Effects of Near-Field Earthquakes on Seismically Isolated Bridges under Bi-Directional Loading Blank line 11 Blank line 11 p Cem Yenidogan, & Mustafa Erdik Bogazici University, KOERI, Istanbul, Turkey Blank Line 9 pt Micheal Constantinou The State University of New York at Buffalo, New York ,USA Blank line 11 p Blank line 11 pt SUMMARY: Major effort in the design or assessment process of structures is based on conducting nonlinear time history analysis. In general framework, seismic hazard assessment and structural analysis have to be performed, respectively. The performance-based earthquake engineering requires reliable assessment of long-period ground motion particularly for seismically isolated structures, liquid storage tanks, long bridges such as cable-stayed, suspension bridges and structures that are designed to deform beyond the elastic range. Seismic isolation is one of the innovative techniques which can be used in the design of new bridges or retrofitting of existing bridges. This study aims to review and discuss issues related with the selection and scaling procedures for seismically isolated structures which have fundamental period in the long period range. Selection and scaling of real earthquake records of long period structures have to be based on the seismicity of the region, seismic hazard assessment as well as characteristic of the structure. Moreover, bi-directional loading is utilized for the analytical models rather than using unidirectional analysis which leads to crude estimation. The selected bridge for the dynamic analysis is a continuous, three-span, cast-in-place concrete box girder structure with a 30- degree skew. The two intermediate bents consist of two circular columns with a cap beam on top. The geometry of the bridge, section properties and foundation properties are assumed to be same as in the original bridge in the FHWA example. Sliding type of seismic isolation devices are implemented into analytical bridge model. Blank line 10 p Keywords: Seismic Isolation, Selection and Scaling of records, Near-Field Effects Blank line 11 pt Blank line 11 pt 1. INTRODUCTION Blank line 11 Earthquakes have significant effects on structures in seismically prone regions and it is mandatory to design conventional earthquake-resistant bridges based on concentration of significant inelastic action (energy dissipation) in the selected structural components during the design process. Bridges are categorized as simple structures however they constitute the critical part of the transportation systems and their desired performance during earthquake is essential. Thus, designers have to minimize risks, and maintain the functionality of bridges after an expected earthquake. Major seismic events demonstrated that even newly constructed bridges by contemporary seismic provisions were damaged in California, Japan, Turkey, New Zealand, Central and South America. Poor performance of bridges were attributed to several reasons such as design philosophy adopted, lack of good detailing, and erroneous definition of probable expected ground motion in seismic hazard studies. The 1971 San Fernando earthquake revealed the inadequacy of both 1965 AASHTO Design guideline and previous seismic provisions in that era. The 1971 San Fernando, 1989 Loma Prieta, and 1994 Northridge earthquakes lead to the development of AASHTO Guide Specifications for Seismic Isolation Design 1991, 1999 and AASHTO LRFD Bridge Design Specifications 2007, 2010 respectively. Due to emphasized importance of bridges, new innovative rehabilitation methods have to be applied to structural members to protect them from major seismic events. Among those proposed innovative systems, seismic isolation has been implemented for buildings, bridges, liquid storage tanks (LNG tanks), nuclear power plants, offshore platforms and electrical substations. The deck of the bridge has the largest portion of the mass and piers should transfer the lateral load which is induced by ground