The truss and under-rafter elements serve as the basis for the construction of roofs. The supporting system of the roofing is roof beams and trusses. Rafter beams are a support for truss elements. They are used in the construction of coatings in one-story multi-span industrial buildings, in residential buildings in the construction of attic floors.
Types of rafters and trusses
The reliability of the entire roofing system fully depends on the strength and solidity of the supporting truss and under-rafter structure. It is exposed to a large number of various external loads.
Rafter beams are products made from one solid element that receives 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 truss roofing systems.
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 connections of the rods. Such structures are used in roofing systems withroof trusses.
Depending on the material of manufacture, truss structures can be divided into:
- Reinforced concrete.
- Steel.
- Wooden.
- Reinforced wood systems.
Reinforced concrete and steel beams and trusses are most commonly used in the construction of industrial buildings and structures. Wooden and reinforced wood elements are actively used for the installation of not only industrial roofs, but also in the construction of roofs in residential buildings.
In industrial construction, truss structures cover 12-meter, 18-meter, 24-meter and 30-meter spans between columns. With a six-meter step of truss structures, the under-rafter beam elements and trusses serve as intermediate supporting elements for them.
By type of cross-section, the beams are divided into:
- Rectangular.
- T-shaped.
- I-beam.
- Box beams.
In individual housing construction, structures designed to support the truss system are not often used. They are mainly used in the construction of attic rooms.
Reinforced concrete truss beams
Reinforced concrete products are used for roofing with a slight slope, as well as pitched roofs. They are made at reinforced concrete factories, where prestressing of beams with steel reinforcement is immediately applied. Types of fittings used:
- Rods with periodicprofile reinforced.
- Wire bundles made of extra strong wire.
- Twisted wire strands.
- String armature.
The form distinguishes between rafter beams with parallel and non-parallel belts. Their calculation is based on the load exerted by the rafter beam, which rests pointwise on the middle of the rafter element, and the load from 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 to support trusses and trusses, if their step width is 6 meters, and the installation width of the middle columns is 12 meters. Installation of rafter beams is carried out on columns, they are fixed by welding embedded parts. In the middle of the rafter beams and at their ends, special support areas are made with embedded sheets and anchor bolts for installing truss structures.
Have a tee or I-beam cross-section with a bottom shelf and a trapezoid shape. The lower shelf is reinforced in places where rafters are to be installed.
The length of the rafter beams is mainly 12 meters, sometimes 18 meters or 24 meters 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.
Reinforced concrete truss truss
Reinforced concrete roof trussesused in the construction of pitched roofs. They have the shape of a trapezoid, which has two belts: the lower one is of a horizontal type and the upper one is of a broken structure. At the moment, the most relevant are bezskorny trusses made of precast concrete.
For the reliability of fastening the truss trusses, the supporting sections of the truss elements are reinforced. Racks at the supports are provided for the installation of floor slabs. Racks and the lower belt of the truss truss are made with prestressing. Concrete grades 300-500 are used for production.
As in the beam version, embedded metal elements are provided in the trusses for fastening to columns and supports.
Steel purlins
Steel systems have lengths of 12 meters, 18 meters, 24 meters, and can be produced in lengths of 48 meters. Structurally, they are similar to the used truss beams. Consist of two belts: upper and lower. The top one rests on the mounting table on the column and is bolted to it. The lower chord of the beam is attached to the column with horizontal battens.
Steel truss truss
Made with parallel top and bottom chords. The length is unified and is 12 meters, 18 meters, 24 meters. Depending on the type of roof truss, the height of the truss structure can be 3.13 meters, 3.27 meters or 3.75 meters.
Mounting to the columns is done with the help of support posts, on which the roof trusses are supported.
Currently inindustrial construction began to use lightweight steel options for the manufacture of trusses. For example, tubular systems or beams with thin walls. Thanks to this design, the trusses are lighter, the consumption of steel for their manufacture is reduced, and the time for their installation is reduced.
Wooden truss systems
Wood structures designed to support the truss system on them have a sufficiently high strength and resistance to many aggressive influences. They are durable when used in structures with normal conditions for temperature and humidity. In industrial construction, they are used in buildings where there are environments unfavorable for reinforced concrete and metal.
By design, rafter elements are distinguished in the form:
- Beams.
- Farm.
- Arok.
- Ram.
If the length of the spans in the building is up to 18 meters, then a wooden rafter beam is used. In buildings where the spans are quite large - up to 30 meters, it is more expedient to use wooden truss trusses. Arches and frames made of wood as rafters are not often used.
Wooden purlin
In industrial construction, beams glued from boards are most often used. Such structures are much stronger and more durable than solid products, it is possible to manufacture various forms of rafter beams. Products from structural glued beams are widely used in the construction of roofing systems. Solid round logs can withstand more severe loads, but they are much inferior to glulam in terms of bending strength.
The joint of the wooden purlin can be seen in the photo below.
The cross section of the beam can be rectangular or I-beam. The top and bottom chords of the purlin can be parallel, or a gable top chord and a horizontal or broken bottom chord. With a span of up to 15 meters, I-beams with walls made of boards or plywood and bar stiffeners are often used.
Wooden truss truss
The main materials for the manufacture of wooden truss trusses are beams, boards or logs. Fastening of elements is possible with the help of metal hardware, plates. Glued wooden truss trusses are widely used. Their belts are made solid in width. With this manufacturing method, threaded toothed spikes and grooves similar in shape are made at the ends of the connecting elements. Glue is applied to the entire joint surface, then the structural details are pressed.
Reinforced wood truss beams and trusses
Wooden truss elements are reinforced in order to strengthen them. Steel or fiberglass are used as reinforcement. The reinforcing part is attached inside the wooden one with epoxy glue. Rebar prestressing is sometimes used.
Installation
Installation of rafter beams and trusses is carried out as follows. Reinforced concrete under-rafter beam elements and trusses are welded directly to the column heads with the help of embedded metal parts. It is possible to fasten them with bolts. Reinforced concrete consoles or metal tables on rafters serve as support platforms for load-bearing truss structures.
Steel trusses are attached to the columns from the side to the metal over-column with the lower belt. Its height is 0.7 m. Trusses are fastened to each other by upper belts. The rafter trusses rest on the tables of the truss trusses and on the patellas fixed on the columns.
Wooden truss beams in the roofing device look like this (see photo).
Installing gable roof beams
When there are two load-bearing internal walls in a building, the rafter system is installed on the rafters. In this case, they rest on a bed, by means of racks of bars, as well as internal walls. Usually these are two sub-rafter beam structures placed along the roof. This option is used with a height from the ceiling to the roof ridge from 1.4 meters to 2.5 meters. In this case, enough free space is formed 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 increaseroof space. The truss, under-rafter systems and the roof here play the role of external walls and coverings.
Rafter and truss beams can be seen in the photo in the article.
First of all, the extreme truss beams are laid on the Mauerlat on both walls of the house. With the option of a wooden house, instead of a Mauerlat, the upper crown of the log house is used. The beams must be strictly parallel to each other, you can check this by measuring the distance between their ends diagonally. The 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 required for the beam, then the rafter beams are spliced.
Then you need to stretch the ropes between the laid beams at both edges, and align them in level. At a distance of one meter from the extreme, the next rafter beam is installed. A board is also laid down on the opposite side. Be sure to check their horizontal position. Thus, the remaining rafter beams are laid along the entire length of the wall.
To align the protrusions of the boards outside the walls, 0.5 meters are measured on each extreme beam, a rope is pulled. Marks are made on the middle beams along the rope, the excess ends are sawn off. Further, load-bearing rafters are installed on the sub-rafter system.
GOSTs for truss structures
GOST 20372-2015, which entered into force on January 1, 2017, applies to the manufacture of reinforced concrete sub-rafter structures. According to this document, forproduction, heavy or light structural concrete is used. Steel truss trusses are produced in accordance with GOST 27579-88. It has certain and glued I-beam dimensions. GOST 19425-74.
