Low-damage design of bridge superstructures for accelerated bridge construction
Thesis DisciplineCivil Engineering
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
Many bridges in New Zealand are reaching the end of their service life, some have been damaged in earthquakes and are in need of retrofit or replacement. Expansion of the transport network also constantly necessitates the construction of new bridges. A major issue caused by bridge retrofit or construction is traffic disruption, and the resulting direct and indirect economic losses. Prefabrication of bridge components is a solution that decreases on-site construction time, reduces traffic delays and improves work-zone safety and construction quality.
In the case of reinforced concrete bridges, prefabrication has been somewhat limited in seismic areas, mainly due to a lack of reliable connections between prefabricated elements that can provide sufficient ductility. It is often more economical to design structures to respond to design level seismic loading inelastically (e.g. plastic hinging), a behaviour which is only possible through provision of sufficient ductility through proper detailing, especially in the connections between elements. Lack of ductility can result in catastrophic failures of structures, examples of which were seen during the 1971 San Fernando, 1994 Northridge and 1995 Kobe earthquakes.
In a bridge structure, the seismic demand is usually addressed through inelastic action of the piles and piers while the superstructure remains elastic. This study aims to develop a low-damage dissipative rocking superstructure system which contributes to the response of the bridge to transverse seismic loading through inelastic action. Therefore, the elements where the majority of the seismic inertia originates from, i.e. the superstructure, contribute to damping of the input seismic energy and self-centering of the structure. The proposed superstructure features unbonded post-tensioning and axial or U-shaped flexural plate mild steel dissipaters in the superstructure to achieve these objectives.
The study was performed in experimental and analytical phases. The experimental phase consisted of quasi-static cyclic testing of a large-scale precast bridge featuring the proposed system. The specimen demonstrated a reliable performance, showing no signs of degradation in stiffness or damping ratio, and demonstrating its ability for energy dissipation and self-centering. In the analytical phase, a method for finding the force-displacement relationship of superstructures with dissipative rocking connections subjected to transverse loading was developed based on the revised monolithic beam analogy (rMBA). A direct displacement based design procedure for dissipative rocking superstructures was demonstrated based on a parabolic displacement profile, and the yield displacement and neutral axis depth was found for a large number of possible configurations. Finally, the results of experimental testing were extrapolated to a number of superstructure configurations and target ductility levels using the described DDBD and rMBA methods. The results provide an insight into the effects of geometry and target ductility in the equivalent damping and base shear demand of the proposed system. These tools can be adapted by practitioners for implementation of dissipative rocking connections in bridge superstructures.