Bending timbers

from Daizen News, 15 October 2011

Some clients desire the look of bent timber. To achieve this, we decide first whether the timber is structural or not.

The strength of structural timber depends at a very basic level on the fibre connections in the wood itself. If the fibres are severed by a cut-out curve, the wood can no longer act as a structural member.

Our approach to a structural curve is to start with solid timber sliced into the optimal thickness. The exact thickness depends on the radius of the curve. We then bend the timber (using a vise like the one below) and achieve the desired final thickness by laminating the slices.

For curves, we use Free of Heart Center (FOHC) timbers. The laminations are very hard to spot, since we slice from the same grain. We test this by asking our colleagues to find the individual slices in the final laminated wood. Over 95% of the people we show our solid bent lamination to think it is a natural bend. This bent timber retains the integrity of the wood’s fiber connection. Further, since the wood is sliced longitudinally, it is in fact more stable than solid timber, which can twist and warp. Note the twist in the straight timber below, where the curved timber has no twist.

Remembering this distinction in approach should help in design. If timber is non-structural, a cutout from wide timber should be free of heart center for best appearance. If the timber is to be structural, it will likely require slicing, bending, and relaminating.