Collector or SLRS?

[ChipB]

See the attached sketch.
The middle bay members are obviously to be designed using the 2.0 overstrength of collectors. My question is concerning the braced bays. There are varying opinions around the office that the struts are part of the SLRS and not collectors. Therefore they are not to be designed with the 2.0 overstrength.

My thoughts on it is that they are to be designed with the 2.0 overstrength.
The reasoning behind this thought is:
The connections are designed with the 2.0 overstrength factor to force the yield state of the bracing system and not the failure of the connection. If the strut were allowed to reach the yield state, it would create a K-Brace situation which is not allowed in seismic connections. Therefore, it should be designed as a collector.

Also, keep in mind, any seismic forces which are not taken out by the bracing system on one side, will be transferred across to the bracing system on the other side. In other words, if the forces at the joint don't add to zero, forces are being tranferred, therefore it is a collector.

What are your thoughts?

 

[JLNJ]

This is an elevation, not a plan, right?

It seems to me that if these members "drag" forces into your braced bays then they need to be designed with the overstrength factor.

 

[ChipB]

JLNJ,
Yes, you're looking at an elevation.
It's the struts in the braced bay that are in question whether they are collectors or part of the SLRS.
Thanks for your input,
Chip

 

[JAE]

ChipB - I'm not sure I understand the distinction you are trying to make between SLRS and collectors. If you look in the Glossary of the AISC Seismic Specification, the definition of SLRS includes collectors.

 

[ChipB]

Right. But the braces aren't designed for an overstrength factor of 2. Just their connections.

Collectors themselves are designed for an overstrength factor of 2.

 

[JAE]

Got it.

I would agree with you that they are collectors. The intent is that you have a relief valve for the forces. With the diagonal braces not designed with the overstrength factor - you have created the necessary "weak-plastic" link in the system. The horizontal struts are indeed collectors and do not have to be weak links since you already have them in the diagonal braces.

 

[haynewp]

I was looking at something similar recently, I say they are collectors. The load collects from one end of the horizontal to the end where the brace attaches. Here is some add'l commmentary I came across on the SFRS and collectors:

http://www.seaoc.org/bluebook/sample_pdfs/401010SFRS_smpl1.pdf

 

[youngstructural]

I'm glad I re-read your post; I thought you were arguing against applying the overstrength factor!

I'm not familiar with the terminology in use, but your rationale and theory are sound. For what it's worth, I agree as well: You must apply the overstrength factor to any load path in which you do not want a risk of plastic yielding.

Cheers,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

 

[ChipB]

Thanks guys!