huangyhg

r=3 and steel design
i have gotten into the habit of performing my lateral analysis of buildings with the response modification coefficient = 3, which falls under the "structural systems not specifically detailed for seismic resistance" of the ibc code (table 1617.6.2).
my question is, since i have to develop seismic and wind lateral forces and apply them to the the building per asce 7 guidelines, how does using r = 3 help me when i design a steel building.
i am having difficulty in remembering what using r=3 helps me to avoid in terms of the aisc specification, and i can not seem to find it within either the ibc or aisc codes.
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special seismic detailing is what you can avoid. with r=3 you design it just like you would design any other building
it gets you out of having to do all of the seismic detailing required for r>3. see the seismic provisions for structural steel buildings. we do the same thing to get out of the detailing.
i went back and dug into my design information and found what i was looking for. page 6.1-15 of the aisc seismic provisions for structural steel buildings states, "when the seismic resonse modification 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.
it further states, in the "user note" that designing with r=3 is generally restricted to sdc a, b, or c.
thanks for your responses.
yes, use r=3 whenever possible.
marinaman,
the trade off with a steel moment-resting frame system, which can have a r value of from 3 to 8 depending on how you design & detail the system, is to balance the increased seismic loads from using r=3 with the 2.67 times larger loads (8 divided by 3). the r=3 system require larger members and different connections than the r=8 system. the r=8 system may or may not require more engineering time and cost.
of course, if you practice in an area of the country that has a lower ss & s1 then the seismic loads might not govern the design so you are ok.

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