Lateral Bracing of Steel Beams - Lateral-Torsional Buckling From Simple Bending

[krus1972]

In typical structural steel construction, shear connections are commonly used on the ends of beams. These shear connections are typically double (clip) angles welded to a beam and bolted to another beam. The distance to the first bolt, from the top of the beam, is usually 3" and additional bolts are usually spaced at 3" within the shear.

My question is, what is the maximum distance from the top of the beam to the first bolt can we go just before it no longer will brace the beam the bolt is connecting to? For example, if you have a W24 beam framing into another W24 beam and the first connection bolt is 10" down from the top, you cannot consider the top flange braced.

I would appreciate any help or any AISC reference that discusses the maximum distance from the top of the beam to the first bolt in order to brace the connecting beam against lateral-torsional buckling resulting from simple bending.

 

[KootK]

A common rule of thumb is to provide a connection that restrains at least 60% of the web depth. I don't recall if/where that lives in past/present versions of AISC 360.

 

[WARose]

I've heard everything from 50 to 75% (of web depth). But I think it's kind of a moot point in most circumstances: in order for a point to be braced it must restrained for lateral displacement or torsional (in the case of LTB). Torsionally, the average clip angle connection has no strength due to the web onf the beam/girder. (And in many cases, the web doesn't the distorsional stiffness anyway.) So you are looking at lateral restraint.

 

[slickdeals]

This is not a direct answer to the question, but AISC has a recommendation of 50% of the T-dimension to provide stability during erection. Refer to page 10-9 of AISC 14th edition.

 

[BridgeSmith]

AASHTO also sets a minimum depth of 50% of the web depth for diaphragms or cross frames bracing bridge girders. 75% is the preferred depth. However, for plate girders, AASHTO also requires the connection plates for the diaphragms be the full depth and be connected to the flanges.