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SCBF - Intersection of Bracing

SCBF - Intersection of Bracing

(OP)
Working on a SCBF and looking for a little guidance with the intersection of the bracing. Working on an X-braced configuration, and at the intersection of the braces one will be discontinuous. Trying to work out the details of the intersection, one is the thru-plate going the continuous brace, should it be welded to the brace? I was concerned of a weld transmitting load through the walls of the brace and not through the plate. Also, should the plates and welds be designed for the loads indicated in F2.6C of the provisions? In the provisions it seems like they are talking specifically about brace-to-column/beam connections.

RE: SCBF - Intersection of Bracing

I'm a big fan of avoiding this problem entirely by having the x-brace intersection at an intermediate floor:

http://www.pacosteel.com/wp-content/uploads/2016/0...

However, if it can't be avoided the prevailing detail appears to be weld the knife plate to the opposite brace tube.

https://www.researchgate.net/profile/Dawn_Lehman/p...

https://i.pinimg.com/originals/48/69/44/48694491e6...

Have not run through such a brace connection design, so I'd have to dig into the code to see what's required. I imagine the post-buckled load case is going to be a huge driver of the design forces on this connection. Also, I would suspect that overstrength is required as failure of this weld would cause a change in the assumed load path and the buckling capacity of the braces.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries
https://www.facebook.com/AmericanConcrete/

RE: SCBF - Intersection of Bracing

I would suggest not providing the X-brace unless you simply have no other way. The connection should be treated as a brace to column connection with some changes. You will need to provide cover plates to make up for the reduced area of the tubes and plate should be designed for the forces of F2.6c.

RE: SCBF - Intersection of Bracing

Agree with TME and sandman about switching to chevron bracing if possible. It is generally cheaper since you have 3 connections per bay rather than 5, especially if you can use a two-story X to mitigate the unbalanced brace force on the floor beam. Some tests have shown that ductility of the x-braced system is also less than other systems due to the tendency of inelastic buckling to concentrate in one half of the discontinuous brace. If you do use X-bracing, I recommend using two gusset plates that sandwich the joint rather than a single plate slotted through the continuous brace. This benefits the economy and performance of the connection in a few ways:

1. It eliminates the slots in both the continuous and discontinuous braces.
2. It eliminates the need to reinforce the braces for net section fracture since the slots are eliminated.
3. It allows you to maintain flexural continuity of the discontinuous brace across the joint, which improves the ability of tension brace to keep the compression brace from buckling out-of-plane. When designing the brace, use half the brace length with K=1.0 for initial sizing and K=0.7 for upper bound compression strength. The shorter unbraced length allows you to use smaller, more compact braces that better dissipate energy in an earthquake.

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