Crane Monorail Unbraced Length

[ARCADEng]

Does anyone know of research or code allowances to support increasing the Lc for a hanger supported monorail crane loaded from the bottom flange? The idea is that since supports are on top and load is on bottom, the beam would not really be in pure bending, moving the neutral axis up and reducing the compression felt. Unless, of course, this doesn't happen and the "roller" support model allows the top flange to go into compression just as though it were supported from below on a pin and roller.

Otherwise, can I reasonably say the unbraced length is from the point of application of the load to the nearest support?

Help is much appreciated.

 

[hokie66]

I don't think so. There is some research that shows that bottom flange loading tends to stabilise the top flange because the top flange has to follow the path taken by the load, but I don't know of a definitive reduction you can apply. Certainly, you can't take the unbraced length as the distance from load to support.

 

[csd72]

There will be a lateral component that will tend to destabilise the beam - take the full length.

 

[ARCADEng]

Thank you, I'll be taking the full length without reduction.

 

[jike]

I agree with the others.

Loading on the bottom flange tends to right the beam if the compression flange wants to buckle to the side. Whereas, loading on the top flange does just the opposite. There are some coefficients that someone has developed to take into account the position of load (ref. Salmon & Johnson) but they are not used in normal design practice. I also use the full length for unbraced length. Incidently, the neutral axis does not move.

An additional question might be how to handle lateral forces applied to the bottom flange. I have seen some engineers go thru a complicated torsional analysis for this but what happens when you get to the hanger?