Three-dimensional behaviour of brickwork masonry arch bridges

Masonry arch bridges form a pivotal part of the transport networks of the UK and many other countries worldwide. Nonetheless, these historical structures face substantial challenges in terms of their safety and functionality. Factors such as climate change and natural hazards contribute to the continuous deterioration of materials, while growing demands are imposed upon them by modern traffic systems. Despite extensive research conducted on masonry arch bridges over the past two decades, the understanding of their fundamental behaviour remains limited, especially concerning their three-dimensional (3D) response characteristics. This dissertation aims to comprehensively assess 3D behaviour and failure mechanisms of brickwork masonry arch bridges under different loading scenarios, improving the resilience of these critical infrastructure assets.

The dissertation initiates with the characterisation of material and mechanical parameters of masonry arch bridges. Initially, small-scale material level tests are conducted to determine the properties of masonry units and mortar, as well as the internal friction angle and cohesion of the backfill materials. Then, the compressive strength and Young’s modulus of masonry prisms, as well as unit-mortar bond properties are experimentally assessed at the component level. Further, large-scale shear box tests are carried out to identify the frictional properties existing between the masonry specimens with different bonds and backfill materials. Additionally, both static and highcycle fatigue three-point bending tests are conducted on masonry flat arches. The experimental results highlight the inherent variability of masonry properties. Moreover, crack propagation and load-deflection curves obtained from the flat arche tests reveal a rapid degradation in stiffness when load magnitude changes.

Utilizing the materials characterised, a full-scale masonry arch bridge is constructed in the laboratory. The bridge is well instrumented with various sensors to capture its 3D response under loads. Static and cyclic (three loading cycles) patch loads are applied to nine locations on the backfill surface, including both centric and eccentric locations. The magnitudes of the load are increased gradually from 150 kN (approximately 22-25% of the peak load that the bridge is expected to carry) to 250 kN (~50% of the ULS), until the bridge failure. Crack evolution, accumulation of damage, load-deflection responses of the arch barrel, in-plane and out-of-plane deflection of the spandrel walls are analysed.

In addition to the contacting gauges, digital image correlation (DIC) is utilised to monitor the initiation and propagation of full-field strain/cracking in the spandrel wall during these tests. Comparing DIC data with readings from displacement gauges, the results confirm that DIC can measure the deformations of the untreated surface with satisfied accuracy, using the inherent brick bond patterns and natural characteristics of the masonry surface as tracking points. DIC results provide valuable insights into the cracking behaviour of the masonry arch bridge throughout the entire loading-unloading process, offering critical information on the load levels that induced the onset and the activation of hinges. Additionally, the interaction between the arch barrel and spandrel wall, as well as the crack mechanism in the spandrel wall, are analysed.

Moreover, along with the loading tests, hammer-induced vibration response of the bridge are collected, and modal parameters of the bridge with different levels of damage are identified using stochastic subspace identification (SSI) method. Also, numerical analysis is performed to identify the mode shapes of the bridge. Full life-cycle vibrationbased assessment is carried out by analysing the correlation between stiffness degradation and variations in the bridge’s modal parameters. The results demonstrate that the first-order frequency had the highest sensitivity to the damage accumulation in the masonry arch bridge. The evolution of early-stage damage, the formation of the first hinge, and the fully activation of a four-hinge mechanism are successfully captured.