Lu and Cb to use for lifted girders with overhangs 7

[AlanLord]

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?

 

[TehMightyEngineer]

This is one of those areas that is not covered by the AISC steel code. Assuming this a lifting beam is the beam loaded on the bottom flange and supported (hung) on the top flange? Are you designing the beam per ASME Below the hook standard for lifting beams?

Maine EIT, Civil/Structural.

 

[AlanLord]

I'm concerned about my girder, because it was lifted by a contractor - he lifted it using one crane using two cables without a lifting beam, and with no lateral restraint anywhere; the girder is 182' long, 10' depth; the load is self-weight exclusively, 900 Lb/ft unfactored

 

[Once20036]

Looks to me like the girder is in negative flexure for the entire length.
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.

 

[KootK]

If this is just a post-humus damage evaluation, I'd just look for evidence of local buckling and/or plastic deformation. It's hard to imagine there would have been enough load cycles to affect fatigue.

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.

 

[AlanLord]

Thanks guys, your comments much appreciated.

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.

 

[SAIL3]

I also would tend to use the full length for Lu and Cb=1 and also add an impact factor to the dead load. Depending on the angle of the cables there also will be an axial load in the girder between the pick points causing an initial moment based on the offset of the lift lug from the centroid of the girder and combined with the deflection may or may not cause a problem...

 

[AlanLord]

Yup, I included such a lift-point "pinching" effect for completeness, but it's not critical.

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' !

 

[AlanLord]

Here is a picture of the girder. Notice the splice smack in the middle between the lift points - with the flange forces reversed!

 

[KootK]

I suspect that the K=3 value may originate with this document from CISC: Link.

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.

 

[KootK]

Some poor bastard did a thesis on this and, by the sound of it, didn't get too far. I give you the Bitchcreek bridge: Link. Seriously. Apparently camber, sweep, rotational restraint, and a bunch of other stuff comes into play too.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[WillisV]

You might want to take a look at the software UT Austin developed for bridge girder lifting:

https://fsel.engr.utexas.edu/software/

 

[AlanLord]

Thanks guys. At least you guys have something down there. In Canada, we don't have much guidance. Our S6 code is basically copied off AASHTO!

 

[dhengr]

AlanLord:
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?

 

[Hokie93]

A spreadsheet developed by Dr. Cris Moen at Virginia Tech may be of interest in investigating lifting beams. The spreadsheet was mentioned in the article "Beam Deflections and Stresses During Lifting" by Plaut, Moen and Cojocaru, which was published in the fourth quarter 2012 issue of AISC Engineering Journal. The Engineering Journal article is available at

http://www.aisc.org

and is a free download for AISC members. The spreadsheet is available free of charge at

http://www.moen.cee.vt.edu.

Scroll to the bottom of the page and click on "Tools" to access the spreadsheet.

 

[AlanLord]

Again, thank you all for your wonderful comments. Attached is a good article on the subject of beam lifting.

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.