Reginald DesRoches

SEISMIC RETROFIT OF SIMPLY SUPPORTED BRIDGES USING SHAPE MEMORY ALLOYS

Recent earthquakes in the United States and Japan have highlighted the vulnerability of bridges to collapse due to excessive movement at the intermediate hinges and abutments. This study investigates the effectiveness of shape memory alloy restrainer bars to reduce the seismic vulnerability of bridges. Full-scale tests of shape memory alloy (SMA) restrainer bars are conducted to determine their force-deformation and energy dissipation characteristics. The 25.4 mm diameter bars are subjected to cyclical strains up to 8%, with minimum residual deformation. The effectiveness of the SMA restrainer bars in bridges is assessed through nonlinear analyses of a typical multi-span simply supported bridge. The SMA restrainer bars are effective in limiting the relative displacement at the piers and abutments. In addition, the bars are shown to be very effective for near-field ground motion.

Analytical Seismic Fragility Curves for Typical Bridges in the Central and Southeastern United States

Seismic fragility curves for classes of highway bridges are essential for risk assessment of highway transportation networks exposed to seismic hazards. This study develops seismic fragility curves for nine classes of bridges (common three-span, zero-skew bridges with non-integral abutments) common to the central and southeastern United States. The methodology adopted uses 3-D analytical models and nonlinear time-history analyses. An important aspect of the selected methodology is that it considers the contribution of multiple bridge components. The results show that multispan steel girder bridges are the most vulnerable of the considered bridge classes while single-span bridges tend to be the least vulnerable. A comparison of the proposed fragility curves with those currently found in HAZUS-MH shows a strong agreement for the multispan simply supported steel girder bridge class. However, for other simply supported bridge classes (concrete girder, slab), the proposed fragility curves suggest a lower vulnerability level than presented in HAZUS-MH.

Seismic Response Control Using Shape Memory Alloys : A Review

Shape memory alloys (SMAs) are a class of alloys that possess numerous unique characteristics. They offer complete shape recovery after experiencing large strains, energy dissipation through hysteresis of response, excellent resistance to corrosion, high fatigue resistance, and high strength. These features of SMAs, which can be exploited for the use in control of civil structures subjected to seismic events, have attracted the interest of many researchers in structural engineering over the past decades. This article presents an extensive review of seismic applications of SMAs. First, a basic description of two unique effects of SMAs, namely shape memory and superelastic effect, is provided. Then, the mechanical characteristics of the most commonly used SMAs are discussed. Next, the material models proposed to capture the response of SMAs in seismic applications are briefly introduced. Finally, applications of SMAs to buildings and bridges to improve seismic response are thoroughly reviewed.

SHAPE MEMORY ALLOYS IN SEISMIC RESISTANT DESIGN AND RETROFIT: A CRITICAL REVIEW OF THEIR POTENTIAL AND LIMITATIONS

Shape memory alloys (SMAs) are a class of materials that have unique properties, including Youngs modulus-temperature relations, shape memory effects, superelastic effects, and high damping characteristics. These unique properties, which have led to numerous applications in the biomedical and aerospace industries, are currently being evaluated for applications in the area of seismic resistant design and retrofit. This paper provides a critical review of the state-of-the-art in the use of shape memory alloys for applications in seismic resistant design. The paper reviews the general characteristics of shape memory alloys and highlights the factors affecting their properties. A review of current studies show that the superelastic and high-damping characteristics of SMAs result in applications in bridges and buildings that show significant promise. The barriers to the expanded use of SMAs include the high cost, lack of clear understanding of thermo-mechanical processing, dependency of properties on temperature, and difficulty in machining.

Bridge Functionality Relationships for Improved Seismic Risk Assessment of Transportation Networks

Relationships between bridge damage and the resulting loss of functionality of the bridge are critical to assessing the impact of an earthquake event on the performance of the transportation network. This study addresses this data need by use of a Web-based survey of central and southeastern U.S. Department of Transportation bridge inspectors and officials. Results of the 28 responses are analyzed and offer a link between various types of bridge component damage and the expected level of allowable traffic carrying capacity due to closure decisions and repair procedures. This data is utilized to assess the probability of meeting various damage states, expressed in terms of restoration of functionality, and subsequently facilitate the refinement of component limit-state capacities for analytical fragility curve development. The bridge functionality relationships and methodology outlined serve as the basis for refinement of critical tools in the seismic risk assessment framework and improved assessment of transportation network performance.

Unseating prevention for multiple frame bridges using superelastic devices

Unseating of bridge spans due to excessive relative hinge opening is a common problem for bridges subjected to strong ground motion. Various unseating prevention devices have been developed in both the United States and Japan to try to reduce the likelihood of collapse due to unseating. This paper presents the results of the evaluation of unseating prevention devices using nitinol shape memory alloys (SMAs). Superelastic SMAs have the ability to remain elastic under very large deformations, due to a solid-state martensitic transformation. This unique property leads to enhanced performance of the adaptive superelastic unseating prevention device, compared with conventional devices used in the United States and Japan. To assess the effectiveness of the devices, nonlinear time history analyses are performed on a typical multiple frame reinforced concrete box girder bridge using a suite of representative ground motions. The results show that for multiple frame reinforced concrete box girder bridges the adaptive superelastic devices are very effective in limiting the relative hinge displacement and preventing unseating, compared with the conventional steel cable restrainers.

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.

Large scale testing of nitinol shape memory alloy devices for retrofitting of bridges

A large scale testing program was conducted to determine the effects of shape memory alloy (SMA) restrainer cables on the seismic performance of in-span hinges of a representative multiple-frame concrete box girder bridge subjected to earthquake excitations. Another objective of the study was to compare the performance of SMA restrainers to that of traditional steel restrainers as restraining devices for reducing hinge displacement and the likelihood of collapse during earthquakes. The results of the tests show that SMA restrainers performed very well as restraining devices. The forces in the SMA and steel restrainers were comparable. However, the SMA restrainer cables had minimal residual strain after repeated loading and exhibited the ability to undergo many cycles with little strength and stiffness degradation. In addition, the hysteretic damping that was observed in the larger ground accelerations demonstrated the ability of the materials to dissipate energy. An analytical study was conducted to assess the anticipated seismic response of the test setup and evaluate the accuracy of the analytical model. The results of the analytical simulation illustrate that the analytical model was able to match the responses from the experimental tests, including peak stresses, strains, forces, and hinge openings.

Shape Memory Alloy Tension/Compression Device for Seismic Retrofit of Buildings

A tension/compression device is developed for applications as bracing elements in buildings. The device is designed to allow Nitinol forms, such as helical springs or Belleville washers, to be used in compression. The device allows both overall extension (tension) and compression while subjecting the Nitinol to an optimum deformation mode. It is possible, due to the versatility of the design, to adjust the force and stroke of the device without changing the overall configuration. This new device is subjected to a cyclic loading protocol that tests the Nitinol element’s ability to recover large deformations. The effect of different Nitinol configurations and a cyclic loading history are evaluated in the study. The results show that Nitinol helical springs produce good recentering and damping behavior while Nitinol Belleville washers show good potential to form the basis for a Nitinol damping device.

Retrofitted Bridge Fragility Analysis for Typical Classes of Multispan Bridges

Retrofitted bridge fragility curves provide a powerful tool for assessing the effect of retrofit measures on the seismic performance of different bridge types under a range of loading levels. Traditional methods for retrofit assessment typically evaluate the effectiveness of retrofit based on the performance of individual components. However, the use of fragility curves for retrofitted bridges has the ability to capture the impact of retrofit on the bridge system vulnerability. Using three-dimensional nonlinear analysis, fragility curves are developed for four common classes of multispan bridges and five retrofit methods. The results show that the effectiveness of retrofit is a function of bridge type and damage state. General conclusions of the influence of the different retrofit measures on the fragility of each class of typical bridges in the Central and Southeastern United States, as well as the fragility parameters, are presented. The results from this work can be used to enhance regional seismic risk assessment and can form the basis for retrofit cost-benefit studies.