Development of long span stress-laminated timber arch bridges.

For the last 2 years the authors have been involved in optimisation of the performance of stresslaminated timber (SLT) structures by utilising the strength properties of timber in an arching action for use as vehicle and pedestrian bridges. During that time 9 permanent bridges have been built and 7 have been load tested. The tested bridges were a half-scale of a 12m span bridge (i.e. 6m span) and a full-scale bridge of 15m span. Laboratory tests have also been carried out on a series of thirdscale models with 3 different arch profiles and with 2 flat decks to study the interactions of friction and arching profiles with different pre-stressing tensions. It has become clear that the design rules for flat SLT decks do not necessarily apply to arches. Furthermore a study of the structural behaviour of a full-scale long span SLT arch structure (20m span bridge) is being carried out at Napier University, so that static and dynamic responses to vandal loadings are examined and evaluated. Contracts are being let for the construction of 4 new arch bridges of spans between 20m and 26m. The extensive testing programme, augmented by analytical work aims to develop reliable design procedures for arch structures using UK softwood.