Reinforcing an existing LVL-Flitch Beam
Reinforcing an existing LVL-Flitch Beam
(OP)
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
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






RE: Reinforcing an existing LVL-Flitch Beam
Weld a plate to the bottom and angles to the top, effectively creating an I beam (built-up girder) Ditch the LVLs (I would be surprised if they were continuous anyways)
Pull it all out and replace with a steel beam
Pull out the LVLs one at a time and replace them with C12s, steel plates or MC 12s or similar.
Of course, brace the beam before you do anything drastic.
RE: Reinforcing an existing LVL-Flitch Beam
Or you could build a pair of steel trusses in place using angles for chords and flats for web members. The bottom chord could turn inward but the top chord would project outward.
BA
RE: Reinforcing an existing LVL-Flitch Beam
RE: Reinforcing an existing LVL-Flitch Beam
RE: Reinforcing an existing LVL-Flitch Beam
RE: Reinforcing an existing LVL-Flitch Beam
Just to make it easier on the workers who have to install this. I am just thinking about how I would do it if in my house. A channel is a much more efficient bending shape than a plate, and surely an appropriately sized channel would do the job. Another advantage of a channel is that its shear centre is outside the web, so eccentricity should not be an issue.
Probably the best reason to use a member on one side only is to avoid the difficulty of matching holes on both sides. The holes in steel side members would be punched in the shop, and it is very difficult to align two pieces and drill the holes perfectly in this arrangement.
RE: Reinforcing an existing LVL-Flitch Beam
RE: Reinforcing an existing LVL-Flitch Beam
Mike McCann
MMC Engineering
RE: Reinforcing an existing LVL-Flitch Beam
RE: Reinforcing an existing LVL-Flitch Beam
RE: Reinforcing an existing LVL-Flitch Beam
RE: Reinforcing an existing LVL-Flitch Beam
RE: Reinforcing an existing LVL-Flitch Beam
RE: Reinforcing an existing LVL-Flitch Beam
Many shops use magnetic drill presses to drill the holes so they could just stack the plates and get them to align.
I agree. Drilling thru 1/4" plate is easy with decent, sharp bits. Get a Drill doctor to keep it sharp.
RE: Reinforcing an existing LVL-Flitch Beam
There is no problem in matching the holes in two pieces of steel in the shop. The difficulty I was pointing out is that it is very difficult on site to maintain the drill alignment perfectly so as to hit the holes on the opposite side when there is a member sandwiched between. For example, drilling holes in posts which are set in a stirrup is tricky, usually resulting in a bit of reaming and drilling from both sides.
RE: Reinforcing an existing LVL-Flitch Beam
Too many people try to make things more complicated than they are.
RE: Reinforcing an existing LVL-Flitch Beam
RE: Reinforcing an existing LVL-Flitch Beam
RE: Reinforcing an existing LVL-Flitch Beam
RE: Reinforcing an existing LVL-Flitch Beam
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.
RE: Reinforcing an existing LVL-Flitch 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
RE: Reinforcing an existing LVL-Flitch Beam
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.
RE: Reinforcing an existing LVL-Flitch Beam
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.