Monitoring to extend the service life of
steel bridges
Gunnstein T. Frøseth, Anders Rønnquist and Ole Øiseth
Overview
Presentation
• Background and motivation
• Measurement system at Stjørdalselva
• Preliminary results and conclusions
• Further work
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Background
Norwegian Railway network
• Increasing axle loads and
speeds
• 1000 steel bridges
• 500 build before 1930
• Not designed against fatigue
How long can this bridge be kept in service? What bridge should
we prioritize?
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Motivation
Service life estimation, steps
i.
Simple models, conservative assumptions.
ii. Detailed models, more information gathering.
iii. Advanced methodology, reliability methods, monitoring, NDT
testing.
iv. Replacement / strengthening
Load, structural and damage models necessary in all
steps
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Motivation
• Numerical models predict response, but how well?
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Motivation
Uncertainty in service life estimation based
on numerical simulations
Uncertainty fatigue resistance
Model uncertainty (damage
model)
Model uncertainty (response)
• Removed by monitoring
• Reduced on general basis by
experience
-> Monitoring necessary to achieve high accuracy prediction
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Stjørdalselva railwaybridge
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Open deck truss bridge
35m span
Completed 1902
Double stringer configuraton
Riveted structure
Modelling questions:
• Load distribution on stringers?
• What level of model error?
Monitoring questions:
• Sensor placement?
• Number of sensors?
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Measurements at Stjørdalselva
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From 01.june to 17.september
Strain measurements
Acceleration measurements
Temperature measurements
WIM axle load measurements
Strain measurements
• 93 straingages mounted
• Half the bridge
• Fatigue critical components
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Stringers
Crossgirders
Angle connections
Welded components
Strain measurements
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Strain measurements
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Temperature measurements
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RTD sensors
Above and below deck
Bridge behavior
Sensor drift
Accelerations
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20 triaxial sensors
Main trusswork
Magnets
Dynamic properties
Model updating
Axle load measurements
• Independent WIM station
• Each train is identified by EVN number
• Calibrated with train during monitoring period
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Data logging
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Local and remote storage
Streaming data to office
Triggering reduce storage
Sampling rate:
– Strains 800 Hz
– Accelerations 400 Hz
– Temperature 1 Hz
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Data logging
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General Results
• More than 40 train passages a day
• Stress levels in nominal locations are low
• Stress in hotspots (welds) are above fatigue limit
– Fatigue an issue at hotspots at current traffic conditions
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Results – stringers
s
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Results – stringers
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Results – stringers
«Modified» connection at
midstringer.
Gap
Not discovered at periodic
inspection.
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Preliminary conclusions
Nominally identical components have similar
response
– Can reduce number of measurement points
– Can utilize easily accessible mounting points
– Even load distribution on double stringers
But components must be identical
– Special attention to any modifications
– Arms length inspection necessary
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Further work
• Comparison of measurements and model prediction
– Quantify model uncertainty/error
– Validate numerical model
• Optimize monitoring scheme
– Sensor location, detect fault?
– Number of sensors to reach confidence level
• Monitoring of other bridges
– Generalize results
• Can we detect the «Fault» in other measurements?
– Measurement vehicle?
– Accelerations?
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Questions?
• Big thanks to:
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