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Knee Brace forces from gravity loads

Knee Brace forces from gravity loads

(OP)

I have a four story steel moment frame and want to use knee braces for seismic resistance.
The project is in Seismic Zone B, and will be detailed per AICS 360
The knee braces will be at all beam column connections, and some of those beams are long and carry a lot of vertical load.
The work points of the knee braces are about 3 feet

My issue is,
by static analysis adding knee braces will in effect make the beam-column connection fixed.
A moment of roughly M = w * L*L / 12 would have to be resolved by the brace and it's connections.
This results in my brace carrying a force due to gravity loads that is 3 times more than the force attributed to the seismic load.

So does the knee brace have to be designed for the gravity loads that make a "fixed" connection, or are knee bracing only intended to be designed for lateral loads.
Any Code or paper reference would be appreciated.

RE: Knee Brace forces from gravity loads

Depends on when the knee braces are installed relative to the dead loads. Sometimes you can dictate to install them after some of the dead loads are in place.

But once in place - any and all subsequent loading will affect the knee braces as they exist, have stiffness and are part of the structural framing.

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RE: Knee Brace forces from gravity loads

Knee braces, as JAE noted, depend on when they are installed, as well as what they are intended to do.

The use can be two-fold. One, for lateral restraint, obviously, but the second is to reduce the effective gravity span of the beam it frames to, allowing a smaller member to be used.

Mike McCann, PE, SE (WA)


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