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

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. The truss support pressure Ff is transmitted from the truss support flange through planed or milled surfaces to the column support plate or the support table of the truss truss. Support flange For clear support, it protrudes 10-20 mm below the packaging of the support unit. The area of ​​the flange end is determined from the crushing condition (if there is a fit). The upper chord of the truss is structurally attached to the supra-column gusset using bolts of rough or normal accuracy. To ensure that the unit cannot absorb forces from the supporting moment and ensures the articulation of the joint, the holes in the packaging are made 5-6 mm larger than the diameter of the bolt.

Horizontal forces from the supporting moment H1>=M1/hОП are perceived by the attachment points of the upper and lower chords. The latter additionally absorbs the force from the thrust of the HP frame. In most cases, the support moment of the truss has a minus sign, and the force H1, like HP, presses the flange of the lower chord assembly to the column. The stresses along the contact surface are small and do not need to be checked. If the force H=H1+HP lifts the flange away from the column (with a positive sign of the moment), then the bolts securing the flange to the column work in tension and their strength should be checked taking into account the application of force eccentric relative to the center of the bolt field.

The seams attaching the flange to the gusset perceive the support reaction of the truss Ff and the eccentrically applied force H (the center of the seam does not coincide with the axis of the lower chord). Under the influence of these forces, fillet welds work to shear in two directions.

If the line of action of the force H1 does not pass through the center of the flange, then the seams and bolts are calculated taking into account the eccentricity.

In the case of large supporting moments and if it is necessary to increase the rigidity of the crossbar-column interface, it is advisable to weld the connection of the upper chord to the column.

The support of rafter trusses on sub-rafter trusses is carried out in most cases using a hinged system. With continuous trusses, to ensure the rigidity of the unit, it is necessary to cover the upper chords of the trusses with an overlay designed to absorb the force from the supporting moment. In the lower chord assembly, this force presses the truss flange to the stand, and no additional elements are required to perceive it

Design and calculation of the bases of eccentrically compressed columns with a through section. The column base must be designed separately, with traverses.

It is necessary to determine the dimensions of the slabs under the branches, the thickness of the slabs, the height of the traverses from the condition of attachment to the branches with welded seams, and also check the seams attaching the traverse to the slab. Designing bases under branches is similar to designing bases for centrally compressed columns. The design forces are the greatest forces in the branches in the lower section of the lower part of the column. In addition, it is necessary to check whether there is a combination of loads at which tensile forces appear in any branch of the column. When determining the design combination of forces in this case, the forces from a constant load should be taken with a factor of 0.9. If any combination of loads results in a tensile force in the branch, then it must be absorbed by the anchor bolts. The condition for the strength of the column fastening in this case is N in ≤ nф Rba Аb n x f / x in, (11.1) where nф is the number of foundation bolts of the tensile branch; Rba – their design resistance /1/; Аb n – calculated cross-sectional area of ​​the bolt /1/; xv – distance from the center of gravity of the branch to the center of gravity of the column section; xf is the distance from the line of action of the resultant forces in the foundation bolts of the branch to the center of gravity of the column section. The value xv is taken constructively. Calculation and design of the base

The longitudinal force and bending moment, which make up an unfavorable combination, are taken according to table. 5 for “embedded” sections.

where is the calculated compressive strength of the foundation material (for concrete class)

Fig.8 Column base

Section 1 ─ cantilever

where ─ load on a slab 1 m wide;

─ console crash.

Section 2 ─ support on 4 sides

where ─ coefficient, determined by adj. 4 Table 1 depending on the ratio of the short fixed side to the free edge

Section 3 ─ support on 3 sides

where ─ coefficient, determined by adj. 4 Table 1 depending on the ratio of the short fixed side to the free edge Thickness of the base plate

therefore we accept

Determination of standard and design bending moments and shear forces for crane beams.

The maximum moment occurs in a section close to the middle of the span. To determine the greatest bending moments and shear forces, we install the cranes in the most unfavorable position (Fig. 6.2.1).

The greatest bending moment from the vertical pressure of the wheels of two overhead cranes:

Where –𝛾 n =0.95 is the reliability coefficient for the intended purpose;

- 𝛾 f =1.1 - load reliability factor;

K d =1.1 – dynamics coefficient, for the operating mode of the overhead crane 7K.

The design moment taking into account the dead weight of the crane structures is equal to:

where a =1.05 is a coefficient that takes into account the influence of the own mass of crane structures on the value of the maximum bending moment.

The calculated bending moment from horizontal forces is equal to:

Fig.6.2.1. Determination of forces M max and Q max when loading a crane beam

two four-wheel cranes.

According to the instructions of the standards, the crane beam is loaded with a load from two bridge cranes as close as possible, while the loads on the hooks are nominal, and the trolleys are very close to this row of crane beams (Fig. 5).

To determine the maximum bending moments in the crane beam acting in the vertical and horizontal planes, Winkler's rule is used.

Design of an interface between crane beams and a column of industrial buildings

In the support nodes of crane beams on columns, large vertical and horizontal forces are transferred. The vertical pressure of split crane beams is transmitted to the column, usually through the protruding milled end of the supporting rib (Fig. 15.17, A). The supporting rib is calculated and designed in the same way as for conventional beams (see Chapter 7, § 5).

In continuous beams, vertical pressure is transmitted through supporting ribs attached to the lower chord, and a gasket is placed between the chord and the column support plate (Fig. 15.17.6).

In continuous crane beams, a negative (downward) reaction occurs on the support of an adjacent, unloaded span. The anchor bolts attaching the beam to the column must be designed to withstand this force.

To absorb the horizontal transverse impacts of the cranes, additional elements for fastening the beams to the columns are installed (Fig. 15.18, a). These elements rely on horizontal force Hi

If there are several fastening elements (for example, rods and linings securing brake structures to the column), horizontal pressure F T distributed between them in proportion to the hardness. In addition to the load-bearing capacity reserve, each fastening element can be counted on full pressure F?.

When designing attachment points for crane structures to columns, the features of their actual operation should be taken into account. As the crane passes, the beam bends and its supporting section rotates through an angle φ (Fig. 15.18.6). Under the influence of temperature influences (especially in hot shops), crane structures are lengthened (shortened), which leads to horizontal displacements of the supporting sections relative to the columns. As a result, the fastening elements receive horizontal movements A n.

Due to the compression of the supporting section of the beams and the compression of the spacers under the supporting ribs, the fastening elements also receive a vertical displacement Av(see Fig. 15.18,6). If the fastening structures are sufficiently rigid and prevent compression and rotation of the supporting sections, then large forces arise in the fastening elements caused by movements An And Av, which, with repeated repeated loading, leads to fatigue failure of the fastening elements. This is confirmed by the results of field surveys.

Therefore, the design of fastening beams to columns in the horizontal direction must ensure the transfer of horizontal transverse forces, while allowing freedom of rotation and longitudinal displacement of the supporting sections.

In order to ensure freedom of longitudinal and vertical movements of fastening elements, two types of nodes are used. In nodes of the 1st type, transverse horizontal impacts are transmitted through elements (thrust strips) tightly fitted to the column flanges, which allow freedom of movement of the supporting sections due to slipping (Fig. 15.19, a). Since over time the contact surfaces become dented and play forms in the connection, it is advisable to fasten the thrust elements (to be able to replace them) with high-strength bolts. In type 2 nodes, beams are attached to columns using flexible elements. With low rigidity of these elements, additional forces arising in them from movements An And Av, small. Sheet elements or round rods are used as flexible fastenings. In the node shown in Fig. 15.19.6, horizontal transverse forces are perceived by flexible round rods. For large horizontal loads, each beam can be secured with two or three bolts located one above the other. The advantage of such fastening is the possibility of straightening the beams and the ease of its replacement.

In buildings with special operating mode cranes, when calculating fastening elements, it is recommended to take into account additional forces arising from movements A n

The bending moment in the fastening element arising from movements is determined as in a beam with clamped ends (see Fig. 15.18.0):

From the skew of the supporting rib of the beam, additional horizontal force is also transmitted to the fastening Not(see Fig. 15.18, d), arising due to the displacement of the resultant reference pressure F R from the beam axis:

According to experimental studies, the value of e can be taken equal to 1/b of the width of the support rib b .

In buildings with large temperature differences (unheated buildings, hot shops), when calculating fastening elements, one should also take into account the forces arising from temperature influences, or design fastenings that provide freedom of movement (for example, with the transmission of forces through thrust elements).

Checking the local stability of the crane beam wall

The wall of the crane beam experiences local compressive stresses as a result of the movement of overhead crane wheels along the crane rails. The wall of the crane beam is also reinforced with paired transverse stiffeners, the maximum distance between which should not usually exceed a = 1; 1.5; 2 m (Fig. 12). dangerous section = 4*a-0.5hw

We check the local stability of the beam wall of the middle compartment, see:

Normal voltage in the “dangerous” section of the compartment

The calculated bending moment in the span section is equal to

The calculated shear force in the support compartment is equal to

Averaged tangential stresses in the “dangerous” section of the compartment

Rice. 6.5.1. To the calculation of the stability of the wall compartments of the crane beam

Local compressive stresses:

Where g f 1=1.1 – coefficient of increase in vertical concentrated force on an individual wheel of an overhead crane;

– design load on the crane wheel without taking into account dynamism;

cm – conventional length of propagation of local compressive stresses;

c– coefficient accepted for welded beams equal to 3.25;

I р, f =I р +I f– the sum of the crane rail’s own moments of inertia I r= 1083.3 cm of the upper chord of the crane beam If.

Critical normal voltage:

AND for. 77

kgf/cm 2,

where is the coefficient determined from the table. 25.

Determine the conditional flexibility of the beam wall

in accordance with clause 7.10, the beam wall must be strengthened with transverse stiffeners. The distance between the main transverse ribs should not exceed cm. We take the distance between the transverse stiffeners a=1.5 m.

The maximum distance between transverse stiffeners (in axes) is set depending on the conditional flexibility.

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.

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.

The truss support pressure F f is transmitted from the truss support flange through planed or milled surfaces to the column support plate or the support table of the rafter truss. For clear support, the support flange protrudes 10-20 mm below the packaging of the support unit. The area of ​​the flange end is determined from the crushing condition (if there is a fit).

The upper chord of the truss is structurally attached to the supracolumn gusset using bolts of rough or normal precision. To ensure that the unit cannot absorb forces from the supporting moment and ensures the articulation of the joint, the holes in the packaging are made 5-6 mm larger than the diameter of the bolt.

Supporting unit for the truss on the column from above (hinge connection)

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.

Horizontal forces from the supporting moment H1>=M 1 /h OP are perceived by the attachment points of the upper and lower chords. The latter additionally absorbs the force from the thrust of the frame H P. In most cases, the support moment of the truss has a minus sign, and the force H 1, like H P, presses the flange of the lower chord assembly to the column. The stresses along the contact surface are small and do not need to be checked. If the force H=H 1 +H P lifts the flange away from the column (with a positive sign of the moment), then the bolts securing the flange to the column work in tension and their strength should be checked taking into account the application of force eccentric relative to the center of the bolt field.

The seams attaching the flange to the gusset perceive the support reaction of the truss F f and the eccentrically applied force H (the center of the seam does not coincide with the axis of the lower chord). Under the influence of these forces, fillet welds work to shear in two directions.

If the line of action of the force H1 does not pass through the center of the flange, then the seams and bolts are calculated taking into account the eccentricity.

In the case of large supporting moments and if it is necessary to increase the rigidity of the crossbar-column interface, it is advisable to weld the connection of the upper chord to the column.

When rigidly connecting trusses with columns (supported from the side), for ease of installation it is advisable to use sub-trusses with a downward support brace (with another solution it is difficult to place the truss between the flanges of the column). The support pressure of the truss truss is transmitted through a planed end from a table welded to the wall of the column. The flange of the support unit is attached to the wall of the column with bolts of normal accuracy. The lower chord of the rafter truss is made shortened (so that it does not need to be brought inside the column) and is attached with an overlay to the edges of the column.

The support of rafter trusses on sub-rafter trusses is carried out in most cases using a hinged system. With continuous trusses, to ensure the rigidity of the unit, it is necessary to cover the upper chords of the trusses with an overlay designed to absorb the force from the supporting moment. In the lower chord assembly, this force presses the truss flange to the stand, and no additional elements are required to perceive it.

The design of trusses begins with drawing axial lines that form the geometric diagram of the truss.

Then the contours of the rods are drawn so that the axial lines coincide with the centers of gravity of the sections. For asymmetrical sections (Ts, corners), axle references are rounded to 5 mm.

When the section of the chord along the length of the truss changes, one center line of the chords is taken in the geometric diagram and the chord elements are tied to it. For the convenience of supporting adjacent elements (for floor trusses - flooring or purlins), the upper edge of the chord is kept at the same level. The places where the cross-section of the belts changes is moved away from the center of the unit in the direction of less force. The grating rods are cut normal to the axis of the rod; For large rods, bevel cutting can be allowed to reduce the size of the gussets. To reduce welding stresses in the gussets, the grid rods are not brought to the belts at a distance equal to ≥ six times the thickness of the gussets, but not more than 80 mm. A gap of at least 50 mm is left between the ends of the joined elements of the truss chords, laid with overlays.

The thickness of the gussets is selected depending on the current forces (Table 7.2). If there is a significant difference in the forces in the grid rods, two thicknesses can be adopted within the sending element. The permissible difference in the thickness of the gussets in adjacent units is 2 mm.

The dimensions of the gussets are determined by the required length of the seams for fastening the elements. It is necessary to strive for the simplest outlines of the gussets in order to simplify their production and reduce the number of trimmings.

It is advisable to unify the sizes of the gussets and have no more than one or two standard sizes per truss. Trusses with a span of 18 - 36 m are divided into two sending elements with enlarged joints in the middle nodes. For ease of assembly and manufacturing, it is advisable to design so that the right and left half-trusses are interchangeable.

Farms from paired corners

In trusses with rods made of two corners, assembled by a brand, the nodes are designed on gussets that are inserted between the corners. The lattice rods are attached to the gusset with flank seams (Fig. a).

The force in the element is distributed between the seams along the butt and leg of the angle in inverse proportion to their distances to the axis of the rod:

where b - corner shelf width;

z 0 - the distance from the center of gravity of the corner to its butt.

a – fastening the brace to the gusset; b – intermediate node;

c, d – support of purlins and slabs

Figure - Truss nodes from paired corners

For rolled angles in practical calculations, the values ​​of the coefficients a 1 and a 2 can be taken from the table.

To reduce stress concentration, the ends of the flank welds are brought out to the ends of the rod by 20 mm (Fig. a). It is recommended to attach gussets to the waistband using continuous seams of minimal thickness. The gussets extend beyond the edges of the waist corners by 10...15 mm (Fig.b). The seams attaching the gusset to the belt, in the absence of nodal loads, are calculated on the difference in forces in adjacent panels of the belt (Fig.b) N = N 2 – N 1. In the place where purlins or roofing slabs rest on the upper chord (Fig. c), the gussets are not brought up to the butts of the waist corners by 10...15 mm.

Table - Distribution of forces between seams along the butt and feather

To attach the purlins, a corner with holes for bolts is welded to the upper chord of the truss. In places where large-panel slabs are supported, if the thickness of the chord corners is less than 10 mm at a truss pitch of 6 m and less than 14 mm at a truss pitch of 12 m, the upper chord of the trusses is reinforced with overlays t = 12 mm to prevent bending of the shelves. To avoid weakening the section of the upper chord, do not weld the linings with transverse seams.

If a concentrated load is applied to the unit (Fig. c), then the seams attaching the gusset to the belt are designed for the combined action of longitudinal force (from the difference in forces in the belts) and concentrated load. Conventionally, force F is transmitted to the seam sections l 1 and l 2. Stress in the seams from this effort

; (1)

from longitudinal force

,

where S l w is the total length of the seams for attaching the belt to the gusset.

The strength of the seam is checked for the combined action of forces according to the formula

When calculating nodes, k f is usually specified and the required seam length is determined.

Truss gussets with a triangular lattice should be designed in a rectangular shape, and with a diagonal lattice - in the form of a rectangular trapezoid.

To ensure smooth transfer of force and reduce stress concentration, the angle between the edge of the gusset and the grid element must be at least 15°. The joints of the belts must be covered with overlays made from corners (Fig.a) (with the same thickness of the belts) or sheets (Fig.b). To ensure that the corners work together, they are connected with gaskets. The distance between the gaskets should be no more than 40 i for compressed elements and 80 i for stretched ones, where i is the radius of inertia of one corner relative to the axis parallel to the gasket. In this case, at least two gaskets are placed in the compressed elements.

o - with corner overlays, b - with sheet overlays

Rice. - Truss nodes with a change in the section of the belt:

If the corners are not connected by spacers, then during the calculation each corner is considered separately, and its flexibility is determined based on the minimum radius of inertia i min for one corner.

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

When the trusses are freely supported on the underlying structure, the support unit is shown in Fig. The pressure of the truss F R is transmitted through the plate to the support. Area Apl is determined by the load-bearing capacity of the support material:

where R op is the calculated compressive resistance of the support material.

The slab bends due to the resistance of the support material in the same way as the column base slab.

The base plate is attached to the support with anchor bolts. The support unit is constructed similarly when supporting the truss at the level of the upper chord (Fig. b).

In case of hinge coupling, the simplest one is to support the truss on the column from above using an additional stand (patella) (see figure).

The truss support pressure is transferred from the truss support flange through the milled surfaces to the column support plate. For clear support, the support flange protrudes 10...20 mm below the gusset of the support assembly. The area of ​​the flange end is determined from the crushing condition: А³F R / R p ,

where R p - design resistance of steel to end surface crushing (if there is a fit).

Figure - Freely supported truss

Rice. – Supporting the truss on the column from above

The upper chord of the truss is structurally attached to the gusset of the supracolumn with bolts of rough or normal accuracy (accuracy class C or B). To ensure that the assembly cannot absorb forces from the supporting moment and ensures articulation of the interface, the holes in the gussets are made 5...6 mm larger than the diameter of the bolts.

To design a rigid truss-column interface, it is necessary to attach the truss to the column from the side (Fig.). With a rigid coupling, in addition to the support pressure F R, a moment M arises in the node. These forces are transmitted separately.

The support pressure F R is transmitted to the support table. The support table is made from a sheet t=30...40 mm or, with a small support pressure (F R ≤200...250 kN) from corners with a cut flange. The support flange is attached to the column flange with bolts of rough or normal precision, which are placed in holes 3...4 mm larger than the diameter of the bolts, so that they cannot absorb the support reaction of the truss in the event of loose support of the flange on the support table.

Rice. - Connection of the truss to the column from the side

The moment is decomposed into a pair of forces N = M / h op, which are transmitted to the upper and lower chords of the truss. In most cases, the support moment has a minus sign, i.e. directed counterclockwise. In this case, the force N presses the flange of the lower chord assembly against the column. The voltages on the contact surface are small and do not need to be checked. The bolts are installed structurally (usually 8 bolts with a diameter of 20...24 mm). If a positive moment occurs in the support unit, then the force pulls the flange away from the column and the bolts should be checked for tension.