Improving the performance of bolted joints in composite structures using metal inserts

As the use of fibre reinforced polymer materials in bridge construction is becoming more popular, appropriate joining techniques, particularly for field joints, are necessary. Bolted joints are a common method for joining fibre reinforced polymer structures. The main advantage of bolted joints is their detachability, but they have a number of shortcomings. On the one hand, hole clearances, which are needed to facilitate on-site assembly, reduce the stiffness and joint efficiency. On the other hand, it is not possible to rely on the beneficial effects of bolt pre-defined tension loads, i.e. load transfer in friction, due to the considerable losses of bolt tension caused by the creep deformation in the composite material. To tackle these problems, a solution utilising metallic inserts in the hole is proposed in this paper. A series of experimental tests have been conducted to investigate the effect of inserts on the bolt-tension relaxation, the stiffness and the load-bearing behaviour of joints. Finite element analyses were also employed. The study demonstrates several benefits of the inserts: the bolt tension relaxation is minimised, the load transfer by friction may be feasible to be utilised in the bridge service state and the joint efficiency is increased in terms of stiffness and strength.