Design - Beam Strip Raft.

The beam strip raft is described in section 9.4.7 and consists of downstand beams in two directions tied together by a ground bearing slab. The beam and the slab are designed as separate elements which are combined together to finalize the rafted design.

These rafts are used where the bearing capacity below the beams is relatively good as is the bearing capacity of the ground below the slab and therefore there is no need to design the total raft foundation when the two are linked  in the final drawing.

The two are generally linked because of the added performance from the two separate elements when they are cast monolithic. The beams may be required due to the point loads from column structures around the edge of the raft  or within the body of the raft and beams are designed to span horizontally between these point loads. Similarly the raft slab is designed to float on the ground between the beams but since the bearing capacity where these rafts  are adopted is relatively good then a nominal design incorporating a top and/or bottom mesh is all that is required  in the slab.

These foundations are generally used in areas where quite shallow sand deposits occur below the topsoil and where there is no need to go to excessive depths around the edges of these rafts for heave or other problems. They can also be used where the strata changes slightly from perhaps clayey sands to sandy/silty clays.

Sizing of the design
The sizing of the sections is carried out by treating the beam strips as independent beams, designed previously.

These two parts for the foundation are then tied as shown in Fig. 13.39.

Beam strip raft.
Fig. 13.39 Beam strip raft.

Any necessary adjustments that the engineer may feel are required due to the changes in behaviour resulting from combining the elements are then made in the detailing of the raft. For example, the linking together will generally improve the raft performance by reducing the stresses in the two elements from those applicable if they acted alone.

However, there will be some occasions, for example, when a local heavy load occurs on a downstand, where a local detail could become critical due to the change in behaviour, and additional reinforcement or a slight adjustment to a detail may be needed.

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