The main bearing roofing elements are laminated or hanging rafters. The total weight of the entire load on the roofing system allows you to divide them into types of permanent and temporary (short-term, long-term and special). The weight of all materials for the construction of the roof should be included in the calculation during the erection of the truss system, therefore its corresponding resistance to long-term loads is required.
Figure 1. Construction of wall rafters supported by the walls of a house.
Short-term roof loads include: the weight of a person, special equipment for the service or repair of the roof system. The special type of loads include the influence of seismic activity. The tree for the rafters take mostly coniferous wood, which has undergone special and additional processing with antiseptics and flame retardant.
The constant load is the weight of the whole structure of hanging rafters, which includes:
Starting the construction of the roof, it is necessary to pay special attention to which rafters will be used, either hung or hanging. When choosing them, one should compare the shape of the roof with the dimensions of the structure itself and the type of its roof. The roof at the same time can have a gable or single slope type. This implies the use of rafters with different ways of mounting during installation.
Figure 2. Map of snow loads.
In buildings with a single or dual type of roof, suspended rafters are used. This type of elements is a construction of small sizes or bars, for which two walls of the building are used as a support, or else three points, including a ridge with two walls.
The ridge element, i.e. the cant, is reinforced with the help of the uprights. The use of certain options, after the built house has the necessary overall dimensions, that is, more than 6 meters, is associated with the emergence of the need for building walls or pillars.
The walls of the house serve as supports at the very ends of the construction of walling rafters, and the average level of the structure should be supported by additional supporting elements (racks, beams, various girders, etc.), which are intermediate. This is shown in fig. 1. Accordingly, they can be installed in buildings equipped with column supports or an average level of load-bearing walls.
The overlap type of rafters is installed at a distance of no more than 6.5 meters from one type of support to another. The presence of additional supporting elements can allow to make the width of a large size, which should be covered with the help of wedged rafters, the size of which is up to 12 meters. Sometimes the opening is blocked by two adjacent supports with a width of no more than 15 m.
In buildings made of timber, upper crowns of wood are provided as supports for the truss legs. Support frame legs is the top trim. If the width of the building is the same everywhere, then the roof with the slope view of the rafters comes out with the least weight. Instead of ordinary wood constructions, the use of iron forms and beams is advantageous.
If the roofing rafter system has rafters that create conditions for normal pressure on the walls, which is horizontal and arching, then the name of the rafter legs will be indiscriminate suspended rafters. It is permissible to mount them in three ways, eliminating the possibility of an unsafe level of wall loads:
Figure 3. Estimated snow load.
To install suspended rafters and spacers - this means arranging roofing systems, the level of pressure on the system which will be transmitted to the walls (if the rafters are attached to them).
If we take into account all three possible ways of fastening non-supporting types of rafters, changing the rafters on rigid, fixedly connecting them with bars or mauerlat, you can create internal stress in the system, expressed in the bursting type of pressure on the walls.
In fact, expansion types of slant rafters are a transitional scheme in the installation conditions of slantless slats of a slant type to hanging elements.
Figure 4. Map of wind loads.
Calculations for various wind load indicators are related to a specific type of terrain. To calculate the snow or wind loads, you can use the formulas (1) and (2):
S = Sg * µ B (1),
where Sg is the indicator for calculating the weight of the snow cover per square meter of the horizontal surface of the plot of land to be taken into account in accordance with the table (Fig. 2). Table values from I to VIII correspond to the areas indicated on the map of snow loads by region. The scheme for the calculation of the table is shown in Fig. 3.
µ - coefficient indicating the transition from the total weight of the entire snow cover on the ground to the snow load on the coating.
The coefficient µ is determined by the angle of inclination of the roof slope:
Figure 5. The values of the correction factor, which depends on the height of the structure and the type of terrain.
W = Wo * k (2),
where Wo is an indicator of standard wind load values taken into account in the formula using tabular data for the calculation (Fig. 4);
k - a coefficient depending on the terrain conditions, takes into account the change in wind pressure along the height, it is determined, taking into account the table values (Fig. 5). The letter "A" in the table means the presence of open coasts of different water bodies, desert, tundra, forest-steppe or steppe. By "B" is meant the presence of an urban area, forest or other area with irregularities or barriers greater than 10 m in height.
Figure 6. The device of the hanging trusses supported on the walls of the house.
For the overlap of various spans that reach more than 7 m in length, in most cases, hanging rafters are used, as shown in fig. 6. Their device is associated with the adoption of the wall for a fulcrum. A special tightening is used to prevent the design of the rafter legs from moving apart.
In terms of its operational properties, the hanging truss system is based on possible compressions and bends. At the same time, the system has disintegrating abilities transmitted to the walls. In order to reduce these efforts, puffs (steel or wood) are used, which connect the rafter legs. They can be placed at the base of the truss system and above it.
The higher the fastening of the special tightening is, the more reliable the hanging rafters should have.
For spans greater than 8 m, a beam (strut) having struts can be used. The pluses of this roofing system include the ability to overlap large gaps.
Hanging truss systems are used for buildings that have a lightweight construction of walls. When constructing hanging type rafters, they can usually rest with their ends on the construction of the wall of the building, as well as the mauerlat, which is a basic type of timber. The supporting structures of the intermediate type are not foreseen.
The rafter system is fixed with clamps to the walls of the building. There is a tension stress, which is not transmitted to the walls, but remains within the prescribed limits of the shape of a triangle, which includes the tightening and both elements of the hanging trusses. The tightening should be strengthened on the mauerlate. At the same time, the rafters with a tightening do not rest on the construction of the walls, but on the power plate.
When installing hanging rafters use gash. So the rafter leg is attached to the mauerlat itself, fixing it with bolts or nails. The device of hanging trusses is completed when they are butt-to-end joined in a ridge and reinforced with wooden overlays.
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