Unbraced Length of Hanging Beam 7

[amcraig729]

What is the unbraced length of a continuous steel I-beam that is picked from the top flange at 2 points?

I have a 200ft long bridge girder that will be picked by 2 cranes using beam clamps to the top flange, each crane will support the girder 40' from an end so there is a 120' center span. We are currently debating what Lb should be used in the LTB equations.

Some say distance between inflection points (90')
Some say distance from crane supports (120')
Some say full length of beam (200')

I tend to agree with 120' with the appropriate Cb factor. It seems from other forums that the rule of thumb for inflection points has been proven incorrect. The distance between supports feels right, but nothing is actually bracing the beam at this location? I usually use the full length for these type lifts to be conservative, but that will not work here. I need to be more correct to prevent an overly conservative answer.

 

[straub46]

Todd Helwig from UT actually did a presentation on Cb factors at the 2016 steel conference, and one of the situations that was examined was lifting a bridge girder that is suspended at 2 points from a crane. The recommendation is to use the full length of the beam for Lb, but to adjust Cb based off where the support points are. For your case, the Cb factor suggested from studies is 2.0

Here's a link to the proceedings, the session was N49. The content on lifting girders starts around 46:00.

https://www.aisc.org/content.aspx?id=42100

Go Bucks!

 

[maxdistortion]

I'm a structural eng but work as a mechanical (all my experience is mechanical).

Although I can't help, I'll share a story. In my city, a contractor set a bridge girder which bowed elastically as they picked it. Once they set it, it would seem that the girder became constrained at the ends, and because it was bowed, once it subdued to its self weight and could not expand axially it buckled, the girder had been braced to adjacent girders, and the force caused those to buckle too. In their case the unrestrained length seemed to have less to do than the elastic deformation and the way they addressed it. They lost their shirts.

Thanks @straub46 for the link, I'll watch it during lunch

 

[Lomarandil]

The short answer is that I haven't found a short answer. (note, I haven't watched Helwig's presentation yet.. maybe he has it)

Kitipornchai, Yura and Ricker have all weighed in with some suggestions, but all of their methods have limitations. You can always do a finite element buckling analysis (AISC's erection engineering webinar had a good example of this recently), if the application merits it.

There are also a few good threads here on Eng-tips that discuss the topic. Here is one:

http://www.eng-tips.com/viewthread.cfm?qid=387114

Our company deals with this from time to time. Our conclusion right now is that the distance between lifting points is unconservative (because of the lack of true restraint), and that taking the full length of the beam might still be overly simplistic. As mentioned in that other thread, inflection points just don't check out. Thankfully, our cases have had enough margin that we haven't had to sharpen the pencil much more than that.

 

[SlideRuleEra]

I watched the excellent video with interest. Discussion of location for the pickup points was especially notable to me. As a 1970's bridge contractor, I had to decide where to attach slings for beam placement. After consideration, I made an educated guess that the best place was at the two points that produced minimum bending moment in the suspended beam (about 21% of the beam's length from each end). From the video, looks like those points are as good as any... and better than most. Surprised the author of the video did not mention the significance of those points.

Thanks for the link to the video.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[BUGGAR]

I can understand why you would want to know if you are guilty.

 

[WARose]

Be advised that the normal LTB buckling allowables are not applicable because the support conditions for suspended cables are not what they are based on.

Two excellent papers that deal with this are:

'Distortion Buckling of Steel Beams', Structural Engineering Report No. 185, University of Alberta Department of Civil Engineering, By: Essa & Kennedy, April 1993.

'Buckling of Suspended I-Beams', ASCE Journal of Structural Engineering, Vol.116, No. 7, July, 1990, by: Dux & Kitipornchai.


IIRC, they address a suspended beam with a cable scenario like the one you describe.

 

[Lomarandil]

Thanks WARose.. that was the Kitipornchai article I was thinking of, but didn't have my reference list handy. I'll have to check out the Essa and Kennedy paper.