Bent beam flange repair
Bent beam flange repair
(OP)
I have a steel roof truss with top and bottom chord members fabricated from W12x96 shapes (A992 steel). After erection, but before roof decking was installed, there was a crane accident which resulted in the 1-ton overhaul ball swinging around like a yo-yo and impacting both the top and bottom chord flanges.
I have found quite a bit of info regarding heat straightening similar damage on a larger scale when it comes to bridge girders (bracing, jacking, and heating), but I'm not sure about the practicality of these methods on so shallow a member.
The worst top chord damage is a "dip" in one side of the top flange approximately 12" long and 3/4" to 1/2" deep. The worst bottom chord damage is also in the top flange and about 12" long, but it is almost 2-1/2" deep. I have attached photos of each.
Any advice or pointer to literature would be greatly appreciated.
Dan

I have found quite a bit of info regarding heat straightening similar damage on a larger scale when it comes to bridge girders (bracing, jacking, and heating), but I'm not sure about the practicality of these methods on so shallow a member.
The worst top chord damage is a "dip" in one side of the top flange approximately 12" long and 3/4" to 1/2" deep. The worst bottom chord damage is also in the top flange and about 12" long, but it is almost 2-1/2" deep. I have attached photos of each.
Any advice or pointer to literature would be greatly appreciated.
Dan







RE: Bent beam flange repair
That is, the web and lower flange are still straight and not visibly distorted in plan nor in section?
RE: Bent beam flange repair
RE: Bent beam flange repair
I'd also let the top chord go on account of the damage being too minor to worry about in my opinion. A reasonable check might be to see if the flange in bending could resist the upward and downward thrusts created by the kinks.
I do tend to be quite liberal when it comes to dinged up trusses.
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: Bent beam flange repair
With an orange heat, the steel upper flange will be soft enough to move with little resistance. BUT!!! the beam itself will be softened as well, and so you need to support it with something "cold" from below. A doubled WF under the bent upper beam could work.
A WF brace over the top of the upper flange might be better. Then, instead of "Pushing" the upper flange from below, you pull it up towards the brace WF that is above the upper flange with the equal of large C-clamps. A bit better control that way..
You really don't want to jack "up" the top flange by pushing down on the lower flange just at that one spot - that will get you a bent upper flange and a distorted lower flange.. And, you don't want to straighten the upper flange by all force resisted and "anchored" only with the structural bolts at both ends.
RE: Bent beam flange repair
I don't believe this top chord would straighten itself out under very many circumstances, but such things are possible in a plastic yielding state.
As others have said, be sure this is a problem before you try to fix it.
RE: Bent beam flange repair
RE: Bent beam flange repair
1) I can rarely envision consequences arising in pure tension members as I've mentioned above.
2) compression member damage that doesn't modify kL/r or alter the location of the centroidal axis location strikes me as minor in a member dominated by compression. If the top chord were a forklift damaged column in a warehouse, most engineers would pass it in my experience.
3) If margins are such that we can't handle a little ding in such a big thing, we're all doomed anyhow. It's a truss, not a piano.
4) I have a general sense that many "repairs" either cause more damage than good or, at best, are neutral and costly. I use them sparingly. CEL and I debate this philosophy from time to time as few share it.
These are all broad generalizations riddled with exceptions of course.
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: Bent beam flange repair
RE: Bent beam flange repair
RE: Bent beam flange repair
RE: Bent beam flange repair
RE: Bent beam flange repair
at a minimum bend (ie heat and hammer) it back to flat; preferribly, since all this plastic working will mess with the allowables, nest a plate/angle to relieve the flange some.
another day in paradise, or is paradise one day closer ?
RE: Bent beam flange repair
1.) I completely agree with you about the bottom flange, if it never goes into compression it should not be an issue.
2.) As for the top chord, if there was any distortion whatsoever in the member the P-Delta effects could be pretty significant. Especially if that member was not designed for flexure, and even if flexure was considered the bent flange will result in a reduced I and reduced moment capacity.
3.) Looking at the truss as a whole, if the impact twisted, displaced or distorted the truss in any way globally (not necessary damaging the members themselves), an eccentric load could also be generated that way, causing unwanted bending in the truss.
4.) The lack of redundancy in a truss also causes me to err on the side of caution when dealing with damaged truss members.
Like you said a lot of this is case dependent and there are numerous exceptions to each of these. However, at the very least I feel that when dealing with a damaged truss compression member, one should tread cautiously when looking to "call ok, leave as is"
RE: Bent beam flange repair
I'm likely desensitized by my previous experience in the wood truss industry. I once went to a site where most of my truss diagonal webs had been removed to make space for attic storage. When I inquired about that, the builder told me that he'd been doing that for twenty years and had it on good authority that the diagonal webs were "just for shipping".
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: Bent beam flange repair
I tend to side with Koot on this one if there is no net uplift on the truss.
Mike McCann, PE, SE (WA)
RE: Bent beam flange repair
RE: Bent beam flange repair
Regardless, you still need to be sure that there is a problem before you start looking for a fix.
The other fact that many engineers fail to understand is just how complex the actual state of stress of the steel in such a section is... If you do repair, don't worry about any residual stress and strains. Once the section goes plastic none of that matters, and the plastic strength is what you are designing to...
RE: Bent beam flange repair
A few helpful references from the guys/gals who gave me a hand:
http://www.fhwa.dot.gov/bridge/steel/
http://www.eng-tips.com/viewthread.cfm?qid=106922
And finally, my post:
http://www.eng-tips.com/viewthread.cfm?qid=379060
RE: Bent beam flange repair