lateral beam bracing and anchorage

huangyhg

lateral beam bracing and anchorage
hey,
so i need to clear up some confusion here.
let's say we have a typical building roof.  it has main support girders, and purlins (or beams) that frame into the compression flange of the girders.  both the girders and beams are at the same top of steel elevation.  on top of the beams is a deck, which is tack welded to the beams below. let's say as well that the roof is braced with horizontal cross bracing members along appropriate lines.
so if i wanted to design the girders for simple bending, can i use the purlins as lateral support?  as far as i can see in what i read, i only can if the purlins are properly anchored.  
so, i am assuming then, that because the deck is only being tacked to the beams and girders, we can't really count on it as lateral support, and thus the lateral unsuported length of the girders is much greater than it appears, with it only being laterally supported at points where the bracing ties in.  and as for the purlins, it would be best to consider those laterally unsupported along it's entire length?

but the whole thing is "anchored" to a lateral system, right?  additionally, a simple shear connection from beam to girder typically has enough rotational strength/stiffness to function as a rotational restraint without the top flange translational bracing.
i am not sure what you are concerned about.  decks are never "tacked" to beams (purlins), but rather are screwed, puddle welded, or power fastened.  so in the system you describe, the beams and girders are fully braced.
daveatkins