Chevron Brace

[MacGruber22]

Reviewing steel shop drawings and calcs and some of the braced frames have chevron braces. The CD's (not designed by my company) show typical details for their braced frame connections to be designed by the detailer. However, their chevron bracing connection (mid-span of beam) says that the joint shall be analyzed/designed using the Uniform Force Method. I don't think that the UFM rational procedure applied to this type of connection. I don't even know how they can attempt to design the joint with UFM, as the nomenclature applies to the beam-column interface.

What should I be watching out for when reviewing these calcs?

 

[WillisV]

http://www.aisc.org/designexamples

PDF page 685 - Design Example II.C-5.

 

[KootK]

I think that you could produce a UFM design if, and only if, you move the brace work point to the underside of the beam flange. That's unusual, however, and the beam would have to be designed to suit.

For the usual case where the work point is above the beam bottom flange, there will need to be in plane moment on the gusset plate in order for an FBD of the gusset to be in equilibrium. This leads you down the unpleasant road of having to assess combined stresses, plate buckling, and all the other voodoo that ensues. The best reference that I know of for this is the AISC Seismic Design Manual.

I'm not a steel connection expert by any means. Hopefully SteelPE, Ferrell, etc will catch wind of your post and chime in.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[JoshPlumSE]

Note:
The version of the AISC examples before Feb 2013 does NOT have the correct procedure for example II.C-5. The one WillisV linked to is fine. But, the older version of the examples is not quite right for chevron braces.

 

[KootK]

Can you elaborate on the problem Josh? I'm curious but I'll probably never get around to comparing the two documents.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[JoshPlumSE]

I don't remember all the differences. But, a few years ago we were working on getting Chevron Braces implemented into our RISAConnection program and found some issues with the older version of the example. We sent our concerns over to AISC's steel solution center and they indicated that they were reworking the example. They gave us a draft copy.

It was only when reviewing the link that WillisV posted that I realized AISC had updated that example.

The most noticeable difference is in the figures that show you sections a-a and b-b. If that is included on a single image then you are looking at the old / incorrect example. If you have two separate images showing you the forces for each section, then you are looking at the new / correct example.

 

[sbisteel]

The AISC example problems that were released during the 13th edition actually used the UFM. When they released the 14th edition they introduced this alternate analysis.

Has anyone ever seen a derivation of the Mprime moment (at the center of the gusset plate along line b-b)?

That little bit has mystified me since the day I first saw it.

 

[PAstructural08]

When I review shop drawings, I don't look and see how they designed the connection (that's the detailer's job, it's why he is getting paid). It doesn't matter to me what procedure they use. Items I look for are as follows:

1. Did they use the posted load on the brace which was on the structural drawing.
2. Did they use the right type of bolt (SC or bearing) and the right diameter.
3. Did they follow your general notes.
4. I sometimes do a quick check and make sure they have the right number of bolts.

If it's an unusual connection perhaps you had a detail on the structural drawing, did they follow that.

Keep in mind you are not checking the drawings; only reviewing.

 

[slickdeals]

AISC sent me Dr. Thornton's markups. See thread507-322006

 

[MacGruber22]

Good info, everyone.

PAstructural08: This was one of the "unusual" connections in addition to many severe copes, blocks, and extended shear plates, so I was instructed to go through the calcs in a more rigorous way for those conditions. As far as the detailer's engineer following the EOR's detail for the brace frame - that was the confusion I ran into. The EOR's detail for the chevron brace connection called for analysis/design based on the UFM, and showed the work point, vectors, and nomenclature that applies to the UFM. I was looking to check that the detailer's engineer followed the EOR details, but I don't think they should have in this particular case. In the end, the detailer's engineer didn't use UFM, but another rational approach. The good thing is that the horizontal brace beam was over-sized for gravity loads, so additional connection moment was a non-issue. Also: I get various options from engineers regarding the depth of calc review they perform - some take a few more steps into the calc details to sleep better at night. We did find improper calculations of extended shear plates by going through the calcs, so I don't think it is unusual to take the extra care when warranted.