Oak beams in medieval frameworks: constraints and advantages for restoration
Résumé
During the fire of Notre-Dame de Paris, timber frames coming from the Middle ages period, in the nave and choir, were burnt. It was decided to proceed to an identical reconstruction, but many questions arise about the timber frame. Is wood outdated as a material for beams? Is oak wood the best solution for timber frame? Are we able to find similar long slender oak beams in today French forest? Is it safe to use green wood for the building?
In fact, the forested area in France is the same today as in the Middle ages period, but two times higher than during the 19 th century. Results from the forest inventory proves that 40 million slender oak trees are available in order to sort around 1500 trees for the restoration.
Cathedral heritage provided a demonstration of the high efficiency of carpenter’s knowhow. Most of oak beams kept their functionality for more than 800 years. This is a nice experimental result for a very long mechanical testing. Wood, emerged as material 300 million years ago as a result of long time evolution, is used to build trees, which are a collection of wooden beams able to resist flexure forces, for a long time.
Technical know-how about timber frame is old in human societies and was improved during the Middle ages, but scientific knowledge about wood as an anisotropic material is very recent. The background in mechanics for such material has been developed in early 20 th century and observation tools for material structures at the nanometre scale appeared in late 20 th century. Wood can be considered as an archetypal hightech material using polymer-with-additive technology combined to nanostructured fibrecomposite within a honeycomb-like micro-structured material. This explains why nature solution for tree beams is very efficient in terms of bending resistance per weight. The mechanical behaviour of wooden beams is highly sensitive to the orientation of the main direction of anisotropy (grain direction) relative to bending forces. Natural beams in trees are built by a kind of 3D printing process so that grain orientation at beam periphery is always optimum. Mechanical bending performance of these beams (round-wood), in spite of branching, is equivalent to the performance of standard clearwood specimens documented in databases. Squaring at the minimum this round-wood should be done while keeping grain angle closest to zero-degree value, which is performed by specific axe-hewn log squaring. The result is “coarse” slightly curved beams much more resistant than “nicely” straight sawn beams with local grain angle above 10°.
Using directly green wood in the building process is effective for time of building and energy consumption. The drying of wood induces long deep longitudinal cracks and a reduction in beam section but a bigger increase in wood mechanical performance. If the green frame resists, the dried one will resist all the following years. Finally, today carpenters have kept the building know-how and there are many modern
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