Jason McCormick

Seismic Vibration Control Using Superelastic Shape Memory Alloys

Superelastic NiTi shape memory alloy (SMA) wires and bars are studied to determine their damping and recentering capability for applications in the structural control of buildings subjected to earthquake loadings. These studies improve the knowledge base in regard to the use of SMAs in seismic design and retrofit of structures. The results show that the damping properties of austenitic SMAs are generally low. However, the residual strain obtained after loading to 6% strain is typically <0.75%. In general, it is shown that large diameters bars perform as well as wire specimens used in non-civil-engineering applications. The results of a small-scale shake table test are then presented as a proof of concept study of a SMA cross-bracing system. These results are verified through analytical nonlinear time history analysis. Finally, a three-story steel frame implementing either a traditional steel buckling-allowed bracing system or a SMA bracing system is analyzed analytically to determine if there is an advantage to using a SMA bracing system. The results show that the SMA braces improve the response of the braced frames.

Testing of Superelastic Recentering Pre-Strained Braces for Seismic Resistant Design

Over the past decade, the use of shape memory alloys (SMAs) in passive control devices has been explored. Nevertheless, some aspects in regards to the cyclic behavior of SMAs and the effect of pre-straining need to be clarified. In this study, small-scale shake table tests have been performed to explore the effectiveness of SMA bracing systems as compared to steel bracing systems. The reduced-scale experimental results imply that SMAs used in braces are more effective in controlling the response of a steel frame compared with a traditional bracing system. A finite element model (FEM) of the frame is developed in order to compare the analytical results with the shake table tests. Further, the effect of pre-straining the SMA braces is evaluated through both experimental and analytical studies. The results show that pre-straining improves the performance of the frame compared to the nonpre-strained case. However, as the level of pre-straining increases above approximately 1.0% to 1.5%, the benefits of pre-straining decrease compared with low-to-moderate pre-strain levels.

Effect of mechanical training on the properties of superelastic shape memory alloys for seismic applications

The objective of this study is to evaluate the effect that mechanical training has on the properties of NiTi based shape memory alloys. The unique mechanical behavior of shape memory alloys, which allows the material to undergo large deformations while returning to their original undeformed shape through either the shape memory effect or superelastic effect, has shown potential for use in seismic design and retrofit applications for civil engineering structures. However, cyclic loading has been shown to degrade the energy dissipation capacity and decrease the recentering capability of the material due to fatigue effects. It has been recommended that mechanical training of superelastic shape memory alloys prior to use in applications can limit these fatigue effects. A factorial experimental design is employed to explore the optimal number of mechanical training cycles, strain level of training, and the effect of the loading rate after training in order to minimize the degradation in the loading plateau stress, residual strain, and equivalent viscous damping properties. The results presented can serve as a guide to optimizing the properties of NiTi shape memory alloys for seismic applications. The ability to obtain stable properties of shape memory alloys under a specified training schedule further supports the eventual implementation of the material into actual building and bridge systems as seismic design and retrofit devices.

Effect of cyclic modeling parameters on the behavior of shape memory alloys for seismic applications

The cyclic behavior of shape memory alloys (SMAs) in their austenitic form is studied to determine the most appropriate method of modeling in terms of both accuracy and ease of implementation. Four different models for SMA behavior are evaluated: (a) a simple nonlinear elastic model, (b) a trigger-line model, (c) a one-dimensional thermomechanical model, and (d) a one-dimensional thermomechanical model which accounts for the behavior of SMAs under cyclic loading. Using a two degree-of-freedom bridge model with SMA restrainers and a single degree-of-freedom building model with SMA cross-braces, the effect of using the different models on the seismic response of the bridge and building is evaluated. Using a suite of nine earthquake ground motions, the displacement response histories with the four different models are compared. The results illustrate that although the models show quite different behaviors for the SMAs, the resulting responses of the bridge and building are insensitive to the type of model used. For most of the ground motion records used, the difference in the maximum displacement for the four models was less than 15%. This study lends support to the use of more simplified models when evaluating the effectiveness of the SMAs for seismic response modification.

Damping Properties of Shape Memory Alloys for Seismic Applications

Shape memory alloys (SMAs) in martensitic form and austenitic form are studied to evaluate their damping potential for applicat ions in earthquake engineering. Shape memory alloy wires and bars are subjected to cyclical loading s similar to that expected during a seismic event. The damping properties of the shape memory alloy s in both the martensitic form and austenitic form are c ompared with respect to bar size, loading rate, and maximum strain cycle. The results show that in the superelastic form, the damping properties of shape memory alloys are generally low, ranging from 2%-7% equivalent viscous damping. The damping properties generally peak at 4%5% strain and begin to degrade at larger strains . In the martensitic form, shape memory alloys have significant energy dissipation with equivalent viscous damping ratios ranging from 15%-25%. Strain rate effects are evaluated by loading the bars at rates up to 2Hz (maximum strain rate of 7.90% per second). The results show that increased strain rates lead to a red uction in the energy dissipation capabilities of shape memory alloys in both forms. The effectiveness of using shape memory alloys for damping applications is evaluated through a comparison with current passive energy dissipation technology in the field of earthquake engineering showing the viability of using shape memory alloys for passive damping applications.

Cyclic Testing of Welded HSS-to-HSS Moment Connections for Seismic Applications

AbstractHollow structural sections (HSS) are currently underutilized in seismic moment frame systems as possible alternatives to typical wide-flange members. To explore the potential use of HSS moment connections in seismic moment frame systems and evaluate connection detailing requirements, experimental tests of welded unreinforced and reinforced HSS-to-HSS connections are conducted under increasing cyclic rotations to failure. The behavior of the unreinforced connections is limited by their inability to isolate inelasticity to the beam member and panel zone region leading to eventual fracture at lower than anticipated drift levels. However, through plate and external diaphragm plate reinforced HSS-to-HSS moment connections are able to move the location of inelasticity into the HSS beam member resulting in desired beam plastic hinging. Although the unreinforced connections have limited ductility, the reinforced connection detailing leads to stable degradation of the moment capacity at increasing drift le...

Properties of large-diameter shape memory alloys under cyclical loading

This study evaluates the properties of superelastic shape memory alloys under cyclical loading to asses their potential for applications in seismic resistant design and retrofit of civil engineering structures. Shape memory alloy bars are tested to evaluate the effect of bar size (diameter) and loading history on the strength, equivalent viscous damping, and recentering properties of the shape memory alloys in superelastic form. The bars are tested under both quasi-static and dynamic loading. The results show nearly ideal superelastic properties can be obtained in large diameter shape memory alloy bars. However, comparing these results to previous studies, the more common wire form of the shape memory alloys show higher strength and damping properties compared with the large bars. The recentering capabilities (based on residual strains) are not affected by the section size of the bar. Overall, the damping potential of superelastic shape memory alloys is low for large diameter bars, typically less than 7% equivalent viscous damping. Degradation of the superelastic properties of the shape memory alloys occurs for cyclical strain greater than 6%, leading to increased residual strains and reduction in energy dissipated. Finally, strain rate effects are evaluated by subjecting the shape memory alloys to loading rates representative of typical seismic loadings. The results show that increased loading rates lead to slight decreases in the equivalent damping, but have negligible effect on the recentering of the shape memory alloys.

Evaluation of HSS-to-HSS Moment Connections for Seismic Applications

Hollow structural sections (HSS) are currently underutilized in seismic applications in structures. To address this, a connection design methodology is derived for both unreinforced and reinforced, fully welded, HSS-to-HSS moment connections. Experimental testing of two unreinforced HSS-to-HSS connections with unmatched and matched beam and column widths is conducted under cyclic loads to failure. The hysteretic behavior shows that these connections are limited in their ability to isolate inelasticity to the beam member and panel zone region. A finite element parametric study also is used to better understand the effect of different parameters on internal and external reinforced HSS-to-HSS connection performance under cyclic loads. Based on the finite element models, reinforced connections show the ability to develop plastic hinging in the beam members with a normalized moment capacity greater than unity.

Large-scale testing of hollow structural sections for seismic applications

ABSTRACT Hollow structural sections (HSS) are highly efficient members which have been underutilized for bending applications, such as beam members, in regions of high seismicity. To increase the use of HSS beam members, a better understanding of their ability to form stable plastic hinges without a dramatic decrease in strength is needed. This paper describes an experimental study focusing on the behavior of HSS members under cyclic bending with respect to various depth-to-thickness ratios. Five U.S. stock HSS beam sections are tested ranging in size from HSS203.2x101.6x6.4 to HSS304.8x152.4x6.4. Increasing cyclic displacement cycles up to rotations of 0.08 radians are applied. These tests show that the depth-to-thickness ratio plays an important role in the stable deformation behavior of these members. Depth-to-thickness ratios larger than 48.5 result in decreased secant stiffness and energy dissipation capacity with continued cycling. The overall results indicate that depth-to-thickness ratios below 19.9 provide more stable inelastic bending behavior suggesting their viability as beam members in seismic moment frames.