There are many aging bridges in service in the US and around the world that were not constructed to modern seismic design codes and standards. As a result, they are expected to perform poorly during intense earthquake ground shaking. Older reinforced concrete (RC) bridges, in particular, are often supported on poorly detailed and seismically deficient columns. The most common deficiencies in these columns are lap-splices located in the plastic hinge region above the footing, insufficient lap-splice length, and inadequate transverse reinforcement that does not adequately confine the concrete core. These three features result in non-ductile response of RC columns that can lead to bridge collapse during strong earthquake ground shaking. To improve the seismic performance of existing RC bridges, a novel retrofit method was developed for RC columns with vintage reinforcing details targeting their inherent deficiencies. The retrofit was applied to square RC columns and combines titanium alloy bars (TiABs) (Ti-6Al-4V) with conventional concrete. TiABs exhibit high ductility and limited strain hardening, provide corrosion resistance and ease of fabrication. In the retrofit method, continuous TiAB spirals are used to provide lateral confinement in the lap-splice region and supplemental longitudinal TiABs are added to provide a redundant flexural load path to resist seismic induced moment. The TiABs are installed using conventional construction industry practices and thus provide an effective and economical retrofit solution.
An experimental program was carried out to assess the performance of the retrofitting method. A total of fourteen (14) specimens were tested under reversed cyclic lateral loading. The specimens with TiAB retrofits produced seismic performance that was as good as or even better than modern designs. This was evident in the form of strength and ductility enhancement and increased hysteretic energy dissipation. The ability to achieve improved seismic performance from older existing structures provides an effective alternative to the need for replacement. The TiAB retrofit solution also allows visual inspection of the column after an earthquake to assess damage, a benefit not available from most other alternatives. Analytical column models were developed in OpenSees to predict the individual specimen response. The experimental and analytical results and studies were used to develop design recommendations for TiAB seismic retrofits of square RC columns. The retrofit design methods are presented with their application to an in-service RC bridge on an interstate highway in Oregon that has seismically deficient details.