Topic Thread

  • 1.  Wind Loads for Drift Check

    Posted 07-21-2022 04:46 PM
    ASCE 7 has moved away from drift checks based on a fixed ratio of design wind loads.  The motivation for this is understandable.  As the commentary (Section CC1.2 in ASCE 7-10) suggests, different projects may have different risk tolerance for serviceability failure.  For example, I am working on a Risk Category II building which has hand-painted frescoed interiors.  Excessive drift would be a real issue for the artwork, so we used a 50-year mean recurrance interval (MRI), bringing the serviceability loads to roughly 0.6W.

    However, note f in IBC (2018) Table 1604.3 allows for a standard 0.42 factor on ultimate C&C loads to check drift, equivalent to roughly a 10-year MRI.  This is consistent with the 0.7 factor on ASD wind loads from the Commentary to ASCE 7-05 Appendix C, which was removed in ASCE 7-10.  With the intent of ASCE 7-10 and later to calculate serviceability wind loads based on a selected MRI, the IBC provision seems increasingly like a loophole: I can't imagine an SER or client specifying an MRI less than ten years.  Is there any discussion of removing this note from IBC?  Should there be?  What are your opinions on using a fixed coefficient versus a mean recurrance interval approach?

    This article in Structure Magazine touches on this:

    Christian Parker EIT, P.E., A.M.ASCE
    Structural Project Engineer
    Washington DC

  • 2.  RE: Wind Loads for Drift Check

    Posted 09-04-2022 10:06 AM
    Christian, as you note the  0.42 factor in IBC is historic and ties back to ASD design.  The ASCE 7 committee has not discussed updating this in IBC, but it could become a topic of discussion for the ASC 7-28 Committee, which will be formed in a few months.  Look for a call for volunteers interested in becoming committee members in the next several months, in venues such as Structure magazine, Modern Steel, Modern Concrete, Civil Engineer, and other publications.  As a committee member, you could bring this discussion to the committee.

    Ronald Hamburger P.E., F.SEI
    Simpson Gumpertz & Heger
    Oakland CA

  • 3.  RE: Wind Loads for Drift Check

    Posted 27 days ago
    Thank you, Ron.  I'll look out for that announcement.

    Does anyone have an opinion either way about the IBC vs ASCE 7 approach to service wind?  Using a fixed factor is simpler, but ASCE's MRI method ties the drift check to more tangible performace objectives.

    Christian Parker P.E., M.ASCE
    Structural Project Engineer
    Washington DC