Cold Formed Steel and Steel Systems Not Specifically Detailed for Seismic Resistance 2

[MattB123]

In ASCE 7-05 and 7-10 Table 12.2-1, I use "Steel Systems not Specifically Detailed for Seismic Resistance" with R-3 as my seismic force-resisting system whenever I can. My question is, does "steel" refer only to structural steel? Or, can cold formed steel (CFS) systems use it too? I have CFS walls with flat strap bracing resisting my lateral forces. This option is included within "Bearing Wall Systems" (although, not included in "Building Frame Systems"), but like I said I would prefer to use the system not specifically detailed...
I understand that sometimes "steel" can refer to either structural steel or CFS; sometimes it is meant to apply only to structural steel. How does it apply in this case?

Thank you,

--Matt B

 

[structSU10]

structural steel is governed by AISC 360, and cold form is governed by AISI.ASCE Table 12.2 does point you to section 14.1. From here, the section 14.1.2.2 exception under structural steel discusses the R=3 system and designing to AISC 360 instead of AISC 341. section 14.1.3 for cold formed steel refers to AISI S100 and such for design. The cold for section does not refer to an R=3 system at all - I would say you have to follow AISI provision for seismic design of cold form structures using the appropriate R value.

 

[sponton]

I assume it means both since the requirements to comply with are those of ASCE section 14.1 which include cold formed steel, just make sure it complies with what it says there.

 

[MattB123]

Thank you both for your responses! structSU10, the cold formed section does not refer to an R=3 system, but it does not specifically exclude it either...and this is the way the code it often written, it's very particular about inclusions and exclusions. The references AISC 360 and AISI S100 provide provisions for design and detailing, but ASCE 7 is used to gather the loads. And, that's what I'm trying to nail down here. As you pointed out, table 12.2-1 directs us to section 14.1, which talks about both structural steel and CFS. So, it seems that "steel systems not specifically detailed...." would apply to CFS system too. It sounds like sponton interprets it the same way. Help me understand if I am misinterpreting your response.

Thank you!

--Matt B

 

[structSU10]

I am saying cold formed is treated a little differently. I think the exception in 14.1.2.2.1 points to that by saying "...for systems designed and detailed in accordance to AISC 360 and need not be designed and detailed in accordance with AISC 341." AISC is strictly a hot rolled steel document. The code is vague, and within the section 'cold formed light frame construction' an R = 3 or less is mentioned in the commentary section C14.1.4.2, which state you must meet the requirements of S213, with certain exceptions if R = 3 or less. I am not well versed in R=3 or less within AISI S213, but I believe there are still special provisions you much meet.

I think AISI may be what holds the answer; it seems you can use R=3 but it may make more sense to use a higher R depending on detailing requirements. I have usually used the higher R allowed to reduce the seismic base shear.

 

[MattB123]

you make a good point about reducing the base shear with a higher R value. the trade off of course comes in the detailing. I will respond again if I find anything more definitive.

Thank you,

--Matt B

 

[TLHS]

A building using cold formed members for major framing is generally going to be so light that the R value isn't all that important. The base shear ends up being really small regardless. I probably wouldnt bother trying to up the R value if it required any extra engineering work to accomplish it.

 

[structSU10]

To clarify I looked at AISI S213 - it is similar to AISC with. For seismic design category A thru C, you can use an R = 3 and get away from special detailing considerations. (I found a commentary to clarify section C1.1)