Naardertrekvaart
Measuring on a bridge is essential to ensure safety, detect early signs of wear and optimize maintenance. It helps to identify structural problems before they become serious, thereby extending the life of the bridge. Continuous measurement supports cost-effective maintenance and public safety.
Many bridges built around 1960 used a half-joint system, which has several advantages, such as reducing internal stresses due to differential settlement of the piers, while maintaining a level surface. Over time it has become apparent that this type of construction can unfortunately cause problems.
In order to gain a better understanding of the behaviour of half-joint bridges, a Structural Health Monitoring (SHM) system was installed on the Naardertrekvaart bridge to provide live measurements and an early warning system. The bridge was equipped with an SHM system in 2021.
Somni, together with CT deBoer, was given the opportunity to install this system by De Jong Zuurmond and RWS (the Dutch authority for infrastructure and water management in the Netherlands).
The construction
The Naardertrekvaart bridge is built using a so-called half-joint construction. Half-joint construction is a structural technique commonly used in bridge engineering to create a connection between two concrete elements, such as a bridge deck and supporting beams, in a way that efficiently manages load distribution and accommodates structural movement. In this method, a notch or 'half joint' is made in one of the concrete elements into which a corresponding part of another element fits, forming an interlocking connection.
The half-joint allows a slimmer design and can reduce the overall height of the structure, which is particularly beneficial in cases where clearance is limited, such as over roads or rivers. This technique also improves the aesthetic appearance of the bridge by maintaining a slim profile without compromising structural integrity.
However, the use of half-joints presents significant challenges in terms of durability and maintenance. The design creates stress concentrations and potential weak points that can be susceptible to cracking, water ingress and corrosion, particularly in aggressive environments. These factors require careful design considerations, including adequate reinforcement, drainage systems and protective measures, to ensure the long-term performance and safety of the bridge.
System setup
The system consists of a number of components. The most important of these are the tilt sensors and the displacement sensors.
The tilt sensors are mounted under each support beam (11 per table side) on the lower lip of the half joint. In this way, an incipient crack can be detected immediately as the support lip begins to make a small angular displacement when an incipient crack occurs. In this installation, the tilt sensor can even detect an angular rotation of 4 milli-degrees.
To make the system even safer, four displacement sensors are also installed on each side of the table to detect the vertical displacement of the lip in relation to the table.
As with all Somni sensors, these are both full fiber optic sensors. This means that there is only one fiber optic cable going to these sensors, which ensures a long service life. The sensors are then read by an interrogator. This device sends light to the sensors and measures the wavelength of the reflected light to calculate the reading for each sensor. The data is then transmitted to a computer which prepares the data for storage in the cloud and processes it for possible alarms. The data and alarms can be made available on all types of devices such as computers and mobile phones.
Results
Besides good, robust sensors for such a system, it is also important to get the right data out of the system.
This bridge carries a lot of traffic, which naturally causes vibrations. In many cases it is these vibrations that you want to map and this is what Somni specialises in. In this case, however, they did not want to detect these vibrations. So it was ensured that these vibrations would not interfere with the very small angular displacements that needed to be measured.
Of course, the structure constantly deforms under the influence of weather and temperature. This is also easy to observe with this type of sensor, but also by measuring the temperature at many points on the structure. These variations are also eliminated when assessing the condition of the structure.
All in all, a very fruitful collaboration between all parties has resulted in a very reliable condition monitoring system for this bridge.
Somni Corporation bv
Rotterdamseweg 183C
2629 HD Delft
The Netherlands
+31 702001967