Staad Foundation is Uneconomical in sizing calculation

I have designed isolated foundation in staad , 

The sizing of foundation is coming very high , eventhough my foundation pressure is very less ie) 40 kN/m2 , allowable is 200 kN/m2.

In-terms of sliding and overturning , i am getting minimum of 3.02 ratio.

Can u please suggestion on it , why i am getting more size.

 I am attaching my staad file along with this Master.xmlMaster.sfa

Parents
  • Since you did not mention which footing your question pertains to, we tested it for just footing 1.

    For this footing, there are load cases where the vertical load is small and overturning moment is large. Since you have specified the minimum area in contact for service and ultimate load cases to be at least 90%, the footing size needs to be large enough to meet that condition for every load case included in the job. However, large footing sizes will also result in small values of soil pressures, and large factors of safety in sliding and overturning.

    If you do the following:

    a) Change the design type to Calculate Dimension
    b) Change the minimum footing size to 1.5 m X 1.5 m
    c) Reduce the minimum required contact area percentage to 50% or lower
    d) Change the multiplier on soil bearing capacity for ultimate load cases from 1 to 1.7

    you may get smaller footing sizes.

Reply
  • Since you did not mention which footing your question pertains to, we tested it for just footing 1.

    For this footing, there are load cases where the vertical load is small and overturning moment is large. Since you have specified the minimum area in contact for service and ultimate load cases to be at least 90%, the footing size needs to be large enough to meet that condition for every load case included in the job. However, large footing sizes will also result in small values of soil pressures, and large factors of safety in sliding and overturning.

    If you do the following:

    a) Change the design type to Calculate Dimension
    b) Change the minimum footing size to 1.5 m X 1.5 m
    c) Reduce the minimum required contact area percentage to 50% or lower
    d) Change the multiplier on soil bearing capacity for ultimate load cases from 1 to 1.7

    you may get smaller footing sizes.

Children