Wednesday, January 9, 2013

Unreinforced Concrete Strips.

The unreinforced strip footing requires slightly better ground conditions than trench fill to maintain trench stability during construction of the masonry over it.

The adoption of a thin strip means that the trenches tend to remain open longer during construction than in the case  of the trench fill solution to allow bricklayers or masons to work from within the trenches. However, the overall cost of the work often proves less than for trench fill and on many sites proves to be easily achieved. It has the added advantage of more easily accommodating services but suffers  similar disadvantages to trench fill in active mining areas.

The choice between trench fill and concrete strips usually depends upon cost.

The width of the strip is generally the nearest suitable excavation standard bucket width to that of the design
width required from the calculations. However, for deep strips the working space required for bricklayers can  determine the width required. The thickness is generally selected to be greater than the overhang (i.e. this is based upon a 45° dispersion of load through the mass concrete, see Fig. 11.18).

 Fig. 11.18 Unreinforced strip.

Where this guidance would give a thinner strip than that practical from a construction point of view, or that desirable from a performance requirement, a greater nominal thickness is used. Longitudinal bending considerations, particularly where the strip requires to be continuous below door openings etc. (see Fig. 11.19), is one of the situations which may demand a thicker strip than that given  by the general 45° line. However, this would only apply if dispersion of load along this length of footing is required to reduce the bearing pressure.

Fig. 11.19 Continuous strip through opening.

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