The lidded cellular raft is described previously and due to its formation tends to be a little less stiff than the true cellular raft. The design calculations however follow similar lines with the exception that the cross-section of the beams tends to be restricted to inverted T and L shapes.
The advantage of this form over the pure cellular raft is that the upper slab can be detailed to allow it to be re-levelled should the floor tilt or distortion become excessive for the building’s use. Also in some locations the top of the lidded raft can be constructed in precast units and may prove more economic, avoiding the possible need for permanent formwork.
The raft is usually designed as a number of intersecting inverted T beams taking advantage of the lower ground slab as the flange of the T but ignoring the upper slab which could be constructed in timber joists and boards or other form to suit the design requirements (see Fig. 13.37).
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| Fig. 13.37 Lidded cellular raft. |
Remember that the strength advantage of the T beam can only be used for midspan, where compression occurs in the bottom of the beam. The section below the column must be designed as rectangular.
The detail at the seating of the upper floor depends upon the need for re-levelling and the possible number of times adjustments may need to be made.
As explained in section 9.4.6, the upper floor of the lidded raft is a separate structure to the main inverted T and L beams forming the concrete raft.
Sizing the design
The design procedure is similar to that of the cellular raft except that the upper deck is simply designed to span as a floor between the up-standing ribs. The remainder of the design follows the same procedure as before with the exception already mentioned that the element sections become inverted T or L beams rather that I or box sections.
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