Structural Health Monitoring System of a Concrete Cable-Stayed Bridge

The cable stayed system is equipped in 80 force sensors [6]. Over each single deck 20 cables are under the SHM observation. In Table 1 a comparison between the beginning average and the last average force value is presented. Each sensor has its number and code which informs the SHM user about its location. Numbers of the sensors W1-W20 are for the cables from the southern side of the pylon (Prague direction) and numbers W21-W40 are for the northern cables (Warsaw direction; see Fig. 2). The letter L in the sensor code means the left deck, and the P letter means the right deck. Then the letter W means that the sensor is located on the internal cable row of the deck, the letter Z refers to the external cables. The sensor is installed on the reference strand in each cable. The cables were installed using the Isotension method, which guarantees the same force in each strand. If the number of strands in cable is known – a simple calculation allows to define the force in a whole cable.

During the analysis for each measured cable the maximum, minimum and average monthly force value was saved. It was the basis for creating a global overview how the forces in 80 cables have been changing for the first 5 years. Generally a decrease of the force has place. Moreover, during summer the force is increasing, and in winter it is lower again. Figure 6 shows an example how the force in the longest stay-cable is changing. Figure 7 shows the same for the shortest cables.

Figure 6.

Figure 7.

According to the Figures 5 and 6 and Table 1 it is visible, that the decrease of the force in the longest cables is between 6.4% and 11.8%, whereas in the shortest the differences are between 7.0% and 16.8%. The biggest change had place in the middle cable W12-PW and was about 21.7%. In cables W12-LZ and W28-PZ the change was about 0.2%. Some sensors are not working properly, like W24-PZ/F and W32-PZ/F. The decrease of forces in cables is a natural process caused by shrinking and creeping of the concrete elements of the bridge.

Furthermore, the temperature changes of the whole construction in summer and winter are seen as the local extreme values on the diagrams. A similar change is visible in a day/night cycle, which is shown in Figure 8. The change of forces for each cable row is shown in Figure 7.

Figure 8.

The measurements from the first 5 years are a basis for an advanced durability assessment of the cable stays in bridges under live loads.

Between August 2011 and December 2015 the extreme monthly values of the angular displacements were measured. In the orthogonal direction (Y in the sensor code) to the pylon surfaces the displacement were measured on 3 levels (Fig. 10): –

on the bottom of the pylon, sensors: P0-L/Tt/Y, P0-P/Tt/Y,

In the pylon surface (X in the sensor code) only rotation on the top were measured: –

sensors: P30-L/Tt/X, P30-P/Tt/X.

Table 2 shows the comparison with the allowed values. Measured angular displacements are below maximum designed values.

The upper cross-beam of the H-pylon of the Rędziński Bridge is exposed to a big torsion moment. The designer of the bridge decided to construct a steel box inside [2]. Moreover, the cross-beam was pre-stressed with 18 cables. To have a constant overview of the stresses in the structure, sensors were installed inside and outside the box, at the steel and concrete surface. Figure 9 shows the localization of each sensor set. The diagrams in Figure 10 and 11 show, that stresses are slowly increasing in the structure – minus means compressing. The yellow and blue lines describe the values of sensors installed under 60 degrees to the bolt axis. The green line is for the sensor installed in the direction of the cross-beam axis.

Figure 9.

Figure 10.

Figure 11.

The 5 years analysis enables a comparison between temperatures in the main structural elements of the bridge. It is an important issue to the Polish Standards, because there is no information about temperature distribution for cable-stayed bridges. Information from the SHM system can be in this case a basis for creating the national attachments for the upcoming Eurocode edition.

A diagram in Fig. 12 shows how the average temperature changed in the cables, deck and pylon. A period of improper work of sensor is visible – the orange line. In the Polish Standard PN-85/S-10030 for Bridges – the temperature changes for steel elements are from -25°C till 55°C and for the concrete elements from – 15°C till 30°C. The Table 3 shows that the temperatures in deck and pylon were higher than allowed. Furthermore short-term temperature changes are also well visible. The diagrams below show how the temperature is changing in the deck structure and pylon between 3^rd of April 2016 and 16^th of April 2016.

Figure 12.

Figure 13.

Figure 14.