Dynamic performance and characteristics of highway bridges

Abstract

Dynamic testing of highway bridges is motivated by two concerns. One is the commonly reported observation from motorists and pedestrians of alarming vibrations, and the other is the possibility of using dynamic response data to assess the structural condition of a bridge and to track structural performance through long term monitoring. Regarding the first concern, measurements have been made on a variety of bridges, including cable-stayed and steel arch bridges and post tensioned concrete expressway flyovers. These measurements are reported here. These vibrations are generally at levels of less than 0.05g, and occur at frequencies in bands of 2-5Hz and around 10Hz and represent structural displacements of at most a few mm. While such vibrations may cause alarm to certain users they cannot cause structural distress and can be used for structural assessment. Regarding the second concern, NTU is developing a system combining modal characterisation, obtained via a complete dynamic survey, with finite element modeling techniques to produce a physically realistic representation identifying elements or areas in a bridge which are not functioning in the manner expected or assumed. Some results from application of this finite element updating technology to a cable stayed bridge are reported. In addition, a program of long term dynamic and static response monitoring of one highway bridge has begun. The aim is to use the data to develop optimal instrumentation strategies and data reduction techniques to detect performance anomalies and point to structural causes