State of the Art Review on Bridges Structural Health Monitoring (Applications and Future Trends)
New bridges offer opportunities for developing complete structural health monitoring systems for bridge inspection and condition evaluation of the bridges. Existing bridges provide challenges for applying state of the art in structural health monitoring technologies to determine the current conditions of the structural element, connections and systems, to formulate model for estimating the rate of degradation, and to predict the existing and the future capacities of the structural components and systems. Advanced health monitoring systems may lead to better understanding of structural behavior and significant improvements of design, as well as the reduction of the structural inspection requirements. Great benefits due to the introduction of structural health monitoring are being accepted by owners, managers, bridge engineers, etc. This chapter provides health monitoring applications to bridges in different spots of the world as well as future trends and research needs. Probably, the most advanced applications of health monitoring are developed in the western parts of the world, specifically, in the United States and Canada. In the following sub sections, the recent applications of health monitoring techniques in North America are presented. PROC EEDIN GS OF THE 11 H ICCAE 11 CON FERENC E, 19 21 APRIL, 2016 WS(1) 4/4 2 1APPLICATIONS IN UNITED STATES Alampalli, et. al. [Error! Bookmark not defined. studied continuous monitoring of two steel bridges over the Conrail mainline tracks in Rochester, N.Y. These bridges were built in 1963. The monitoring system was included as a part of rehabilitation contracts. Altogether five inclinometers, 22 accelerometers, and five strain gauges were installed in these two bridges. All these were connected with to a remote host computer. Natural frequencies, mode shapes, damping ratios, modal assurance criteria, etc. were then computed for use in condition monitoring and assessment. A long term ambient vibration survey on the Fred Hartman Bridge in Texas (fig 1) is in progress primarily to monitor stay cable vibration [1] Other objectives include efforts to better understand the overall wind performance and the modal characteristics of the structure. The bridge site is prone to extreme climate conditions, such as high speed winds, rain, thunderstorms and temperature changes throughout the test program. The instrumentation system consists of the following components: (1) two three axis anemometers at the deck level, (2) propeller vane anemometer at the south tower top, (3) 19 two axis accelerometers installed on stay cables, (4) 8 displacement transducers installed on stays, (5) 8 strain gauges measuring strains on guide pipes, (6) 4 dampers on various cables, (7) 2 load cells installed on dampers, (8) 5 accelerometers, 4 one axis and a two axis, installed on the bridge deck 4g range) (9) 2 rain gauges (0.01' resolution) (10) Temperature probe and barometer, (11) 4pole Bessel filters set to 10 Hertz, and (12) Windows based Pentium PC with data acquisition and remote communication software. FIGURE 1 FRED HARTMAN CABLE STAYED BRIDGE. All of the instruments were continuously monitored using a remotely interrogated selftriggering system that records files on the basis of exceeding threshold motion and wind PROC EEDIN GS OF THE 11 H ICCAE 11 CON FERENC E, 19 21 APRIL, 2016 WS(1) 4/4 3 levels. The recorded data files contain 5 minute time histories that are sampled at 40 Hertz and are stored on high capacity removable disks for further processing. Approximately 10,000 trigger files have been recorded during the three years of the test program, and more are continuously being collected. Analyzing such a large number of data files demands the extensive use of automated procedures. However, the use of automated procedures must be carefully controlled to ensure that flawed or questionable data are not blindly processed. The recorded files are initially processed to determine the statistical features of each record. These features include the mean, standard deviation and other higher moments of the data as well as 1 minute average wind speeds, wind directions, accelerations and displacements, all of which are automatically added to a database. The second stage of processing involves the statistical determination of modal parameters by means. Each 5 minute time history is divided into 1 minute sections, which are processed to obtain the power spectrum of deck and stay accelerometers. One of the primary tasks in this process was the identification of structural frequencies and mode shapes under a wide range of meteorological and operating environments. fig 2 shows the histogram of deck frequencies for modes up to 1 Hz that was obtained using this database. The locations of peaks in this histogram indicate the presence of modal frequencies. The modal frequencies are distributed in specific ranges along the frequency axis. Modes 1 and 2 are first bending modes with in phase and out of phase motions of the decks, respectively. Similarly, modes 3 and 4 are differently phased second bending modes. A distribution of 0.05 Hz in frequencies for these modes, corresponding to 17% and 12% of the mean values respectively, can be seen in these histograms. This range of values suggests that the inherent variability of modal frequencies should be carefully considered in applications, which use modal characteristics. FIGURE 2. HISTOGRAM OF DECK MODES [1] PROC EEDIN GS OF THE 11 H ICCAE 11 CON FERENC E, 19 21 APRIL, 2016 WS(1) 4/4 4 In a structural identification effort, the frequencies and mode shape estimates have been compared to a finite element (FE) model of the structure, showing reasonable agreement. Another objective was to obtain measurements of these large amplitude vibrations at full scale. Numerous events of large amplitude vibration were recorded before the installation of a vibration mitigation system. As a preliminary analysis of the collected data, statistics were routinely computed for each channel in every record and stored in a relational database to allow for easy analysis of correlations between the various statistical quantities using queries. Observations of the vibration characteristics and their correlation with wind speed, wind direction, and rainfall have been presented using the statistics in this database. The bulk of the large amplitude responses occur for wind speeds between 16 and 45 km/h; the precise limits vary from stay to stay. Aktan et. al. [2] implemented a health monitoring scheme to Commodore Barry Bridge (fig 3 and 4) They reviewed the design and shop drawings for the bridge, inspection reports and relevant reports and documentation for the bridge to identify the state of the structure including its performance and maintenance history. A site visit was also performed to visually examine and verify the condition and locations of any special or complex member and connection details, retrofits to the structure, boundary conditions, and to establish access requirements for any instrumentation work. A three dimensional FE model of the bridge was, then, constructed to assist with identifying the critical regions and behavior mechanisms of the bridge's structural systems and to estimate the limits of the forces, strains, tilts, displacements and accelerations that may be necessary to measure. The theoretical model was calibrated through system identification procedures to permit reliable simulations based on the data from a health monitoring implementation. The data needed for system identification of the bridge and subsequent calibration of the FE model were obtained from controlled experiments conducted on the bridge. These experiments included ambient vibration monitoring of the through truss spans and a controlled load test using heavy cranes. PROC EEDIN GS OF THE 11 H ICCAE 11 CON FERENC E, 19 21 APRIL, 2016 WS(1) 4/4 5 FIGURE 3. COMMODORE BARRY BRIDGE THROUGH TRUSS STRUCTURE. FIGURE 4. 3D ANALYTICAL MODELING OF THE BRIDGE SUBSTRUCTURE. The obvious and less understood phenomena that would be monitored for a successful and long term health monitoring implementation were determined. It is important to recognize the possible impacts of humidity, wind, temperature, radiation, long term movements, tilts, slips and settlements on the intrinsic strains and forces. Non linearity and boundary and continuity conditions and energy dissipation mechanisms were recognized in the design of the health monitoring system. The sensing and data acquisition systems for the measurements were selected based on the phenomena needing measurement. The individual sensor and data acquisition components were selected from a number of proven off the shelf sensors, signal conditioning and data acquisition systems based on their physical, electrical and thermodynamic behavior data. It is imperative that these data were verified through calibration studies to permit reliable interpretation of the acquired measurements. The Commodore Barry Bridge health monitoring system integrated streaming digital video PROC EEDIN GS OF THE 11 H ICCAE 11 CON FERENC E, 19 21 APRIL, 2016 WS(1) 4/4 6 images that monitor the traffic moving over critical areas of the bridge and temperature, displacement, tilt, strain and acceleration measurements. Being connected to a local area network, the Commodore Barry Bridge health monitoring system permits a combination of continuous, event based and time based programmable as well as manually controlled on line data acquisition modes. Video cameras, wind, temperature, radiation and humidity measurement, vibrating wire based displacement, tilt and strain measurement and the high bandwidth strain, displacement and acceleration sensors for high speed responses are interrogated by a dedicated data acquisition system. Wireless operation is a simple step from the copper optical fiber network communication mode. The system is designed to operate in a programmed mode in which the in