CONFEDERATION BRIDGE ICE FORCE MONITORING

1
CONFEDERATION BRIDGEICE FORCE MONITORING

• T.G. Brown
• Department of Civil Engineering, Schulich School
  of Engineering, University of Calgary, Calgary,
  Alberta, Canada

2
Outline

• Why monitor ice forces?
• How to measure ice forces?
• What we have experienced?
• What we have measured and what it means?
• What does it all mean?
• Where are we now?

3
Why?

• Uncertainty regarding design ice forces
• 16 to 33 MN for 100-year design force
• Uncertainty regarding behaviour of ice against
  conical piers
• Provide information as input to maintenance
  programme for ice-breaking cones
• Opportunity to conduct research on ice
  interactions with full-scale offshore structures

4
How?

• Measure ice forces
• Measure ice pressures
• Observe behaviour of ice against piers
• Measure related ice and environmental parameters

5
Measuring Ice Forces

• Confederation Bridge, we measure
• Tilt response of piers
• Acceleration response of piers
• Advantages
• Highly accurate
• Dynamic sensitivity
• Disadvantages
• Response contaminated by structure response to
  ice and other dynamic load effects

6
Kinematic Instrumentation
Biaxial Tiltmeter
Accelerometers
7
Ice pressures

• Choice of panel
• Dynamic response
• Accuracy
• Cost/coverage
• Strain-gauged
• 56 m2
• Limited accuracy

8
Behaviour

• Closed-Circuit TV
• Time-lapse recording
• Focus on two piers
• Initially analogue
• Now digital

9
Pier Elevation
10
Panels
11
How much ice?

• 3000 km ice/year on each pier
• Typically, 3 ridges/km
• Ridges with keels to 16 m deep,
• Consolidated layers to 3m thick.

12
Ice Ridges
Sail
Consolidated Layer
Keel
13
Important Ice Characteristics

• Consolidated layer thicknesses
• Keel depths
• Floe sizes
• Rubble pile size and frequency
• Keel mechanical properties
• Consolidated layer mechanical properties

14
Consolidated Layer
15
Keel Depths
16
Keel profile
17
Velocities
18
Rubble Piles
19
Ice Behaviour

• Characteristics of Interactions
• Flexural failure (circumferential and radial
  cracks)
• Downward failure
• Ploughing of ridges
• Fracture
• Limit driving force
• Rubble under ice sheet

20
Fracture
21
Ride-Up
22
Maximum ride-up
23
Rubble Piles
24
Ice Loads - Process

• Initial event identification (load 0.75 MN)
  from tiltmeter average file.
• Process includes check for presence of ice,
  correction for baseline shift in tiltmeters, and
  correction for effect of wind.
• The tiltmeters respond to all tilt deformations
  of the piers, including those associated with
  temperature and solar radiation.

25
Ice Loads raw tilts
26
Ice Loads corrected for wind
27
Loads
28
Ice Conditions
29
Annual Loads
30
Relation between load and FDD
31
Loads

• Thousands of events identified
• Maximum event magnitude (17-sec average
  assessment) 7.25 MN
• 100-year design loads
• Developer 16 MN
• Independent Engineer 30 MN
• Govt of Canada 15.2 MN

32
Relation between Load and Keel Depth
33
Relation between Load and Consolidated Layer
Thickness
34
Detailed Analysis
35
Panel Pressures - Shaft
36
Panel Pressures - Shaft

• Shaft Pressures
• Very intermittent
• Far fewer pressure records than would be expected
  given number of keel interactions, and extent of
  any given interaction
• This accounts for the recognized deficiencies of
  the panels in responding to very localized
  pressures
• At average ice speeds, keel interactions should
  last from 1 to 3 minutes

37
The Event!
38
April 4, 2003 Morning Events
39
Interaction
40
Interaction two ridges
41
1023 Event
Note Peak load 8.5 MN compared to 7.2 MN from
average file
42
Conclusions

• Winters have been less severe than the conditions
  used for design
• Observations indicate that the pier design is
  effective in mitigating ice forces and clearing
  ice
• Several forms of interaction behaviour, not
  considered in design, have been observed
• Rubble piles are always present in interactions
  ride-up also occurs but not in the form
  considered in the algorithms
• Keel depths have been greater than predicted
• Consolidated layer thicknesses are similar to
  those predicted, without the extremes
• Ridge keels do not contribute significantly to
  the load largely as a function of pier shape
• Loads are lower than predicted, and are lower
  than those predicted by the algorithms
• One extreme event has been experienced, and the
  resulting loads were within the design spectrum,
  and were lower than would have been predicted

43
Acknowledgements

• Those who have helped to initiate and maintain
  the instrumentation system
• And those who have maintained the ice monitoring
  programme

44
Acknowledgements

• Those who have funded the program
• Natural Sciences and Engineering Research Council
• Strait Crossing Bridge Limited
• Public Works and Government Services Canada
• Program for Energy Research and Development
  (PERD)
• Exxon-Mobil
• Marathon Oil

45
Acknowledgements

• And my students
• Eric Lemee ridge keel loads
• Derek Mayne flexural failure
• Dambar Tiwari database
• Mohamed El Seify ridge load models
• Drubha Tripathi local pressures
• Susan Tibbo loads (detailed analysis)
• Keely Obert ridge keels
• Noorma Shrestha - database
• For without them, none of this would be possible

46

• Questions?