Unbraced Length of a Cantilever 1

[Soundwallguy]

Hi All:

Been a long time since I had to calculate Unbraced Length of a Cantilever in bending. Some literature says to multiple the length of the cantilever by two, others say just use the length of the cantilever. Any concensus?

 

[JAE]

Sorry about the bad calc. Attached (I think) is a more correct calculation. This includes three methods:

Method 1 - use AISC provisions without the Kc factor and with a calculated Cb applied.

Method 2 - use AISC provisions without the Kc factor and with Cb = 1.0.

Method 3 - use Kc method with Cb = 1.0.

 

[BAretired]

JAE:

BAretired - I'm not trying to argue with you but if the end of the cantilever is braced against rotation (which it usually is for my structures) I still can't see using 2L.

I agree with you. If the both ends of the cantilever are laterally braced top and bottom, the effective or unbraced length is L.

My earlier comment was referring to a cantilever with no brace at the free end. Like you, I don't like it, but some engineers do it anyway.

BA

 

[hokie66]

I agree that it is not a worry if the end of the cantilever is well braced, but the unbraced condition is what you have in the case of a cantilevered monorail beam.

Another problem is architects. A current fad is for some architects to want to build cantilevered awnings with tapered tee sections, finishing at the tip with little depth. As wind load can bend the members either way (not to mention snow load for some of you cold climate folks), these things are a real challenge in negotiation.

 

[Sufjan]

hokie, this is exactly what I was dealing with last week. A 12' "blade" supporting a glass roof canopy and mega snow loads. The vertical dimension at the tip of the blade is a mere 3", depth at the support is 16". No bracing of the entire length. Fixed connection at the support.

 

[JAE]

Well, with a monorail you have bottom flange loading and the load tends to correct any LTB. The article you linked to, hokie66, shows very small Kc values for that condition so you get much higher moment capacities with bottom flange loading.

 

[hokie66]

Sufjan,
I sympathize. So far, I have been able to convince the architects that a flange is necessary for most of the length. Some engineers seem to deal with this by using thick plates for the web, but I see some along the street that baffle me as to how somebody "got it to work", if indeed they did.