R=3 and Steel Design

[marinaman]

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.

 

[slickdeals]

special seismic detailing is what you can avoid. With R=3 you design it just like you would design any other building

 

[Lion06]

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.

 

[marinaman]

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.

 

[vandede427]

yes, use R=3 whenever possible.

 

[OldPaperMaker]

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.