Bottom Flange Restraints for Seismic Loads

[structuralecstasy]

Hello all,

I am working on a steel structure in a high-seismic zone area, and the question arose about restraining the bottom flange for lateral-torsional buckling since the vertical seismic loads are large and reversible. As a general rule of thumb at my company, we use 3" as a maximum difference in TOS (or BOS) for the shorter beam to laterally brace the corresponding flange of the beam it frames into.

My question does not directly have to do with that rule of thumb, but rather if anyone has an opinion on whether the compression flange of the beam could actually buckle during a seismic event. It seems to me that the extremely short load durations at the peak response of the structure would not be long enough to cause the compression flange to buckle, especially since the load will alternate between the positive and negative moment several times during the event. Is compression flange buckling a sudden catastrophic event, or does it need some time to deform and twist the member first?

My current design has top flange restraints (same TOS) and bottom flange restraints where the incident beams are 3" or less deeper than the collector beam. I have a few cases where the bottom flanges are 4" or 5" apart and I will have a better peace of mind justifying that the bottom flange of the deeper beam is braced by the shorter member if there is a small likelihood of the compression flange actually buckling during a seismic event.

Thanks for your help!

 

[JAE]

I guess I don't see the concern for bottom flange bracing against seismic loads. First, the vertical seismic load shouldn't be THAT large or even enough to overcome the dead load (but maybe I'm wrong).

Even if it does overcome the dead load and cause compression in the bottom flange, your connecting beams should be adequate to brace the collecting beam against LTB. Yura (at UT Austin) has shown that rotational restraint is almost as effective as flange transitional bracing for LTB. And the bracing force/stiffness isn't that large anyway that your gravity connecitons would be adequate.

 

[ishvaaag]

In reality this thing is calculable, I think precisely by method proposed by Yura, for it all amounts to the required bracing of a compression flange loaded to some load level, which may be furnished through restraint to rotation.

The general geometry in the area won't initially be as deformed as to not be feasible to think the concrete parts in the floor be able to provide the (almost) fixity required to provide the rotational restraint, and all is then a matter of if the web as a cantilever is strong enough as to provide -in concurrence of the other standing forces- the required strength and stiffness to act as an effective stiffener for the standing compressive force in the bottom flange.

 

[unclesyd]

JAE,
Depending on your location, the vertical loadings probably should be considered to be large. Check the report on the Northridge, California Earthquake. Though I'm not a structural engineer,I remember seeing some heavy damage in the photographs and discussions of same caused by vertical motion. I think they had a large movements in both the horizontal and vertical direction. I don’t think anyone had considered the vertical movement would be as large as it was. If I understood it right, the movement was not the rolling wave but a vertical uplift. I imagine that California codes have put a little more in their structural codes concerning vertical movement.
I believe the report was put out by the AWS. I was looking at some very bad weld failures in the connections. There were some connections that you could see apparent reversals at the connection in the vertical direction.