Strengthening of timber structures using pre-stressed carbon fibre lamellas

The refurbishment of old buildings often comes hand in hand with an increase of the dead- and live load. This and the higher safety factors often make a reinforcement of the old beams necessary. The use of steel is often not regarded as an adequate solution in a timber construction. This because of the increase of the structures self weight, the appearance and the corrosion in marine environments. The use of CFRP allows methods, which are nearly invisible. In order to gain the needed contribution of the reinforcement to the service limit state a solution involving pre-stressing is requested otherwise the amount of carbon fibre would be very high and therefore the method not cost efficient. Pre-stressed carbon fibres are commercially used to strengthen concrete and steel structures (especially old structures in cast iron). Until now the methods were not successfully applied to timber this mostly because of the delaminating of the carbon fibre strip due to the concentrated shear stress at the beams end. Moreover, compared to the steel, the FRP composites show a different structural behaviour and their utilisation for the in-situ external bonding has not yet been fully explored. In order to gain quantitative and qualitative knowledge on structural adhesive and fibre-reinforced polymer (FRP), suitable for in-situ post-strengthening of timber structures, the authors investigated in Cooperation with SIKA and EMPA an application method using pre-stressed carbon fibre strips. In this method the timber beam is cambered prior to the installation of the FRP lamella. This cambering is done with a in the height adjustable prop placed in the middle of the beam. The pre-stress force in the FRP is not constant. It peaks in the middle of the beam were it is mostly needed and is zero towards the ends were high stress would causes delamination. A calculation model was developed and verified. Thus, this paper reports on the latest progress made on the development of the in-situ strengthening of timber beams with pre-stressed carbon fibres.