LIFE-CYCLE MONITORING AND MAINTENANCE OF BRIDGES: THE ROLE OF REMOTE SHM SYSTEMS

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1 Istanbul Bridge Conference August 11-13, 2014 Istanbul, Turkey LIFE-CYCLE MONITORING AND MAINTENANCE OF BRIDGES: THE ROLE OF REMOTE SHM SYSTEMS K. Islami 1, G. Moor 2 and N. Meng 3 ABSTRACT To minimise costs and inconveniences due to damage during the entire life-cycle of bridges, it is necessary to study in depth the causes of such damage. The standard methods of investigation and safety evaluation are sometimes insufficient, and the demand for improved information and better understanding continues to increase. Structural health monitoring (SHM), in particular, which can be used to measure high-frequency movements, accelerations, settlements, forces, etc., is being increasingly used for such purposes. SHM systems can provide very detailed information on structural behaviour and actual conditions, and enable uncertainties associated with material properties and structural capacity to be reduced. This paper presents recently installed automated SHM systems, demonstrating their usefulness and ease of use and the enormous gains in efficiency they offer over manual monitoring methods. One application relates to the monitoring of a number of bridges in a mining area in Australia. Due to the risk of ground settlements, it was decided that the structures should be remotely monitored by a permanent SHM system with an event notification feature to notify the responsible engineers should predefined threshold values be exceeded during ground settlements. As a result of the use of the SHM system, ground conditions have been confirmed to be stable on an ongoing basis, enabling the bridges to remain open to traffic a typical benefit of SHM in the management and maintenance of bridges. 1 Head of Structural Health Monitoring, Mageba SA, Switzerland. 2 General Manager, Mageba USA LLC, USA 3 Head of Sales, Mageba SA, Switzerland Islami K, Moor G, Meng N. Life-cycle monitoring and maintenance of bridges: The role of remote SHM systems. Proceedings of the Istanbul Bridge Conference, 2014.

2 Life-Cycle Monitoring And Maintenance Of Bridges: The Role Of Remote SHM Systems K. Islami 1, G. Moor 2 and N. Meng 3 ABSTRACT This paper presents recently installed bridge SHM systems which measure high-frequency movements and thus enable a proper understanding of the bridges behaviour to be developed. SHM systems can provide very detailed information on structural behaviour and actual conditions, and enable uncertainties associated with material properties and structural capacity to be reduced. In particular, the measured data is analysed to eliminate the very prominent effects of temperature variations, enabling the lesser effects of traffic etc. to be much more easily assessed and thus facilitating, for example, more reliable damage detection. The case studies presented demonstrate the usefulness and ease of use of modern SHM systems, and the enormous gains in efficiency they offer over alternative manual monitoring methods. Introduction The intrinsic weakness of some structural elements, deterioration occurrences and the updating of structural codes have shown that many existing bridges are structurally inadequate and have a need to be upgraded to comply with current seismic codes. In the case of newer structures, the use of advanced finite element method (FEM) analysis has pushed design and use of materials to the limit, so a direct verification of the actual behaviour of a structure can provide much-needed confidence in a structure s performance. Design of complex bridge structures, largely an analysis-based activity, is today carried out using highly developed computer software, so it is not surprising that other analysis-based activities on the same structures can very often also be optimised using similar technology. Such technology generally takes the form of a Structural Health Monitoring (SHM) system. SHM systems typically consist primarily of a data acquisition unit such as that shown in 1 Head of Structural Health Monitoring, Mageba SA, Switzerland. 2 General Manager, Mageba USA LLC, USA 3 Head of Sales, Mageba Mageba SA, Switzerland Islami K, Moor G, Meng N. Life-cycle monitoring and maintenance of bridges: The role of remote SHM systems. Proceedings of the Istanbul Bridge Conference, 2014.

3 Figure 1, and a number of sensors, together with a power source and data transmission capabilities as may be required. SHM systems may be temporary and portable, for short-term applications, or permanent, for long-term applications. A schematic representation of a permanent system is shown in Figure 2 (Spuler et al, 2012). Figure 1 - Acquisition unit of a typical SHM system, with connections to all sensors Figure 2 - Schematic representation of a typical permanent SHM system Monitoring of the twin suspension bridges of the city of Halifax, Canada When planning renovation works, it is often of great benefit to know precisely how the bridge has performed in the past. Indeed, analysis of the actual bridge can be sure to provide more relevant data than any scale model in a laboratory or computer model on a hard drive. By precisely assessing a structure s condition and diagnosing any problems identified, an SHM system can help optimise the selected solution. The city of Halifax, capital of the Canadian province of Nova Scotia, relies heavily on two structures in particular - the Angus L. Macdonald Bridge and the A. Murray MacKay Bridge (Figure 3), both of which connect the city across the sea inlet that divides it in two. Having

4 been opened to traffic in 1955 and 1970 respectively, both structures have already provided several decades of service. The A. Murray MacKay Bridge was renovated in recent years, and similar renovation works, with similar changes to the bearing support of the bridge deck, are currently being planned for the Angus L. Macdonald Bridge. The bridge is proposed to receive an entire new deck, and computer modeling of the deck, verified by measured data, will play a key role in the design process. Figure 3 - The Angus L. Macdonald Bridge and A. Murray Mackay Bridge It was determined that an SHM system should be used to measure and record the movements and rotations of the deck of the Angus L. Macdonald Bridge at its expansion joints, providing the data needed by the finite element modeling. An example of some of the data provided by the system, for a specified period of time, is presented in Figure 4. This particular data enables rotations and displacements (both longitudinal and transverse) to be easily related to temperature, and to wind strength and direction. Further analyses have been performed on the data of these structures in order to enable the effects of environmental changes on bridge deck displacements to be eliminated if desired for example, to gain a clearer understanding of the effects on deck displacements of other influences such as traffic. Figure 4 - Example of data recorded during a 1-year period The principal movements were correlated to temperature during the monitored period (Figure 5). Generally, all locations show high dependency on temperature, but the presented sensor in particular shows a linear correlation. Consequently, the plot shows a decreasing trend of displacement when temperature decreases. This important aspect of the bridge s behaviour must also be evidenced by the structure s finite element model. To observe the distribution of movements, a very useful means is offered by histograms. Figure 6 shows on a histogram the number of times a value of measurement arose during one

5 year of monitoring at a single sensor. It is also desirable to verify whether the effects of various environmental variables measured in situ influence the static or dynamic behaviour of the structure. Therefore, it is important to eliminate the influence of these factors, so that small changes due to damage can be detected. This is made possible by the use of regression models which can determine the static variables starting from a predefined input. Such techniques of monitoring, of statistical modeling of the response of structures, and gathering and processing data in real time are important, especially in the context of particularly sensitive structures. Measured Displacement vs. Temperature Displ. [m] Temperature [ C] 30 Figure 5 - Temperature effects on displacement measurements Displ. Histogram - Measured - Sensor Times appeared Displ [m] Figure 6 - Histogram with distribution of measurements To improve damage detection at a bridge s expansion joints, environmental effects can be eliminated using a regression model such as that shown in Figure 7, generated from the correlation between temperature or humidity and the movements of the bridge (Islami & Modena, 2013). The resulting movements recorded by the system do not then include movements resulting from temperature or humidity, enabling abnormal influences (due to damage or other unexpected events) to be easily recognised. This is illustrated by Figure 7, which shows on the lower graph, with normalised values, the bridge movements that are due to traffic etc. and not due to temperature. Any abnormal shift on this graph would be immediately recognisable, enabling the need for repair or preventative action to be assessed.

6 The SHM system also provided a very interesting insight into the movements of the deck of the A. Murray Mackay Bridge. It established that the deck moves a great deal in the longitudinal direction, with accumulated movements of up to 25 km per year at a single expansion joint (Figure 8). Such enormous accumulated movements are extremely rare for such structures, and many times higher than the movements experienced by the decks of the great majority of long span bridges. Normalized Values Measures Model -4 Jun 12 Aug 12 Oct 12 Nov 12 Jan 13 Mar 13 Apr 13 Jun 13 Aug 13 Sep 13 Date Measured Frequency -5 Jun 12 Aug 12 Oct 12 Nov 12 Jan 13 Mar 13 Apr 13 Jun 13 Aug 13 Sep 13 Date Date Figure 7 Left: Regression model showing measured and estimated displacement. Right: Displacement, before and after the elimination of environmental effects For comparison, the movements of the existing deck of the neighboring Angus L. Macdonald Bridge are estimated by the same monitoring system to amount to less than 700 m per year at each joint (Figure 9). This gives a strong indication of how the movements of the new deck of the Angus L. Macdonald Bridge will increase if it is supported in the same way as the previously renovated deck of the A. Murray Mackay Bridge. The understanding of deck movements provided by the SHM system will thus play a crucial role in supporting the planning of renovation works of one deck and maintenance and possible adaptation works of the other. For instance, all mechanical parts of the bridge, such as its bearings and expansion joints, must be capable of accommodating and withstanding these extreme movements; and ways of controlling and minimising the movements may also be considered for example, through the use of hydraulic dampers. By recognising and understanding the challenge, the responsible engineers are well equipped to plan and implement effective, durable solutions. Norm. Displacement Norm. Displacement Measured and Environmental Effect-free Displacements Measures without Env. Effects -4 Jun 12 Aug 12 Oct 12 Nov 12 Jan 13 Mar 13 Apr 13 Jun 13 Aug 13 Sep 13 Figure 8 - Accumulated movements (per month) on the A. Murray Mackay Bridge

7 Figure 9 - Accumulated movements (per month) on the Angus L. Macdonald Bridge Monitoring the settlement of Seven Bridges on the Hunter Expressway, Australia The Hunter Expressway, recently constructed in Branxton, New South Wales, is situated in a mining area. The bridges on the new road are at risk of settlement due to mining subsidence, so it has been decided to permanently monitor seven structures. The structures are mainly concrete box girder bridges with lengths of 155m, 155m, 337m, 257m, 257m, 200m and 200m respectively (Figure 10). Figure 10 Views of the monitored structures of the Hunter Expressway

8 The principal purpose of this project is to measure the differential settlements of piers and abutments and the horizontal displacements of abutments. A permanent monitoring system has been installed to fulfil this purpose. The system s principal sensors include: ultrasonic sensors to measure horizontal movements at the abutments; hydrostatic tube balance sensors to measure differential settlements at piers and abutments (Figure 11); and air temperature and humidity sensors and structural temperature sensors in order to relate the movements to environmental effects. All measurements are accurately gathered in a double database and immediately visible in the project s SHM web interface. Figure 12 shows one of the cockpits that the end-user can access day or night. The measurements on two bridges are shown: west and east structures. On each abutment or pier, the following data is measured and recorded: At the abutments: displacement in longitudinal and transverse direction of the expansion joint, settlements and temperature At the piers: settlements relative to the right or left abutment and temperature Additionally, the level of the reservoir is measured to ensure an appropriate accuracy. Figure 11 Installation of the monitoring system inside the concrete box girders of the Hunter Expressway

9 Figure 12 Web interface cockpit of the Hunter Expressway SHM system Not only can the responsible engineers check settlements at any time, they can also view the entire time history of the measurements graphically (Figure 13). Any abnormal settlements will be immediately recognisable, enabling safety precautions to be implemented. To ensure that the information is immediately recognised by the engineers, even when not in the office, the system also includes an alarm feature which will provide immediate notification of such an occurrence by or SMS.

10 Figure 13 Data records for one-month settlement measurements at one bridge Figure 14 Data records for half-year settlement measurements at another bridge

11 It is interesting to observe the displacements and settlements in Figure 14, where six months of measurements are presented. These show that longitudinal displacements at the bridge s expansion joints are highly dependent on temperature, while settlements at every pier and abutment are not influenced by environmental parameters, as might be expected. Happily, since the installation took place, the SHM system has been able to confirm that no significant settlement has occurred. Conclusions SHM systems offer many benefits over traditional manual observation and measurement methods. They are typically much more efficient, having far lower running costs, and are capable of an extraordinary level of detail and accuracy, e.g. in measuring high-frequency vibrations that would scarcely be registered by human touch. They can also be set up to operate 24 hours a day, 7 days a week, for as long as required, and can thus be relied on to immediately record and report unexpected / serious events, no matter when they might occur. Whether used during installation, inspection, maintenance or replacement works, or to facilitate assessment of unexpected events or planned modifications, automated monitoring systems are thus sure to be increasingly used on bridges in years to come. References 1. Chen, C. & Jianchi, Z. 2012, Bridge Design & Engineering (Bd&e) Issue 66. London. 2. Islami K. & Modena C., 2013, Life-cycle assessment by dynamic diagnosis and long term monitoring of old bridges in Northeastern Italy. ISHMII Hong Kong. 3. Spuler, T., Moor, G. & Berger, R Automated structural health monitoring filling a knowledge gap. Proc. 3rd fib International Congress. Washington D.C.