Structural monitoring and modal testing of bridges and viaducts: now it is possible to correlate the response with the real mass in transit

The awareness, also following some tragic collapses, of the state of vulnerability and aging of the large number of works of art present on ordinary or motorway roads, is bringing more and more the attention of these infrastructures’ managers towards IBM ( Intelligent Bridge Monitoring ) systems for the surveillance and monitoring of bridges and viaducts in operation.

Intelligent monitoring systems involve the installation of one sensor network of different types, including accelerometers, inclinometers, strain gauges, displacement sensors, crackmeters, distance meters, etc. that positioned in specific points allow to detect the dynamic response of the structure through the continuous monitoring of the same.

The processing of the large amount of data collected allows to obtain the dynamic identification or modal test (Modal Testing or Modal Identification) of the structure and thus determine multiple real dynamic structural parameters , as generated on the basis of real operating data. Dynamic identification can be carried out according to OMA (Operation Model Analysis) techniques that consider only the response of the “bridge system” or according to EMA (Experimental Model Analysis) techniques that also consider the response of the system, but correlating it to the forcing action insistent on the structure .

It is clear that, in the case of bridges and viaducts, the load represented by the mass in transit instantaneously in the different points of the structure, represents the main component of the forcing action (“system entrance“), but it is equally clear how problematic it is to be able to know this given in a precise, continuous way, referenced in space and time and referring to real traffic rather than to measurement campaigns carried out in a short time interval and often also with a fleet of vehicles in transit specifically prepared for the purpose and not at all representative of real traffic in transit.

Although electronic technology, in its optical and electromagnetic declinations, has for some time now made it possible to detect numerous quantities relating to traffic with devices such as cameras, radar, lidar and coils, allowing to calculate the number of vehicles in transit, their speed and type, none of these systems is still able to determine the mass actually in transit on a stretch of road with precision and continuously over time.

The enabling technology for the resolution of this age-old problem is represented by the use of a weigh-in-motion system, placed on the bridge deck or in the immediate vicinity both upstream and downstream.of the work , which detects in real time and continuously the mass in transit on the totality of the lanes and carriageways characterizing the specific stretch of road network.

Sabbioncello Bridge connecting Croatia to Europe.

BISON weigh in motion solution, the tool that allows you to know anytime the load on bridges and viaducts

The dynamic weighing system BISON, developed and produced by the Italian company IWIM, allows to calculate the real transit mass insistent on the work, thanks to the processing of weight and kinematic data detected by its system. It is therefore finally known both the space-time distribution of the real mass in transit and the composition of the same in terms of the type of means that can generate different tip loading situations.

The user, by accessing a web interface in the cloud, can independently process the load status in the moments of his interest insisting on different sections of the work thanks to spatial goals that can be defined at will and which can therefore represent the entire bridge or the subdivision of the same into spans or sections under examination .

The processing will therefore dynamically return the trend over time of the load insistent on the work , highlighting the transfer of the load from one section of the structure to another in the defined time interval of interest.

Since BISON dynamic weighing system detects the weight of each axle shaft and the geometries of the vehicles in transit in terms of width and distance of the axles, the distribution of the load in transit is represented as a temporal evolution of the contact areas of the individual wheels, also highlighting whether they are single or twin axes.

Thanks to the detection of the weights for each lane, the load distribution will be returned referenced in two dimensions allowing to evaluate the extent of the stress insistent on the system both along a longitudinal plane and transversal to the structure and therefore to the road surface.

Double BISON on the new Genoa Bridge.

In order to allow the structural engineers to perform the temporal and functional correlation between the forcing action and the response of the system through specific modeling software, the result of the processing of the space-time distribution of the mass in transit is made available through a file of data interchange in JSON format that allows a quick and simple interface with the calculation software.

The system for calculating the mass in transit offered by BISON system also produces reports in aggregate form that allow you to finally have knowledge, at a higher level, of the actual total load in transit and highlight individual situations or repetitive patterns overload giving the possibility to highlight specific correlations with the composition of traffic. Such reporting is configured as an irreplaceable support tool in the forecasting and planning of maintenance interventions both in terms of technical and economic resources and inconvenience to traffic. Finally, a further plus is represented by the possibility of correlating the mass actually transited and the wear and tear of the infrastructure in the medium to long term.

Lastly, it is important to highlight how the function of calculating the real mass in transit is not strictly linked to the presence of a work of art such as a bridge or a viaduct, but it can also be used to determine the mass in transit in a spatial interval near the weighing system also on roads on land, in trenches or embankments.