PSI - Issue 64

6

Author name / Structural Integrity Procedia 00 (2019) 000–000

M. Saiid Saiidi et al. / Procedia Structural Integrity 64 (2024) 2021–2027

2026

Fig. 8. (left) NiTiCo bar specimen dimensions; (right) NiTi bar fracture under cyclic loads.

The study of NiTiCo bar characteristics is near completion. It includes the cyclic response under various temperatures as well the fatigue behavior of NitTiCo and other types of SMA bars. The current analytical studies consist of moment curvature analysis of full-scale reinforced concrete columns with conventional concrete and reinforcing steel materials and equivalent NiTiCo-reinforced columns that incorporate ECC in the plastic hinge. The former to provide recentering and the latter to minimize the plastic hinge damage. The reference conventional column is a cantilever member with a diameter of 1.2 m. The clear cover is 51 mm. The column is reinforced with 18-#11 (diameter= 35.8 mm) longitudinal mild steel Gr. 60 bars (specified yield strength= 420 MPa). The transvers reinforcement consists of #5 (15.9 mm) hoops spaced at 152 mm. The properties of the reference column were based on a full-scale model that was tested on a shake table by Schoettler et al. (2015). Scale model tests are envisioned as part of the current project to utilize NiTiCo bars and determine their recentering effectiveness based on comparison with the cyclic response of conventional columns. 4. Summary and Conclusions Much interest has been shown by bridge earthquake engineering researchers and design engineers to explore the use of novel materials in lieu of conventional concrete and reinforcing steel in improving the seismic performance of bridges subjected to strong earthquakes. Unlike innovations in conventional construction materials that have been incremental, the adaptation of novel materials in bridge engineering is considered a disruptive approach. Considering the severity of the economic impact and failure consequence in bridges, this adaptation needs to be science based with ample data to support the premise of the potentially superior performance of novel materials. The study presented in this article demonstrated the various stages of progress in building confidence in the behavior of shape memory alloys that specifically are made with Nickel and Titanium (NiTi). It was shown that initial research was focused on determining whether superelasticity of bars can transform concrete beam behavior into a superelastic response as well. Because the ultimate goal of taking advantage of superelasticity was to provide recentering capability for bridge columns in high seismic zones, in follow up studies the cyclic load response of columns individually, piers, bridge models was investigated, with eventual application in an actual bridge in Seattle, Washington. The high cost of large diameter NiTi bars provided incentive to explore higher strength SMAs that incorporated Cobalt in addition to Nickel and Titanium. This paper showed a sample data on material characteristics studies of NiTiCo bars that is in progress. The relatively high yield strength of NiTiCo is expected to reduce the amount of SMA bars and make columns more cost effective and easier to construct. Acknowledgements The studies summarized in this article were funded by US Federal Highway Administration, the National Science Foundation, and the state departments of transportation in Washington State, Nevada, California, and Alaska. The authors would like to thank SAES Smart Materials and Headed Reinforcement Corp. (HRC) for producing the SMA bars and the couplers used in this study. Mr. Ed Little of Fiber Matrix is thanked for his assistance in providing the ECC mix. Mr. Don Newman and Brendan Morris are thanked for the construction of the test models. The laboratory staff including Dr. Patrick Laplace, Chad Lyttle, and Todd Lyttle are thanked for their invaluable assistance with testing.