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CONTENTS
Volume 2, Number 2, June 2011
 

Abstract
The inelastic earthquake response of non-symmetric, braced steel buildings, designed according to the EC3 (steel structures) and EC8 (earthquake resistant design) codes, is investigated using 1, 3 and 5-story models, subjected to a set of 10, two-component, semi-artificial motions, generated to match the design spectrum. It is found that in these buildings, the so-called

Key Words
asymmetry; eccentricity; torsion; multistory steel buildings; braces; earthquake inelastic response; plastic hinge model.

Address
M.T. Kyrkos and S.A. Anagnostopoulos: Dept. of Civil Engineering, University of Patras, 26500 Patras, Greece

Abstract
In a companion paper as well as in earlier publications, it has been shown that in asymmetric frame buildings, designed in accordance with modern codes and subjected to strong earthquake excitations, the ductility demands at the so called "flexible" edges are consistently and substantially higher than the ductility demands at the "stiff" edges of the building. In some cases the differences in the computed ductility factors between elements at the two opposite building edges exceeded 100%. Similar findings have also been reported for code designed reinforced concrete buildings. This is an undesirable behavior as it indicates no good use of material and the possibility for overload of the "flexible" edge members with a consequent potential for premature failure. In the present paper, a design modification will be introduced that can alleviate the problem and lead to a more uniform distribution of ductility demands in the elements of all building edges. The presented results are based on the steel frames detailed in the companion paper. This investigation is another step towards more rational design of non-symmetric steel buildings.

Key Words
asymmetry; eccentricity; torsion; multistory steel buildings; braces; earthquake; inelastic response; plastic hinge model.

Address
M.T. Kyrkos and S.A. Anagnostopoulos: Dept. of Civil Engineering, University of Patras, 26500 Patras, Greece

Abstract
Dynamic response of two adjacent single degree-of-freedom (SDOF) structures connected with friction damper under base excitation is investigated. The base excitation is modeled as a stationary white-noise random process. As the force-deformation behavior of friction damper is non linear, the dynamic response of connected structures is obtained using the equivalent linearization technique. It is observed that there exists an optimum value of the limiting frictional force of the damper for which the mean square displacement and the mean square absolute acceleration responses of the connected structures attains the minimum value. The close form expressions for the optimum value of damper frictional force and corresponding mean square responses of the coupled undamped structures are derived. These expressions can be used for initial optimal design of the friction damper for connected structures. A parametric study is also carried out to investigate the influence of system parameters such as frequency ratio and mass ratio on the response of the coupled structures. It has been observed that the frequency ratio has significant effect on the performance of the friction damper, whereas the effects of mass ratio are marginal. Finally, the verification of the derived close from expressions is made by correlating the response of connected structures under real earthquake excitations.

Key Words
adjacent structures; dynamic response; earthquake; friction damper; optimization; system parameters.

Address
C.C. Patel and R.S. Jangid: Dept. of Civil Engineering, Indian Institute of Technology Bombay, Powai, Mumbai-400076, India

Abstract
Rational scaling of design earthquake ground motions for tall buildings is essential for safer, risk-based design of tall buildings. This paper provides the structural designers with an insight for more rational scaling based on drift and input energy demands. Since a resonant sinusoidal motion can be an approximate critical excitation to elastic and inelastic structures under the constraint of acceleration or velocity power, a resonant sinusoidal motion with variable period and duration is used as an input wave of the near-field and far-field ground motions. This enables one to understand clearly the relation of the intensity normalization index of ground motion (maximum acceleration, maximum velocity, acceleration power, velocity power) with the response performance (peak interstory drift, total input energy). It is proved that, when the maximum ground velocity is adopted as the normalization index, the maximum interstory drift exhibits a stable property irrespective of the number of stories. It is further shown that, when the velocity power is adopted as the normalization index, the total input energy exhibits a stable property irrespective of the number of stories. It is finally concluded that the former property on peak drift can hold for the practical design response spectrum-compatible ground motions.

Key Words
scaling of ground motion; design earthquake ground motion; maximum acceleration; maximum velocity; peak drift; earthquake input energy; Arias intensity; velocity power; acceleration power; resonant motion.

Address
I. Takewaki: Dept. of Architecture and Architectural Engineering, Kyoto UniversityKyotodaigaku-Katsura, Nishikyo-ku, Kyoto 615-8540, Japan
H. Tsujimoto: Nippon Steel Corp., Futtsu 293-8511, Japan

Abstract
In this study, the subspace stochastic realization theories (SSR model I and SSR model II) have been applied to a real bridge for estimating its dynamic characteristics (natural frequencies, damping constants, and vibration modes) under ambient vibration. A numerical simulation is carried out for an arch-type steel truss bridge using a white noise excitation. The estimates obtained from this simulation are compared with those obtained from the Finite Element (FE) analysis, demonstrating good agreement and clarifying the excellent performance of this method in estimating the structural dynamic characteristics. Subsequently, these methods are applied to the vibration induced by both strong and weak winds as obtained by remote monitoring of the Kabashima bridge (an arch-type steel truss bridge of length 136 m, and situated in Nagasaki city). The results obtained with this experimental data reveal that more accurate estimates are obtained when strong wind vibration data is used. In contrast, the vibration data obtained from weak wind provides accurate estimates at lower frequencies, and inaccurate accuracy for higher modes of vibration that do not get excited by the wind of lower intensity. On the basis of the identified results obtained using both simulated data and monitored data from a real bridge, it is determined that the SSR model II realizes more accurate results than the SSR model I. In general, the approach investigated in this study is found to provide acceptable estimates of the dynamic characteristics of highway bridges as well as for the vibration monitoring of bridges.

Key Words
structural health monitoring; system identification; ambient vibration; bridge dynamic characteristics; subspace stochastic realization theory.

Address
Md. Rajab Ali: Graduate School of Science and Technology, Nagasaki University1-14, Bunkyo machi, Nagasaki 852-8521, Japan
Takatoshi Okabayashi: Dept. of Civil Engineering, Faculty of Engineering, Nagasaki University1-14, Bunkyo machi, Nagasaki 852-8521, Japan


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