Which American Code contains clauses for minimum contact area limit between footing and soil?

This is not a SFA software-related problem. But this limit has to be set manually when we use SFA.

But I didn't find any provisions for it in US codes.

In VERIFICATIONMANUAL, 75% is used.

Parents
  • Thank you for your support.
    We are looking forward to new version of SFA.
    As you answered, when we use SFA to model, with no data imported from Staad Pro, the current version of SFA can identify Gk to be dead load, but the default value of dead weight factor is 1.
    So by default load factor of Gk is the value we define in table of "generate load combinations", γ, for example. If we overrided value of dead weight factor from 1 to X, the resultant load factor of Gk would be the production of γ and X.
    But what made us confused is special rules for mat foundation. In previous version of SFA, weight of soil is not considered for mat foundation as it is not designed as rigid foundation.
    So I hope you can explain to us the different definitions of dead weight factors for mat foundations and other types of foundations.
    I still want to add a question. In the following picture, it says SFA checks soil bearing capacity under load combinations of service ultimate state. But in calculation report, SFA also checks it for strength ultimate cases.
    Is this step intended to get the net soil reaction presussure?



  • Zhao,

    Your question:

    But what made us confused is special rules for mat foundation. In previous version of SFA, weight of soil is not considered for mat foundation as it is not designed as rigid foundation.

    Answer:

    As far as I remember, all versions of SFA have considered the weight of soil as an additional load on mat. In older versions, the soil weight was applied by converting it to an equivalent selfweight of the mat. You can find the details here.

    https://communities.bentley.com/products/ram-staad/f/ram-staad-forum/103571/selfweight-of-mat-foundations-in-staad-foundation-advanced/599428#599428

    In the current version, the program applies the soil weight as a pressure load on the mat. For each load case, the pressure is calculated as

    dead load factor * Unit weight of soil * height of soil on top of mat

    SFA creates a STAAD.Pro file (.STD file) containing the finite element model of the mat. If you open that model in STAAD.Pro, you can see the pressure value applied as an element pressure load for each load case on all the elements that constitute the mat.

    -----------------------------------

    Your question:

    In the following picture, it says SFA checks soil bearing capacity under load combinations of service ultimate state. But in calculation report, SFA also checks it for strength ultimate cases.
    Is this step intended to get the net soil reaction presussure?

    Answer:

    We agree that we need to mention in the manual that the recent versions of the program check the soil pressure for strength load cases too. The pressure needs to be calculated for the strength load cases because the bending moments and shear force on the footing are based on the soil pressure.



  • When checking footing shear capacity and calculating reinforcement area, net soil reaction is used. That is the reason why SFA calculates Pmax under ultimate load combinations(Then minus the pressure induced by selfweight of soil and footing).

    But should we compare Pmax with soil bearing capacity in order to conclude if dimension of footing is enough? I don't think so.

    And there is also no need to calculate contact area percentage under ultimate load combinations.

    But if Chinese Code is selected, this step is not included in calculation sheet.

    This is a bit confusing.

Reply
  • When checking footing shear capacity and calculating reinforcement area, net soil reaction is used. That is the reason why SFA calculates Pmax under ultimate load combinations(Then minus the pressure induced by selfweight of soil and footing).

    But should we compare Pmax with soil bearing capacity in order to conclude if dimension of footing is enough? I don't think so.

    And there is also no need to calculate contact area percentage under ultimate load combinations.

    But if Chinese Code is selected, this step is not included in calculation sheet.

    This is a bit confusing.

Children
  • Zhao,

    Your question:

    When checking footing shear capacity and calculating reinforcement area, net soil reaction is used. That is the reason why SFA calculates Pmax under ultimate load combinations (Then minus the pressure induced by selfweight of soil and footing).

    Answer:

    That is correct.

    -----------------------------------------------------------

    Your question:

    But should we compare Pmax with soil bearing capacity in order to conclude if dimension of footing is enough? I don't think so.

    Answer:

    That is a choice left to the user. We agree that in most engineers do not check the ultimate soil pressures against a bearing capacity (factored by a number between 1 and 2 typically). But some companies do. So, we have provided that option.

    --------------------------------------------------------------
    Your question:

    And there is also no need to calculate contact area percentage under ultimate load combinations.

    Answer:

    This too is the user's choice. Some users have told us that for ultimate load cases too, they want to ensure that the loss of contact is within a reasonable limit, especially if the structure is supporting costly equipment like turbines. If it is not a matter of concern to you, you can ignore it.
    ---------------------------------------------------------------
    Your question:

    But if Chinese Code is selected, this step is not included in calculation sheet. This is a bit confusing.

    Answer:

    We will look into it.



    Answer Verified By: ZHAO BIAO 

  • Thank you very much.

    I need to confirm, in SFA, if checking soil bearing capacity and contact area is optional or compulsory.

    If it is compulsory, we have to create 2.sfa models.

  • Yes, these are compulsory checks at present. However, in order to ensure that they do not govern, specify a large number (2 or greater) as the multiplying factor, and a small value like 5% for contact area for ultimate load cases. Later this year, we will have a version of the program where these checks are optional. You do not need to create 2 separate models.



    Answer Verified By: ZHAO BIAO 

  • I am using version 9.5.062. I do not see how to turn off contact area check for combined footing. So I set min contact area as 5% and set overhang, thickness and calculate width. The width is increased until there is no negative pressure. How do I get it the uplift and calculate rebar and width for my minimum width of 24"

  • It looks like the e = M/P <= L/2 is the controlling factor