Omega Factor for Equipment Anchored to Concrete 4

[nutte]

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?

 

[JAE]

Would you not just use the Ω_o of your overall building?

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[haynewp]

You need the 3rd print supplement to ASCE 7-10.

http://skghoshassociates.com/SKGAblog/viewpost.php?id=38

 

[nutte]

Haynewp, thank you! That did it.

JAE, I didn't think so, and the 3rd print supplement confirms my understanding.

 

[JAE]

nutte - what did you get from the errata...just curious as to your conclusions.

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[JAE]

Never mind - I see the addition of the omega's in the table.

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[wannabeSE]

Ironically, Ω₀ was added to the tables with supplement No. 1, but 13.3.1 still says the overstrength factor does not apply.

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.

 

[nutte]

You're right, section 13.3.1 in ASCE 7-10 says the overstrength factor Omega doesn't apply. That begs the question: why did they put it in the tables? To say these provisions are confusing and self-contradictory is an understatement.

 

[JAE]

My guess would be (and it's a guess) is that they forgot to take the language out from 13.3.1.

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[SAIL3]

I personally would use the omega factor on the design of the anchors, at least....IBC 1621.1.7 (2000 version) reduced the Rp for post-installed shallow anchors for component attachment. Required by code or not, I usually use the omega factor for critical elements ie. anchor bolts, col splices etc....design seismic loads are just an approximation at best...

 

[haynewp]

More discussion on the omega confusion:

http://skghoshassociates.com/SKGAblog/viewpost.php?id=43

http://skghoshassociates.com/SKGAblog/viewpost.php?id=49

 

[nutte]

Thanks again. It's a shame that we need multiple posts and discussions from various industry experts just to interpret the building code requirements.

 

[birthi]

I just ran into the same problem, and thanks to all of your contributions and clarifications I was able to resolve it (and learn something). Again, the plan checker was right and I was wrong. I hate when that happens!

Herbert Birthelmer
Civil Engineer (Structural)
California C 78306

 

[kauaiboi04]

Section 13.3.1 does say that Omega should not be used, but it is in reference to vertical seismic forces. Omega should be used for anchorage forces developed by horizontal seismic acceleration.

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

 

[structuralsteelhead]

kauaiboi04, I cannot concur to your conclusion that the permission to drop the use of Omega is in reference to the requirement for consideration for the additional vertical force. I feel it is rather just another provision of Section 13.3.1 for the design of the component, along with the additional vertical force consideration. With regards to anchorage, I believe you may have some merit as to the correlation between the old .4 anchor capacity requirement and the newer omega factor requirement in ACI 318-11 App. D. I agree with you that the omega's were added to table 13.5-1 and table 13.6-1 to account/coordinate with the newer ACI App. D provisions, otherwise we would be designing anchors for over 2.5 times what we did a code cycle ago. I would be careful to make sure you apply the omega factor in the load combinations in Section 12.4.3 and not afterwards (not sure what you meant,..."have been calculating the anchorage forces using CH.12 and then applying Omega to the reaction" You may end up with non-conservative results. For instance, let's just say your result is no net uplift for your anchorage tension case as a result in not using omega within your respective load combinations and then you multiply the Omega,...result still zero/no net uplift. Omega must be applied within the load combination, not after. Not as simple as multiplying your old anchorage spreadsheet results by the omega factor for example. You may be doing this already,...was just thrown a bit by your wording regarding about when you applied the omega factor. In general, ACI 318 Appendix D covers anchorage and doesn't care whether your structure is a component or non-building structure. So, ASCE Chapter 13 needed that Omega factor (Ch. 15 already had it, of course) addition since Section 13.4.2.1 sends you there to cover anchorage provisions to concrete. Further, use of the omega factor is a viable option of D.3.3.4.3 to easily satisfy more lightly loaded anchorage situations such as mechanical and electrical units where no appreciable tension force may occur even with the omega factor. It also appears to be the quickest, most economical design option to keep our design fees closer to what mechanical and electrical contractors would expect to see for a quick M/E pad and anchorage design.

 

[humanengr]

WannabeSE,
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.

 

[wannabeSE]

humanengr,
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 .

 

[birthi]

It's a pity that the same rigor that is expect from us in applying the codes is apparently not applied to the code development process. This became very obvious to me specifically with ASCE Ch 13 & 15 (in projects related to battery storage racks and water tanks). Add to that the attempt to correlate with IBC, and if you really want to get confused, add the IRC to the mix.


Herbert Birthelmer
Civil Engineer (Structural)
California C 78306