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CONTENTS
Volume 9, Number 1, July 2015
 

Abstract


Key Words


Address


Abstract
In this paper Fragility Curves (FCs) relevant to existing RC framed building types representative of the Italian building population designed only to vertical load and regular in-plan have been derived from an extensive campaign of non-linear dynamic analyses. In the generation of the FCs, damage states according to the EMS98 scale have been considered while the intensity measure has been defined by adopting an integral parameter, such as the Housner intensity. FCs have been generated by varying different parameters, including building age, number of storeys, presence and position of infill panels, plan dimensions, external beams stiffness and concrete strength. In order to verify the effectiveness of the damage prediction, comparisons were made between the results obtained from the proposed FCs with those deriving from both prominent fragility studies available in the technical literature and damage distributions observed in past earthquakes. Results show that damage grades obtained by adopting the proposed FCs are generally lower than those provided by the other approaches considered. A comparison with real damage data, shows that the proposed FCs generally estimate more severe damage distributions than those observed in past earthquakes, although they give lower differences with respect to the other approaches.

Key Words
existing buildings; reinforced concrete; seismic vulnerability; fragility curves; nonlinear dynamic analyses; Housner intensity

Address
A. Masi, A. Digrisolo and V. Manfredi: School of Engineering, University of Basilicata, viale dell\'Ateneo Lucano, 85100 Potenza, Italy

Abstract
The main purpose of this work is to compare different criteria for the seismic strengthening of RC framed buildings in order to find the optimal combinations of these retrofitting techniques. To this end, a numerical investigation is carried out with reference to the town hall of Spilinga (Italy), an RC framed structure with an L-shaped plan built at the beginning of the 1960s. Five structures are considered, derived from the first by incorporating: carbon fibre reinforced polymer (FRP)-wrapping of all columns; base-isolation, with high-damping-laminated-rubber bearings (HDLRBs); added damping, with hysteretic damped braces (HYDBs); FRP-wrapping of the first storey columns combined with base-isolation or added damping. A three-dimensional fibre model of the primary and retrofitted structures is considered; bilinear and trilinear laws idealize, respectively, the behaviour of the HYDB, providing that the buckling be prevented, and the FRP-wrapping, without resistance in compression, while the response of the HDLRB is simulated by using a viscoelastic linear model. The effectiveness of the retrofitting solutions is tested with nonlinear dynamic analyses based on biaxial accelerograms, whose response spectra match those in the Italian seismic code.

Key Words
seismic retrofit of framed buildings; fibre-reinforced-polymer structure; base-isolated structure; added-damping structure; nonlinear dynamic analysis

Address
Fabio Mazza: Dipartimento di Ingegneria Civile, Università della Calabria, 87036 Rende (Cosenza), Italy

Abstract
The present paper aims at evaluating damage and collapse behavior of low-rise buildings with unidirectional mass irregularities in plan (torsional buildings). In previous earthquake events, such buildings have been exposed to extensive damages and even total collapse in some cases. To investigate the performance and collapse behavior of such buildings from probabilistic points of view, three-dimensional three and six-story reinforced concrete models with unidirectional mass eccentricities ranging from 0% to 30% and designed with modern seismic design code provisions specific to intermediate ductility class were subjected to nonlinear static as well as extensive nonlinear incremental dynamic analysis (IDA) under a set of far-field real ground motions containing 21 two-component records. Performance of each model was then examined by means of calculating conventional seismic design parameters including the response reduction (R), structural overstrength (Ω) and structural ductility (µ) factors, calculation of probability distribution of maximum inter-story drift responses in two orthogonal directions and calculation collapse margin ratio (CMR) as an indicator of performance. Results demonstrate that substantial differences exist between the behavior of regular and irregular buildings in terms of lateral load capacity and collapse margin ratio. Also, results indicate that current seismic design parameters could be non-conservative for buildings with high levels of plan eccentricity and such structures do not meet the target “life safety” performance level based on safety margin against collapse. The adverse effects of plan irregularity on collapse safety of structures are more pronounced as the number of stories increases.

Key Words
torsional buildings; mass irregularities; damage; collapse; performance-based; IDA

Address
Salar Manie: Department of Civil Engineering, Islamic Azad University, Sanandaj Branch, Sanandaj, Iran

Abdoreza S. Moghadam and Mohsen Ghafory-Ashtiany: International Institute of Earthquake Engineering & Seismology (IIEES), Arghavan, North Dibajee St., Farmanieh, Tehran, Iran

Abstract
The effectiveness of tuned mass dampers (TMDs) in reducing the seismic response of civil structures is still a debated issue. The few studies regarding TMDs on inelastic structures indicate that they would perform well under moderate earthquake loading, when the structure remains linear or weakly nonlinear, while tending to fail under severe ground shaking, when the structure experiences strong nonlinearities. TMD seismic efficiency should be therefore rationally assessed by considering to which extent moderate and severe earthquakes respectively contribute to the expected cost of damages and losses over the lifespan of the structure. In this paper, a method for evaluating, in a life-cycle cost (LCC) perspective, the seismic effectiveness of TMDs on inelastic building structures is presented and exemplified on the SAC LA 9-storey steel moment-resisting frame benchmark building. Results show that the LCC concept may provide an appropriate alternative to traditional performance criteria for the evaluation of the effectiveness of TMDs and that TMD installation on typical existing middle-rise buildings in high seismic hazard regions may significantly reduce building lifetime cost despite the poor control performance observed under the most severe seismic events.

Key Words
structural control; tuned mass dampers (TMD); existing structures; nonlinear dynamic analysis; life-cycle cost analysis; cost-effectiveness

Address
Emiliano Matta: Department of Structural, Geotechnical and Building Engineering, Politecnico di Torino, C.so Duca degli Abruzzi 24, Turin, Italy

Abstract
Based on the reconnaissance of buildings in Dujiangyan City during 2008 Wenchuan earthquake, China, structural damage characteristics and the spatial distribution of structural damage are investigated, and the possible reasons for the extraordinary features are discussed with consideration of the influence of urban historical evolution and spatial variation of earthquake motions. Firstly, the urban plan and typical characteristics of structural seismic damage are briefly presented and summarized. Spatial distribution of structural damage is then comparatively analyzed by classifying all surveyed buildings in accordance with different construction age, considering the influence of seismic design code on urban buildings. Finally, the influences of evolution of seismic design code, topographic condition, local site and distance from fault rupture on spatial distribution of structural damage are comprehensively discussed. It is concluded that spatial variation of earthquake motions, resulting from topography, local site effect and fault rupture, are very important factor leading to the extraordinary spatial distribution of building damage except the evolution of seismic design codes. It is necessary that the spatial distribution of earthquake motions should be considered in seismic design of structures located in complicated topography area and near active faults.

Key Words
Wenchuan earthquake; spatial distribution; building damage; near-fault earthquake; topography

Address
Junfeng Jia, Nianhua Song, Zigang Xu: Key Laboratory of Urban Security and Disaster Engineering of Ministry of Education,
Beijing University of Technology, Beijing 100124, China

Junfeng Jia, Nianhua Song, Zigang Xu: Beijing Collaborative Innovation Center for Metropolitan Transportation, Beijing 100124, China

Zizhao He: Department of Civil and Environmental Engineering, University of California, Los Angeles, USA

Yulei Bai: Department of Civil and Environmental Engineering, Hong Kong, Polytechnic University, China

Abstract
In this study, the coupled effects of magnetic field, stress and thermal field on gravity waves propagating in a liquid layer over a solid surface are discussed. Due to change in temperature, initial hydrostatic stress and magnetic field, the gravity-sound Rayleigh waves can propagate in the liquid-solid interface. Dispersion properties of waves are derived by using classical dynamical theory of thermoelasticity. The phase velocity of gravity waves influenced quite remarkably in the presence of initial stress parameter, magneto-thermoelastic coupling parameter in the half space. Numerical solutions are also discussed for gravity-Rayleigh waves. In the absence of temperature, stress and magnetic field, the obtained results are in agreement with classical results.

Key Words
gravity-Rayleigh waves; magnetic field; thermoelasticity; initial stress

Address
Rajneesh Kakar: Faculty of Engineering & Technology, GNA University, Hargobindgarh, Phagwara, 163/1, C-B, Jalandhar-144022, India

Shikha Kakar: Department of Electronics, SBBS University, Padhiana, 163/1, C-B, Jalandhar-144022, India

Abstract
In conventional seismic design, structures are assumed to be fixed at the base. To reduce the impact of earthquake loading, while at the same time providing an economically feasible structure, minor damage is tolerated in the form of controlled plastic hinging at predefined locations in the structure. Uplift is traditionally not permitted because of concerns that it would lead to collapse. However, observations of damage to structures that have been through major earthquakes reveal that partial and temporary uplift of structures can be beneficial in many cases. Allowing a structure to move as a rigid body is in fact one way to limit activated seismic forces that could lead to severe inelastic deformations. To further reduce the induced seismic energy, slip-friction connectors could be installed to act both as hold-downs resisting overturning and as contributors to structural damping. This paper reviews recent research on the concept, with a focus on timber shear walls. A novel approach used to achieve the desired sliding threshold in the slip-friction connectors is described. The wall uplifts when this threshold is reached, thereby imparting ductility to the structure. To resist base shear an innovative shear key was developed. Recent research confirms that the proposed system of timber wall, shear key, and slip-friction connectors, are feasible as a ductile and lowdamage structural solution. Additional numerical studies explore the interaction between vertical load and slip-friction connector strength, and how this influences both the energy dissipation and self-centring capabilities of the rocking structure.

Key Words
damage avoidance; energy dissipation; rocking structure; slip-friction; timber shear walls

Address
Wei Yuen Loo, Pierre Quenneville and Nawawi Chouw: Department of Civil and Environmental Engineering, Faculty of Engineering, The University of Auckland, 20 Symonds St, Auckland, New Zealand

Abstract
Due to earthquakes, many structures suffered extensive damages that were attributed to the torsional effect caused by mass, stiffness or strength eccentricity. Due to this type of asymmetry torsional moments are generated that are imposed by means of additional shear forces developed at the vertical resisting structural elements of the buildings. Although the torsional effect on the response of reinforced concrete buildings was the subject of extensive research over the last decades, a quantitative index measuring the amplification of the shear forces developed at the vertical resisting elements due to lateraltorsional coupling valid for both elastic and elastoplastic response states is still missing. In this study a reliable index capable of assessing the torsional effect is proposed. The performance of the proposed index is evaluated and its correlation with structural response quantities like displacements, interstorey drift, base torque, shear forces and upper diaphragm’s rotation is presented. Torsionally stiff, mass eccentric singlestory and multistory structures, subjected to bidirectional excitation, are considered and nonlinear dynamic analyses are performed using natural records selected for three hazard levels. It was found that the proposed index provides reliable prediction of the magnitude of torsional effect for all test examples considered.

Key Words
torsional effect; mass eccentric; torsionally stiff; center of rigidity; nonlinear dynamic analyses

Address
Chrysanthi G. Stathi, Nikolaos P. Bakas, Nikos D. Lagaros and Manolis Papadrakakis: Institute of Structural Analysis & Antiseismic Research Department of Structural Engineering, School of Civil Engineering, National Technical University, Athens 9, Iroon Polytechniou Str., Zografou Campus GR-157 80, Athens, Greece

Abstract
Site response analysis is an important topic in earthquake engineering. A time-domain numerical method called as one-dimensional (1D) finite element artificial boundary method is proposed to simulate the homogeneous plane elastic wave propagation in a layered half space subjected to the obliquely incident plane body wave. In this method, an exact artificial boundary condition combining the absorbing boundary condition with the inputting boundary condition is developed to model the wave absorption and input effects of the truncated half space under layer system. The spatially two-dimensional (2D) problem consisting of the layer system with the artificial boundary condition is transformed equivalently into a 1D one along the vertical direction according to Snell’s law. The resulting 1D problem is solved by the finite element method with a new explicit time integration algorithm. The 1D finite element artificial boundary method is verified by analyzing two engineering sites in time domain and by comparing with the frequency-domain transfer matrix method with fast Fourier transform.

Key Words
seismic site response analysis; layered half space; oblique incidence; Snell’s law; finite element method; artificial boundary condition

Address
Mi Zhao, Houquan Yin, Xiuli Du and Lingyu Liang: The Key Laboratory of Urban Security and Disaster Engineering of Ministry of Education, Beijing University of Technology, Beijing 100124, China

Jingbo Liu: Department of Civil Engineering, Tsinghua University, Beijing 100084, China

Abstract
A partial (hybrid) seismic isolation scheme for precast girder bridges in the form of a “buffergap- elastomeric bearings” system has been endorsed in the literature as an efficient seismic design system. However, no guides exist to detail an optimal gap size for different configurations. A numerical study is established herein for different scenarios according to Euro code seismic requirements in order to develop guidelines for the selection of optimal buffer-gap arrangements for various design cases. Various schemes are hence designed for ductile and limited ductility behavior of the bridge piers for different seismic demand levels. Seven real ground records are selected to perform incremental dynamic analysis of the bridges up to failure. Bridges with typical short and high piers are studied; and different values of initial gaps at piers are also investigated varying from a zero gap (i.e., fully locked) condition up to an initial gap at piers that is three quarters the gap left at abutments. Among the main conclusions is that the as-built initial gaps at piers (and especially large gap sizes that are ≥1/2 as-built gaps at abutments) do not practically reduce the seismic design demand and do not affect the reserve capacity of the bridge against failure for bridges featuring long piers, especially when these bridges are designed a priori for ductile behavior. To the contrary, the “buffergap- elastomeric bearings” system is more effective for the bridge schemes with short piers having a large difference between the stiffness of the bearings and that of their supporting (much stiffer) squat piers, particularly for designs with limited ductility. Such effectiveness is even amplified for the case of larger initial as-built gap sizes at piers.

Key Words
buffer; gap; elastomeric bearings; seismic response; EC8

Address
Mousa M.N. Farag, Sameh S.F. Mehanny and Mourad M. Bakhoum: Department of Structural Engineering, Faculty of Engineering, Cairo University, Gamaa Road, Giza, Egypt

Abstract
In this study, Homotopy Perturbation Method (HPM) is used to solve the nonlinear oscillators with damping. We have considered two strong nonlinear equations to show the application of the method. The Runge-Kutta’s algorithm is used to obtain the numerical solution for the problems. The method works very well for the whole range of initial amplitudes and does not demand small perturbation and also sufficiently accurate to both linear and nonlinear physics and engineering problems. Finally to show the accuracy of the HPM, the results have been shown graphically and compared with the numerical solution.

Key Words
Homotopy Perturbation Method (HPM); nonlinear vibrations; damping

Address
Mahmoud Bayat, Mahdi Bayat: Department of Civil Engineering, Mashhad Branch, Islamic Azad University, Mashhad, Iran

Iman Pakar: Young Researchers and Elite Club, Mashhad Branch, Islamic Azad University, Mashhad, Iran

Abstract
This paper presents an experimental study to assess the effectiveness of using ferrocement to strengthen deficient beam-column joints. Ferrocement is proposed to protect the joint region through replacing concrete cover. Six exterior beam-column joints, including two control specimens and four strengthened specimens, are prepared and tested under constant axial load and quasi-static cyclic loading. Two levels of axial load on column (0.2fc’Ag and 0.4fc’Ag) and two types of skeletal reinforcements in ferrocement (grid reinforcements and diagonal reinforcements) are considered as test variables. Experimental results have indicated that ferrocement as a composite material can enhance the seismic performance of deficient beam-column joints in terms of peak horizontal load, energy dissipation, stiffness and joint shear strength. Shear distortions within the joints are significantly reduced for the strengthened specimens. High axial load (0.4fc’Ag) has a detrimental effect on peak horizontal load for both control and ferrocement-strengthened specimens. Specimens strengthened by ferrocement with two types of skeletal reinforcements perform similarly. Finally, a method is proposed to predict shear strength of beam-column joints strengthened by ferrocement.

Key Words
reinforced concrete; beam-column joints; strengthening; ferrocement; composite; cyclic behavior

Address
Bo Li, Eddie Siu-shu Lam: Department. of Civil & Environmental Engineering, The Hong Kong Polytechnic University, Hong Kong

Bo Wu: School of Architectural and Civil Engineering, South China University of Technology, Guangzhou, China

Ya-yong Wang: Institute of Earthquake Engineering, China Academy of Building Research, Beijing, China

Abstract
The primary objective of this study is to summarize results from previous experimental tests on laboratory specimens of RC/steel frames with masonry infills, in order to develop fragility functions that permit the estimation of damage in typical non-structural components of RC frame buildings, as a function of attained peak interstory drift. The secondary objective is to derive loss functions for such non-structural components, which provide information on the probability of experiencing a certain level of monetary loss when a given damage state is attained. Fragility curves and loss function developed in this study can be directly used within the FEMA P-58 framework for the seismic performance assessment of RC frame buildings with masonry infills.

Key Words
masonry infills; damage states; fragility functions; repair costs; loss functions; FEMA P-58

Address
Donatello Cardone and Giuseppe Perrone: School of Engineering, University of Basilicata, Viale Ateneo Lucano 10, 85100 Potenza, Italy


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