Study On Superelastic SMA Isolator And Its Application To Lattice Grid Structure

In recent years, spatial structures have been widely used in long span buildings in our country. Because of large sizes and complex mechanical characteristics, seismic performance of spatial structures has received more and more attention. Studies have shown that seismic isolation technique has proven to be an effective means for reducing seismic response of structures. Three types of isolation bearings including rubber isolator, sliding isolator and isolators with added energy device are main control components all over the world. Over the past two decades, such isolation bearing have been successfully applied to multi-story buildings, high rise buildings and bridges. However, current technologies present some limitations, such as problems r Elated to durability and geometry restoration after strong events.To develop high performance isolation devices, shape memory alloy(SMA) has been introduced to seismic control of structures. Considering the features of dynamic property and seismic response, a new type of isolation device consisting of a flat sliding bearing and several super Elastic h Elical springs entitled as SMA spring-friction bearing(SFB) is proposed. In this study, mechanical experiments and theoretical analysis have been conducted on the proposed SFB device. The main contents and achievements are as follows.(1) A new type of SFB device is developed. In this hybrid isolation bearing, the flat sliding surface is aimed at supporting the gravity loads of superstructure while allowing large displacement due to the low friction between steel-teflon interface, and the role of the superelastic SMA helical springs is to provide re-centering capacity to depress maximum and residual displacement. According to the working principle of the SFB, a theoretical model is established. The corresponding computation model of the key parameters in the theoretical model is derived. Numerical simulation of SFB devices with different design schemes is carried out, and the major characteristics of hysteretic behavior of the SFB are mastered.(2) Large scale super Elastic helical springs with two type of Ni Ti SMA(Ni50.8Ti49.2 and Ni51.0Ti49.0) are fabricated. Cyclic tension-compression tests on the SMA helical springs under different loading conditions are conducted to study the influence of cyclic number, loading frequency and displacement amplitude on the hysteretic behavior. The results show that the SMA helical springs exhibit stable hysteretic curves, exc Ellent re-centering performance and large deformation capacity. The remarkable properties make the SMA spring an ideal subcomponent for use in re-centering devices.(3) Two types of SFB devices(SFB I and SFB II) are designed and manufactured. In such isolation devices, the SMA helical springs with and without pre-compression are utilized to form different functions. The pseudo-static tests with several cases are carried out. The hysteretic curves of the two types of SFB device under various vertical pressures, loading frequencies and displacement amplitudes are obtained. The results show that the SFB devices can provide full and stable hysteretic loops, proving their great potentials for re-centering isolation for engineering structures.(4) Based on the theoretical model of the SFB, hysteretic behaviors of SFB I and SFB II are numerically simulated by using the finite element(FE) software ABAQUS. The solid element and connector element are respectively employed to simulate the nonlinear behavior of the flat sliding bearing and the SMA helical spring. The hysteretic responses of the FE model are compared with those of realistic SFB model. The results show that the numerical results match closely with the experimental data, proving the validity of the proposed numerical model of the SFB.(5) A passive control system of lattice grid structures with SFB is established. The re-centering isolation devices based on working mechanism of SFB II are designed. Computational model of the controlled lattice grid structure is established with SAP2000 software. Considering the mechanical properties of the nonlinear elements provided by SAP2000, a parallel system of the friction element, the multi-linear elastic element and the Wen element are used to reproduce the mechanical behavior of the proposed SFB. Numerical simulation of seismic response of the isolated lattice grid structure is carried out. The results indicate that the SFB can reduce the structural response effectively, and that the displacement of the SFB can be limited to the design range.