Compression Perpendicular to Wood - Sill Plates 1

[BSVBD]

I've read a 2018 and a 2019 thread, which were both helpful, BUT... did not touch what I've been wondering since my curiosity originated over a decade or two ago.

In general wood frame commercial and multi-family residential construction, I frequently specify that supporting shoulders of girders, beams and headers, are to interrupt top plates and sill plate AND finally, must bear on top of a 1/4" steel plate between top of concrete foundation and bottom of shoulders.

If supporting wall of the header or beam is parallel to header or beam, I WILL add whatever quantity of shoulders to satisfy the alllowable perp-to-grain of the pressure treated sill ALSO applying the NDS Wet-Service Factor, as well as others, "IF" I deem necessary.

Unfortunately, I have not devised a way to (reasonably) overcome the allowable perp-to-grain within supporting walls perpendicular to supported header or beam.

With Mechanical Fasteners we generally apply a 4:1 or 5:1 Safety Factor.
With Concrete & Masonry we apply a 0.2 or 0.25 or whatever percentage of fc per application.
With Steel we apply 0.66, 0.6, 0.4 or whatever to Fy per application.
With Wood... thru NDS... I've only used what I've considered an allowable, as provided by NDS, and only applied whatever "Use" Factors, as well as personal and judgment factors I choose.

THE QUESTION... Has anyone come up with a formula (or other) to determine an allowable Compression Perp-To-Grain for wood "AFTER" the actual point load has exceeded the allowable, SIGNIFICANTLY ENOUGH, to crush the wood, extrude to air gaps, and compress it by 1/16"? 3/32"? 1/8"?

I "BELIEVE", the Compression Perp MUST increase for every 1/32" or whatever, provided we are not doubling or tripling the allowable, which, would THEN exceed the allowable parallel, for the shoulder, which leads to a failure preceding this thread.

At this time I use my own judgment when my actual compression is close enough for "ME" to accept it.

Any thoughts?

Thank you all!

 

[Eng16080]

According to Section 4.2.6 of the NDS code, the compression perp. to grain design values are based on a deformation of 0.04". That section also provides an equation for perp. to grain at a deformation limit of 0.02." Perhaps you could extrapolate between the two to find what you're looking for. Further, Section 4.2.6 of the commentary provides additional information which may be helpful to you.