Double channel with torsional restraints 2

[jdonville]

Situation: Back-to-back 10" channel sections are joined with spacer plates which are welded to the backs of the channels. The composite double-channel beam is simply supported, and is loaded at discrete points.

The point loads are such that the spacers need to act as torsional restraints so that yielding under flexure is the critical strength limit state.

The nearest restraints to one point load are located at 18 inches one direction and 12 inches the other.

How will the torsion be distributed to the restraints?

I think that the torsion will be distributed based on geometry - the closest restraint will get the most torsion - but I'm unsure as to the correct relation, and would appreciate some guidance.

 

[Ron]

The farther away from the supports, the more likely the torsional geometry will change. I agree that you can distribute based on basic geometry, just note that that torsion near the center will be greater (assuming your support condition includes the top and bottom of the beam).

How much space is between the channels? Proper welding would require a rather large gap.

 

[BAretired]

If the load is centrally applied between the channels, they each tend to rotate so that the top flanges come together and the bottom flanges separate. If each channel twists an angle theta, then the angle of twist per unit length on one side of the load is 18/theta and on the other, 12/theta and the torsional moment going to the restraints is proportional to the angle of twist per unit length, so I agree with you.

BA

 

[jdonville]

Ron and BARetd:

5-inch spacing between the channels - we have verified that the welds can be physically performed and inspected.

I dug into my 1st year mechanics text and confirmed the ratios - similar to the distribution of shear on a simply-supported beam from a concentrated load.

Thanks to you both.

J

 

[cntw1953]

If you have 5" space in between channels, you may want to use batten plates welded to the flanges instead of spacers. Just a thinking.

 

[jdonville]

cntw1953,

We are using 5"x9.5"x20mm plates as the spacers. The plates get fillet welded both sides to the backs of the channels.

J

 

[BAretired]

Another option is to use a 5" channel x 9.5" long welded on the heel and toe.

BA