Sill Plate Crushing @ Ground Floor on Concrete Foundation Wall - How Concerned Should I Be?

[Norby_acn]

I am working on a bit of a unique wood structure. Multiple stories, primarily storage, many wood posts carrying a fairly high load to the foundation.

I've sized the sill plate at the ground floor to not crush when carrying the load of these heavy posts. The post sizes are set in stone and cannot be altered. After a back and forth with the owner/contractor, the sill plate needs to be white oak in order to not crush, while all other wood on the project is SYP, and they've agreed to this. Now, they are coming back and wanting to adjust a few things on stories above which results in the load on the posts increasing to the point where the sill plate crushes, even as white oak.

I am aware that I can use a steel bearing plate to transmit out load a bit, and that will likely be the route I take, but as I was considering the problem, a question came to mind. Why do I care if the sill plate, bearing continuously on the top of the foundation wall "crushes"? I am aware that NDS perpendicular to grain compression values are linked to 0.04" of deformation as a limit and that the commentary states that exceeding this "does not lead to structural failure". I also do not have any brittle finishes at all to be concerned about. If the sill plate "crushes", what it is really doing is compressing a bit and the material can't really go anywhere because it is continuously bearing on concrete. So I'm wondering if this is something I am being overly concerned about.

I'm curious as to what some other, more experienced, design professionals would consider and do when facing this consideration?

Thank you for any and all advice!

 

[TheDW]

Any reason you can't skip the sill plate altogether, and go post-steel plate-foundation?

 

[MR_E30]

Daywalker has a great point. Also, if you still need a sill plate for siding attachment (or something along those lines) you could always run sill plates between the columns/posts.

 

[Norby_acn]

Skipping the sill plate altogether is a possibility in a lot of locations where I don't have lateral braces coming down. Where I do have braces, I am relying on the plate to drag a certain amount of lateral load into the foundation wall. There are a lot of braces, so the load from any particular brace isn't huge, but does take several spaced adhesive anchors to distribute.

 

[XR250]

Have you taken into account the bearing area factor in the NDS? Might be enough to make the white oak work. Honestly, if there is not sheetrock involved, and you were close, I would not worry about it. My experience has been that the lumber ends up crushing a lot more than the design values would indicate.

 

[phamENG]

I would be concerned, especially if you have multiple levels. Are your stair/elevator towers made of CMU or concrete? If so, and you have 3 or more levels, you already need to start thinking of differential movement between structural materials. By allowing crushing to occur, you are introducing quite a bit of additional vertical movement to the post and may create serviceability issues for finishes, sills and varied materials, etc.

But the sole plate of adjacent walls to the post and attached directly to the slab/foundation with a Simpson post base, steel plate, or something similar.

 

[dhengr]

Norby_acn:
Don’t forget, wooden material near grade and on the conc. has to be pressure treated, and oak doesn’t treat worth a damn, assuming you can even find P.T. oak. The bottom portion of the posts should probably be treated, as they rest on the conc. You might use some standoff bases from Simpson or Mitek (USP), but they are pretty expensive and you haven’t bothered to tell us what the post sizes or loads are. What’s the big secret? It always astonishes me that so much, like column sizes, types locations and other important structural details are predetermined and sacrosanct before the engineer even gets a word in edgewise, and then, come what may, he should learn to live with that and design around it. We always used to help the Arch. and G.C. with the structural system and layout, during a prelim. design phase, so they didn’t get in to deep before we determined that it couldn’t be done for less than a billion dollars. Those same high concentrated loads have to go up through the floor framing above, does that work out o.k? Then, build infill walls in btwn. the posts and beams above. Ypu probably have to pay some attention to how the beams above might load the infill walls, and account for that in the wall design. That may lighten the post loads a bit.