1 INCOHERENT SOIL-STRUCTURE INTERACTION (SSI) EFFECTS FOR A 242M LONG CONCRETE BRIDGE FOUNDED ON DEEP PILES Mircea CONȚIU 1 , Dan M GHIOCEL 2 and Dan CREȚU 3 ABSTRACT The paper presents the results of a project of the new research center in earthquake engineering entitled “Dan Ghiocel International Research Center” at Technical University of Civil Engineering Bucharest, Romania. The paper presents a seismic SSI study that has been conducted for the newly designed Fartec Bridge in Brasov, Romania. The bridge is a 242m long concrete structure, divided into 8 spans, each of 30m length, and has deep foundations on drilled piles. The structure is typical for Romania and highly used throughout the country. Seismic input was defined by a set of three- directional spectrum compatible acceleration histories generated based on the Romanian seismic design spectrum for the Fartec area. The seismic SSI were performed using the state-of-the-art ACS SASSI software. The SSI analyses were performed for both coherent (synchronous) and incoherent (non-synchronous) seismic inputs. The main focus of the study is to evaluate the effects of the seismic motion incoherency of the Fartec bridge dynamic response. The paper includes comparisons of results obtained using the Eurocode 8 analysis procedures and the state-of-the-art seismic SSI analysis using ACS SASSI. The final conclusions indicate the limitations of the current bridge design SSI modeling and the simplified motion spatial variability modeling in Eurocode 8. INTRODUCTION Most types of civil engineering structures are in direct contact with the soil, and therefore their behavior is affected the soil dynamic properties. This soil-structure interaction (SSI) between the ground and the structure is important during earthquakes, because it may change the seismic behavior of the structure completely. In some cases, the consideration of SSI may be favorable, reducing the forces on the structure and, thus, the execution cost, or it could be unfavorable and not considering it could have devastating results. The seismic SSI effects may be neglected for small and light structures build on stiff soils, such as rocks, but becomes very important for larger and heavier structures, such as nuclear power-plants, concrete bridges or even high-rise buildings. Recent earthquakes have highlighted the necessity to include SSI effects in the design of structures. The 1995 Kobe earthquake has severely damaged bridges along the Hanshin Expressway. This shows that the response of the structure is influenced by the interaction between the foundations and the soil, which, in this particular case, had a negative role (Mylonakis et al., 2000). The goal of this paper is to compare the current practice in the seismic design of bridges, according to Eurocode 8, and the state-of-the-art seismic SSI analysis methods, developed specifically 1 Ph.D. Candidate, Technical University of Civil Engineering, Bucharest, Romania, contiu.mircea@gmail.com 2 Chief of Engineering, Ghiocel Predictive Technologies, Inc., Rochester, New York, and Adjunct Professor with Case Western Reserve University, Cleveland, Ohio, USA, dan.ghiocel@ghiocel-tech.com 3 Professor, Technical University of Civil Engineering, Bucharest, Romania, cretud@utcb.ro