Strengthening and experimental assessment of old wooden beams

Because of artistic, historical or cultural reasons, the conservation of old timber structures is becoming more and more important and desirable, and often emphasis is placed more on conservation than on economic aspects. On the other hand (e.g. in many residential buildings) the lowest possible cost in order to obtain durability, serviceability and compliance with building regulations is frequently the main requirement: the cost of strengthening or repair becomes then the guiding parameter in choosing between various criteria for performing work on existing timber structures.

Some aspects of restoration work are peculiar to timber structures, and usually require the specific expertise of a wood technologist. His specic contribution may include the following subjects:

. identication of wood species;

. evaluation of durability of the timber members in their present conditions (according to wood species, heartwood distribution, environmental conditions, etc.), and any need for preservative treatment in order to ensure the desired durability;

. assessment of the actual structural performance of the timber members.

The better knowledge provided by the wood scientist makes it possible nowadays to conserve structural members that until a few years ago would have simply been removed and substituted. Also, when the original structure does no longer meet the minimum safety requirements, a good knowledge of structural timber often makes it possible to take action so that old members may still contribute to the global loadbearing capacity. Further aspects that the wood expert should especially take care of are:

. the compatibility between the wood of the member and the new materials used;

. the inuences of the repair on the serviceability, reliability and durability of both the repaired members and the whole structure.

Timber members aected by heavy decay or damage can be strengthened through repair works.

It should, however, be noted that many old timber structures are oversized, considering the structural requirements. After a detailed inspection and careful assessment it is therefore possible that (in spite of past damage suered by the members) residual cross sections are still sucient to provide a loadbearing capacity complying with present and anticipated service conditions. In such cases there should be no obstacle to report: "no repair work needed", allowing work to concentrate on the prevention of further decay.

The loadbearing capacity of a timber structure needs in certain cases to be improved through appropriate structural consolidation, in order to comply with increased performance requirements (e.g. modifications in the use of the structure). Compatibility between dierent materials should always be a major concern; this is specially true for materials used for repairing and/or strengthening timber structures. Whenever glued or rigid connections are made, special care is needed in order to avoid stresses (and possible failures) caused by dierential deformations related to the following[:

. differential shrinkage between wood and non-hygroscopic materials;

. differential shrinkage between dierent anatomical directions of wood;

. different thermal expansion coecients between wood and metals;

. different displacements between structural parts or members with dierent sttnessess.

Other compatibility problems may arise from chemical factors, such as corrosion of steel fasteners caused by wood extractives under high moisture conditions. Finally, condensation of moisture (and consequent decay problems) is often a result of poor compatibility situations, such as:

. wood placed in close contact with metals or stone featuring a dierent heating or cooling rate, under insucient conditions of ventilation;

. possible condensation of moisture within timber pieces, at the interface between wood and resin.