NDS Effective Length for Cantilever Beam

[cal91]

If I have a beam, main span = 20', cantilever on only one side = 5' with a point load at the end of the cantilever... assume beam is weightless and no other loads are present. Bottom of beam is braced at supports only.

What is my governing unbraced length, lu ?

5' or 20'?

Also, does it not make a difference if there is continuous lateral restraint at the top of the cantilever section, since this is bracing the tension side?

 

[cal91]

I thought this would've been an easy question! No takers?

 

[bhiggins]

The bending moment is equal for both spans at the cantilever support. Therefore Lu will be the worst case scenario between the two spans which is 20'. Your effective length Le will be greater than 20' and will need to be multiplied by the coefficients in the NDS manual. The continuous top restraint has no affect on your bottom unbraced length.

 

[cal91]

Thanks, for confirming what I thought. The top restraint having no effect on the cantilever seems over conservative to me. But the code is the code...

 

[Eagleee]

ECCS rules for member stability have some guidelines for something similar to your situation I think, but then again you are using imperial units so I assume you are not using Eurocodes...if you can get a hand on the book though it may help with the overview. The picture below is from appendix D.7 in that publication...the factors shown in the picture are subsequently used to calculate the critical moment. I quote the paragraph in the book just above from where the graph is given: 'It should be especially mentioned, that there are
remarkable differences between the fixed cantilever and the cases where an adjacent single beam is present.'