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.