Bridge monitoring using the HBM measurement technology

Monitoring solution with 472 sensors provides long time control of bridge performance

The Žeželj Bridge was an arch bridge over the Danube River for railway and road traffic in Novi Sad, Serbia. It was built in 1961 by a famous civil engineer, Branko Žeželj. A unique construction of pre-stressed concrete at the time, many people had doubts about the durability of the construction. So, in order to reassure them, Branko rowed a boat under the bridge during the test loads. However, during the NATO bombings in 1999, the bridge was bombed on 12 occasions, and finally, after multiple direct hits, it was destroyed on 23.4.1999. It was the last standing bridge in Novi Sad during the NATO campaign, the “Operation Noble Anvil”.

A new bridge that connects people, countries and economy

A New Žeželj Bridge is being built again and the construction is expected to be complete by the end of 2017. Its design will remain similar to that of the old bridge, but the material will consist of steel arches. It will be 474 m long and will have two railway tracks for an international corridor as well as two road traffic lanes and two bicycle and pedestrian lanes. The European Union is participating in the funding of this project since it is a vital transportation line for the Balkan states and EU.

Modern designs require efficient monitoring concepts

The behavior of railway and traffic bridges requires the most complex monitoring system due to the varying type and intensity of loads and also because a bridge's structural safety is important in everyday life. The Department for Civil Engineering and Geodesy (FTN - The Faculty of Technical Sciences, Novi Sad) in cooperation with TRC PRO (Technical Research Center) have been using HBM’s measurement technology for the safe construction of the new Railway Traffic Bridge. Structural Health Monitoring is performed through a set of activities that includes observation, data acquisition, transfer and analysis of data acquired by long time measurement during the bridge's exploitation. The goal for monitoring the bridge’s structural health is to form a database for tracking the behavior of the bridge's structure in order to avoid any potential deterioration in the bridge's safety and performance (the bearing capacity, stiffness, serviceability and durability).

Reliable sensor technology

The basic instruments used for the measurement are strain-gauges, which are attached directly to the structure of the bridge or as sensing element in the force sensors. Strain-gauge based sensors have the advantage of high long-term stability. This is important in this project because they are operational during the whole lifetime of the new bridge and it is not possible to stop the monitoring for recalibration. In addition to this, the HBM strain gauges of the LY41-series, which are used in this application, offer a good temperature compensation, which is a must, as the structure is faced with several weather and temperature conditions. For data acquisition, HBM's PMX system is being used. After the installation, the measuring point is covered to prevent damage and electromagnetic interference (EMI).

Critical bridge sections

This project comprises of a distributed multichannel configuration for permanent observation of the critical bridge sections:

Experimental stress analysis (durability testing): 4 mid span zones, 8 support zones and 2 zones during launching;
Force measurement in hangers: 12 hangers;
Vertical displacements: 12 mid span zones and 8 support zones;
Longitudinal displacements: 4 support zones;
Inclinations: 12 mid span zones and 9 support zones;
Acceleration, frequency and damping parameters: 12 mid span zones and 9 support zones and
Temperature measurement in the bridge structure: 12 mid span zones and 9 support zones.

472 sensors installed

The total number of installed sensors and transducers is 472:

328 HBM strain gauges (LY41-series) in 14 sections (the experimental stress analysis),
80 force transducers (force monitoring in hangers)
12 displacement transducers in 4 movable support sections (for horizontal displacements),
32 bi-axial inclinometers (for bi-axial inclination and vertical displacement from acceleration measurements) with embedded tri-axial accelerometers
and temperature sensors in 20 structural sections and one section of central pier zone.

Adjustments can be made directly at the measurement point

The measurement and data acquisition and control system has to fulfill several tasks: Fast and reliable measurements, easy set-up, real-time calculations, diagnosis information, no additional software installation and efficient pricing. All this can be achieved with the PMX measurement and control system. As an additional protection against dust and EMI, the PMX data acquisition unit was mounted in a cabinet, which is a cost-effective solution and also includes the power supply. In the first step, all signals are sampled with 19.2 kHz, which guarantees a high measurement-bandwidth and evaluation of the measurement signals. All adjustments to the PMX system are handled via the standard Ethernet-interface and the internal PMX webserver. This solution offers a great benefit: Adjustments can be done directly at the installation point or via the Ethernet network at the control room or even remotely if additional WiFi is available. So each engineer is able to have a real-time view of the application and the status of the test. An integrated operator-control prevents unsecure operations.

Data storage concept

The requirements captured several tasks. For long-term monitoring, all data from the PMX systems are transmitted via the Ethernet-network in the bridge to a data-server for evaluation and control. But, as a backup, each PMX has an internal data-logger and stores the data with a time-stamp in its internal memory. So the measurement-data won’t get lost in case a breakdown of the Ethernet network. After the re-establishment of the network, the data can be transferred from each PMX station to the data-server.

Building a condition monitoring network

The PMX monitoring systems run all over the bridge, which is a distance of approx. 500 meters. But to get valuable measurement results, the sample rate of each PMX system and each measuring channel must be synchronized. Therefore, each PMX system offers an option of synchronization.

Powerful real-time calculation with the PMX Smart functions

In the second step, the desired and relevant information has to be calculated in real time. This can be done with the Smart-functions of PMX, which can be used easily by the operator without having any advanced software skills. The Smart-functions include several mathematical functions like a pocket calculator, logical functions, process evaluations up to controller functions like PID-controller.

Real-time diagnosis with PMX monitoring system

A detailed and deeper diagnostic is stored in the PMX Log-file. This file is stored in the internal memory of the PMX. It covers all upcoming errors in the device and measurement, as well as any changes in the parameterization done by the operators. This allows a 100% coverage of the entire testing and measurement process.

Data collection and evaluation

The engineer team defined in the requirements an easy-to-use, robust and powerful DAQ and evolution software. “The subsystem for data processing (multipurpose computer system for data acquisition, transfer and processing of data) acquired from HBM’s PMXs uses a custom developed user friendly HBM software for data presentation in HBM catman”, says Hotimir Licen of TRC PRO. With catman, the complete data storage and visualization can be realized. Different graphical user-panels and data-loggers can be set-up for different monitoring tasks. In addition, an event monitoring system with combined email send-out or push-notification to a mobile phone is possible. This offers an immediate feedback from the monitoring task and the controller of the bridge can react and set the right traffic sign to regulate the traffic on the bridge to lower the stress on the bridge.

“We found the PMX WebServer to be a very helpful tool for parameterization, setup and control. We didn’t need any additional software and could use the standard WebBrowser on our PCs, tablet or smartphone. Moreover, it is permanently available in real-time over the whole bridge area”

Hotimir Licen from TRC PRO

Good results also with limited budgets

Since the budget for structural health monitoring was limited, the PMX platform was chosen as a solution due to the excellent price-performance ratio. According to Hotimir Licen from TRC PRO, it offered the following advantages:

Excellent accuracy and high resolution
The ability to measure both static and dynamic events
A universal system for strain gauge and inductive bridge sensors
Stress analysis with hardware temperature compensation of each channel in half bridge configuration.
A distributed system with an Ethernet interface
A synchronized measurement of all channels via NTP time synchronization
A stand-alone operation with data storage on a local USB disk at the PMX system and remote access on a PC with the HBM catman software.