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  • 1.  Seismic Design Forces: ASCE7-16, Chp 13.4.1

    Posted 01-09-2023 03:15 PM
    For a mechanical system with R > 6.0, is it permitted to use the larger R-value as long as you are also using the overstrength factor, omega when determining design anchorage forces?

    For example: 1.2DL + 6/9ΩEh for a system that uses R=9 compared with 1.2DL + ΩEh for a system that uses R=6

    Scott Dawson P.E., M.ASCE
    PACE Engineers, Inc.
    Lake Oswego OR

  • 2.  RE: Seismic Design Forces: ASCE7-16, Chp 13.4.1

    Posted 01-10-2023 12:43 PM

    Hi Scott,

    I think I understand your logic but no, I don't see a path in the standard for what you're proposing.  The use of Ω as given in Chapter 13 is stated to be just for anchorage to concrete and masonry, which you may or may not have (last paragraph of  Either way, the limit on R is independent of this and based on the notion that piping systems with large R (i.e., R > 6) have an "inherent ductility and overstrength" (see commentary C13.4.1), which the anchorages may not have.  If you happen to be anchoring into concrete or masonry then I'd interpret that you're left with both (i.e., the limit on R and using Ω as required). 

    Hope this helps.

    Keith MacBain Ph.D., P.E., M.ASCE
    Associate & Chief Analyst
    Geiger Engineers, Campus 2
    Suffern NY

  • 3.  RE: Seismic Design Forces: ASCE7-16, Chp 13.4.1

    Posted 01-10-2023 02:38 PM

    When determining anchorage forces for mechanical or electrical equipment under ASCE 7-16, both the Rp and Omega-0 coefficients should be taken from the appropriate entry line in Table 13.6-1.  No further modification is appropriate.  You may wish to refer to ASCE 7-22 which contains updated procedures for determination of equipment anchorage forces.  The updated procedure, based on recent research, considers the strength and system type of the supporting structure, factors not previously considered.

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