Checking vs. Stress Cracks in Wood Columns

[dhesington]

I recently inspected a 120,000sf, 70+ year old building with 360 6x8 southern pine columns. Approximately 85-90% of the columns had surface cracks. Nearly 100% of those occured on the 8" side of the column. The cracks ranged from a <1/16" to over a 1/2". The larger ones extending from 1 1/2" to 3" deep in the column. Nearly 100% of the cracks no matter size and depth extendend the full height of the column.

My question is are these cracks natural checks or are they a result of stress over time. We're still in the proccess of charting the columns on plan to see if there is a pattern, but my preliminary calculations suggest that some of the tallest columns could have been orginally stressed upwards of 95% of the orginal capacity when built depending on the grade of the wood which makes an argument for stress. On the other hand, the cracks have all of the qualities of checks (except for the size of some).Only 2 of the columns out of 360 had clearly split at the bottom.

I've attached a picture of one of the columns for reference (can attach more). I've read the other recent posts on the topic which have been helpful, but am still curious as to what other people's opinions of the cracks might be.

 

[msquared48]

How heavily are the columns loaded? 300 to 600 psi should be no worries.

If the cracking is all the way through, then obviously the l/d is compromised and the allowable load needs to be reduced (unless through bolts are added to regain the integrity of the columns), but at only 1.5" deep, I do not think it is necessary. I have seen too many DF and HF columns in this shape or worse that are still structurally functional.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[Ron]

This appears to be a natural shrinkage checking in rough sawn timber. The fact that they occur on one or two sides indicates a preferential grain orientation, likely related to the location the timber was taken from the log.

Measure the width of each face at multiple points. If consistent, it is not likely load related as there would be some inconsistent bulging under compressive load (due to variations in the wood properties along the length, resulting in variations in Poisson's ratio at various points).

Agree with Mike...probably not a big deal structurally.

 

[dhengr]

If I didn’t see anything worse than that I don’t think I’d worry much. They have been there for 70+ years, probably seen some near historic loads and haven’t failed yet. That looks like pretty normal checking or shrinkage cracking on those size timbers. The general thinking is that you haven’t lost anything in the way of compressive material area in the member, despite the checking. If the crack got big enough, or went all the way through the member, you might want to think in terms of some change in l/r, that is two 4x6's vs. a 6x8, or some such. If, on inspection, they do not look like new cracks, and there is no other distress in connections, significant bowing, etc. I wouldn’t worry much. Also, the fact that this kind of cracking exists in almost all of the columns might lead you be believe that it’s a fairly common material or sawing or grain orientation phenomenon, which it is. Those kinds of members are all sawn out of the center of the tree, thus they all act the same way in terms of checking, some a little more than others.