Reinforcing an existing LVL-Flitch Beam 1

[sanford1]

Hello,
I have an existing 25' beam consisting of (2) 1.75”x11.25” LVL engineered wood beams with a ¼” steel flitch plate sandwiched between the LVLs. The beam has a few (basically none) small carriage bolts holding it together. The beam has been in place for approx. 10 years and is starting to show signs of rotation / torsional buckling and moderate deflection. Calcs are showing that two 3/8 plates would need to be added and throughly bolted, however drilling the existing beam would be labor intensive. Widening the beam a great deal will be difficult due to an existing door and window on each side of the support column. Adding a mid-span column isn't an option.

Does anyone have any suggestions on alternative designs to tackle this problem?

Thanks

 

[dhengr]

Connect2:
You play this problem by eye and ear, it takes a fair amount of engineering judgement. And, your example, an 8" x 16" beam 60+ years old is a pretty extreme example. In any case, you do the jacking slowly and in steps, letting things rest and adjust for a day btwn. each step. Look and listen around and above the beam for cracking and the like. The whole structure above the beam has to reacclimate to the upward (new) beam position. Sht.Rck., plaster, and the like will creep with the beam over time, but crack due to jacking which is too quick. I would generally try to get all the deflection out of the old beam, or without a good explanation of what’s happening, I’ll be blamed for not fixing the problem. I usually try to jack the existing beam something above zero delta, because the whole thing will still settle again, before it brings the bolts/bolt holes into play and really starts to share the load with the new reinforcements.

 

[MikeHalloran]

Has anybody wrapped a beam?

By which I mean, jack the camber out as discussed, then tightly wrap the existing wood/flitch/wood beam with metal sheets, e.g. galvanized steel, then wrap that with Band-It or similar stainless strapping bands every few inches.

( An inexpensive manual tensioning tool can pull really hard on the straps. Once all the straps are nicely tight, the same tool cuts them off cleanly, and you close the retaining tabs on the coupling clip with a hammer. It can be made to look pretty neat.)

No drilling, no aligning of holes, no bolts to tighten, no loss of section in the wood.

You could also trap additional steel flitch plates outside the wood and inside the galvanized wrapping, adding extra uh, flitch, without drilling.

Something to think about.

No, I haven't done it.
I got to thinking about it after reading an account of a historic court battle between auto body builders championing all-wood vs all-steel bodies. Both parties agreed that the strongest possible auto body would comprise a hardwood frame tightly wrapped with formed steel sheet. Both parties neglected to mention to the court that nobody had ever actually built an entire body that way, but lab tests of representative prototype sections produced impressive results. ... i.e., the wood added a tiny bit of composite action to the steel, but more important, kept the steel from buckling.






Mike Halloran
Pembroke Pines, FL, USA

 

[connect2]

dhengr,
Well actually that type of creep deflection is not as odd to come across as you might think. We do alot of historic timber structures, mostly churches. we have alot of engineering logic defying pictures. It would take about as long to get the deflection out as it took to get it, but regardless you can never re-orientate the wood cellulose fibers back into the original position. Once it's gone past it's elastic limit it's done. We run into alot of people who feel that just 'plating' the existing deflected beam is sufficient, with LVL's to, soooo... Generally we find we have to remove the beam and replace, or put a new beam just above it, and also we have to search through the structure above to see where the load has gone and what's happened to the joinery and structure above. Timber beams and strengthening and stiffening with new steel plates, aka 'Flitch" beams is trickey and deflected shape is one of the first things you want to consider before heading down that road.

Mike - Carbon Fiber wrapping is not so far out as you think, although I have no knowledge of it and timber.

 

[dhengr]

Connect2:
Sounds like you are using exactly the type of engineering judgement and experience I was thinking of. And, I wasn’t ignoring timber framing, I kinda suspected that an 8x16 beam might be in that category. You’ll get no argument from me about what you said in your last post.