ASCE 7 Drift Check for Seismic and Wind Loading

ASCE 7 Drift Check for Seismic and Wind Loading

Many engineers raise queries about the load case or load combination that shall be adopted for the control of drift under seismic analysis. Ever since strength-level (as opposed to service-level) design earthquake forces were introduced in the 1997 Uniform Building Code (UBC), as indicated by a load factor of 1.0 on E in strength design load combinations, it has been required that drift computations be done directly under those strength-level forces. The drift limits were adjusted accordingly. You never reduce the strength-level design earthquake forces to service-level forces for the purposes of drift computation. This is true of the 1997 UBC, all editions of the IBC, and all editions of ASCE 7 since 1993. The language cited from ASCE 7-10 is telling us exactly this.

In term of drift computation under wind load cases, here is no drift limit requirement in the code for wind design. There are various design office practices and suggested limitations, but no code requirements. Appendix C to ASCE 7 addresses serviceability considerations and includes recommendations for drift limitations for wind design as well as the respective wind loads that should be used for the purpose of checking drift.

We deal mostly with concrete buildings and this is what we would recommend: A top deflection (total, not inter story) equal to building height over 500 or less is acceptable. Anything more is too flexible. We have seen people go down to H/400, or even H/350 (particularly with steel buildings). That’s already quite flexible. If you were thinking of H/200, that simply won’t do. In concrete, of course, much depends on how the deflection is calculated. One needs to make realistic stiffness assumptions. Also, for drift check, most engineers consider it reasonable to use wind loads corresponding to a shorter return period than is used in the strength design of a structure.

Here below is the requirement of drift computation under UBC 97:

ubc 97 drift

 

The commentary CC.1.2 on ASCE 7-05 Appendix C titled Serviceability considerations is non-mandatory, which is also mentioned in footnote f to 2012 IBC Table 1604.3. This commentary discusses the wind load to use in drift computation. Note that while, in seismic design, drift refers to interstory drift, in ASCE 7 Appendix C, drift is taken as synonymous with lateral deflection.

In ASCE 7-05, the Commentary to Appendix C recommended the following load combination for use in serviceability check:

D + 0.5L + 0.7W

ASCE 7-05 used to specify 50-year mean recurrence interval (MRI, commonly referred to as return period) wind load (considered to be service-level wind load), 1.6 times which was used for the strength design of structures. The multiplication by 0.7 was to shorten the MRI on the wind load, because the use of 50-year MRI was felt to be conservative. In ASCE 7-10, strength-level wind is directly specified, the load factor on the effects of which in Strength Design load combinations is 1.0; it is brought down to service level through multiplication by 0.6 (= 1/1.6) in ASD load combinations. Thus 0.7 times service-level W of ASCE 7-05 is 0.7×0.6 = 0.42 times the strength-level W of ASCE 7-10. This explains Footnote f to Table 1604.3 of the 2012 IBC.

 

ASCE 7-05 CC.2.1

Post Author: Zahi Baroudi

3 thoughts on “ASCE 7 Drift Check for Seismic and Wind Loading

  • […] ASCE 7 Drift Check for Seismic and Wind Loading […]

  • Peter Placzek

    (May 23, 2016 - 5:47 PM)

    Nothing mentioned about the post elastic drift magnification factor 0.7R
    Nothing mentioned about service level wind deflection loads, usually 1 in 20 year wind (not the limit state wind loads 1 in 500 year)

    Your articles are consistently skimming over the subject.

    • Ramsey

      (May 23, 2016 - 9:03 PM)

      Hello Peter,

      Thanks for your contribution. Kindly note that this article came after receiving several queries about under what load combination set the drift shall be conducted.
      Regarding the 0.7R it’s mentioned in the standard snap , maybe you missed it.
      In term of other articles, it will be a pleasure if you can give us an example of what articles skim over the subject.

      Stay tuned and thank you again 🙂

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