What are types of raft foundation? Define according to rigid mat deign.

One of the common types of raft foundation is the flat concrete slab 0.75 to 2.00 m thick with a two way reinforcing both at the bottom and at the top. Conventional or rigid method can be used where columns are regularly spaced. The method assumes a rigid mat so that the pressure diagram is linear. If the line of action of the resultant of the vertical loads passes through the centroid of the raft, the upward pressure can be taken as uniform. The weight of the raft is not considered in the structural design. The mat is divided into convenient strips as shown in Figure 3.28 in both x and y directions.

Rigid Mat Deign

The strips will have several columns and each strip is considered as a combined footing. The depth of the raft foundation is determined on the basis of punching shear as for a spread foundation and columns adjacent to the edges are likely to control the design. The raft will have a uniform depth based on the most critical section. Pattern of reinforcement are very conservative in rafts to allow for uncertainties in the design. A grid of both top and bottom reinforcement bars rr and yy directions may be placed throughout the mat.