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MCEER Bulletin, Volume 22, Number 1, December 2008

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Volume 22, Number 1, December 2008

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New Technical Reports

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Statistical and Mechanistic Fragility Analysis of Concrete Bridges

By M. Shinozuka, S. Banerjee and S.H. Kim, MCEER-07-0015, September 10, 2007, 218 pages, $35.00

This report elaborates on the seismic performance of reinforced concrete bridges subjected to earthquake ground motion. Probabilistic, statistical and mechanistic aspects of bridge damageability are integrated in the form of two-parameter lognormal fragility curves. To simulate general patterns of the progressive nature of bridge damage and failure mechanisms, the study performs nonlinear time history analyses of typical California RC bridges by finite element method (FEM). The analyses demonstrate that under normal conditions, the most prominent bridge damage is the formation of plastic hinges at the ends of bridge columns. A nonlinear static procedure is also performed to assess seismic vulnerability of the bridge. The results from these two methods are in good agreement. Furthermore, the effect of ground motion directionality on bridge fragility characteristics is demonstrated, and the results indicate that it may have a significant influence on bridge seismic damageability.

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Three-Dimensional Modeling of Inelastic Buckling in Frame Structures

By M. Schachter and A. Reinhorn, MCEER-07-0016, September 13, 2007, 268 pages, $35.00

The main purpose of this research is to develop a formulation for three-dimensional frame structures with geometric and material nonlinearities subjected to static and dynamic loads. A corotational formulation capable of macro-modeling geometric nonlinearities that considers the plasticity of the cross sections was developed. Results from shake table tests provided data to verify the new formulation and computational model. The frame chosen was a “zipper frame,” which is a chevron braced frame where columns link the midpoints of the beams at the brace connections. By comparing the predictions of the new analytical model with the data obtained from the “zipper frame” experiment, the capability of the formulation to predict inelastic buckling is verified.

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Modeling of Seismic Wave Scattering on Pile Groups and Caissons

By I. Po Lam, H. Law and C-T. Yang, MCEER-07-0017, September 17, 2007, 122 pages, $25.00

This report documents practical modeling procedures for wave propagation problems adopted in the bridge engineering community for seismic design and retrofit of long span bridges. The research developed a procedure to perform wave scattering problems in the time domain considering all elements of boundary issues to minimize wave reflection and refraction. The solutions were compared against a traditional frequency domain approach. The time domain procedure was shown to address not only free field conditions but also foundation systems subjected to earthquake loadings. The work was extended to address pile group foundations and gravity caissons under dynamic loading conditions. The effects of wave scattering can now be more easily included in future design procedures.

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Bridge Foundations: Modeling Large Pile Groups and Caissons for Seismic Design

By I. Po Lam, H. Law and G. R. Martin (Coordinating Author), MCEER-07-0018, December 1, 2007, 148 pages, $25.00

Current FHWA-sponsored research on large pile groups and caissons is synthesized and summarized into this design-oriented report on the seismic design of bridge foundations for long span bridges. The information is suitable for both retrofit and design applications. Characterization of ground motions for analyses, including methods of generating spectrum compatible time histories, spatial variations of ground motion, and effects of local soil conditions on site response, is presented. Modeling approaches for large pile groups are discussed. The use of p-y curves and associated pile groups, development of linearized Winkler springs, and the assembly of pile group stiffness matrices are discussed and illustrated with examples. Modeling approaches such as the linear stiffness matrix, nonlinear lumped springs and nonlinear distributed springs are discussed for large caissons, which can be employed as foundation piers in many over-water bridges. Case histories of the San Francisco-Oakland Bay Bridge East Span Project and the Tacoma Narrows Bridge are presented to illustrate the modeling approaches.

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Principles and Performance of Roller Seismic Isolation Bearings for Highway Bridges

By G.C. Lee, Y-C. Ou, Z. Liang, T. Niu and J. Song, MCEER-07-0019, December 10, 2007, 182 pages, $30.00

This report presents a new roller seismic isolation bearing for use in highway bridges. The bearing uses rolling motions of cylindrical rollers between sloping surfaces to achieve seismic isolation. The bearing is characterized by a constant spectral acceleration under horizontal ground motions and by a self-centering capability, which are two desirable properties for seismic applications. The former ensures that resonance between the bearing and horizontal earthquakes will not occur while the latter guarantees that the bridge superstructure can self-center to its original position after an earthquake. Principles of the bearing under vertical loading and earthquake excitation are analytically and experimentally investigated. Two prototype roller isolation bearings have been developed, one with and the other without a built-in friction device for supplemental energy dissipation. A design example of the bearing for use in a bridge in a region of high seismicity is presented.

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Centrifuge Modeling of Permeability and Pinning Reinforcement Effects on Pile Response to Lateral Spreading

By L.L. Gonzalez-Lagos, T. Abdoun and R. Dobry, MCEER-07-0020, December 10, 2007, 382 pages, $35.00

This report presents the results of a study of the soil-structure response of pile groups subjected to liquefaction induced lateral spreading. The first section presents the experimental results and analyses of six centrifuge tests that investigated the effects of soil permeability on the response of pile foundations subjected to lateral spreading. In the centrifuge models simulating the liquefiable coarse sand layer, the piles bounced back after a couple of cycles of shaking. However, in models that simulate a liquefiable fine sand layer, the piles never bounced back, reaching maximum displacements and bending moments six times larger than for the coarse sand layer. The second part presents experimental results and analyses of four centrifuge tests that were conducted to study the reinforcing or pinning effect pile groups have on reducing lateral spreading. An analysis approach developed in this study proved to be very useful in understanding the reinforcement effect pile groups have on liquefaction induced lateral spreading.

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Damage to the Highway System from the Pisco, Peru Earthquake of August 15, 2007

By J.S. O’Connor, L. Mesa and M. Nykamp, MCEER-07-0021, December 10, 2007, 100 pages, $30.00

This report presents the results of a field investigation undertaken in September 2007 following the M 8.0 August 15, 2007 Pisco, Peru earthquake. It provides a brief description of the event and damage to the highway system. It includes information from government reports issued immediately after the event provided by leaders in Peru’s academia, and the authors’ field observations. The report documents the performance of structures designed according to AASHTO specifications and provides an assessment of the adequacy of the standards used at the time of construction.

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Experimental Seismic Performance Evaluation of Isolation/Restraint Systems for Mechanical Equipment
Part 2: Light Equipment Study

By S. Fathali and A. Filiatrault, MCEER-07-0022, December 13, 2007, 220 pages, $35.00

This report describes shake table experiments on an air-handling unit in two different configurations: supported by six isolation/restraint systems and rigidly-mounted. The testing included seismic and system-identification tests, and incorporated different input-motion amplitudes and different isolation/restraint system properties. Experimental results showed that limiting the displacement of the equipment by the restraint components of the isolation/restraint systems resulted in amplification of the equipment acceleration responses. Based on these results, reducing the gap size is recommended to improve the seismic performance of these systems in areas of high seismicity. The test results also showed that higher amplification of acceleration responses should be expected for light and flexible equipment than for rugged and heavy equipment. A companion report, MCEER-07-0007, focuses on heavy mechanical equipment.

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Fragility Considerations in Highway Bridge Design

By M. Shinozuka, S. Banerjee and S.H. Kim, MCEER-07-0023, December 14, 2007 192 pages, $30.00

This report integrates statistical and analytical methods for the seismic performance evaluation of highway transportation networks. Bridge damageability is expressed in the form of fragility curves, represented by two-parameter lognormal distribution functions, in which fragility parameters are estimated through the maximum likelihood procedure. Empirical fragility curves are developed utilizing bridge damage data obtained from past earthquakes, particularly the 1994 Northridge earthquake. They are then used to construct a mechanistic model which calibrates analytical damage states with empirical damage data so that analysis becomes consistent with past experience. Analytical fragility curves are generated through nonlinear static and dynamic procedures for typical bridges in Los Angeles and Orange counties. The application of fragility curves in the performance-based design of bridges is demonstrated and a design acceptance criteria is suggested that verifies the target performance level of a newly designed bridge under a prescribed level of seismic hazard.

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Performance Estimates for Seismically Isolated Bridges

By G.P. Warn and A.S. Whittaker, MCEER-07-0024, December 30, 2007, 334 pages, $35.00

This research investigated key assumptions in the equation for calculating displacements in seismically isolated bridges (Equation 3 of the 1999 AASHTO Guide Specifications), and the validity of testing protocols for full-scale seismic isolators as specified in AASHTO 1999. To facilitate response-history analysis, earthquake ground motions were collected and organized into eight bins that characterized seismic hazard using the mean and median spectrum. These were used to calculate the maximum design displacement using the static analysis procedures given in AASHTO 1999. Nonlinear response-history analysis was performed considering a simple isolated bridge model and twenty combinations of isolator properties subjected to unidirectional and bidirectional seismic excitation. The results were mined to determine maximum isolator displacements and energy demands imposed on seismic isolators during maximum earthquake shaking.

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Seismic Performance of Steel Girder Bridge Superstructures with Conventional Cross Frames

By L.P. Carden, A.M. Itani and I.G. Buckle, MCEER-08-0001, January 7, 2008, 264 pages, $35.00

This report describes a series of cyclic and shake table experiments on a 2/5 scale model of a simply supported steel girder bridge superstructure, with two steel plate I-girders, a reinforced concrete deck slab and single angle cross frames. The experiments were conducted to determine the effect of transverse seismic loading in regions of high seismicity. It is shown that the superstructure deforms in a flexural-torsional mode with transverse seismic forces distributed along the span and resisted at the supports. At the supports, the loads are transferred from the deck slab into the girders through the shear studs located near the girder supports or through a top chord connecting the deck slab to the end cross frames. The transverse loads are then distributed to the base of the girders through the end cross frames and into the substructure through the bearings and transverse bearing restraints. Each of these critical components should be designed for transverse seismic loading.

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Seismic Performance of Steel Girder Bridge Superstructures with Ductile End Cross Frames with Seismic Isolators

By L.P. Carden, A.M. Itani and I.G. Buckle, MCEER-08-0002, January 7, 2008, 278 pages, $35.00

The end cross frames of steel plate girder bridges are critical elements in the transverse seismic load path of the superstructure. It has been suggested that if these cross frames were to yield in a ductile manner, the elastic base shear in the bridge would be substantially reduced. This report describes experimental and analytical studies on a two girder bridge superstructure to investigate the validity of this proposition. Two types of end cross frames are studied: ductile single angle braces, and buckling restrained braces. The buckling restrained braces resulted in 20% to 30% smaller drifts in the superstructure than with X-braces at the same level of base shear, a result that is attributed to better energy dissipation. Removing some shear studs near the supports of the girders, and allowing the transverse shear to be transferred into the end cross frames using the top chord, allows the girders to “rock,” enabling considerable transverse drifts in the girders without distress to the slab-girder connection. Lead-rubber isolation bearings were also used to reduce the seismic base shear, and shown to give a greater reduction in the base shear than was possible with the ductile end cross frames.

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Analytical and Experimental Investigation of a Controlled Rocking Approach for Seismic Protection of Bridge Steel Truss Piers

By M. Pollino and M. Bruneau, MCEER-08-0003, January 21, 2008, 484 pages, $60.00

This report presents the results of an analytical and experimental study on a seismic design (or retrofit) strategy that allows uplift and rocking of steel truss piers on their foundation. Displacement- based steel yielding devices and velocity-dependent viscous dampers, installed at the uplifting location, were used to control system response. The behavior of 2- and 4-legged bridge steel truss piers was considered. Methods to predict their response under multiple components of seismic excitation were evaluated using nonlinear, inelastic time history analyses. Experimental investigations included shake table testing of a rocking pier with the added devices to verify the analytical methods and further investigate the dynamic response. Overall system behavior and the methods of response prediction were shown to be reasonably accurate.

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Linking Lifeline Infrastructure Performance and Community Disaster Resilience: Models and Multi-Stakeholder Processes

By S.E. Chang, C. Pasion, K. Tatebe and R. Ahmad, MCEER-08-0004, March 3, 2008, 154 pages, $30.00

This report examines how lifeline infrastructure performance in disasters can be linked to the disaster resilience of a community. The scope is limited to the social and economic dimensions of resilience, and focuses on the Los Angeles Department of Water and Power. The research links infrastructure performance and community resilience through two channels: quantitative modeling and development of decision-support tools; and exploration of the role of community engagement in defining performance goals. The research develops a new simulation model of direct economic loss from lifeline disruption and a model to estimate the demand for public shelter. A second line of research explores issues related to how these socio-economic impacts can be considered in mitigation decision-making by utilities. The report provides background, quantitative models, preliminary community input, and recommendations for a process by which utilities and communities can assess and improve their disaster resilience.

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Modal Analysis of Generally Damped Linear Structures Subjected to Seismic Excitations

By J. Song, Y.L. Chu, Z. Liang and G.C. Lee, MCEER-08-0005, March 4, 2008, 226 pages, $35.00

Motivated by the need for a systematic approach for seismic evaluation and design of structures with supplemental damping, a general modal analysis method considering over-damped modes is developed and described in this report. The method deals with a unified formulation used to evaluate most structural response quantities of interest, such as displacements, velocity, interstory drifts, story shear, damping forces and absolute accelerations, etc. In addition, a novel general real-valued transformation matrix is established to decouple the equations of motion of a generally damped structure in terms of real-valued modal coordinates. Also, two general modal combination rules for the response spectrum analysis, GCQC and GSRSS, are formulated. To enable the new rules to be applicable to the practicing earthquake engineering community, a conversion procedure to construct an over-damped mode response spectrum compatible with the given 5% standard design response spectrum is established.

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System Performance Under Multi-Hazard Environments

By C. Kafali and M. Grigoriu, MCEER-08-0006, March 4, 2008, 246 pages, $35.00

This study presents a new methodology to assess the performance of structural/nonstructural systems subjected to multiple hazards during their lifetime, and to identify an optimal strategy from a collection of design alternatives. The methodology is probabilistic in nature since the intensity and arrival time of different hazards such as earthquakes and hurricanes, the loads acting on the system due to a hazard event such as ground accelerations and wind velocities, and some system characteristics, are generally uncertain. Consequently, probabilistic models are developed to characterize the natural hazards that could occur at a given site at single/multiple points. These models specify the random arrival times of individual events at a site during a reference time, the random properties of the hazards under considerations at these times, and the random loads acting on the system due to each event. The models are implemented in computer programs and the life-cycle risk analysis methodology is illustrated through numerical examples.

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Mechanical Behavior of Multi-Spherical Sliding Bearings

By D.M. Fenz and M.C. Constantinou, MCEER-08-0007, March 6, 2008, 182 pages, $30.00

This report describes the principles of operation and development of cyclic force-displacement relationships for three variations of multi-spherical sliding bearings: the double FP bearing, the triple FP bearing, and the modified single FP bearing. The force-displacement relationships of devices with increasingly complex behavior were determined by extending the fundamental principles of operation that apply to sliding on a single concave surface. It was shown that each device is capable of exhibiting displacement-dependent adaptive behavior, i.e., desirable changes in stiffness and damping over the course of motion. These changes are determined by the relative values of each surface’s coefficient of friction, effective radius of curvature, and displacement capacity. Since all are predefined design parameters, their behavior is controllable by the engineer. In addition, it was shown that in cases where multiple surfaces are of equal friction, the complexity of behavior exhibited by these devices is reduced. This is important because bearings of smaller plan dimension can still use familiar and proven methods of analysis and design, resulting in significant cost savings.

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Post-Earthquake Restoration of the Los Angeles Water Supply System

By T.H.P. Tabucchi and R.A. Davidson, MCEER-08-0008, March 7, 2008, 140 pages, $25.00

This report describes a discrete event simulation model of post-earthquake restoration developed for the Los Angeles Department of Water and Power (LADWP) water supply system. For a particular earthquake, the model uses information about damage to the system and the resulting hydraulic flow obtained from the GIRAFFE model that was also developed for the LADWP system. Throughout the simulation, the model interacts with GIRAFFE to receive updates of the system functionality at specific times as the restoration process proceeds and damage is repaired. Several different types of output are provided, including system and subregion restoration curves; spatial distribution of restoration; material usage; crew usage; average time each customer is without water; and time to restore the system and subregions to 90%, 98%, and 100%. Ten simulations of the restoration model were run using real damage data from the 1994 Northridge earthquake as input, and the results were compared to the actual restoration that took place following Northridge. The average spatial distribution of restoration roughly matches what occurred in 1994.