Early recognition of structural problems

This freeway bridge, completed in 1968, shows significant signs of corrosion damage resulting from its age and exposure to chlorides from thawing salts. In particular the rapid increase of heavy truck traffic was not foreseeable at the time of construction. For this reason the bridge is presently being replaced by a new bridge. The Engineering Office MBK in Darmstadt was requested to measure the values at the site.

The prestressed concrete bridge is 567 m long, a maximum of 52 m high and is subdivided into 12 separate segments each with a length of approx. 47 m. The primary values to be measured were defined as the changes in the width of the fissures at the 11 coupling joints between the segments in the lanes and one crack on the bottom of the bridge. Moreover a number of temperatures, the moisture in the concrete, the pressure below the asphalt and the expansion of the bridge at the abutments are measured. All totaled there are 40 measuring points. The values are recorded continuously, stored locally and transferred to the responsible point via GSM. An alarm is given when the individual specified limits are exceeded. A video camera is used to record image data allowing the cause of the overload (heavy load, traffic jam) to be identified.

The decentralized arrangement of the measuring points over a total length of approx. 1.3 km (the two shafts beneath the lanes are not connected) made it necessary to use a modular, distributed measuring system. The carrier frequency amplifiers used are the e.bloxx A6 units, which can be connected to the inductive displacement sensors used as well as to the strain gauges. e.bloxx A1 modules are used for DC voltages. The modules are located in the vicinity of each coupling joint and are connected to the local computer over a serial RS485 interface. The measuring modules record the expansion values at a scanning rate of 500 Hz and the minimum and maximum values are already calculated in the module. This makes it possible to reduce the volume of data right in the module. The measured values are recorded with the MW package from MBK. A total of 96 values are generated per second, amounting to 3.2 Mbytes per day. A new file is generated every day and transferred during the night. Moreover the maximum values for the vibration widths are categorized into classes, evaluated and saved in the form of Excel tables.