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CONTENTS
Volume 15, Number 5, November 2018
 

Abstract
Damages to buildings affected by a near-fault strong ground motion are largely attributed to the vertical component of the earthquake resulting in column failures, which could lead to disproportionate building catastrophic collapse in a progressive fashion. Recently, considerable interests are awakening to study effects of earthquake vertical components on structural responses. In this study, detailed modeling and time-history analyses of a 12-story code-conforming reinforced concrete moment frame building carrying the gravity loads, and exposed to once only the horizontal component of, and second time simultaneously the horizontal and vertical components of an ensemble of far-field and near-field earthquakes are conducted. Structural responses inclusive of tension, compression and its fluctuations in columns, the ratio of shear demand to capacity in columns and peak mid-span moment demand in beams are compared with and without the presence of the vertical component of earthquake records. The influences of the existence of earthquake vertical component in both exterior and interior spans are separately studied. Thereafter, the correlation between the increase of demands induced by the vertical component of the earthquake and the ratio of a set of earthquake record characteristic parameters is investigated. It is shown that uplift initiation and the magnitude of tensile forces developed in corner columns are relatively more critical. Presence of vertical component of earthquake leads to a drop in minimum compressive force and initiation of tension in columns. The magnitude of this reduction in the most critical case is recorded on average 84% under near-fault ground motions. Besides, the presence of earthquake vertical components increases the shear capacity required in columns, which is at most 31%. In the best case, a direct correlation of 95% between the increase of the maximum compressive force and the ratio of vertical to horizontal „effective peak acceleration (EPA)‟ is observed.

Key Words
earthquake vertical component; reinforced concrete; moment resisting frames; axial column force; shear capacity; near-fault earthquake; statistical correlation

Address
Mokhtar Ansari: Department of Civil Engineering, Bozorgmehr University of Qaenat, Qaen, Iran
Masoud Ansari: Department of Civil Engineering, Semnan University, Semnan, Iran
Amir Safiey: Glenn Department of Civil Engineering, Clemson University, Clemson, SC, USA

Abstract
The virtual forces of the original local flexibility method are restricted to inducing stress on the local parts of a structure. To circumvent this restriction, we developed a pseudo local flexibility (PLFM) method that can successfully detect damage to hyperstatic beam structures using fewer modes. For this study, we further developed the PLFM so that it could detect damage in plate structures. We also devised the theoretical background for the PLFM with non-local virtual forces for plate structures, and both the lateral and rotary degree of freedom (DOF) measurements were considered separately. This study investigates the effects of the number of modes, the actual location that sustained damage, multiple damage locations, and noise in modal parameters for the damage detection results obtained from damaged numerical plates. The results revealed that the PLFM can be used for damage detection, localization, and quantification for plate structures, regardless of the use of the lateral DOF and/or rotary DOF.

Key Words
pseudo local flexibility method; rotary DOF; long gauge; plate; damage detection

Address
Ting Yu Hsu and Chao Lun Liu: Department of Civil and Construction Engineering, National Taiwan University of Science and Technology, Taipei, Taiwan

Abstract
The buildings of architectural and cultural heritage are mostly built with stone or brick wall elements, which are connected using limestone or limestone cement mortar, without a full knowledge of the mechanical properties of masonry structures. The compatibility of heritage masonry buildings with valid technical specifications and the rules for earthquake resistance implies the need for construction work such as repairs, strengthening or reconstruction. By strengthening the masonry buildings, ductility and bearing capacity are increased to a level, which, in the case of the earthquake design, allows for some damage to happen, however the structure retains sufficient usability and bearing capacity without the possibility of collapse. Comparison between traditional and modern techniques for seismic strengthening of masonry buildings is given according to their effects, benefits and disadvantages. Recent Croatian provisions provided for heritage buildings enabling deviation of technical specifications are discussed.

Key Words
technical provisions for buildings; masonry buildings of architectural heritage; strengthening techniques

Address
Marijana Hadzima-Nyarko, Gordana Pavic and Tanja Kalman Sipos: Faculty of Civil Engineering and Architecture Osijek, University of J.J. Strossmayer, 31000 Osijek, Vladimira Preloga 3, Croatia
Naida Ademovic: Faculty of Civil Engineering in Sarajevo, University of Sarajevo, Patriotske lige 30, Sarajevo, Bosnia and Herzegovina

Abstract
Buckling-restrained braces are passive control devices with high level of energy dissipation ability. However, they suffer from low post-yield stiffness which makes them vulnerable to severe ground motions, especially near-field earthquakes. Among the several methods proposed to improve resistance of BRB frames, mega-brace configuration can be a solution to increase frame lateral strength and stiffness and improve distribution of forces to prevent large displacement in braces. Due to the limited number of research regarding the performance of such systems, the current paper aims to assess seismic performance of BRB frames with mega-bracing arrangement under near-field earthquakes via a detailed probabilistic framework. For this purpose, a group of multi-story mega-BRB frames were modelled by OpenSEES software platform. In the first part of the paper, simplified procedures including nonlinear pushover and Incremental Dynamic Analysis were conducted for performance evaluation. Two groups of near-fault seismic ground motions (Non-pulse and Pulse-like records) were considered for analyses to take into account the effects of record-to-record uncertainties, as well as forward directivity on the results. In the second part, seismic reliability analyses are conducted in the context of performance based earthquake engineering. Two widely-known EDPbased and IM-based probabilistic frameworks are employed to estimate collapse potential of the structures. Results show that all the structures can successfully tolerate near-field earthquakes with a high level of confidence level. Therefore, mega-bracing configuration can be an effective alternative to conventional BRB bracing to withstand near-field earthquakes.

Key Words
buckling restrained braces; mega-bracing; probabilistic assessment; near-fault records; fragility assessment; pushover analysis

Address
Sajad Veismoradi: School of Civil Engineering, Iran University of Science & Technology, Tehran, Iran
Ehsan Darvishan: Young Researchers and Elites Club, Roudehen Branch, Islamic Azad University, Roudehen, Iran

Abstract
This paper aims to investigate the seismic performance of the prestressed steel strips retrofitted RC beam-column joints. Two series of joint specimens were conducted under compression load and reversed cyclic loading through quasi-static tests. Based on the test results, the seismic behavior of the strengthened joints specimens in terms of the failure modes, hysteresis response, bearing capacity, ductility, stiffness degradation, energy dissipation performance and damage level were focused. Moreover, the effects of the amount of the prestressed steel strips and the axial compression ratio on seismic performance of retrofitted specimens were analyzed. It was shown that the prestressed steel strips retrofitting method could significantly improve the seismic behavior of the RC joint because of the large confinement provided by prestressed steel strips in beamcolumn joints. The decrease of the spacing and the increase of the layer number of the prestressed steel strips could result in a better seismic performance of the retrofitted joint specimens. Moreover, increasing the axial compression ration could enhance the peak load, stiffness and the energy performance of the joint specimens. Furthermore, by comparison with the specimens reinforced with CFRP sheets, the specimens reinforced with prestressed steel strips was slightly better in seismic performance and cost-saving in material and labor. Therefore, this prestressed steel strips retrofitting method is quite helpful to enhance the seismic behavior of the RC beam-column joints with reducing the cost and engineering time.

Key Words
reinforced concrete structure; beam-column joint; retrofitted methods; prestressed steel strips; experimental study; seismic performance

Address
Yong Yang, Yang Chen, Zhan Chen, Niannian Wang and Yunlong Yu: School of Civil Engineering, Xi

Abstract
A typical viable technique to decrease the seismic response of liquid storage tanks is to isolate them at the base. Base-isolation systems are an efficient and feasible solution to reduce the vulnerability of structures in high seismic risk zones. Nevertheless, when liquid storage tanks are under long-period shaking, the base-isolation systems could have different impacts. These kinds of earthquakes can damage the tanks readily. Hence, the seismic behaviour and vibration of cylindrical liquid storage tanks, subjected to earthquakes, is of paramount importance, and it is investigated in this paper. The Finite Element Method is used to evaluate seismic response in addition to the reduction of excessive liquid sloshing in the tank when subjected to the long-period ground motion. The non-linear stress-strain behaviour pertaining to polymers and rubbers is implemented while non-linear contact elements are employed to describe the 3-D surface-to-surface contact. Therefore, Nonlinear Procedures are used to investigate the fluid-structure interactions (FSI) between liquid and the tank wall while there is incompressible liquid. Part I, examines the effect of the flexibility of the isolation system and the tank aspect ratio (height to radius) on the tank wall radial displacements of the tank wall and the liquid sloshing heights. Maximum stress and base shear force for various aspect ratios and different base-isolators, which are subjected to three seismic conditions, will be discussed in Part II. It is shown that the composite-base isolator is much more effective than other isolators due to its high flexibility and strength combined. Moreover, the base isolators may decrease the maximum level pertaining to radial displacement.

Key Words
base isolation; liquid storage tank; earthquake; composite-elastomeric; seismic response

Address
A. Shahrjerdi: Department of Mechanical Engineering, Malayer University, Malayer, Iran
M. Bayat: Department of Civil Engineering, Aalborg University, Denmark

Abstract
The influences of initial damage paths and aftershock (AS) spectral shape on the assessment of AS collapse fragility are investigated. To do this, a four-story ductile reinforced concrete (RC) frame structure is employed as the study case. The farfield earthquake records recommended by FEMA P695 are used as AS ground motions. The AS incremental dynamic analyses are performed for the damaged structure. To examine the effect of initial damage paths, a total of six kinds of initial damage paths are adopted to simulate different initial damage states of the structure by pushover analysis and dynamic analysis. For the pushover-based initial damage paths, the structure is \"pushed\" using either uniform or triangle lateral load pattern to a specified damage state quantified by the maximum inter-story drift ratio. Among the dynamic initial damage paths, one single mainshock ground motion or a suite of mainshock ground motions are used in the incremental dynamic analyses to generate a specified initial damage state to the structure. The results show that the structure collapse capacity is reduced as the increase of initial damage, and the initial damage paths show a significant effect on the calculated collapse capacities of the damaged structure (especially at severe damage states). To account for the effect of AS spectral shape, the AS collapse fragility can be adjusted at different target values of

Key Words
aftershock; mainshock; collapse fragility; damage path; spectral shape; RC frame structures

Address
Yang Liu: College of Architecture and Environment, Sichuan University, Chengdu 610064, P.R. China; School of Civil Engineering, Harbin Institute of Technology, Harbin 150090, P.R. China
Xiao-Hui Yu: Key Lab of Structures Dynamic Behavior and Control of the Ministry of Education, Harbin Institute of Technology, Harbin 150090, P.R. China; Key Lab of Smart Prevention and Mitigation of Civil Engineering Disasters of the Ministry of Industry and Information Technology,
Harbin Institute of Technology, Harbin 150090, P.R. China;
School of Civil Engineering, Harbin Institute of Technology, Harbin 150090, P.R. China
Da-Gang Lu: Key Lab of Structures Dynamic Behavior and Control of the Ministry of Education, Harbin Institute of Technology, Harbin 150090, P.R. China; Key Lab of Smart Prevention and Mitigation of Civil Engineering Disasters of the Ministry of Industry and Information Technology,
Harbin Institute of Technology, Harbin 150090, P.R. China;
School of Civil Engineering, Harbin Institute of Technology, Harbin 150090, P.R. China
Fu-Zi Ma: School of Civil Engineering, Harbin Institute of Technology, Harbin 150090, P.R. China

Abstract
In this paper a limit analysis based procedure for the rapid evaluation of the in-plane seismic capacity of masonry arch bridges is carried out. Attention has been paid to the effect of the backfill on the collapse load. A parametric investigation has been performed by varying the rise/span ratio and the results have been compared with those obtained by finite element modelling. The comparison highlights the conservative feature of the proposed model in terms of ultimate loads and a good agreement in terms of collapse mechanisms.

Key Words
masonry bridges; limit analysis; seismic capacity; backfill

Address
Marco Breccolotti, Laura Severini, Nicola Cavalagli, Federico M. Bonfigli and Vittorio Gusella: Department of Civil and Environmental Engineering, University of Perugia, via Goffredo Duranti 93, 06125, Perugia, Italy

Abstract
Allowing structures to uplift in earthquakes can significantly reduce or even avoid the development of plastic hinges within the structure. The permanent deformations in the structure can thus be minimized. However, uplift of footings can cause additional horizontal movements of a structure. With an increase in movement relative to adjacent structures, the probability of pounding between structures increases. This experimental study reveals that the footing mass can be used to control the vertical displacement of footing and thus reduce the horizontal displacements of an upliftable structure. A four storey model structure with plastic hinges and uplift capability was considered. Shake table tests using ten different earthquake records were conducted. Three different footing masses were considered. It is found that the amplitude of footing uplift can be greatly reduced by increasing the mass of the footing. As a result, allowing structural uplift does not necessary increase the horizontal displacement of the structure. The results show that with increasing footing weight, the interaction between structural and footing response can increase the contribution of the higher modes to the structural response. Consequently, the induced vibrations on secondary structure increase.

Key Words
structural uplift; scaling; dimensional analysis; low-damage seismic design; induced vibration

Address
X. Qin and N. Chouw: Department of Civil and Environmental Engineering. The University of Auckland, Auckland Mail Centre, Private Bag 92019, Auckland 1142, New Zealand

Abstract
This paper proposes a new analytical formulation for computing the seismic input at various levels of a structure in terms of floor response spectra. The approach, which neglects the dynamic interaction between primary structure and secondary element, is particularly useful for the seismic assessment of secondary and non-structural elements. The proposed formulation has a robust theoretical basis and it is based on few meaningful dynamic parameters of the main building. The method has been validated in the linear and nonlinear behavior of the main building through results coming from both experimental tests (available in literature) and parametric numerical analyses. The conditions, for which the Floor Spectrum Approach and its simplified assumptions are valid, have been derived in terms of specific interval ratios between the mass of the secondary element and the participant mass of the main structure. Finally, a practice-oriented formulation has been derived, which could be easily implementable also at code level.

Key Words
floor response spectra; nonlinear behavior; dynamic filtering effect; seismic analysis; secondary and nonstructural elements

Address
Stefania Degli Abbati, Serena Cattari and Sergio Lagomarsino: Department of Civil, Chemical and Environmental Engineering, University of Genoa, Via Montallegro 1, 16145, Genoa, Italy


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