Steel flitch plate with pressure treated wood beam 1

[Ben29]

I am working with a contractor who installed a beam that is not structurally adequate for the loading. (long story) The contractor asked me to provide a structural design to reinforce the existing pressure treated 2-ply 2x12 beam. He wants me to provide a design for a beam repair whereby a steel plate is bolted to the outside face of the beam (like a flitch beam). Due to the corrosive nature of pressure treating, I am concerned if this is possible. If I specified a galvanized steel plate, do you think that would work?

 

[phamENG]

You need to check the treatment specs and find out what chemical was used if you want to assess corrosion risk. Generally speaking, though, galvanizing will do the job.

If you're putting this in a place where the wood needed to be treated, the plate should probably be galvanized anyway.

 

[jayrod12]

The preserved wood foundation manual indicates that fasteners used in their framing should be stainless steel or hot-dipped galvanized.

So I'm on board with galvanized, assuming it is hot-dipped, and also assuming that all cuts/holes are made in the plate prior to dipping. All bolts shall be similarly stainless steel or hot-dipped.

 

[Ben29]

pharm: The structure is an enclosed space that is elevated ~4ft above the ground and is open below. The contractor took it upon himself to design the floor framing (long story). He provided PT floor framing throughout.

Thank you both for the response. I will move forward with the HDG plate.

 

[phamENG]

No problem, Bern. Glad we could help.

 

[XR250]

Honestly, i have seen plenty of 20 year old exterior decks with painted flitch beams and only minor corrosion. This may be due to the less corrosive nature of old-school treated wood though.

 

[phamENG]

XR - probably. The switch to ACQ resulted in a lot of premature hardware failures.

 

[jayrod12]

I have seen what the new preservative can do to improperly treated fasteners, it's scary. The steel plate itself likely has enough mass to it that you could add a bit of extra meat and get away with painted.

 

[Ben29]

The contractor is pushing back and wants to use an oil-based primer on the steel.

 

[phamENG]

Have him prove (to your satisfaction) that the chemistry of the treatment used will not imperil the steel if it's coated like that. If he can't, then make him do it your way if he wants you to sign off on it.

 

[jayrod12]

So he used all treated material for the framing, but just wants to prime the steel? Has he ever seen primed steel in humid place? It rusts so fast.

 

[gte447f]

He's just being cheap. I'm sure he has some sob story about long lead times for hot dip galvanizing. He sounds like a jack leg, since he created this problem in the first place. Don't let him have any say whatsoever in influencing your design decisions. Tell him its your way or the highway.

 

[Enginerdad]

Seems like any sort of barrier between the two would be sufficient to prevent corrosion. Have him put a layer of roofing felt, sill gasket, literally anything that will keep the plate from touching the wood. Galvanizing a single or pair of plates would be uneconomical, so I understand his reluctance to do that. If the beam isn't exposed to weather, which it sounds like it isn't, then the barrier will remain intact as long as it's sandwiched between the beam and plate.

 

[XR250]

Seems like any sort of barrier between the two would be sufficient to prevent corrosion. Have him put a layer of roofing felt, sill gasket, literally anything that will keep the plate from touching the wood. Galvanizing a single or pair of plates would be uneconomical, so I understand his reluctance to do that. If the beam isn't exposed to weather, which it sounds like it isn't, then the barrier will remain intact as long as it's sandwiched between the beam and plate.

Some peel and stick membrane might do the trick. Around here, there is no upcharge for HDGing a single piece. It is done by the pound.

 

[HouseBoy]

I often specify a two part paint strategy as an alternative to HDG.
I'd also be concerned about the bolts and the bolt HOLES (need to be good about getting the paint in there after drilling).