BSVBD
Structural
- Jul 23, 2015
- 463
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!
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!
