Wood sill plate bearing stress perpendicular to the grain question
Wood sill plate bearing stress perpendicular to the grain question
(OP)
We are designing a 5-story wood-framed apartment building. The bearing wall design is being governed by the limit state of wood bearing on the sill plates – perpendicular-to-the-grain compression stress. It appears that this is a serviceability limit state (not a strength limit state). We are trying to minimize the impact of this limit state driving the stud spacing. We are exploring the use of a better grade wood for the sill plates, but another thing we are wondering is whether we would be justified in backing off on the live load used for checking bearing on the sill plates. If the bearing perpendicular to the grain is a serviceability limit state, then would it not be reasonable to use a lower (realistic) live load to check bearing perpendicular to the grain on the sill plates? (Perhaps 10 psf versus 40 psf.) This would be similar to using code wind loads to design the lateral-load-resisting system in a building, but using lower wind pressures to check drift. Any thoughts would be appreciated.






RE: Wood sill plate bearing stress perpendicular to the grain question
Also the duration of load factor for this condition is not allowed. Per my knowledge, this is due to the fact that wood crushes rather than deforms under compression loads perpendicular to grain.
As for using the bearing factor, I typically would not use this as it limits the use of doubling the spacing of the studs by doubling up the studs.
As for reducing the live loads, the area of applied loads to an individual stud would be so small as to make such a reduction not an allowable option.
As for your question on reducing live loads for a serviceability limit state. I would not consider accepting the responsibility of doing this.
Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.
RE: Wood sill plate bearing stress perpendicular to the grain question
I agree with woodman, an individual stud has a small tributary area, making reduction of the live load, per code, not acceptable.
RE: Wood sill plate bearing stress perpendicular to the grain question
If you've taken the precaution of using engineered wood for the joists to limit story to story shrinkage, you really should follow through and use the better grade plates at the lower levels to limit crushing too.
I have to agree with the two posters above.
Mike McCann
MMC Engineering
http://mmcengineering.tripod.com
RE: Wood sill plate bearing stress perpendicular to the grain question