Abstract: Monitoring displacement of in operation bridges is practically challenging but potentially very useful for condition assessment and decision support. The primary difficulties are in finding fixed physical reference points and, for the majority short span bridges under normal operation, the mm-level magnitudes of displacement under normal operating conditions (e.g. standard truck loading). With rare possibility for physical connection between a reference and a bridge, non-contacting technologies such as GPS need to be used. Other options include total station and more exotic technologies of laser interferometer and radar have also been tried. There are drawbacks for each technology related to limited sample rate (for total station) and signal to noise ratio (for GPS) while radar and laser are expensive and require specialist users. With advances in computing power, optics-based systems are becoming popular, relying on a standard lens but with capability to track multiple positions with potential to recover deformation with high spatial resolution. This paper reports the experiences of the authors exploring the suitability of a commercially available optics-based system in terms of spatial and temporal resolution and sampling and in challenging field conditions required for long term monitoring. For example issues such as stability of camera mounting (e.g. in wind) and varying lighting conditions while not problematic in a laboratory govern performance in the field. The paper tracks a sequence of experiments moving from lab to field, ultimately moving up to a field test on a road bridge in Devon. In each case the capabilities and limitations of the system have been critically examined. The study has defined both limitations and capabilities, while defining best approaches for use and at the same time providing some useful performance data on the subject bridges.
2. OUTLINE
Purpose of the Field Testing
Test Structure – Exe North Bridge
Instrumentation
Results and Discussions
1
3. PURPOSE OF THE TEST
Field testing can potentially reveal hidden
reserves of strength in a bridge,
In particular we were interested in identifying
how did the real transverse distribution factors
compare to those prescribed in assessment codes
This will be examined by placing a truck at
various transverse positions on the deck and
examining the variation in strain transversely in
a beam and slab bridge
2
4. TEST STRUCTURE – EXE NORTH BRIDGE
3
SUPERSTRUCTURE
Composite type bridge
12 girders
Steel beams embedded
into RC I-girders
4 traffic lanes
Non-continuous spans,
simply supported
A
A
6. 5
A
A
STRAIN MEASUREMENTS (¼ SPAN)
Four- axle Field Test
32 tonne truck 04:00 am
Sensor 5
Sensor 2
Sensor 7
Start End
7. 1
A
A
STRAIN & DISTRIBUTION FACTORS (¼ SPAN)
DF=
ε𝑖
ε𝑖
Girder Numbers
LoadDistributionFactors
Lane – 2
Lane – 3
Lane – 4
Girder Numbers
Microstrain
Lane – 1Sensor 5
Sensor 2
Sensor 7
Sensor 5
Sensor 2
Sensor 7
8. 8
A
A
NEXT STEPS FOLLOWING EXPERIMENTAL TESTING
Now that we had the experimental data, we wanted
to compare with a theoretical study
Finite Element Model (FEM) of the bridge were
developed and calibrated based on the experimental
analysis
Bridge structural behaviour under static loading were
investigated
9. 9
A
A
FE MODEL AND BOUNDARY CONDITIONS
Boundary Conditions modelled
Pin – Roller
Pin – Pin
Partially
Restrained
1)
2)
3)
Beams only
Beams & Slab
10. Boundary Conditions
Girder Numbers
Microstrain
Girder Numbers
MicrostrainMicrostrain
Girder Numbers
Lane – 1
Lane – 2
Lane – 3
Lane – 4
Girder Numbers
Microstrain
1
ANALYTICAL & EXPERIMENTAL COMPARISON (¼ SPAN)
Field Test
Pin - Roller
Pin - Pin
Partially
Restrained
Girder NumbersLoadDistributionFactors
12. CONCLUSIONS
Change in boundary conditions can change the stress in load
carrying elements significantly (~50%)
Load is more uniformly distributed when support conditions are
free to move in longitudinal direction.
Change in boundary conditions can increase transverse load
distribution factors slightly
Field testing is an important topic for bridge evaluation and can
reveal any hidden strength reserves 13
13. ACKNOWLEDGEMENT
This research project has received funding from
the European Union’s Horizon 2020 research and
innovation programme under the Marie
Skłodowska – Curie grant agreement No. 642453
Devon County Council
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