R=2.5 detailing requirements

[TheLinker]

I have a short (5ft or less) frame supporting a mechanical equipment skid in SDC D. My intent is to avoid seismic detailing per AISC 341 and use the higher R value since the skid and framing are not too heavy.

I am using ASCE 7-05 table 15.4-1 for non-building structures similar to buildings with an OMF in the transverse direction and OMF (knee brace system) in the longitudinal direction.

My question is this, if I use the OMF w/ permitted height increase (to 100ft) and R = 2.5 value, the detailing requirements indicate AISC 341. My understanding is AISC 341 detailing is for R equal or greater than 3. So would this approach and R value require any of AISC 341 detailing or would AISC 360 be acceptable?

The OMF w/ unlimited height with R=1.0 can be detailed to AISC 360 but I have a 2.5 times higher base shear to deal with and this may cause issues. Am I stuck with R=1 if I want to detail to AISC 360 only?

Appreciate any feed back.

 

[Gumpmaster]

I'd push for calling it a nonbuilding structure that's not similar to buildings. That opens up Table 15.4-2 for you.

Even better, call it a mechanical component and use Table 13.6-1. You get ap=1, Rp=2.5.

A heavy equipment skid really doesn't resemble a building.

 

[wannabeSE]

I agree with Grumpmaster. The AISC 341-05 scope says:"When the seismic response coefficient, R, is taken as 3 or less, the structure is not required to satisfy these provisions, unless specifically required by the applicable building code."

 

[TheLinker]

Thanks Gumpmaster and Wannabe,

I can see designing the connections per Chp 13, but not the support frame as the skid is > 25% of the mass weight (more like 60%). The support frame is a typical 4 column structure (14'x 20' x 5' tall) with moment connections in one direction and vert bracing in the other, its about as building like as it gets. If anything I would think reverting back to Chp 12 for the structure would make sense, but since it is an unoccupied skid / structure, I feel ok with using Chp 15.

I am still confused on why ASCE indicates AISC 341 for R=2.5. I agree 341 states that for R < 3 those provisions do not apply and I feel it can be detailed to 360 beacuse of that. I may pose the question to Ask AISC just to be sure.

 

[Gumpmaster]

So, AISC 341 doesn't say "it doesn't apply" for R<3. It says it doesn't apply "unless specifically required", which, in this case, ASCE 7 specifically requires it. Bummer for you.

That's why it would be nice to get you out of an OMF. The nice part about OMF is that you don't have to follow the prequalified connection requirements. There are other, less onerous requirements. There are definately some disconnects between chapters 12, 13, & 15.

Good Luck.