Unbraced Length of a Cantilever
Unbraced Length of a Cantilever
(OP)
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?
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?






RE: Unbraced Length of a Cantilever
RE: Unbraced Length of a Cantilever
AISC design or some other specification?
And unbraced length in regards to strength correct?
If so - the length of the cantilever is the unbraced length. Recommendations are to brace the tension side of the cantilever end to avoid twisting. Cb = 1.0 (for AISC specification).
RE: Unbraced Length of a Cantilever
RE: Unbraced Length of a Cantilever
RE: Unbraced Length of a Cantilever
ht
RE: Unbraced Length of a Cantilever
I believe this came out of AISC commentary.
RE: Unbraced Length of a Cantilever
hokie66, that was an interesting link to the MSC article.
RE: Unbraced Length of a Cantilever
I attach the Mathcad 2000 Professional worksheet.
By the way and what read I seem to have found 6th guide better as a collection of references than as a collection of procedures, at least comparatively with previous guides. This quite likely will render the book less useful to get direct results for those not having the resources to delve in the references themselves.
RE: Unbraced Length of a Cantilever
RE: Unbraced Length of a Cantilever
However, for the unsupported length, it depends on the lateral support mechanism spacing, if any, for both flanges, not just the compression flange, for the tension side can roll over too, even if the compression flange is braced.
Mike McCann
MMC Engineering
http://mmcengineering.tripod.com
RE: Unbraced Length of a Cantilever
But for strength I've never heard of using 2L for Lb.
RE: Unbraced Length of a Cantilever
If the moment varies from zero at the end to a maximum at the support, L may be used because the average moment is less than or equal to half the maximum moment.
If the support is not completely fixed about both axes, this simplification is not justifiable and the equivalent length should be greater than L.
BA
RE: Unbraced Length of a Cantilever
RE: Unbraced Length of a Cantilever
RE: Unbraced Length of a Cantilever
The length of beam between cantilever support and end doesn't "know" that it is a cantilever but rather just a segment of beam with braces at each end.
I still don't see using 2L. It is not in the AISC specification that I can see and I've never seen it in journals or texts.
RE: Unbraced Length of a Cantilever
Also, there is some discussion about this relating Cb on page 16.1-271 at the top (black book).
RE: Unbraced Length of a Cantilever
RE: Unbraced Length of a Cantilever
These are based upon a W10x26 beam with 8, 12, and 16 ft. spans.
Using the Kc method from Modern Steel Construction, the values of Mn for the beam were determined based upon the method given with Cb = 1.0 per the article.
For the Mn values derived strictly from the AISC specification (without the Kc method) I used Lb = span of cantilever and the Cb values were calculated and included (the spec says to use Cb = 1.0 for cantilevers where the free end is unbraced.
I only looked at conditions where the support was laterally braced as well as the tip as most all cases we brace members that way.
From this, the values are about the same except when the span gets a longer (12 - 16 ft). A 16 ft. cantilever is quite long for a W10 but could happen I guess.
For the other cases where the support is not totally braced or the tip is not totally braced, the Kc method really punishes you.
I also don't think this Kc method includes the fact that in many cases the top flange of the cantilever is tied into the slab system and thus the connection helps resist - to a point - rotational twisting of the beam. We usually don't consider that anyway but it does help.
RE: Unbraced Length of a Cantilever
RE: Unbraced Length of a Cantilever
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.
RE: Unbraced Length of a Cantilever
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
RE: Unbraced Length of a Cantilever
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.
RE: Unbraced Length of a Cantilever
RE: Unbraced Length of a Cantilever
RE: Unbraced Length of a Cantilever
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.