Lateral Force Resisting System - Braced frame or drag strut?

[Daniel-PRE]

Okay everybody, I did a quick search and couldn't find anything on it in the forums. If you have a high roof and a low roof, can you call the low roof X-brace the braced frame designed to SCBF and call the high roof diagonal just a drag strut / collector designed to the over-strength factor? I believe I've done this for small changes in height, but this is basically a one to one where the low roof is at 20' and the high roof is at 40'.

Thanks everyone!

 

[Luceid]

Feels more like a braced frame at each story with a horizontal discontinuity - and probably simpler that way? Would you rather design that upper diagonal to overstrength or as a braced frame itself?

 

[JAE]

I'd be tempted to do this and have two separate braces.

 

[driftLimiter]

Seems like an in-plane offset vertical irregularity to me. Interesting though what is the natural height of the building? How many stories? tough questions.

Prudent designer would have 1 lateral system for the tall portion and another for the lower portion.

 

[JAE]

I like what you suggested driftlimiter - a high-low roof system in high seismic certainly suggests an expansion joint and two separate buildings.

 

[Daniel-PRE]

This is a one story building with two roof heights; the high roof on this side of the building can't have a full height brace from high roof to floor because of architectural needs, so the load needs to transfer to the low roof.

JAE, you recommend bracing at "each story," but if it isn't a floor that the bottom of the brace comes to, this would be more like a multi-story braced frame, right? Can you do a MT-SCBF without a brace at the lower level or would that be disallowed?

Luceid, designing the upper brace as a drag strut and just putting overstrength on it would be a lot easier, because there are less requirements on the detailing for collectors than there are for SCBF elements. HOWEVER, is that right to do that? It's dragging diagonally, not in plane with the diaphragm it's supposed to be collecting from.

Edit: The height of the main building is 20' and the height of the high roof is about 35'.

 

[WesternJeb]

I would consider this a drag strut with a vertical irregularity.

It sounds like you need to put your foot down and request more framing, to me.

 

[Daniel-PRE]

WesternJeb, drag struts wouldn't usually play into if there is a horizontal or vertical irregularity, right? Isn't it usually dependent on the seismic resisting elements of the specified system? I would assume a vertical in-plane offset if it were a high brace offset from a low brace, but if it's just a drag it's not "really" a brace.

 

[WesternJeb]

Forget the semantics and think about how your system will act. To follow that, it will act as you detail it.

You have a high roof with a high diaphragm load. If the member you design is rigid as all get out, then it will act more of a collector/transfer member into your ductile system. But that top has to be STRONG to get those forces down, similar to how we design penthouses.

If you design this member to yield and help dissipate energy as you transfer it into your larger system, then it will be included as part of your SCBF.

 

[Daniel-PRE]

WesternJeb, I guess I just get hung up on using the diagonal brace as just a drag strut designed to over-strength because that would be like designing to an R of 1 for the brace which isn't allowed in buildings. In some cases, you can go down to an R of 1.25, but that's limited by SDC and wouldn't be allowed for SDC D and up. If this is a building, not a non-building structure, they don't want you to use such a low R and that's what we'd effectively be doing in this case.

I understand the general idea of following stiffness and creating the system that does the dissipation, but I think there are reasons why these are limited by the code. It would be so much easier if the architect would just give a little wall to carry this down... May have to go fight for that.

 

[WesternJeb]

It sounds like you need to put your foot down and request more framing, to me.

I agree with everything you have said. I just don't see how you can create a ductile system with what your current snips shows which is why I was trying to reason with your drag strut idea. See above quote.