Lu and Cb to use for lifted girders with overhangs
Lu and Cb to use for lifted girders with overhangs
(OP)
For a girder lifted by crane at 0.4L and 0.6L points, giving 0.4L cantilever overhangs, what unsupported lateral length should I use? A lot of people say 3 x the overhang length; is this correct? The girder is loaded only under its own self-weight.
For the same girder, is the moment magnifier factor to use Cb = 1.0 , or something else?
For the same girder, is the moment magnifier factor to use Cb = 1.0 , or something else?






RE: Lu and Cb to use for lifted girders with overhangs
Maine EIT, Civil/Structural.
RE: Lu and Cb to use for lifted girders with overhangs
RE: Lu and Cb to use for lifted girders with overhangs
Cb always = 1.0 for cantilevers that are not braced at the tip.
I'd check it based on the actual length (182 ft)
I don't think that I would factor the load because this is a very clearly defined load.
Assuming this beam is simply supported and looking at the "just built" condition, get a positive moment roughly 50% greater than the negative moment during lifting.
Are there intermediate supports in the final condition?
Is there some evidence that the girder was compromised by this lifting operation?
Was the contractor told he needed some specific rigging for the girder?
With the information available, I don't see much cause for concern.
RE: Lu and Cb to use for lifted girders with overhangs
It's a monstrous girder. When you lift these things, do you typically consider the lift points to be points of rotational restraint?
If the beam were lifted on the flat, is there any chance that it would remain elastic? Probably not, given that we're having this discussion.
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Lu and Cb to use for lifted girders with overhangs
Yes, the girder was in negative flexure its whole length; the girder flanges show no signs of distress, but it's just that it's unnerving to see theoretical overstress when checking AASHTO and Canadian code (I'm in Montreal) LTB provisions.
The girders however have layovers all over the place tho, up to 2 inches, I wonder if it was due to any LTB warping.
RE: Lu and Cb to use for lifted girders with overhangs
RE: Lu and Cb to use for lifted girders with overhangs
in the available literature on girder lifting (all American, by the way), I keep seeing Cb = 1.0, and Lu = Total L. And AASHTO load combo IV is used: 1.5D;
But some engineers up here in Canada - even the steel fabricators - told me to use Lu = 3 x the overhang, giving a longer Lu (218') than the total girder length of 182' !
RE: Lu and Cb to use for lifted girders with overhangs
RE: Lu and Cb to use for lifted girders with overhangs
Two things make it very difficult to know what is technically correct for the unbraced length here:
1) The rotational restraints provided by the lift points are very much non-linear rotational springs.
2) At the splices, you lose torsional stiffness, albeit very locally.
I'm surprised that there isn't some kind of accepted reference for this.
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Lu and Cb to use for lifted girders with overhangs
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Lu and Cb to use for lifted girders with overhangs
RE: Lu and Cb to use for lifted girders with overhangs
RE: Lu and Cb to use for lifted girders with overhangs
Probably, as good a reference text as any on this general subject is “Guide to Stability Design Criteria for Metal Structures,” Edited by T.V. Galambos. It also lists reams of reference materials at the end of each chapter for further reading. I don’t think I’d get all hung up in trying to find some steel design code methods, except as they relate to the general theory on the subject. LRFD, load factors and the like are too complicated by a bunch of extraneous b.s. which may or may not apply to your problem. Set your own factor of safety, and understand the general theory on the subject, and the general theory doesn’t change as it crosses the boarder.
RE: your 1DEC14, 14:06 post..., what does this statement mean? “The girders however have layovers all over the place tho, up to 2 inches, I wonder if it was due to any LTB warping.” Do you actually see any damage from the lift?
RE: Lu and Cb to use for lifted girders with overhangs
RE: Lu and Cb to use for lifted girders with overhangs
The statement “The girders have layovers all over the place" means that, say for five girders, each has different top and bottom flange lateral offsets at the supports, one beam's top flange may be offset 1.5" east, an adjacent one 2" west, etc, and the offsets are not consistent; basically the girders are not plumb in their theoretical axes; we're investigating fabrication and erection errors that may have contributed to this problem, in addition to skew effects; there is a 39° skew in tne bridge, which does not help.