How far below/above the compression flange is a lateral restraint effective?

[BendingMoment123]

Hey guys

So for example, if I have a 6m I-beam (beam 1) with a depth of 200mm (not including the flange thickness here) and I connect an adjacent beam (beam 2) with a full depth of 175mm to the center of the web of beam 1 at mid span, how effective would beam 2 be in reducing the laterally unsupported length of the I-beam?

Thanks

 

[human909]

Pictures please! If a picture is "worth a thousand words" you are only giving us 6% of that picture. Please give us more than that.

 

[BendingMoment123]

 

[1503-44]

I would not consider any material outside the flange itself. First, any web moment of inertia contributing to lateral dimensional stiffness is very, very small number. Secondly, flange buckling at the farthest distance from the web would occur well before any web stiffness ever started to mobilize. Since buckling is a rapid process, by the time any web stiffness is called upon to resist and reach the lateral brace at mid web depth, the game is effectively over. Once buckling has started, it would be very difficult to stop by any method. You'd only be limiting damages at best.

If you make the connection through a full depth stiffener plate, then fully effective.

 

[KootK]

For many conventional shear connections, beam 2 would be considered to provide torsional bracing to beam 1. As such, it's unimportant how far beam 2 is below beam 1. The effectiveness off such bracing can be checked using appendix 6 of the AISC steel construction manual for US practice.

For true lateral bracing (not torsional) of a simple span beam, the bracing will become less effective the further it is placed from the compression flange. The rate at which that effectiveness diminishes will be a function of the slenderness of the beam being braced.

 

[countervail]

There is a document I think called "Common cases of restraint" by the IStructE which gives guidance on when LTB restraints become ineffective there is also plenty of guidance in various bridge loading codes (where deeper beams with dropped shallow beams are very common)