seismic force-resisting system

huangyhg

seismic force-resisting system?
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,
use smallest r for conservative answer.
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.
see attached thread for a similiar discussion.
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?