Can you tell me (for USA) which code (or standard or guide) and in it where exactly I can see the distance required for the raft foundation, as far as how much it should extend horizontally beyond the outermost columns of the building? (In other words, how much larger it should be (as a minimum or a specified ratio etc...) than the building footprint as defined by the outer columns) I checked ACI 318, IBC, NEHRP document and cannot see it. Unless I missed it. Please help.
Raft foundation doesn't necessarily have to extend byond the exterior column. On occasion a column may start at 2nd or 3rd floor even outside of the footprint of the raft, having the floors above hanging out.
The foundation has to be designed to handle different position of the columns.
There are several methods to design raft foundation.
design the foundation as a rigid mat loaded with column loads, that is resolve to one vertical load applied at a certain point not necessarily coinciding with the CG of the raft. Then calculate soil's reaction and settlement.
design the raft as a flexible mat being supported on flexible soil, settling more under heavier columns. Again either assuming the soil acting like many springs or assuming it as a flexible media.