Shear Modulus and Damping Ratio of Cohesive Soils

This article presents results from a laboratory investigation into the dynamic characteristics of reconstituted and undisturbed cohesive soils by means of resonant-column tests. In particular, results showing the influence of various soil parameters, such as confining stress, overconsolidation ratio, void ratio, plasticity index, calcium carbonate content, and time of confinement on shear modulus and damping ratio at small and high shear strains are presented and then discussed. Relationships for the small-strain shear modulus, the degradation of shear modulus at high strains, and the increase of damping ratio at high strains over its small-strain value are proposed. Finally, the practical implications of the results in the context of seismic site response analysis are discussed.     

Seismic Reliability Assessment of a High-Voltage Disconnect Switch Using an Effective Fragility Analysis

This article deals with evaluation of the seismic vulnerability of a high-voltage vertical disconnect switch, one of the most vulnerable elements of electric substations. The main objective of the research is to evaluate the seismic fragility of the apparatus using a new effective method. By combining standard reliability methods for time-invariant problems with the response surface technique, this original procedure called “EFA” (Effective Fragility Analysis) permits the evaluation of fragility curves using a very limited number of numerical simulations. On the basis of experimental tests, to determine the mechanical characteristics of the disconnect switch components (ceramic, joints, etc.) the fragility curves of the equipment analyzed are carried out. The results are discussed and compared with the results of Monte Carlo simulations, which confirm the reliability of the procedure.     

Analysis of Carbon Fiber Sheet-Retrofitted RC Bridge Columns Under Lateral Cyclic Loading

In recent years, the use of carbon fiber sheet (CFS) to provide lateral confinement for enhanced ductility and strength of reinforced concrete bridge columns has been increasing. While the monotonic behavior of CFS-confined concrete has been studied extensively, its cyclic response has not been fully understood. Most of the available studies are experimental investigations, hence there is a need to develop an analytical model to simulate the experimental results. Analysis of the hysteretic behavior of CFS-retrofitted circular columns is presented in this article using the fiber element that is based on cyclic constitutive models of longitudinal reinforcement and concrete confined by both CFS and tie reinforcement. The analysis was verified based on available cyclic test data and the analysis provides good agreement with the experimental results. Results show that flexural strength and ductility of columns wrapped with CFS increases as CFS ratio increases. However, as tie reinforcement ratio increases, there is no much difference on the hysteretic response for low tie reinforcement ratios. Using the fiber element analysis, the effect of CFS retrofit on the seismic response of a 7.5 m tall prototype pier built in the 1970s to 1980s is also clarified.     

Effects of Vertical Load,Strain Rate and Cycling on the Response of Lead-Rubber Seismic Isolators

Lead-rubber isolators represent a valid and economic solution for the seismic isolation of bridge structures and modern manufacturing techniques make available large devices. Velocity effects on small to medium-scale isolators have been discussed by several authors (e.g., Clark et al., 1997 Clark, P. W., Aiken, I. D. and Kelly, J. M. 1997. Experimental studies of the ultimate behavior of seismically isolated structures, Berkeley, CA: Earthquake Engineering Research Center, University of California. Report No. UCB/EERC-97/18 [Google Scholar]; Thompson et al., 2000 Thomson, A. C., Whittaker, A. S., Fenves, G. L. and Mahin, S. A. 2000. “Property modification factors for elastomeric seismic isolation bearings”. In Proceedings of the 12th World Conference on Earthquake Engineering, New Zealand: Auckand. January [Google Scholar]) as well as included in reports of experimental programs (e.g., CERF, 1999 CERF. Civil Engineering Research Foundation, Summary of evaluation findings for the testing of seismic isolation and energy dissipation devices. CERF Report No. 40404. 1999.  [Google Scholar]). Only recently, however, the behavior of large devices was validated under full-scale displacements, loads, and velocities.

In this article, results obtained from an experimental investigation on the effects of axial load and strain rate on the performance of a full-scale lead-core elastomeric bearing for bridge applications, are reported. The bearing response was analyzed with particular attention to the variation of critical performance characteristics in order to produce a set of information that could be implemented in a physically motivated numerical model.

The results, in line with additional tests performed on similar full-scale bearings at the Caltrans SRMD Testing Facility at the University of California San Diego, indicate a moderate effect of the applied vertical load but a significant effect of the strain rate and cycling on all the significant response parameters. This information should be taken into account by designers, particularly when high component of velocities are associated with the expected seismic motion. A simplified numerical model is proposed for the assessment of lead-rubber bearing performance.     

Bayesian Updating of Seismic Demand Models and Fragility Estimates for Reinforced Concrete Bridges with Two-Column Bents

Probabilistic models have been developed in a previous study by the authors to estimate the seismic deformation demands on structural components of reinforced concrete (RC) bridges with two-column bents. However, such models should be updated to reflect the latest laboratory of field data. Using a Bayesian approach, this article updates a currently available probabilistic model for the deformation demands of columns in bridges with two-column RC bents. The updated model incorporates information from newly available experimental data from shake table tests conducted based on a record of the 1994 Northridge Earthquake for a structural system with three bents with two columns per bent. The updated model is more accurate than the previous one in predicting the deformation demand of bridges with two-column RC bents and reduces the statistical uncertainty due to the addition of new data. As an application, fragility estimates for an example bridge are computed using the updated model both at the component (column) and system (bridge) levels.     

Use of Pushover Analysis for Predicting Seismic Response of Irregular Buildings: A Case Study

Structural irregularity undermines capability of conventional methods for 2D pushover analysis to closely approximate results from inelastic dynamic analysis. In recent years, different methods have been developed to overcome such limitation and their suitability has been checked with reference either to idealized building models or to geometrically simple tested structures. In this paper, suitability of one such method, proposed by Fajfar et al. [2005] Fajfar, P., Maru?i?, D. and Perus, I. 2005. Torsional effects in the pushover-based seismic analysis of buildings. Journal of Earthquake Engineering, 9(6): 831–854. [Taylor & Francis Online], [Web of Science ®] , [Google Scholar], is evaluated considering an existing school building which presents both vertical and plan irregularities. Types of irregularity encompass not only those usually considered by seismic codes but also those deriving from a bad conceptual design and construction inaccuracies, very frequent at the year of construction (1974). It is found that, even under such complex irregularity conditions, this ‘modified’ pushover analysis correlates well results from inelastic dynamic analysis almost up to failure, since, in most cases, its predictions of interstorey drifts and plastic rotations are conservatively close to values from inelastic dynamic analysis. Even failure mechanism, consisting of a floor mechanism at the third level, is correctly predicted, thus demonstrating adequacy of such method for actual framed structures.     

Multi-Objective Evolutionary Seismic Design with Passive Energy Dissipation Systems

The problem of multi-objective seismic design optimization is examined within the context of passive energy dissipation systems. In particular, a genetic algorithm approach is developed to enable the evaluation of the Pareto front, where maximum inter-story drifts and maximum total accelerations, both important measures for damage, serve as objectives. Here the cost of the passive system is considered as a constraint, although it could be included instead as a third objective. Hysteretic, viscoelastic and viscous dampers are all considered as possible design strategies, as well as the weakening plus damping concept. Since different types of passive systems are included, diversity of the Pareto front becomes a key issue, which is addressed successfully through an innovative definition of fitness. The multi-objective framework enables the evaluation of trade-offs between the two objectives and, consequently, provides vital information for the decision maker. Furthermore, the results presented offer valuable insight into the characteristics of optimal passive designs for the different objectives. Some of these characteristics confirm results reported elsewhere, while others are presented here for the first time.     

Seismic Performance and Modeling of a Half-Scale Base-Isolated Wood Frame Building

The concept of base isolation is a century old, but application to civil engineering structures has only occurred over the last several decades. Application to light-frame wood buildings in North America has been virtually non existent with one notable exception. This article quantitatively examines issues associated with application of base isolation in light-frame wood building systems including: (1) constructability issues related to ensuring sufficient in-plane floor diaphragm stiffness to transfer shear from the superstructure to the isolation system; (2) evaluation of experimental seismic performance of a half-scale base-isolated light-frame wood building; and (3) development of a displacement–based seismic design method and numerical model and their comparison with experimental results. The results of the study demonstrate that friction pendulum system (FPS) bearings offer a technically viable passive seismic protection system for light-frame wood buildings in high seismic zones. Specifically, the amount and method of stiffening the floor diaphragm is not unreasonable, given that the inter-story drift and accelerations at the upper level of the tested building were very low, thus resulting in the expectation of virtually no structural, non structural, or contents damage in low-rise wood frame buildings. The nonlinear dynamic model was able to replicate both the isolation layer and superstructure movement with good accuracy. The displacement-based design method was proven to be a viable tool to estimate the inter-story drift of the superstructure. These tools further underscore the potential of applying base isolation systems for application to North America's largest building type.     

Regional Seismic Risk Assessment of Bridge Network in Charleston,South Carolina

This article presents the results of a seismic risk assessment of the bridge network in Charleston, South Carolina and the surrounding counties to support emergency planning efforts, and for prioritization of bridge retrofit. This study includes an inventory analysis of the approximately 375 bridges in the Charleston area, and convolution of the seismic hazard with fragility curves analytically derived for classes of bridges common to this part of the country. State-of-the-art bridge fragility curves and replacement cost estimates based on region-specific data are used to obtain economic loss estimates. The distribution of potential bridge damage and economic losses are evaluated for several scenario events in order to aid in the identification of emergency routes and assess areas for investment in retrofit. This article also evaluates the effect of uncertainty on the resulting predicted economic losses. The findings reveal that while the risk assessment is very sensitive to both the assumed fragility curves and damage ratios, the estimate of total expected economic losses is more sensitive to the vast differences in damage ratio models considered.