HSS Columns in Moment Frames SDC - C

[profplan]

I'm designing a 2 story steel framed building with moment frames each direction on the east coast which could end up in either SDC C or D. The frames use HSS square 77" columns and wide flange beams for the frames. Assuming it is SDC C, would I be able to use R=3 and treat this as a non-seismic design application? Under what conditions?

If not, for R>3, would I be able to use HSS columns in the moment frames and satisfy the requirements of AISC 341?

 

[MrHershey]

ASCE 7 doesn't have any limits on R=3 steel systems in seismic design category C. So if you end up that way, you should be clear and don't need to worry about AISC 341.

If you get stuck in SDC D, then you'll probably end up as either an intermediate or special moment frame, based on your height. You may want to switch to wide flange columns in that case, as the options for prequalified moment frame connections under AISC 358 are pretty limited. Think there's just one that uses HSS columns and it's proprietary. You may be able to verify other options if there's been testing done that matches your scenario, but would check with AHJ on what their review process they'll want to do for that is (if any).

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[Shu Jiang PEng SE]

ASCE7-10 Table 12.2-1 specify that R=3 for "STEEL SYSTEMS NOT SPECIFICALLY DETAILED FOR SEISMIC RESISTANCE,EXCLUDING CANTILEVER COLUMN SYSTEMS", and it can be built without height limit for seismic design categories "B" and "C".

Section 14.1.2.2.1 in ASCE7-10 specifies that "The response modification coefficient, R, designated for “Steel systems not specifically detailed for seismic resistance, excluding cantilever column systems” in Table 12.2-1 shall be permitted for systems designed and detailed in accordance with AISC 360 and need not be designed and detailed in accordance with AISC 341.

Therefore you do not need to detailing the structure per AISC341 if the building is seismic design category "C" and response modification coefficient R=3 is chosen in your design.

 

[profplan]

MrHershey,

Thanks for the advice. Height of the building is typical 2 story, about 30 ft tall. I guess that does not help much to change the SDC?

 

[MrHershey]

No, SDC is based on ground motion and occupancy/risk category. Actual building properties don't really factor into it. Your building could be 300 ft tall and it would have the same SDC for a given ground motion and risk category.

If you're using light-frame construction and your dead loads are low enough you may be able to qualify for an exception that allows ordinary moment frames in SDC D up to 35 ft tall. Look at 12.2.5.7 in ASCE 7-05. Don't know your construction type, maybe you'll get lucky. I don't think I've personally ever been able to qualify for the exception.

 

[Shu Jiang PEng SE]

I agree that seismic design category is determined by site seismic data and occupancy importance. Once the seismic design category is decided per the mentioned two factors, the designer has to determine what type of structure (for example braced or moment frame. Each type of structure is subjected to the limitation of structure height and applicable design code per Table 12.2-1 in AISC 7.

Detailing the structure per AISC341 is not necessary if the building is seismic design category "C" and response modification coefficient R=3 designated for “Steel systems not specifically detailed for seismic resistance, excluding cantilever column systems” is chosen in design.

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For two story building with height 30 feet, the type of the structure can be either moment frame or braced frame if the seismic design category is C. You calculate the seismic loads using equivalent laters l load method per R=3 and combine seismic loads with the all other loads to analyze the structure, then check the strength of members and connections based on the calculated forces per AISC360.

 

[profplan]

Thank you all for the posts.

It is a steel framed building and I am planning to use steel moment frames in both directions for the LLRS however may try to supplement with braced cores at the building ends, if necessary.

The other building feature is a 2 story clear height garage attached to the 2 story building which complicates the system selection particularly for seismic. 2 story perimeter frames are possible but no internal frames for the clear height to roof. It is considered an emergency facility which I understand shifts the SDC to the next more stringent. What combination systems would be preferred or recommended - particular considerations for analysis/design?

I'll review again the geotechnical information to see the site class information. My understanding is it is often possible to lower the site class by performing additional investigation.