8. Modal Irregularity in Continuous Reinforced Concrete Bridges. Detection, Effect on the Simplified Seismic Performance Evaluation and Ways of Solution

This chapter presents the preliminary results of an ongoing investigation on the modal irregularity in structures, particularly in reinforced concrete viaduct-like bridges, and the effect of this structural/demand characteristic on the seismic performance evaluation of such structures using simplified methods of analysis. The main objective of this chapter is to understand modal structural irregularity and its effects on the performance results obtained from approximate elastic analysis procedures prescribed by most codes or simplified nonlinear analysis methods using modal spectral analyses. It is shown that modal irregularity may be present in bridges with relatively close modes, and it may also occur when, in the inelastic range, the instantaneous modes of the bridge change their composition and that this irregularity may lead to erroneous results, particularly when a modal combination rule is involved. To overcome this problem, a new analysis method for the seismic performance evaluation of bridges exhibiting modal irregularity is presented. This method has as origin a simplified seismic evaluation method based on the performance of a reference single degree of freedom system derived from the capacity curve of the bridge, calculated using evolutive modal spectral analyses with their corresponding dissipated hysteretic energy correction. To show the application of this method, the seismic performance of two bridges, one considered regular and the other with evident modal irregularity, subjected to an earthquake record of two different intensities, one that keeps them within the elastic range of behaviour and the other that takes them into the inelastic range. Finally, the validity of this approximation is shown by comparing the “exact” seismic performance of the bridges, obtained by nonlinear step-by-step analysis, with the corresponding performances obtained using the simplified method. To show the validity of the procedure proposed for the construction of the capacity curve of a bridge, a comparison of results obtained from conventional force pushover analysis, evolutive modal spectral analysis with a correction for hysteretic energy dissipation and the incremental dynamic analysis; using as reference the last one and stressing the potential and limitations of the second.