Structural Health Monitoring (SHM) of the 25 de Abril Bridge in Lisbon, Portugal
The suspended 25 de Abril Bridge, located in the estuary of the Tejo River, which connects the cities of Lisbon and Almada. This bridge has a total length of 2277 m, with three suspended spans: a central suspended span of 1012 m, two suspended lateral spans of 483 m each and tree non-suspended lateral not longer than 100 m. The North access is made through a prestressed concrete viaduct comprising 13 spans while the south span is made by a railway tunnel and road access.
The bridge deck is composed of a rigid beam, suspended by 1344 vertical hangers which, in turn, are suspended by 4 main cables, supported at piers P2 and P5 and at pylons P3 and P4. The cross-section of the deck’s stiffening truss, supports the road traffic at the upper chord level, with 6 roadway lanes, and the railway traffic on the lower chord, with 2 railway lines. All piers and pylons consist of steel trusses, being the single exception the P7, which was built as a concrete hollow section. The pylons P3 and P4 are 180 m high and, as happens for all piers, support the suspended truss at a height of 60 m. The pylons P3 and P4 are supported by concrete foundations while P2, P5, P6 and P7, by piles. The abutment P1 and the cable’s anchorage structures consist of concrete hollow elements, founded directly on the rock.
?Structural Health Monitoring Data Acquisition
The monitoring system installed in the 25 de Abril Bridge comprises 8 types of sensors and allows acquiring the structural behavior and the imposed actions, operational (traffic) and environmental (temperature and wind).
The representation of the monitoring system is presented. The data is acquired at 500 samples per sensor per second, after which is filtered and decimated to distinct sample rates according to measurement type.
The data acquisition system is composed of 13 data concentrators (controllers) which measure more than 200 sensors distributed on the bridge, all connected through RS-485 on fiber optic cabling.
Each of these concentrators, located in the key areas of sensor location, are connected by an Ethernet network built also on fiber optic with ring geometry for redundancy.
An industrial computer located also on site controls de acquisition from the concentrator network and establish the permanent link to LNEC’s server, to ensure real-time availability and data processing. The sensors installed on the deck’s stiffening truss consist of: thermo compensated strain gages installed on the chord, to measure the stress and overall force distribution, accelerometers, to measure the natural frequencies and mode shapes, displacement transducers, to capture the displacements between the bridges and the piers as well as those between road stringers and the transversal suspended frames, thermometers installed on the locations of the strain gages.
The Data Acquisition solution is based on Gantner Instuments, starting two decades ago with E.bloxx I/O modules and gradually update last years to Q.bloxx I/O modules with controllers Q.gates & Q.station 101
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On the section over P1, four railway pads were installed under the rail, to allow detecting trains travelling over the bridge, and on two triaxle ultra-sound anemometers were installed to allow estimating wind distribution applied on the bridge. Concerning the sensors installed on the pylons, 6 sections comprise strain gages for stress measurement, tilt-meters are installed on the top section, to allow for the indirect control of the cable’s cell displacements, and accelerometers are installed.
The Remote and Distributed Data Acquisition system was developed by the LNEC - Laboratório Nacional de Engenharia Civil
(Portuguese National Laboratory for Civil Engineering)
Engineering Sales Partner : PerfectDecibel, https://perfectdecibel.pt/
Data Acquisition Measurement : Gantner Instruments,