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Checks and Splits in Post Frame Timbers

Checks and Splits in Post Frame Timbers

Checks and splits in post frame timbers (wall columns) are often misunderstood when assessing a structure’s condition. There are two means where checks and splits can form in wood elements: during seasoning, or drying, and during manufacture.
Development of checks and splits after installation occurs after wall columns have dried in place. Usually these were installed green, especially after a recent pressure preservative treatment. Due to their size, it’s not practical for timbers to be kiln dried. Some are air dried for a period of time prior to installation, but mostly they are installed green, and therefore, are allowed to dry in place.
During the seasoning process, stresses develop in wood as a result of differential shrinkage often leading to checking, splitting and even warping. Wood fiber separation results in checking and splitting. Due to innate wood characteristics, it shrinks and swells differently. Generally wood shrinks (or swells) approximately twice as much tangentially to annual rings as compared to radially. Additionally, during initial drying process timber outside inevitably dries quicker than interior, causing differential stresses to develop.

Combined effects of these drying stresses in a post often, and sometimes inevitably, result in formation of a check or a split. Since wood’s weakest strength property is tension perpendicular to grain (similar to how wood is split with an ax), drying stresses can result in a check or split forming in a radial direction across annual rings. However, while these seasoning characteristics may initially appear as problematic, they likely are not. It is important to remember as wood dries, it becomes stronger. Furthermore, the development of these seasoning characteristics is, quite often, normal. Most importantly, both are accounted for in derivation of design values for timbers and are also accounted for in applicable grade rules.

A check is separation in wood fibers across annual rings of a piece of wood and a split is a separation of wood fibers across annual rings but through a piece of wood. A third type of fiber separation, known as a shake, occurs along annual rings and is generally a naturally occurring phenomenon in standing trees, not a result of seasoning. There are several types of checks and splits defined and handled in grading rules for timbers.
In evaluation of post frame timber columns normal checks and splits can often be interpreted as problematic by some design professionals with respect to allowable design values. However, in most cases they are not. There are instances, however, where a check or split may reveal an important issue or a problem. For example, a relatively large split across a severe slope of grain.

Still concerned? In many locations glu-laminated post frame building columns are available, usually at a slight premium. Individual members of glulams have been dried prior to fabrication and pose little chance of checks or splits.

Timbers Checking In

One of our clients, Matt from Chapel Hill, NC, sent us a few photos showing a concern of “timber checking” to Justine, the Hansen Pole Buildings Productions Wizard (she loves it when I come up with interesting titles for what she does).

 Here is what Matt had to say:

“Been a while since we spoke.  First off, want to say we are very happy with our new barn.  Turned out great.  Going to be photographing it soon.  Attached a few pics in this email.

 I do have a concern I wanted to get your feedback on.  Many of the wood columns have pretty large cracks in them.  Not sure what is normal and what may be an issue.  We did do an interior buildout in the part of the barn.  Not sure if that created an excessive load or not.  But are also similar cracks in the barn area that doesn’t have buildout.

 Let me know what your engineer thinks.”

I live life for clients who are happy with their new buildings. There are lots of them, which makes life very rewarding.

What the client is seeing is called “checks”.  These are a separation of the wood normally occurring across or through the rings of annual growth and usually as a result of seasoning.

Checks occur because drying stresses exceed the tensile strength of the wood perpendicular to the grain, and they are caused by tension stresses which develop in the outer part, or shell, of timbers as they dry around the still wet and swollen core. Surface checks usually develop early in drying because the lumber surfaces dry too quickly as a result of low relative humidity.

In timbers, surface seasoning checks are not limited. Columns are subjected to primarily tension and compression loads, so surface seasoning checks have a negligible effect on all strength properties.

Matt’s post frame building happens to be a rather unique monitor barn, with a roof only side shed along one side and a partial loft. I’m looking forward to seeing the photos of the completed project, I hope you are as well.