Early warning monitoring system of modular expansion joints based on dynamic behaviour

One of the existing modular expansion joints of the Martinus Nijhoff Bridge in the Netherlands was monitored with the purpose of extending its service life for on year until replacement in 2011. Based on an analysis of the failure mechanisms of this (type of) joint, strain gauges were used as sensors and applied at centre and crossbeams. Chosen because of their high reliability, these sensors constitute a major part of the system costs. All major abrupt failure modes would lead to increased strains and vibrations and could be detected by increased magnitude in the measurements. Measurement frequency was set at 2000 Hz. Extreme values of the measured strains determined in 1 minute were stored. They were analyzed daily and compared to the values measured the day before. Criteria on (in)significant increase in the strains were determined based on empery, expert opinion and later with the results of over rolling tests. The system operated flawless over 1 year. After accounting for large fluctuations in traffic loads due to holidays) no alarms were issued by the system. Visual inspections affirmed that no abrupt failure of the joint occurred. Yet from a thorough evaluation of the measurements and the results of the over roll tests it could be concluded that the system worked properly