In coating structures greatest distribution received two design solutions: with and without the use of longitudinal girders. In the first case, light load-bearing elements are laid along the trusses in increments of 1.5 or 3 m - purlins on which small-sized roofing slabs rest (Fig. 1); in the second, large-sized slabs or panels are placed directly on the trusses, combining the functions of purlins and slabs (Fig. 2).

Coverage by purlin

The simplest purlins are beams made from rolled channels or I-beams (with a step roof trusses 6 m). The purlins are installed on the upper chord of the truss at its nodes.

For coverings along the purlins of unheated buildings, small-sized reinforced concrete slabs with an asphalt screed (leveling layer) and a roofing felt carpet (Fig. 3, a), corrugated asbestos-cement sheets of reinforced profile, corrugated sheets made of steel or aluminum alloys(Fig. 3, b), as well as flat steel sheets 3-4 mm thick (Fig. 3, c).

Rice. 3 Roofing by purlins

For warm roofs, profiled steel decking, reinforced cement and asbestos cement boards are widely used as roofing elements laid along purlins.

Steel profiled flooring (Fig. 4, a) is made of galvanized steel with a thickness of ∂=0.8; 0.9 and 1 mm, width B=680, 711 and 782 mm, profile height h=40, 60 and 80 mm and length up to 12 m.

Profiled sheets are laid along purlins, usually located every 3 m in a split or continuous pattern. The sheets are attached to the purlins with self-tapping bolts (Fig. 4, b) with a diameter of 6 mm. The sheets are connected to each other along long side combined rivets d=5 mm (Fig. 4, c), installed every 300 mm and allowing riveting to be done on one side of the deck (Fig. 4, d).

The weight of the profiled sheet is 0.1 – 0.15 kN/m².

Rice. 4 Warm roof with profiled steel decking

a – profiled flooring; b – self-tapping bolt; c – combined rivet; g – angle roofing

Continuous purlins located on the roof slope bend in two planes. The vertical load q can be decomposed into qᵪ, acting in the plane of greater rigidity of the purlin, and the pitch component qᵧ (Fig. 5, a). Although at small slopes the pitch component is small, due to the low rigidity of the run relatively y-y axes her tensions turn out to be high. To reduce the bending moments from the pitched component, the purlins are secured with round steel ties with a diameter of 18-22 mm (Fig. 5, b), which reduce the design span of the purlin in the plane of the ramp. The tie rods are placed between all purlins, with the exception of the ridge purlin. In the panels at the ridge, the strands go obliquely and are attached to the truss or to ridge run near supports.

The components of the load on the run qᵪ and qᵧ, depending on the angle of inclination of the roof slope, are determined: qᵪ = qcosa and qᵧ = qsina

The values ​​of bending moments in the plane of less rigidity of the purlin depend on the number of strands (Fig. 5, c). With a truss pitch of 6 m, one strand is usually installed; with a truss pitch of 12 m or a steep slope, it is better to install two.

When installing one strand, the bending moment in the plane of the slope is located as a supporting moment in a two-span continuous beam (in the same section where Mᵪ is maximum). The values ​​of bending moments when installing one and two strands are given in Fig. 5, c.

Rice. 5 Calculation of runs

a – load action diagram; b – decoupling of the girder in the plane of the slope with strands; c – determination of the calculated forces in the run

The highest stresses in the run due to the combined action of bending in two planes:

The strength of the purlins is checked using the formula, taking into account plastic deformations:

If the roofing decking is attached rigidly to the purlins and forms a continuous sheet (for example, a flat steel sheet welded to purlins; steel profiled decking is attached to the purlins with self-tapping bolts, and the decking sheets are connected to each other with rivets), then the pitched component will be perceived by the roofing panel itself. In this case, there is no need for ties and the girders can be calculated only for the load qᵪ. The general stability of the purlins is not checked, since their stability is ensured by roofing slabs or decking resting on them along their entire length.

The deflection of the purlins is checked only in the plane of its greater rigidity. It should not exceed 1/200 of the span (of normal load). The purlins are attached to the truss chords using short corners, strips, and bent elements made of sheet steel. Some options for fastening the purlins are shown in Fig. 3.

With a truss pitch of 12 m, the use of continuous purlins increases the consumption of steel per 1 m² of coverage and then through purlins are used. Through purlins are calculated as trusses with an appropriate lattice system and a continuous top chord. The upper belt of the girders works in compression with bending (in one plane, if there is no slope component of the load, or in two planes), the remaining elements experience longitudinal forces.

Non-running coating

For non-running coatings, they are widely used various types large-panel unified reinforced concrete slabs with a width of 1.5 and 3 m and a length of 6 and 12 m. The height of the slabs with a span of 6 m is 300 mm, with a span of 12 m – 450 mm. The disadvantage of large-panel reinforced concrete slabs is their large dead weight (1.2 - 2.4 kN/m²), which leads to heavier load-bearing structures of the building (trusses, columns, foundation).

The desire to lighten a warm large-panel roof leads to the search for others constructive solutions panels using bent profiles, profiled flooring, aluminum, lightweight insulation.

For cold roofs, large-sized panels are used more often, since their design is quite simple.

RAFTER TRUSS DIAGRAMS

Schemes of trusses used in building coverings can be quite diverse. Depending on the design of the roof, its slope is assigned. When using corrugated asbestos-cement, steel or aluminum sheets for roofing, in order to prevent water from flowing between the seams of the sheets, its slope must be at least ⅟₇ for metal roofs and ¼ for asbestos-cement roofs. In the case of rolled or steel roofs (δ = 3-4 mm) with welded seams, the slope may be less than ⅛ - ⅟₁₂. Roofs with a slope of 1.5%, which are usually designed with roll coating and protection thin layer fine-grained gravel on bitumen mastics.

The type of truss lattice is determined by the design of the covering, as well as the presence of loads applied to the lower chord ( dropped ceilings, communications, overhead transport, etc.). Typically, the size of a truss panel is a multiple of 3 m. When choosing a truss truss design, architectural considerations are also taken into account.

Rice. 6 Schemes of roof trusses

a – gable; b – single-pitched

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STATE ELEMENTAL ESTIMATED STANDARDS FOR CONSTRUCTION WORK - COLLECTION 10 - WOODEN STRUCTURES - GESN-2001-10 (approved... Relevant in 2018

Table 10-01-082 Laying purlins along trusses

Scope of work:

01. Laying of covering elements with cutting, fitting and fastening.

02. Antiseptic treatment of the upper edges of the purlins.

Meter: 1 m3 of wood in the structure

Laying purlins along trusses:

10-01-082-1	from boards
10-01-082-2	from beams

Resource code	Name of cost elements	Unit measured	10-01-082-1	10-01-082-2
1	Labor costs of construction workers	person-hour	14,39	15,04
1.1	Average job level		3,9	4
2	Driver labor costs	person-hour	0,36	0,36
3 MACHINES AND MECHANISMS
400001	Flatbed vehicles with a carrying capacity of up to 5 tons	mach.-h	0,21	0,21
021141	Truck-mounted cranes when working on other types of construction (except main pipelines) 10 t	mach.-h	0,15	0,15
331531	Electric circular saws	mach.-h	0,13	0,14
4 MATERIALS
101-0181	Construction nails with a flat head 1.8-60 mm	T	0,0075	0,0007
101-0783	Forgings from square billets weighing 2.825 kg	T	0,002	0,001
102-0059	Lumber coniferous species. Edged boards 4-6.5 m long, 75-150 mm wide, 44 mm thick or more, grade I	m3	1,01	-
102-0061	Softwood lumber. Edged boards 4-6.5 m long, 75-150 mm wide, 44 mm thick or more, grade III	m3	0,04	0,03
101-1777	Antiseptic paste	T	0,0012	0,0015
102-0023

Rafter trusses serve to support roof structures and absorb the loads acting on them. The roof structures, together with the trusses and ties, form the roof covering. The main purpose of the roofing covering is to protect the premises from atmospheric influences(snow, rain, cold, etc.). Roof trusses are mostly supported by steel or reinforced concrete columns.

In industrial buildings, a distinction is made between warm and cold roofs.

Warm roofs consist of load-bearing slabs, insulation, asphalt screed and waterproofing carpet made of rolled material - roofing felt.

There are two types of coating- with runs and without runs. In the first type of coating, standard prefabricated reinforced concrete slabs, reinforced foam concrete and reinforced foam silicate slabs (combining the functions of a load-bearing element and insulation), etc. can be used as load-bearing slabs.

These slabs are laid on purlins that rest on trusses at the nodes, transferring the load to them. The most common slab length that defines a truss panel is 3 m.

The second type of coating (non-purlin), more common, consists of large-panel reinforced concrete slabs 6 m long, resting directly on the upper chords of the trusses.

The slabs are attached to the truss belt by welding corners (short pieces) to it, which are concreted into the slab. On large panel slabs Insulation (if they do not simultaneously perform thermal insulation functions), screed and roofing felt carpet are also laid.

Cold roofs are used in hot shops and cold buildings. They are made from corrugated asbestos-cement sheets or, in some cases, from corrugated steel along purlins. Along with this, cold roofs can be constructed like warm roofs using reinforced concrete slabs, but without insulation.

To ensure water drainage, the roof is given a slope, which mainly depends on the roofing material. Typically, roof slopes are assigned:

The roof slope is usually created by installing an inclined upper chord of roof trusses.

There are gable and single-pitch roofs.

Purlins can be made of steel (solid rolled or lattice) and reinforced concrete. Rolled purlins made from channels or I-beams are heavier than lattice ones, but are much simpler and cheaper to produce, which explains their primary use. Rolling purlins are installed on the inclined upper chord of the trusses; being located at an angle to the plane of action of the force, they are subject to oblique bending.

The calculation of the purlins is carried out for the load caused by the weight of the roof and snow.

This load is decomposed into components along the main axes of the purlin section: q x - normal to the slope, q y - along the slope (slope component); these components bend the purlin in two planes.

The maximum design moments are respectively equal to:

It is obvious that at the extreme two points of the cross section of the run, the stresses from the simultaneous action of M x and M y will be summed up. In this case, the total voltage should not exceed the design resistance multiplied by the operating conditions coefficient:

It is necessary to strive to ensure that the stress σ y is significantly less than the stress σ x, since beam profiles have a well-developed moment of inertia about the x-x axis and a relatively small moment of inertia about the y-y axis.

To reduce the stress o from the pitched component, the purlins are usually secured with ties placed in the middle of the span in the roof plane and thereby halve the value l y. The tie rods facilitate alignment of the purlin line during installation, improving the conditions for laying reinforced concrete slabs.

In oblique bending, taking into account the development of plastic deformations is allowed only for the component load acting in the plane of greatest rigidity, i.e., in the plane of the greatest moment of inertia.

Thus, the working formula for checking the voltage in a rolling run has the form

In addition, it is also necessary to check the deflection of the purlin in the plane of its greatest rigidity.

Attaching the rolled purlins to the trusses is carried out using black bolts: channel purlins - using angle shorts welded to the truss chord or, like I-beam purlins, - using strips welded to the purlins from below and bolted directly to the upper chord of the trusses.

I-beam purlins can also be attached using angle shorts.

Lattice purlins are used with round steel gratings (rod purlins). Bar purlins can be installed vertically and obliquely, i.e. perpendicular to the belt of trusses (for roof slopes up to 1:7).

In all cases, and especially when the purlins are in an inclined position, it is necessary to secure them with ties in the lateral direction (both the upper and lower chords); connections can be made of wire d = 6 mm.

The use of lattice purlins is especially advisable for spans of more than 6 m.

Roof trusses often have lanterns, that is, structures designed to illuminate rooms with overhead light and for natural ventilation(aeration). Lanterns for lighting purposes are made when the light area of ​​the windows in the walls is not enough 1 .

Lanterns can be located along the building (longitudinal lanterns) and across the building (transverse lanterns). Metal window frames or blinds are suspended from the lantern structures. The entire load from the pavement, consisting of dead load and snow, is transferred to the roof trusses either through purlins and lanterns, or directly through large-panel reinforced concrete slabs, and it is assumed that the loads act strictly in the plane of the trusses.

In reality this is not the case; the load is applied with some eccentricity caused by design necessity. This circumstance, as well as the need to prevent longitudinal bending of the upper compressed chord of the trusses, requires the installation of connections that do not lie in the plane of the truss.

Contacts are arranged by: horizontal - in the plane of the upper chord, vertical - between the trusses, placing them at the ends of the building (or temperature block); this creates rigid panels.

Intermediate trusses are connected to rigid panels by purlins or spacers. Question about the arrangement of connections in industrial buildings discussed in the section.

1 Building codes and rules (SNiP), II-B.4 and II-B.5.

"Design of steel structures"
K.K. Mukhanov

The covering of the building consists of the roof (enclosing structures), load-bearing elements(purlins, trusses) on which the roof rests, and connections along the covering. In addition, to illuminate the premises with overhead light and their natural ventilation, lanterns resting on trusses are installed in the covering system of multi-span buildings. Depending on the purpose of the premises, roofs can be warm or cold. By design, coverings with and without purlins are distinguished.

The covering with purlins consists of steel purlins, installed in the nodes of trusses, on which asbestos-cement corrugated sheets are laid for cold roofs, and galvanized steel profiled decking for warm roofs (Fig. below). The domestic industry produces decking with a height of 40,60 and 80 mm from sheets with a thickness of 0.8-1 mm. The cross-sectional size of profiled decking (Fig. below) depends on the load on the coating, the width is 680-845 mm, the length is up to 12 m. Factories can produce decking of unlimited length, but due to transportation conditions and ease of installation, the length is limited.

Profiled steel decking in a package (a) and cross-section of profiled decking (b)

The profiled flooring is laid along purlins, usually located at the nodes of the truss every 3 m. The flooring sheets are attached to the purlins with self-tapping bolts with a diameter of 6 mm, and the sheets are connected to each other along the long side with special combined rivets with a diameter of 5 mm (Fig. below).

Fasteners

Both types of connections allow you to carry out fastening work while being on one side of the deck. Steel consumption per 1 m 2 of covering is 10-16 kg (from the flooring only).

The main advantage of profiled steel decking is the low dead weight of the roof. This is achieved in combination with the use of effective lightweight insulation (phenolic foam, polyurethane foam, polystyrene foam) with a density of up to 50 kg/m3. In Fig. Below is a roofing assembly with profiled decking.

Roofing assembly using profiled steel decking

1 - profiled flooring; 2 - vapor barrier; 3 - insulation;

4 - waterproofing carpet; 5 - combined rivets;

6 - self-locking bolts

Non-roof warm roofing is usually made from large-panel reinforced concrete slabs, directly laid on the upper chords of roof trusses.

The size of such slabs is 3x6 or 3x12 m (rarely 1.5x6 or 1.5x12 m). It depends on the distance between the trusses (truss pitch). The slabs have embedded parts that are welded to the truss chords. A vapor barrier, insulation, leveling asphalt screed and a waterproofing carpet made of several layers of roofing material on bitumen mastic are laid on reinforced concrete slabs. The main disadvantage of coverings made from large-panel reinforced concrete slabs is the large load from its own weight (up to 2.5 kPa), which increases the total consumption of metal by bearing structures building.

When laying reinforced concrete slabs on the belt corners of trusses, these corners, with their small thickness, are reinforced with overlays t = 12 mm. This must be done if the thickness of the belt corners is less than 10 mm with a truss pitch of 6 m and less than 14 mm with a truss pitch of 12 m. To drain water, the roof is given a slope, which depends on the roofing material. For roll materials this slope is 1/8-1/12 subject to protection roll roofing layer of bitumen mastic with embedded gravel, the slope can be 1.5% (almost flat roof). Cold roofs from metal sheets Usually they have a cut of 1/5, and from asbestos-cement sheets 1/3.5.

Roof purlins can be solid (rolled and cold-formed) or lattice. Rolled purlins made from channels or I-beams are heavier than lattice ones, but are much simpler and cheaper to produce, which explains their predominant use. The most common purlins with a truss pitch of 6 m are made from channels that are simply attached to the truss chords (Fig. below). They work better than I-beams in oblique bending.

The disadvantage of a channel girder is that it has a narrower flange than an I-beam of the same number or the same bearing capacity. Purlins located on an inclined upper chord are subject to oblique bending due to vertical loads. They are calculated for the load from the weight of the roof and snow. This load q is decomposed into components along the main axes of the girder section: q x = qcosa - perpendicular to the slope; q y =qsina along the slope (Fig. below). Due to the low rigidity of the purlin relative to the y-y axis, even a small bending moment along the slope M y causes large stresses in it and creates the need for a significant increase in the cross section of the purlin. To reduce the unfavorable influence of slope components (loads q y), tie rods are placed between the girders in the plane of the slope (Fig. below).

With steep roof slopes or heavy loads, as well as with a truss pitch of 12 m, two strands are usually installed, and with flat slopes and a truss pitch of 6 m - one. The installation of strands transforms the girder in the plane of the slope from a single-span beam into a two- or three-span continuous beam, which significantly reduces the bending moment M y from the pitched components q y. IN vertical plane the purlin operates as a single-span beam.

Thus, the calculated bending moments in the run can be determined from following formulas(pic. below):

with one strand:

M x = q x l 2 /8; M y = q y l 2 /32;

with two strands (in a theoretically dangerous section of the purlin, coinciding with the junction of the strands):

M x = q x l 2 /9; M y = q y l 2 /90;

where l is the span of the purlin (equal to the pitch of the trusses).

The rods are made of round steel with a structural diameter of 18 - 22 mm.

The deflection of the purlins is checked only in the plane of its greatest rigidity (relative to x-x axis), it should not exceed 1/200 of the span.

To the calculation of runs

A - general form; b - load action diagram; c - design diagrams

Rolled purlins are attached to the trusses with bolts of normal accuracy using angle shorts welded to the truss belt (see figure above).