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CONTENTS
Volume 12, Number 4, April 2017
 

Abstract
In this paper a simple approach is presented for spectral matching of ground motions utilizing the wavelet transform and a recently developed metaheuristic optimization technique. For this purpose, wavelet transform is used to decompose the original ground motions to several levels, where each level covers a special range of frequency, and then each level is multiplied by a variable. Subsequently, the vibrating particles system (VPS) algorithm is employed to calculate the variables such that the error between the response and target spectra is minimized. The application of the proposed method is illustrated through modifying 12 sets of ground motions. The results achieved by this method demonstrate its capability in solving the problem. The outcomes of the VPS algorithm are compared to those of the standard colliding bodies optimization (CBO) to illustrate the importance of the enhancement of the algorithm.

Key Words
spectrum-compatible ground motions; wavelet transform; response spectrum; vibrating particles system algorithm

Address
Centre of Excellence for Fundamental Studies in Structural Engineering, Iran University of Science and Technology,
Narmak, Tehran, P.O. Box 16846-13114, Iran


Abstract
To estimate the structural seismic demand, some methods are based on an equivalent linear system such as the Capacity Spectrum Method, the N2 method and the Equivalent Linearization method. Another category, widely investigated, is based on displacement correction such as the Displacement Coefficient Method and the Coefficient Method. Its basic concept consists in converting the elastic linear displacement of an equivalent Single Degree of Freedom system (SDOF) into a corresponding inelastic displacement. It relies on adequate modifying or reduction coefficient such as the inelastic deformation ratio which is usually developed for systems with known ductility factors (C_µ) and (C_R) for known yield-strength reduction factor. The present paper proposes a rational approach which estimates this inelastic deformation ratio for SDOF bilinear systems by rigorous nonlinear analysis. It proposes a new inelastic deformation ratio which unifies and combines both C_µ and C_R effects. It is defined by the ratio between the inelastic and elastic maximum lateral displacement demands. Three options are investigated in order to express the inelastic response spectra in terms of: ductility demand, yield strength reduction factor, and inelastic deformation ratio which depends on the period, the post-to-preyield stiffness ratio, the yield strength and the peak ground acceleration. This new inelastic deformation ratio (C_n) is describes the response spectra and is related to the capacity curve (pushover curve): normalized yield strength coefficient (n), post-to-preyield stiffness ratio (α), natural period (T), peak ductility factor (µ), and the yield strength reduction factor (R_y). For illustrative purposes, instantaneous ductility demand and yield strength reduction factor for a SDOF system subject to various recorded motions (El-Centro 1940 (N/S), Boumerdes: Algeria 2003). The method accuracy is investigated and compared to classical formulations, for various hysteretic models and values of the normalized yield strength coefficient (n), post-to-preyield stiffness ratio (α), and natural period (T). Though the ductility demand and yield strength reduction factor differ greatly for some given T and n ranges, they remain take close when n>1, whereas they are equal to 1 for periods T>1s.

Key Words
deformation ratio; yield strength; reduction factors; ductility; inelastic spectra; Pushover; normalized yield strength coefficient; seismic design

Address
Benazouz Chikh, Nacer Laouami, Youcef Mehani, Abderrahmane Kibboua: National Earthquake Engineering Research Center, CGS, Rue Kaddour Rahim, BP 252 Hussein-Dey, Algiers, Algeria

Ahmed Mebarki: University Paris-Est, Laboratoire Modelisation et Simulation Multi Echelle (MSME), UMR 8208 CNRS, 5 Bd Descartes, 77454 Marne-La-Vallee, France

Moussa Leblouba: Department of Civil & Environmental Engineering, College of Engineering, University of Sharjah, P.O. Box 27272 Sharjah, United Arab Emirates

Mohamed Hadid: National School of Built and Ground Works Engineering (ENSTP), 01 Rue SidiGaridi, Vieux Kouba 16003, Algiers, Algeria

Djilali Benouar: University of Sciences& Technology Houari Boumediene (USTHB), Faculty of Civil Engineering, BP 32, 16111 El-Alia / Bab Ezzouar, Algiers, Algeria

Abstract
This paper proposes a quick and simplified method to describe masonry vaults in global seismic analyses of buildings. An equivalent macro-element constituted by a set of six trusses, two for each transverse, longitudinal and diagonal direction, is introduced. The equivalent trusses, whose stiffness is calculated by fully modeled vaults of different geometry, mechanical properties and boundary conditions, simulate the vault in both global analysis and local analysis, such as kinematic or rocking approaches. A parametric study was carried out to investigate the influence of geometrical characteristics and mechanical features on the equivalent stiffness values. The method was numerically validated by performing modal and transient analysis on a three naves-church in the elastic range. Vibration modes and displacement time-histories were compared showing satisfying agreement between the complete and the simplified models. This procedure is particularly useful in engineering practice because it allows to assess, in a simplified way, the effectiveness of strengthening interventions for reducing horizontal relative displacements between vault supports.

Key Words
vault; macro-element; equivalent stiffness; truss; churches

Address
Linda Giresini, Mauro Sassu: Department of Energy, Systems, Territory and Constructions Engineering Largo Lucio Lazzarino, 1, Pisa, University of Pisa, Italy

Christoph Butenweg: Lehrstuhl für Baustatik und Baudynamik, Mies-van-der-Rohe-Stra

Abstract
Vibration control using a tuned mass damper (TMD) is an effective technique that has been verified using analytical methods and experiments. It has been applied in mechanical, automotive, and structural applications. However, the damping of a TMD cannot be adjusted in real time. An excessive mass damper stroke may be introduced when the mass damper is subjected to a seismic excitation whose frequency content is within its operation range. The semi-active tuned mass damper (SATMD) has been proposed to solve this problem. The parameters of an SATMD can be adjusted in real time based on the measured structural responses and an appropriate control law. In this study, a stiffness-controllable TMD, called a leverage-type stiffness-controllable mass damper (LSCMD), is proposed and fabricated to verify its feasibility. The LSCMD contains a simple leverage mechanism and its stiffness can be altered by adjusting the pivot position. To determine the pivot position of the LSCMD in real time, a discrete-time direct output-feedback active control law that considers delay time is implemented. Moreover, an identification test for the transfer function of the pivot driving and control systems is proposed. The identification results demonstrate the target displacement can be achieved by the pivot displacement in 0-2 Hz range and the control delay time is about 0.1 s. A shaking-table test has been conducted to verify the theory and feasibility of the LSCMD. The comparisons of experimental and theoretical results of the LSCMD system show good consistency. It is shown that dynamic behavior of the LSCMD can be simulated correctly by the theoretical model and that the stiffness can be properly adjusted by the pivot position. Comparisons of experimental results of the LSCMD and passive TMD show the LSCMD with less demand on the mass damper stroke than that for the passive TMD.

Key Words
semi-active control; variable stiffness; mass damper; discrete-time optimal linear-quadratic regulator (LQR) control; direct output-feedback, time-delay compensation, leverage theorem; position control; servo motor

Address
Department of Civil Engineering, National Cheng Kung University, No.1, University Road, Tainan, Taiwan

Abstract
Soft or extreme soft storeys in multi-storied buildings cause localized damage (and even collapse) during strong earthquake shaking. The presence of such soft or extremely soft storey is identified through provisions of vertical stiffness irregularity in seismic design codes. Identification of the irregularity in a building requires estimation of lateral translational stiffness of each storey. Estimation of lateral translational stiffness can be an arduous task. A simple procedure is presented to estimate storey stiffness using only properties of fundamental lateral translational mode of oscillation (namely natural period and associated mode shape), which are readily available to designers at the end of analysis stage. In addition, simplified analytical expressions are provided towards identifying stiffness irregularity. Results of linear elastic time-history analyses indicate that the proposed procedure captures the irregularity in storey stiffness in both low- and mid-rise buildings.

Key Words
modal analysis; mass participation; open storey; soft storey; storey stiffness

Address
Department of Civil Engineering, Indian Institute of Technology Madras, Chennai 600036, India

Abstract
Using X-braced frames in steel structures is a current procedure to achieve good strength against lateral loads. Study on mid-connections of X-braces and their effects on frame behavior is a subject whose importance has been more or less disregarded by researchers. Experimentally inspecting models involves considerable expense and time; however, computer models can be more suitable substitutes. In this research, a numerical model of X-braced frame has been analyzed using finite element software. The results of pushover analysis of this frame are compared with those of the experimental test. With the help of computer model, the effects of different mid-connection details on ductility and lateral strength of the frame are inspected. Also performances of bolted and welded connections are compared. Taking into account ductility and strength, this study suggests details of a decent pattern for the mid-connection.

Key Words
steel structures; X-brace; mid-connection; ductility; lateral strength

Address
Department of Civil and Environmental Engineering, Shiraz University of Technology, Shiraz, Iran

Abstract
When studying the vibration of a suspension bridge based on the traffic-bridge coupled system, most researchers ignored the contribution of the pavement response. For example, the pavement was simplified as a rigid base and the deformation of pavement was ignored. However, the action of deck pavement on the vibration of vehicles or bridges should not be neglected. This study is mainly focused on establishing a new methodology fully considering the effects of bridge deck pavement, probabilistic traffic flows, and varied road roughness conditions. The bridge deck pavement was modeled as a boundless Euler-Bernoulli beam supported on the Kelvin model; the typical traffic flows were simulated by the improved Cellular Automaton (CA) traffic flow model; and the traffic-pavement-bridge coupled equations were established by combining the equations of motion of the vehicles, pavement, and bridge using the displacement and interaction force relationship at the contact locations. The numerical studies show that the proposed method can more rationally simulate the effect of the pavement on the vibrations of bridge and vehicles.

Key Words
traffic-pavement-bridge coupled system; bridge; vehicle; road surface

Address
Yin Xinfeng, Liu Yang: School of Civil Engineering and Architecture, Changsha University of Science & Technology, Changsha 410004, Hunan, China

Kong Bo: Department of Civil and Environmental Engineering, Louisiana State University, Baton Rouge, Louisiana, 70803, USA


Abstract
This paper summarizes estimated seismic response results from three-dimensional nonlinear inelastic time-history analyses of some steel buckling-restrained braced (BRB) structures taking into account soil-structure interaction (SSI). The response results involve mean values for peak interstorey drift ratios, peak interstorey residual drift ratios and peak floor accelerations. Moreover, mean seismic demands in terms of axial force and rotation in columns, of axial and shear forces and bending moment in BRB beams and of axial displacement in BRBs are also discussed. For comparison purposes, three separate configurations of the BRBs have been considered and the aforementioned seismic response and demands results have been obtained firstly by considering SSI effects and then by neglecting them. It is concluded that SSI, when considered, may lead to larger interstorey and residual interstorey drifts than when not. These drifts did not cause failure of columns and of the BRBs. However, the BRB beam may fail due to flexure.

Key Words
buckling-restrained braces; seismic response; soil-structure interaction; three-dimensional steel structures

Address
Antonios K. Flogeras: Engineer Consultant, Zakinthou 34, GR-26441 Patras, Greece

George A. Papagiannopoulos: Department of Civil Engineering, University of Patras, GR-26504 Patras, Greece


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