Expert: Arshad Khan - 2/20/2010

Question
QUESTION: Salam.
i have modelled & analysed a G+10 storey structure in Staad-pro by providing plates for the raft foundation. My Question is, after analysing the structure the Base pressure i get from the plate result is approximately 238 KN/M2, where as the required SBC as per the soil test report is 200 KN/M2. In this situation what would you suggest me ?

ANSWER: Hi Bilal,

If the 238 kN/m2 is fairly localised, then you need not worry too much as this will re-distribute due to differential settlement, to a more even lower pressure. However, if the pressure is over a wider area, you may need to thicken up the raft (or make it larger in area/on plan) so as to spread the load more evenly and reduce the high pressure zones.

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

QUESTION: Thanks for the reply.

I have one more question regarding the same subject.
what if, the area of Raft is Restricted? for example, Axial Load (Unfactored) From all the columns including 10% self weight of Raft is 131285 KN & the Area of Raft is 500 M2. the net upward pressure is 262.57KN/m2. where as the SBC of soil according to the soil test report is 200KN for a foundation depth of 3M below the NGL.   
I am working in Oman & here the soil condition is loose sandy soil.
it will be great to know the answer for the above question in detail.
& also forward me if you have any procedure for the design of raft for above conditions as i have to submit all the necessary design data to the municipality for its approval.
By the way do u have any Spreadsheet for the same? if yes, then please forward it to me on my email id  

Answer
Hi Bilal,

In that case you have 2 options only- drop the raft by incorporating a basement, or use piles.

Rafts are quite complex to design, so you will need to use a suitable FEA software. Here are some sources for further info:

http://www.oasys-software.com/information/case_studies/aspire_tower.shtml

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