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CONTENTS
Volume 2, Number 1, March 2011
 

Abstract
During very strong earthquakes, seismically isolated buildings may experience large horizontal relative displacements, which may lead to poundings if an insufficiently wide clearance is provided around the building. This paper investigates, through numerical simulations, the effectiveness of using rubber bumpers, which could be attached at locations where it is likely to have impacts, in order to act as shock-absorbers. For the simulation of the dynamic behavior of such rubber bumpers during impacts, a nonlinear force-based impact model, which takes into account the finite thickness of the rubber bumpers, has been developed. Subsequently, a series of parametric analyses are performed to assess the effect of the gap size, the earthquake characteristics and the thickness, compressive capacity and damping of the bumpers. The stiffness of the moat wall is also parametrically considered during poundings of a seismically isolated building, as another potential mitigation measure for poundings of seismically isolated buildings.

Key Words
poundings; seismic isolation; shock-absorbers; bumpers; rubber; damping.

Address
Panayiotis C. Polycarpou and Petros Komodromos: Dept. of Civil and Environmental Engineering, University of Cyprus
75 Kallipoleos Str., P.O. Box 20537, 1678 Nicosia, Cyprus

Abstract
This paper discusses a mechanical model for the vulnerability assessment of old masonry building aggregates that takes into account the uncertainties inherent to the building parameters, to the seismic demand and to the model error. The structural capacity is represented as an analytical function of a selected number of geometrical and mechanical parameters. Applying a suitable procedure for the uncertainty propagation, the statistical moments of the capacity curve are obtained as a function of the statistical moments of the input parameters, showing the role of each one in the overall capacity definition. The seismic demand is represented by response spectra; vulnerability analysis is carried out with respect to a certain number of random limit states. Fragility curves are derived taking into account the uncertainties of each quantity involved.

Key Words
capacity spectrum; fragility curves; uncertainties; damage assessment; seismic vulnerability.

Address
Luisa Carlotta Pagnini: Dept. of Civil, Environmental and Architectural Engineering - DICAT, University of Genova, Italy
Romeu Vicente: Dept. of Civil Engineering - DECIVIL, University of Aveiro, Portugal
Sergio Lagomarsino: Dept. of Civil, Environmental and Architectural Engineering - DICAT, University of Genova, Italy
Humberto Varum: Dept. of Civil Engineering - DECIVIL, University of Aveiro, Portugal

Abstract
One-dimensional rod theory is very effective as a simplified analytical approach to large scale or complicated structures such as high-rise buildings, in preliminary design stages. It replaces an original structure by a one-dimensional rod which has an equivalent stiffness in terms of global properties. The mechanical behavior of structures composed of distinct constituents of different stiffness such as coupled walls with opening is significantly governed by the local variation of stiffness. Furthermore, in structures with setback the distribution of the longitudinal stress behaves remarkable nonlinear behavior in the transverse-wise. So, the author proposed the two-dimensional rod theory as an extended version of the rod theory which accounts for the two-dimensional local variation of structural stiffness; viz, variation in the transverse direction as well as longitudinal stiffness distribution. This paper proposes how to deal with the two-dimensional rod theory for structures with setback. Validity of the proposed theory is confirmed by comparison with numerical results of computational tools in the cases of static, free vibration and forced vibration problems for various structures. The transverse-wise nonlinear distribution of the longitudinal stress due to the existence of setback is clarified to originate from the long distance from setback.

Key Words
simplified analytical method; extended rod theory; two-dimensional stiffness of structures; setback; nonlinear stress distribution; preliminary design for buildings; dynamic analysis.

Address
Hideo Takabatake: Dept. of Architecture, Kanazawa Institute of Technology Institute of Disaster and Environmental Science 3-1 Yatsukaho, Hakusan, Ishikawa Prefecture, 924-0838, Japan
Fumiya Ikarashi and Motohiro Matsuoka: Kanazawa Institute of Technology, 924-0838, Japan

Abstract
A new method for the seismic design of plane steel moment resisting frames is developed. This method determines the design base shear of a plane steel frame through modal synthesis and spectrum analysis utilizing different values of the strength reduction (behavior) factor for the modes considered instead of a single common value of that factor for all these modes as it is the case with current seismic codes. The values of these modal strength reduction factors are derived with the aid of a) design equations that provide equivalent linear modal damping ratios for steel moment resisting frames as functions of period, allowable interstorey drift and damage levels and b) the damping reduction factor that modifies elastic acceleration spectra for high levels of damping. Thus, a new performance-based design method is established. The direct dependence of the modal strength reduction factor on desired interstorey drift and damage levels permits the control of deformations without their determination and secures that deformations will not exceed these levels. By means of certain seismic design examples presented herein, it is demonstrated that the use of different values for the strength reduction factor per mode instead of a single common value for all modes, leads to more accurate results in a more rational way than the codebased ones.

Key Words
modal strength reduction (behavior) factor; equivalent linear modal damping ratios; damping reduction factors; interstorey drift; damage; seismic design; steel moment resisting frames.

Address
George A. Papagiannopoulos: Dept. of Civil Engineering, University of Patras, 26500 Rio-Patras, Greece
Dimitri E. Beskos: Dept. of Civil Engineering, University of Patras, 26500 Rio-Patras, Greece
Office of Theoretical and Applied Mechanics, Academy of Athens, 4 Soranou Efessiou str. 11527 Athens, Greece

Abstract
In this paper a simple mathematical model is presented for estimating the natural frequencies and corresponding mode shapes of a tall building with outrigger-belt truss system. For this purposes an equivalent continuum system is analyzed in which a tall building structure is replaced by an idealized cantilever continuum beam representing the structural characteristics. The equivalent system is comprised of a cantilever shear beam in parallel to a cantilever flexural beam that is constrained by a rotational spring at outrigger-belt truss location. The mathematical modeling and the derivation of the equation of motion are given for the cantilevers with identically paralleled and rotational spring. The equation of motion and the associated boundary conditions are analytically obtained by using Hamilton\'s variational principle. After obtaining non-trivial solution of the eigensystem, the resulting is used to determine the natural frequencies and associated mode shapes of free vibration analysis. A numerical example for a 40 story tall building has been solved with proposed method and finite element method. The results of the proposed mathematical model have good adaptation with those obtained from finite element analysis. Proposed model is practically suitable for quick evaluations during the preliminary design stages.

Key Words
tall buildings; shear beam; outrigger-belt truss system; Hamilton principle; free vibration; natural frequency.

Address
Mohsen Malekinejad and Reza Rahgozar: Dept. of Civil Engineering, University of Kerman, Kerman, Iran


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