With rigid coupling, the truss is attached to the column at nodes 1 and 7 (Fig. 25). The design forces M, N, Q are taken from the table of combinations in the frame section under consideration. The bending moment M is replaced by a pair of forces H = M/h 0 . The calculated forces at the interface between the truss and the column are presented in Fig. 29.

The vertical support reaction N is transmitted to the column support table through the truss support flange in node 1 (Fig. 30).

Fig. 29 Calculated forces in the support node Fig. 30 Rigid coupling

trusses with a rigid connection to the column of the truss with the column in node 1 (Fig. 25)

The width of the support flange b fl is taken structurally according to the size of the column flange. Flange length l fl is determined by the size of the gusset, which is limited by the lengths of the weld seams for attaching the support brace and the lower chord.

The calculation of these seams along the back and leg of the corner is discussed in section 3. In accordance with the design rules, the gusset must describe these seams. For ease of installation, it is necessary to provide a gap of 150 mm between the lower belt and the support table. The thickness of the support flange tfl is determined from the condition of collapse from the influence of the vertical support reaction N:

t fl ≥ (R p – see Appendix 1)

Two vertical seams attaching the support flange to the gusset perceive the support reaction N and the horizontal forces (H+Q) applied by the eccentricity e. The strength of these seams is ensured if

where τ WN = ; τ W Н Q = ; τ WM = ;

l W = l fl – 1 cm – estimated length of the vertical weld;

e – eccentricity of application of forces Q and N (distance from l W to the axis of the lower chord of the truss).

Vertical welds securing the support table to the column l st are calculated for the influence of N. Due to possible discrepancies in the contact surface of the flange and the table, a coefficient of 1.2 is introduced.

l st ≥

The legs of the welds are taken according to the thickness of the joined elements.

The upper support unit 7 (Fig. 31) perceives the force N. seams of fastening the belt to the gusset along the butt and feather of the waist corner:

;

From the gusset, the force N is transferred to the flange through two vertical seams. If the design rules are followed, the strength of these seams will be ensured. The flange is attached to the column with 4 bolts, which must be selected based on tensile strength. With the calculated tensile strength of the bolts R bt (Appendix 3), the required cross-sectional area of ​​each of the 4 bolts will be A b required ≥ N / 4R bt. The diameter of a standard bolt is selected according to the required area.

Fig. 31 Rigid coupling of a truss with a column at node 7

The flange bends like a beam, clamped with bolts and loaded with a concentrated force N. design bending moment:

Where b is the distance between the bolts in plan (Fig. 31).

Flange bending moment

Where a is the height of the flange,

t fl – its thickness.

From the condition of bending strength σ = M /W fl ≤ R y γ C the required flange thickness is determined

t fl ≥ .

The thickness of the flange is taken according to the assortment and must be at least 20 mm based on rigidity.

With a hinged connection, the truss rests on the column from above in node 1 (Fig. 25). The hinged support diagram is shown in Fig. 32.

Fig. 32 Scheme of hinged Fig. 33 Truss support unit

supporting a truss on a column

The calculation of welds for attaching corners to the gusset using forces S 1 , S 2 and S 3 is similar to the above. A feature of the calculation of this unit (Fig. 33) is the transfer of the vertical support reaction V from the truss to the column. This support reaction is transmitted via a flange. The flange thickness is determined from the collapse condition:

(R p – see Appendix 9)

The width of the flange b fl is taken structurally according to the cross-sectional dimensions of the corners of the truss and the head of the column. From the end of the flange, V is transferred to 2 vertical welds attaching the flange to the gusset. The estimated length of each seam is:

Other design solutions for hinged units supporting the truss on columns are possible.

BIBLIOGRAPHY

1. SNiP 2.01.07-85 Loads and impacts / Gosstroy USSR - M.: CITP Gosstroy USSR, - 36 p.

2. SNiP P-23-81*. Steel structures / Gosstroy USSR. - M.: CITP Gosstroy USSR, 1990, - 96 p.

3. Metal structures. General course: Textbook for universities. - 6th ed. / Under general ed. E.I. Belenya. - M.: Stroyizdat, 1985. - 550 p.

Applications

Appendix 1. Table 1*

A diagram of the assembly is drawn onto the paper: the axes of the elements converging in it, then the contours of the elements, starting from the belt (Fig. below). The lines of the centers of gravity of the elements are combined with the axial lines of the diagram.

When centering to draw the contours of the corners (in trusses with rods made of paired corners), the corner backing is set aside from the center lines, the distance Z 0 rounded to 5 mm from the center of gravity to the backing, determined from the assortment. In the opposite direction from the axis, the distance (b - Z 0) is laid off. The same applies to sections of other shapes. After drawing the outline of the elements, they show the cut of the corners of the lattice so that in the welded joints between the edges of the belt and the elements of the lattice there is a gap of 40-50 mm to reduce the harmful effect of shrinkage of the seams in the gussets (Fig. below).

Centering light truss units

It is advisable to maintain the same distance between the edges of adjacent lattice elements in nodes and between the edges (ends) of adjacent seams securing the linings at the joints of the belt. The corner is usually cut perpendicular to the axis. It is permissible to cut off part of the corner flange, but not further than the beginning of the rounding, which allows you to slightly reduce the size of the gusset.

It is recommended to weld the braces only with flank seams along the butt and feather, constructively bringing them to the end of the rod to a length of 20 mm. You should strive for the simplest outline of the gusset (rectangle, rectangular trapezoid, parallelogram, etc.). Attaching the gusset to the belt, if the joint of the belt is not arranged in the node, must be designed for the resultant forces N of all lattice elements adjacent directly to the nodal gusset. With a straight belt, this resultant is equal to the difference in forces in the adjacent panels of the belt (N = N 2 -N 1 figure above). If a concentrated load F is applied to the belt corners in a node (which is present in the upper nodes of rafter trusses), then the seams attaching the gusset to the belt are calculated for the equal force from the concentrated load and the difference in forces in adjacent panels. With a load F perpendicular to the belt, the resultant

N = √N 2 -N 1 2 +F 2

Welds are applied on both sides - on the side of the butt and the feather - along the entire length of the junction of the gusset with the belt. For this purpose, the edge of the gusset is moved outward by 10-15 mm (Fig. above). However, it is not always structurally convenient to extend the entire gusset beyond the edge of the chord, for example, when installing purlins attached to corner shorts along the upper chord (see figure above), or overlays on which reinforced concrete slabs rest (figure below). In this case, part of the gusset is not brought to the edge of the corners by 10-15 mm. Thus, the main working design seams in this case will be the seams placed at the feather. The usual design of intermediate welded joints (without a belt joint) of light trusses with rods from paired angles is shown in Fig. above (upper belt) and fig. above (lower belt).

When changing the sections of the belts, it is necessary to join the belt corners. As a rule, the joint is located in a node, and part of the gusset can be used as a joint element.

In the case of using corners with different flange thicknesses in the truss belt, the factory joint of the belts is made using sheet overlays and gussets (Fig. below).

Joining of belts using sheet overlays

It is believed that 70% of the force at the joint is transmitted through the linings, the remaining 30% is transmitted through the gusset, and a part of the gusset with a width of no more than 2b is included in the work (where b is the width of the flange of the smaller corner). To include the gusset in the joint work, it is continued by the knot. Usually the joint is moved towards the panel with less force by 500 mm.

In trusses with belts made of T-beams obtained by longitudinal dissolution of wide-flange I-beams, and lattice rods from paired angles, it is necessary to have nodal widening in order to obtain the required length of welds. To do this, a gusset is attached to the wall of the tee using a butt seam (Fig. below).

Truss knots with belts made of T-bars and a lattice of paired corners

The butt weld is calculated for shear from the sum of the calculated forces in the adjacent braces, designed on the axis of the belt. The joints, as in the corner truss, are moved towards the panel with less force by 500 mm. They are performed with the introduction of vertical sheet inserts and horizontal overlays (Fig. above).

Rafter trusses can be supported by reinforced concrete columns, brick walls or elements of the steel frame of industrial buildings - steel columns. An example of the design of a truss support unit when resting it on a reinforced concrete column from above is shown in Fig. below. The rigid connection of the truss with the steel column of the building frame is shown in Fig. below.

Supporting a truss on a reinforced concrete column

a - trapezoidal; 6 - triangular

Rigid connection between a truss and a steel column

a - plan the end of the supporting rib; N - expansion

According to transport conditions, trusses of large spans (more than 18 m) are divided into separate sending elements, assigning enlarged (assembly) joints in the middle of the span. As a rule, enlarged joints are made using horizontal and vertical sheet overlays. Horizontal pads overlap the waist corners and the flange of the tee, transmitting 70% of the force at the joint, and vertical pads join the gussets and walls of the tee, transmitting 30% of the force at the joint. Ribs are welded to the vertical overlays in the trusses from the corners to attach the ties. Similar ribs in trusses with belts made of T-bars are attached to the posts. At the junction of the upper chord of the trapezoidal truss, the horizontal plate has an inflection. Examples of the implementation of light truss units with enlarged joints are presented in Fig. below.

Enlargement units for light truss belts

a - diagram of the farm; b—the upper of the brands; in—the lower of the paired corners

In rods whose cross-section is made up of two corners or any other profiles, it is necessary to install connecting spacers that ensure the joint operation of the profiles as a single section.

All joints are designed for a force that is 20% more than the actual one. This is explained by some vagueness in the operation of knots with joints. Vertical seams should be designed for the combined action of vertical support pressure and bending moment caused by the eccentric application of longitudinal force relative to the center of gravity of the seams.

In hydraulic valves, elements of braced trusses are often taken from welded brands. This leads to some peculiarities in the design of nodes.

In such assemblies, to attach the rods to the gussets, simultaneously butt and fillet flank welds or only butt welds are used. An example of the implementation of a flat shutter assembly is shown in Fig. below.

Flat hydraulic valve assembly

1,2 - longitudinal and transverse connections

In the case of attaching rods with two types of welds, the wall of the welded tee is attached using a butt weld, and the flange is attached with four flank seams, for which a slot is first made in the flange for the length of the seam and a width 1 mm greater than the thickness of the gusset.

Metal trusses are often used for the construction of utility, industrial and commercial buildings. Metal rafter systems have a number of advantages; on the other hand, when constructing private houses they are too expensive for the owner. The need for them arises only when you need to make a super-strong roof or build a complex structure. But even then, homeowners prefer combined rafter systems - some of the elements in them are made of wood, the rest are made of metal.

Material for constructing a metal truss structure

Typically, all elements are made of profiled metal - angles, I-beams, channels. They can have a wide variety of shapes: hardware can be rectangular, trapezoidal, triangular or more complex geometry.

When constructing production workshops, metal rafter systems are often mounted on sub-rafter rectangular trusses, also made of metal (channels or thick-walled square pipes). Reinforced concrete pads or individual reinforced concrete or metal columns also serve as supports.

1) Lower chord of the truss;2) Upper chord of the truss;3) Brace;4) Nodal gusset;5) Sheet overlay;6) Roof bearing Z - profile (thickness 1.5; 2mm);7) M12 bolt; M16 (according to calculation);

The individual components and elements of the rafters are connected using steel gussets, which are secured by welding or bolting.

Corners are used to make rafters directly. Corners with sides of the same size go to the lower chord of the truss, and the upper part of the structure is made from versatile corners. The corners are welded in such a way as to form a brand.

To connect the elements of the system, structures are made from T-shaped or cross-shaped corners. Fastenings for rafters are made of sheet steel, angles or iron strips.

When constructing private houses and small outbuildings, bent ones are used as the main material for the rafter system. Such systems turn out to be much lighter, and at the same time have sufficient strength.

Metal truss installation technology

The main advantages of steel trusses compared to steel trusses are durability, special strength, industrial style and ease of installation.

Metal rafters can be up to 50 meters in length, have a relatively low weight, and are not subject to deformation due to sudden temperature changes. Their installation is carried out in strict accordance with the detail drawings, which contain wiring diagrams indicating the brands of individual structural elements. Therefore, at the site, all structural parts are marked. In addition, usually all elements are equipped with mounting holes.

During assembly, these holes make it possible to prepare joints for welding without the use of clamps, wedges or clamps - the parts to be connected are fixed with conical and through mandrels. If there are no such holes, the easiest way to pre-fix the elements to be joined is to tack (short seams tightened using clamps).

Most metal truss elements are welded or bolted. Bolted connections are the simplest; black bolts are used to secure purlins, trusses, braces, and half-timbered structures. The reliability of such a connection depends on the degree of tension of the bolts. This work is usually carried out by two installers, tightening the nuts with special wrenches with long handles or pneumatic ones.

Welded connections are mainly used when it is necessary to obtain the most rigid connection possible.. Columns and trusses, crane beams and columns, as well as column joints are welded together. Before welding, the individual structural elements are connected using rough mounting bolts. Then, to obtain the required rigidity, they are welded together. Especially critical connections are made using rivets.

During installation, temporary connections are first made; only after final alignment and assembly of structures are all installation elements finally secured.

Installation of metal trusses is carried out using jib cranes. To prevent the trusses from swaying, use paired manual guys. They also help guide the truss during installation. Before removing the slings from the truss, it must be secured at least

half of the bolts specified in the project.

If it is mounted on reinforced concrete columns or brick walls, it is secured with anchor bolts. The installation of trusses begins in that part of the frame where the installation of connections is provided. The first two trusses are secured, without removing the braces, with all the design connections and purlins. Only after all bolted connections are securely tightened and all joints are welded can the trusses be braced.

If the installation is carried out by a crane with a large lifting capacity, it is better to install the trusses with enlarged blocks.

Column joints

Typically, column joints are made above the crane beams, in the above-crane part of the structure. Long columns (over 18 m) are transported in fragments. Then they are assembled and welded, sometimes welding is done using special metal plates, which are bolted and welded to the parts being connected. The ends of the main and crane parts of the column are carefully joined, fixed and welded together. Both fragments are connected with a scarf for reinforcement.

Connection of columns with crane beams

When installing on a column (on its base plate), support the vertical edge of the crane beam and tighten the connection with bolts. Then they make additional fastening of the beam with brake structures to the above-crane part of the column, tighten the bolts and make an extended weld.

Connection of columns with trusses

When a rigid connection between the column head and the truss truss is required, an overlay is mounted at the junction point, which is connected to the truss belt and the base plate with columns. A bolted connection is used, then the entire structure is welded. The lower chord (base) of the truss is supported with a gusset on the mounting table and finally attached to the column using bolts and welding. In the case of hinged support, the upper chord of the truss is attached to the column, rigidly connecting the gusset and the plates welded to the column.

Installation of columns

Before the column begins to be installed, mounting axial marks are applied to its shoe (support sheet). A temporary ladder is attached to the column; scaffolding (at the junction of trusses and crane beams). After this, the sling is secured and the lifting begins.

The column, at the installation site, is placed on anchors and supported on strictly horizontal support beams or pads. Then they combine the marks on the support sheets with the marks on the embedded parts of the foundation, level the column and temporarily secure it.

Columns no more than 12 meters high are fixed using bolted connections, and taller columns (or columns with narrow shoes) are additionally fixed with braces, which are not removed until final installation. It happens that in order to securely fasten a column, it is necessary to fill the shoe with concrete mortar - this should be done only after the column has been finally aligned and secured.

If the design does not provide for connections between the first and second mounted columns, they should still be secured with temporary connections. Temporary connections can be removed only after all other columns have been finally installed.

Installation of crane beam structures

Crane beams are mounted on a console or crane branch of a metal beam and connected by welding or bolts. Before transportation to the installation site, special devices for preliminary fastening are installed on it. Guys are attached to the ends of the beam, which allow you to adjust its position and direct the crane beam to strictly defined places on the column consoles. Crane beams are installed in the design position, focusing on the axial marks that are marked on it and on the column consoles.

They are finally installed and secured after checking their position with geodetic instruments. Crane beams are welded to embedded parts that are mounted on columns.

Safety precautions

Installation of metal trusses can only be carried out by qualified installers and slingers who have permission to work at height. Each of them must undergo safety training before starting work. When installing, you should wear helmets and gloves, follow the rules for working with lifting mechanisms, and when working at heights, use a mounting belt.

Metal roof trusses allow the construction of large-area roofs with high quality and in a short time. Today there is no alternative to them in industrial construction.

Due to the limited length of rolled products, as well as due to transport conditions, trusses of large spans (l > 18 m) have to be divided into separate sending elements, assigning assembly joints, as a rule, in the middle of the span.

When designing joints, it is necessary to follow the basic joining rule: The cross-sectional area of ​​the butting elements must be no less than the cross-sectional area of ​​the joined elements. The joints of truss chords can be located both in nodes and in panels. The location of the belt joint in the knot is more convenient, since in this case part of the gusset is used as a joint element.

The simplest joint design is to overlap the waist angles with butt angles of the same profile. Figure a shows a welded joint, and figure b shows a riveted joint of the lower chord of the truss. In a welded joint, the flanges of the butt angle are trimmed in order to avoid concentration of seams at the feathers, as well as for a more uniform transfer of force.

The joint of the upper chord, usually located at the ridge of the truss, can be carried out similarly to the joint of the lower chord, covering it with bent butt corners. Figure a shows such a joint, with the gusset extended upward to attach the lantern structure. This joint, which essentially repeats the idea of ​​riveted joints, also received another solution, shown in figure b.

Here the T-section of the gusset completely compensates for the cross-section of the two corners. It is advisable only to designate the size h in such a way that the center of gravity of the T-shaped gusset coincides with the axis of the waist corners; in case of mismatch, it is necessary to check the gusset not only for compression, but also for bending from a moment equal to the axial force in the belt multiplied by the eccentricity of the force relative to the center of gravity of the gusset.

For the convenience of applying seams at the edges of the corners of the belt, the width of the horizontal bar should not exceed 2h. The design of the joint according to figure b is convenient for installation due to the presence of a horizontal table on which the lantern structure is installed.

Support nodes

Rafter trusses can rest on brick walls, reinforced concrete columns or elements of the steel frame of an industrial building - steel columns or trusses. The design of attaching trusses to steel columns and sub-rafter trusses is discussed in detail in Chapter. IX.

Supporting trusses on reinforced concrete columns.

An example of supporting a truss on a reinforced concrete column is shown in the figure. The base plate, usually 16 - 20 mm thick, is attached to the column with anchor bolts with a diameter of 22 - 24 mm; The dimensions of the slab are determined based on the calculated compressive resistance of the support material. The holes in the base plate are made 2 - 3 times larger than the diameter of the anchor bolts, taking into account possible inaccuracies in laying the latter.

For trusses with a span of up to 36 m, the requirement for mobility of support fastenings is usually not imposed.

Details

As already indicated, the compressed elements of the trusses, consisting of two corners, must be connected to each other with small connecting strips in the spaces between the gussets.

Otherwise, under the influence of the longitudinal compressive force N, each corner receiving force N/2 can bend independently of one another, since a single corner has a minimum radius of gyration relative to the axis ξ significantly less than the radius of gyration

General design requirements. The design of trusses begins with drawing axial lines that form the geometric design of the structure, in accordance with the configuration of the truss and its main dimensions. The centerlines of elements converging at nodes must intersect at the center of the node.

The contours of the rods are applied to the axial lines, which are tied to the axes at the centers of gravity of the section, while in welded trusses the distance from the center of gravity to the butt (binding) is rounded up to a whole number multiple of 5 mm. In trusses with bolted connections, the corners are tied to the axles along the marks closest to the butt.

When the section of the chord along the length of the truss changes, one center line is adopted in the geometric scheme, while the upper edge of the chord is kept at the same level for ease of support of adjacent elements. The displacement of the truss chord axes when changing the section may be ignored if it does not exceed 1.5% of the smaller height of the chord section.

The gussets used to form the truss nodes are of a simple shape in order to simplify their production and reduce the number of trimmings.

The gussets are produced beyond the edges of the waist corners by 15 - 20 mm for the possibility of applying welds. In places where purlins are installed, attached to corner shorts, and in places where the belt is reinforced with overlays, when supporting reinforced concrete slabs on the upper chord, the gusset is not brought (recessed) to the edge of the corners by 10 - 15 mm.

The procedure for designing and calculating truss nodes is as follows:

1) draw the center lines of the elements so that they converge in the center of the node;

2) “tie” waist corners to the center lines. To do this, determine the size Z o from the center of gravity of the corner to the butt according to the assortment and round it according to the rounding rule to 5 mm, thereby obtaining the distance from the corner butt to the center line. Apply the contour lines of the lattice rods in the same way. The distance between the edges of the lattice elements and the belt in nodes (a) should be taken equal to 6t - 20 mm, but not more than 80 mm (here t = gusset thickness, mm);

The design of the truss support units depends on the method of coupling the truss with the column.

With a hinged connection, the simplest is to support the truss on the column from above using an additional post (supracolumn). With this solution, it is possible to support the trusses on both a metal and reinforced concrete column. The knot of support of the truss on the truss is solved in a similar way.

With a rigid connection, the truss is usually adjacent to the column on the side.

The support pressure F f is transmitted to the support table. The support table is made from a sheet t = 30...40 mm with a small support pressure (F f< ф. Опорный фланец крепят к полке колонны на болтах грубой или нормальной точности, которые ставят в отверстия на 3-4 мм больше диаметра болтов, чтобы они не могли воспринять опорную реакцию фермы в случае неплотного опирания фланца на опорный столик.

.


^ Fig. 38. Truss support units: 1 – stiffener; 2 – support stand; 3 – rib with oval holes; 4 – rib for attaching a vertical connection; 5 – washer
The welds attaching the support rib to the assembly gusset are designed to transmit the support reaction of the truss:
, (30)
Where l w is the estimated length of the weld, equal to the height of the gusset minus 1 cm.
To fix the position of the unit on the column, the support rib is connected with normal precision bolts to the support post, which in turn is bolted and then welded to the head of the column. The gusset of the upper truss assembly is also connected to the support post through a special rib with oval holes. Oval holes allow the upper unit of the truss to move relative to the support post and thereby ensure free support of the truss, i.e., without the appearance of a support moment. Vertical braces are also attached to the ribs of the support post.