Jenn-Shin Hwang

Direct displacement-based design for building with passive energy dissipation systems

This paper presents a seismic displacement-based design method for new and regular buildings equipped with passive energy dissipation systems (EDS). Using the substitute structure approach for the building structure and simulating the mechanical properties of the passive energy dissipation devices (EDD) by the effective stiffness and effective viscous damping ratio, a rational linear iteration method is proposed. A target displacement is at first specified and then the corresponding design force, strength and stiffness are obtained. Comprehensive procedures for displacement-based design of several buildings with passive energy dissipation systems are presented. The results are verified by dynamic inelastic time history analysis. Based on the study, it is found that the proposed displacement-based design method is straightforward and can accurately predict the nonlinear behavior of buildings equipped with passive energy dissipation systems.

An equivalent linear model of lead-rubber seismic isolation bearings

An equivalent linear model for the seismic analysis of base-isolated bridges with lead-rubber bearings (LRB) is established in this paper using an identification method. Recognizing that the inelastic displacement spectra with constant ductility ratios of an earthquake ground motion can be approximated by the equivalent elastic spectra with appropriate effective period shifts (or effective stiffness) and equivalent damping ratios, the effective stiffness and equivalent damping ratio of lead-rubber isolation bearings are determined based on the identification of the inelastic displacement response spectra of 20 earthquake ground motions. In the identification formulation, a nominal strain hardening ratio of the LRB is used. The proposed equivalent linear model is characterized as a modification of the current equivalent linear model provided by the AASHTO isolation guide specifications so that the proposed model can be readily applied to the practical analysis and design. Numerical comparisons indicate that the proposed model in general can predict comparably accurately with the current practical methods.

Equivalent elastic seismic analysis of base-isolated bridges with lead-rubber bearings

Abstract The specifications for the seismic analysis of base-isolated bridges have recently been provided by the American Association of State Highway Transportation Officials (AASHTO). The specified effective stiffness and equivalent damping ratio for an equivalent elastic system to the base-isolated bridge are evaluated. Based on this study it is found that these parameters of the equivalent elastic system are unrealistically represented. An alternate approach derived from an empirical method is then proposed for the determination of the effective period shift and equivalent damping ratio. The proposed empirical method is validated using an inelastic seismic analysis method. In addition, an analysis procedure incorporated with the emperical method is formulated to determine the maximum inelastic responses of the base-isolated bridges with lead-rubber bearings subjected to the AASHTO design earthquake and recorded ground motions. The effects of the stiffness (or flexibility) of bridge abutments and column bents on the seismic responses of base-isolated bridges are considered in the formulation. The results obtained from the proposed analysis procedure are compared with those determined from an inelastic seismic analysis method.

Comparison of distribution methods for viscous damping coefficients to buildings

A simple and convenient method often adopted by practising engineers designing supplemental viscous dampers to a building is to calculate damping coefficients of viscous dampers corresponding to a desired added damping ratio. To facilitate the design, various methods for distributing damping coefficients along the height of the building are compared in the study. In the article, two non-repetitive distribution methods are proposed and compared with some often adopted methods and a repetitive simplified sequential search algorithm. Numerical studies of three planar frames in which two are vertically irregular have indicated that all distribution methods may result in similar seismic responses if added damping ratio are the same. Nevertheless, compromising among a few design factors such as the total added damping coefficient, maximum damper force at one storey, total added damper force, control of storey drift and total computational efforts, one of the two proposed methods distributing the damping coefficient only to ‘efficient storeys’ may provide one of the better choices for the practical design of viscous dampers.

Seismic response prediction of HDR bearings using fractional derivative Maxwell model

Shaking table tests are conducted for one type of high damping rubber (HDR) bearing in this study. The bearing is strained up to a maximum shear strain of approximately 200%. The equivalent linear characteristics are determined according to AASHTO specifications based on the steady-state responses of sinusoidal tests. An equivalent linear analysis is performed to predict the seismic responses of the test structure and the results are compared with the measured responses. Besides, the fractional derivative Maxwell model of the bearing is formulated and an explicit integration scheme is implemented for solving the equation of motion of the test structure subject to ground motions. Two sets of numerical parameters involved in the fractional derivative Maxwell model are identified with respect to the best fit to the dynamic amplification function and phase angle determined from the sinusoidal tests. According to the comparison of the predicted and measured seismic responses of the test structure, it is found that the parameters should be determined from the best fit to the phase angle rather than the dynamic amplification function. For practical applications, the fractional derivative Maxwell model is equipped with an iteration procedure.

Composite damping ratio of seismically isolated regular bridges

In this paper seismically isolated regular bridges are modelled as two-degree-of-freedom systems in the longitudinal or transverse direction. The equivalent linear characteristics, including the effective stiffness and equivalent viscous damping ratio of isolation bearings, are used to formulate the equivalent linear theory of the seismically isolated regular bridge. The system composite damping ratio, attributed to the equivalent viscous damping ratio of isolation bearing and the viscous damping ratio of bridge column bents or piers, is obtained both by classical damping and nonclassical damping assumptions. The parametric influences including those of the mass ratio, stiffness ratio, viscous damping ratio of bridge column bents and equivalent viscous damping ratio of isolation bearings on the composite damping ratio are investigated. The composite damping ratio formulated in the study is compared with that obtained from the modal strain energy method. In addition, the parametric influences on the system composite damping ratio of isolated bridges are compared with those of isolated buildings.

Practical Analysis of Bridges on Isolation Bearings with Bi‐Linear Hysteresis Characteristics

Various equivalent elastic models specified in current bridge engineering practices for the seismic analysis of base-isolated bridges are summarized and evaluated. Two additional methods proposed by the California Department of Transportation (CALTRANS) are validated based on their predictions of the maximum inelastic seismic responses of base-isolated bridges. The CALTRANS proposed methods are implemented with an empirical model for the determinations of the effective stiffness and equivalent viscous damping ratios of isolation units and base-isolated bridges. A modal strain energy method combined with the concept of component energy ratio is utilized to formulate the “composite damping ratio” of an entire base-isolated bridge. A five-span regular bridge subjected to four design earthquakes and ten recorded ground motions is employed to investigate the accuracy of prediction using various equivalent elastic methods.

A refined model for base-isolated bridges with bi-linear hysteretic bearings

Recognizing that the inelastic displacement spectra with constant ductility ratios of an earthquake ground motion can be approximated by the equivalent elastic displacement spectra with the consideration of appropriate effective period shifts (or effective stiffness) and equivalent damping ratios, a refined equivalent linear model for the seismic analysis of base-isolated bridges with bi-linear hysteretic bearings is established in this paper using a system identification method. The parameters necessary to completely define a bi-linear hysteresis loop such as the elastic stiffness, yielding force, strain harden ratio and ductility ratio are all considered in the modeling. Eighteen California, one Washington and one Japan earthquake ground motions are used for the identification. The equivalent linear model established through the identification process is characterized as a modification of the current equivalent linear model provided by the AASHTO isolation guide specifications. Numerical comparisons indicate that the proposed model in general can predict accurately compared against the inelastic solutions and, therefore, the proposed model can be applied to the practical analysis of base-isolated bridges.

A fractional derivative model to include effect of ambient temperature on HDR bearings

Based on a previous application of the fractional derivative Kelvin model to the seismic response prediction of high damping rubber (HDR) bearings, the effect of ambient temperature is incorporated into the formulation of the model in this study, considering the variation of ambient temperature may significantly influence the mechanical characteristics of the bearings. Shaking table tests are conducted for a test structure composed of a steel deck isolated by four HDR bearings. The bearings were tested at various temperatures ranging from 0 to 28°C. Identified from the results of sinusoidal tests, the effective shear modulus is expressed in terms of maximum shear strain and temperature of the bearing. The numerical constants involved in the fractional derivative Kelvin model are represented by functions of ambient temperature. The extended model is validated by comparing the predicted seismic responses of the test structure with those measured from the earthquake tests conducted at various temperatures.

A study of reinforced concrete bridge columns retrofitted by steel jackets

Abstract In this paper, theoretical and experimental results of two as‐built circular reinforced concrete (RC) bridge columns and two columns retrofitted with steel jackets are presented. A constitutive model for concrete confined by a steel jacket is proposed. The proposed model is implemented into a sectional analysis to predict the lateral load‐deformation relationship of retrofitted columns. 2/5 scaled RC bridge columns are designed based on the standard details of the existing bridge columns mostly built in late 1980s and early 1990s, in Taiwan (Ministry of Transportation and Communication, 1987; 1995). The columns are expected to have a flexural failure mode during severe ground shaking. Displacement‐controlled cyclic loading tests were conducted to obtain the seismic performance of the columns. The experimental results showed that the bridge column retrofitted with steel jacketing could greatly improve seismic performance measured based by the strength and ductility. The analytical results showed that the proposed constitutive model, implementing sectional analysis, could well capture the lateral force‐displacement relationship of the bridge columns retrofitted with steel jackets.