GNSS (GPS) Monitoring of Dynamic Deflections of Bridges: Structural Constraints and Metrological Limitations

Stathis C. Stiros

doi:10.3390/infrastructures6020023

Abstract

The advent of modern geodetic satellite techniques (GNSS, including GPS) permitted to observe dynamic deflections of bridges, initially of long flexible ones, and more recently of short, essentially stiff bridges with modal frequencies > 1 Hz, and with small SNR (signal-to-noise ratio), even SNR < 1. This was an enormous progress, but not without problems. Apart from monitoring results consistent with structural models, experimental data and serviceability criteria, there exist some apparently unexplained cases of stiff bridges for which there have been claimed apparent dynamic deflections too large for common healthy structures. Summarizing previous experience, this article: (i) discusses structural constraints, experimental evidence, and serviceability limits of bridges as constraints to GNSS monitoring; (ii) examines a representative case of careful monitoring of a reinforced concrete road bridge with reported excessive dynamic deflections; and (iii) explains such deflections as a result of a double process generated by large reflective surfaces of passing vehicles near the antenna; first corruption/distortion of the satellite signal because of high-frequency dynamic multipath, and second, shadowing of some satellites; this last effect leads to a modified observations system and to instantaneously changed coordinates and deflections. In order to recognize and avoid such bias in GNSS monitoring, a strategy based on practical rules and structural constraints is presented.

Highlights

Deformations of the deck of bridges, in the form of deflections from their original surface, produces discomfort, even hazard of collapse, and are among the parameters characterizing bridge performance
The aim of this paper is (i) to examine certain constraints in the output and the applicability of GNSS bridge monitoring imposed by structural data; (ii) recognize certain pitfalls of the GNSS monitoring, mostly in form of apparent excessive deflections of stiff bridges; and (iii) explain the causes of such pitfalls and (iv) propose some practical rules to avoid them
Total failure occurred when the vertical deflection reached approximately 300 mm [21]. These results indicate that deflections of structurally healthy, stiff bridges can hardly exceed 1 cm in amplitude, and deflections of the order of tens of centimeters are associated with structural damage

Summary

Introduction

Deformations of the deck of bridges, in the form of deflections from their original surface, produces discomfort, even hazard of collapse, and are among the parameters characterizing bridge performance. Satellite geodetic techniques-GPS [4] This was a real breakthrough, because till for lack of the necessary measuring technology, it was extremely difficult, if not impossible, to measure static and especially dynamic bridge deflections, mostly for long bridges. Till recently, it was possible to measure static deflections of bridges using conventional survey techniques, either through leveling along the deck of the bridge, or through observations at distances of usually up to a few hundred meters from stations not lying on the bridge (for example from the banks of a river or from nearby islets) for distances of up to several hundred meters. The main possibility to measure dynamic deflections at the midspan of a bridge was to use LVDT extensometers

Objectives

Discussion

Conclusion