Flitch Plate - Plate Size

[bookowski]

Typically flitch plate beams that I see have the steel plate slightly smaller than the adjacent wood members, what is the reason for this? Is it for shrinkage? With this configuration you need to transfer all the shear back into the wood at the beam ends, why not instead keep the bottom of plate flush with bottom of wood and get direct force transfer from both (i'm assuming a bearing condition).

 

[MiketheEngineer]

You can. And many do.

Its just that many wood joists are marginally correct in size and/or "crown". I think the milling tolerances allowed are +/- 1/8'' - maybe 1/4''. I have seen over 1/4'' variations in joists SIDE BY SIDE. Why do you think TrusJoist is selling the heck out of the I-Joists. They are dead nuts on and carpenters, builders, home owners LOVE them. I even know a few builders who have switched to TimberStrand 2x4's in their kitchens and baths BECAUSE hanging drywall, cabinets, sinks, showers is so much easier - and very few call backs!! And the price is not that bad!!

 

[msquared48]

Depending on the framing conditions at the column, presence of a transverse beam, etc., you may not be able to bear the flitch plate on the column.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[bookowski]

Thanks. I am using LVL's, it sounds like there is no reason (at least in my case) to not keep b.o.plate flush with b.o.wood.

 

[hokie66]

If you are bearing on wood at the end, you probably still need to transfer the shear back into the wood side members, as the width of the steel will typically not be enough.

 

[dhengr]

Bookowski:
It seems to me that there are about seven (+/- a dozen) variables which enter into your question. Wood shrinkage and size variability are two of them; then you have the drilling, bolting and fit-up issues, plus slippage as bolts come into bearing on the steel and wood; finally, if the plate extends beyond the bearing edge of the wood for any reason, it acts more like a knife edge than a narrow, stronger, bearing surface. Most of the time, in this application, you are trying to make a wooden beam span, carry load and deflect better; you are working with wood framing and the beam bearings are not the real problem, but you make them work too, maybe an added stud under each end; and this generally involves compression parallel or perpendicular to the grain, which the small steel edge just overpowers. That’s some sort of a bastard bearing joint where the steel crushes the wood, either parallel to (on a col.) or perpendicular to (on a beam or sill pl.) the grain, and then the wooden side members pick up the rest, and this adds to total settlement in the joint. I’m sure you can find exceptions to the above: the knife edge (flitch pl.) on a steel bearing plate on conc. blk. or poured conc., but then you have a pretty tippy beam; maybe steel flitch pl. to steel beam or column. But, basically you are trying to strengthen and stiffen the middle 2/3rds of a wooden beam which should actually be deeper to span and carry the loads. Otherwise, you want to hide the steel within the wood so the carpenters can frame around it. You are correct that LVL’s shrink less across the grain, and they do have higher compressive strength perpendicular to the grain, but you’ll still get fit-up movement in the system.

 

[jdgengineer]

I typically add a steel bearing plate that is welded to the flitch plate at the bearing ends, rather than rely on transfer the load back to the wood for bearing. To me it seems like a more direct load path. Essentially, then the wood beams are there only to stabilize the steel plate from buckling, rather than using them structurally. Maybe this is conservative, but I haven't had any push back from a contractor on it.