Expert: Veto Varma - 3/21/2006

Question
What are the equations that are to be used to calculate the width of foundations. BS specifications would be helpful.

Answer
Hello Mark,

The foundation dimensions are mainly governed by soil bearing capacity and the load carried by the foundation. Again the type and location of foundation governs the calculation. If you specify your questions further (type and location of foundation, soil condition etc.) I can give you exact answer, but for general purpose here is the discussion.

For a simple spread footing, Load divided by soil capacity will give you idea about the area of the footing. Sometimes one of the dimension is already fixed depending on the location of footing e.g. boundary line in the building, adjacent footings, clearances etc., Then other dimension you can arrive at easily. Or if you are free to choose try square footing.

Once you arrive at decision of approximate length and width, check it with the one way/two way shear capacity giving you the thickness and shear reinforcement details.

Again this is a general idea and design decision depends on the exact conditions on hand. Feel free if you have further questions.

Regards
Veto

P.S. I will be away from March 21 to 26.