There are many existing bridges around the world that were designed without consideration for seismic effects. Many of these bridges were designed before modern earthquake engineering design standards and practices existed and thus are expected to perform poorly during strong ground shaking. Common structural deficiencies are found in their reinforced concrete (RC) substructures, including columns and footings. The most common deficiencies include reinforcing steel lap-splices within the plastic hinge region, insufficient lap-splice length, and poor lateral confinement due to insufficient transverse reinforcement. The effect of these details results in non-ductile response of the member that produces damage, residual drift, and can lead to bridge failure. Recent research has demonstrated the effectiveness of a novel retrofit method that uses titanium alloy bar (TiAB) ligaments and continuous spirals that provide both confinement and an alternative flexural load path that enables the retrofitted column to endure large drifts with sustained and predictable ductile response, high energy dissipation, reduced residual drift due to self-centering capacity, and no loss of axial load capacity. Although the retrofit is a viable option for improving seismic performance of columns, there are limited data that consider or incorporate the effects of soil-structure interactions on retrofit performance and design.
An experimental program was executed to evaluate reverse-cyclic performance of square RC columns retrofitted with TiABs on simulated soil. All specimens were designed and constructed to be representative of full-scale column-footings identified within the Oregon Department of Transportation bridge inventory. Eight (8) tests were conducted on three (3) specimens under sustained axial load and applied lateral loading. Different soil simulant constraints were used to isolate and quantify their effects on the structural performance. An analytical model of the soil subgrade and embedment was developed to predict the global response of the specimens under different boundary conditions.
Retrofitted columns exhibited either no damage or very minor damage due to activation of rocking foundation conditions with soft soil conditions. Higher column bar flexural stresses were observed in the case of stiffer soil conditions. In all cases, inelastic response of the soil simulant was observed. A single specimen that was unretrofitted was observed to fail with the soil simulant and then retrofitted with the TiABs. It was evident in subsequent tests that the retrofitted column possessed sufficient flexural strength and stiffness to activate the rocking foundation mechanism which thereby limited demands in the structural elements. The findings of this study demonstrate that interactions between the column-footing-soil should be properly accounted for to ensure the desired design outcome when considering and implementing seismic retrofit strategies on bridge substructures.