Crane Runway: Lateral torsional buckling with bottom flange support

[wadavis]

With an underslung overhead crane:

Does a runway beam have any additional lateral stability from the bridge girder acting as a link to the lightly loaded opposite runway beam? Does this link provide any stability in addition to that already provided by the load being underslung (located below the beam center-line)?

My takeaway is that the link beam resists lateral movement of the runway beam, the underslung load resists torsion of the runway beam. The combined resistance should be some sort of cumulative. But I'm looking for the input of others here.

The material I'm using for underslung loads references Helwig (1997), anyone know if it has useful commentary on this question?

_{Helwig, T. A., Frank, K. H., and Yura, J. A. (1997)."Lateral-torsional Buckling of Singly Symmetric I-Beams." Journal of Structural Engineering 123(9), 1172-1179.}

Structural, Alberta

 

[Agent666]

Can you provide a sketch of the configuration of the 'link beam' and the loading scenario?

Generally if the link beam is an additional member tying the two runway girders together (i.e. not the actual crane assembly):-
To have any effect in improving lateral torsional buckling from flexural actions (i.e reduction of the effective length for flexural buckling) it must either prevent twist of the cross section,
or prevent lateral deflection of the critical flange (compression flange),
or prevent both of these effects.

This is essentially the definition of an effective restraint for flexure in many modern codes

Generally if a separate beam is slung between two parallel beams with the requirement that it is to improve the flexural capacity of the runway girders, then the separate link beam as you term it must prevent the twisting of the cross section (i.e. must be flexurally 'stiff' and have appropriate connections).
With no other external bracing/restraints the two girders can essentially deflect laterally together, so the only option for improvement is to provide restraint via preventing the twist of the girder cross section at the connection to the link beams. Note you can use adjacent members to provide the bracing via their minor axis stiffness, but in general you might need 20-30 times the stiffness of the original member about its minor axis to force a higher mode of buckling such that the connection to the other members acts as an effective restraint to the girder in question. In some cases you can justify the use of a channel top hat or similar to both provide bracing to the girders under purely vertical crane loads and to resist lateral loading from the crane assembly (for example crane surge loads when loads is being moved laterally).

Your code of choice can then be used to quantify the reduction, usually a factor on the length of the member for working out the capacity.