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Wood Bearing Plate for Wood Trusses

Wood Bearing Plate for Wood Trusses

Wood Bearing Plate for Wood Trusses

(OP)
The typical project is located within the upper Midwest and great lakes area.

SPF #2 and DFL #2 are the only two wood species that will even be considered.

The typical scenario is wood roof trusses bearing on horizontal wood bearing plate(s).

1. Can a wood bearing plate, Compression Perpendicular To Grain, be increased 1.15x for two month load duration?
2. In the past, I've pondered:

Regardless of wood species, load duration factors, adjustment factors and bearing components (am I missing anything?); when the design has left the paper and reality is in place, if the actual truss reaction were to exceed the allowable compression perpendicular to grain by a small amount, perhaps less than 100 psi, just how much would the wood crush? 1/32"? 1/16"? 1/8"? And if the wood actually DID begin to crush, wouldn't the compression perpendicular actually increase as the wood is crushing, and thus, soon terminate the crushing? Is there any way to determine this?

Considering the previous paragraph, is it a major concern if the compression perp to grain exceeds the published allowable by a minimal amount? If so, what IS a minimal amount? Is a small amount really worth upgrading the wood to a more expensive species?

I realize there are others critical factors to consider, such as, whether the bearing plates are supported by wood studs or a continuous grouted CMU bond beam, for example. I further realize we can specify truss bearing enhancers, a wider bearing plate, a denser species... anything else? By the way, what IS the allowable compression perp for plywood? I don't see this in NDS.

I'm asking this with the intent to be economical, yet, above all, safe, without being unreasonable. Every contractor will accuse us of designing bomb shelters. But, on the other hand, I do like to consider a budget and I don't like to oversize only for simplicity, if not warranted. I do realize what I am inquiring about will not likely throw any project out of budget, but, every consideration is a contribution.

Thank you all!

RE: Wood Bearing Plate for Wood Trusses

If the wood truss is adequate in bearing, I don't think the bearing plate should be a problem.

BA

RE: Wood Bearing Plate for Wood Trusses

1. It depends on the type of load. If it's purely dead load that you're checking, no. If you're including roof snow load, then yes, it can be increased by 15% because the load isn't there permanently.
2. The crushing happens differently depending on the type of wood being used. Typically, the crushing can cause several issues with the most critical being weakening of the fibers thus reducing the bending capacity. If there is continuous support under the plate, like a CMU wall, then deflection caused by the plate crushing is the main concern and not a decrease in bending capacity. As far as I know, there is no way to quantify how much the wood will crush. In the past, I've used mixed species between the studs and plates, so I can get a higher bearing perpendicular to grain. I've even had a couple jobs where we used engineered lumber plates.

The values for plywood vary based on the number of plies. I believe there are several documents from AWC or APA that outline the design of plywood based on the number of plies.

RE: Wood Bearing Plate for Wood Trusses

i'm thinking, over an end of the day beer mind you, that generally crushing perp to grain does not get a duration of load increase just like E doesn't. I'll pull out the ole NDS in the morning and eat my words if need be.

RE: Wood Bearing Plate for Wood Trusses

This thread contains a ton of useful info on this topic: Link.

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: Wood Bearing Plate for Wood Trusses

Triangled,

You are right; no load duration factor for perp to grain compression.

RE: Wood Bearing Plate for Wood Trusses

If perpendicular to grain compression was a real problem, stud wall bottom plates would typically be over stressed when studs are fully loaded.

BA

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