Unbraced Lengths 3

[eit09]

What is the correct way to Specify Unbraced length in Risa 3D? Say we have a W12 beam spanning 12' with two C6's framing into the W12 flush with the top flange. (Local vert axis is y and local horizontal axis z and longitudinal axis x) The only nodes on the W12 are where the C6's frame in. In Risa I would leave Lb yy "blank", Lb zz "segment", Lcomp top "segment", Lcomp bot "blank". Also if we changed the channels to C12's I would change the blank input for Lcomp bot to segment also. I ran across the help section in Risa and it says Lcomp top = Lbyy for a member framing flush with the top flange of the member. Does anyone agree with the way I specify unbraced lengths or is the way the Risa info more correct?

 

[JoshPlumSE]

The Lbyy is the weak axis buckling of the member. It is NOT buckling in the local y direction.... rather it is buckling about the local yy axis. Therefore, this would either be manually set to the largest distance between changels or to segment.

Lbzz is the strong axis buckling of the member. This would probably be left blank (if you use C6's) as it should probably be the full length of the member.

Lcomp-top will default to Lbyy if it is not otherwise entered. But, it could be set to segment as well. If you have continuous diaphragm on the top of the member then you could make an argumen that this could actually be set to zero.

Lcomp-bot would probably be the full length of the member (for the C6's). But, if you use C12's then it could be set to segment as well. I should point out that for a simply supported beam, Lcomp-bottom should never affect the calculations as it only comes into place for negative bending.

 

[JAE]

I might add that Lbyy and Lbzz are used in compression/tension calculations (i.e. kl/r) for axial loads.

Lcomp-top and Lcomp-bott are used for flexural beam calculations (i.e. moment capacity).

 

[eit09]

JoshPlum,

So the Lbyy is buckling in the local z axis and the Lbzz would be buckling in the local y axis? ie.. Say I had a vertical brace at the center of a beam that is 12' long then the Lbzz would be 6, or what do you mean by local yy axis?

 

[JoshPlumSE]

For a wide flagne beam the local yy axis is the axis that is parallel to the web of the beam and id generally pointed upwards.

A moment ABOUT the local yy axis would produce weak axis bending in the wide flange. Therefore, euler buckling about the local yy axis would also be related to the weak axis moment of inertia.