Beam Design in Chevron Braced Frame

[bbartlet]

I am designing a two story ordinary braced frame in Iowa, that is governed by wind load. I remember seeing something about needing to design the beams in a chevron braced frame to be able to support their trib loads without using the chevron braces as a support point (effectively cutting the span in half). Is this only for seismically controlled SCBF's or is this for all chevron braced frames? Or can anyone point me in the direction of where I might find this information?

Thanks in advance.

 

[frv]

I'm not familiar with Iowa, but just because a design is governed by wind load does not mean you don't have to detail it for seismic.

If you are using an R>3, then it precisely because you are using seismic detailing that you can reduce your base shear (i.e., use a higher R).

Having said that, however, if you are in SDC A or are using an R=3, you do not have to detail it for seismic.

 

[bbartlet]

We are in SDC A. We do not typically have to detail for seismic.

My question is whether or not it is acceptable to design the chevron braces to also support the floor beam. I thought I had read that beams need to be designed to span the full length of the frame (in case the braces fail) but I have not been able to find where I may have seen it. I was hoping that if it is written somewhere that it was only a requirement for buildings in higher seismic design categories.

If it is acceptable, I'd like to use the chevron bracing to help support the floor because it greatly reduces the size of my beam (W16x36 vs. W24x76) in an area where space is already tight.

Has anyone else seen a requirement similar to this?

 

[frv]

Yes. The beam being designed to support the entire gravity load is only a seismic requirement (See AISC 341).

 

[JoshPlumSE]

The philosophy behind that requirement is that one of the braces will fail in compression first. It will stop accepting load, but as more lateral force (whether from wind or seismic) is applied the tension brace will actually start to pull down on the beam... possibly resulting in collapse.

Therefore, if you want your structure to remain standing in a post-event scenario your beams have to at least be capable of resisting the gravity loads applied to them.

This is a seismic specific design requirement. But, the BEHAVIOR behind the code requirement is independent of what type of loading you would have. I'm not saying that you have to design to this code provision... just that it is good to understand the seismic code requirement more fully before you dismiss it.

 

[haynewp]

I don't know why there is not more of a force support requirement from the beam because of that tension brace pulling on it post buckling of the compression brace.

 

[JoshPlumSE]

Haynewp -

Actually, there is a larger force requirement for that beam in the seismic provisions. In those provisions, you have to design the beam for the fulll weight of gravity load + the difference between the yield force of the tension brace and the post-buckling compression strength of the compression brace. So, the seismic guys are desiging for that tension brace pulling down on the beam.

I didn't mention that portion of the seismic requirements though. That's because I would never consider using that requirment for a Seismic Design Category A structure. But, the requirement for the beam to support the full gravity load is something I personally would enforce...

To me the gravity load provision makes sense for any structure. I've seen maintenance workers remove braces so that they could fit some equipment throught the frame or drive a forklift or such. You don't want the beam to collapse when they do something like that!!

 

[haynewp]

Makes sense, I didnt have the seismic spec with me to check for that requirement.

 

[InDepth]

Well, I think you are also missing the constructibility point...have the beam span full length so that the contractor doesn't have to shore or have to install the braces prior to placing deck and concrete. Plus, why add additional gravity load to your braces...they should be designed for wind only.....plus if they are damaged in a wind event they can be easily cut out a replaced without having to shore.
If you still want the W16x just add another beam adjacent to cut the gravity trib load.

 

[ToadJones]

I typically "deactivate" bracing when designing for gravity loads. It is very important to do so when designing columns in braced frames as the braces can attract load and you wind up with under-designed columns

 

[LonnieP]

Out in seismic land the structurals are now adding a modist steel column down to the foundation to resolve issues related to differential brace loads. They call this a "Zipper frame" because of it resembles a zipper. This makes the calc's much simpler, until the PHDs get involved anyway.

 

[SAIL3]

TJ..very good point

 

[ToadJones]

Lonnie-
Not sure I follow you, care to elaborate?

Sail-
This is another downfall of blindly using FEM software.

 

[sandman21]

A picture of a Zipper column can be found on page 6.1-180 AISC 341-05, during post elastic, brace yielding/brace buckling, there will be a downward load on the beam, in lieu of the beam taking the load a column is added, you can either develop the load into the stories above or take out the compression and the foundation.