When Bridges Move: GPS-Based Deflection Monitoring

April 1, 2006 By: Gethin Wyn Roberts, University of Nottingham, Xiaolin Meng, University of Nottingham, Chris Brown, Brunel University Sensors

Kinematic GPS trials measure 3D displacements— to milimeter precision — of Scotland's Forth Road Bridge.

Major bridges are designed to accommodate specific levels of traffic and wind loading, as well as to withstand deterioration over time. But it's not unusual for them to experience levels greater than those initially anticipated. As a result of these forces and thermal action, the large structures often move anywhere from a few decimeters to nearly a meter. While engineers use theoretical models to design the structures, they rarely compare the models with actual movement measurements. To factor in these real-world conditions, engineers are turning to a new application of an established technology.

Global Positioning Systems

GPS is a U.S. owned-and-operated satellite-based positioning system. The 28 satellites, orbiting Earth at an altitude of 20,000 km, transmit binary code modulated on carrier waves.

The conventional way of measuring ranges with known coordinates is to measure the time of flight of the timing codes. By knowing the time required for the signal to reach the GPS receiver from the satellite, you can calculate the range, and therefore calculate the coordinate of the GPS receiver based on simultaneous ranges from a number of satellites. This identifies a position accurate to within ~10 m.

Kinematic GPS

In addition to the timing code, survey-grade GPS receivers use the carrier signal to determine ranges. The carrier has a resolution of ~1 mm and a resulting 3D positional precision of ~1 cm.

Carrier phase positioning requires a reference GPS receiver to which the user's GPS receiver is coordinated. A variety of processing methods are used in this approach; one is kinematic GPS. Using this method, the roving receivers, whose coordinates are unknown, are positioned relative to reference receivers with known coordinates. Data processing can be carried out on the fly (OTF) in real time.

Kinematic GPS is ideal for monitoring large structures because it generates 3D coordinates at a typical rate of 10 Hz, producing 3D movement data for specific locations on the structure. In addition, the resulting movement measurements are in absolute terms (i.e., the coordinates can be repeated), and the GPS equipment requires minimal setup and calibration. Other devices, such as accelerometers and strain gauges, take longer to install and, at best, result in relative displacement data (i.e., the coordinates may not be repeatable in future trials).

The GPS-based system provides 3D coordinates with corresponding time, making it possible to derive the frequency of structural movements. Typically, the first natural frequency of long suspension bridges is 0.1 Hz; second- and third-order frequencies have been detected as well. You can analyze deflections in real time and compare them with the limitations of the bridge, and also analyze the frequencies of the movements and compare them with predicted values. GPS-derived data can be used to validate predictive models of the structure, and you can use the constant comparison of the models with the real movements to analyze structural health.

Putting GPS to the Test

From February 8–10, 2005, staff from the University of Nottingham's Institute of Engineering Surveying and Space Geodesy (IESSG) and from Brunel University's (West London) School of Engineering and Design investigated the use of GPS to establish the magnitude and frequencies of a bridge's deflections. The field research was conducted on the Forth Road Bridge, near Edinburgh, Scotland. The bridge has an overall length of 2.5 km and a main span length of 1,005 m. Traffic over the bridge has steadily increased from 4 million vehicles in 1964 to more than 23 million in 2002. In addition, the heaviest commercial vehicles weighed 24 tons in 1964; the current limit is 44 tons.

The project staff conducted trials at seven GPS-receiver locations on the bridge during a 46 hr. period. The bridge's GPS receivers were coordinated relative to two reference receivers located adjacent to the bridge, on the southern end viewing platform. The GPS receivers on the bridge were located on the east side mid span, ¼ span, ¾ span, and ⅜ span, as well as on the west side mid span and the top of the two southern towers. The receivers gathered data at a rate of 10 Hz. In addition, data from nearby Ordnance Survey's Active Station Network were downloaded at a rate of 1 Hz for future processing.

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