## Unbraced Length, Lb, of bottom flange in compression....

## Unbraced Length, Lb, of bottom flange in compression....

(OP)

Say I have a cantilevered W-section beam with a backspan of 25 feet and the cantilever extending 8 feet out over a column support. There is uniform loading (due to concrete slab, etc.) along the entire length of the beam, and there is a substantial concentrated load at the end of the cantilever. Obviously the top flange of the beam is continuously laterally braced by the metal deck/concrete slab (Lb=0), but what is the unbraced length of the bottom flange if it is in compression due to negative moment over the column? Is it the distance from the column support (which braces the bottom flange through the cap plate connection and beams framing in perpendicular at this location) to the inflection point in the backspan, where moment changes from negative back to positive?

Thanks for any feedback.

Thanks for any feedback.

## RE: Unbraced Length, Lb, of bottom flange in compression....

If the bottom flange is unbraced for the whole cantilever length then Lb could be twice the cantilever, i.e. 16 ft.

However, any stiffening running from the slab connection to the bottom flange would cut this down to the distance between stiffeners.

Good luck, Helmut

http://www.engs-comp.com/

## RE: Unbraced Length, Lb, of bottom flange in compression....

## RE: Unbraced Length, Lb, of bottom flange in compression....

The second design is in the span: check the negative moment at the support with unbraced length = the span (25 feet) and Cb calculated per AISC method. Also check positive moment (if applicable) with Lb = 0 and Cb = either calculated or 1.0.

## RE: Unbraced Length, Lb, of bottom flange in compression....

Guide to Stability Design Criteria for Metal Structures, Vth ed., Theodore V. Galambos, John Wiley and SOns, New York, 1998, at page 207.

If

the root of the cantilever is continuous

and

the tip of the cantilever is torsionaly restrained by bracing both top and bottom flanges

then the effetive length to be considered for the cantilever is

1.5·L if the load falls in top flange

0.8·L if the load falls in bottom flange.

So there being explicit precision in one competent technical reference like this I would go for this approach, in spite of my feeling it being clearly overconservative, at least for most cases.

For my entertainment I once did a Quattro sheet covering the lateral bracing of the bottom flange by the mere vertical stiffness of the part of the web protruding from one partly embedding floor assumed to deliver full fixity at the base of the protruding part, but I don't locate it now. When found I might made it available to you for comment. Hope I will post something about this.

## RE: Unbraced Length, Lb, of bottom flange in compression....

I will send it free to whomever asks it via e-mail, for whatever the use, study, comment or whatever you may want do with it.

er, as long you are not many in which case would devise a site for download.

Write me to

mom@arrakis.es