How STAAD Foundation Design for Flexure

Hello,

In designing combined footings, how does STAAD Foundation compute the “Ultimate Moment (Mz)” and “Critical Moment for Distribution Steel (Mx)”? I attached my calculation sheet for reference.

Thank you in advance!


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  • Please go through verification there are many examples showing on theory behind software computation

  • Hello,

    Yes, I already checked the verification manual but I just need a few more details as to how the moments at the critical sections are determined or computed. I am reviewing “US Combined Foundation 1” (pp. 322 to 329) found in the said manual. More specifically, I need guidance in answering the ff. questions:

    1. Can you provide more details as to how Mz (sagging), Mz (hogging), and Mx are computed? (shown in the attached photos)

    2. I am wondering as to why the governing Mz (sagging) of 2,523 kip-in is beyond the maximum moment of 2,414.89 kip-in found in the moment diagram? (see attached photo of moment diagram)


  • 1)to Calculate SF and BM diagrams, SFA considers all four corner pressures for that respective load case. Now this is shifted from 2D- to 1D ( average out top left and bottom left corner pressures to get pressure at left edge center-line and in the similar way calculate pressure at right edge center-line. Now considering the pressure trapezoid is pushing upward, Self weight soil weight etc are pushing downward and xial loads and moments are acting in respective directions, we can easily draw SF and BM diagram.

    2) Maximum Mz sagging moment used for design should be generally less than maximum sagging moment Mz in BM diagram, as maximum sagging moment occurs at top of pedestal, where for design we use column face moment

  • Mx calculation in combined footing is a bit different than Mz calculation in combined footing. In Mz calculation, SFA takes the center line approach and calculate the actual Mz at  different places. This approach is not safe for Mx calculation. As, for some cases, the L/W ratio of combined footing is very high , So, if we consider center line for Mx direction, it may under design to one end (Please ref the attached sketch). Therefore, Mx = Maximum corner pressure x Footing Length x (Foortng Width - column width)/2  x (Foortng Width - column width)/4

  • Hello, 

    I tried using that formula in the current STAAD file that I have. However, I computed Mx to be 263.6 kNm but the software computed it to be 212kNm.

    Mx = 187.4588 kN x 5m x (2m-0.5m)/2 x (2m-0.5m)/4

    Mx = 263.6kN

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