Structural Analysis of Historical Constructions - Modena, Lourenço & Roca (eds) © 2005 Taylor & Francis Group, London, ISBN 04 15363799 Historical railway bridges: tests and numerical analysis M. Ferraioli, P. Malangone, M. Rauci & A. Zambrano Second University of Naples, Ilaly ABSTRACT: A procedure based on experimental and theoretical analyses to evaluate the response ofrailway bridges under dynamic loading is presented. In particular an ancient railway bridge under service on the AvelIino- Rocchetta-Foggia line in the Southem part ofItaly was considered. A simplified procedure to study the vehicle- bridge interaction with a simple continuous beam model is proposed. The calibration ofthe model was carried out at first with the study ofthe technical reports, the visual inspection, the material tests and the dynamic tests. Then an identification procedure to characterize the presence of structural non-linearities is applied. FinalIy, the calibrated model of the bridge was used to perform modal analysis, to evaluate the magnification facto r of the bridge and the acceleration response of the vehicle. INTRODUCT ION The procedures for the analysis of existing bridges are usualIy based on field experiments which are the most reliable mean to determinate their dynamic char- acteristics. In fact the dynamic properties are only marginalIy influenced by variations in loading while the static properties may sutTer strong variations with the load pattem. As a consequence, during the last years many studies focused on the possibility ofusing the vibration characteristics to evaluate the structural health. In the simplest form, the identification tech- niques generalIy use the natural frequencies and the mode shapes obtained with field tests (ambient vibra- tion, forced vibration, free vibration, tratTic vibration, earthquake response measurements). These dynamic properties alIow the control ofthe construction quality, the validation and the improvement ofstructural mod- eIs, the assessment of damage (Salawu & Williams 1995, Kou & De Wolf 1997, Capecchi & Ve stroni 1999, Castiglioni et aI. 2002). The approaches based on the modal analysis techniques in frequency domain are stilI dominant in the model updating philo sophy (Ewins 1986). GeneralIy, the finite element model of the structure is validated with the comparison between the eigenproperties deriving from the model and the eigenproperties calculated from the moda I tests. Then the interaction among the train, the support- ing track and the bridge has to be properly modeled. In fact this interaction generalIy produces the ampli- fication of displacements, strains and stresses in the structural members (Hurty & Rubinstein 1967). This interaction problem between vehicles and bridge has attracted much attention due to the large increase in the proportion of heavy vehicles and high-speed vehicles in railway tratTic. FinalIy, the dynamic nature ofthe train loading has to be considered because it produces some particular etTects. First of alI, the sudden changes in the bridge loading due to train speed create heavy inertial etTects on the structure. Then changes in the axial load on the bridge occur as a consequence both of the rough- ness of the railroad and of the irregularity of the train wheels. FinalIy, the repeated sequences of the loads due to the nearly constant spacing of the train axles can produce both resonance phenomena and huge and dangerous vibrations. After all the dynamic behavior ofthe bridge is influenced by some main factors: a) the natural frequency ofthe structure; b) the wheel bases of the train; c) the train speed; d) the structural damp- ing; e) the type ofbridge span; f) the spacing oftrain axles; f) the track structure; g) the wheel imperfections; h) the roughness of the rail road. In this paper so me simple models for dynamic inter- action between vehicle and bridge were characterized to study the dynamic response ofthe bridge under the moving train loading, and to evaluate the acceleration inside the vehicle. The interaction between the mov- ing train and the bridge was modeled neglecting the rolIing and yawing of the railway vehicles, constrain- ing the rail on the bridge deck and considering the damping characteristics ofthe vehicle suspension. An equivalent beam model for the bridge was calibrated on the basis of the experimental results. This simple model accounts for the vehicle-bridge interaction, but cannot precisely represent two or three-dimensional behaviour, particularly in the case ofmoving train with paths that are not along the centerline of the bridge. 1019