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Unbraced length of temporary steel sitting on existing steel
2

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

Quote (canwesteng)

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?

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

(OP)
I'm thinking if I make a base wider than the beam is tall and add web stiffeners at the ends, that is enough to by inspection state it can't roll over at the ends.

RE: Unbraced length of temporary steel sitting on existing steel

I'd be inclined to agree.

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

Regarding that end detail, you might not even need to do anything special.

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

Neat. Thanks for sharing that Lomarandil. I figured that we were talking about no bottom flange connection here. A couple of clamps is nothing though if everything's spatially accessible.

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

(OP)
Good reference to have - unfortunately the steel this sits on is already covered in checker plate.

RE: Unbraced length of temporary steel sitting on existing steel

AISC under the general tab, (pg. 2-13 for AISC 360-05) talks about this. really you use appendix 6 checks to see if the web is stiff enough to restrain the beam (I think they give you the equation to use if I remember right), otherwise provide an endplate or bearing stiffener.

RE: Unbraced length of temporary steel sitting on existing steel

(OP)
Is that in the spec or the manual? I have the spec in front of me but can't seem to find it.

RE: Unbraced length of temporary steel sitting on existing steel

Its in the manual.

RE: Unbraced length of temporary steel sitting on existing steel

(OP)
Wish the CSA guys could offer us so much guidance... For those interested, the provision states that for beam ends supported on bearing plates, the beam can be built into concrete or masonary, the top flange can be stabilized, or end plate or transverse stiffeners extending to the top flange k-distant welded to the web and bottom flange can be provided. The beam is also to be anchored to the support. More or less what I had in mind anyway.

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