桥梁外文文献.docx

1、桥梁外文文献AUTOMATIC DEFLECTION AND TEMPERATURE MONITORING OFA BALANCED CANTILEVER CONCRETE BRIDGEby Olivier BURDET, Ph.D.Swiss Federal Institute of Technology, Lausanne, SwitzerlandInstitute of Reinforced and Prestressed Concrete SUMMARYThere is a need for reliable monitoring systems to follow the evolu

2、tion of the behavior of structures over time.Deflections and rotations are values that reflect the overall structure behavior. This paper presents an innovative approach to the measurement of long-term deformations of bridges by use of inclinometers. High precision electronic inclinometers can be us

3、ed to follow effectively long-term rotations without disruption of the traffic. In addition to their accuracy, these instruments have proven to be sufficiently stable over time and reliable for field conditions. The Mentue bridges are twin 565 m long box-girder post-tensioned concrete highway bridge

4、s under construction in Switzerland. The bridges are built by the balanced cantilever method over a deep valley. The piers are 100 m high and the main span is 150 m. A centralized data acquisition system was installed in one bridge during its construction in 1997. Every minute, the system records th

5、e rotation and temperature at a number of measuring points. The simultaneous measurement of rotations and concrete temperature at several locations gives a clear idea of the movements induced by thermal conditions. The system will be used in combination with a hydrostatic leveling setup to follow th

6、e long-term behavior of the bridge. Preliminary results show that the system performs reliably and that the accuracy of the sensors is excellent.Comparison of the evolution of rotations and temperature indicate that the structure responds to changes in air temperature rather quickly.1. BACKGROUNDAll

7、 over the world, the number of structures in service keeps increasing. With the development of traffic and the increased dependence on reliable transportation, it is becoming more and more necessary to foresee and anticipate the deterioration of structures. In particular,for structures that are part

8、 of major transportation systems, rehabilitation works need to be carefully planned in order to minimize disruptions of traffic. Automatic monitoring of structures is thus rapidly developing.Long-term monitoring of bridges is an important part of this overall effort to attempt to minimize both the i

9、mpact and the cost of maintenance and rehabilitation work of major structures. By knowing the rate of deterioration of a given structure, the engineer is able to anticipate and adequately define the timing of required interventions. Conversely, interventions can be delayed until the condition of the

10、 structure requires them, without reducing the overall safety of the structure.The paper presents an innovative approach to the measurement of long-term bridge deformations. The use of high precision inclinometers permits an effective, accurate and unobtrusive following of the long-term rotations. T

11、he measurements can be performed under traffic conditions. Simultaneous measurement of the temperature at several locations gives a clear idea of the movements induced by thermal conditions and those induced by creep and shrinkage. The system presented is operational since August 1997 in the Mentue

12、bridge, currently under construction in Switzerland. The structure has a main span of 150 m and piers 100 m high.2. LONG-TERM MONITORING OF BRIDGESAs part of its research and service activities within the Swiss Federal Institute of Technology in Lausanne (EPFL), IBAP - Reinforced and Prestressed Con

13、crete has been involved in the monitoring of long-time deformations of bridges and other structures for over twenty-five years 1, 2, 3, 4. In the past, IBAP has developed a system for the measurement of long-term deformations using hydrostatic leveling 5, 6. This system has been in successful servic

14、e in ten bridges in Switzerland for approximately ten years 5,7. The system is robust, reliable and sufficiently accurate, but it requires human intervention for each measurement, and is not well suited for automatic data acquisition. One additional disadvantage of this system is that it is only eas

15、ily applicable to box girder bridges with an accessible box.Occasional continuous measurements over periods of 24 hours have shown that the amplitude of daily movements is significant, usually amounting to several millimeters over a couple of hours. This is exemplified in figure 1, where measurement

16、s of the twin Lutrive bridges, taken over a period of several years before and after they were strengthened by post-tensioning, areshown along with measurements performed over a period of 24 hours. The scatter observed in the data is primarily caused by thermal effects on the bridges. In the case of

17、 these box-girder bridges built by the balanced cantilever method, with a main span of 143.5 m, the amplitude of deformations on a sunny day is of the same order of magnitude than the long term deformation over several years.Instantaneous measurements, as those made by hydrostatic leveling, are not

18、necessarily representative of the mean position of the bridge. This occurs because the position of the bridge at the time of the measurement is influenced by the temperature history over the past several hours and days. Even if every care was taken to perform the measurements early in the morning an

19、d at the same period every year, it took a relatively long time before it was realized that the retrofit performed on the Lutrive bridges in 1988 by additional post-tensioning 3,7,11 had not had the same effect on both of them.Figure 1: Long-term deflections of the Lutrive bridges, compared to defle

20、ctions measured in a 24-hour period Automatic data acquisition, allowing frequent measurements to be performed at an acceptable cost, is thus highly desirable. A study of possible solutions including laser-based leveling, fiber optics sensors and GPS-positioning was performed, with the conclusion th

21、at, provided that their long-term stability can be demonstrated, current types of electronic inclinometers are suitable for automatic measurements of rotations in existing bridges 8.3. MENTUE BRIDGESThe Mentue bridges are twin box-girder bridges that will carry the future A1 motorway from Lausanne t

22、o Bern. Each bridge, similar in design, has an overall length of approximately 565 m, and a width of 13.46 m, designed to carry two lanes of traffic and an emergency lane. The bridges cross a deep valley with steep sides (fig. 2). The balanced cantilever design results from a bridge competition. The

23、 100 m high concrete piers were built using climbing formwork, after which the construction of the balanced cantilever started (fig. 3).4. INCLINOMETERSStarting in 1995, IBAP initiated a research project with the goal of investigating the feasibility of a measurement system using inclinometers. Prel

24、iminary results indicated that inclinometers offer several advantages for the automatic monitoring of structures. Table 1 summarizes the main properties of the inclinometers selected for this study.One interesting property of measuring a structures rotations, is that, for a given ratio of maximum de

25、flection to span length, the maximum rotation is essentially independent from its static system 8. Since maximal allowable values of about 1/1,000 for long-term deflections under permanent loads are generally accepted values worldwide, developments made for box-girder bridges with long spans, as is

26、the case for this research, are applicable to other bridges, for instance bridges with shorter spans and other types of cross-sections. This is significant because of the need to monitor smaller spans which constitute the majority of allbridges.The selected inclinometers are of type Wyler Zerotronic

27、 1 9. Their accuracy is 1 microradian (rad), which corresponds to a rotation of one millimeter per kilometer, a very small value. For an intermediate span of a continuous beam with a constant depth, a mid-span deflection of 1/20,000 would induce a maximum rotation of about 150 rad, or 0.15 milliradi

28、ans (mrad).One potential problem with electronic instruments is that their measurements may drift overtime. To quantify and control this problem, a mechanical device was designed allowing the inclinometers to be precisely rotated of 180 in an horizontal plane (fig. 4). The drift of each inclinometer

29、 can be very simply obtained by comparing the values obtained in the initial and rotated position with previously obtained values. So far, it has been observed that the type ofinclinometer used in this project is not very sensitive to drifting.5. INSTRUMENTATION OF THE MENTUE BRIDGESBecause a number

30、 of bridges built by the balanced cantilever method have shown an unsatisfactory behavior in service 2, 7,10, it was decided to carefully monitor the evolution of the deformations of the Mentue bridges. These bridges were designed taking into consideration recent recommendations for the choice of th

31、e amount of posttensioning7,10,13. Monitoring starting during the construction in 1997 and will be pursued after the bridges are opened to traffic in 2001. Deflection monitoring includes topographic leveling by the highway authorities, an hydrostatic leveling system over the entire length of both br

32、idges and a network of inclinometers in the main span of the North bridge. Data collection iscoordinated by the engineer of record, to facilitate comparison of measured values. The information gained from these observations will be used to further enhance the design criteria for that type of bridge,

33、 especially with regard to the amount of post-tensioning 7, 10, 11, 12, 13.The automatic monitoring system is driven by a data acquisition program that gathers and stores the data. This system is able to control various types of sensors simultaneously, at the present time inclinometers and thermal sens