The slip sandwich raft is designed on the assumption that the two halves of the raft – on either side of the centreline – are moving away from each other (tension), or towards each other (compression). The maximum horizontal force across the centreline of the raft, arising from the horizontal strains in the underlying ground, is equal to the maximum frictional force which can be transmitted across the slip-plane into one half of the raft.
Loads are as Design Example 1 (see Fig. 13.15):
|Fig. 6.15 Raft detail – low-rise/lightly loaded buildings.|
Horizontal force across raft centreline
The raft is 10.0 m × 12.0 m. With reference to Fig. 13.30, the total ultimate vertical load on one half of the raft is
located at the level of the underside of the raft thickenings to act as a slip-plane (see Fig. 13.30). The raft will be assumed to behave as illustrated in Fig. 6.14 and Fig. 13.27.
|Fig. 6.14 Effect of ground strain on raft.|
The Coal Authority guidelines(4) recommend the use of a coefﬁcient of friction of µ= 0.66 for a sand slip-plane. The length of the centreline is B = 10.0 m. The horizontal force per metre length across the centreline of raft is therefore given by
Provide high yield reinforcement to resist this force in tension such that
Design for raft compression
The same tensile force calculated above can also act in com- pression. By inspection the raft concrete can accommodate this magnitude of compressive stress.