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  • 1.  Wind Load for Story Drift

    Posted 05-07-2019 10:42 AM
    Edited by Tirza Austin 05-07-2019 10:41 AM
    ASCE 7-05 includes the following in the commentary:

    "Use of the factored wind load in checking serviceability is excessively conservative.  The load combination with an annual probability of 0.05 of being exceeded, which can be used for checking short-term effects is D + 0.5L + 0.7W"

    Since this code predates the transition of the wind maps from ASD to ultimate, the load factors for W are as follows:

    LRFD: 1.6
    ASD: 1.0
    Serviceability (drift only): 0.7

    In the 2010 and 2016 versions, however, the load factors shift to ASD, and the corresponding section in the commentary has changed to:

    "Use of the nominal (700-year mean recurrence interval (MRI) or 1,700-year MRI) wind load in checking serviceability is excessively conservative.  The following load combination, derived similarly to Eqs. CC-1a and CC-1b, can be used to check short-term effects:
    D + 0.5L + Wa
    in which Wa is wind load based on serviceability wind speeds in Figs. CC-1 through CC-4."

    The commentary further states that: "Some designers have used a 10-year MRI (annual probability of 0.1)  for checking drift under wind loads for typical buildings, whereas others have used a 50-year MRI or 100-year MRI for more drift-sensitive buildings.  The selection of the MRI for serviceability evaluation is a matter of engineering judgment that should be exercised in consultation with the building client."

    This is much more ambiguous than the previous code.  I have seen references recommending:
     - Use of 50-year MRI with a load factor of 0.6
     - Use of any selected MRI, then further reducing V by 0.75
     - Use of 10-year MRI with no load factor
     - Use of ultimate loads with a drift limit of H/175 instead of H/400 (close to a load factor of 0.42 = 0.6 * 0.7)

    Generally the outcomes are similar, but not universally.  ASCE tells us to coordinate with the client for MRI, but on small projects where the client has little knowledge of engineering, this may not be realistic.

    Further, the new codes appear to be more conservative than 7-05.  For a Risk Category II building, the ultimate design wind speed is about 50% higher than the minimum (10-year) MRI per Fig. CC-1, resulting in a ratio of service pressure to ultimate pressure of .44, or slightly greater than the ratio of load factors in '05.  This suggests that even a building designed to a 10% annual probability of exceeding the serviceability limit in ASCE 7-10 would be equal to or better than any building designed to the previous prescriptive code.  Are standards for serviceability is increasing?  What service loads should we use for story drift on a typical building?

    Christian Parker EIT, A.M.ASCE
    Structural Project Engineer
    Washington DC
    (202)628-1600 EXT 190

  • 2.  RE: Wind Load for Story Drift

    Posted 05-08-2019 01:28 PM
    Edited by Tirza Austin 05-08-2019 04:03 PM
    The conference paper I wrote in 2011 "Deflection Limits in Tall Buildings-Are They Useful?" discusses this.  Because ASCE 7 is primarily focussed on life safety and not serviceability, this has never been codified.  It really is a question of risk and performance for the building owner. 

    If you are designing a building with a custom facade, then the facade supplier will need to know what the movements are and design for those.  If you have especially valuable or brittle finishes in the building, then you will need to pay extra attention to this.  Otherwise, following precedent is the best advice, even if this varies.

    Rob Smith C.Eng,P.E.,S.E.,M.ASCE, FICE, MIStructE
    San Francisco CA