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Splits in 19th Century Timber

Splits in 19th Century Timber

Splits in 19th Century Timber

I am evaluating an early 1800's timber framed church in New England. The roof is framed with timber trusses about 13 feet on center. Trusses are generally in poor shape with conditions throughout the building. The most widespread condition is severe splitting of timber pieces. I'd say 40% of the chords, webs, and columns have a relatively severe split. I have a reference that refers to an article in the Engineering News Record from July of 1944 by "V. Ketchum, et al" entitled "are timber checks and cracks serious?" for guidance on evaluating splits. I cannot track this article down. Does anybody have a copy of it?

RE: Splits in 19th Century Timber

I've read Ketchum's article that you mention, though I'm not sure if I have a copy. I just moved this weekend and have yet to find my deodorant, much less my antiquarian engineering boxes :)

I do remember, however, the gist of his article and my other reading on the same subject. I'm assuming here that:
a) Your checks/splits are actually checks/splits and are longitudinal along the member, along the grain.
b) Your trusses are still acting as trusses and none of the members have become bending members with non-negligible flexure or shear forces.

As the AITC article that boo1 links mentions, for axial compression only members, if the check doesn't extend through the member and effectively split (pun intended) the member into two separate members, it does not affect the compressive capacity much if at all. If it is actually a split and not a check, and the formerly single member is now two members you'll need to consider them as such as the AITC article describes. If its an axial tension only member then checks/splits theoretically shouldn't affect the capacity at all, as long as the end connections aren't compromised.

You mentioned columns in your post. A check/split right at the ends of your columns could allow the 'along the grain' end bearing area to be reduced, which could cause column eccentricity and/or localized 'perpendicular to grain' bearing failure in your supported chord.

In my experience, short of rot or mechanical damage, you'll find most of your issues in 19th century trusses to be in the joints and connections. The checks/splits can add to or initiate some of those issues, however, by allowing connection/joint designs that depended on 'perpendicular to grain' dimensional stability to slip once the check/split opens up. Or by allowing moisture/insects into members that can then cause hidden member rot, or corrosion of hidden metal components. Once joints begin to slip you'll induce flexure and shear in your truss members and what I said about checks/splits above no longer applies.

RE: Splits in 19th Century Timber

Timber checks. Period. It always has & even our new codes consider this. People who don't know timber get bent out of shape about things they don't understand. Learn the difference between checks, splits, shakes, etc. & then learn to evaluate a structure based on what's going on. Your best teacher (besides good books by Hoad, the CWC, and the AWC, etc.), is to spend some time in old buildings: mills, barns, etc. & look at what the timbers have done & what they are doing. Don't listen to Chicken Little; the sky might not be falling. Assess what is really there.

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