The two-day technical program will feature presentations on a variety of topics in structural dynamics and earthquake engineering. The speaker list includes local and international perspectives, representatives from both academia and industry, and experts that are well-established or more up-and-coming. A number of Professor Chopra’s former students and close collaborators will be featured. Speakers and abstracts are presented below in no particular order.
Livermore Software Technology Corporation
Two major issues in the earthquake analysis of concrete dams are (i) modelling the unbounded foundation rock and impounded water, and (ii) incorporating the earthquake excitation into the model. Ideally, these should be modelled directly in the time domain to allow non-linear analysis of the dam, but current state-of-the-practice is either rigorous and linear analysis or non-linear but ad-hoc and approximate analysis.
This talk will present a novel method that adopts a wave-scattering view of the problem, and resolves both the above issues, allowing accurate and inexpensive non-linear analysis of the dam. The effective seismic input method for soil-structure interaction is extended to dam-water-foundation rock systems, and perfectly matched layers are used to model the unbounded domains efficiently. The talk will show how these two methods can be woven together to form an effective analysis procedure.
In addition, the talk will also feature the speaker’s personal recollections of his long association with Professor Chopra.
Coupled Finite Element Simulation of Earthquakes and Tsunami Inception: Case Study of the 2011 Tohoku-Oki Earthquake and Tsunami
Hamerschlag University Professor
Civil and Environmental Engineering
Carnegie Mellon University,
In this presentation, I address the coupled nature of earthquakes and the resultant tsunamis through a case study of the 2011 Tohoku-Oki event. We have achieved this by means of simulations carried out using Hercules, the parallel octree-based finite element earthquake simulator developed by the Quake Group at Carnegie Mellon University. As an improvement to the original Hercules code, we have incorporated acoustic wave propagation in the ocean into our simulations to capture the generation and the offshore propagation of tsunami waves.
Sub-oceanic earthquakes and the tsunamis triggered by seismic faulting are naturally coupled events. Yet, traditionally they have been studied separately. Some recent studies have addressed the coupled nature of these two events, mostly through weakly-coupled simulations in which the feedback from the ocean waves to the solid domain were ignored. Such feedback can be significant, especially for the slow rupture events of which the 2011 Tohoku-Oki earthquake is a major example. Here, we briefly describe the mathematical formulation of the coupled problem and its numerical implementation using finite elements, and subsequently focus on the generation and offshore propagation of the tsunami waves generated by the fault slip of the Tohoku-Oki earthquake, using a seismic velocity model that is a combination of two publicly available seismic velocity models of Japan. To assess the validity of this methodology, we compare qualitatively results from the simulations with corresponding recorded values.
Division of Dam Safety and Inspections
Federal Energy Regulatory Commission
United States Department of Energy
Falls Church, Virginia
The Federal Energy Regulatory Commission (FERC) first issued its Engineering Guidelines for Arch Dams in 1999. Over the last 20 Years, advances in the reliability and availability of non-linear structural analysis codes, and in our understanding of arch dam failure modes, has led to a re-write of these guidelines. This presentation will cover new FERC Arch Dam Engineering Guidelines that are currently being drafted and the philosophy underlying them.
.Juan Carlos de la Llera Martin
Dean of the Faculty of Engineering and
Professor of Structural and Geotechnical Engineering
School of Engineering
Pontificia Universidad Catolica de Chile
The role that elastic and inelastic building torsion plays in the collapse of structures is still a topic that probably has not reached a real closure. In practice, plan asymmetry still remains basically out of the control of structural engineering hands, and many times compliance with the current seismic code provisions is a sufficiently strong argument to leave everyone involved in a project sufficiently confident. Looking in retrospective, did we really learn enough about building torsion in the nineties to make everyone today so comfortable with it? Was building torsion a real problem, or was it a mirage solved by simply improving our structural analysis models? Have we seen evidence of torsional effects again in some of the most recent building collapses and damage? Do seismic codes correctly account for accidental and natural eccentricities today?, or are there ways of to controlling naturally building torsion? The work of professor Anil Chopra and his students undoubtedly pushed forward the knowledge of this complicated phenomenon in the 80’s and 90’s, but it is quite impressive to still see the engineering community debating in the realm of codes about the best way to account for accidental and natural torsion more than 25 five years later. Although from a more philosophical standpoint, this article deals with some of the most serious historical misinterpretations of building torsion, and it attempts to demystify some of the complex issues around it. It does not promise a closure, since perhaps the problem was wrongly posed from the beginning.
Challenges and Progress Toward the Assessment of Residual Capacity for Earthquake-Damaged Concrete Buildings
Kenneth J. Elwood
MBIE Chair in Earthquake Engineering
Department of Civil and Environmental Engineering
University of Auckland, New Zealand
The 2010-11 Canterbury Earthquakes have provided many lessons related to concrete buildings, and continue to influence codes and standards internationally, but perhaps the most striking lesson is the potential for widespread demolition of generally good-performing buildings. With a transformation of the urban environment resulting from demolition of over 60% of concrete buildings in the Central Business District, questions have been raised about the acceptability of this outcome and the reasons for demolition. Damage to concrete buildings in Wellington from the 2016 Kaikoura Earthquake has further highlighted the importance of understanding the residual capacity of damaged buildings for the resilience of a community. These experiences in New Zealand have raised fundamental questions about the ubiquitous life-safety performance objective in international building codes. The presentation will summarise ongoing studies related to demolition/repair decisions in Christchurch, large scale testing of RC beams to assess the residual capacity of a plastic hinge, and the need to benchmark our codes to a repairability, in addition to a collapse, limit state.
Seismic Vulnerability Functions Accounting for Cumulative Damage Estimates Based on Structural Health Indicators
Faculty of Engineering
Universidad Nacional Autonoma de Mexico
Mexico City, Mexico
A criterion is proposed for the estimation of seismic vulnerability functions of structures affected by seismic excitations, taking into account information derived from indicators of global and local structural damage. Its final aim is the generation of information needed to support optimum repair and maintenance strategies within a life-cycle framework. Attention is focused on the following concepts:
a) Influence of accumulated damage on the seismic reliability and expected damage functions;
b) Estimation of global and local cumulative seismic damage indexes, on the basis of the probabilistic relations between those indexes and global and local structural health indicators; in this case, the ratio of dominant frequencies of the transfer functions of the power spectral densities of roof displacement to ground acceleration, and the index measuring the damage accumulated from previous earthquakes; and
c) Updating of seismic reliability and expected damage functions, taking into account information derived from the observed value of the structural health indicators adopted
The scope of this study has been limited to symmetric multistory buildings subjected to a single component ground motion. Additional studies must be developed to cover buildings with in-plan eccentricities subjected to two simultaneous horizontal components or, more generally, including also a vertical component. They should also cover buildings with irregular distributions of mass, strength and/or stiffness along their height.
Attention in this study is focused on the influence of cumulative damage on seismic vulnerability functions for the systems considered. However, the local structural health indicators proposed can also be used as efficient tools for local damage identification, oriented to local repair decisions.
Faculty of Civil Engineering and Geodesy
University of Ljubljana
In late 1990’s the structural engineering community has developed a new generation of design and rehabilitation procedures that incorporates performance-based engineering concepts. Nonlinear analysis was introduced into the seismic design methodology. A practice-oriented approach, appropriate for adoption in seismic guidelines and codes, is the nonlinear static (pushover) analysis in combination with inelastic response spectra. Very important contributions to the development of this approach have been made by Chopra and Goel. One of the pushover-based methods is the N2 method, developed at the University of Ljubljana. The method has been adopted in the European standard for earthquake resistant structures Eurocode 8 in 2004. The application of the basic pushover-based analysis is limited to structures which vibrate predominately in a single mode. The extended N2 method, which will most probably be adopted in the revised Eurocode 8, takes into account higher modes in elevation and in plan (torsion) by enveloping the results of the pushover analysis and elastic modal response spectrum analysis. The pushover-based analysis allows the analyst to develop a good feeling on the structural response, thanks also to the visual representation, so it became quite popular in Europe, whereas nonlinear response history analysis has been restricted to the most important structures, due to its complexity and several issues not yet completely solved.
In the presentation, after a brief discussion of advantages and weaknesses of the pushover-based methods compared to other analysis methods, the extended N2 method will be summarized. Its use for the risk-targeted analysis, intended for adoption in the next generation of Eurocode 8, will be shown.
Member Models for Practical Seismic Evaluation and Rehabilitation of Concrete Buildings Using Nonlinear Response History Analysis
Professor and Director of Structures Laboratory
Department of Civil Engineeirng
University of Patras
Recent developments are highlighted concerning simple models which give those properties of concrete beams, columns (rectangular or circular) and walls (with a variety of cross-sectional shapes) which are important for practical nonlinear response-history analysis in terms of the geometric parameters of the member, the strength of its materials and the axial load. The properties concerned are the effective stiffness to the yield point, the yield moment and the corresponding deformations, the cyclic rotation capacity and parameters of the hysteresis laws which control hysteretic damping. The models cover members with deformed or plain bars, continuous or lap-spliced in the plastic hinge region. Members retrofitted with concrete or Fiber Reinforced Polymers are also included. The models have been developed or calibrated using a database of several thousand member tests, the largest and most diverse of its kind. They are physically-based or of the (semi-)empirical sort. Their example applications to structures Pseudo-dynamically tested at full scale with or without retrofitting, give the opportunity to examine the effect of using the initial or the tangent stiffness in the Rayleigh damping model and to contribute to the ongoing deliberations on this open issue with proposals.
President and Principal
Quest Structures, Inc.
Risk and uncertainty are intrinsic in water resources management activities. Uncertainty arises from the lack of knowledge and randomness of the loads, imperfect information about the way the dam responds to those loads, and limited information about what the resulting consequences would be. The aim of seismic risk assessment is to identify loading conditions, potential failure modes, and consequences, and to estimate the probabilities for each event. The overall process consists of hazard definition, seismic fragility, consequence analysis, and integration of the three. Among these components, the seismic fragility defined as the conditional probability of dam failure as a function of earthquake ground motion intensity is the focus of this presentation.
Contemporary methods of calculating probability of dam failure involve using an event tree to identify and evaluate the sequence of events that could lead to dam failure, and subjective estimation of probabilities that rely on the skill and judgment of the participants. The presentation provides a quantitative methodology for developing seismic fragility that removes limitations inherent in the contemporary methods and formally incorporates both the aleatory variability and epistemic uncertainty in the analysis. The method uses nonlinear dynamic analyses in combination with the Latin Hypercube Simulation (LHS) to develop the probability of dam failure. The presentation then discusses the application of the methodology to seismic vulnerability evaluations of a concrete dam and design of a strengthening scheme to reduce the risk of dam failure. The dam had been identified as one of the dominant contributors to the seismic risk associated with a nuclear power plant located 1.4 km downstream of the dam.
Development of Improved Procedure to Estimate Seismic Forces in Ancillary Systems Supported on Piers and Wharves: A Connection to Earthquake Engineering Concepts Presented by Professor Chopra
Rakesh K. Goel
Associate Dean and Professor of Civil Engineering
College of Engineering
California Polytechnic State University
San Luis Obispo, California
Engineering practice currently uses procedures described in ASCE-7 standard to estimate seismic forces in ancillary systems supported on other structures. This procedure first amplifies the ground acceleration to estimate acceleration at the point of attachment of the ancillary system and then further amplifies it due to its flexibility of the ancillary system. The ASCE-7 procedure estimates the peak acceleration at the point of attachment of the ancillary system to be three times the peak ground acceleration when the ancillary system connects at the top of the primary system regardless of the system’s dynamic properties.
We can idealize piers and wharves, which are generally one-level structures, as much simpler single-degree-of-freedom (SDF) systems. For such SDF system, the peak acceleration at the top of the system is equal to peak of the total acceleration or “true” acceleration. Professor Chopra first explored the connection between “true” acceleration and spectral (or pseudo-acceleration) in his book – Dynamic of Structures: Theory and Applications to Earthquake Engineering – now in its fifth printing and showed that peak value of the “true” acceleration is essentially equal to the spectral acceleration for systems with low value of damping. This connection was demonstrated based on fundamentals of how SDF system respond to ground shaking and by comparing “true” and spectral acceleration spectra for a selected ground motion. We use this concept presented by Professor Chopra to develop a simple and more accurate procedure to estimate seismic forces in ancillary systems supported on piers and wharves.
Design Coordinator for Panama Canal Expansion Project and
Vice President of Engineering
Panama Canal Authority
Republic of Panama
The presentation describes the Panama Canal’s main components built during 1904-1914, and an additional dam built in the 1930s. The Panamax, defined as the largest that can navigate through the original locks, is introduced to explain the motivation for the Canal Expansion Program (2009-2016). It then describes the new Post-Panamax lock structures and earth dams built during the Expansion Program. Following is brief summary of the process by which the seismicity of the area was characterized (1991-2009) and the impact it had on the design of the new structures.
Professor Chopra provided valuable support during the process of converting seismic hazards into acceptable Risk Design Criteria. He also provided concepts with respect to the appropriate process to review the designs of the Design-Build contractor. Given the increase in seismic risk identified by the seismic characterization process, Panama’s Building Design Code required adjustments. Professor Chopra recommended the appropriate consulting team required for this purpose.
In 2010, the consulting team met in Panama with members of the Panamanian Structural Code Committee and addressed the adaptation of the seismic hazards to buildings. In general, higher risk levels were admissible, when compared to locks and dams that contain the Canal’s reservoir. The process also included the definition of an appropriate capping level to the accelerations, to be complemented by the introduction of stricter ductility requirements.
Civil Engineering Program
Northern Cyprus Campus, Middle East Technical University
A parametric study is conducted to investigate the dynamic shear amplification factor (DAF) in low-to-mid-rise frame–wall systems in which the reinforcement curtailment along the height matches the required code strength. The increase in the shear force that the wall must resist is customarily specified independently of the capacity of the frame system of which it is a part, but the wall is not an isolated cantilever element but it interacts with the surrounding frame system. The level of frame–wall interaction is varied by changing the wall index, defined as the ratio of the total wall area to the floor plan area, in a generic frame–wall system, and its correlation with the DAF is investigated. Wall index values ranging in the 0.2 percent to 2 percent interval are selected. Walls with lengths of 3 m, 5 m and 8 m are used in the design of model buildings with 4, 8 and 12 stories. Shear–flexure beam continuum formulation is used in design and modeling. The global behavior is analyzed using nonlinear response history procedure using spectrum compatible ground motions. It is found that the primary source of amplification is the level of inelastic demand on the system. Walls designed for code-specified force reduction factor R= 6 experienced an average base shear force amplification in the order of 1.64 with standard deviation of 0.19 with respect to the design shear force. Amplification diminishes with decreasing R. An expression for the dynamic amplification factor as a function of the number of stories and force reduction factor R is proposed.
Senior Water Resources and Geotechnical Engineer
GEI Consultants, Inc.
Rancho Cordova, California
Dams are an important piece of infrastructure that provides benefits to people all over the world, such as water supply, hydroelectric power, flood control, recreation and others. However, the consequence of a dam failure could be devastating and most dams throughout the world are over 50 years old and do not necessarily meet today’s standards. Therefore, careful evaluation of dams is critical to the safety of people that live downstream of these dams. Many of the largest dams in the world are concrete type
structures of varying size and type. Methodologies continue to improve regarding the design and evaluation of concrete dams. This presentation will review the history of evaluations and the important contributions that Professor Anil Chopra made to the advance of concrete dam design
Next Generation Structural Engineering: Redefining the Role of the Structural Engineer in Changing Times
President and CEO
Computers and Structures, Inc.
Walnut Creek, California
In this short talk, Ashraf will share his view of the future of the structural engineering profession as we approach the next quarter of the 21st Century.
As the world is quickly changing, so must our profession in order to keep up with advancements in materials, construction techniques, computers, and more. Ashraf will talk about how artificial intelligence (AI), machine learning, cloud-based project collaboration, and mobile workplaces are defining the future. How will the structural engineering profession adjust to these and other new technology-driven ways of the world?
Ashraf will discuss how the structural engineer’s education and role must change if our profession is to thrive in these rapidly-changing times. He demonstrates why students need to be exposed to the arts, public speaking, human psychology, and marketing so that they enter the professional world ready to lead, inspire, and motivate. Only with this foundation will students be able to fully leverage the limitless potential that our amazing profession has to offer!
Professor of Civil Engineering
Division of Engineering and Applied Science
This talk will cover features and observations related to nonlinear analysis of frame buildings subjected to strong earthquake ground motions. The two main topics of the presentation are efficient modeling/solution techniques and appropriate damping formulations.
Neal Simon Kwong
Assistant Professor of Civil Engineering
The Cooper Union
New York, New York
The Conditional Mean Spectrum (CMS) is often employed to select ground motions for intensity-based assessments of tall buildings. However, the seismic demands determined by response history analyses with ground motions selected to match a single CMS may be unconservative whenever multiple vibration modes significantly influence the response. Existing solutions to this problem include analyzing the building with: (i) a Uniform Hazard Spectrum, (ii) several CMS’s, and (iii) a complete risk-based assessment. To minimize computational effort while preserving accuracy, an alternative engineering solution is developed, which is based on a simplified version of the Generalized CMS where two specified conditioning spectral accelerations share a common hazard level. The results from a realistic case study suggest that the proposed spectrum provides seismic demands that are as accurate and precise as those obtained from analyzing the structure with multiple CMSs while simultaneously reducing the computational effort by a factor of two or more.
Peter L. Lee
Associate Director and
Skidmore, Owings & Merrill LLP
San Francisco, California
Professor Anil K. Chopra, a member of the world renowned faculty at UC Berkeley’s division of SEMM, has taught structural dynamics and earthquake engineering for 46 years. Apart from performing cutting edge research that has greatly advanced knowledge and practice in the fields of structural and earthquake engineering, the institution has trained and graduated scores of engineers each year who have gone on to be productive members of the structural engineering community. We, Peter Lee and Neville Mathias, are just two representatives of the vast pool of graduates who are deeply indebted to UC Berkeley and its structural faculty for the knowledge it has passed on to us that has shaped our thinking and professional approach at SOM where we have had the opportunity to work on some of the most exciting and challenging structures in the world. We would like to share a few of these projects with you on this day in celebration of Professor Chopra’s retirement symposium. We appreciate the opportunity to reflect on what we learned at UCB, how it has influenced our careers and work, and led to contributions to innovation at SOM. This presentation will highlight technical design challenges in structural and earthquake engineering on representative career project work both domestic and international. For these building type project structures, emphasis will be given to the implementation of innovative concepts and technologies based on an understanding of structural behavior to achieve enhanced seismic performance objectives, as well as, the importance of design integration with architectural building constraints, components and systems.
Department of Civil, Geological and Mining Engineering
The effects of transient uplift pressure response acting in cracks in concrete gravity cracks during earthquakes, as well as post-seismic uplift pressure intensity and distributions, for structural stability assessment are still the subject of active debate. The seismic failure mechanisms of concrete dams are first presented. The lack of knowledge regarding the seismic uplift pressures is discussed by comparing the effects of seismic uplift pressures according to different dam safety guidelines. The phenomenological processes of water pressure variations in concrete cracks during earthquakes and laboratory tests to measure transient water pressures inside small cracked concrete specimens subjected to cyclic motions are then presented. The results show that the water pressure is reduced during crack opening while it is increase during crack closing. The magnitude of pressure drops during crack opening and pressure increase during crack closing is shown to depend on the loading frequency. Historical evidences regarding post-earthquake uplift pressures in concrete dams are then reviewed. It is indicated that post-earthquake uplift pressures in seismic cracks depend on (a) the initial conditions along the crack plane (either compression or tension), (b) the cyclic damage of the crack walls that controls the crack hydraulic conductivity, and (c) the post-earthquake drain efficiency. Finally, user friendly seismic structural analysis tools developed to assess the seismic cracking and sliding response of concrete gravity dams while considering different modelling assumptions for the seismic as well as the post-seismic uplift pressures are briefly presented (CADAM-2D, 3D; RS-DAM). These structural analysis tools include Professor’s Chopra simplified method for seismic analysis of gravity dams, widely used by the profession.
Byron and Elvira Nishkian Professor of Structural Engineering, and
Director, NSF/NHERI Center for Computational Modeling and
Simulation of the Effects of Natural Hazards on the Built Environment
This presentation examines the evolution of computational simulation in earthquake engineering starting with the speaker’s early work with Anil Chopra and
Vitelmo Bertero that focused on computationally simulating the nonlinear response of reinforced concrete buildings damaged during strong earthquakes. The presentation then follows the speaker’s activities to develop of more advanced numerical models to predict the transient behavior, including failure, of reinforced concrete and steel structures, components and materials when subjected to generalized loading.
The adaptation of such numerical methods to experimentally based simulation by means of advanced hybrid techniques is then discussed. The presentation ends with a discussion of current work to extend high performance computing and performance-based evaluation methods to simulate damage, and economic and other
consequences, to complete cities during strong earthquake.
Civil, Environmental and Construction Engineering
University of Central Florida
The uplifting and rocking of slender, free-standing structures when subjected to ground shaking may limit appreciably the seismic moments and shears that develop at their base. While the unparalleled seismic performance of rocking isolation has been documented with the through-the-centuries survival of several free-standing ancient temples; and careful post-earthquake observations in Japan during the 1940’s suggested that the increasing size of slender free-standing tombstones enhances their seismic stability; it was Housner (1963) who elucidated a size-frequency scale effect and explained that there is a safety margin between uplifting and overturning and as the size of the column or the frequency of the excitation increases, this safety margin increases appreciably to the extent that large, free-standing columns enjoy ample seismic stability. This talk revisits the important implications of this post-uplift dynamic stability and explains that the enhanced seismic stability originates from the difficulty to mobilize the rotational inertia of the free-standing column. As the size of the column increases, the seismic resistance (rotational inertia) increases with the square of the column size; whereas, the seismic demand (overturning moment) increases linearly with size. The same result applies to the articulated rocking frame given that its dynamic rocking response is identical to the rocking response of a solitary free-standing column with the same slenderness; yet, larger size. The talk concludes that the concept of rocking isolation is a unique seismic protection strategy for large, slender structures such as tall bridges—not just at the limit-state but also at the operational state.
Experiences from 1994 Northridge and 1995 Kobe – Comparison in Seismic Performance of Steel Moment Frames Between the United States and Japan
President of Kobori Research Complex and Counselor of Kajima Corporation. Tokyo, Japan
and Professor Emeritus, Disaster Prevention Research Institute (DPRI), Kyoto University, Kyoto, Japan
I stayed in UC Berkeley in 1994, and it was the summer of that year when my friendship with Professor Chopra began. Ten years later, he kindly invited me to the editorship of Earthquake Engineering and Structural Dynamics (EESD), and since that time we continue to work together for the running of EESD. I have been engaged in research on seismic analysis and design of steel building structures and development of experimental techniques like hybrid simulation and large-scale shaking table testing.
Among those topics, I wish to present a subject of seismic performance of steel moment frames in the US and Japan. Stimulation on this subject stemmed from 1994 Northridge and 1995 Kobe earthquakes in which many of the US and Japanese steel moment frames sustained serious damage particularly in the welded beam-to-column connections. I participated in the post-Northridge project commonly named “SAC Joint Venture” and also led a Japanese national project that investigated the Kobe damage. Many similarities did exist in the type of damage disclosed in the two earthquakes, but their engineering solutions differed significantly. In the US, rather than minutely modifying connection details for better performance of beam-to-column connections, adopted were the approaches in which forces exerted into the connection were lessened. In Japan, most of the efforts were placed to improve the details (welding, bolting, holes, etc.) by which the connections were made stronger and more ductile. The differences were found to be closely associated with the fabrication and construction processes exercised in the two countries. The summary of my learning from those experiences: “Design” is important, but “Fabrication/Construction” is most influential in the decision making among multiple engineering solutions. In my presentation, I will touch upon an overview of steel damage in the two earthquakes, discussions made in respective countries to respond to the damage, test and analysis done in post-earthquake projects, multiple alternatives devised in the projects, the differences of adopted solutions between the two countries, and finally my subjective view for the reasons.
Civil Structural Engineer
US Bureau of Reclamation
University of Colorado, Boulder
The Bureau of Reclamation (Reclamation) implements the use of advanced numerical modeling through finite element (FE) analysis in performing structural assessments of concrete dams. Capabilities of numerical modeling allow for estimating the inherent structural behavior of a dam under various loading conditions. The scope of an analysis is ideally based on structural potential failure mode(s) (PFM) with respect to seismic loading and routine examination results. Upon completion of an analysis, Reclamation engages a technical team to estimate the risk posed to the downstream population and potential justification for additional actions or studies to address PFMs with elevated risks.
Evaluation of a slab and buttress dam exemplifies justification for considering both stream and cross-canyon directional loading utilized in a 3-dimensional (3D) FE evaluation context. Based on the innate components of a slab and buttress dam, structural response could be different due to upstream-downstream loading, cross-canyon loading or both. Identifying these relationships as well as utilizing the FE results suggest the appropriate level of analysis and enhance the information for the subsequent risk analysis.
Based on Reclamation’s State-of-the-Practice guidelines, the dam, foundation, and reservoir were incorporated in the FE model, along with consideration of the condition of the dam based on design documentation and available studies. This presentation will include the evaluation for the dam with a linear elastic FE model under (1) static and (2) seismic loading conditions. The presentation will conclude with additional studies recommended for further evaluation of the slab and buttress dam to assess the structural stability, and computational efficiency.
Research and Development Affecting Seismic Analyses of Concrete Dams at the Bureau of Reclamation, USA
Larry K. Nuss, P.E.
Senior Structural Engineer
Nuss Engineering, LLC
The Bureau of Reclamation’s has about 370 facilities in the western USA that include some of the largest concrete dams in the country (i.e. Hoover (727 feet), Morrow Point (465 feet), Yellowtail (525 feet), and East Canyon (265 feet)). Structural analysis of concrete dams has advanced from the trial load method to now advanced non-linear finite element methods. This paper presents some research and development associated with the stability study of Hoover Dam from the mid-1980s to 2000 that affected the direction of current structural analysis practices
Substantial topics affecting the seismic stability of concrete dams needed to be studied: hydrodynamic interaction, dam-foundation interaction, spatially variations in ground motions, nonlinear features like contraction joints, appropriate material properties, and damping. Each topic studied improved the understanding of the performance of concrete dams to earthquakes. Reclamation incorporated these developments into their dam safety process with the goal of performing the most realistic structural analyses possible for concrete dams. This way decisions about the safety of their inventory of dams were based on state-of-the-art practices and reliable field data.
Dynamic Testing of Large Scale Dams and Correlation with Numerical Predictions using 2D and 3D programs Developed by Dr. Chopra and his Collaborators
Chair of the Department of Civil Engineering
Université de Sherbrooke
Researchers at the earthquake engineering and structural dynamic research group at the University of Sherbrooke, Canada, under the direction of Prof. P. Paultre, have carried out several large scale dynamic tests on bridges, buildings and dams since the 1990s. This presentation will focus on ambient and forced vibration tests on large concrete dams located in Canada, France and Switzerland. Results from these tests were used throughout the years to correlate with numerical predictions obtained from specialized 2D and 3D programs, developed by Dr. Chopra and his collaborators, for earthquake analysis of large dams. The combination of experimental data recorded on the dams, along the foundation as well as inside the reservoir, enabled a thorough investigation of the interactions between each of those components during seismic events. During the 2000s, correlation studies were also carried out with actual earthquake data recorded at different arch dam sites in Switzerland, in collaboration with Dr. G. Darbre at the Swiss Federal Office for Energy. Results obtained with 3D models developed with dif
ferent versions of the EACD (Earthquake Analysis of Concrete Dams) program, and their ability to predict the recorded data, showed the significance of water compressibility, of damping and of dam-reservoir-foundation interaction in general, thus emphasizing the importance of Dr. Chopra’s work in the field of dam engineering.
Juan Carlos Reyes
Department of Civil and Environmental Engineering
Universidad de los Andes, Bogotá-Colombia
Seismic design/assessment of structures strongly relies on the proper quantification of the mass that is present at the time of occurrence of a major earthquake. ASCE/SEI 7-10 establishes that for seismic analyses of structures the minimum portion of the live load to be included as inertia is 25% in the case of facilities used for storage. This percentage reduces to 10% in guidelines that are applicable to marine structures (POLB 2012) and to zero in bridge design standards (AASHTO 2012). Such provisions are typically not questioned because live loads may not be so significant and because of the accepted perception that only a minor portion of the live load is likely to be present at the time of occurrence of a seismic event. However, for pile-supported container yards, in which live loads are nearly permanent and could exceed the dead load by a factor of 2.0 or more, the proper selection of seismic mass becomes very critical. The contribution of live loads to inertial forces depends on how much energy is dissipated through sliding and rocking of containers.
This investigation has two phases. The first phase consisted on a numerical study using a lumped-parameter model that describes the seismic behavior of a single-degree-of-freedom (SDF) structure supporting a rigid block with the possibility to slide. After evaluating its capability using finite element software and shake table test results, the numerical model was implemented in a statistical methodology to quantify the portion of the block’s mass that should be considered as inertia in the seismic design of one-story storage structures. Various structural periods T, friction coefficients μ, block-to-structure mass ratios α, response modification factors R were included in a parametric study, which involved thousands of analysis cases. In addition, two seismic hazard levels were considered consistent with service and extreme conditions. It was found that the portion of live load that should be included as inertia in seismic design increases significantly with T, μ, and R. However, the variable that best correlates with seismic mass is the total acceleration A_max experienced by the SDF platform alone. If A_max is small, the blocks may behave as rigidly attached to the structure, so their total mass should be included as inertia in the seismic analysis of storage facilities; this may be the case of structures subjected to service ground motions or designed for R values larger than 3. Finally, a design expression is proposed to estimate the portion of the live load to be included as inertia in function of the maximum total floor acceleration, the live load to structure self-weight ratio and the friction coefficient at the block-structure interface.
The second phase of the research includes results of 154 shake table tests conducted using a 1:15 scale single story model that supported a block with the possibility to slide and/or rock. The experimental program involved five block-to-structure mass ratios, two block aspect ratios, and two seismic hazard levels. Drift demands on the model were measured to be higher when the structure supported a squat block as compared to a slender block with the same mass. The implementation of a statistical methodology to quantify the effect of the blocks on the seismic response of the one-story laboratory model showed that for the conditions of the experimental program: a) squat blocks had a larger portion of their mass effectively contributing to inertial forces on the structure as compared to slender blocks; b) the portion of the block’s mass that was effective as inertia does not exhibit a clear correlation with the block-to-structure mass ratio (even though drift demands on the model were consistently higher for heavier blocks); c) the live load percentages that are prescribed in current codes and standards may significantly underestimate the contribution of live load to inertial forces, especially for service-level ground motions. The design expression developed in the first phase of the research was favorably compared against the experimental results. Numerical modeling was carried out to extend the conclusions and observations to conditions beyond those of the experimental program.
Lessons Learned from 3-Dimensional Shake Table Testing of a Full-Scale Seismically-Isolated Building
Department of Civil and Environmental Engineering
University of Nevada Reno
Through a Memorandum of Understanding between the U.S. Network for Earthquake Engineering Simulation (NEES) and Japan’s National Institute of Earth Science and Disaster Prevention, a full-scale shaking table test of a 5-story base-isolated was carried out at Japan’s Hyogo Earthquake Engineering Research Center (E-Defense) in 2011. The building was tested with two different isolation systems (triple pendulum bearings and a hybrid system of lead-rubber bearings and low-friction rolling cross-linear bearings) and in the fixed-base configuration. The tested building had a realistic floor system, nonstructural components (suspended ceilings, sprinkler piping and interior walls), and furnishings, and was subjected to strong earthquake shaking. The tests served as a full-scale proof of the concept of seismic isolation to protect the building from damage in very strong earthquake shaking; for instance, displacement demands across the isolation system were more than twice what has been observed in any previous earthquake event. However, the nonstructural components and furnishings were not completely protected from damage, and the tests showed that these items were sensitive to the vertical component of ground shaking, which is unaffected by the seismic isolation system. While the overall performance of the isolation systems was very impressive when considered against other available options for seismic protection, the tests highlight the challenge of designing a building to remain immediately operational following a large earthquake. This presentation will summarize the test program, present the major findings, and discuss future directions in research and design practice.
Validation of Nonlinear Finite Element Analysis Methods used in Evaluating Hydraulic Structures Subject to Extreme Loading Conditions
Division of Safety of Dams
California Department of Water Resources
As finite element analysis methods continue to advance the state of practice, they provide additional insight into the nonlinear three-dimensional dynamic response of hydraulic structures subject to extreme loading conditions. Simultaneously, the inclusion of advanced modeling features necessary to simulate nonlinear behavior and damage introduces room for additional error. Confidence in results can therefore only be gained once analysis methods have been validated and evaluated using direct measurements of known behavior.
This presentation highlights some of the recent modeling progressions and compares nonlinear modeling methods to more traditional analysis techniques. Analysis methods are also evaluated using direct measurements of known behavior and results focused on comparing physical recordings of dynamic response to model calculations. The findings presented are intended to demonstrate how finite element analysis methods in modern commercially available can capture wave propagation, site response, and structural response. The comparisons demonstrate the capability of current modeling techniques to re-create a known earthquake and simulate dynamic response of a dam subject to seismic loading.
Executive Portfolio Manager
Hoboken, New Jersey
Have you ever wondered what goes into writing and building a successful engineering textbook? In honor of Professor Anil Chopra’s retirement, Holly Stark, Dr. Chopra’s long time acquisitions editor for his textbook, Dynamics of Structures, will share personal insights and perspectives based on her experiences, about the writing, developing, and publishing process for University level textbooks and course materials. Topics will range from ideas, prospectus development to peer reviewing, production processes (such as figures, tables, photos, art, composition, design and content for pedagogical impact) all the way through to potential marketing activities and considerations.
Department of Hydraulic Engineering
The presentation focuses on the seismic response analysis of high concrete dams. Two topics will be discussed. One is the seismic analysis procedure of dam-water-foundation systems. A comprehensive analysis model developed by the research group at Tsinghua University is presented, which simultaneously takes into account radiation damping effect of semi-unbounded canyons, dynamic interaction of dam-water, opening of contraction joints, seismic damage cracking of concrete, and strengthening of dam. The earthquake response of the Pacoima dam to the 1994 Northridge earthquake was presented to verify the developed model. The other one is the numerical simulation of ground motion at dam sites. A large-scale numerical simulation of seismic ground motion from source rupture to dam canyons is introduced. The characteristics of near-field ground motion at dam sites are simulated considering the effect of source mechanism, propagation media, and local site.