beam lateral bracing 1

[haynewp]

Are there any cases where a metal deck being welded to the top flange of a beam be considered NOT good enough to provide continuous lateral support for this top flange? (example if it was a very long span?)Are there any additional erection notes others are adding to the drawings to be sure the deck is properly attached for the braced assumption?
Also, since stl jst manufacturers specify minimum bracing per joist type and span, does anyone know if that means they wouldn't consider the deck weld as bracing the top compression chord?
thanks

Will

 

[ishvaaag]

One can guess there are cases where your guesses come true. A ponding last span pulling the top chord is no good thing to see from LTB viewpoint. So in some sense, it is seen is the extant lateral stiffness (in the failing case above negative stiffenss since pulling) what matters. Maybe the manufactures specifying bracing is a neat way of getting out of these problems, and that would mean that effectively, for some cases they think (and have seen) the deck not being bracing enough.

Anyway, attached decks with equal spans at both sides won't displace (nor much must do end spans) in service or even strength level behaviour if not failed (when the secondary catenary load path develops). And continuous bracing is very effective to prevent buckling, since only small amounts of bracing is required.

Theodore V. Galambos Guide to Stability Design Criteria for metal structures 5th ed treats braced beams of your problem kind in section 13.5.3 at p.507. The question is that the answer there given makes the strength linked to one effective shear modulus of the deck, which is maybe tantamount to your won question.