What is ring shake?
Ring shake is a particular form of splitting in timber that occurs when annual growth rings separate from one another. The split therefore follows the curve of a growth ring, rather than running along the radius of the trunk as an ordinary drying check does. It is also known as cup shake; the Dutch term is ringscheur.
How to spot it
Look at the end grain of a beam or log:
- Ring shake — a crescent or moon-shaped split following the curve of a growth ring, sometimes running right round a whole ring
- Radial or drying check — a straight split from the pith towards the bark, across the rings; by far the most common defect and usually harmless
On a sawn member, ring shake can be almost invisible on the face while the timber is already separated internally over a large area.
Causes
The precise cause is not always a single thing. Contributing factors include sharply varying growth rates between successive years, which leaves weak bonding between rings; wind loading and shock on the standing tree; frost; and stresses released during felling and conversion.
Why it is serious
Ring shake is far more dangerous than an ordinary drying check, because it lies exactly in the plane where horizontal shear in a bending member is greatest: in the neutral axis, near the support. The timber can therefore fail well below its design load, with little visible warning.
Practical tips
- Inspect the end grain. When buying structural timber, and when surveying existing beams, always check the beam ends; that is where it matters
- Grading. Ring shake is a rejection criterion in the visual strength grading of structural timber (BS 4978 and EN 1912). Buy graded, stamped timber with a strength class such as C16 or C24
- Glulam barely suffers from this, because the laminations are selected and glued
- If you are unsure about an existing beam, have a structural engineer assess the residual capacity and, if necessary, strengthen the bearing zone with steel or timber flitch plates
Related terms
- Shrinkage crack
- Beam
- Dowel
- Formwork
- Reinforcement
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