Fixity of Braced Beam column Connection

[mfritze]

Hello,

If I am designing a diagonal brace for a corner connection with a beam shear connection to the column, how does the brace affect the end condition of the beam?

The brace gusset attaches the beam flange to the column flange --doesn't this fix the beam beyond simple shear and transfer beam moment into the column?

Thanks,

 

[jayrod12]

It does. However generally we ignore this fixity.

If you post a sketch showing proper beam, column and brace sizing, we could provide further input.

 

[BAretired]

What is the purpose of a diagonal brace if not to take moment?

BA

 

[txeng91]

I see where you are going with this, there is a connection from the beam flange to the column. However, I have never heard of anyone analyzing the beam/column connection as anything other than pinned, especially if there is only a gusset on the top or bottom side of the beam. In reality, it wouldn't induce any moment ont the column but there might be some additional axial load carried by the brace instead of the column, but the column should be a lot stiffer than the brace so I would think that assuming the column takes all of the gravity load would be ok, but if you wanted to be conservative you could design the bracing system to take a portion of the reaction at the end of the beam.

 

[BAretired]

I assume we are talking about a knee brace at 45[sup]o[/sup] which connects to the column a short distance below the underside of beam. If so, any compression in the brace puts moment in the column.

Alternatively, if we are talking about a brace which extends from underside of beam to bottom of column, that is a different kettle of fish. Perhaps a sketch is in order.

BA

 

[wannabeSE]

BAretired - I think the question is about the beam-column-brace connection for concentric braced frames.

mfritze - since the braced frame is stiffer than the "moment frame" created by the gusset at the beam-column connection, the load is assumed to go to the brace and a braced frame is analyzed with pinned connections. However, more attention to this connection is required for Special Concentric Braced Frames used in high seismic areas. Seismic drift can be very large and the beam-column-gusset must withstand large rotations.

 

[BAretired]

We seem to be talking at cross purposes on this thread. Going back to the original post:

If I am designing a diagonal brace for a corner connection with a beam shear connection to the column, how does the brace affect the end condition of the beam?

It restrains rotation of the beam.

The brace gusset attaches the beam flange to the column flange --doesn't this fix the beam beyond simple shear and transfer beam moment into the column?

Yes, it does, a substantial moment and it cannot be ignored. The connection at each end of the brace may be pinned, but axial force in the brace puts moment in the column.

BA