Seismic risk analysis including soil – structure interaction (SSI) of harp type cable stayed bridge with support flexibility is presented which can be used for preliminary estimate of its probability of failure. The risk analysis procedure uses the format of probabilistic risk analysis (PRA) and considers the band limited white noise at the bedrock as the seismic input. The bridge deck is modeled as a beam supported on springs at different points. The coupled stiffness matrix of the springs is determined by a separate 2D static analysis of cable-tower-deck system in which flexibility of the tower base due to soil-structure interaction is included. Damping due to soil is incorporated by the equivalent modal energy method. The response of the bridge deck is obtained by the response spectrum method of analysis for multi-degree of freedom system. The PRA includes uncertainties of responses due to the variation in ground motion, material property, modeling and method of analysis, and uncertainties of the capacity due to the variation of ductility factor and damage concentration effect. Failure mode of the bridge is assumed to be bending failure of the bridge deck at the point of maximum bending moment. Probability of failure of the bridge deck is determined by First Order Second Moment (FOSM) theory of reliability analysis. A three span double plane cable stayed bridge is used as an illustrative example. The fragility curves for the bridge deck failure are obtained under a number of parametric variations. The parameters include, base flexibility, degree of correlation of ground motion, angle of incidence of earthquake, ratio of the components of ground motion. Study shows that flexible base condition provides significantly less value of probability of failure as compared to the fixed base. Further, angles of incidence, degree of correlation and ratio of components of ground motion have considerable effects on the probability of failure.
structural response
Seismic Demand Analysis of Inelastic SDOF Systems Designed According to the NTC 2008 for L’Aquila Earthquake
he study investigates on the inelastic response of equivalent Simple Degree Of Freedom systems (SDOF) to some L’Aquila earthquake accelerograms recorded in the epicentral area. According to recent statistical analysis, L’Aquila earthquake is considered as a seismic event having a return period of about 475 years. The aim of the paper is to give a first interpretation to the high damage levels (IX-X MCS) due to the earthquake, in spite of its moderate magnitudo (ML = 5.8) and the correct historical seismic classification of the stroked area. The study thus analyzes the inelastic seismic demand of a SDOF, designed according to the NTC08, to four near-fault accelerograms recorded during the L’Aquila earthquake main shock, by varying SDOF capacity in terms of strength, ductility and viscous damping, in order to assess the performance targets imposed by the same NTC08. Two well- known damage indices have been used to estimate seismic effects on built environment: Park & Ang and low-cycle fatigue indices. The results highlight the high seismic inelastic demand, well beyond the limit specified in NTC08, both for the design of new buildings and for seismic upgrading of existing ones. This clearly shows the need for a revision of the actual seismic hazard map taking into account the near-fault and site amplification effects, which are not considered within the seismic hazard methods adopted by the NTC08.
For this paper is available an extended abstract after the text in Italian