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Citation Information : Architecture, Civil Engineering, Environment. Volume 11, Issue 4, Pages 69-77, DOI: https://doi.org/10.21307/ACEE-2018-055
License : (BY-NC-ND-4.0)
Received Date : 25-June-2017 / Accepted: 04-October-2018 / Published Online: 08-February-2019
In this paper a system of the Rędziński bridge will be described. The Rędziński bridge is the biggest object along the A8 motorway around the city of Wrocław and the biggest concrete cable-stayed bridge in Poland. For the purposes of the bridge monitoring a system of 222 sensors was installed. Results from the first 5 years of work of the SHM will be presented below.
For over the last decade an intensive development of SHM systems has appeared in Poland and some of them are installed in the following bridges:
– the Solidarity Bridge over the Vistula River in Płock (2007), which is the biggest cable-stayed bridge in Poland, made of steel;
– the John Paul II Bridge over the Vistula River in Puławy (2008) – one of the largest arch bridges in Poland, made of steel;
– the Rędziński Bridge (Fig. 1) over the Odra River in Worcław (2011), which is the biggest concrete cablestayed bridge in Poland, constructed along the motorway A8.
Furthermore SHM systems are not only installed on bridges. Under a constant observation is also the roof structure of the National Football Stadium in Warsaw or the road surface on motorway A4 which is built in the region of underground mine damages.
The Rędziński Bridge  was open to traffic on 31st August 2011 and is the biggest bridge along the motorway ring-road of Wrocław. It is a four-span cablestayed bridge situated over the Odra River. The spans are 50 m + 2 x 256 m + 50 m long (Fig. 2) The two separated concrete decks are connected to a single 122 m high concrete pylon located on the Rędziński Island. The stay cable system consists of 160 stays. The decks were built with the longitudinal launching method .
For the purposes of the bridge monitoring a system of 222 sensors was installed (Fig. 4). The system is saving data concerning stresses in the concrete elements like the pylon and the decks, it is measuring the forces and accelerations in 80 cable-stays, furthermore it is collecting data about the temperature in the bridge elements with comparison of the current weather conditions. All parameters are measured at the same time and saved in 6 local servers. The dynamic values are registered with the frequency of 100 Hz. The database is available via internet through a professional application. The application (Fig. 5) is equipped with an alert module that informs the user about some dangerous or strange behaviours of the bridge elements. Moreover, it is equipped with a 3D-model of the bridge, where the user can check the precise location of each sensor.
In 2016 a first overview of the registered data from August 2011 till December 2015 was made . The analysed sensors were divided into 3 groups:
– for the cable stayed system: forces, temperatures and acceleration sensors,
– for the deck: stresses, temperatures and acceleration sensors,
– for the pylon: stresses, temperatures, acceleration and displacements sensors.
Some of the results are presented below in detail.
The cable stayed system is equipped in 80 force sensors . 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.
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.
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,
– on the pylon’s cross-beam, sensors P17-L/Tt/Y, P17-P/Tt/Y,
– on the top of the pylon, sensors: P30-L/Tt/Y, P30-P/Tt/Y.
In the pylon surface (X in the sensor code) only rotation on the top were measured:
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 . 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.
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 3rd of April 2016 and 16th of April 2016.
All measurements were taken in real weather conditions and under real loads by the SHM system. Such an overview gives the opportunity to compare the measured values (stresses in concrete and steel elements, the displacements of the pylon and the deck, the change of forces in cable stays) with each other. A long term observation of the force in cable stays with an additional dynamic analysis made with an FEM-model can be a first assessment of the fatigue durability of steel in these structural elements. SHM systems are an innovative research method, because they not only give the opportunity to a constant supervision of the bridges, but also enable the engineers and researches to work with reliable measured data. Such investigations are a valuable contribution to modern civil engineering.