Modal analysis of the suspension footbridge over river Vrbas in Banja Luka

Abstract

When evaluating modal properties of lively structures as footbridges, numerical modelling should be used with caution. This is due to model uncertainties mostly related to material parameters, boundary conditions, the effects of the non-structural elements, and also other inevitable differences between as-built and designed structures. Differences between the dynamic behaviour of the as-built structure and the numerical finite element model are best visible after performing vibration testing and modal properties identification based on experimentally obtained results. Conclusions regarding reasons for these differences can be useful for further understanding of vibrational behaviour of footbridge structures and can help designers in better predicting dynamic properties of such structures in the design stage. This paper describes a lively full-scale footbridge over the Vrbas river in Banja Luka, Prijecani sub-area, Serbian Republic, Bosnia and Herzegovina, its modal properties estimation based on ambient vibration measurements and numerical modelling. Identification of modal properties was performed using Ambient Modal Testing applying Multiple Test Setup Measurement Procedure and Frequency Domain Decomposition method. The ambient vibration measurement technique for the subject footbridge, which gave a good quality of the results obtained, is presented. Analysis of the obtained experimental results is given, as well as their comparison with numerically estimated values in the finite element model. Despite some irregular features of the subject footbridge, correlation is obtained, and conclusions are made regarding the effects of numerical model parameters on structural dynamic properties.