Rok Mesar and Matjaž Sokol (Project manager)

Tuesday, June 27, 2023 · 0 min read

Cestel d.o.o.

Dynamic Structural Monitoring of a Highway Bridge

Bridges are significant elements of our everyday life. They connect people and enable the vital flow of goods and services. Any damage or, in the worst-case scenario, a collapse can have tragic consequences. Engineers develop new structural health monitoring systems to avoid such scenarios. The Slovenian company Cestel runs an innovative project to dynamically identify the bridge status correlating its structural response and the traffic load. The monitoring system includes Dewesoft sensors.

Below the main road bridge at Tomačevo is a small world normally only visited by the occasional dog walker or jogger. Plenty of tags cover the bridge pillars. This is where the guys from Cestel have a job to do.

Cestel is a Slovenian-based company. It is one of the world’s leaders in high-speed weigh-in-motion, traffic analysis, and bridge assessment. In 2022, the company started a structural health monitoring research project using a novel approach to understand the performance of bridges over time. Cestel dynamically identifies the bridge status based on the correlation between the structural response - natural frequencies, modal shapes, and damping ratios - and the registered traffic load.

Matija Mavrič, head of sales and marketing at Cestel, says: “We always strive for improvements and are developing new applications to upgrade the existing technology and its usefulness. Our Bridge WIM Diagnostic solution will be unique. In addition to providing information about vibrations, bridge movements, cracks, and weather effects, it also gives information about the weight of the trucks on the bridge.

Bridges are aging around the world and Bridge owners are wondering if they are still safe. That is why we need permanent structural health monitoring. Structural health monitoring systems are usually based on sets of various sensors and devices:

  • Data acquisition devices.

  • Sensors (accelerometers, temperature sensors, strain gauges, wind sensors, etc).

  • Software for calculating and presenting the data collected. 

Following some tragic bridge collapses in recent years, the issue is becoming a high priority worldwide. The European Union has initiated the BIM4CE project - Bridge monitoring using real-time data and digital twins for Central Europe. The project is part of Interreg, a European cross-border initiative for regional development in nine EU Member States: Austria, Croatia, Czech Republic, Germany, Hungary, Italy, Poland, Slovakia, and Slovenia.

The issue

Regular maintenance is crucial for ensuring the safety and longevity of bridges. Neglected maintenance can lead to structural defects and other issues that can compromise bridge integrity and increase the risk of collapse. Regular inspection or monitoring is necessary to address problems before they become critical and potentially catastrophic. 

A combination of issues most often causes bridges to collapse. They might relate to insufficient maintenance or ageing, construction or design flaws, manufacturing errors, or unexpected structural or design-related issues. In other cases, collapses happen due to accidents, fires, or natural phenomena like floods or earthquakes.

Some recent bridge collapses

Bridge collapses are dangerous and can result in injury, loss of life, and damage to property and infrastructure. Bridges must be designed, built, and maintained properly to ensure their safety and prevent collapses from occurring.

Recent examples of notable bridge collapses include:

  • Morbi, India (2022) - In October, a pedestrian bridge, a century-old cable suspension bridge across the river Machchhu collapsed, killing 135 people and injuring many more. 

  • Øyer, Norway (2022) - In August the Tretten bridge carrying county road 254 over the river Gudbrandsdalslågen collapsed after only ten years and two months in operation. 

  • Weymouth, UK (2020) - In June, a 90-year-old footbridge collapsed, causing several people to fall into the water below. Fortunately, no one was seriously injured.

  • Mirepoix-sur-Tarn, France (2019) - In November, a suspension bridge for pedestrians and bicycles over the Tarn River collapsed, killing two people. A truck exceeding the weight limit was crossing at the time of the collapse.

  • Nanfangao, Taiwan (2019) - In October, the Nanfangao Bridge collapsed, killing six people and injuring several others. The collapse caused significant disruption to the local fishing industry.

  • Mumbai, India (2019) - In July, a portion of the Gokhale Bridge collapsed during heavy rains, killing two people and injuring several others. The collapse caused significant traffic disruptions.

  • Venice, Italy (2018) - In October, a portion of the historic Ponte Morandi collapsed, but fortunately, no one was injured.

  • Genoa, Italy (2018) - The Morandi Bridge collapsed in August, killing 43 people. The collapse caused significant disruption to traffic and transportation.

  • Miami, USA (2018) - In March, a pedestrian bridge under construction at Florida International University in Miami collapsed, killing six people and injuring several others.

These incidents demonstrate the potential dangers of bridge collapses and the need for regular inspections, maintenance, and investment in infrastructure to ensure their safety.

The Tomačevo bridge

The Tomačevo Bridge is a two-lane bridge spanning over the Sava River. It is part of road no. 104, one of the most used highways between Ljubljana and its northern suburbs. The bridge consists of two separate structures. There are two lanes in each direction. It is every day crossed by thousands of commuters and heavy vehicles transporting cargo and goods from industrial areas to the capital.

Figure 1. The Tomačevo Bridge - view from the air.

The reinforced concrete bridge was built in 1982. It has seven spans and is 204 m long. Each structure is 11 meters wide and carries two lanes for vehicular traffic, a sidewalk, and a bicycle lane. Structurally, the bridge consists of:

  • Eight lines of concrete piers.

  • A concrete deck.

  • Two concrete beams.

  • A sidewalk.

  • A parapet.

Figure 2. Tomačevo bridge longitudinal section.
Figure 3. Tomačevo bridge cross-section.

Bridge instrumentation

Bridge monitoring provides several benefits that can help ensure the safety and longevity of bridges. Here are some of the main benefits:

  • Early detection of structural problems: Detecting structural issues before they become severe allows engineers to take corrective action before the problem worsens.

  • Improved maintenance planning: The data can help engineers prioritize maintenance activities and ensure the effective use of resources.

  • Enhanced safety: Monitoring can help identify potential safety hazards before they become critical. 

  • Cost savings: By catching problems early, engineers can address them before the fixing becomes more costly and reduce expensive repairs or replacements. 

  • Improved data collection and analysis: Data on the performance and behaviour of a bridge helps refine design and construction techniques for future projects.

Overall, bridge monitoring is an important tool for ensuring the safety and longevity of bridges. It provides valuable insights that can help improve the design and construction of future infrastructure projects.

The Structural Bridge-WIM Diagnostic solution is a new concept bringing WIM sensors and low-noise 3-axial MEMS accelerometers together. 

The instrumentation core of this new bridge monitoring concept is the Cestel SiWIM bridge Weigh-in-Motion (WIM) system in combination with Dewesoft 3-axial accelerometers. The idea is to dynamically identify the bridge status based on the correlation between the structural response and the detected traffic load. Along with the vehicle loads, several other parameters are also monitored:

  • temperatures, 

  • strains, 

  • wind speed

  • vertical displacements.

We installed the structural dynamics and weight monitoring system on one of the bridge’s two directions, under the lanes going towards Ljubljana.

The sensors

SiWIM is a fully portable Weigh-in-Motion system (WIM). The WIM technology enables weighing vehicles while in motion - in free-flow traffic. It offers a non-intrusive and efficient method for collecting data on vehicle weights and traffic patterns. It is used to support a range of transportation and infrastructure management applications. 

This technology involves the installation of sensors on the bottom side of the bridge. These can measure the dynamic tire loads of vehicles passing, and the measurements enable the calculation of the vehicle weights.

Bridge-WIM systems (B-WIMs) apply for various purposes, including commercial vehicle enforcement, traffic data collection, and pavement & bridge management. In commercial vehicle enforcement, B-WIM systems can identify overweight or overloaded vehicles, which can cause damage to roadways and bridges and pose a safety risk to other drivers. The system, combined with a camera, provides overview photos of heavy vehicles, including license plate detection and recognition. 

Figure 4. An IP Camera mounted on the Tomačevo bridge provides overview photos of heavy vehicles. The services can include license plate detection and recognition.

Traffic data collected from WIM systems can provide valuable information on vehicle traffic patterns supporting transportation planning and management. Finally, WIM data allows the assessment of the structural condition of infrastructure and the identification of areas needing maintenance or repairs.

SiWIM generally measures strains on the major longitudinal parts of the bridge to provide records describing the behaviour of the structure under the moving vehicle load. SiWIM ST-504 strain gauge sensors installed on the bottom of the bridge’s road decks supply the measurements. No parts of the system are visible on the road surface. 

Figure 5. SiWIM ST-504 - the Cestel SiWIM sensor.
Figure 6. Typical installation of a SiWIM system.

IOLITEiw-3xMEMS-ACC is a 3-axial, low-noise (25 μg√Hz spectral noise density) accelerometer with integrated DAQ and EtherCAT interface. The device is fully waterproof with IP67 protection. The device can measure structural accelerations in X, Y, and Z, static inclinations, and displacements.

Figure 7. IOLITEiw-3xMEMS-ACC.
Figure 8. IOLITEiw-3xMEMS-ACC installed on the Tomačevo bridge.

Cestel uses a Deflection Multi Meter (DMM) to measure the vertical deflection of a bridge in real-time (10 Hz sampling rate) based on an optical reference level created by a laser and a target. The laser is mounted on a stable (non-moving) surface, while the target is where the deflection is to be measured. The device has a measurement range of 160mm, a resolution of 0,5 mm, and measures a distance of up to 350 meters.

Figure 9. DMM sensor and laser reference.

The Deflection Multi Meter (DMM) is a new level measuring device for monitoring the condition of large load-bearing structures such as bridges.

The measuring device is based on a flat laser, in our case Leica Rugby 830. Combined with DMM units, it measures the deflection of a structure from several points. Real-time measurement makes it unique compared to other measurement methods. The DMM device is suitable for measuring the deflection of a bridge during a test load or for long-term monitoring of the structure.

The sensors are connected in series and read by a computer via an RS-485 cable, which also supplies the system operating voltage. The MODBUS protocol facilitates data transfer, which allows DMM units to connect to other measuring systems. Results are viewable with any measurement program that can work with the MODBUS protocol.

In many applications, DMM units can be attached to the structure with clamps or magnets, making the system very flexible and quick to install.

To monitor ambient and asphalt temperatures PT100 temperature sensors are installed into the asphalt and under the bridge.

Monitoring software

The solution on a combination of several software products:

  • SiWIM is a system to collect traffic data. It identifies the behaviour of a bridge through its actual influence line and load distributions, which are crucial for converting traffic loads into load effects and axle loads into moments and shear forces.

  • DewesoftX is test and measurement acquisition software for signal processing, data recording, analysis, and visualization.

  • Dewesoft Historian is a database software service for long-term and permanent monitoring. It provides storage in an InfluxDB time-series database.

  • Dewesoft Artemis OMA is a software suite for analyzing the structural dynamics of civil engineering structures, operating machinery, and any structure difficult to excite in a controlled manner. A set of modal parameters, such as mode shapes, natural frequencies, and damping ratios, can be determined for the operating structures acquiring only the output response data.

Measurement system setup

Figure 10. Tomačevo Bridge system architecture.

The PT100 temperature sensors connect to the IOLITEi-1xSTG. The IOLITEi-1xSTG is a multipurpose EtherCAT-based signal conditioning system. It can acquire signals from voltage and current, as well as full, half, and quarter bridge output sensors, and has settable voltage and current excitation. 

IOLITEi-1xSTG amplifiers are also used to connect to the SiWIM ST-504 full-bridge strain gauge sensors. 

For measuring accelerations, the system uses low-noise density capacitive triaxial MEMS sensors. The IOLITEiw-3xMEMS-ACC EtherCAT device embeds these sensors. 

A microprocessor inside the IOLITE device transmits the samples. The samples are sent to the DewesoftX software running on Windows or to any controller running EtherCAT master on any platform - in this case, to the SiWIM system.

The DMM sensor outputs a MODBUS signal. It is connected directly to the DewesoftX software through a Dewesoft-Modbus-Client plugin.

The data processed by the DewesoftX and SiWIM systems go to the Dewesoft Historian database. A monitoring client (Graphana) is then used for high-level dashboards that display the monitored data: 

  • temperatures, 

  • traffic loads,

  • accelerations, and 

  • vertical displacements. 

This software is web-based and cross-platform - it can be accessed from any system.

The raw acceleration data from the DewesoftX are sent over FTP to Dewesoft Artemis for real-time OMA and damage detection.

The system includes two 300 Ah Li Fe batteries and four 150 W solar panels for power supply. 

Sensor positions

On large structures like bridges, the selection of sensors, their location, and installation represent several challenges:

  • Location and sensor types are selected based on the design of the bridge and its current state.

  • The placement of sensors is tricky. Usually, installation is done under the bridge deck or, in some cases, embedded into the deck/piers or other elements.

  • Cable routing is complicated due to heights and long distances.

  • The environmental conditions are harsh.

The Italian company, ESSEBI, has been operating for 30 years in structural diagnostics and monitoring. The company’s core business is the implementation of operational modal analyses, i.e., of the output-only type, which it has carried out since the first applications. Essebi now provides significant Dewesoft field support as a project integrator for many SHM plants scattered throughout Italy. 

In this project, Essebi advised on the choice of transducers and the configuration of the measurement points - see Figure 10. In particular, the company focused on the SHM and WIM integration. 

Essebi is developing correlation algorithms for the two types of measurement. Employing the two most popular modal shape extraction programs, Simcenter Test Lab and Artemis, the company in different periods carried out three double-blind modal analyses of the bridge. These analyses highlight a low-frequency variability with temperature significant to track over time to evaluate the maintenance.

Figure 11. Sensor positions.

All in all, Cestel has installed the following sensors:

  • SiWIM ST-504 sensors.

  • One deflection multi-meter (DMM).

  • 14x IOLITEiw-3xMEMS-ACC accelerometers.

  • 5x IOLITEi-1xSTG devices: three units for ST-.504 and two units for temperature sensors.

The IOLITE devices are daisy-chained with a standard Ethernet network cable. EtherCAT protocol allows the devices to be easily distributed across large distances. Devices can span 50m node-to-node with only one cable between them for signal, power, and synchronization. 

EtherCAT communication between devices ensures 1 us synchronization between the samples taken from different devices in the chain. The distance between devices does not influence the precision of the synchronization.

Figure 12. Daisy-chained IOLITE devices.

Installation of sensors and cable rooting was carried out by the experienced CESTEL staff. To reach the most remote places a lifting platform was used.

Figure 13. The team installing the sensors and routing the cables.
Figure 14. SiWIM ST-504, IOLITEiw-3xMEMS-ACC, and laser reference system in situ.

Monitoring and Measurements

Monitoring results, including loads, accelerations, deflections, and temperatures, are presented through the Dewesoft Grafana web interface. 

Figure 14. A screenshot from Dewesoft Grafana shows the truckloads and measurements of deflections, strains, and temperatures.
Figure 15. Data showing a truck of 200+ tons crossing the bridge.

On the 9th of March 2023, a 13-axle truck of +200 tons crossed the bridge at around 8:02 PM. It caused the bridge to deflect around 3,6mm.

Figure 16: The 200+ tone, 13-axle truck crossing the bridge.

The system performs OMA (Operational Modal Analysis) automatically. The data is remotely accessible in web browser interfaces developed for this purpose. Information about modal shapes, natural frequencies and damping ratios, and deviations from normalized values are displayed.

Figure 17. Example of Online Dewesoft-Artemis OMA.


CESTEL has started an innovative research project to dynamically identify the bridge status correlating its structural response and the traffic load. Several sensors, including SiWIM, accelerometers, strain gauges, and temperatures, were installed on the bridge.

Sensors were easily deployable along the bridge due to single-cable daisy chaining. The system stores the data in Dewesoft Historian and makes them available in the Dewesoft Grafana web GUI. For interpretation purposes, it performs online OMA.

For us, the research project means the development and testing of various sensors and software. At the final stage, it will offer the market a unique solution for online structural monitoring of bridges in correlation with axle loads from the traffic flow. The Dewesoft structural health monitoring solutions, including DAQs and software, helped us quickly set up a running demo system and speed up our research. Great tools!

Matija Mavrič, head of sales and marketing at Cestel