Example 5: Comparison between flat and ribbed rafts

Description of the problem:

A ribbed raft may be used where the distance between columns is so great that a flat raft requires excessive depth, with resulting high bending moments. Consequently, the volume of concrete is reduced. A ribbed raft consists of a stiffened slab by girders in x- and y-directions. The girders on the raft may be either down or up the slab. Ribbed rafts can be used for many structures when a flat level for the first floor is not required. Such structures are silos, elevated tanks and various other possible structures. Although this type of foundation has many disadvantages if used in normally buildings, still uses by many designers. Such disadvantages are: the raft needs deep foundation level under the ground surface, fill material on the raft to make a flat level. In addition, a slab on the fill material is required to be constructed for the first floor. The use of the ribbed raft relates to its simplicity in analysis by traditional manners or hand calculations. Particularly, if the columns are arranged in lines. The ribbed raft generally leads to less concrete quantity than the flat raft, especially if the columns have heavy loads and large spans.

In this example two types of rafts, flat and ribbed rafts, are considered as shown in Figure (49). The length of each raft is L = 14.3 [m] while the width is B = 28.3 [m]. Each raft carries 15 column loads and a brick wall load of p = 30 [kN/m] at its edges. Width of ribs is chosen to be bw = 0.30 [m] equal to the minimum side of columns, while the height of ribs including the slab thickness is chosen to be hw + hf = 1.0 [m]. Column dimensions, reinforcement and loads are shown in Table (53). A thin plain concrete of thickness 0.20 [m] is chosen under the raft and is not considered in any calculation.