Chapter 2

Why do timber frames fail? 2/5.

2.2 Materials failure - early failure due to greenness/ warping/ springing out/ shrinkage - inherent faults.

Materials understanding was not far-reaching and carpenters were blinkered by Guild practices (Swanson.H, 1983) from looking closely at other professions to gain insight into the mechanical properties of green timber. Green timber, that is timber freshly cut and used without being seasoned, was employed because the hand tools available in the mediaeval period were only strong enough to cut soft materials. However, green timber has a huge water content that increases the weight up to ten-fold from a fully dried out sample. The carpenter probably associated weight with strength.

If the building was constructed from green timber over a typical period of six months it might then be subjected to a rapid drying out during a hot summer. In this case the joints would all shrink, the pegs loosen, the dovetails spring and any number of timbers warp. This would probably not be disastrous but would need addressing. Carpentry joints could be firmed up with larger pegs and wedges and perhaps some auxiliary braces added.

However, what is more likely is that the drying out would be prolonged over several decades and that it would occur differentially, the south side of the building drying first with all its concomitant problems. By then the frame has been buried in the internal décor and even rendered externally. Wholesale repairs would no longer be an easy option and so localised iron-work repairs would be installed to arrest any further movement. This approach would have to be refreshed periodically.

There is also the possibility of inherent faults, not detected in the green stage in the timber, leading to an early failure under loading, drying out or mechanical damage.

2.3 Loading failures

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