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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 + 6/9ΩE_h for a system that uses R=9 compared with 1.2DL + ΩE_h for a system that uses R=6

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Scott Dawson P.E., M.ASCE
PACE Engineers, Inc.
Lake Oswego OR
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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 13.3.1.1).  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

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Keith MacBain Ph.D., P.E., M.ASCE
Associate & Chief Analyst
Geiger Engineers, Campus 2
Suffern NY
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Original Message:
Sent: 01-09-2023 02:19 PM
From: Scott Dawson
Subject: Seismic Design Forces: ASCE7-16, Chp 13.4.1

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 + 6/9ΩE_h for a system that uses R=9 compared with 1.2DL + ΩE_h for a system that uses R=6

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

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.

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Ronald Hamburger P.E., F.SEI
Principal
Simpson Gumpertz & Heger
Oakland CA
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Original Message:
Sent: 01-09-2023 02:19 PM
From: Scott Dawson
Subject: Seismic Design Forces: ASCE7-16, Chp 13.4.1

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 + 6/9ΩE_h for a system that uses R=9 compared with 1.2DL + ΩE_h for a system that uses R=6

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