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CONTENTS
Volume 9, Number 3, September 2015
 

Abstract
In the last decade, displacement-based (DB) methods have become established design procedures for reinforced concrete (RC) structures. They use strain and displacement measures as seismic performance control parameters. As for other simplified seismic design methods, it is of great interest to prove if they are usually conservative in respect to more refined, nonlinear, time history analyses, and can estimate design parameters with acceptable accuracy. In this paper, the current Direct Displacement-Based Design (DDBD) procedure is evaluated for designing simple single degree of freedom (SDOF) systems with specific reference to simply supported RC bridge piers. Using different formulations proposed in literature for the equivalent viscous damping and spectrum reduction factor, a parametric study is carried out on a comprehensive set of SDOF systems, and an average error chart of the method is derived allowing prediction of the expected error for an ample range of design cases. Following the chart, it can be observed that, for the design of actual RC bridge piers, underestimation errors of the DDBD method are very low, while the overestimation range of the simplified displacement-based procedure is strongly dependent on design ductility.

Key Words
Performance-Based Design; Displacement-Based Design accuracy; multi-span simply supported bridge RC piers; SDOF systems; equivalent viscous damping

Address
Giovanni Tecchio, Marco Dona and Claudio Modena: Department of Civil, Architectural and Environmental Engineering (DICEA), University of Padua, via Marzolo 9, 35131 Padua, Italy

Abstract
Current design codes and technical recommendations often provide rough indications on how to assess effective stiffness of Reinforced Concrete (R/C) frames subjected to seismic loads, which is a key factor when a linear analysis is performed. The Italian design code (NTC-2008), Eurocode 8 and ACI 318 do not take into account all the structural parameters affecting the effective stiffness and this may not be on the safe side when second-order P-Δ effects may occur. This paper presents a study on the factors influencing the effective stiffness of R/C beams, columns and walls under seismic forces. Five different approaches are adopted and analyzed in order to evaluate the effective stiffness of R/C members, in accordance with the scientific literature and the international design codes. Furthermore, the paper discusses the outcomes of a parametric analysis performed on an actual R/C building and analyses the main variables, namely reinforcement ratio, axial load ratio, concrete compressive strength, and type of shallow beams. The second-order effects are investigated and the resulting displacements related to the Damage Limit State (DLS) under seismic loads are discussed. Although the effective stiffness increases with steel ratio, the analytical results show that the limit of 50% of the initial stiffness turns out to be the upper bound for small values of axial-load ratio, rather than a lower bound as indicated by both Italian NTC-2008 and EC8. As a result, in some cases the current Italian and European provisions tend to underestimate second-order P-Δ effects, when the DLS is investigated under seismic loading.

Key Words
effective stiffness (in R/C); R/C frames; columns; beams; seismic design

Address
Francesco Micelli, Leandro Candido, Marianovella Leone
and Maria Antonietta Aiello: Department of Innovation Engineering, University of Salento, via per Monteroni - 73100 Lecce, Italy

Abstract
A new constitutive model for the representation of the seismic behaviour of steel bars including hardening phenomena is presented. The model takes into account relative slip between bars and concrete, necessary for the estimation of the structural behaviour of r.c. elements and of the level of strain induced by earthquakes on bars. The present work provides the analytical formulation of the post-yielding behaviour of reinforcements, resulting in a continuous axial stress-slip relationship to be implemented in engineering software. The efficacy of the model is proved through the application to a cantilever column, for whose bars the constitutive law is derived.

Key Words
relative slip; hardening effects; cyclic behavior; stress-slip relationship; reinforcing bars

Address
Franco Braga,Rosario Gigliotti: Department of Structural Engineering and Geotechnics, University of Rome

Abstract
This paper aims at developing the knowledge on the seismic behavior of dowel beam-to-column connections, typically employed in precast buildings in Europe. Despite the large diffusion of the industrial buildings, a high seismic vulnerability was exhibited by these structures, mostly due to the connection systems deficiencies, during some recent earthquakes (Emilia 2012, Turkey 2011). An experimental campaign was conducted on a typical dowel connection between an external column and a roof beam. In this paper, the performed cyclic shear test is described. According to the experimental results, the seismic response of the system is evaluated in terms of strength, stiffness and failure mechanism. Moreover, the complete damage pattern of the test is described by means of the instrumentations records. The connection failure occurred due to the concrete cover failure in the column (splitting failure). Such a mechanism corresponds to a negligible energy dissipation capacity of the connection, compared to the overall seismic response of the structure. The experimental results are also compared with the results of a similar monotonic shear test, as well as with some literature relationships for predicting the strength of dowel connections under horizontal (seismic) loads.

Key Words
dowel connection; precast buildings; experimental test; cyclic behavior; seismic response

Address
Gennaro Magliulo, Marianna Ercolino, Maddalena Cimmino, Gaetano Manfredi: Department of Structures for Engineering and Architecture, University of Naples Federico II, Via Claudio 21, 80125 Naples, Italy

Vittorio Capozzi: Enel Green Power, Rio de Janeiro, Brazil

Abstract
Over the past few decades, seismic retrofitting of structural systems has been significantly improved by the adoption of various methods such as FRP composite wraps, base isolation systems, and passive/active damper control systems. In parallel with this trend, probabilistic risk assessment (PRA) for structural and nonstructural components has become necessary for risk mitigation and the achievement of reliable designs in performance-based earthquake engineering. The primary objective of the present study was to evaluate the effect on piping fragility at T-joints due to seismic retrofitting of structural systems with passive energy-dissipation devices (i.e., linear viscous dampers). Three mid-rise building types were considered: without any seismic retrofitting; with distributed damper systems; with optimal placement of dampers. The results showed that the probability of piping system failure was considerably reduced in a Multi Degree of Freedom (MDOF) building retrofitted with optimal passive damper systems at lower floor levels. This effect of damper systems on piping fragility became insignificant as the floor level increased.

Key Words
passive dampers; piping; fragility; SSSA; earthquake

Address
Woo Young Jung: Department of Civil Engineering, GangNeung-WonJu National University, GangNeung, Korea

Bu Seog Ju: Department of Civil Engineering, North Carolina State University, Raleigh, USA

Abstract
The thermomagnetic effect on plane wave propagation at the liquid-solid interface with nonclassical thermoelasticity is investigated. It is assumed that liquid-solid half-space is under initial stress. Numerical computations are performed for the developed amplitude ratios of P, SV and thermal waves under Cattaneo-Lord-Shulman theory, Green-Lindsay theory and classical thermoelasticity. The system of developed equations is solved by the application of the MATLAB software at different angles of incidence for Green and Lindsay model. The effect of initial stress and magnetic field in the lower half-space are discussed and comparison is made in LS, GL and CT models of thermoelasticity. In the absence of magnetic field, the obtained results are in agreement with the same results obtained by the relevant authors. This study would be useful for magneto-thermoelastic acoustic device field.

Key Words
initial stress; reflection; refraction; relaxation time; temperature; magnetic field

Address
Rajneesh Kakar: Chotti Baradari, 163/1, Jalandhar, India

Shikha Kakar: Department of Electronics, SBBSIET, Padhiana, India

Abstract
The scale and nature of the recent earthquakes in the world and the related earthquake disaster index coerce the concerned community to become anxious about it. Therefore, it is crucial that seismic lateral load effect will be appropriately considered in structural design. Application of seismic isolation system stands as a consistent alternative against this hazard. The objective of the study is to evaluate the structural and economic feasibility of reinforced concrete (RC) buildings with base isolation located in medium risk seismic region. Linear and nonlinear dynamic analyses as well as linear static analysis under site-specific bi-directional seismic excitation have been carried out for both fixed based (FB) and base isolated (BI) buildings in the present study. The superstructure and base of buildings are modeled in a 3D finite element model by consistent mass approach having six degrees of freedom at each node. The floor slabs are simulated as rigid diaphragms. Lead rubber bearing (LRB) and High damping rubber bearing (HDRB) are used as isolation device. Change of structural behaviors and savings in construction costing are evaluated. The study shows that for low to medium rise buildings, isolators can reduce muscular amount of base shears, base moments and floor accelerations for building at soft to medium stiff soil. Allowable higher horizontal displacement induces structural flexibility. Though incorporating isolator increases the outlay, overall structural cost may be reduced. The application of base isolation system confirms a potential to be used as a viable solution in economic building design.

Key Words
aseismic building; base isolation; nonlinear dynamics; linear dynamic analysis; seismic excitation; structural insinuation; economic insin

Address
A. B. M. Saiful Islam, Mohd Zamin Jumaat, Rasel Ahmmad and Kh. Mahfuz ud Darain: Department of Civil Engineering, University of Malaya, Kuala Lumpur, Malaysia

Kh. Mahfuz ud Darain: Architecture discipline, Khulna University, Khulna, Bangladesh

Abstract
It is usual in design and assessment of structures to isolate the effects of vertical and horizontal excitations by ignoring their coupling effects. In this situation, total structural response is obtained by employing the well-known combination rules whereby independent assumed response components of earthquakes are combined. In fact, the effects of the simultaneity of the ground motion components are ignored. In this paper, the effect of vertical excitation on horizontal response of structures, the coupling of vertical and horizontal responses, has been evaluated. A computer program is prepared to perform nonlinear dynamic analysis based on the derived governing equations of coupled motions. In the case of simultaneous excitation the results show significant increases in spectral displacement in some periods of vibration in comparison to only horizontally excited systems. Moreover, whenever ratio of the vertical peak ground acceleration to horizontal one become larger, the significant increase in horizontal spectral displacements are observed.

Key Words
simultaneous excitation; coupled horizontal and vertical components; vertical earthquake

Address
Hosein Ghaffarzadeh and Ali Nazeri: Department of Civil Engineering, University of Tabriz, Tabriz, Iran

Abstract
Seismic isolation devices are commonly used to mitigate damages caused by seismic responses of structures. More damages are created due to progressive collapse in structures. Therefore, evaluating the impact of the isolation systems to enhance progressive collapse-resisting capacity is very important. In this study, the effect of lead rubber bearing isolation system to increase the resistance of structures against progressive collapse was evaluated. Concrete moment resisting frames were used in both the fixed and baseisolated model structures. Then, progressive collapse-resisting capacity of frames was investigated using the push down nonlinear static analysis under gravity loads that specified in GSA guideline. Nonlinear dynamic analysis was performed to consider dynamic effects column removal under earthquake. The results of the push down analysis are highly dependent on location of removal column and floor number of buildings. Also, seismic isolation system does not play an effective role in increasing the progressive collapse-resisting capacities of structures under gravity loads. Base isolation helps to localize failures and prevented from spreading it to intact span under seismic loads.

Key Words
base isolation; seismic loads; progressive collapse; column removal; push down analysis; nonlinear time history analysis

Address
Hamid R. Tavakoli, Fahime Naghavi and Ali R. Goltabar: Department of Earthquake Engineering, Babol University and Technology, Babol, Iran

Abstract
The level of ductility is determined by depending on the intended use of the building, the region

Key Words
seismic isolation; isolators; dynamic analysis in the time do

Address
Mohammad Manzoor Nasery, Mustafa Ergun, Sevket Ates and Metin Husem: Karadeniz Technical University, Department of Civil Engineering, 61080, Trabzon, Turkey

Abstract
This paper investigates inelastic seismic demands of the normal component of near-fault pulselike ground motions, which differ considerably from those of far-fault ground motions and also parallel component of near-fault ones. The results are utilized to improve the nonlinear static procedure (NSP) called Displacement Coefficient Method (DCM). 96 near-fault and 20 far-fault ground motions and the responses of various single degree of freedom (SDOF) systems constitute the dataset. Nonlinear Dynamic Analysis (NDA) is utilized as the benchmark for comparison with nonlinear static analysis results. Considerable influences of different faulting mechanisms are observed on inelastic seismic demands. The demands are functions of the strength ratio and also the pulse period to structural period ratio. Simple mathematical expressions are developed to consider the effects of near-fault motion and fault type on nonlinear responses. Modifications are presented for the DCM by introducing a near-fault modification factor, CN. In locations, where the fault type is known, the modifications proposed in this paper help to obtain a more precise estimate of seismic demands in structures.

Key Words
forward directivity; pulse-like ground motion; near-fault earthquakes; nonlinear static procedure; displacement coefficient method; inelastic displacement ra

Address
A. Esfahanian and A.A. Aghakouchak: Faculty of Civil and Environmental Engineering, Tarbiat Modares University, P.O. Box 11365-9313, Tehran, Iran


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