Crane Runway Lateral Bracing Requirements

[levey12]

I am in the preliminary design phase for a 50 ton crane system with crane runway beam spans varying from 35ft to a max. of 82ft. I would appreciate any advice for the lateral bracing requirements for both the top and bottom flanges of the crane runway beams. The rail beams are simple span.

For the compression flange it seems that lateral bracing should be provided to account for the effects of LTB. However, I have some questions pertaining to the bottom flange bracing requirements.

Per AIST Technical Report No. 13, there is a requirement for all crane runway girders with spans of 36ft and over in building classifications A, B, and C or runway girder spans of 40ft in class D buildings to have bottom flange bracing. Is this requirement included to address web sidesway buckling? If the design of the crane runway beams includes the proper checks (serviceability, biaxially bending of top flange, web yielding, web crippling, sidesway buckling, etc.) does the bottom flange still have to be braced? I know that this is a requirement per AIST Tech. Report No. 13 and not for some other codes, including Canadian standards.

Thank you in advance for any helpful information

 

[JLNJ]

When the lateral forces are applied at the top of the crane rail there is an overturning force on the crane girder. It seems to me that the bottom flange bears part of this force to right the overturning. The force may not be big, but the bottom flange is often small and will see a large deflection under lateral load for longer spans. Combine this with some mis-alignment eccentricity and you might have a significant bottom flange horizontal load.

Most people I run into seem to ignore this, apply the lateral load at the top flange of the girder, and call it good.

 

[ToadJones]

Bottom flange bracing helps this. If you brace the top and bottom to a back up truss, you essentially have a box section (space frame) that is very torsionally rigid.