Beam Rotation (Precast) 2

[astructurale]

I am designing a beam that will pick up (2) other beams. The supporting beam is pocketed for the (2) beams, so there is torsion induced in the beam. I have designed the section for the shear and torsion, and it is adequate. My question is how to I make sure that the beam will not rotate such that the (2) beams will have a tendency to slip off the supporting beam. I have placed a welded top connection for connecting the (2) beams to the supporting beams, but I just want to make sure there will not be a tendency for rotation that is not negligible.

I have discovered an equation for deflection due to torque
= tl^2/8GJt for "fixed both ends"(PCI Handbook 6th ed)
t=torque
l=span of beam
G=shear modulus
J=torsion constant

Is this the correct way of doing this?

 

[jike]

Can you create a moment connection at the end of the two supported beams such that the twisting angle of the supporting beam is limited by the end rotation of the other two beams?

 

[astructurale]

The opposite end of the 2 beams is supported by columns with a doweled connection, which allows for some beam end rotation.

The end at the supporting beam could likely be designed that way. I do not like welding the bottom of the beams and it is recommended practice not to do so. However, I believe a welded connection at the top and bottom of the supported beams to the supporting beam would be OK if I still allow the column end of the supported beams to be free to rotate.

So you are saying the I can use the supporting beams which are applying the torque as a means to also brace the supporting beam from rotation. Is that a correct description?

(This is similar to using "kick back" support (a steel brace) to tee stems for long spandrels to prevent out of plane rotation, right?)

 

[JAE]

jike's point is a good idea. Just remember to consider long term shrinkage and creep (longitudinally down the supported beams).

 

[jike]

I am not a precaster but I believe they use this technique to resist twist. You are using the moment capacity of the supported beams to resist the twist of the supporting beam. It is a self limiting rotation if the proper connection can be made.

I assume the rotation is only in one direction. If so, can you use a combination of grout on top and a weld plate on the bottom to resist the rotation? The erector may have to shore the beam or use wedges until the grout is cured.

I am not familiar with a "kick back" support.