Development of Sheathed Strand for Bridges
Jagan Mohanraj Norwegian bridge conference, November 2013
Presentation Overview
1) Introduction
 Strands for structural applications
 Corrosion protection
 Motivation for product development
2) Sheathed strand system for structures
 Development steps
3) Summary
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Structural Applications
Products for Structural Applications
Spiral strand
Locked coil strand
• Helical spun round wires in several layers
• Helical spun Z‐shaped wires over spiral strand
• Torsionally balanced
• Torsionally balanced
• Nom. tensile strength of wire up to 1770 MPa •
(1960 MPa for offshore mooring applications)
•
• Rope modulus of about 170 kN/mm2
•
• 75% metallic fill factor
•
Nom. tensile strength of shaped wires is 1570 MPa
Modulus of about 165 kN/mm2
89% metallic fill factor
Superior corrosion protection, higher axial stiffness and better clamping capabilities
Factory fabricated assemblies ready for installation.
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Corrosion barriers – Design & Manufacture
Corrosion resistance of structural strands
Galvanising of wires
Zinc or Galfan®
(Zn‐Al)
Locked surface due to full lock wires
Blocking material during stranding
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Coating the strand
Outer Corrosion Protection – Bridge deck
• Paint system for strands ‐ application of a primer
followed by few layers of paint.
• Requires moveable or fixed scaffolding over the
entire length.
• Significant overall cost.
• Progress of work is dependent on weather
conditions.
• Traffic disturbances.
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Development of corrosion protection for structural strands
sheathed
Year
since 1885
1970
1981
1991
2013
Objective was to develop factory fabricated, cost effective
sheathed cable assemblies ready for installation.
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Plastic sheathed strands for bridges
Benefits of plastic sheathing of structural strands:
1) Additional corrosion protection
2) Paint coatings not necessary
3) Cost‐intensive scaffoldings not required
4) Robust during handling on site
5) Sheathing applied in factory environment
6) Installation can be done independent of weather
7) No restrictions for NDE inspections
8) Periodic maintenance not necessary
9) Different colours to suit the architectural design
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Sheathed cables for offshore Deep Water Mooring Systems
FPSO Turret Moored
SPAR
Semi-submersible
• Operate in water depths of up to 3000 metres
• Design life of cable up to 40 years
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Development steps of sheathed bridge cables
1) Sheath plastic
2) 5) Sheathed Sheathing process
Inspection methods
locked coil strand system
3) Socketing
4)
Internal testing
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1) Sheath plastic for structural applications Requirements of sheath plastic for structural strands:
1. Durable and physical properties meet application demands.
2. Chemically stable to blocking compound used in stranding.
3. Impermeable for moisture, air, pollution, salt etc.
4. No embrittlement or softening between ‐40˚ and +60˚C.
5. Interface with the rope at service temperature.
6. Resistant against UV degradation – durable for min. 50 yrs.
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1) Plastic sheathing – colour durability
• Radiation energy affects the long‐term UV stability of the colour.
• Radiation energy varies with geographical location.
• Colour recommendation for sheathing depends on radiation energy level.
Average radiation energy measured in KWh/m2 year
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1) Plastic sheathing – colour recommendation
Average radiation energy for Europe measured in KWh/m2 year
Colour recommendation for 60 years + durability:
Grey (RAL 7000‐7047), White and black (RAL 9001 – 9018), Yellow (1000‐2000), Blue (5014‐5019), Orange (2003‐2012), Green (6000).
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1) Plastic sheathing – flame retardant
Flame Retardants & Smoke Suppressants
• Most plastics are FLAMMABLE
• Flame retardant plastic for increased resistance to ignition, reduce
rate of flame spread and reduce smoke emission
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2) Sheathing process for strands
• Special extrusion process ‐ interstices in the
outer layer of the strand are filled with molten
plastic.
“Tube down” of the molten plastic
• To ensure no water ingress which could lead to
subsequent in‐service corrosion.
• Corrugations in the inside of the sheathing
provide significant resistance to inhibit slip
between the sheathing and the rope at any
stage.
• Sheath adherence test up to 40% of strand
MBL.
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2) Sheathing opportunities ‐ 1
Double sheathed strands
• Offer more corrosion protection
• Additional UV protection
• Resistance to crack propagation.
• Visual aid to indicate areas of damage in
sheathing
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2) Sheathing opportunities ‐ 2
Ropes with Anti-vibration Sheathing
• Novel extrusion process developed in 2011.
• Helical strake integral with sheathing.
• Varying strake pitch along the rope length.
• Eliminate self excited vibrations (strumming) in strands for bridge hangers and stay
cables.
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3) Socketing of sheathed strands
• Approved standard termination process for sheathed spiral strand.
• Socket termination procedure for sheathed full locked coil strand.
Sheathed strand
Plastic boot
Socket
• Special socket design to accommodate sheathed strand inside the socket.
• Joint between sheathed strand and socket sealed with a plastic boot to prevent in‐service corrosion.
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4) Tests on sheathed strands
• Accelerated UV test on sheathed plastic ‐ Combination of heat,
moisture and UV light to accelerate the degradation process in
plastic.
• Sheath adherence test up to 40% of strand MBL.
• Modulus and breaking load tests on unsheathed and sheathed
locked coil strands
• Tension‐ tension fatigue tests on unsheathed and sheathed locked
coil strands
• No adverse effects as a result of plastic sheathing operation.
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5) Inspection methods for sheathed strands
NDT techniques during sheathing • Spark tester – check for gaps or holes in sheathing.
• X‐ray gauge – sheathed diameter & concentricity.
• 100% inspection.
Onsite visual or robotic examination
• Sheathing conditions.
• HD photography over the entire cable
surface.
Image: AAS Jakobsen / Norway
Image: Diagnose Institute Boue / Germany
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5) Inspection methods for sheathed strands
Magneto‐inductive examinaton
• Inner and outer wire breaks and cracks.
• Condition of corrosion.
Image: DMT / Germany
Ultrasonic examination
Endoscope
• Wire breaks.
• Heavy corrosion of the outer wires at
the transition to the cast socket.
• Examination of cable root inside the
anchorage compartment.
Image: DMT / Germany
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5) Inspection methods for sheathed strands
Sheathed Cable inspection • Sheath structural cables with a translucent / transparent stripe
• Aid visual inspection for signs of degradation, such as broken wires
and/or corrosion.
• Axial stripe co‐extruded within an opaque sheathing.
• Coloured to suit the customer’s preference.
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Summary
1) Sheathed spiral strands and locked coil strand offer new
opportunities for the design of bridges.
2) Corrosion protection of structural further enhanced by
sheathing methods.
3) Sheathed strands have the advantage to reduce overall
costs.
4) Double sheathing with contrasting colour provide visual
aid to indicate areas of damage in sheathing and increase
the longevity of the structural strands.
5) Extrusion of continuous helical strakes integral to the
sheathing offer a more cost‐effective solution to VIV
problems.
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Lusail Bridge in Qatar
Images: COWI
August 2013
• ‘Q’ shaped rings 50m above the sea, main span of 200m and width of 28m.
• Main cable ‐ 6 locked coil strands of diameter of 140mm and 43 sheathed locked coil
strands of diameter 70mm.
• Sheathed locked coil strand for hangers to increase corrosion protection.
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Photographs of sheathed cables (display samples)