Case Study in Extending the Life of a Bridge - Fatigue Experiment in the Netherlands
The second work package of BridgeMon, WP2, aimed to develop a technique to accurately monitor the traffic loading on a bridge and calculate the remaining fatigue life.
The ability to obtain such information on loading and resistance will assist bridge owners in making better founded and more economic life-cycle maintenance decisions.
A Structural Health Monitoring (SHM) system was combined with measurements of the actual traffic loading on the bridge to facilitate more realistic fatigue damage calculations. The SHM system used the ‘virtual monitoring’ concept which ‘virtually’monitors the parts of the bridge that are not monitored directly, through simulations using a calibrated Finite Element (FE) model.
The overall goal of WP2 of the BridgeMon project was to test a Structural Health Monitoring system that utilises traffic loading measurements, and the virtual monitoring concept, on a steel bridge in the Netherlands. A cable stayed bridge was selected for testing, one part of which was instrumented with the SiWIM® Bridge-WIM system to collected traffic information. Additional strain gauges were installed on the main cable-stayed portion of the structure. Using information gained on-site and in the design drawings, a detailed Finite Element (FE) model of the bridge was developed. The information from the two measurement systems was used to calibrate and validate this numerical model of the bridge. The calibrated FE model of the bridge was then used to assess the level of fatigue damage at critical locations. This virtual monitoring concept allows a fatigue damage calculation to be carried out for any location on the bridge, without the requirement for the installation of sensors at every specific location.
Fatigue assessment requires an accurate estimation of lifetime traffic load. Given the cost involved in monitoring a bridge for its lifetime, traffic measurements are generally only collected for a limited period of time. In this study a month of traffic measurements, collected by the SiWIM® system, were used to simulate traffic flow scenarios based on the observed patterns in the measured traffic. Using this so-called “scenario modelling”, accurate projections of traffic flow can be simulated by examining the nature of the actual traffic at the chosen site. This approach is extremely useful for carrying out a fatigue calculation, as accurate site-specific traffic loading can be simulated for the duration of the service life of the bridge.
The combination of a validated numerical model of the bridge and the actual traffic loading allowed a detailed prediction of the present fatigue damage of the bridge and an estimate of the remaining fatigue life. The comparison between damage resulting from scenario modelling versus the EuroCode load model clearly indicated that the EuroCode Load model is excessively conservative and using the virtual monitoring method which is site-specific, can facilitate maintenance optimisation and lifecycle cost reductions through avoidance of unnecessary repair and/or replacement on serviceable bridge structures. This approach provides bridge owners/managers with an important decision making tool when deciding on possible future rehabilitation/renovation strategies.