Torsion in Underhung Crane Runway

[chrisrosebud2001]

I am designing an underhung crane and currently have a cross section consisting of a built up wide flange and channel cap system. The channel has been provided for bending in the out of plane direction. However, I am running to the issue of resolving the torsional forces. The torsional stress could be potentially very high on this section and there doesn't really seem to be a good way to resolve the torsion as I am unable to attach anything to the bottom flange. Does anyone have any advice. Also, I have assumed that the torsional properties are similar to that of a wide flange for example J = sum bt^3/3. Would the torsional properties be much differen for this section?

 

[ToadJones]

How are the runway beams supported?

 

[ToadJones]

problem with assuming that the Channel takes out of plane bending is that since it is on the top flange and the side thrust forces are all the way down on the bottom, the channel really doesn't help.

I generally assume that the bottom flange has to take all of the side thrust loading and ignore torsion. Basically the same assumption made for top running crane girder design.

Be sure also to check the combined stress in the bottom flange from the local wheel loads. CMAA has a set of equations for this.

The reason I ask about how the beams are supported is that generally underhung beams are hung from hangers that allow the beams to sway somewhat. This reduces the torsional stresses.

 

[chrisrosebud2001]

The crane is supported by hangers, but there are also kickers at each hanger support as well.

 

[ToadJones]

Usually those kickers are only to brace the the hanger itself, not the bottom flange. That being true the bottom flange can still sway.

 

[chrisrosebud2001]

After looking further, I believe I have overestimated my lateral forces as well. I was referring to AISC design guide #7 which really only applies to rail supported cranes. This crane is supported on the bottom flange similar to a monorail. However, I am having trouble finding the design side thrust for this situation. Any good references? CMAA 74 reccommends approximately 5% of the wheel load, but I believe this is also for a rail supported crane.

 

[charliealphabravo]

could we get a sketch of the section?

there is a method in AISC for calculating the torsional properties of built up sections.

 

[ishvaaag]

Not very promising the article, 20% of load

http://www.integritycrane.com/docs/Design_20of_20Monorail_20Systems.pdf

A wise to the matter opinion...

http://www.peo.on.ca/DIMENSIONS/septoct2004/Letters.pdf

leaves it a 10% (but no code)

If I find something normative will add later.

 

[ishvaaag]

More info from a previous thread here

http://www.eng-tips.com/viewthread.cfm?qid=44818