raft foundation for low cost housing

huangyhg

raft foundation for low cost housing
i am involved in low costt housing (40 sqm)in south africa. i am looking for expert advice to design an economical raft foundation for many different kinds of soil conditions.

the main criteria is bearing pressure, the lower the allowable pressure, the stiffer the raft has to be.
in the uk it is usual to design the edge as a beam for spanning 3m and cantilevering 1.5m (note rafts in mining areas would be subject to more onerous design cases). the beam width is set to suit the allowable bearing pressure with the moment in the slab determined by resolution of the forces about the centroid of the beam. internal beams are treated similarly.
an aspect ratio of 2:1 (length is twice the width) as a guide with beams crossing to ensure stiffness.
the main issue is of course your local soil conditions. in the uk soils tend to be soft, cohesive and wet so the edge beam needs to be set at a minimum 450mm below ground for frost protection. this is probably not the case in sa. also, we tend to use cavity masonry which is much heavier than timber and provides more onerous loading conditions.
however, a typical raft construction for a two storey masonry dwelling would be 150mm thick rc with 8mm high tensile rebar at 200mm c/c each direction (mesh) in the top of the slab with a 600mm wide 'boot' edge beam reinforced with 8mm high tensile links @ 200 c/c and 12mm dia longitudinal bars (4top and 4 btm) with 450mm internal beams under loadbearing walls. this would be typically on a soil with an allowable bearing pressure of 50-75 kn/m2 and would be constructed on a gas/water proof membrane and a minimum 150mm well graded well compacted granular hardcore.
hope this helps
thank you,i appreciate your advice.
this is single storey buildings, but the principles are applicable.