Expert: Arshad Khan - 10/19/2010

Question
QUESTION: Mr. Khan,
How does the design a raft foundation that will support generators, equiptments, etc. defers from the conventional shallow foundation design that support column, and if you can what are the steps to be considered in the raft foundation design? Thank you in advance and thank you also for this website. keep it up.

ANSWER: Hi Pablito,

The design of a raft foundation for generators, equipment, etc is similar to designing a raft for a building, but with an additional input required for the dynamic effects due to vibration, inertia, etc.

As for the design on rafts, I have answered this before, if you browse my past answers:

A raft foundation is designed like an upside-down flat slab, with the upwards subgrade reactions being equal and opposite to the self weight of the raft, the walls, the columns and of course the applied live loading. So, if you have a column in the centre of the raft, you will have concentrated rebar around its footprint in the bottom layers. You need to have subgrade modulus spring stiffnesses incorporated, in accordance with recommendations from the ground investigation report.

See these website for more details:

http://www.pdf-search-engine.com/optimal-cost-design-of-raft-foundation-pdf.html


http://environment.uwe.ac.uk/geocal/foundations/Fountype.htm#RAFT



http://rapiddigger.com/raft-foundation-design-example/


---------- FOLLOW-UP ----------

QUESTION: Mr. Khan,
Thank you again for the reply.
If the raft foundation is to be designed as an upside-down flat slab, what will be the support conditions at the perimeter? Will it be continuous or discontinuous? Will the equivalent upward subgrade reactions the loadings to be used in the analysis or just the applied downward loadings w/o self weight of the raft?
Thank you very much.
Regards,
Pablito

Answer
Hi Pablito,

The support conditions at the ends will be discontinuous (taking account of any basement walls), but you will still have the springs supporting the whole raft, including the edges and you need to apply both raft self weight as well as live loading, column loading, wall laoding, etc. And if you have a high water table for a basement situation, you also need to apply water up-lift pressure under the raft.