Seismic Force-Resisting System?

[cdi12]

I am doing ASCE 7-05 seismic calculations of cooling tower support steel structure (see attachment) and I am not sure what seismic force-resisting system it is. Ordinary steel concentrically braced frame (R=3.25) or Special steel concentrically braced frame (R=6) or Steel system not specifically detailed for seismic resistance (R=3)? Seismic design category is “D”. Thank you,

 

[civilperson]

Use smallest R for conservative answer.

 

[ARLORD]

Steel systems not specifically detailed for seismic resistance is Not Permitted for Cat D.

Looks like you have some sort of an eccentric braced frame. Since you are Cat. D you should be using AISC 341, see ASCE 7-05, Section 14.1.3. I would look through AISC 341 and modify your bracing to match one of their braced systems, and design accordingly.

 

[SteelPE]

See attached thread for a similiar discussion.

http://www.eng-tips.com/viewthread.cfm?qid=228959&page=8

 

[cdi12]

Thank you all for valuable comments.

According to AISC 341-05 Commentary C11.1 OMF Knee-Brace Systems – I can design tower framing in one direction as ordinary moment frames with R=3.5. How about other direction? Any suggestions?