practically not used at present.

More acceptable is polygonal outline with a fracture of the belt at each node (e).

Rafter truss - choosing a scheme

It corresponds fairly closely to the parabolic outline of the moment diagram and does not require the manufacture of curvilinear elements. Such trusses are sometimes used to cover large spans and in bridges, i.e. in structures delivered to the construction site in bulk (from individual elements). For coating trusses of ordinary buildings, supplied for installation, as a rule, in the form of enlarged sending elements, due to the complexity of manufacturing, these trusses are currently not used. You can only find them in old buildings built before the 50s.

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

Farms with parallel belts in their outline they are far from the moment diagram and in terms of consumption they are not economical.

However, equal lengths of lattice elements, the same layout of nodes, the highest repeatability of elements and parts and the possibility of their unification contribute to the industrialization of their production. Due to these advantages, parallel chord trusses have become a staple for roofing buildings.

Farms triangular in shape rational for cantilever systems, as well as for beam systems with a concentrated load in the middle of the span (rafter trusses).

Grid systems

The choice of lattice type depends on the load application pattern, the outline of the chords and design requirements. Thus, in order to avoid bending of the belt, places where concentrated loads are applied should be reinforced with lattice elements. To ensure compactness of the units, it is advisable to have the angle between the braces and the belt in the range of 30...50°.

To reduce the labor intensity of manufacturing, the truss should be as simple as possible with the least number of elements and additional parts.

Triangular system lattice has the smallest total length of elements and the smallest number of nodes. There are trusses with ascending and descending support braces. If the support brace goes from the lower support node of the truss to the upper chord, then it is called ascending. When the brace is directed from the support node of the upper chord to the lower one - downward. In places where concentrated loads are applied (for example, in places where roof purlins are supported), additional racks or hangers can be installed. These racks also serve to reduce the estimated length of the belt. Racks and suspensions work only on local loads.

The disadvantage of a triangular lattice is the presence of long compressed braces, which requires additional steel consumption to ensure their stability.

In a braced lattice system all braces have forces of one sign, and racks have forces of another. Thus, in trusses with parallel chords, with an ascending brace, the posts are stretched and the braces are compressed; when descending, it’s the other way around. Obviously, when designing trusses, one should strive to ensure that the longest elements are in tension, and compression is absorbed by the short elements. A diagonal lattice is more metal-intensive and labor-intensive compared to a triangular lattice, since total length there are more lattice elements and in it more nodes. The use of diagonal lattice is advisable for low truss heights and large nodal loads.

Truss lattice used for off-node application of concentrated loads to the upper chord, as well as when it is necessary to reduce the estimated length of the belt. It is more labor-intensive, but by eliminating the bending work of the belt and reducing its design length, it can reduce steel consumption.

If the load on the truss can act in both one and the other direction (for example, wind load), then it is advisable to use cross lattice.

Rhombic and semi-diagonal gratings 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. They are rational for large truss heights and when structures operate under significant lateral forces.

A combination of different types of lattice is possible in one farm.

According to the method of connecting elements At the nodes, trusses are divided into welded and bolted. In structures manufactured before the 50s, riveted joints were also used. The main types of trusses are welded. Bolted connections, as a rule, with high-strength bolts are used in assembly units.

Reinforced concrete trusses and some heavy steel trusses can be constructed without bracing with rigid joints.

The height of the trusses is h= (1/5 – 1/4)L, the height of trusses with parallel chords and trapezoidal trusses is h= (1/6 – 1/8)L. The slope of the braces is 35 0 – 45 0.

Steel trusses.

Depending on the span and the magnitude of the acting load, light trusses are conventionally distinguished with sections of elements made of simple rolled or bent sections (with forces in the rods N<500кН и пролетом до 50 метров) и тяжелые фермы с элементами составного сечения (N >500kN), capable of covering spans up to 100 meters. Light steel trusses are designed for spans of 18, 24, 30, 36 meters with a standardized panel size of 3 m, heights of 2.25 m, 2.4 m, 3.15 meters (taking into account the dimensions of cargo transported by rail).

Spatial rigidity is ensured by installing horizontal and vertical connections. Purlins and floor slabs also contribute to providing rigidity.

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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 δ compresse arise in the upper part of the section, and tensile stresses δ ras 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 an I-section, channels, T-bars and angles are obtained, which are reassembly can form the original I-beam, box or cross.

We will continue to remove “extra” material from the beam and reduce its weight without losing its load-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 “extra” material from the body of the beam, but by a simpler method of 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.

How to make a rafter truss?

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 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). 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. Truss - geometrically unchangeable rod system with hinged units. 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.

Typically, wooden trusses made from long boards. 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.

Rafter trusses are rigid structures designed to construct a roof. They transfer the load from the sheathing with the roof lying on it to the walls of the house.

Traditionally they are made of wood. Currently, to facilitate private construction, ready-made wooden roof trusses are produced.

Basic elements of a roof truss.

Rods- elements (racks, braces...) forming a lattice structure.

Nodes— points of connection of rods.

Belts- longitudinal elements of the truss located along its span.

Truss (structure)

Upper and lower belt.

Truss lattice- formed by rods.

Truss height- the distance between the centers of gravity of the section of the belts.

Panel length— the distance between adjacent belt nodes.

Operating principle of a truss truss.

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. It’s a completely different matter if you make a regular triangle from the rods. Now, no matter how much you press, the structure will only be able to fold if you break one of the rods or tear it away from the others. This design 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.

It is important to know that since any rods work better in compression-tension than in fracture, the load on the truss should be applied at the points of connection of the rods.

In fact, the truss rods are usually connected to each other not through hinges, but rigidly.

A metal truss is made of 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-beam 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

A metal truss has the qualities of rigidity and strength that are provided by its shape. The most common option is the one that contains rods, among them there are parallel directed elements that have a zigzag shape. Thanks to this arrangement, even with insignificant material consumption, the resistance of the system increases many times.

Main structural elements

A metal truss consists of racks, braces, and lattice. The nodal connection of the components is made by joining one element to another. The lattice rods are attached to the belts using welding or shaped elements. In addition to rafters, there may also be sub-rafters. They are used as a support for load-bearing floors and structures, which is true if there is a greater distance between the columns than between the beams.

Types of trusses based on lattices and belts

A metal truss can be classified according to the geometry of the chords and the type of lattice. If we talk about the outline of the belt, then it can contain elements arranged in parallel, that is, it can have a sufficient number of design advantages.

Parts are repeated with the greatest frequency, which is due to the uniform lengths of the rods for the lattice and belts, the same patterns of nodes, as well as the smallest number of joints, which makes it possible to unify the 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 improve 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 a design 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

If you decide to arrange metal trusses, drawings of which are presented in the article, then it is worth using the triangular system, which acts as the most effective option in the case of parallel belts. This is also true for the trapezoidal outline. It is possible to use this system in a grid with a triangular outline, in which the longest elements are stretched to the greatest extent. Compared to a triangular one, such a lattice is the most complex in design and also requires significant consumption of material.

Features of calculations

Installation of metal trusses is carried out only after proper calculation of the system, which takes into account the load based on the weight of the roof, drainage systems, lights, and fans. It is important to take into account the own weight of the supporting structure. Temporary loads include wind pressure, the weight of people, snow, and overhead transport. Wind load must be taken into account in the slope of the farm, starting from 30 degrees. It is important to take into account periodic loads such as hurricanes and seismic disturbances.

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 tack mounting is completed, semi-automatic or manually, you can begin 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

Metal trusses will be installed depending on the roof slope. Before starting work, it is necessary to understand the dependence of this indicator on the design of the system. Thus, the angle will be equal to the limit from 6 to 15 degrees if the truss has a trapezoidal shape.

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 should have a triangular shape, metal constructions the trusses 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, an even number of panels should be made in each half, the length of which 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 in this case Long braces of the central panels can be eliminated, reducing 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 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.

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 construction profile. By dividing the I-section, channels, tees and angles are obtained, which, when reassembled, can form the original I-beam, box or cross.

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.

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 loads from suspended ceiling transmitted 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 girth rod.

Where is it used?

As already mentioned, a truss is an indispensable element in construction. With its help, builders facilitate the construction of the structure and reduce the consumption of 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 in the 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 of supporting structures, as well as types of chords of 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 node layout, 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 teach 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 are thereby 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 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 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. For it, ordinary wooden boards are used, attached to each other at an angle, which, in turn, form a lattice.

Metal truss. Metal constructions

Design features of a metal truss

Main structural elements

Types of trusses based on lattices and belts

Grid systems

Features of calculations

Work on the manufacture and connection of elements

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 tack welding is completed semi-automatically or manually, you can begin 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.

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.

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:

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 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.

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 - 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 loads, 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]

This section of the article has not been written.

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 designs- pavilions, stage structures, awnings and podiums;

The fuselage of an aircraft, 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]

This section is not completed.

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]

This section is not completed.

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 all are found 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. In modern practice they use computer programs, most of which are based on the knot cutting method. Sometimes the calculations use the Henneberg rod replacement method.

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