Design for Seismic Tie Beams

Per ACI 318-08 section 21.12.3.2 – “Grade beams designed to act as horizontal ties between pile caps or footings shall be proportioned such that the smallest cross-sectional dimension shall be equal to or greater than the clear spacing between connected columns divided by 20 but need not be greater than 18 inches. Closed ties shall be provided at a spacing not to exceed the lesser of one half the smallest orthogonal cross-sectional dimension and 12 inches.”

But ACI is not telling us that what force these tie beams should be designed for, so here is the IBC 2009 section to tell us that:

IBC2009 section 1809.13 for shallow foundations and 1810.3.13 for deep foundations:

for SDC C, D, E or F, ties shall be capable of carrying, in tension or compression, a force equal to the lesser of the product of the larger pile cap or column design gravity load times the seismic coefficient, Sds,divided by 10, and 25 percent of the smaller pile or column design gravity load. (there is more in this section so please read the code before applying this to your case)

In simple words Tie beam force; FT = Larger of ( Pu_large x Sds /10 , Pu_small x 25%)

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I am structural Engineer working in a reputable firm in Los Angeles

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Posted in ACI318-08, Design Code, IBC 2009, Seismic, structural engineering, structures
11 comments on “Design for Seismic Tie Beams
  1. Rana Waseem says:

    Dear friend,
    You blog about structural engineering is superb. I am a Structural Engineer and i run a blog too. It is http://www.ranawaseem.com, soon I am going to launch software applications. I think we both can link exchange and put logos of our forums on both the blogs. Looking forward to hear from you. Thanks
    Rana Waseem (Pakistan) 3yrs of exp
    Structural Engineer
    Working in Saudi Arabia
    waseemrana8@gmail.com

  2. KK says:

    Hi,
    On a different subject: For crack width calculation is ACI 318 or ACI 224R is used? Crack width calculated by ACI is more stringent than calculated by BS Code. Pl advise.

    • zstructures says:

      I dont completely understand your question, but as far as i know is that BS code first specifically mentions about the crack width and the reason is their equations which already include the consideration of cracking.

  3. karthick says:

    Hi,
    I am a structural engineer,can anybody please explain whether the tie beam can be connected at pile cap bottom level.

    Also kindly explain is there any structural implication on the pile and the cap.

    Regards,
    Karthick.k

  4. tirumala says:

    Hi;
    my name is Tirumala;
    can you provide reference for design of Foundation.

  5. tirumala says:

    Hi;
    My name is Tirumala;
    Can you provide reference for design of Foundation Tie as per ACI 318-08.

  6. Ryan says:

    Sir can I get your email add?

  7. Ryan Layug says:

    Sir can I get your email add?

  8. Ramy says:

    Hi,
    Tie beams should also resist differential settlement of footings.
    So do we design them for the mentioned axial force in addition to bending moments and shear due to differential settlement?

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