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News Release from: AV Technology | Subject: Complex monitoring system
Edited by the Manufacturingtalk Editorial
Team on 09 February 2007
Complex monitoring system provides 3D
picture
Structural monitoring specialist AV Technology is carrying out a challenging two year bridge monitoring project for Lancashire County Council.
Structural monitoring specialist AV Technology is carrying out a challenging two year bridge monitoring project for Lancashire County Council (LCC) on a 150 m section of the Centenary Way viaduct carrying the A682 dual carriageway through the centre of Burnley The downhill route taken by the A682 at this point is particularly complex with the viaduct spanning two different roads, the Liverpool Canal and several buildings
This article was originally published on Manufacturingtalk on 21 Dec 2004 at 8.00am (UK)
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Over this relatively short section the two carriageways are supported on six concrete piers and two abutments, in close proximity to residential and commercial buildings.
There are a total of 32 bridge bearings supporting the bridge deck with the carriageways having their fixed bearings at the lower east abutment.
All the bridges in the county have a General Inspection every two years, with a Principal Inspection or Special Inspection being carried out as deemed necessary.
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It was observed during a recent Special Inspection on the Centenary Way viaduct that some of the bearings were showing little or no sign of movement.
Normally a grease line shows the limits of movement but there was very little evidence of this, prompting the need for further investigation by LCC engineers.
In parallel studies are being carried out to assess the impact of 40 tonne capacity trucks on such bridges.
Given the complexity of the road structure and its location, it was estimated that the cost to replace all the bearings could run into millions of pounds.
Before doing this it was therefore decided to carry out further long term tests to ascertain the exact nature of the problems.
This relatively low cost monitoring project will provide LCC bridge engineers with the detailed data to allow them to determine the most cost effective course of action, possibly involving the replacement of only some of the bearings.
The key measurands in the project are temperature, strain and displacement.
AVT are employing four separate measuring technologies in order to obtain a clear and comprehensive three dimensional picture of how the viaduct and supporting structures are behaving during changing environmental and operational conditions.
Results are being obtained from both permanent solar panel powered instrumentation and portable geodetic optical monitoring equipment.
Carrying out a comprehensive long term monitoring program at such locations requires careful planning and as their divisional director Neil Parkinson explains: "Access to the bridge sections and piers involves working in both private and public areas as well as adjacent to open water.
We are also utilising surrounding buildings and structures to provide datum target points for the optical monitoring.
We are used to working in confined or awkward places but this project presented a number of new and diverse challenges for AVT.
To cover all eventualities we prepared a rigorous health and safety plan to conform with the Construction, Design and Management (CDM) regulations.
We also needed to arrange traffic management on the surrounding roads whilst we used high access equipment to fit the sensors, optical targets and data loggers.
There is no permanent power available and it was not practical to use batteries or lay temporary cables both from a safety and vandal point of view.
As a result all the permanent instrumentation is powered from two solar panels fitted on the south facing side of the carriageway." AVT have been responsible for installing and commissioning the permanent vibrating wire strain gauges (VWGs), linear displacement transducers (LVDTs), electrolevel tilt meters and data logging equipment, whilst they are using the expertise of specialist surveying company Survey Operations for the optical monitoring.
The four vibrating wire strain gauges are fixed to the underside of the carriageway, 150 mm from the bearing plates and measure compressive/tensile stresses in the deck structure.
VWGs provide 'conveniently packaged' strain measuring units, which are extremely robust and easy to use, making them ideal for such long term outdoor applications.
The units used in this project are hybrid pre-tensioned units with built in thermistors, specially made for AVT by gauge Technique International .
The six LVDTs are fitted to five of the piers and the west abutment to measure the relative horizontal movement between the vertical supports and the deck.
The electrolevel beam sensors, which are designed to monitor differential movement and rotation in structures, are fitted vertically in pairs, one above the other, on two of the piers.
The beam sensor consists of an electrolytic tilt sensor attached to a 2 m long aluminium beam.
Movement of the structure changes the tilt of the beam and the output of the sensor.
The beam sensors can detect a change in tilt as small as one second of arc, and have a repeatability of +/-0.015 mm/m.
Data from the sensors is collected every five minutes and averaged so that two hours worth of data is available for each stored data point, thereby giving 12 data points for each 24 hour period.
The data is automatically downloaded each day at a set time at AVT's offices in Stockport by connecting to the GSM modems incorporated within the Campbell Scientific multi-channel CR1000 data loggers.
Optical monitoring is being used to give an overall picture of any movements across the complete structure.
Measurements are taken using a Leica Total Station which uses laser technology to give typically 1 mm positioning accuracy.
The primary mounting position for the Total Station is on top of a wall running north to south underneath the bridge.
There are eleven primary targets on the structure and four reference targets on three surrounding buildings and a bridge.
The latter are used to check and compensate for any changes in the absolute position of the Total Station mounting points.
All the primary targets are not in the line of sight of the primary mounting position so an additional mounting position and cross reference point have been provided at suitable locations outside the envelope of the viaduct.
Preliminary results taken during the particularly hot summer showed that the majority of bearing movement (due to temperature) occurs at the west abutment.
At the other locations, the bearings tend to stick, causing rotation of the vertical concrete piers and increased compressive stresses in the bridge deck.
Later data shows that some movement occurs in a staccato stick-slip manner, releasing the stored energy.
This occurs as discrete step changes, affecting all piers at the same time.
The movements are progressive, not reversing after temperature cycling. Request a free brochure from AV Technology ...
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