Torsion of top flange restrained monorail beam

[awiab]

What is the way to consider a torsion of monorail beam when the concentrated lateral load is applied to bottom flange and only the top flange is supported. The beam is for monorail so stiffeners can't be added.

 

[TehMightyEngineer]

Check the out-of-plane flexural strength of the web for the torsional load. I suspect you can at a minimum consider a length of web 2-times the beam depth to be engaged; if not a lot more. Check the bolts for this torsional load plus prying; or better yet make your monorail have thick enough flanges to avoid prying. This has the double benefit of helping to prevent flange bending issues under the monorail wheels. In many ways S-sections work better than W-sections for monorails due to the thicker flange root.

If there's any chance that the load will be deliberately swinging, lifted off center, or otherwise have deliberate lateral and torsional loads put on the monorail then I would consider a deeper beam so that you can use partial depth stiffeners. It doesn't take much for a millwrong to overload a monorail by side loading.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

http://www.americanconcrete.com

 

[Shu Jiang PEng SE]

Following is how to consider lateral load on crane or monorail beam specified in section 4.9.4 AISC7-10.

"The lateral force on crane runway beams with electrically powered trolleys shall be calculated as 20 percent of the sum of the rated capacity of the crane and the weight of the hoist and trolley. The lateral force shall be assumed to act horizontally at the traction surface of a runway beam, in either direction perpendicular to the beam, and shall be distributed with due regard to the lateral stiffness of the runway beam and supporting structure."

The strength of the monorail beam can be approximately checked this way

Mx/Sx+My/Sy,t <= allowable stress
Mx/Sx+My/Sy,c <= allowable stress
Mx is vertical bending moment
My is lateral bending moment
Sx, section modulus of the beam along x-x
Sy,t section modulus of the beam tension part along y-y
Sy,c section modulus of the beam compression part along y-y

 

[damorim]

I recommend you read the discussion in this previous post (link below). It goes over a very similar situation. Generally, the consensus seems to be that beams with bottom flange loading can be treated similarly to beams with shear centre loading. The loading on the bottom flange produces a stabilizing effect that acts to prevent LTB of the beam.

http://www.eng-tips.com/viewthread.cfm?qid=93822