OMF in Seismic Cat D AISC 341

[captb]

Hi, I have the situation of designing an OMF in a seismic category D. The OMF will only be supporting a DL=15psf and a reduceable RLL=20psf. I would like to avoid designing per AISC 341 so I would like to design as a "Steel system no specifically detailed for seismic resistance" with an R=3. The section of the code that allows me to use this system is ASCE 7-05 Section 12.2.5.7. However, it reads,

"steel ordinary moment frames in structures assigned to Seismic Design Category D or E NOT MEETING the limitations set forth in section 12.2.5.6 are permitted within light-frame construction up to a height of 35ft..."

the problem is that because of the light DL and LL i meet the limitations of section 12.2.5.6 which will put me back at an R=3.5 which will still require me to design for AISC 341. My question is, am i understanding correctly that having a DL and LL <20psf requires higher seismic design considerations than one with DL>20psf but <35psf? or am i reading it incorrectly and I have the option of designing for an R=3 or 3.5 as long as I meet the 35ft requirement?

Thanks in advance.

 

[DST148]

Ref ASCE 7-05: Per Table 12.2-1, Steel systems not specifically detailed for seismic resistance are not permitted in SDC D, E, and F.
Per section 14.1.3 - Steel structures assigned to SDC D, E, and F shall be designed and detailed in accordance with AISC 341 for structural steel and AISI Lateral for light-framed steel construction.
OMF is not permitted in SDC D. However, there are exceptions and they are listed in 12.2.5.6 and 12.2.5.7. Even if you meet the criterion for the exceptions, you would still need to comply with AISC 341.

 

[captb]

I understand that we must meet the detailing requirements of AISC 341 regardless, but do we have to design using it when the R=3?

 

[DST148]

For OMF ( or any structural steel LFRS), R = 3, SDC - D, provisions of AISC 341 would apply.