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CONTENTS
Volume 7, Number 6, December 2014
 

Abstract
The seismic characteristics of two semi-gravity reinforced concrete cantilever retaining walls are examined via an experimental program using an outdoor shake table (one with and the other without concrete masonry sound wall on top). Both walls are backfilled with compacted soil and supported on flexible foundation in a steel soil container. The primary damages during both tests are associated with significant lateral displacements of the wall caused by lateral earth pressure; however, no collapse occurs during the tests. The pressure distribution behind the walls has a nonlinear trend and conventional methods such as Mononobe-Okabe are insufficient for accurate pressure estimation.

Key Words
retaining wall; sound wall; shake table test; seismic loads; earth pressure

Address
Erin Mock: Alta Vista Solutions, Inc., 3260 Blume Dr., Suite 500, Richmond, CA 94806, U.S.A

Lijuan Cheng: Department of Civil and Environmental Engineering, University of California, Davis, One Shields Ave., Davis, CA 95616, U.S.A

Abstract
A semi-active control platform comprising the mechanical model of magnetorheological (MR) dampers, the bang-bang control law and damage material models is developed, and the simulation method of steel plate shear wall (SPSW) and optimization method for capacity design of MR dampers are proposed. A 15-story steel frame-SPSW structure is analyzed to evaluate the seismic performance of nonlinear semiactive controlled structures with optimal designed MR dampers, results indicate that the control platform and simulation method are stable and fast, and the damage accumulation effects of uncontrolled structure are largely reduced, and the seismic performance of controlled structures has been improved.

Key Words
steel plate shear wall; magnetorheological (MR) damper; control platform; nonlinear analysis; seismic damage control;

Address
Longhe Xu and Zhongxian Li: School of Civil Engineering, Beijing Jiaotong University, Beijing 100044, China

Yang Lv: School of Civil Engineering, Tianjin University, Tianjin 300072, China

Abstract
Storage racks are used worldwide in industries and commercial outlets due to the advantage of lighter, faster erection and easy alteration of pallet level as required. The studies to understand the behaviour of cold formed steel pallet racks, under seismic action is one of the emerging area of research. The rack consists of perforated uprights and beams with hook-in end connector, which enables the floor height adjustments. The dynamic characteristics of these racks are not well established. This paper presents the dynamic characteristics of 3-D single bay two storey pallet rack system with hook-in end connectors, which is tested on shake table. The sweep sine test and El Centro earthquake acceleration is used to evaluate the seismic performance of the cold formed steel pallet racks. Also an attempt is made to evaluate the realistic dynamic characteristics by using STAAD Pro software. Modal analysis is performed by incorporating the effective moment of inertia of the upright, which considers the effect of presence of perforations and rotational stiffness of the beam-to-upright connection to determine the realistic fundamental frequency of pallet racks, which is required for carrying out the seismic design. Finite element model of the perforated upright section has been developed as a cantilever beam through which effective moment of inertia is evaluated. The stiffness of the hook-in connector is taken from the previous study by Prabha et al. (2010). The results from modal analysis are in good agreement with the respective experimental results.

Key Words
pallet rack; shake table test; cold formed steel; modal analysis; effective moment of inertia

Address
Saravanan M., Marimuthu V., Prabha P., Surendran M., and Palani G.S.: Scientist, CSIR-Structural Engineering Research Centre, Chennai, India

Abstract
Aim of the paper is the definition of optimal design parameters characterizing the isolation system of a bridge, both in the case of elastomeric (VI) and sliding bearings (SI), having viscoelastic or rigid-plastic behavior, respectively, installed between the piers and the deck. The problem is treated by means of an analytical approach. Using the frequency response analysis, a simple procedure is proposed to determine the optimal value of the viscous coefficient or the yield displacement of the isolators. The adequacy of the proposed procedure is finally verified through time-history analyses performed on a practical case under natural earthquakes.

Key Words
seismic response; isolated bridge; optimal design; damping; elastomeric bearings; sliding bearings

Address
Daniele Losanno, Mariacristina Spizzuoco, Giorgio Serino: Department of Structures for Architecture and Engineering, University Federico II,
Via Claudio 21, 80125, Naples, Italy

Abstract
The size of spread footings was found to be unnecessarily large from some actual engineering practices constructed in Taiwan, due to the strict design provisions related to footing uplift. According to the earlier design code in Taiwan, the footing uplift involving separation of footing from subsoil was permitted to be only up to one-half of the foundation base area, as the applied moment reaches the value of plastic moment capacity of the column. The reason for this provision was that rocking of spread footings was not a favorable mechanism. However, recent research has indicated that rocking itself may not be detrimental to seismic performance and, in fact, may act as a form of seismic isolation mechanism. In order to clarify the effects of the relative strength between column and foundation on the rocking behavior of a column, six circular reinforced concrete (RC) columns were designed and constructed and a series of rocking experiments were performed. During the tests, columns rested on a rubber pad to allow rocking to take place. Experimental variables included the dimensions of the footings, the strength and ductility capacity of the columns and the intensity of the applied earthquake. Experimental data for the six circular RC columns subjected to quasi-static and pseudo-dynamic loading are presented. Results of each cyclic loading test are compared against the benchmark test with fixed-base conditions. By comparing the experimental responses of the specimens with different design details, a key parameter of rocking behavior related to footing size and column strength is identified. For a properly designed column with the parameter higher than 1, the beneficial effects of rocking in reducing ductility and the strength demand of columns is verified.

Key Words
The size of spread footings was found to be unnecessarily large from some actual engineering practices constructed in Taiwan, due to the strict design provisions related to footing uplift. According to the earlier design code in Taiwan, the footing uplift involving separation of footing from subsoil was permitted to be only up to one-half of the foundation base area, as the applied moment reaches the value of plastic moment capacity of the column. The reason for this provision was that rocking of spread footings was not a favorable mechanism. However, recent research has indicated that rocking itself may not be detrimental to seismic performance and, in fact, may act as a form of seismic isolation mechanism. In order to clarify the effects of the relative strength between column and foundation on the rocking behavior of a column, six circular reinforced concrete (RC) columns were designed and constructed and a series of rocking experiments were performed. During the tests, columns rested on a rubber pad to allow rocking to take place. Experimental variables included the dimensions of the footings, the strength and ductility capacity of the columns and the intensity of the applied earthquake. Experimental data for the six circular RC columns subjected to quasi-static and pseudo-dynamic loading are presented. Results of each cyclic loading test are compared against the benchmark test with fixed-base conditions. By comparing the experimental responses of the specimens with different design details, a key parameter of rocking behavior related to footing size and column strength is identified. For a properly designed column with the parameter higher than 1, the beneficial effects of rocking in reducing ductility and the strength demand of columns is verified.

Address
Hsiao-Hui Hung, Kuang-Yen Liu and Kuo-Chun Chang: National Center for Research on Earthquake Engineering, Taipei, Taiwan

Kuo-Chun Chang: Department of Civil Engineering, National Taiwan University, Taipei, Taiwan

Abstract
In this paper, a systematic technique is proposed for the optimal placement and design of nonlinear dampers for building structures. The concept of Output Frequency Response Function (OFRF) is applied to analytically represent the output frequency response of a building frame where nonlinear viscous dampers are fitted for suppression of vibration during earthquakes. An effective algorithm is derived using the analytical representation to optimally determine the locations and parameters of the nonlinear dampers. Various numerical examples are provided to verify the effectiveness of the optimal designs. A comparison of the vibration suppression performance with that of the frame structure under a random or uniform damping allocation is also made to demonstrate the advantages of the new designs over traditional solutions.

Key Words
optimal damper placement; nonlinear damper; frame structure; earthquake loading; vibration control

Address
Kohei Fujita, Masatoshi Kasagi and and Izuru Takewaki: Department of Architecture and Architectural Engineering, Kyoto University, Kyoto 615-8540, Japan

Zi-Qiang Lang, Guo Penfei and Izuru Takewaki: Department of Automatic Control and Systems Engineering, University of Sheffield,
Mappin Street, Sheffield S1 3JD, UK

Abstract
The 2011 off the Pacific coast of Tohoku Earthquake brought serious damage around the Tohoku district in Japan, and much human life and fortune were lost. Bridges were damaged by this earthquake. It was the most serious damage that the superstructures of bridges were flowed out by tsunami. Earthquakes of the same scale are predicted in other areas of Japan. It is necessary to take measures for bridges near coast. In order to understand the tsunami force acting on the bridge, hydraulic model experiments was conducted. In addition, this paper focused on fairing that is effective in wind resistant stability. Installing fairing to bridges has been verified by experiments whether it is possible to reduce the force of tsunami.

Key Words
tsunami; Kesen-Bridge; fairing; hydraulic model experiment, mechanism of tsunami action for bridge

Address
Takahiro Abukawa: Department of Design, CHODAI, Sendai 984-0051, Japan

Yuto Nakamura and Akira Hasegawa: Department of Civil Engineering, Hachinohe Institute of Technology,
Hachinohe 031-8501, Japan

Abstract
An iterative hybrid structural dynamic reliability prediction model has been developed under multiple-time interval loads with and without consideration of stochastic structural strength degradation. Firstly, multiple-time interval loads have been substituted by the equivalent interval load. The equivalent interval load and structural strength are assumed as random variables. For structural reliability problem with random and interval variables, the interval variables can be converted to uniformly distributed random variables. Secondly, structural reliability with interval and stochastic variables is computed iteratively using the first order second moment method according to the stress-strength interference theory. Finally, the proposed method is verified by three examples which show that the method is practicable, rational and gives accurate prediction.

Key Words
structure; interval load; random strength; strength degradation; hybrid model; dynamic reliability

Address
Yongfeng Fang: School of Mechanical Engineering, Bijie University, Bijie 551700, China

Jianbin Xiong: School of Computer and Electronic Information, Guangdong University of Petrochemical Techno
logy, Maoming, 525000, China

Kong Fah Tee: Department of Civil Engineering, University of Greenwich, Kent ME4 4TB, UK

Abstract
Pounding of adjacent structures are always a notable reason for damages after strong ground motions, but it is already unforeseen detail in newly constructed structures. Thus, several approaches have been proposed in order to prevent the pounding of structures. By using optimally tuned mass dampers, it is possible to decrease the displacement vibrations of structures. But in adjacent structures, the response of both structures must be considered in the objective function of optimization process. In this paper, two different designs of Tuned Mass Dampers (TMD) are investigated. The first design covers independent TMDs on both structures. In the second design, adjacent structures are coupled by a TMD on the top of the structures. Optimum TMD parameters are found by using the developed optimization methodology employing harmony search algorithm. The proposed method is presented with single degree of freedom and multiple degree of freedom structures. Results show that the coupled design is not effective on multiple degree of freedom adjacent structures. The coupled design is only effective for rigid structures with a single degree of freedom while the use of independent TMDs are effective on both rigid and flexural structures.

Key Words
adjacent structures; structural control; tuned mass damper; optimization; harmony search; pounding

Address
Sinan Melih Nigdeli and Gebrail Bekdaş: Department of Civil Engineering, Istanbul University, 34320 Avc

Abstract
A sophisticated story-wise stiffness identification method for a shear building structure is applied to the case where the shear building is subjected to an actual micro-tremor. While the building responses to earthquake ground motions are necessary in the previous method, it is shown that micro-tremors can be used for identification within the same framework. This enhances the extended usability and practicality of the previously proposed identification method. The difficulty arising in the limit manipulation at zero frequency in the previous method is overcome by introducing an ARX model. The weakness of small SN ratios in the low frequency range is avoided by using the ARX model together with filtering and introducing new constraints on the ARX parameters.

Key Words
system identification; story-wise stiffness identification; ambient vibration data; shear building; ARX model

Address
Ayumi Ikeda, Kohei Fujita and Izuru Takewaki: Department of Architecture and Architectural Engineering, Kyoto University, Kyotodaigaku-Katsura, Nishikyo-ku, Kyoto 615-8540, Japan

Abstract
A damage-based seismic design procedure for steel frame structures is formulated as an optimization problem, in which minimization of the initial construction cost is treated as the objective of the problem. The performance constraint of the design procedure is to achieve \"repairable\" damage state for earthquake demands that are less severe than the design ground motions. The Park–Ang damage index is selected as the seismic damage measure for the quantification of structural damage. The charged system search (CSS) algorithm is employed as the optimization algorithm to search the optimum solutions. To improve the time efficiency of the solution algorithm, two simplifying strategies are adopted: first, SDOF idealization of multi-story building structures capable of estimating the actual seismic response in a very short time; second, fitness approximation decreasing the number of fitness function evaluations. The results from a numerical application of the proposed framework for designing a twelve-story 3D steel frame structure demonstrate its efficiency in solving the present optimization problem.

Key Words
damage-based design methodology; steel frame structures; Park-Ang damage index; charged system search algorithm; equivalent SDOF system; fitness approximation

Address
A. Kaveh, M. Kalateh-Ahani and M. Fahimi-Farzam: Centre of Excellence for Fundamental Studies in Structural Engineering, Iran University of Science and Technology, Narmak, Tehran 16844, Iran

Abstract
This study is aimed to investigate the seismic performance of low-rise precast wall system with base isolation. Three types of High Damping Rubber Bearing (HDRB) were designed to provide effective isolation period of 2.5 s for three different kinds of structure in terms of vertical loading. The real size HDRB was manufactured and tested to obtain the characteristic stiffness as well as damping ratio. In the vertical stiffness test, it was revealed that the HDRB was not an ideal selection to be used in isolating lightweight structure. Time history analysis using 33 real earthquake records classified with respective peak ground acceleration-to-velocity (a/v) ratio was performed for the remaining two types of HDRB with relatively higher vertical loading. HDRB was observed to show significant reduction in terms of base shear and floor acceleration demand in ground excitations having a/v ratio above 0.5g/ms-1, very much lower than the current classification of 0.8g/ms-1. In addition, this study also revealed that increasing the damping ratio of base isolation system did not guarantee better seismic performance particularly in isolation of lightweight structure or when the ground excitation was having lower a/v ratio.

Key Words
high damping rubber bearing; seismic base isolation; precast wall; damping ratio; passive earthquake mitigation

Address
Patrick L.Y. Tiong: Seismology and Earthquake Engineering Division, Doshin Rubber Products (M) Sdn. Bhd., Lot PT 34252, Jalan Sekolah, Rantau Panjang, 42100 Klang, Selangor, Malaysia

Azlan Adnan, Ahmad B.A. Rahman: Department of Structure and Materials, Faculty of Civil Engineering, Universiti Teknologi Malaysia, 81310 Skudai, Johor, Malaysia

Abdul K. Mirasa: Department of Civil Engineering, Faculty of Engineering, Universiti Malaysia Sabah, 88400 Kota Kinabalu, Sabah, Malaysia

Abstract
Damping is one of the parameters that control the performance of structures when they are subjected to seismic, wind, blast or other transient shock and vibration disturbances. By adding supplemental viscous dampers, the energy input from a transient deformation is absorbed, not only by the structure itself, but also by the supplemental dampers. The aim of this study is to evaluate the values of both damping and ductility reduction factors for steel moment resisting frames with supplemental linear viscous dampers. Two-dimensional finite element models have been established for a range of low to mid rise buildings with different parameters: number of floors; number of bays; and number of dampers with different supplemental damping ratios (from 5% to 30%). A parametric study has been performed using time history analyses and a well-documented research method (N2-method). In addition, an equation has been proposed for each reduction factor based on regression analysis for the obtained results. The results of the Time history analyses are compared with those of a modified N2-method. Moreover, a comparison with values specified in the European code EC8 and the Egyptian code ECP-201 has been performed.

Key Words
seismic force reduction factor, steel frame; viscous damper; damping; ductility

Address
M. Hassanien Serror, R. Adel diab and S. Ahmed Mourad: Department of Structural Engineering, Cairo University, Egypt

Abstract
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. This high-performance seismic behavior is inherent in the design of ancient temples with emblematic peristyles that consist of slender, free-standing columns which support freely heavy epistyles together with the even heavier frieze atop. While the ample 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 George Housner who 50 years ago elucidated a size-frequency scale effect that explained the \"counter intuitive\" seismic stability of tall, slender rocking structures. Housner\'s 1963 seminal paper marks the beginning of a series of systematic studies on the dynamic response and stability of rocking structures which gradually led to the development of rocking isolation—an attractive practical alternative for the seismic protection of tall, slender structures. This paper builds upon selected contributions published during this last half-century in an effort to bring forward the major advances together with the unique advantages of rocking isolation. The paper concludes that the concept of rocking isolation by intentionally designing a hinging mechanism that its seismic resistance originates primarily from the mobilization of the rotational inertia of its members is a unique seismic protection strategy for large, slender structures not just at the limit-state but also at the operational state.

Key Words
seismic protection; rocking frame; recentering; moment of inertia; earthquake engineering

Address
Nicos Makris: Division of Structures, Department of Civil Engineering, University of Patras, 26500 Patras, Greece

Abstract
Seismic isolation has been established as an effective earthquake-resistant design method and the lead rubber bearings (LRBs) are among the most commonly used seismic isolation systems. In the scientific literature, a sharp bilinear model is often used for capturing the hysteretic behaviour of the LRBs in the analysis of seismically isolated structures, although the actual behaviour of the LRBs can be more accurately represented utilizing smoothed plasticity, as captured by the Bouc-Wen model. Discrepancies between these two models are quantified in terms of the computed peak relative displacements at the isolation level, as well as the peak inter-storey deflections and the absolute top-floor accelerations, for the case of base-isolated buildings modelled as multi degree-of-freedom systems. Numerical simulations under pulse-like ground motions have been performed to assess the effect of non-linear parameters of the seismic isolation system and characteristics of both the superstructure and the earthquake excitation, on the accuracy of the computed peak structural responses. Through parametric analyses, this paper assesses potential inaccuracies of the computed peak seismic response when the sharp bilinear model is employed for modelling the LRBs instead of the more accurate and smoother Bouc-Wen model.

Key Words
seismic isolation; base-isolation; lead rubber bearings; bilinear model; Bouc-Wen model

Address
seismic isolation; base-isolation; lead rubber bearings; bilinear model; Bouc-Wen model

Abstract
This is paper presents the results of an analytical study aimed at evaluating the effect of narrow-banded mainshock/aftershock seismic sequences on the response of structures built on very soft soil sites. Due to the scarce availability of recorded seismic sequences in accelerographic stations located in the lake-bed of Mexico City, artificial narrow-banded sequences were employed. In the first part of this study, a parametric investigation was carried out to identify the mainshock/aftershock ground motion features that have detrimental effects in the seismic performance of equivalent single-degree-of-freedom systems representative of framed-buildings that house standard and essential facilities. In the second part of this work, the seismic response of two (8- and 18-story) steel-moment resisting frames that house essential facilities is examined. It is concluded that buildings with fundamental periods of vibration longer than the dominant period of the mainshock can experience a significant increment in their inter-story drift demands due to the occurrence of an aftershock.

Key Words
seismic sequences; displacement-based design; soft soils; performance-based design

Address
Gerardo Díaz-Martínez and Amador Terán-GilmoreDepto. de Materiales, Universidad Autónoma Metropolitana-Azcapotzalco, 02200 México D.F., México

Jorge Ruiz-García: Facultad de Ingeniería Civil, Universidad Michoacana de San Nicolás de Hidalgo, Edificio C, Planta Baja,
Cd. Universitaria, 58040 Morelia, México

Abstract
This research has been conducted in order to investigate the effects of peak ground velocity (PGV) of near-field earthquakes on base-isolated structures mounted on Single Friction Pendulum (SFP), Double Concave Friction Pendulum (DCFP) and Triple Concave Friction Pendulum (TCFP) bearings. Seismic responses of base-isolated structures subjected to simplified near field pulses including the forward directivity and the fling step pulses are considered in this study. Behaviour of a two dimensional single story structure mounting on SFP, DCFP and TCFP isolators investigated employing a variety range of isolators and the velocity (PGV) of the forward directivity and the fling step pulses as the main variables of the near field earthquakes. The maximum isolator displacement and base shear are selected as main seismic responses. Peak seismic responses of different isolator types are compared to emphasize the efficiency of each one under near field earthquakes. It is demonstrated that rising the PGVs increases the isolator displacement and base shear of structure. The effects of the forward directivity are greater than the fling step pulses. Furthermore, TCFP isolator is more effective to control the near field effects than the other friction pendulum isolators are. This efficiency is more significant in pulses with longer period and greater PGVs.

Key Words
near field ground motions; simplified pulse models; seismic isolation; friction Isolators

Address
H.Tajammolian: Structural Engineering, Structural Engineering, Faculty of Civil Engineering, Amirkabir University of Technology, Tehran, Iran

F.Khoshnoudian: Faculty of Civil Engineering, Amirkabir University of Technology, Tehran, Iran

S.Talaei and V.Loghman: Earthquake Engineering, Faculty of Civil Engineering, Amirkabir University of Technology, Tehran, Iran

Abstract
Ground motion modification is extensively used in seismic design of civil infrastructure, especially where few or no recorded ground motions representative of the design scenario are available. A site in Los Angeles, California is used as a study site and 28 ground motions consistent with the design earthquake scenario are selected. The suite of 28 ground motions is scaled and modified in the time domain (TD) and frequency domain (FD) before being used as input to a bilinear SDOF system. The median structural responses to the suites of scaled, TD-modified, and FD-modified motions, along with ratios of he modified-to-scaled responses, are investigated for SDOF systems with different periods, strength ratios, and post-yield stiffness ratios. Overall, little difference (less than 20%) is observed in the peak structural accelerations, velocities, and displacements; displacement ductility; and absolute accelerations caused by the TD-modified and FD-modified motions when compared to the responses caused by the scaled motions. The energy absorbed by the system when the modified motions are used as input is more than 20% greater than when scaled motions are used as input. The observed trends in the structural response are predominantly the result of changes in the ground motion characteristics caused by modification.

Key Words
time domain modification; frequency domain modification; ground motion characteristics; bilinear SDOF system; structural seismic response analyses

Address
Clinton P. Carlson, Dimitrios Zekkos and Jason P. McCormick: Department of Civil and Environmental Engineering, University of Michigan, Ann Arbor, Michigan 48109, USA

Abstract
The main objective of critical excitation methods is to reveal the worst possible response of structures. This goal is accomplished by considering the uncertainties of ground motion, which is subjected to the appropriate constraints, such as earthquake power and intensity limit. The concentration of this current study is on the theoretical optimization aspect, as is the case with the majority of conventional critical excitation methods. However, these previous studies on critical excitation lead to a discontinuous power spectral density (PSD). This paper introduces some critical excitations which contain proper continuity in frequency domain. The main idea for generating such continuous excitations stems from the combination of two continuous functions. On the other hand, in order to provide a non-stationary model, this paper attempts to present an appropriate envelope function, which unlike the previous envelope functions, can properly cover the natural earthquakes\'accelerograms based on multi-peak conditions. Finally, the proposed method is developed into the multiple-degree-of-freedom (M.D.O.F) structure

Key Words
random vibrations; continuous critical excitation; envelope functions; power spectral density

Address
S. Hooman Ghasemi: Auburn University, USA

P. Ashtari: Civil Engineering, Zanjan University, Iran


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