Galecopperbrug (case study)
The Galecopperbrug (located in Utrecht, The Netherlands) is an important bridge in the Dutch road network (A12). It is a steel cable-stayed bridge with a span of 240 meters in total, of which 180 meters over the Amsterdam-Rijnkanaal. Altough one speaks about the Galecopperbrug, it are basically two separate. The first one, holding the traffic from west to east, was constructed in 1971. The second bridge was constructed in 1976. Both bridges hold six lanes of traffic (devided over a main road and a parallel road).
An important aspect of this bridge are the staycables. Each bridge consists of 2 pylons, both supporting 2 stay-cables. These stay-cables start at the anchorage just below deck at the abutment, pass the top of the pylon and are anchored again at a second anchorage below deck (above water). Each stay-cable consists of 6 locked-coil cables (76 mm diameter). These locked-coil cables on their turn are constructed out of 200+ wires of 4-8 mm in diameter.
The case study described here is related to the detection and monitoring of possible wire breaks. The capacity of the bridge is largely determined by the capacity of the stay-cables. The capacity of the stay-cables on their turn depends on the number of wires of the strands that are still intact. Regular inspection showed that, due to corrosion caused by rain, wires of the stay-cable can break and that wires have broken in the past. When multiple wires break, this affects the capacity of the strand. To ensure a sufficient capacity of the strands, frequent inspection of the strands takes place. This inspection is done visually, and is only done by examining the outer layer of the strands in the last three meters of the strand. On top of this, after detection of the first corrosion a climate chamber is built around the ends of the strands, thereby making it almost impossible for water to intrude. By keeping the water out, in combination with sustaining correct humidity and temperature, it is tried to prevent additional corrosion and thus additional wire breaks. In addition to the visual inspection also acoustic monitoring takes place. This monitoring system is installed in order to detect a wire break, based on the assumption that a wire break will lead to a high amount of (acoustic) energy in the strand. Apart from the acoustic monitoring system some additional measurements and monitoring has taken place in the past: 1) bypass are placed at a few strands. These bypasses consist of two clamping parts that are clamped onto the strand using bolts. Two steel bars are attached to this clamp and to the anchorage, thereby taking over a part of the force in the strand. The forces in these bolts, as well as the forces in the two steel bars, are continuously monitored. 2) force measurements are performed in the past, to determine the force in the stay-cables. To this end the taut-string method is applied. For those measurements the stay-cable is brought into vibration, and the vibration of the cable is measured using accelerometers. 3) multiple strain gauge measurement campaigns have taken place in the past.
Visual inspection of the strands still takes place frequently. On top of this an acoustic monitoring system is installed to detect acoustic activity in the strands. Each strand is equipped with one sensor at its end. These sensors are monitored continuously, and if activity above a certain threshold level (traffic noise) is detected, the signal is stored for later evaluation. By comparing the measured signals with prior knowledge on acoustic energy due to wire breaks (literature, lab testing), the acoustic signals that indicate a wire break can be isolated. If the acoustic signals give reason to, an additional visual inspection can take place. As described in the previous section, apart from the acoustic monitoring and visual inspection regarding wire breaks, additional measurements have taken place in the past. Cable forces are measured using the taut string theory and the forces in the bypasses are continuously monitored using ring force meters.
This case study was contributed by Stefan Verdenius of TNO. Last edited by technical staff.