Earthquake engineering for concrete dams: Analysis, design, and evaluationAnil K.Chopra. Wiley‐Blackwell, 2020, 320 pp., $135.00 USD

Limited historical evidences, dating back to the sixty's (i.e., the 1967 unexpected Konya gravity dam damage), have shown that large concrete gravity and arch dams will be subjected to severe cracking and displacements in the case of major earthquakes. Their catastrophic destruction potential, from a sudden release of the energy stored from high water reservoir, is phenomenal and must be prevented. Seismic structural safety assessment of concrete dams is one of the most challenging problems in earthquake engineering because of the complex seismic wave propagation phenomena affecting massive structures bounded by water and fractured rock media of quasi-infinite spatial extent with quite different mechanical impedance characteristics. Chopra's new book presents in a single document, with no equivalent in the technical literature, the fundamental concepts, their mathematical framework, and their applications, which have become central to earthquake engineering for structural safety assessment of concrete dams. Those were developed over more than 50 years of seminal research by the author, in collaboration with his graduate students, while developing rigorous computational finite element methodologies to model dam-water-foundation interactions that are in current use worldwide. It is shown that rigorous dam-water-rock modeling may avoid unnecessary strengthening measures deemed necessary by less sophisticated approaches using, for example, massless foundation models. Following, Chapter (1) Introduction, the book consists of three parts divided into 13 chapters: Part I Gravity dams contains seven chapters; (2) Fundamental Mode Response of Dams Including Dam–Water Interaction; (3) Fundamental Mode Response of Dams Including Dam–Water–Foundation Interaction; (4) Response Spectrum Analysis of Dams Including Dam–Water–Foundation Interaction; (5) Response History Analysis of Dams Including Dam–Water–Foundation Interaction; (6) Dam–Water–Foundation Interaction Effects in Earthquake Response; (7) Comparison of Computed and Recorded Earthquake Responses of Dams; Part II Arch Dams consists in four chapters; (8) Response History Analysis of Arch Dams Including Dam–Water–Foundation Interaction; (9) Earthquake Analysis of Arch Dams: Factors to Be Included; (10) Comparison of Computed and Recorded Motions; (11) Nonlinear Response History Analysis of Dams; Part III Design and Evaluation presents three chapters; (12) Design and Evaluation Methodology; (13) Ground-Motion Selection and Modification; (14) Application of Dynamic Analysis to Evaluate Existing Dams and Design New Dams. Chapter 1 introduces the complexities in modeling dam-water-foundation interactions and discusses the limitations of the traditional approaches using seismic coefficients, Westergaard added water masses, and massless foundation. Two rigorous alternatives are developed in subsequent chapters: (1) The "substructure method" formulated in the frequency domain using semi-unbounded water and rock continua, therefore limited to linear systems, and the (2) "direct finite element method (FEM)" formulated in the time domain using truncated FE water and rock domains with wave-absorbing boundaries. The direct FEM can be extended to nonlinear behavior and implemented using commercial FE programs for industrial applications. Chapter 2 presents rigorous frequency domain (FD) compressible and incompressible solutions for hydrodynamic pressures generated by dam-water interaction. Chapter 3 extends the FD formulation and its solution to include interactions with the foundation modeled as an unbounded viscous elastic half-space. Chapter 4 presents a simplified 2D response spectra analysis (RSA) procedure, derived from extensive parametric analysis from the "substructure method," and suitable for gravity dams. It is based on the dynamic response in the first mode of the dam-water-foundation system with a static correction for higher mode effects. Chapter 5 presents 2D gravity dam-water-foundation frequency domain response history analysis (RHA) that was implemented in the EAGD-84 computer program. Chapter 6 provides a systematic assessment of the effect of key input parameters on computed seismic dam displacements and stresses, including the range of substructure natural frequencies where water compressibility could be ignored. Chapter 7 presents back analyses, using the 2D substructure method of two gravity dams (Tsudura, Japan, 117 m high; Koyna, India) that were subjected to major earthquakes. Chapter 8 develops the 3D generalization of the substructure method, implemented in the computer program EACD-3D, and suitable for RHA of arch dams. Chapter 9 discusses the factors to be included in earthquake response analysis of arch dams through the analysis of four existing dams including (i) dam-water interactions (compressible vs. incompressible), (ii) dam-foundation interactions (massless vs. mass; with different stiffnesses), and (iii) the spatial variations of ground motions along the dam-foundation contact. The massless foundation model is shown to grossly overestimate internal stresses while ignoring water compressibility could over or underestimate internal stresses. Chapter 10 provides comparisons between the computed and actual seismic responses of two arch dams. The importance of computing the effective damping ratio of the dam-water-foundation systems using, for example, the half-power bandwidth is discussed indicating that the ″overall damping in a numerical model should not exceed 5% unless a larger value was measured at a particular dam.″ Chapter 11 is devoted to nonlinear 3D RHA of dams (cracking, joint movements) using the direct FEM. The earthquake input mechanism is done via three 1D deconvolutions of the longitudinal, transverse, and vertical components the free-field ground motions to all foundation boundaries. Comparisons between the substructure method and the direct FEM, through frequency response functions and the earthquake response of the Morrow Point arch dam (142 m high), are used to show the excellent accuracy that could be achieved using the direct FEM. This chapter closes with a nonlinear analysis of Morrow Point dam using the commercial FE program ABAQUS. Chapter 12 first reviews current dam safety guidelines regarding the selection of the earthquake return period to ensure an adequate safety margin against loss of reservoir. Because of significant uncertainties in the current state-of-the-practice (state-of-the-art) to predict nonlinear response of dam-water-foundation systems, Chopra advocates that design criteria should be more stringent to remain basically undamaged during a maximum design earthquake with 10,000-year return period. If this is not economical, more precise limits should be put on the acceptable damage level with more explicit requirements than ″no uncontrolled release of reservoir.″ Chapter 13 advocates that the Uniform Hazard Spectrum (UHS), currently defined in building codes, is not the best candidate to define the target spectrum for safety assessment because it is too conservative, being unrepresentative of an individual ground motion (GM) spectrum. Instead, an optimal CMS (Conditional Mean Spectrum)-UHS composite spectrum is conveniently defined using only two conditioning periods, the longest and shortest of interest. Ground motion selection, amplitude scaling and spectral matching procedures to define target spectrum compatible GM are reviewed. The book closes with Chapter 14 that provides an overview of the practical applications of the dynamic analysis procedures presented in the book (i) for the seismic evaluation of Folsom gravity dam using EAGD-84 (in 1989), (ii) the design of Olivenhain gravity dam using EAGD-84 (in 2003), (iii) the seismic evaluation of Hoover arch dam using SAPIV and EACD-3D-1996 (in 1998) and, (iv) the seismic design of Dagangshan dam using a direct FEM (in 2015). The whole book is well-written with clear explanations of the physics behind the mathematical formulations to enhance the comprehension of the material presented. The book contains several recommendations to define key modeling parameters, such as damping, to perform reliable dynamic modeling and simulations of dam-water-foundation systems. The limitations of different methods and assumptions for modeling of dam-water-foundation interactions, ranging from the simplest one, to the more sophisticated FEM are comprehensively illustrated through comparative parametric and sensitivity analyses. To appreciate the technical content of the book, the reader should have the background of a first course in structural dynamics including frequency domain analysis as well as wave propagation in elastic media. In fact, in several instances, the book refers to the very popular textbook Dynamic of Structures: Theory and Application to Earthquake Engineering by the author.1 The book, also containing several case studies, is a very valuable resource for graduate students, researchers and practicing engineers interested in learning and applying state-of-the-art earthquake engineering and structural dynamics principles to design new concrete dams or assess the safety of existing ones.