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CONTENTS
Volume 1, Number 2, June 2010
 

Abstract
The ground acceleration measured at a point on the earth's surface is composed of several waves that have different phase velocities, arrival times, amplitudes, and frequency contents. For instance, body waves contain primary and secondary waves that have high frequency content and reach the site first. Surface waves are composed of Rayleigh and Love waves that have lower phase velocity, lower frequency content and reach the site next. Some of these waves could be of more damage to the structure depending on their frequency content and associated amplitude. This paper models critical earthquake loads for single-degree-of-freedom (SDOF) inelastic structures considering evolution of the seismic waves in time and frequency. The ground acceleration is represented as combination of seismic waves with different characteristics. Each seismic wave represents the energy of the ground motion in certain frequency band and time interval. The amplitudes and phase angles of these waves are optimized to produce the highest damage in the structure subject to explicit constraints on the energy and the peak ground acceleration and implicit constraints on the frequency content and the arrival time of the seismic waves. The material nonlinearity is modeled using bilinear inelastic law. The study explores also the influence of the properties of the seismic waves on the energy demand and damage state of the structure. Numerical illustrations on modeling critical earthquake excitations for one-storey inelastic frame structures are provided.

Key Words
critical excitation; earthquake acceleration; seismic waves; inelastic structures; ductility; damage indices.

Address
Abbas Moustafa: Dept. of Civil Engineering, Minia University, Minia 61111, Egypt
Kohei Ueno and Izuru Takewaki: Dept. of Urban & Environmental Engineering, Kyoto University, Kyotodaigaku-katsura, Kyoto 615-8540, Japan

Abstract
The vertical axis through the modal center of rigidity (m-CR) is used for interpreting the code torsional provisions in the design of eccentric multi-story building structures. The concept of m-CR has been demonstrated by the author in an earlier paper and the particular feature of this point is that when the vertical line of the centers of mass at the floor levels is passing through m-CR, minimum base torsion is developed. For this reason the aforesaid axis is used as reference axis for implementing the code provisions required by the equivalent static analysis. The study examines uniform mixed-bent-type multistory buildings with simple eccentricity, ranging from torsionally stiff to torsionally flexible systems. Using the results of a dynamic response spectrum analysis as a basis for comparisons, it is shown that the results of the code static design are on the safe side in torsionally stiff buildings, but unable to predict the required strength of bents on the stiff side of systems with a predominantly torsional response. Suggestions are made for improving the code provisions in such cases.

Key Words
asymmetric structures; design eccentricities, modal analysis.

Address
George K. Georgoussis: Dept. of Civil and Construction Engineering, School of Pedagogical and Technological Education (ASPETE), N. Heraklion 14121, Attica, Greece

Abstract
This paper presents an appraisal of four nonlinear static procedures (CSM, N2, MPA and ACSM) employed in seismic assessment of plan-irregular buildings. It uses a three storey reinforced concrete plan-irregular frame building exemplifying typical older constructions of the Mediterranean region in the early 1970s that was tested in full-scale under bi-directional pseudo-dynamic loading condition at JRC, Ispra. The adequacy and efficiency of the simplified analytical modelling assumptions adopted were verified. In addition, the appropriate variants of code-prescribed NSPs (CSM and N2) to be considered for subsequent evaluation were established. Subsequent parametric studies revealed that all such NSPs predicted reasonably well both global and local responses, having the benchmark values been determined through nonlinear dynamic analyses using a suit of seven ground motions applied with four different orientations. The ACSM, however, predicted responses that matched slightly better the median dynamic results.

Key Words
Seismic assessment; 3D irregular SPEAR building; nonlinear static procedures; numerical simulation.

Address
R. Bento and C. Bhatt: Instituto Superior Tecnico, Technical University of Lisbon, Lisbon, Portugal
R. Pinho: Dept. of Structural Mechanics, University of Pavia, Via Ferrata 1, Pavia 27100, Italy

Abstract
The paper presents a methodology for developing earthquake damage and loss scenarios for urban areas, as well as its application to two cities located in Mediterranean countries, Grevena (in Greece) and Duzce (in Turkey), that were struck by strong earthquakes in the recent past. After compiling the building inventory in each city, fragility curves were derived using a hybrid approach previously developed by the authors, and a series of seismic scenarios were derived based on microzonation studies that were specifically conducted for each city (see companion paper by Pitilakis et al.). The results obtained in terms of damage estimates, required restoration times and the associated costs are presented in a GIS environment. It is deemed that both the results obtained, and the overall methodology and tools developed, contribute towards the enhancement of seismic safety in the Mediterranean area (as well as other earthquake-prone regions), while they constitute a useful pre-earthquake decision-making tool for local authorities.

Key Words
earthquake damage scenarios; vulnerability; loss assessment; fragility curves; capacity curves; hybrid methodology; restoration time.

Address
A.J. Kappos, G.K. Panagopoulos, A.G. Sextos, V.K. Papanikolaou and K.C. Stylianidis: Dept. of Civil Engineering, Aristotle University of Thessaloniki, GR-54124, Thessaloniki, Greece

Abstract
The main objectives of this work were to investigate the effects of the nonlinear behavior of the isolation pads on the seismic response of bridges with rubber bearings, and to identify when base isolation improved their seismic performance. To achieve these objectives a parametric study was conducted designing a set of bridges for three different soil types and varying the number of spans, span lengths, and pier heights. The seismic responses (accelerations, displacements and pier seismic forces) were evaluated for three different structural models subjected to three earthquakes with different dynamic characteristics. The first represented bridges without base isolation; the second corresponded to the same bridges including now rubber bearings as an isolation system, with linear elastic behavior that shifted the natural period of the bridge by a factor of 2 to 4. In the third model the seismic response of bridges supported on lead-Rubber bearings was studied accounting for the nonlinear behavior of the lead. The results show clearly the importance of the nonlinear behavior on the seismic performance of the bridges.

Key Words
lead-rubber bearings; base isolation; bridges; nonlinear behavior.

Address
Olmos B.A.: Facultad de Ingenieria Civil, Universidad Michoacana de San Nicolas de Hidalgo, Mexico
Roesset J.M.: Department of Civil Engineering, Texas A&M University, College Station, USA


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