The basis of the design when installing roofs are the rafters and sub-rafter elements. The bearing system of the roofing is rafters and trusses. Sub-rafter beams are a support for rafter elements. They are used in the installation of coatings in single-storey multi-span industrial buildings, in residential buildings with the installation of attic floors.
Types of roof beams and trusses
The reliability of the entire roofing system fully depends on the strength and thoroughness of the supporting rafter and sub-rafter structure. A large number of various external loads appear on it.
Sub-rafter beams are products from one solid element that accepts an external load, distributing it along its entire length. In this case, the greatest stresses occur at the ends of the beam. It is used in beam rafter systems for roofing.
The truss truss is a complex composite structure that is assembled from separate rods rigidly connected to each other. Loads occur only in the nodal joints of the rods. Such structures are used in roofing systems with truss trusses.
Depending on the material of manufacture, the truss structures can be divided into:
- Reinforced concrete.
- Steel.
- Wooden.
- Reinforced wooden systems.
Reinforced concrete and steel beams and trusses are most often used in the construction of industrial buildings and structures. Wooden and reinforced wood elements are actively used for the device not only production roofs, but also in the construction of roofs in residential buildings.
In industrial construction, the 12-meter, 18-meter, 24-meter and 30-meter spans between the columns are covered with truss structures. With a six-meter pitch of the truss structures, the truss beam elements and trusses serve as intermediate supporting elements for them.
By the type of cross-section, the beams are divided into:
- Rectangular
- Tauris.
- I-beams.
- Box beams.
In individual housing construction, structures designed to support the rafter system are not often used. Basically, they are used for attic rooms.
Reinforced concrete sub-rafters
Reinforced concrete products are used for roofing with a slight slope, as well as pitched roofs. They are manufactured at precast concrete plants, where the prestressing of the beams with steel reinforcement is immediately applied. Types of fittings used:
- Reinforced hardened rods.
- Extra heavy wire wire bundles.
- Twisted wire strands.
- String reinforcement.
The form distinguishes between rafters with parallel and non-parallel belts. Their calculation is based on the load that the rafter beam has, resting pointwise on the middle of the rafter element, and the load on the weight of the beam itself, distributed along its length. Products are made with sling holes designed for mounting and lifting, sometimes mounting loops are used instead.
They are installed in the middle rows of structures for supporting rafter beams and trusses if their step width is 6 meters and the installation width of the middle columns is 12 meters. Installation of roof beams is carried out on the columns, they are fixed by the method of welding embedded parts. In the middle of the rafters and at their ends, special supporting areas with embedded sheets and anchor bolts for installing rafters are made.
They have a T-shaped or I-shaped cross-section with a lower shelf and a trapezoid shape. The lower shelf is reinforced in places where the installation of rafters is supposed.
The rafters are generally 12-meter long along the length of the beam, sometimes 18-meter or 24-meter products are used. The height in the center is 1.5 meters, in the places of supports - 0.6 meters. The width of the lower shelf is 0.7 meters. Must have certain I- beam dimensions. GOST 19425-74.
Subsurface reinforced concrete farm
Reinforced concrete trusses are used for the installation of pitched roofs. They have the shape of a trapezoid, in which there are two belts: the lower one is of horizontal view and the upper one is of broken structure. At the moment, the most relevant are mowing farms made of precast concrete.
For reliability of fastening of truss trusses, supporting sections of truss elements are reinforced. To install the slabs, racks are provided at the supports. The racks and the lower belt of the truss truss are made with prestressing. For the production of concrete grades 300-500 are used.
As in the beam version, in the roof trusses for fastening to columns and supports, embedded metal elements are provided.
Steel beams
Steel systems are 12 meters long, 18 meters long, 24 meters long, and can be produced 48 meters long. Structurally, they are similar to the rafters used. Consist of two belts: upper and lower. The upper one rests on the mounting table on the column and is bolted to it. The lower belt of the beam is attached to the column by means of horizontal bars.
Steel truss truss
They are made with parallel upper and lower belts. The length is standardized and is 12 meters, 18 meters, 24 meters. Depending on the type of truss truss, the height of the truss structure can be 3.13 meters, 3.27 meters or 3.75 meters.
Fastening to the columns is carried out using support stands, on which the trusses are supported.
Currently, lightweight steel truss manufacturing options have begun to be used in industrial construction. For example, tubular systems or beams with thin walls. Thanks to this design, the trusses are lighter, steel consumption for their manufacture is reduced, and the time for their installation is reduced.
Wooden truss systems
Structures made of wood, designed to support the rafter system on them, have a sufficiently large strength and resistance to many aggressive influences. They are durable when used in buildings with normal conditions in temperature and humidity. In industrial construction, they are used in buildings where there are unfavorable environments for reinforced concrete and metal.
Structurally, there are sub-rafter elements in the form:
If the span in the building is up to 18 meters, then a wooden beam is used. In buildings where the spans are large enough - up to 30 meters, it is more expedient to use wooden trusses. Wooden arches and frames are not often used as truss elements.
Wooden rafter beam
In industrial construction, beams glued from boards are most often used. Such designs are much stronger and more durable than whole products, it is possible to manufacture various forms of roof beams. Products from structural glued beams are widespread in the installation of roofing systems. Solid round logs can withstand more severe loads, but they are much inferior to glued beams in bending strength.
The docking unit of the wooden roof beam can be seen in the photo below.
The cross section of the beam is rectangular or I-beam. The upper and lower belts of the truss beam can be parallel, or a gable upper belt and a horizontal or broken lower belt. With a span of up to 15 meters, I-beams are often used with walls of boards or plywood and squared stiffeners.
Wooden truss truss
The main materials for the manufacture of trussed wooden trusses are beams, boards or logs. Fastening of elements is possible with the help of metal hardware, plates. Glued wooden sub-rafter trusses were widely used. Their belts are made continuous in width. With this manufacturing method, threaded gear spikes and grooves similar in shape are made at the ends of the connecting elements. Glue is applied to the entire surface of the joint, then the structural details are pressed.
Reinforced wooden roof trusses and trusses
Reinforced wooden truss elements in order to strengthen them. As reinforcement, steel or fiberglass is used. The reinforcing part is fixed inside the wood using epoxy glue. Sometimes prestressing reinforcement is used.
Installation
Installation of rafters and trusses is carried out as follows. Reinforced concrete sub-rafter beam elements and trusses are welded directly to the column heads using embedded metal parts. It is possible to mount them on bolts. Reinforced concrete consoles or metal tables on rafters perform the function of supporting platforms for supporting rafter structures.
Steel trusses are attached to the columns on the side to the metal pillar with a lower belt. Its height is 0.7 m. Farms are fastened to each other by upper belts. Rafter trusses rest on tables of truss trusses and on patella fixed on columns.
Wooden rafters in the roofing device are as follows (see photo).
Installation of beams for a gable roof
When there are two load-bearing internal walls in the building, the rafter system is installed on the rafter elements. In this case, they rely on the bed, by means of racks of bars, as well as internal walls. Typically, these are two sub-rafter beams located along the roof. This option is used with a height from overlapping to the ridge of the roof from 1.4 meters to 2.5 meters. In this case, a sufficiently free space forms under the roof, which can be used as an attic.
The rafter beam, or puff, can be installed directly on the rafters at a distance of one third of the height from the ceiling to the ridge. This option allows you to increase the volume of the interior under the roof. The rafter, sub-rafter systems and the roof here play the role of external walls and coverings.
Rafter and roof rafters can be seen in the photo in the article.
First of all, the extreme rafter beams are laid on the Mauerlat on both walls of the house. In the case of a wooden house, the upper crown of the loghouse is used instead of the Mauerlat. Beams should be strictly parallel to each other, this can be checked by measuring the distance between their ends diagonally. Beams lie with a protrusion of at least 0.5 meters beyond the edges of the perimeter of the house. If the boards are shorter in length than is required for the beam, then the trusses are spliced.
Then you need to pull the ropes between the laid beams on both edges, and align them with the level. The next sub-rafter beam is installed at a distance of one meter from the extreme. The board also lies on the opposite side. Be sure to check their horizontality. In this way, the rest of the rafters are laid along the entire length of the wall.
To align the ledges of the boards outside the walls, 0.5 meters are measured on each extreme beam, a rope is stretched. On middle beams marks are made on the rope, excess ends are sawn off. Further, the rafters are installed on the rafter system.
GOSTs for truss structures
GOST 20372-2015, effective from January 1, 2017, applies to the manufacture of reinforced concrete sub-roof structures. According to this document, heavy or lightweight structural concrete is used for production. Steel truss trusses are produced according to GOST 27579-88. Has a certain and glued I- beam sizes. GOST 19425-74.