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Steel SMRF Beam Bracing

Steel SMRF Beam Bracing

(OP)
I recently looked into lateral bracing of steel beams. Per AISC, I need to brace both top and bottom flanges.

I wanted to check the stiffness of just using stiffener plates to brace the bottom flange when the top flange is fully braced. See attached.

When I check it this way, the stiffness is unreasonably low.

With h = 12", I'm getting stiffness on the order of 100 kips/in. With h = 24", my stiffness is about 200 kips/in.

I would guess my assumptions about the fixed base are wrong, but this doesn't seem too unreasonable to analyze it this way. What do you all think?

RE: Steel SMRF Beam Bracing

There may be an error in your stiffness calculation. Assuming that it is the same plates, one cantilevering 12" and one 24". It is not reasonable to have the 24" cantilever with a higher stiffness. That said, I have a couple other comments.

1. The 2010 edition of AISC 341 changed bracing requirements and torsional bracing is acceptable for SMRFs. You may want to use consider torsional rather than lateral bracing. ( I assume the stiffness is for lateral bracing since the units are kips/in instead if kip-in/rad)

2. I would measure h from the centroid of the bolt group to the centroid of the beam bottom flange (unless you are somehow trying to include some contribution from the plywood deck)

3. It may be difficult to justify the stiffness and strength of the connection to the wood.

RE: Steel SMRF Beam Bracing

(OP)
Ack sorry the first one should be 1000 kips/in (with h = 12").

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