Friday, December 14, 2012

STRUCTURES: Strip footings.

Strip footings are used under relatively uniform point loads or line loads. The main structural function of the strip is  to disperse the concentration of load sideways into an increased width of sub-strata in order to reduce the bearing stress and settlement to an acceptable limit. A cross-section through an unreinforced concrete strip footing showing the assumed dispersion of load is shown in Fig. 9.1.

A further major structural function is to redistribute the loads in the longitudinal direction where the loading is
non-uniform or where the sub-strata resistance is variable (see Fig. 9.2). The width of the strip is usually decided by calculating the width required to limit the bearing stress and choosing the nearest excavator bucket size up from that dimension. From a construction point of view, the strip depth is used as a means of levelling out irregularities in the trench bottom and the width has to absorb the excavation
tolerances which would be unacceptable for the setting out of walls etc.

There are a number of different types of strips which include masonry strips; concrete strips – plain and reinforced; concrete trench fill –  and stone trench fill; reinforced beam strips – Rectangular Beam Strips  and Inverted T Beam Strips, and these are described in the following sections.

Fig. 9.1 Typical strip footing.

Fig. 9.2 Strip footing load spread distribution.

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