Timber Truss Splits and Cracking

No. You need an expert, preferably a structural engineer, to look at them.

Yes, at such times when the structure is built without the benefit of design by a competent professional.

And if it was--GET OUT NOW!

Criminy! It depends. It the truss member is in compression, who cares. If the truss member is in tension, check (that's a bad pun) to see if the splits are parallel to the grain. If they are, and they're not near the bolted connections, again who cares.

Connect with the connections.

Here are some paragraphs from an old book, I had read while considering building a timer framed house, titled "Wood handbook - Wood as an Engineering Material" Forest Service Agricultural Handbook No. 72 (sorry a real book and not a google link),

Definitions:

Shake - A separation along the grain, the greater part of which occurs between the rings of annual growth.Usually considered to have occurred in the standing tree or during felling.

Check - A lengthwise separation of the wood that usually extends across the rings of annual growth and commonly results from stresses set up in wood during seasoning.

The chapter called "Lumber Stress Grades and Allowable Properties: says:

Shake in members subjected to bending reduces the resistance to shear and therefore is limited most closely in those parts of a bending member where shear stresses are highest. In members subjected only to tension or compression, shake does not greatly affect strength; it may be limited because of appearance and because entrance of moisture that results in decay.

While shake indicates a weakness of fiber bond that is presumed to extend lengthwise without limit, checks and splits are rated only by the area of actual opening. An end split is considered equal to an end check that extends through the full thickness of the piece. The effects of checks and split upon strength and the principles of their limitations are the same as for shake.

All that said, the tables in the book seem to over engineer the the sizing of the wood beams to account for these normal occurring properties of wood.

Get a copy of this book and start some backward calculations to discover whether your beams were correctly sized for the loads you now have. If you feel the splits are not very eye appealing, maybe now would be a good time to replace them.

Let us know what you find.

Short answer: It depends.

Longer answer: Wood shrinks circumferentially more than radially, hence stress. When the stress exceeds the wood's cohesion, splits happen. If the "core" is part of the timber, splitting is virtually inevitable. If the timber is used only for compression or tension, splitting is not a problem unless it gets "real bad" [sorry too snow you with technical jargon]. If, however, the timber is used as a beam, there are a bunch of variables that come into play. Most carpenters over-engineer timber frames, so it's often a non-issue. If appearance matters, I sometimes rip a sawcut on a non-visible side of the timber to facilitate splitting where it doesn't matter.

If I need beam strength, I buy or mill "structural" grade timbers. There must be no core in the beam, and no knots which exceed the limits for maintaining tensile integrity.

If the timber is coated with a finish that slows moisture transfer, the violent moisture cycles are reduced and some of the splitting is reduced.

Patrick

Funny - when I saw the roof trusses for my house I asked the same question. I handpicked every piece of lumber for the building until we got to these trusses, and I wasn't too pleased to see they were full of splits, not a nice board in the lot of them. And as one of the carpenters commented "They sure put a lot of faith in them gang nails". Up they had to go anyway, and it was like nailing together a half pot of spaghetti.

Twenty years later the trusses are still holding up, and I don't see any sign that the splits got bigger over time. So I guess they were following the "if.. who cares" rules properly (although, really, weighing a little heavy on the "who cares" part).

Maybe some companies that engineer trusses also get deals on funky lumber, because they can use it anyhow without compromising the structure.