Civil Engineering Reference
In-Depth Information
arose with the arrival of more children (see Figure 8.7). This lean-to's roof, added onto
the existing house, is referred to as a skillion roof.
The direction of forces applied by the roof load
The forces applied to buildings are classified as either live loads or dead loads. The dead
load is the weight of the building itself while the live load includes all other weights and
pressures applied to the building. The live loads would include things such as rain, snow,
wind, earth tremors, furniture and people. The dead load is stagnant but cannot be
ignored, as added pressure is bought to bear on the building by the live load, which
compounds the effect of the dead load. The load applied to a building will ultimately be
transferred to the foundation or ground beneath the building in one way or another. A
well-built structure will pass the force applied by the total load on the house through the
structure of the house to the ground without compromising the stability of the structure.
If, however, the structure has insufficient strength to allow the load to be passed through
it, both the load of the building and the load on the building will demolish it, ultimately
transferring the force to the earth.
Roofs can cause walls to collapse
A badly constructed roof will cause the walls below to collapse. The roof is crucial, as the
force applied to a badly constructed roof will quickly destroy the roof and walls below.
When the load is applied to the roof, there is a natural tendency for the peak of the roof
to drop, thereby forcing the bottom of the rafters to spread. If the top of the opposing
rafters were not fixed together, they would slip past each other, resulting in the peak of
the roof dropping. However, the rafters have to
be joined for there to be strength in the roof.
When the rafters are joined at the ridge but
unsupported the transfer of the load will still be
lateral, however it will be expressed by forcing the
external walls outward, thereby causing them to
collapse, as they are constructed to transfer weigh
vertically to the foundation, not to resist lateral
force and transfer the load to the foundation (see
Figure 8.8). This must be kept in mind at all
times when assembling the roof, for the weight of
the timber alone is sufficient to spread the
external walls.
If the walls were built with a triangular strut
or wall section extending out from the top of the
wall onto the ground, the load would be transferred through the vertical wall and the
angular strut to the earth (see Figure 8.9). You will often see walls like this in old
churches.
Apart from providing the wall with external triangulated struts there are two ways to
overcome the spreading expression of load transfer. By supporting the ridge beam the top
Figure 8.8
Roof loads on an incorrectly
constructed roof will cause the walls to spread
and the building to collapse
