Staad.Foundation - Gross Bearing Capacity

Hi

I have a question with regard to the Gross bearing capacity input. Here it is.

Let us assume that I have a footing which is critical in uplift. Let us also assume that Im using the “Gross Bearing Capacity Input” setting. If the depth of excavation is 2m then please clarify the following

  1. The parameter “Depth of soil above footing” is “2m-footing thickness” for gravity load cases (and)

  2. 0m for the case where there is uplift. (I think this should be 0m because the bearing capacity has been increased already on account of additional soil).

Please clarify if my assumption above is correct. If it is correct, then is there a provision in staad foundation to have the soil weight act only for some load cases and not act for the remaining cases.

Regards

Sriprakash

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  • Hi
     
    Many thanks for your response. But can you please answer my question exactly as I have some doubts regarding the use of soil weight for gravity loading.
     
    Regards,
     
    H.E.Sriprakash Shastry
    Partner
    Aswathanarayana & Eswara
    Consulting Engineers & Architects
    MMPDA Towers First Floor
    No. 184, (Old No. 214), Royapettah High Road
    Royapettah
    Chennai 600014
    Phone: +91 44 43102201
    Fax:     +91 44 43102206
     
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    Sent: 09 May 2014 12:13
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    Subject: RE: [Structural Analysis and Design] Staad.Foundation - Gross Bearing Capacity
     
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    Hi,

    It is always suggested to use net bearing capacity instead of gross bearing capacity. The net bearing capacity is what you need to use directly to find out initial footing size from the bearing capacity concept. IS 6403 instructs to compute net bearing capacity for the foundation design. The effect of depth is being considered in net bearing capacity and which can directly be used for foundation design.

  • The first point you mentioned is correct. The depth of soil above footing would be 2m – footing thickness but this is not just for gravity cases but for all cases including those which may cause uplift. In other words the effect due to the weight of soil and selfweight of the footing does not vary from case to case. For cases where the footing is subjected to overturning forces and moments, the weight of soil and selfweight of footing would counteract the uplift. If the overturning effect is stronger compared to the restoring effect due to soil weight + selfweight of footing, part of the footing would be subjected to uplift. There is no provision to make the soil weight act for some and not for some other cases.  



  • If the overturning effect is stronger compared to the restoring effect due to soil weight + selfweight of footing, part of the footing would be subjected to uplift.

    ...and in this case STAAD Foundation will do the maths by increasing the footing dimension to bring the uplift to the allowable zone (by satisfying the uplift factor of safety defined by the user) if 'Calculate Dimension' option was used, correct?

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