Unbraced length of temporary steel sitting on existing steel
Unbraced length of temporary steel sitting on existing steel
(OP)
I'm designing some temporary steel to sit on a floor during installation of some equipment. I've got a frame of W12s (see attached sketch). I'm wondering what to use for the unbraced length of the temporary beam that supports the other temporary beam. Conservatively, I would think some detail at the ends preventing the beam from rolling over would enough to get away with using the whole length of the beam. Not sure if I can count on the other temporary beams to brace it though - clearly they don't offer lateral restraint but would they offer torsional restraint?






RE: Unbraced length of temporary steel sitting on existing steel
Absolutely. For LTB, once you have rotational restraint, you're good to go. You could use the spacing between infill beams as your unbraced length with the exception of the ends. If you provide restraint then, again, you're golden. If you don't, you'll need to treat the end spans as torsionally cantilevered. I suspect that just gravity weight will give you the restraint that you need at the end. but it's tough to prove that robustly (or without burning an afternoon's worth of fee).
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Unbraced length of temporary steel sitting on existing steel
RE: Unbraced length of temporary steel sitting on existing steel
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Unbraced length of temporary steel sitting on existing steel
Using an old rule of thumb from British Standard 5950 (and referenced in an AISC erection engineering webinar), when you have a rolled shape beam cribbed up on another beam (bottom flange laterally restrained, but the top flange free without stiffeners), you can use Lb'=Lb+2d. This modified length is considered adequate to account for the flexibility of the web and the imperfect restraint at the top flange.
Then just make sure your bottom flange connections have some capacity to count as a torsional brace point. A lot of times, this only requires a couple of C-clamp.
RE: Unbraced length of temporary steel sitting on existing steel
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Unbraced length of temporary steel sitting on existing steel
RE: Unbraced length of temporary steel sitting on existing steel
RE: Unbraced length of temporary steel sitting on existing steel
RE: Unbraced length of temporary steel sitting on existing steel
RE: Unbraced length of temporary steel sitting on existing steel