Pin-pin-free cantilever unbraced length

[sccci]

I am designing a beam to be suspended from two parallel girders. The beam will be attached to the bottom flange of the girders and the free end will cantilever out to support two beams. The suspended beam will be attached to the girders by the top flange. The beams supported by the cantilever will be pinned connections resting on the top flange.


Girder Girder
_____ _____
| |
Load Load | |
| | | |
| | __|__ __|___
__\|/_______\|/_________\/_________\/__



I am having trouble deciding the correct unbraced length to use for designing the cantilever beam. If it were possible, the bottom flange of the cantlevered beam would not be supported along its entire length. Worst case scenario, the unbraced length would be the distance from left load to the right support. Can the unbraced length be considered from the left load to the left support? Any input would be appreciated.

 

[JAE]

If you can install vertical stiffner plates on the web of the supported cantilevered beam, at the two girder connection points, an detail it such that the rotation of the supported beam is restricted by the stiffner plates, then you can count on them as brace points for your bottom, compression flange.

If the bottom flange must be free and clear, say for a monorail hoist or something, then I would say that your full length is the unbraced length to use.

 

[ECSEng]

Depending on the rotational fixity at the supports and whether the whether it is the tension or compression flange that is attached to the support, the effective unsupported length gets very high. There is a reference in one of the british design manuals (which I can't recall the title of at this time) that gives unsupported lengths for the various support conditions of cantilever beams. If I recal correctly, the maximum kl is something like 4 times the length of the cantilever, particularly if the compression flange is not supported. At a minimum, you should probably be using k = 2.

 

[archeng59]

third edition LRFD, spec section C3.4a indicates that bracing shall be attached to the compression flange of a beam, except for a cantilevered member. for a cantilever, a lateral brace is required only at the end of the member near the top flange, which is in tension. I believe Galambos' text, Guide to Stability Design Criteria for Metal Structures, also has information about cantilever beams.

When I design cantilever beams, I assume the unbraced length is equal to the length of the cantilever even if the top flange is braced by a slab or roof deck.

as far as using the stiffeners for lateral bracing, AISC FAQ's has some discussion about lateral bracing and the use of stiffeners to prevent section rotation. here's the link, if it works:

http://www.aisc.org/Template.cfm?Se...tDisplay.cfm&InterestCategoryID=402&FAQID=904

 

[miecz]

Agree with JAE that, if you stiffen the web at the connection points, you may use the unbraced length as the length of your cantilever. I believe then, that Cb cannot be taken greater than 1.0.