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In the case where the centre of the acting force lies in the middle between the bearing points, the load is distributed equally over the bearing points. For the purpose of calculating the load distribution over two bearing points it is convenient to assume that the whole weight acts through this point. In the case of a vertical load only, the centre of the acting force is identical with the centre of gravity. In roof constructions, the centre of the acting force and the location of the bearing (reaction) points are usually not identical. This support design requires additional fixing means and is in practice very difficult to implement.Ģ.1.3 Load distribution of external forces If the wall plate or ridge beam is slanted in line with the roof slope, a pair of reacting forces results, one vertical to the rafter, the other one longitudinal to it. Under normal load conditions (excluding wind) the so-called "bird’s mouth cut" support causes a direct vertical reacting force. This can best be illustrated by two different designs for rafter support: For that reason for every force acting on a body (acting) there has to be a corresponding force (or sum of forces) acting in the opposite direction (reacting).Ī vertical acting force (usually gravity) causes a vertical reacting force a horizontal one (usually wind) causes a horizontal reacting force.Įxample: Reacting forces at a rafter support pointĭepending on the support system, an acting force can either causes a reacting force directly in the opposite direction or a pair of reacting forces with a resultant in the opposite direction. The sum of the forces acting on a body in a state of equilibrium is always zero. Its magnitude depends on the significance of the building and on its location (risk zone).Įvery force acting on a body causes a corresponding reacting force otherwise the body would not be stable. For static calculations the earthquake load is taken into account in the form of an additional fraction of the normal horizontal load. Earthquakes cause a horizontal or vertical movement that results in dynamic forces due to the dead load. Service (repair) load is of little relevance because it does not normally apply at the same time as the maximum wind or snow load.įorces resulting from earthquakes are in fact not additionally-superimposed loads. Snow loads only occur in certain regions, depending on the climate.
DETERMINE THE MAGNITUE AND DIRECTION OF THE ANCHORING FORCE FREE
Snow load Service and repair load Free fall body Stable body Wind can also have a suction effect, causing the roof to lift. In lightweight roofs such as tiles and sheets wind is often the heaviest load. Live loads consist of wind load, in some cases show load and service or repair load. The dead load consists of the weight of the structure and of the cover. A distinction can be made between dead load and live load. The loads that act on a roof structure come from different sources. (Magnitude of forces on roofs see Chapter 2.3)
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It should not be as strong as possible, but as strong as necessary.Ī basic knowledge of building statics enables the builder to develop a structurally correct system and to assess the appropriate dimensions of the structural components. On the other hand, for economical reasons, the structure should not be oversized. The roof must be stable, must not collapse or distort.
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When designing and constructing a roof it is self-evident that it should be robust enough to withstand all the forces that act on it.