Laying beam purlins along trusses. Calculation and installation of wooden roof trusses. Features of arranging rafters on different roofs
1. Cutting roofing felt into strips and laying strips under the mauerlats and linings. 2. Laying mauerlats. 3. Laying corner purlins. 4. Laying support elements. 5. Manufacturing and laying of linings. 6. Installation of support trusses. 7. Installation of lower rafter panels. 8. Strengthening support trusses, lower rafter panels and rafters with wire twists and hammering ruffs into the masonry. 9. Installation of upper trusses. 10. Installation of lathing panels. 11. Installation of fillies. 12. Lathing eaves overhangs in the corners. 13. Removing diagonal joints. 14. Installation of couplers. 15. Manufacturing and installation of struts. 16. Manufacturing and installation of racks for diagonal rafter legs. 17. Making and staging sprigs. 18. Sheathing the rafters with bars. 19. Installation of the ridge board. 20. Device dormer windows with cutting openings in the sheathing. 21. Installation of cuts near pipes.
Table 3
Time standards and prices per 100 m2 of roof slope
B. Installation of trusses and installation of coverings on trusses Scope of work
1. Slinging of structures. 2. Lifting structures using a tower crane and installing them in the design position. 3. Temporary fastening. 4. Unslinging and final fastening.
Table 4
Table 5
Time standards and prices for 1 truss or beam
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Constructions |
N.v. Dist. | |||
|
driver |
carpenters | |||
|
Trusses span, m | ||||
|
Composite beams | ||||
D. Laying purlins, installing decking and lantern frames Scope of work
When laying purlins
1. Manufacturing and fitting of mating joints in beams or composite purlins from boards. 2. Making connections between purlins and trusses. 3. Drilling holes for installing bolts. 4. Laying purlins along trusses with installation of fastenings.
When installing flooring along purlins
1. Reception of packages with materials. 2. Layout of boards or slats along the purlins with cutting at the joints and overhangs. 3. Nailing. 4. Trimming the ends of boards or slats that protrude beyond the edges of the slopes.
When constructing lantern frames
1. Marking and production of elements according to templates with the production of all mates. 2. Marking and drilling bolt holes. 3. Assembling the frame with fitting connections between each other and with the supports. 4. Weighing, alignment and jointing of the installed frame. 5. Bolting and nailing.
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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 | ||||
Through runs
When the truss spacing is 12 meters, continuous purlins become uneconomical; instead, through purlins of various design solutions have become widespread. The most successful are through purlins designed under the roof with a slope of 1.5% on a steel profiled deck. If they are properly secured with round wire ties, they can also be used for roofs with a large slope.
Purlins have triangular shape, their height in the axes is 1.5 meters (Fig. 19). The braces are made of single angles, and the upper chord is made of paired cold-formed channels. The purlin elements are connected to each other by resistance spot or electric arc welding.
When calculating the girder, its upper chord is considered as a continuous three-span beam on elastically subsiding supports, assuming a hinged connection of the lattice elements with the belt. In this case, both full loading of the entire upper chord with a uniform load, and partial loading - at half the span - is taken into account.
Farm connections
A covering consisting only of trusses on which purlins or slabs are laid cannot work normally, since it is a geometrically variable system. Farms may go out of business vertical plane, and the coating will “fold” (Fig. 20, a). If you fix the trusses on supports in a vertical position, they will not tip over, but in this case it is difficult to ensure the stability of the compressed chords from the plane of the truss, since their design length is very large (equal to the span of the truss).
Rice. 19. Through purlins with a span of 12 m: a - ordinary purlin (PR); b - end run (PC)
The purlins cannot prevent the belts from losing stability and move along with them (Fig. 20, b). Finally, trusses with very low lateral stiffness are not able to withstand transverse horizontal loads, such as wind. To ensure normal operation of the coating, connections are made between the trusses, which perform the following functions:
Provide geometric immutability of the coating;
Allows you to control correct position trusses during installation and hold them in the design position during installation and operation of the building;
Reduce the estimated length of the truss chords from its plane;
They absorb horizontal loads (wind, from crane braking).
Rice. 20. Behavior of the covering on trusses in the absence of connections: a - change in geometric shape ((overturning of trusses); b - loss of stability of compressed truss chords; 1 - truss; 2 - run; 3 - upper truss chord
The geometric immutability of the coating is ensured as follows. Of two nearby roof trusses a rigid spatial block is created. For this purpose, transverse (relative to the length of the building) connections are used along the upper chords of the trusses, transverse connections along the lower chords and vertical connections between the trusses (Fig. 21, a).
Vertical braces must be placed in the planes of the truss supports, i.e., at the ends of the block, and in the interval between them: when the trusses span up to 30 meters, in the middle of their length, and when the span exceeds 30 meters, two vertical braces are placed approximately in thirds of the length of the trusses.
In coverings with profiled decking and non-girder coverings using large-sized slabs that form a rigid disk in the plane of the upper chords of trusses, another rigid block solution can be adopted.
Rice. 21. Rigid geometrically unchangeable spatial block of two trusses: 1 - truss truss: 2 - transverse connections along the upper chords of the trusses; 3 - the same for the lower chords of the trusses; 4 - vertical connections
In this case, transverse connections along the upper chords are not arranged; their role in the operation of the building is performed by a hard disk formed by profiled flooring or reinforced concrete slabs. But vertical connections are placed every 6 m along the length of the trusses (Fig. 21, c), which is necessary to increase the rigidity of the block and reduce the free length of the compressed chords of the trusses during the installation process.
Rigid spatial blocks must be installed at the ends of the building or temperature compartment, as well as in the space between these end blocks, if the length of the building or temperature compartment exceeds 144 meters. All intermediate trusses are tied to rigid blocks using spacers and guy wires. Thus, the coating as a whole becomes a spatial geometrically unchangeable system
The connections along the upper chords of the trusses include transverse connections and girders, which in this case act as spacers between the trusses. Cross braces are horizontal braced trusses, the chords of which are the upper chords of the trusses included in the rigid block. The role of the racks is performed by purlins. Tie truss braces are special elements made from angles or pipes.
When bracing a braced truss from corners, a cross lattice system with flexible elements is usually adopted. The brace of such a lattice has a small cross-section of a single corner, and it is not able to perceive compressive forces: the compressed brace bulges and is thus disabled from work. In each panel, only one brace works - stretched. When the direction of forces on the braced truss changes, the braces in this panel change roles. A lattice with flexible braces, the cross-section of which is selected as for tension elements, is more economical than a cross lattice with rigid elements, when the cross-section of all braces must be selected as for compressed rods.
Ties secure some nodes of the upper chords of trusses from their displacement in horizontal plane and thereby reduce the design length of the compressed chords from the plane of the truss. The belt can only lose stability between these fixed nodes, as shown in Fig. 22, a.
The connections along the lower chords of the trusses (Fig. 22, c) consist of cross braces, struts and braces. In addition, longitudinal connections can be additionally arranged.
Cross braces are horizontal braced trusses, the chords of which are the lower chords of the trusses included in the rigid block. The posts and braces of a braced truss are special elements. Cross braces, together with spacers and braces, by means of which the nodes of the intermediate trusses are connected to the nodes of the rigid blocks, reduce the free length of the lower chords of the trusses from their plane. Transverse braced trusses perceive the horizontal wind load acting on the end of the building and transmitted to them by the uprights of the end fencing of the building.
Rice. 22. Diagram of connections from corners in a covering with purlins with a truss pitch of 6 meters: a - connections along the upper chords of the trusses; b - vertical connections between farms; c - connections along the lower chords of the trusses; 1 - truss included in a rigid block; 2 - intermediate truss; 3 - transverse connections along the upper chords of the trusses; 4 - the same for the lower chords of the trusses; 5 - vertical connections; 6 - longitudinal connections along the lower chords of the trusses; 7 - stretch marks; 8 - runs
Longitudinal braced trusses together with transverse braced trusses create a rigid contour in the plane of the lower chords and thereby increase the overall rigidity of the building frame. They perceive wind loads from the half-timbered racks of the longitudinal walls and transfer them to the columns, and also redistribute the forces from the lateral braking of the cranes between the building frames. Finally, longitudinal braced trusses reduce the design length of the lower chords of trusses, which is especially important in the case of rigid coupling of trusses with columns, when compressive forces may arise in the lower chord.
Longitudinal braced trusses are installed along the outer rows of columns in buildings with heavy and very heavy duty cranes; in coverings with rafter trusses; in one- and two-bay buildings with overhead cranes with a lifting capacity of more than 10 tons, and when the bottom elevation of rafter structures is over 18 m - regardless of the lifting capacity of the cranes. In buildings with more than three spans, longitudinal braced trusses should also be placed along the middle rows of columns no less often than across a span in buildings with heavy-duty and very heavy-duty cranes and after two spans in other buildings.
Vertical braces, as already noted, together with transverse braces along the upper and lower chords of the trusses, unite two adjacent trusses into a rigid, geometrically unchangeable block. Typically, vertical connections are made in the form of trusses with parallel chords and a triangular or cross lattice system (Fig. 23).
Rice. 23. Schemes of vertical connections between trusses: A - with a truss pitch of 6 m; b - with a truss pitch of 12 m
QUESTIONS FOR SELF-CONTROL
- 1. What kind of structure is called a truss?
- 2. Give a classification of farms.
- 3. What is the difference between purlin and non-propelled coating.
- 4. What is a purlin called and what is its design.
- 5. Why are connections arranged along the upper and lower chords of trusses?
- 6. What is called a rigid block and where is it installed?
- 7. Draw a diagram of connections in the coating with purlins.
The rafters serve as the basis for the entire roofing structure, and their installation is one of the most important tasks when building a house. The frame of the future roof can be made and installed independently, observing technological features roofs of different configurations. We will present the basic rules for the development, calculation and selection of a rafter system, and also describe the step-by-step process of installing the “skeleton” of the roof.
Rafter system: rules for calculation and development
The rafter system is a load-bearing structure capable of resisting gusts of wind, taking on all external loads and evenly distributing them to the internal supports of the house.
When calculating truss structure The following factors are taken into account:
- Roof angle:
- 2.5-10% - flat roof;
- more than 10% - pitched roof.
- Roof loads:
- permanent - total weight all elements " roofing pie»;
- temporary - wind pressure, the weight of snow, the weight of people carrying out repair work on the roof;
- force majeure, for example, seismic.
Size snow loads calculated based on the climate characteristics of the region using the formula: S=Sg*m, Where Sg- weight of snow per 1 m2, m-calculation coefficient (depending on the slope of the roof). The determination of wind load is based on the following indicators: type of terrain, regional wind load standards, building height.
Coefficients, necessary standards and calculation formulas are contained in engineering and construction reference books
When developing a rafter system, it is necessary to calculate the parameters of all components of the structure.
Elements of the truss structure
The rafter system includes many components that perform a specific function:
Materials for making rafters
Rafters are most often made from coniferous trees (spruce, larch or pine). For roofing, well-dried wood with a humidity level of up to 25% is used.
The wooden structure has one significant drawback- over time, rafters can become deformed, so metal elements are added to the supporting system.
On the one hand, metal adds rigidity to the rafter structure, but on the other hand, it reduces its service life wooden parts. Condensation settles on metal platforms and supports, which leads to rotting and damage to the wood.
Advice. When installing a rafter system made of metal and wood, care must be taken to ensure that the materials do not come into contact with each other. You can use moisture-proofing agents or use film insulation
In industrial construction, metal rafters made of rolled steel (I-beams, T-beams, angles, channels, etc.) are used. This design is more compact than wood, but retains heat less well and therefore requires additional thermal insulation.
Choosing a rafter system: hanging and suspended structures
There are two types of rafter structures: hanging (spacer) and layered. The choice of system is determined by the type of roof, floor material and natural conditions region.
Hanging rafters rest solely on the external walls of the house, intermediate supports are not used. Rafter legs hanging type perform compression and bending work. The design creates a horizontal bursting force that is transmitted to the walls. Using wooden and metal ties you can reduce this load. The ties are mounted at the base of the rafters.
Hanging rafter system often used to create an attic or in situations where roof spans are 8-12 m, and additional supports are not provided.
Layered rafters mounted in houses with an intermediate columnar support or additional load-bearing wall. The lower edges of the rafters are fixed to external walls, and their middle parts are on the inner pier or supporting pillar.
Installation of a single roofing system over several spans should include spacer and layered roof trusses. In places with intermediate supports They install layered rafters, and where there are none, hanging ones.
Features of arranging rafters on different roofs
Gable roof
Gable roof, according to building regulations, has an inclination angle of up to 90°. The choice of slope is largely determined by the weather conditions of the area. In areas where heavy rainfall prevails, it is better to install steep slopes, and in areas where strong winds prevail, flat roofs are installed in order to minimize the pressure on the structure.
Common option gable roof- design with an inclination angle of 35-45°. Experts call such parameters the “golden mean” of consumption. building materials and load distribution along the perimeter of the building. However, in this case attic space It will be cold and it will not be possible to arrange a living room here.
For a gable roof, a layered and hanging rafter system is used.
Hip roof
All roof slopes have the same area and the same angle of inclination. There is no ridge girder here, and the rafters are connected at one point, so the installation of such a structure is quite complicated.
It is advisable to install a hip roof if two conditions are met:
- the base of the building is square in shape;
- in the center of the structure there is a load-bearing support or wall onto which a post supporting the joint can be fixed rafter legs.
Create hip roof It is possible without a rack, but the structure must be strengthened with additional modules - racks and puffs.
Hip roof
Traditional design hip roof assumes the presence of slanted rafters (diagonal) directed towards the corners of the building. The slope angle of such a roof does not exceed 40°. Diagonal runs are usually made with reinforcement, since they account for a significant part of the load. Such elements are made from double boards and durable timber.
The joining points of the elements must be supported by a stand, which increases the reliability of the structure. The support is located at a distance of ¼ of the length of the large rafters from the ridge. Shortened rafters are installed in place of the gable roof gables.
Rafter structure hipped roof may include very long diagonal elements (more than 7 m). In this case, it is necessary to install under the rafters vertical stand, which will rest on the floor beam. You can use a truss as a support - the beam is located in the corner of the roof and fixed to adjacent walls. The truss truss is reinforced with struts.
broken roof
Sloping roofs are usually created to accommodate a larger attic. The installation of rafters with this roofing option can be divided into three stages:
- Installation of a U-shaped structure - supports for purlins that hold the rafter legs. The base of the structure is floor beams.
- At least 3 purlins are installed: two elements run through the corners of the U-shaped frame, and one (ridge purlin) is mounted in the center of the attic floor.
- Installation of rafter legs.
Gable roof: do-it-yourself rafter installation
Calculation of inclination angle and loads
Calculation gable roof Of course, you can do it yourself, but it’s still better to entrust it to professionals in order to eliminate errors and be confident in the reliability of the design.
When choosing the angle of inclination, it is necessary to take into account that:
- 5-15° angle is not suitable for everyone roofing materials, therefore, first choose the type of coating, and then make a calculation of the rafter system;
- at an angle of inclination over 45°, material costs for the purchase of components of the “roofing cake” increase.
Load limits from snow exposure range from 80 to 320 kg/m2. The design coefficient for roofs with a slope angle of less than 25° is 1, for roofs with a slope from 25° to 60° - 0.7. This means that if there are 140 kg of snow cover per 1 m2, then the load on a roof with a slope at an angle of 40° will be: 140 * 0.7 = 98 kg/m2.
To calculate the wind load, the aerodynamic influence coefficient and wind pressure fluctuations are taken. The value of the constant load is determined by summing the weight of all components of the “roofing cake” per m2 (on average 40-50 kg/m2).
Based on the results obtained, we find out the total load on the roof and determine the number of rafter legs, their size and cross-section.
Installation of Mauerlat and rafters
Do-it-yourself installation of rafters begins with the installation of a Mauerlat, which is fixed anchor bolts to the longitudinal walls.
Further construction of the structure is carried out in the following sequence:
Installation of rafters: video
Methods for connecting rafter structure elements: video
In coating structures, two design solutions are most widespread: 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 truss pitch of 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), wavy asbestos cement sheets 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 as roofing elements, laid along purlins, steel profiled flooring, reinforced cement and asbestos cement slabs are widely used.
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 the long side with combined rivets d = 5 mm (Fig. 4, c), installed every 300 mm and allowing riveting to be carried out on one side of the flooring (Fig. 4, d).
The weight of the profiled sheet is 0.1 – 0.15 kN/m².
Rice. 4 Warm roof with steel profiled decking
a – profiled flooring; b – self-tapping bolt; c – combined rivet; g – roofing angle
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 slope component qᵧ (Fig. 5, a). Although at small slopes the pitch component is small, due to the low rigidity of the run relative to the y-y axis, its stresses are large. 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 run obliquely and are attached to the truss or to the ridge girder near the 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 roofing decking is attached rigidly to the purlins and forms a continuous panel (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 weight load-bearing structures buildings (trusses, columns, foundations).
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. In the case of roll or steel roofs(δ = 3-4 mm) with welded seams, the slope can be less than ⅛ - ⅟₁₂. Wide Application find roofs with a slope of 1.5%, which are usually designed with roll coating and protection with a thin layer of fine-grained gravel on bitumen mastics.
The type of truss lattice is determined by the design of the coating, 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
