On the delamination phenomenon in the repair of timber beams with steel plates

This paper presents a non-invasive technique for the repair of ancient wooden floors. Steel plates are glued on one side only by epoxy-adhesive into longitudinal grooves in order to allow the free swelling and shrinkage of the wood in the direction transversal to the plate glueing surface, thus reducing the risk of plate delamination. A set of high strength steel nails provides load transmission from the steel plates to the wooden beam in the case of loss of adhesion due to fire or delamination. This technique was used to repair a precious beam in a 15th-century wooden floor in Palazzo Calini (Brescia, Italy). The technique requires particular attention because it might be affected by the delamination of the glued reinforcement due to stress concentration, which occurs at the end of the repairing element or at any cracks in the repaired beam. Results of experimental studies on delamination phenomenon investigated by means of the Moiré interferometry analysis are also presented. These show that the risk of plate debonding can be markedly reduced by the capability of the sapwood to develop plastic strain. The wooden floor has been monitored for more than fourteen years, confirming the effectiveness of the adopted technique.