Wood failure 2

[ronster]

I just looked at a 11 year old house where a 2x12 first floor joist had a sudden failure leading to a series of (4) 5" deep x 3/8" wide vertical cracks formed in the the mid depth of the 2x12 joist about 8" on center from the support. The cracks stop about 3" from the top and bottom of the joist. There owner said there was a loud pop and the floor moved almost like a small earthquake happened with the epicenter being the end of the joist. The ceramic tile above the joist is delaminated and tented up. The effected end of the joist is about 1/8" lower than the adjacent joists. It apears the crack was triggered by the joist drying out (middle of a severe winter) and most likely due to a failure in the material. The joist is not subject to any concentrated loads. Anybody familar with a failure such as this in a wood?

 

[boo1]

Ronster, this is an odd location for joist failure (at neutral axis).

Where there and axial loads (horizonal shear), possibly from unbalanced fill, stairs ect? Holes drilled, knots? What is the wood type? Basement? extreame weather swings? Rot? What is the span? Blocking?

Was there was standing water in the crawlspace. The changes in moisture content brought about by the presence of this water would be sufficient to cause shrinkage and swelling of the wood framing members supporting the floors and walls?

 

[JAE]

I've seen these in a wood framed house before....investigated it in 1993 when there was a tremendous amount of rainfall during the construction of the house...very wet conditions.

The house was completed, owner moved in and turned on the Air conditioner. The joists essentially dried out after being fully saturated with months of exposure to rain. The cracks were in the joist exactly as you described...in the center depth of the members and vertically oriented.

Cracks were due to shrinkage from moisture variations. Some large joists are cut near the center of the tree where radial and tangential shrinkage is magnified. Usually, shrinkage longitudinally with the grain is smaller.

 

[steve111]

odd, 11 year old house, the framing inside should have reached an equilibrium moisture content long ago, yes some seasonal fluctuation in humidity but ... Where is the floor located in the home? Like the idea of shrinkage but what would have caused the sudden change in moisture content?

 

[JAE]

Steve111 - I agree with you on the 11 yr. old situation. Very curious. Perhaps the cracks were already there a long time ago and were just discovered with the loud movement/noise. Some other issue or problem may exist as these cracks usually do not result in the kind of movement described.

 

[Sawbux]

Sounds to me like there may be a possibility of "compression fractures" in the wood. These sometimes develop due to high winds or during falling of the tree. When the tree is overstressed in compression the cell walls tend to buckle in parallel straight lines such as you describe. If they happen to a young tree, they can become embedded within the piece as the tree continues to grow. These regions are very weak in tension and are very difficult to detect visually. With a magnifying glass you may be able to see them as a light line across the grain.

I have run across these type of failures, mainly in glulaminated beams made from visually graded lumber. Visual grading cannot usually detect these failures, but Machine Stress Rating such as used in more modern beams and trusses can.