Wood sill plate bearing stress perpendicular to the grain question

[cliff234]

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.

 

[woodman88]

Per the Wood Handbook 2010 "Compressive stress perpendicular to grain—Reported as stress at proportional limit. There is no clearly defined ultimate stress for this property."

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.

 

[structSU10]

There also is a clause in the NDS (2005) 4.2.6 that discusses that 0.04" of deformation is assumed at the listed perpendicular to grain bearing stress, and is a practical limit for good service of the structure. You should understand whether this is acceptable or if you have to go to a more stringent criteria. They list 0.73*Fp if 0.02" is more appropriate.

I agree with woodman, an individual stud has a small tributary area, making reduction of the live load, per code, not acceptable.

 

[msquared48]

For a 5 story structure, you should be lookimg at 4X material a@ 16" oc at the bottom floor.

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

 

[wannabeSE]

I have very little experience with wood. If you are in the U.S., you might contact Woodworks and ask if they have any innovative solutions. Also, there website has some articles on midrise wood buildings.

http://www.woodworks.org/technical-support/