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CONTENTS
Volume 3, Number 3, June-August 2012
 

Abstract
This paper presents results for impedance (and compliance) functions and input motions of foundations in a layered half-space computed on the basis of a procedure that combines a consistent transmitting boundary with continued-fraction absorbing boundary conditions which are accurate and effective in modeling wave propagation in various unbounded domains. The effects of obliquely incident seismic waves in a layered half-space are taken into account in the formulation of the transmitting boundary. Using the numerical model, impedance (and compliance) functions and input motions of rigid circular foundations on the surface of or embedded in a homogeneous half-space are computed and compared with available published results for verification of the procedure. Extrapolation methods are proposed to improve the performance in the very-low frequency range and for the static condition. It is concluded from the applications that accurate analysis of foundation dynamics and soil-structure interaction in a layered half-space can be carried out using the enhanced consistent transmitting boundary and the proposed extrapolations.

Key Words
foundation dynamics; soil-structure interaction; consistent transmitting boundary; continuedfraction absorbing boundary condition; layered half-space; wave propagation

Address
Jin Ho Lee and John L. Tassoulas: Department of Civil, Architectural and Environmental Engineering, the University of Texas at Austin, Austin, Texas, USA; Jae Kwan Kim: Department of Civil and Environmental Engineering, Seoul National University, Seoul, Korea

Abstract
In this study, a systematic investigation is carried out on the seismic behaviour of plane moment resisting steel frames (MRF) to repeated strong ground motions. Such a sequence of earthquakes results in a significant damage accumulation in a structure because any rehabilitation action between any two successive seismic motions cannot be practically materialised due to lack of time. In this work, thirtysix MRF which have been designed for seismic and vertical loads according to European codes are first subjected to five real seismic sequences which are recorded at the same station, in the same direction and in a short period of time, up to three days. Furthermore, the examined frames are also subjected to sixty artificial seismic sequences. This investigation shows that the sequences of ground motions have a significant effect on the response and, hence, on the design of MRF. Additionally, it is concluded that ductility demands, behaviour factor and seismic damage of the repeated ground motions can be satisfactorily estimated using appropriate combinations of the corresponding demands of single ground motions.

Key Words
moment resisting steel frames; inelastic seismic analysis; repeated earthquakes; damage accumulation

Address
D. Loulelis: Department of Civil Engineering, University of Patras, GR-67100 Patras, Greece; G.D. Hatzigeorgiou: Department of Environmental Engineering, Democritus University of Thrace, GR-67100 Xanthi, Greece; D.E. Beskos: Department of Civil Engineering, University of Patras, GR-67100 Patras, Greece; Office of Theoretical and Applied Mechanics, Academy of Athens, 4 Soranou Efessiou,GR-11527 Athens, Greece

Abstract
This paper investigates the effectiveness of widely used identification methods to identify the response of seismically isolated structures supported on bearings with bilinear behavior. The paper shows that while both time domain and frequency domain methods predict with high accuracy the modal characteristics of structures isolated by linear isolation system, their performance degrades appreciably when the isolation system exhibits bilinear behavior even when its strength assumes moderate values (say 5% of the weight). The paper also shows that the natural period of isolated structure that results from bilinear isolation systems can be satisfactorily predicted with wavelet analysis.

Key Words
seismic isolation; bilinear behavior; system identification; wavelet analysis

Address
G. Kampas and N. Makris: Division of Structures, Department of Civil Engineering, University of Patras, Greece

Abstract
This paper presents the solution scheme of using the continuous formulation of 1-D linear member for the dynamic analysis of structures consisting of axially loaded members. The context describes specific applications of such scheme to the verification of experimental data obtained from field test of bridges carried out by a microwave interferometer system and velocimeters. Attention is focused on analysis outlines that may be applicable to in-situ assessment for cable-stayed bridges. The derivation of the dynamic stiffness matrix of a prismatic member with distributed properties is briefly reviewed. A back calculation formula using frequencies of two arbitrary modes of vibration is next proposed to compute the tension force in cables. Derivation of the proposed formula is based on the formulation of an axially loaded flexural member. The applications of the formulation and the proposed formula are illustrated with a series of realistic examples.

Key Words
dynamic stiffness matrix; axially loaded member; cable force; cable-stayed bridge; transfer function

Address
Chih-Peng Yu and Chia-Chi Cheng: Department of Construction Engineering, Chaoyang University of Technology, 168 Gifeng E. Rd., Wufeng, Taiwan

Abstract
This paper is intended to show the robustness and capabilities of a coupled boundary element-finite element technique for the analysis of vibrations generated by high-speed trains under different geometrical, mechanical and operation conditions. The approach has been developed by the authors and some results have already been presented. Nevertheless, a more comprehensive study is presented in this paper to show the relevance and robustness of the method which is able to predict vibrations due to train passage at the vehicle, the track, the free-field and any structure close to the track. Local soil discontinuities, underground constructions such as underpasses, and coupling with nearby structures that break the uniformity of the geometry along the track line can be represented by the model. Non-linear behaviour of the structures can be also considered. Results concerning the excitation mechanisms, track behaviour and sub-Rayleigh and super-Rayleigh train speed are summarized in this work.

Key Words
FE/BE model; time domain; HST vibration

Address
A. Romero, P. Galvin and J. Dominguez: Escuela Tecnica Superior de Ingenieria, Universidad de Sevilla, Camino de los Descubrimientos, 41092 Sevilla, Spain

Abstract
There are many types of seismic isolation devices that are being used today for structural control of earthquake response in buildings. The most commonly used are sliding bearings and elastomeric bearings, the latter with or without lead core. An alternative solution is the use of steel springs combined with viscoelastic fluid dampers, which is the case discussed in this paper. An analytical study of a three-story building supported on helical steel springs and viscoelastic fluid dampers, GERB Control System (GCS), subjected to near-fault earthquakes is presented. Several earthquakes records have been obtained by the acceleration network installed in the isolated building and in its non-isolated twin since they were finished. These experimental results are analysed and discussed. The aim is to show that the spring-based system can be an alternative for base isolation of small building located near active faults.

Key Words
building base isolation; spring bearing; viscous damping; near fault earthquakes

Address
Miguel Eduardo Tornello: Regional Center of Technological Developments for Construction, Seismology and Earthquake Engineering (CeReDeTec.), National Technological University,
Rodriguez 273 (5500), Mendoza, Argentina; Mauricio Sarrazin: Department of Civil Engineering, University of Chile, Beauchef 850, Santiago (8370448), Chile

Abstract
The spectral representation method is a quick and versatile tool for the generation of spatially variable, response-spectrum-compatible simulations to be used in the nonlinear seismic response evaluation of extended structures, such as bridges. However, just as recorded data, these simulated accelerations require processing, but, unlike recorded data, the reasons for their processing are purely numerical. Hence, the criteria for the processing of acceleration simulations need to be tied to the effect of processing on the structural response. This paper presents a framework for processing acceleration simulations that is based on seismological approaches for processing recorded data, but establishes the corner frequency of the high-pass filter by minimizing the effect of processing on the response of the structural system, for the response evaluation of which the ground motions were generated. The proposed two-step criterion selects the filter corner frequency by considering both the dynamic and the pseudo-static response of the systems. First, it ensures that the linear/nonlinear dynamic structural response induced by the processed simulations captures the characteristics of the system

Key Words
acceleration simulations; velocity and displacement time series; spatial variation; response spectrum; spectral representation; processing; corner frequency; high-pass filter; nonlinear seismic response; bridges

Address
A. Zerva: Department of Civil, Architectural and Environmental Engineering, Drexel University, 3141 Chestnut Street, Philadelphia, PA 19104, USA; H. Morikawa: Department of Built Environment, Tokyo Institute of Technology, 4259-G3-7 Nagatsuta, Midori-ku, Yokohama 226-8502, Japan; S. Sawada: Disaster Prevention Research Institute, Kyoto University, Gokasho, Uji, Kyoto, 611-0011, Japan

Abstract
Recent earthquakes have damaged some bridges located on the Pacific Coast of Mexico; these bridges have been retrofitted or rebuilt. Based on the fact that the Pacific Coast is a highly active seismic zone where most of the strong earthquakes in the country occur, one fertile and important area of research is the study of the vulnerability of both new and existent bridges located in this area that can be subjected to strong earthquakes. This work is focused on estimating the contribution of some parameters identified to have major influence on the seismic vulnerability of reinforced concrete bridges. Ten models of typical reinforced concrete (RC) bridges, and two existing bridges located close to the Pacific Coast of Mexico are considered. The group of structures selected for the study is based on two span bridges, two pier heights and two substructure types. The bridges were designed according to recent codes in Mexico. For the vulnerability study, the capacity of the structure was evaluated based on the FEMA recommendations. On the other hand, the demand was evaluated using a group of more than one hundred accelerograms recorded close to the subduction zone of Mexico. The results show that the two existent bridges analyzed show similar trends of behavior of the group of bridge models studied. In spite of the contribution that traditional variables (height and substructure type) had to the bridge seismic response, the bridge length was also found to be one of the parameters that most contributed to the seismic vulnerability of these RC medium-length bridges.

Key Words
bridges; fragility curves; bridge vulnerability; RC bridges; medium-length bridges

Address
B.A. Olmos, J.M. Jara and M. Jara: Facultad de Ingenieria Civil, Universidad Michoacana de San Nicolas de Hidalgo, Mexico

Abstract
The objective of the present paper is to review and implement the most recent developments in the Spectral Element Method (SEM), as well as improve aspects of its implementation in the study of wave propagation by numerical simulation in elastic unbounded domains. The classical formulation of the method is reviewed, and the construction of the mass matrix, stiffness matrix and the external force vector is expressed in terms of matrix operations that are familiar to earthquake engineers. To account for the radiation condition at the external boundaries of the domain, a new absorbing boundary condition, based on the Perfectly Matched Layer (PML) is proposed and implemented. The new formulation, referred to as the Multi-Axial Perfectly Matched Layer (M-PML), results from generalizing the classical Perfectly Matched Layer to a medium in which damping profiles are specified in more than one direction.

Key Words
elastic wave propagation; spectral element method; perfectly matched layer; engineering seismology

Address
Kristel C. Meza Fajardo: Departamento de Ingenieria Civil Universidad Nacional Autonoma de Honduras (UNAH), Tegucigalpa, Honduras; Apostolos S. Papageorgiou: Department of Civil Engineering, University of Patras, GR-26500 Patras, Greece

Abstract
This paper presents a displacement-based seismic design method with damage control, in which the targets for the considered performance level are set as displacements and a damage distribution is proposed by the designer. The method is based on concepts of basic structural dynamics and of a reference single degree of freedom system associated to the fundamental mode with a bilinear behaviour. Based on the characteristics of this behaviour curve and on the requirements of modal spectral analysis, the stiffness and strength of the structural elements of the structure satisfying the target design displacement are calculated. The formulation of this method is presented together with the formulations of two other existing methods currently considered of practical interest. To illustrate the application of the proposed method, 5 reinforced concrete plane frames: 8, 17 and 25 storey regular, and 8 and 12 storey irregular in elevation. All frames are designed for a seismic demand defined by single earthquake record in order to compare the performances and damage distributions used as design targets with the corresponding results of the nonlinear step by step analyses of the designed structures subjected to the same seismic demand. The performances and damage distributions calculated with these analyses show a good agreement with those postulated as targets.

Key Words
displacement-based seismic design; damage control; reference single degree of freedom system; modal spectral analysis; nonlinear behaviour

Address
A. Gustavo Ayala, Hugo Castellanos and Saul Lopez: Instituto de Ingenieria, Universidad Nacional Autonoma de Mexico, Ciudad Universitaria, Mexico 04510, D.F.

Abstract
An impact on the ground surface may represent several phenomena, such as a crash of an airplane or an explosion or the passage of a train. In order to analyze and design structures and equipment to resist such a type of shocks, the response spectra for an impact on the ground must be given. We investigated the half-space motions due to impact using the finite element method. We performed extensive parametric analyses to define a suitable finite element model and arrive at displacement time histories and response spectra at varying distances from the impact point. The principal scope of our study has been to derive response spectra which: (a) provide insight and illustrate in detail the half-space response to an impact load, (b) can be readily used for the analysis of structures resting on a ground subjected to an impact and (c) are a new family of results for the impact problem and can serve as reference for future research.

Key Words
impact; shock; spectra; soil; aircraft impact; explosion; heavy tamping; urban vibration

Address
Ioannis V. Constantopoulos: Department of Building, Architecture, and Town Planning, Ecole Polytechnique, Universite Libre de Bruxelles, Avenue Adolphe Buyl 87, 1050 Brussels, Belgium; Department of Applied Mechanics and Strength of Materials, Hellenic Naval Academy, Hatzikyriakou Avenue, 18539 Piraeus, Greece; Yukiko Van Wessem: Tractebel, Avenue Ariane 7, 1200 Brussels, Belgium; Jean-Claude Verbrugge: Department of Building, Architecture, and Town Planning, Ecole Polytechnique, Universite Libre de Bruxelles, Avenue Adolphe Buyl 87, 1050 Brussels, Belgium

Abstract
Current code procedures for stress and stability analysis of new and existing concrete-gravity dams are primarily based on conventional methods of analysis. Such methods can be applied in a straightforward manner but there has been evidence that they may be inaccurate or, possibly, not conservative. This paper presents finite element modeling and analysis procedures and makes recommendations for local failure criteria at the dam-rock interface aimed at predicting more accurately the behavior of dams under hydraulic and anchoring loads.

Key Words
dam; concrete; stability; failure; sliding; overturning; finite element; interface

Address
Chryssis G. Papaleontiou: Department of Civil Engineering and Geomatics, The Cyprus University of Technology, Limassol, Cyprus; John L. Tassoulas: Department of Civil, Architectural and Environmental Engineering The University of Texas at Austin, Austin, Texas, U.S.A.

Abstract
The capacity of pipelines to resist collapse under external pressure and bending moment is a major aspect of deepwater pipeline design. Existing design codes present interaction equations that quantify pipeline capacities under such loadings, although reasonably accurate, are based on empirical data fitting of the bending strain, and assumed simplistic interaction with external pressure collapse. The rational model for collapse of deepwater pipelines, which are relatively thick with a diameter-to-thickness ratio less than 40, provides a unique theoretical basis since it is derived from first principles such as force equilibrium and compatibility equations. This paper presents the rational model methodology and compares predicted results and recently published full scale experimental data on the subject. Predictive capabilities of the rational model are shown to be excellent. The methodology is extended for the problem of pipeline collapse under point load, longitudinal bending and external pressure. Due to its rational derivation and excellent prediction capabilities, it is recommended that design codes adopt the rational model methodology.

Key Words
pipeline; collapse; bending; external pressure; deepwater; interaction; design

Address
Andre C. Nogueira: INTECSEA, Houston, USA

Abstract
An elasto-dynamic model for pile-soil-pile interaction together with a simple plate model is used in this study to assess the effect of flexible foundation slabs on the dynamic response of pile groups. To this end, different pile configurations with various slab thicknessesare considered in two soil media with low and high elastic moduli. The analyses include dynamic impedances and seismic responses of pile-group foundations.The presented results indicate that the stiffness and damping of pile foundations increase with thickness of the foundation slab; however, the results approach those for rigid slab as the slab thickness approaches twice the pile diameter for the cases considered in this study. The results also reveal that pile foundations with flexible slabs may amplify the earthquake motions by as much as 10 percent in the low to intermediate frequency ranges.

Key Words
pile group; foundation slab; pile-soil-pile; interaction; stiffness; damping; earthquake; dynamic; impedance

Address
Amir M. Kaynia: Norwegian Geotechnical Institute (NGI), Oslo, and Norwegian University of Science and Technology (NTNU), Trondheim, Norway

Abstract
It is now widely accepted that the resulting displacement field within elastic, inhomogeneous, anisotropic solids subjected to equipartitioned, uniform illumination from uncorrelated sources, has intensities that follow diffusion-like equations. Typically, coda waves are invoked to illustrate this concept. These waves arrive later as a consequence of multiple scattering and appear at

Key Words
diffuse fields; energy density; Green

Address
Francisco J. Sanchez-Sesma: Instituto de Ingenieria, Universidad Nacional Autonoma de Mexico, Cd. Universitaria,
Coyoacan 04510, Mexico D F, Mexico; Alejandro Rodriguez-Castellanos: Instituto Mexicano del Petroleo, Eje Central Lazaro Cardenas 152, CP 07730, Mexico D F, Mexico; Juan J. Perez-Gavilan: Instituto de Ingenieria, Universidad Nacional Autonoma de Mexico, Cd. Universitaria, Coyoacan 04510, Mexico D F, Mexico; Humberto Marengo-Mogollon: Coordinacion de Proyectos Hidroelectricos, Comision Federal de Electricidad, Av. Rio Mississippi 71, CP 06500, Mexico D F, Mexico; Luis E. Perez-Rocha: Division de Sistemas Mecanicos, Instituto de Investigaciones Electricas, Reforma 113, Col. Palmira, CP 62490, Morelos, Mexico; Francisco Luzon: Depto. Fisica Aplicada, U de Almeria, Canada de San Urbano s/n, 04120-Almeria, Spain

Abstract
Using nondestructive testing techniques to evaluate the length or depth of an existing foundation is an important issue with potential high application values. One of these is to evaluate whether the foundation is broken after severe earthquakes. In this aspect, academic research related to nondestructive evaluation for caisson foundations is rarely reported. The objective of this paper is to study the feasibility of using Sonic Echo method to evaluate the depth of caisson foundations. Two types of caissons, simple cylindrical caisson and compound caisson with chambers, were studied for their responses to the Sonic Echo tests. The study was carried out in numerical simulation with finite element method and experimental way with in-situ tests. A bridge system which spans over Sofong Brook in Taiwan was selected for the tests in situ. The bridge system is still under construction and therefore the effect of different construction stages on the testing results may be studied. In this paper, the parameters to be varied for the studies include the testing locations and the existence of chamber plates, the bottom plate and the top plate. Finally some preliminary conclusions can be reached for a successful test.

Key Words
nondestructive test; caisson foundation; Sonic Echo method; finite element method

Address
Jian-Hua Tong: Faculty of Department of Computer Science & Information Engineering, Hungkuang University, Taichung, Taiwan; Shu-Tao Liao: Faculty of Department of Civil Engineering, Chung Hua University, Hsinchu, Taiwan; Kang-You Liu: Graduate Student of Department of Civil Engineering, Chung Hua University, Hsinchu, Taiwan

Abstract
Dynamic analysis of concrete gravity dam-reservoir systems is an important topic in the study of fluid-structure interaction problems. It is well-known that the rigorous approach for solving this problem relies heavily on employing a two-dimensional semi-infinite fluid element. The hyper-element is formulated in frequency domain and its application in this field has led to many especial purpose programs which were demanding from programming point of view. In this study, a technique is proposed for dynamic analysis of dam-reservoir systems in the context of pure finite element programming which is referred to as the wavenumber approach. In this technique, the wavenumber condition is imposed on the truncation boundary or the upstream face of the near-field water domain. The method is initially described. Subsequently, the response of an idealized triangular dam-reservoir system is obtained by this approach, and the results are compared against the exact response. Based on this investigation, it is concluded that this approach can be envisaged as a great substitute for the rigorous type of analysis.

Key Words
concrete gravity dams; wavenumber; absorbing boundary conditions; truncation boundary

Address
Vahid Lotfi and Ali Samii: Department of Civil and Environmental Engineering, Amirkabir University of Technology, Tehran, Iran

Abstract
Many building foundations are embedded, however it is not easy to compact the backfill around the foundation especially for the deeply embedded ones. The soil condition around the embedded foundation may affect the seismic response of a building due to the weak contact between the soil and the foundation. In this paper, the response accelerations in the short-period range and at the period of 1 second (in the long-period range) for a seismic design spectrum specified in the IBC design code were compared considering perfect and poor backfills to investigate the effect of backfill compaction around the embedded foundation. An in-house finite-element software (P3DASS) which has the capability of horizontal pseudo-3D seismic analysis with linear soil layers was used to perform the seismic analyses of the structure-soil system with an embedded foundation. Seismic analyses were carried out with 7 bedrock earthquake records provided by the Pacific Earthquake Engineering Research Center (PEER), scaling the peak ground accelerations to 0.1 g. The results indicate that the poor backfill is not detrimental to the seismic response of a building, if the foundation is not embedded deeply in the soft soil. However, it is necessary to perform the seismic analysis for the structure-soil system embedded deeply in the soft soil to check the seismic resonance due to the soft soil layer beneath the foundation, and to compact the backfill as well as possible.

Key Words
response acceleration; backfill; embedded foundation; finite-element software of P3DASS; structure-soil system; soft soil.

Address
Yong-Seok Kim: Dept. of Architectural Engineering, Mokpo National University, Republic of Korea; Civil and Environmental Eng., University of California, Los Angeles, 90095, U.S.A

Abstract
We modify the formulation of a recently developed absorbing boundary condition (ABC), the perfectly matched discrete layers (PMDL), to incorporate the excitation coming from the exterior such as earthquake waves. The modified formulation indicates that the effect of the exterior excitation can be incorporated into PMDL ABCs (traditionally designed to treat only interior excitation) simply by applying appropriate forces on the nodes connected to the first PMDL layer. Numerical results are presented to clearly illustrate the effectiveness of the proposed method.

Key Words
perfectly matched discrete layer; absorbing boundary; soil-structure interaction; seismic domain truncation

Address
Murthy Guddati: Department of Civil, Construction and Environmental Engineering, North Carolina State University, Raleigh, NC 27695, USA; Siddharth Savadatti: Faculty of Engineering, University of Georgia, Athens, GA 30605, USA

Abstract
In this paper a novel non-iterative approach is proposed to address the problem of deriving non-stationary stochastic processes which are compatible in the mean sense with a given (target) response (uniform hazard) spectrum (UHS) as commonly desired in the aseismic structural design regulated by contemporary codes of practice. This is accomplished by solving a standard over-determined minimization problem in conjunction with appropriate median peak factors. These factors are determined by a plethora of reported new Monte Carlo studies which on their own possess considerable stochastic dynamics merit. In the proposed approach, generation and treatment of samples of the processes individually on a deterministic basis is not required as is the case with the various approaches found in the literature addressing the herein considered task. The applicability and usefulness of the approach is demonstrated by furnishing extensive numerical data associated with the elastic design UHS of the current European (EC8) and the Chinese (GB 50011) aseismic code provisions. Purposely, simple and thus attractive from a practical viewpoint, uniformly modulated processes assuming either the Kanai-Tajimi (KT) or the Clough-Penzien (C-P) spectral form are employed. The Monte Carlo studies yield damping and duration dependent median peak factor spectra, given in a polynomial form, associated with the first passage problem for UHS compatible K-T and C-P uniformly modulated stochastic processes. Hopefully, the herein derived stochastic processes and median peak factor spectra can be used to facilitate the aseismic design of structures regulated by contemporary code provisions in a Monte Carlo simulationbased or stochastic dynamics-based context of analysis.

Key Words
non-stationary process; design spectrum compatible; inverse problem; Monte Carlo simulation; peak factors; artificial accelerograms

Address
Agathoklis Giaralis: School of Engineering and Mathematical Sciences, City University London, Northampton Square, EC1V0HB, London, UK; Pol D. Spanos: Department of Mechanical Engineering and Material Sciences, Rice University, 6100 Main St, Houston, TX 77251, USA

Abstract
The inelastic earthquake response of existing, reinforced concrete buildings with an open ground story, designed according to the old Greek codes, is investigated before and after their seismic strengthening with steel braces restricted to the open ground stories. The seismic performance evaluation is based on Part 3 of Eurocode 8 for assessment and retrofitting of buildings. Three and five-story, symmetric and non-symmetric buildings are subjected to a set of seven pairs of synthetic accelerograms, compatible with the design spectrum, and conclusions are drawn regarding the effectiveness of the strengthening solutions. Seismic behavior of the selected models confirms results of previous work regarding the insufficient capacity of the open ground stories for design level earthquakes. It is also shown that strengthening only the weak ground story, a choice having the substantial advantage of low cost and continued usage of the building during its seismic retrofitting, can remove the inherent weakness without shifting the problem to the stories above and thus making such buildings at least as strong as those without a weak first story. This partial strengthening is possible for symmetric as well as eccentric buildings, in which torsion plays a further detrimental role.

Key Words
reinforced concrete buildings; open ground story; inelastic earthquake response; seismic evaluation; seismic strengthening; steel bracing

Address
T.A. Antonopoulos and S.A. Anagnostopoulos: Department of Civil Engineering, University of Patras, 26500 Patras, Greece


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