Experimental and numerical investigations on seismic applications of high damping SMAs

There are several different mechanisms for creating a restoring force to return a building structure to plumb after an earthquake. One approach is to allow structure to undergo controlled rocking at discrete locations such as column-base joint or beam-column joints. Another approach is to employ braces or seismic control devices with self-centering capabilities. Due to its inherent nonlinear elastic behavior, shape memory alloys (SMAs) have been considered to develop self-centering braces or devices. Recently, NiTiHfPd alloys that have very high strength (up to 2000 MPa), high dissipation/damping capacity, good cyclic stability and large operating temperature have been developed. This study explores the superelastic response of NiTiHfPd SMAs under various conditions and illustrates their application into seismic applications. In order to collect experimental data, uniaxial tests are conducted on superelastic NiTiHfPd SMAs in the temperature range of -35 ºC to 25 ºC, and at the loading frequencies of 0.05 Hz to 1 Hz with four different strain amplitudes. The effects of loading rate and temperature on superelastic characteristics of NiTiHfPd SMAs are examined. A numerical model that reliably simulates the response of NiTiHfPd SMAs is developed. Then, a four-story moment resisting frame with and without supplementary SMA damping elements is designed and modeled. Nonlinear response history analyses are conducted to assess the performance of NiTiHfPd SMAs in mitigating seismic response and limiting residual drifts of steel frames subjected to strong ground motions.