PSI - Issue 64

M. Saiid Saiidi et al. / Procedia Structural Integrity 64 (2024) 2021–2027 Author name / Structural Integrity Procedia 00 (2019) 000–000

2022

2

behind the cautious approach are economy and reliability. An important deviation from the relatively slow adaptation rate of unconventional materials occurred approximately four decades ago with the utilization of fiber reinforced polymer (FRP) materials in construction because FRPs could improve performance of built facilities. The reliability of the material in structural engineering was investigated beyond its past military application, and suppliers reduced the cost to make it competitive with the more conventional construction material. One of the areas where improvement of performance with conventional materials is not economically feasible is earthquake engineering of bridges. In moderate and high seismic zones, conventional bridges are expected to suffer significant damage making them unserviceable after the earthquake. The damage could be permanent lateral drift, severe failure of concrete and steel materials, or both. It is possible to address the first damage type using metallic superelastic shape memory alloys (SMAs). The more established type of SMAs is an alloy of Nickel and Titanium (NiTi). The material has been used in medical devices and military equipment for over 70 years but in sizes and characteristics that are not proper for structural engineering application. NiTi application in bridge engineering has been studied in the past two decades leading to design guidelines and a showcase three-span bridge in Seattle, Washington, USA. A new generation of superelastic NiTi has emerged that included Cobalt raising its strength substantially. This new material, NiTiCo, has been studied to determine its characteristics for possible adaption in bridges subjected to strong earthquakes. The present article provides a summary of the study and some of the important findings in addition to plans for large-scale bridge column models studies under cyclic loads. 2. Bridge Engineering Research on NiTi The primary attractive feature of NiTi is its superelastic behavior that leads to a flag-shaped stress-strain diagram. This had prompted studies on wires and small diameter elements that were suited for non-civil engineering applications. But early basic material characteristics research on NiTi bars appropriate for civil structures led to useful information by DesRoches and Delemont (2002) and DesRoches and Smith (2003) that provided an impetus for exploring the use of NiTi bars in concrete structure. Saiidi et al. (2007) studied experimentally a series of NiTi reinforced beams under half cycle loading to determine if the superelastic behavior of NiTi bars would provide displacement recovery in concrete beams. It was found that the NiTi bars reduced the residual displacement by over 80 percent. Encouraged by the findings, the ability of NiTi bars to help recover the permanent lateral displacement of bridge columns subjected to strong earthquakes was explored by Saiidi et al. (2009). Similar to the beam experiments, the NiTi bars were threaded at ends and machined to dog-bone shapes to avoid fracture at the threads. A damage resistant cementitious material known as ECC (engineered cementitious composite) was used in the column plastic hinges to minimize the earthquake damage. Fig. 1 shows one of the bars, the column plastic hinge reinforcement, and the column base under 10% drift ratio. The residual displacement was reduced by over 80 percent even under multiple cycles of loading. Although the study demonstrated successful performance of the column, it was felt that two improvements in NiTi are necessary before it is ready for incorporation in real bridges: (1) alternative couplers are needed to avoid machining of NiTi bars, which is both costly and wasteful because not the full section of the bars is utilized to provide flexural strength and (2) larger diameter bars should be studied to provide data on bar diameters that are feasible for use actual bridges. To avoid wasting SMA through machining into dog bone shapes, shear screw couplers were attempted. The NiTi bars were connected to mild steel in the column cage and the mild steel footing dowels. This detail was used in the bottom plastic hinges of two of the columns in a quarter scale model of a four-span bridge that was supported on three, two-column bents. The bridge was tested on shake tables to failure by Cruz and Saiidi (2012). Fig. 2 shows the shear Nomenclature FRP Fiber reinforced polymers NiTi Nickel-Titanium alloy NiTiCo Nickel-Titanium-Cobalt alloy SMA Shape memory alloy