Wood Post Capacities - Compression Perpendicular to Grain

[Charlie Delta Whiskey]

I am in my first year out of college and where I work we use tables similar to those found here to size wood posts, but we never check the compression perpendicular to grain bearing. When I asked the principals why we didn't they didn't really give me a satisfactory answer (we just don't worry about it they said). Is that standard practice? If so why? When would it be necessary to check. Excuse my ignorance. Thanks!

 

[jayrod12]

Technically you should be checking it.

The don't worry about it opinion was explained to me this way however, what exactly is a bearing failure perpendicular to grain? Slight crushing to remove any void space between the wood fibres. After that nothing happens.

The people above me have typically taken the stance that it'll be fine and marched forward.

 

[PittEng88]

Posts are normally not checked for compression perpendicular to the grain because they are axial force members and the forces are assumed to run parallel with the grain of the member. Compression perpendicular to the grain is more of concern for beam bearing or where other members are framing into the post, where there will be a force applied perpendicular to the grain.

I would recommend checking out, Design of Wood Structures ASD/LRFD by Donald E. Breyer. It is an excellent reference for timber design and has helped me out quite a few times.

Hope this clears things up a bit.

 

[jayrod12]

I was under the impression he was talking about the perpendicular to grain compression of the plate below the post.

 

[WARose]

Compression perpendicular to the grain isn't really a consideration for a post (i.e. a member with axial load & bending). About the only time I check it is when we are talking a stud or a truss/rafter bearing on a sill plate.

 

[PittEng88]

If he was talking about bearing on the plates below, then yes I would agree that it should be checked for the plate.

 

[MikeHalloran]

When you need to lift something really heavy for a nontrivial distance, and there is no overhead crane or similar available, you call a rigger.

Said rigger will arrrive with a large quantity of nicely squared but not 'finished' 4x4 or larger timbers, and will use simple devices to elevate your load 4+ inches at a time, and build 'cribbing' underneath your load as they go.
The timbers are stacked neatly in a stable pattern, but not generally fastened to each other or anything else.

The riggers will be careful to make the stacks very neat, so the timbers are not even trivially stressed in bending. They know that, even for softwoods, the compression perpendicular to grain numbers are huge relative to other directions, and that the wood will give some warning before it fails completely. So they don't worry too much about it.

WRT to the referenced table, it's unfortunate that they didn't give units for the loads. I might assume lbf, but I don't like assuming anything. The allowable perpendicular to grain bearing load Pc⊥ in the table gives you an idea of what is possible. I think there may be more comprehensive tables in really old engineering handbooks, though they may assume better typical lumber than the firewood that seems to be standard now.

I don't see the harm in you (quietly) checking a few joints for your concerns, so you will come to understand why your principals don't worry about it.



Mike Halloran
Pembroke Pines, FL, USA

 

[Charlie Delta Whiskey]

I was talking about the post bearing on a plate. Thanks for all the responses. I was just curious if this was typical.

PittEng88-I actually already have Breyer's book from a college course and it really is a very useful reference.

 

[XR250]

It is not a safety issue but can cause cracking in drywall - which customers are never happy about.
Also, if the plate gets wet, they seem to crush more.
I generally check the plate - rarely the post as allowable stress parallel to the grain is pretty high.

 

[dhengr]

Charlie Delta Whiskey:
With all due respect, you should not let your superiors short change you in your learning and mentoring, with such a clipped and half assed answer to a perfectly reasonable question. Ask it again in a slightly different way, be persistent..., how the hell do they ever expect you to learn and become a better engineer on the team if they don’t explain their thinking? Jayrod12 and
XR250 explained it pretty well, but I might add that this crushing, plus cross grain shrinkage can become accumulative as you move up several stories.

The Simpson tables that you attached look at the studs/posts as columns or beam/columns, subject to buckling under load, so to that extent they should be reasonably good as a guide for that purpose. But, we also know that as we follow the load path we always need to be looking for the weakest link, and you found one. So, you deserve a reasonable explanation, not just the brush-off. If you had asked me that question, under most circumstances, you would have gotten extra points, becuase it shows you are thinking through the whole problem, and how the pieces work together.