Omega Factor for Equipment Anchored to Concrete
Omega Factor for Equipment Anchored to Concrete
(OP)
I have a piece of mechanical equipment being anchored to concrete with Hilti post-installed anchors. The relevant codes are IBC 2012, ASCE 7-10, and ACI 318-11. I am in seismic design category D. I need to get the anchors to work per section D.3.3.4.3(d) and D.3.3.5.3(c), which states that the tension and shear loads must come from load combinations with the seismic load E increased by the overstrength factor Omega.
My seismic loads come from chapter 13 of AISC 7-10. Table 13.6-1 has the relevant coefficients (ap and Rp). There is no Omega factor noted here, or anywhere else in chapter 13 that I can find. Does anybody know where to find the required Omega factor for this condition?
In other words, when calculating seismic loads on equipment per ASCE 7-10 chapter 13, what is the overstrength factor Omega?
My seismic loads come from chapter 13 of AISC 7-10. Table 13.6-1 has the relevant coefficients (ap and Rp). There is no Omega factor noted here, or anywhere else in chapter 13 that I can find. Does anybody know where to find the required Omega factor for this condition?
In other words, when calculating seismic loads on equipment per ASCE 7-10 chapter 13, what is the overstrength factor Omega?






RE: Omega Factor for Equipment Anchored to Concrete
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RE: Omega Factor for Equipment Anchored to Concrete
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RE: Omega Factor for Equipment Anchored to Concrete
JAE, I didn't think so, and the 3rd print supplement confirms my understanding.
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RE: Omega Factor for Equipment Anchored to Concrete
Pay attention to ACI D3.3.4.1. When the tension force with 0.9D+E is less than 20% of the anchor capacity, the anchors can be designed for the tensile requirements in D4.1.1 (I read this to mean no Ω0 and no 0.75 penalty for seismic loading). In some cases, an anchor will have no tension with typical load combinations, but this changes when combinations with Ω0 are used.
RE: Omega Factor for Equipment Anchored to Concrete
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RE: Omega Factor for Equipment Anchored to Concrete
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RE: Omega Factor for Equipment Anchored to Concrete
RE: Omega Factor for Equipment Anchored to Concrete
Herbert Birthelmer
Civil Engineer (Structural)
California C 78306
RE: Omega Factor for Equipment Anchored to Concrete
We have had a very good discussion about the use of Omega within our office. The footnote refers to using the over strength load combinations of ch. 12, but in doing this we found that units we have significantly higher tension anchorage forces. We believe this is no necessarily the intent of the code as previously the loads were calculated using standard load combinations of ch. 12 and then a phi factor of 0.4 would be applied per ACI. Our belief is that the Omega factor was added to remove the 0.4 (1/0.4 = 2.5) and therefore have been calculating the anchorage forces using CH.12 and then applying Omega to the reaction. Would love to hear from other practicing engineers about how they are dealing with this situation. Thanks
RE: Omega Factor for Equipment Anchored to Concrete
RE: Omega Factor for Equipment Anchored to Concrete
CODE --> D4.1.1
I believe that is an incorrect interpretation of D.3.3.4.1
What this ACI provision says:
You have the total factored anchor tensile force (not capacity).
Earthquake loading makes up part of that total tensile force.
If the Earthquake component is not more than 20% of the total force,
then you may design to satisfy D.5 and D.4.1.1 (D.3.3.4.3 need not
apply).
Therefore, if 50% (21% for that matter) of the Total Force comes from seismic, and the Total Force is only 20% (or even say, 5%) of the capacity of the anchor D.3.3.4.3 still applies.
In all of the above, we're talking (ACI is addressing) tensile forces.
RE: Omega Factor for Equipment Anchored to Concrete
You are right, I was wrong and stand corrected. A couple weeks ago I was reviewing that section because I had a piece of equipment with ~ 200# tension without Ω0 load combinations and over 1200 lbs tension at the anchors with Ω0. While I understand the intent of the Ω0 being applied to equipment anchorage, I question the actual code implementation of Ω0 (and some of the ap and Rp values for anchoring distribution systems in ASCE 7) .
Some people don't like appendix D. Personally, I become frustrated with D3.3 because it changes with every new ACI 318. Then the IBC might make there amendments and the California Building Code sometimes will modify it further
RE: Omega Factor for Equipment Anchored to Concrete
Herbert Birthelmer
Civil Engineer (Structural)
California C 78306