This study presents a novel type of self-centering damper employing NiTi Shape Memory Alloy (SMA) bars combined with variable friction mechanism for tunable hysteretic behavior and enhanced deformability. The new damper, which is called SMA-based variable friction damper (SMA-VFD), aims to significantly amplify the maximum deformability and energy-dissipation capability compared with the existing self-centering devices with tensioning SMA members. The cyclic response, phase transformation behavior, and in-situ observation of the microstructure of individual SMA bars receiving different heat treatments were first investigated to promote an adequate understanding of the link between the microscopic structure and the macroscopic property of the large size SMA bars. This is followed by a detailed discussion of the SMA-VFD, where theoretical equations as well as three performance parameters, i.e., self-centering factor γ, displacement amplification factor ω, and energy-dissipation factor λ, were derived to predict the mechanical behavior of the damper. Proof-of-concept SMA-VFD specimens were manufactured, where different preloads and friction coefficients were considered which could affect the cyclic performance. A three-dimensional finite-element model was then developed to simulate the nonlinear responses of the test specimens. Both the analytical prediction and numerical simulation exhibited good agreements with the experimental results. Utilizing the validated numerical model, the influence of manufacturing tolerance on the initial stiffness of the proposed damper was particularly investigated to help explain some important findings from the present experimental study. 

Manufacturing, testing and simulation of novel SMA-based variable friction dampers with enhanced deformability

This study presents a novel type of self-centering damper employing NiTi Shape Memory Alloy (SMA) bars combined with variable friction mechanism for tunable hysteretic behavior and enhanced deformability . The new damper, which is called SMA-based variable friction damper (SMA-VFD), aims to significantly amplify the maximum deformability and energy-dissipation capability compared with the existing self-centering devices with tensioning SMA members. The cyclic response, phase transformation behavior, and in-situ observation of the microstructure of individual SMA bars receiving different heat treatments were first investigated to promote an adequate understanding of the link between the microscopic structure and the macroscopic property of the large size SMA bars. This is followed by a detailed discussion of the SMA-VFD, where theoretical equations as well as three performance parameters, i.e., self-centering factor γ, displacement amplification factor ω, and energy-dissipation factor λ, were derived to predict the mechanical behavior of the damper. Proof-of-concept SMA-VFD specimens were manufactured, where different preloads and friction coefficients were considered which could affect the cyclic performance. A three-dimensional finite-element model was then developed to simulate the nonlinear responses of the test specimens. Both the analytical prediction and numerical simulation exhibited good agreements with the experimental results. Utilizing the validated numerical model, the influence of manufacturing tolerance on the initial stiffness of the proposed damper was particularly investigated to help explain some important findings from the present experimental study.

Analytical approach for the design and optimal allocation of shape memory alloy dampers in three-dimensional nonlinear structures

Although controller devices highly improve the seismic performance of civil structures, they usually incur enormous financial costs. A variety of sequential methods for minimizing the cost function have been proposed in the literature. In most of these methods, the control system is optimally designed after the best structural configuration is provided. However, such sequential procedures are unable to yield the best overall design. This paper discusses a combined structural and control optimization approach in which the structural mass and controlled system energy are simultaneously minimized. To this end, a twofold objective function is defined by linearly combining the structural mass and linear quadratic regulator (LQR) performance index. It is shown that the optimal value of this objective function depends on the initial condition, and to circumvent this problem, an alternative objective function is introduced which allows for solving the problem of initial condition dependency. Given the complexity of the problem, a variable neighborhood search (VNS) metaheuristic method is developed. The performance of the developed method is demonstrated through case studies involving the determination of structural responses for shear-type frames. The case studies are presented for evaluating and comparing the effectiveness of the proposed simultaneous technique and the conventional sequential method. The responses of the structures are determined under an array of strong ground motions. The results of the comparative analyses revealed that the simultaneously optimized cases using the proposed methodology had superior dynamic performance compared to the sequentially optimized cases.

Simultaneous optimization approach for combined control–structural design versus the conventional sequential optimization method

Uniform damping ratio-based design method for seismic retrofitting of elastoplastic RC structures using viscoelastic dampers

Summary
This paper proposes a new seismic modification methodology for optimal design of multistory frame structures to avoid damage and attain a robust response to extreme hazards The methodology determines added damping devices of optimal size at strategic locations and modifies story stiffnesses, by solving a newly formulated constrained optimization problem The objective is to minimize a cost function representing total energy while satisfying the equations of motion and allowable interstory drifts for given severe ground motions The proposed methodology combines the classical linear quadratic regulator, which searches for an optimal gain matrix, with an analysis-redesign iterative procedure to produce a robust analysis redesign (RAR) iterative approach A new algorithm approximates the equivalent stiffness to fit the displacement gains At convergence, the RAR procedure will yield optimal sizing and new topology for the designed system The RAR procedure is developed here for shear structures However, RAR can be utilized for complex structures by modeling them first as equivalent shear-type structures having stiffness properties that yield the same maximum interstory drifts for the given records Two numerical examples of shear-type structures are examined These examples use structures already having an “optimal” configuration, which are further optimized using the methodology developed herein In addition, a numerical example comprised of a 10-story moment resisting frame structure is presented to illustrate the use of the equivalent shear structure approach The results of the exemplified retrofitted moment resisting frame structure show great improvements in its dynamic response

Assaf Shmerling

Papers

Seismic retrofit of frame structures using passive systems based on optimal control

Analytical approach for the design and optimal allocation of shape memory alloy dampers in three-dimensional nonlinear structures

Seismic structural design methodology for inelastic shear buildings that regulates floor accelerations

Seismic upgrade of structures using the H∞ control problem for a general system interconnection paradigm

Seismic retrofit of two-way unsymmetric-plan buildings with added masses