Keeping an eye on the effects of heavy goods traffic: Long-term monitoring on a motorway bridge

Increasing heavy goods traffic is a constant endurance test for the approximately 38,000 bridges on the German motorways and main roads. In order to investigate these effects, the Federal Highway Research Institute (BASt) commissioned the Institut für Massivbau (Institute of Concrete Construction) of the Leibnitz Universität Hannover to carry out measurements on a prestressed concrete box girder bridge. Measurement technology from HBM was used over one year to measure e.g. component temperatures, strain and deformation under various traffic situations.

Research project of the Leibniz Universität Hannover

In order to investigate the effects of current heavy goods traffic on the load-bearing structures of a prestressed concrete box girder bridge, the Federal Highway Research Institute (BASt) commissioned the Institut für Massivbau (Institute of Concrete Construction) of the Leibniz Universität Hannover with structural measurements of the box girder structure of a bridge on the A8 motorway. The A8 runs in three segments from the Luxembourg to the Austrian border near Salzburg and is one of the main West-East connections in southern Germany.

The following questions needed to be answered:

• What effects do the increasingly greater total loads of the heavy goods traffic have?
• What conclusions can be drawn from the determination of the actual traffic compositions on the standard loading evaluations?
• Must recommendations for the updating of standards be drawn up?

Test setup

The measurements were implemented in three measurement cross-sections where component, internal and external air temperatures, concrete strains and relative deformation were recorded. The strain of the traverses and the deformation of the corresponding lamella between the traverses were recorded at two points on the roadway joints (RJ). In addition, the deformation and movement of the pot bearings were recorded.

Laser light barriers were mounted on the existing lateral noise protection walls to detect and determine the speed of heavy goods vehicles. This delivered additional measured values with which the positions of the HGV's in the lanes on the bridge could be determined.

Long distances between sensor and amplifier

In total, the measurement system comprised 96 measurement channels. There were great distances within the structure between sensors and amplifiers. The CANbus system was used to bundle the various measuring points. Four CANHEAD modules with CD 1010 carrier-frequency amplifiers were used here.

The measurement channels came together with various connection cards in the amplifier system MGCplus. DC modules, TF modules and connections for PT100 temperature transducers were combined in the MGCplus. In addition to the MGCplus, two Spider8 amplifiers, connected in series to the measurement computer, were connected via the parallel port of the measurement computer. These carried out the deformation and movement measurements for the bearing bodies. Measurement data acquisition was implemented with the software catman®.

Flexible system

Chartered engineer Jan Peter Liebig from the Institut für Massivbau of the Leibniz Universität Hannover on the selection of the measurement technology:

"The decision to use the MGCplus was based on the number of measurement channels required. In addition, the MGCplus system is able from the start, with the use of CANHEAD modules, to bundle numerous measurement channels and transmit data across greater distances. This measurement system also allows us to bring together various sensor signals into a amplifier system due to the flexible use of plug-in cards. The combination of MGCplus and Spider8 was seen as most practical because the catman® software gave us the possibility of combining various amplifiers."

Online-monitoring of measurements via GSM

The measurement period was set for one year. As the measurement object is not in the vicinity of the Leibniz University of Hannover, it needed to be possible to monitor the measurements remotely and, where necessary, to influence them. Two GSM modules were integrated in the measurement system for this purpose. One module is used here for separate control of the power supply for the measurement computer and amplifier, the other for the online-monitoring of the measurements.

1.4 GB measurement data per day

The measurement sensor signals are recorded in three measurement rate groups. A measurement frequency of min. 1200 Hz is required for the very short-term actuation of the RJ. Strains and deformations are recorded with 50 Hz and temperatures with 1 Hz. The measurement data with a channel depth of around 100 million measured values runs to around 1.4 GB per day, so that regular monitoring of the system is necessary.

Requirements for a long-term test

The measurement period of one year posed high requirements for the equipment and the settings of the measurement sensors. Therefore great value was placed on the protection of the measurement equipment against weather influences. The sensors on the RJ needed to be protected against penetration of water and de-icing salts in particular as the RJ had become leaky in some areas during the time the structure has been in use. It was necessary to lay the prestressing steels bare at two points to measure the prestressing steel strains. These measurement points were then resealed once installation was complete.

Calibration of the measurement system

Following completion of all installation work and once all measurement procedures were running smoothly, the measurement system on the bridge was calibrated. Various heavy goods vehicle combinations with different total weights were driven over the bridge at different speeds for this purpose. The measurements during the calibration program were used to draw conclusions about the stress based on continuous traffic. Support was provided during this calibration program by Daimler AG in the form of a 25t flatbed truck, a 40t articulated lorry and a 60t vehicle combination based on modular commercial vehicle concept. The motorway bridge was closed to continuous traffic during calibration so that numerous measurement drives were possible.

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