Seismic Response Modification Coefficient for Aluminum Structures 2

[brut3]

Is there a reference for various aluminum LFRS R Factors similar to what ASCE 7 gives in chapter 12? I have looked in the 2015 Aluminum Design Manual and didn't see it specifically addressed and my google search didn't turn up very much useful information. Any help would be greatly appreciated.

 

[JAE]

A summary of your question - but no definitive conclusion:

https://www.structuremag.org/?p=13567

A Q & A from the Aluminum Association referring to a FEMA P695 procedure to establish an R value:

https://www.aluminum.org/sites/default/files/Designing Aluminum Structures - ADM 2015 FAQs.pdf

 

[strucbells]

What type of structure are you working on? I've designed some aluminum framed stairs lately and used either Chapter 15 if they were freestanding: "All other self-supporting structures, tanks or vessels not covered above..." with an R = 1.25 and Omega of 2 or Chapter 13's "egress stairways" if they were on top of an existing structure.

 

[brut3]

They're freestanding aluminum canopies. They're very light weight and seismic generally will not govern the design. The LFRS will either be a ordinary cantilever column or ordinary moment frame type. I was getting hung up on the fact that both of these systems have limitations of use in SDC D and above. This generally isn't an issue due to the fact the majority of my clients projects have been located in relatively low seismic regions (SDC A - C); however, I could see this becoming an issue if they pick up work in regions of higher seismic activity.

 

[JAE]

You could/should try contacting the Aluminum Association and see if they have any specific resources to share (sell) you as this appears to be a topic that would fall on them to deal with.

All the ASCE 7 provisions don't say much of anything about aluminum.

 

[WARose]

Great input JAE.

Just to eliminate another place to check: I checked some older editions of the Aluminum Design Manual (by The Aluminum Association)....and this topic is not discussed.

Perhaps it would be wise (as JAE says) to contact the The Aluminum Association directly and ask about this. (Doing a search on their site.....I don't see any special publication dealing with it.)

 

[strucbells]

It seems to me that a reasonable approach (and what I've done on my recent projects) is to choose a conservative R value (1.0, 1.25) and take an approach similar to the OMF/OCCS detailing requirements in AISC 341 depending on which LFRS/connections you specify.

The low R shouldn't be too punitive since the structures are so light, and the detailing will provide a reasonable degree of ductility.

 

[EdStainless]

I am not a structural, but I assisted doing a failure analysis on an Al structure after and earthquake.
With Al the modulus is so low compared to other metals that the amount of motion in a quake is significant.
It whipped so much that it buckled.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy

 

[Aesur]

Take a look at: Aluminum.org Document

 

[strucbells]

Ed..that effect should theoretically be captured by P-big delta analysis.

 

[JAE]

Aesur - that was the second link I posted above.

 

[Aesur]

@Jae, whoops, didn't even notice. Thanks for pointing it out.

 

[brut3]

Thanks all, at this point I'm going to move forward using the an R = 1.25 and Omega of 2 from Chapter 15 like strucbells stated above. I'll reach out to the Aluminum Association to see if they could provide me with any additional references, or if nothing else, bring the topic to their attention. If I receive any useful information I'll be sure to update the thread.