LATERAL BRACING

[TA]

IS A STEEL BEAM CONSIDERED LATERAL BRACED IF JOISTS ARE PERPENDICULAR TO THE STEEL BEAM(FLUSH FRAME) AND ATTACHED TO CONTINOUS WOOD BLOCKING WHICH IS SECURE TO THE WEBB? OR MUST THE STEEL BEAM BE BRACED AT THE TOP FLANGE BY SECURING WOOD NAILER TO FLANGE AND PLYWOOD SUBFLOOR ATTACHED?

 

[Ron]

While it can be done I do not like to use wood as lateral bracing, unless the beam is lightly loaded and the wood joists are spaced no more than 16 inches, o.c.

The compressibility of wood, coupled with the inability to affect a proper tensile connection (from the opposite side) limits its effectiveness in preventing flange buckling.

 

[JAE]

Ron,
I see the point about the condition where joists are only coming in from one side. Unless there is a positive connection between the joists and steel beam, the beam could conceivably buckle away from the joists.

But if the joists are on both sides, each providing a positive compressive restraint against sidesway translation, I would think you could consider the beam braced. The small amount of compressibility wouldn't seem to adversely affect the bracing.

If the beam was not heavily loaded, the lateral force, and stiffness required for lateral stability wouldn't be that significant.

What do you think?

We have typically used wood joists as bracing, but admittedly, we usually have the wood on top of the beam.

 

[Ron]

JAE,
I agree with you, just that conditions have to be right for wood to be effective bracing. In most cases it works fine, since as you say, the lateral load to prevent buckling is usually small (less than 500 lbf).

You mentioned the key to getting it to work correctly...make sure both sides in compression at start. Often, this is not done and the result is too much movement capability on one side of the beam.

Ron

 

[FrankHartzell]

I forget the details but many years ago (maybe 30-35 years) there was a catastrophic steel formwork failure somewhere in the northwest USA or Canada. A very complete report of the failure investigation was published. As I recall, one of the main culprits was web buckling of grillage beams which depended on some kind of wood blocking between the beams for web stiffeners. The investigator's analysis proved that the stiffness of the wood blocking was insufficient to prevent the webs from buckling.

I would want to check the joists and joist-steel beam connection details for adequate stiffness, e.g, "Bracing for Stability", Yura & Helweg.

 

[JAE]

It would make sense, bussh, that web buckling wouldn't be resisted properly by wood blocking....the lateral force on a web that buckles would be fairly high. But, web buckling is a local effect, producing high forces locally. Lateral torsional buckling is a "macro" effect which, I believe, wood joists, perpendicular to the beam, could probably take care of.