Unbraced Length of Hanging Beam
Unbraced Length of Hanging Beam
(OP)
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.
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.






RE: Unbraced Length of Hanging Beam
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!
RE: Unbraced Length of Hanging Beam
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
RE: Unbraced Length of Hanging Beam
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.
RE: Unbraced Length of Hanging Beam
Thanks for the link to the video.
www.SlideRuleEra.net
www.VacuumTubeEra.net
RE: Unbraced Length of Hanging Beam
RE: Unbraced Length of Hanging Beam
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.
RE: Unbraced Length of Hanging Beam