ASCE 7 - Seismic Design (R < 3) of Nonbuilding Similar to Building

[ARS97]

I'm using ASCE 7-10 to calculate the seismic loads acting on a structure. The structure is a moment frame (OMF) that supports an elevated bin that stores fly ash. Based on this useage, I classified this structure as a "nonbuilding similar to a building" (Chapter 15 provisions). I came across an interpretation issue.

NOTE - this is Seismic Design Category B and special seismic detaiing requirements are not desired (per AISC 360 only....not AISC 341).

According to ASCE 15.4.1, note 1a, the structural system can be selected from either Table 12.2-1 or 15.4-1. Table 12.2-1 has a category (H) that is for buildings NOT specifically detailed for seismic resistance & R = 3. Table 15.4-1 does not have such a note......instead, Table 15.4-1 allows detailing per AISC 360 under the "unlimited" height category, which requires R = 1 for a OMF.

My interpretation is that the R = 3 (Table 12.2-1 Category H) is a general recommendation for anything that hasn't been specifically detailed for seismic resistance. However, some structures require a R value of less than 3 even if special detailing requirements aren't followed. In essence, Table 15.4-1 overrides Table 12.2-1 in this case. (This is also demonstrated in various structural systems that have R < 3 in Table 12.2-1.)

Am I interpreting this correctly?

 

[Hokie93]

I don't believe your interpretation is correct. The R-values are identical between Table 15.4-1 and Table 12.2-1 for the basic systems. The R-values are different only for the "With permitted height increase" and "With unlimited height" cases. In your case, because you are Seismic Design Category B, you have the option of using R=3 from Table 12.2-1 or R=1 from Table 15.4-1 and avoid the seismic detailing requirements of AISC 341 in either case. Using R=3 is the obvious answer. Had you been in Seismic Design Category D or above, wanted to use an ordinary moment frame, and wished to avoid the seismic detailing requirements of AISC 341, Table 15.4-1 permits that option but 'punishes' you by requiring the use of R=1.

 

[ARS97]

I see it now. I typically deal with low seismic categories (A or B) so my options are usually limitless. In a case where I would have SDC D, I couldn't use a OMF for a building system.....I would have to use a special type of moment frame. However, in this same case, I could use a OMF for a nonbuilding, but if I wanted to still stay with AISC 360 detailing requirements, I'm "punished" by using a lower R value.

Sometimes it just takes another set of eyes......thanks!