What is the useful width and length for the dimensioning of a raft foundation ?

A foundation raft is designed to spread the weight of the building over as large an area as possible area to reduce the bearing pressures on the soil. The raft is designed so that any minor differential settlements do not cause any significant cracking in the building, which would otherwise be the case.

No main provisions or specs for the raft foundation dimensions. You can just use it for large areas and to help with differential settlement. Most of all, the dimensions depends on building area since it should be covered (some engineers extend the raft edges beyond the outer columns). Also depends in your soil report, some recommendations can be found in the soil report with regards of the foundation type.

Thanks for invitation. If raft the whole area is covered so that to prevent differential settlement and increase stability.

since raft foundation is primarily used to avoid differential settlements so it should have minimum dimensions equal to the dimensions of "covered/costructed area"

Hi,

There is no specific recommendation on length and breadth ratios. For a building, it all depends on SBC, soil characteristics, any soft (compressible) soil  layer below the hard strata, soil exploration is to be done to a depth more than building height or twice depending on importance of structure, etc of soil and others ;   the Height of the building/Forces like wind, EQ, History of location, etc . Everything put together experience and expertise of the structural designer. 

So several factors are to be considered on necessity of Raft Foundation which is the costliest foundation.

It will all start with the results of the Soil test. We design the raft depending on the soil strength. a rule of thump, the stronger the soil the less are for the raft dimension.  because we are distributing the lateral loads on larger cross section are.

Thank's for Invitation 

There are no specific dimensions for raft foundation, the dimensions are dependant for the loading on the building, and the type of the soil under foundations .. 

Dimensions of raft foundation mainly depends upon the following factors;

1. Bearing Capacity of soil and recommendations given in soil report.

2. Load of structure coming on soil 

Normally, raft foundation is also proposed when isolated footing areas become more than60% of the covered area so its better to provide a raft foundation.

Agree with the previous answers