Glulam timber arches Strength of splices and reliability-based optimisation

University dissertation from Luleå : Luleå tekniska universitet

Abstract: Arch structures are very sensitive to changes in the distribution of the load as well as to changes in their shape. A slight change in the load distribution may imply internal forces for which the arch is not designed. During the winter 1993-1994 the Nordic Hall in Sundsvall, Sweden was close to a collapse because of an extremely non-uniform distribution of the snow load, which caused splitting failures at the apex splice for two of the arches. This event and these characteristic properties of arch structures give rise to two main questions which are addressed in this thesis: Is the reliability sufficient and as expected if common deterministic methods are used when designing arch structures, without taking into consideration the large influence of the uncertainties in the distribution of the load? How is the arch to be designed to best utilise the timber with a prescribed reliability? The first question is a reliability problem and the other an optimisation problem and together they will form a reliability-based optimisation problem. This thesis can be divided into three parts concerning glulam arches; structural reliability, strength of splices and reliability-based optimisation. The reliability has been analysed by calculation of the reliability indices using a method based on the linear regression of the basic variables on a linearised safety margin. The influence of the assumed statistical distributions of the basic variables on the reliability index has been studied. Splice connections consisting of nailed steel plates have been tested and a method based on linear elastic fracture mechanics has been proposed for designing this type of splices. A method for reliability-based optimisation of glulam arches has been developed by combining an optimisation routine with calculation of the reliability index by a method based on linear regression. The efficiency of using reliability-based optimisation methods when designing glulam timber arches was clearly demonstrated.