Raft foundation is same to that of a floor slab turned upside-down. The distributed ground or soil pressure acts at the bottom surface and is held in equilibrium by the downward acting concentrated forces from columns and walls. The tendons are arranged as in an elevated slab, but with an inverted layout such that the low points are under the columns and walls, and the high points are in the spans.
In setting the slab depth, primary consideration is given to shear resistance and allowable soil pressure. Common L/h values range from 10 to 12, depending upon the modulus of sub grade reaction, the load magnitude and arrangement, the concrete strength, the level of stress, and the allowable differential deformation. Check of the punching shear resistance is generally performed. If the resistance is insufficient, the slab thickness can be increased overall, or just thickened under the columns.
To improve the tendons contribution for shear resistance and local flexural effects in the columns region, most of the tendons in each direction should be located in the column strips.
Raft thickness and pre-stressing requirements selection criteria as follows:
In buildings there are requirements of transfer slabs needed to transfer, at a certain level, some of the vertical loads to other alignments in order to obtain bigger spans at the lower levels, a more cost-effective structure, with smaller column or wall spacing can be envisaged for the upper floors, while at the lower levels, in particular at the ground level, bigger inter-column distances can be adopted, this is generally used at hotels for lobbies or where there is requirement of open space.
In order to transmit the high concentrated forces from the columns or wall, that have no continuity to the foundations, the transfer structures require, design dependant not only on the flexural behavior but, is considerably influenced by shear or punching resistance.
Due to the considerably high forces involved, the transition structure usually requires large depths and great amounts of reinforcement, in this case Post-tensioning is used, in these cases, a very cost-saving way to reduce both depth and reinforcement content & also provides a positive measure against early age shrinkage cracking in concrete.