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CONTENTS
Volume 9, Number 5, November 2015
 

Abstract
Seismic isolation systems decouple structures from ground motions to protect them from seismic events. Seismic isolation devices have been implemented in many full-scale buildings and bridges because of their simplicity, economic effectiveness, inherent stability, and reliability. It is well known that the most uncertain aspect for obtaining the accurate responses of an isolated structure from seismic events is the seismic loading itself. It is needed to know the seismic response distributions of the isolated structure resulting from the randomness of earthquakes when probabilistic designing or probabilistic evaluating an isolated structure. Earthquake time histories are useful and often an essential element for designing or evaluating isolated structures. However, it is very challenging to gather the design and evaluation information for an isolated structure from many seismic analyses. In order to evaluate the seismic performance of an isolated structure, numerous nonlinear dynamic analyses need to be performed, but this is impractical. In this paper, the concept of the stochastic response database (SRD) is defined to obtain the seismic response distributions of an isolated structure instantaneously, thereby significantly reducing the computational efforts. An equivalent model of the isolated structure is also developed to improve the applicability and practicality of the SRD. The effectiveness of the proposed methodology is numerically verified.

Key Words
isolated structure; seismic response distribution; stochastic response database; equivalent model

Address
Seung-Hyun Eem: Disaster Management HPC Research, Korea Institute of Science and Technology Information,
245 Daehak-ro, Yuseong-gu, Daejeon 305-701, Republic of Korea

Hyung-Jo Jung: Department of Civil and Environmental Engineering, KAIST, 291 Daehak-ro, Yuseong-gu,
Daejeon 305-701, Republic of Korea

Abstract
Earthquakes occur as a cluster in many regions around the world where complex fault systems exist. The repeated shaking usually induces accumulative damage to affected structures. Damage accumulation in structural systems increases their level of degradation in stiffness and also reduces their strength. Many existing analytical tools of modeling RC structures lack the salient damage features that account for stiffness and strength degradation resulting from repeated earthquake loading. Therefore, these tools are inadequate to study the response of structures in regions prone to multiple earthquakes hazard. The objective of this paper is twofold: (a) develop a tool that contains appropriate damage features for the numerical analysis of RC structures subjected to more than one earthquake; and (b) conduct a parametric study that investigates the effects of multiple earthquakes on the response of RC moment resisting frame systems. For this purpose, macroscopic constitutive models of concrete and steel materials that contain the aforementioned damage features and are capable of accurately capturing materials degrading behavior, are selected and implemented into fiber-based finite element software. Furthermore, finite element models that utilize the implemented concrete and steel stress-strain hysteresis are developed. The models are then subjected to selected sets of earthquake sequences. The results presented in this study clearly indicate that the response of degrading structural systems is appreciably influenced by strong-motion sequences in a manner that cannot be predicted from simple analysis. It also confirms that the effects of multiple earthquakes on earthquake safety can be very considerable.

Key Words
RC frames; multiple earthquakes; damage accumulation; stiffness and strength degradation

Address
Adel E. Abdelnaby: Department of Civil Engineering, University of Memphis, 3815 Central Ave, Memphis, TN 38152, USA

Amr S. Elnashai: Department of Civil and Environmental Engineering, University of Illinois at Urbana-Champaign, 205 N, Mathews Ave, Urbana, IL 61801, USA

Abstract
This research evaluates the soil conditions for seismic stations situated in Romania using the horizontal-to-vertical spectral ratio (HVSR). The strong ground motion database assembled for this study consists of 179 analogue and digital strong ground motion recordings from four intermediate-depth Vrancea seismic events with Mw

Key Words
soil class; strong ground motion records; Vrancea subcrustal earthquakes; spectral amplification; fundamental frequency

Address
Florin Pavel and Radu Văcăreanu: Seismic Risk Assessment Research Centre, Technical University of Civil Engineering Bucharest, Bd. Lacul Tei no. 122-124, Sector 2, 020396, Bucharest, Romania

Abstract
Despite the fact that the fundamental period appears to be one of the most critical parameters for the seismic design of structures according to the modal superposition method, the so far available in the literature proposals for its estimation are often conflicting with each other making their use uncertain. Furthermore, the majority of these proposals do not take into account the presence of infills walls into the structure despite the fact that infill walls increase the stiffness and mass of structure leading to significant changes in the fundamental period numerical value. Toward this end, this paper presents a detailed and in-depth analytical investigation on the parameters that affect the fundamental period of reinforce concrete structure. The calculated values of the fundamental period are compared against those obtained from the seismic code and equations proposed by various researchers in the literature. From the analysis of the results it has been found that the number of storeys, the span length, the stiffness of the infill wall panels, the location of the soft storeys and the soil type are crucial parameters that influence the fundamental period of RC buildings.

Key Words
fundamental period; infilled frames; masonry; modal analysis; reinforced concrete buildings

Address
Panagiotis G. Asteris, Athanasios K. Tsaris: Computational Mechanics Laboratory, School of Pedagogical and Technological Education, Heraklion, GR 14121, Athens, Greece

Constantinos C. Repapis: Department of Civil Engineering, Piraeus University of Applied Sciences, 250 Thivon and Petrou Ralli Str., Aigaleo 122 44, Athens, Greece

Fabio Di Trapani and Liborio Cavaleri: Department of Civil, Environmental, Aerospace and Materials Engineering (DICAM), University of Palermo, Viale delle Scienze, 90128, Palermo, Italy

Abstract
To investigate the seismic pounding response of long-span bridges with high-piers under strong ground motions, shaking table tests were performed on a 1/10-scaled bridge model consisting of three continuous spans with rigid frames and one simply-supported span. The seismic pounding responses of this bridge model under different earthquake excitations including the uniform excitation and the traveling wave excitations were experimentally studied. The influence of dampers to the seismic pounding effects at the expansion joints was analyzed through nonlinear dynamic analyses in this research. The seismic pounding effects obtained from numerical analyses of the bridge model are in favorable agreement with the experimental results. Seismic pounding effect of bridge superstructures is dependent on the structural dynamic properties of the adjacent spans and characteristics of ground motions. Moreover, supplemental damping can effectively mitigate pounding effects of the bridge superstructures, and reduce the base shear forces of the bridge piers.

Key Words
seismic pounding; isolation device; traveling wave excitation; shaking table test; nonlinear analysis

Address
Qiang Han, Huihui Dong, Xiuli Du and Yulong Zhou: Key Laboratory of Urban Security and Disaster Engineering of Ministry of Education, Beijing University of Technology, Beijing 100124, China
Beijing Collaborative Innovation Center for Metropolitan Transportation, Beijing 100124, China

Abstract
A performance-based design (PBD) procedure, initially proposed for the seismic design of buildings, is tailored herein to the structural configurations commonly adopted in bridges. It aims at the efficient design of bridges for multiple performance levels (PLs), achieving control over a broad range of design parameters (i.e., strains, deformations, ductility factors) most of which are directly estimated at the design stage using advanced analysis tools (a special type of inelastic dynamic analysis). To evaluate the efficiency of the proposed design methodology, it is applied to an actual bridge that was previously designed using a different PBD method, namely displacement-based design accounting for higher mode effects, thus enabling comparison of the alternative PBD approaches. Assessment of the proposed method using nonlinear dynamic analysis for a set of spectrum-compatible motions, indicate that it results in satisfactory performance of the bridge. Comparison with the displacement-based method reveals significant cost reduction, albeit at the expense of increased computational effort.

Key Words
deformation-based design; direct displacement-based design; nonlinear dynamic analysis; bridges; reinforced concrete

Address
Konstantinos I. Gkatzogias and Andreas J. Kappos: Research Centre for Civil Engineering Structures, Department of Civil Engineering, City University London, London EC1V 0HB, United Kingdom

Abstract
This paper investigates the limits and efficacies of the Fiber Reinforced Polymer (FRP) material for strengthening mid-rise RC buildings against seismic actions. Turkey, the region of the highest seismic risk in Europe, is chosen as the case-study country, the building stock of which consists in its vast majority of mid-rise RC residential and/or commercial buildings. Strengthening with traditional methods is usually applied in most projects, as ordinary construction materials and no specialized workmanship are required. However, in cases of tight time constraints, architectural limitations, durability issues or higher demand for ductile performance, FRP material is often opted for since the most recent Turkish Earthquake Code allows engineers to employ this advanced-technology product to overcome issues of inadequate ductility or shear capacity of existing RC buildings. The paper compares strengthening of a characteristically typical mid-rise Turkish RC building by two methods, i.e., traditional column jacketing and FRP strengthening, evaluating their effectiveness with respect to the requirements of the Turkish Earthquake Code. The effect of FRP confinement is explicitly taken into account in the numerical model, unlike the common procedure followed according to which the demand on un-strengthened members is established and then mere section analyses are employed to meet the additional demands.

Key Words
seismic retrofit; FRP strengthening; RC jacketing; confinement effect; deformation capacity

Address
Eleni Smyrou: Department of Civil Engineering, Istanbul Technical University, Ayazaga Campus, Maslak, 34469, Istanbul, Turkey

Abstract
The adequacy of a number of advanced earthquake Intensity Measures (IMs) to predict the structural damage of earthquake resistant 3D R/C buildings is investigated in the present paper. To achieve this purpose three symmetric in plan and three asymmetric 5-storey R/C buildings are analyzed by nonlinear time history analysis using 74 bidirectional earthquake records. The two horizontal accelerograms of each ground motion are applied along the structural axes of the buildings and the structural damage is expressed in terms of the maximum and average interstorey drift as well as the overall structural damage index. For each individual pair of accelerograms the values of the aforementioned seismic damage measures are determined. Then, they are correlated with several strong motion scalar IMs that take into account both earthquake and structural characteristics. The research identified certain IMs which exhibit strong correlation with the seismic damage measures of the studied buildings. However, the degree of correlation between IMs and the seismic damage depends on the damage measure adopted. Furthermore, it is confirmed that the widely used spectral acceleration at the fundamental period of the structure is a relatively good IM for medium rise R/C buildings that possess small structural eccentricity.

Key Words
structure-specific intensity measures; seismic response; nonlinear time history analysis; scalar ground motion IMs; bidirectional excitation; R/C buildings

Address
Konstantinos G. Kostinakis and Asimina M. Athanatopoulou: Department of Civil Engineering, Aristotle University of Thessaloniki Aristotle University Campus, 54124, Thessaloniki, Greece

Abstract
The study considers earthquake shake table testing of bending-torsion coupled structures under multi-component stationary random earthquake excitations. An experimental procedure to arrive at the optimal excitation cross-power spectral density (psd) functions which maximize/minimize the steady state variance of a chosen response variable is proposed. These optimal functions are shown to be derivable in terms of a set of system frequency response functions which could be measured experimentally without necessitating an idealized mathematical model to be postulated for the structure under study. The relationship between these optimized cross-psd functions to the most favourable/least favourable angle of incidence of seismic waves on the structure is noted. The optimal functions are also shown to be system dependent, mathematically the sharpest, and correspond to neither fully correlated motions nor independent motions. The proposed experimental procedure is demonstrated through shake table studies on two laboratory scale building frame models.

Key Words
random vibration; multi-component earthquake support motion; critical excitation models; shake table testing

Address
S. Ammanagi and C.S. Manohar: Department of Civil Engineering, Indian Institute of Science, Bangalore 560 012, India

S. Ammanagi: Bangalore Integrated System Solutions (P) Ltd., Bangalore 560058, India


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