Open Spiral Strand v Full Locked Coil v Parallel Strand Stay Cable

Posted on Tuesday 22nd September 2020

Open Spiral Strand v Full Locked Coil v Parallel Strand Stay Cable

Suitable Bridge Cable Options

When looking at suitable options for structural cables in a Bridge application there are often three main options that are widely used around the world:

  1. OSS- Open Spiral Strand
  2. FLC- Full Locked Coil
  3. PSSC- Parallel Strand Stay Cable

This Blog is intended to share some information regarding the different properties of all three options and the Pros and Cons of OSS and FLC opposed to the PSSC option. Arcus Wire Group are the local partners for Redaelli Tecna S.P.A in Italy who produce OSS and FLC. With years of experience around the world on many major Stadiums and Bridges we feel this knowledge will assist the local market in making an informed decision about the use of OSS and FLC solutions on their next project.

Grayston Bridge in Johannesburg, South Africa

What is OSS

Open Spiral Strand (OSS) – OSS is a single helically wound strand made up exclusively of round wires each of which are nominally ¼” Dia. The open spiral strands are made with hot dip galvanised high strength steel wires. The strand is made up of several layers of round wires helically wound around the core. The typical main properties of the wires conform to EN 12385-10 standard. 

Tensile strength, yield stress, ductility, elongation at breaking, bending, torsion, adherence and wrapping properties are individually tested by Redaelli on each wire. The zinc coating and the wire ductility comply with the EN 10264 class A and other required standards. All OSS Redaelli cables meet or exceed the minimum specifications for fatigue strength defined by EN 1993-1-9 and EN1993-1-11. 

Typically OSS is used for carrying and stabilizing strands for Tensostructures, stays for pedestrian bridges and guy ropes. Redaelli supply OSS strands in 8mm through to 88mm. 

OSS

 

What is FLC

Full Locked Coil (FLC) – FLC is made of outer layers of interlocking Z shaped wires which are helically spun around an OSS core. Full Locked Coil strands have a high metallic cross section - this guarantees a higher breaking load and elastic axial stiffness with a smaller/reduced diameter. The outer "Z"-shaped layers give the rope a smoother profile, which allows for better clamping. The locked external surface, the internal compound Tensofill and the zinc–aluminium alloy coating (Zn95Al5) of each external wire allow for a high corrosion protection - that can be further improved with special waxes and extruded or co-extruded HDPE sheathing with wide set of colors and finishing's. Redaelli has developed an advanced software which, in addition to the company’s total control of the manufacturing process, allows to customize the locked coil ropes to customer’s requirements. Including custom shaped wires for aerodynamic purposes, low torque cables or other special designs. Full Locked Coil strands are suitable for metal or resin socketing. 

Typically FLC is used for carrying and pre-stressing strands for tensostructures., stays for bridges, hangers for suspension bridges and stays and suspension cables for buildings.  Redaelli supply FLC strands in 16mm through to 144mm. 

 FLC

 

What is PSSC?

Parallel Strand Stay Cable (PSSC)- PSSC is a parallel strand system that is often used in larger bridge application where a FLC is no longer cost effective. Other applications include stay cables, suspended roofs and facades. The cables generally consists of a bundle of seven wire strands that combine together in a protective coating to act as a large parallel strand. Essentially you are combining a large amount of individual cables to act as one large structural cable. These have many benefits including flexibility, Modularity and individual corrosion protection. 

PSSC Cross Section

 

Why choose FLC over OSS?

As can be seen from the images above, the OSS is essentially an open construction compared to the FLC which is a closed construction. In many cases the market pricing for both types of cables are almost the same but FLC cables provide the following advantages;

  1. Higher Axial Stiffness (EA). This is because there is a lot more steel in the cross sectional area (A) compared to OSS. The result is a less elastic cable leading to a more stable cable and hence less lively structure in service.
  2. Higher strength to weight ratio. When comparing the same cable diameters FLC is approx. 5% to 10% higher breaking strength which means that potentially a smaller cable diameter and smaller socket end fittings can be selected for the same load carrying capacity.
  3. Superior clamping properties. The smoother outer surface of a FLC gives a greater surface area for friction clamps to attach when compared to the ribbed outer surface of a FLC. A FLC is more resistant to lateral clamping forces meaning it retains its circular shape rather than elliptical out of roundness.
  4. Superior long term fatigue life. Both FLC and OSS are helically spun with layers of individual wires largely in opposing directions. At the points where the layers of wires intersect each other, the round wires have appoint contact as opposed to the FLC Z shaped wires which have a distinct flat surface at the wire intersection point. This greater surface contact area helps resolve the issue of cross cutting that the round wires sometimes suffer from during long term service.
  5. Improved corrosion resistance. A combination of the closed design construction of a FLC cable, with Galfan coated outer Z shaped wires and an internal zinc rich paste blocking compound that Redaelli cables use result in a finished cable that has several stages of corrosion resistance. We would also recommend a further additional corrosion coating of a product called Tensocoat which is essentially a paint system containing micro flakes of Aluminium which, when applied to the outer surface of the cable, become the primary sacrificial corrosion protection mechanism for the cables. In total this results in a cable which is able to resist corrosion for in excess of 100 years, which compares to something like 25 – 30 years for a standard galvanised OSS cable. Please note, these figures are obviously dependent on local environmental conditions of the structure.
  6. Ease of in service inspection. Because there is no plastic ducts or HDPE coating on the cables, the FLC and OSS cables can be readily visually inspected at all points throughout the cable service life. It is also possible to run a FLC and OSS through a non-destructive examination, which is essentially a NDE magnetic head that provides a continuous trace showing any internal imperfections or wire breaks.

 

Why choose FLC over PSSC?

There are several main benefits of FLC over PSSC:

  1. Factory Manufacturing Process
  2. Highly Efficient Cross Section
  3. Circular Outer Shape
  4. Variety of Socket Anchorage Options
  5. Corrosion Protection
  6. Manufactured Ready for Installation On Site
  7. Easy Service Inspection and Maintenance

Factory Manufacturing Process

FLC cables are carefully designed to achieve the correct ratio between the required cable Minimum Breaking Strength and Axial Elastic Stiffness. Cables need to be fabricated at the exact length in controlled factory conditions. The cables are then pre-stretched to reduce and fluctuations once the cable is installed onsite. All cable manufacturing operations are undertaken in a factory controlled environment, using calibrated specialist equipment with exacting tolerances which are controlled under regularly audited ISO 9001 quality systems.

Highly Efficient Cross Section

FLC has a higher Axial Stiffness (EA), this is because there is alot more steel in the cross sectional area (A) compared to a PSSC. The result is a less elastic cable leading to a more stable cable and hence less lively structure in service. FLC Void factor is 0.10 v PSSC Void Factor of 0.80. FLC has a higher strength to weight ratio. When comparing the same cable diameters FLC is higher breaking strength which means that potentially a smaller cable diameter and smaller socket end fittings can be selected for the same load carrying capacity. There are also minimal non-metallic components within the strand thereby reducing the susceptibility of the stay cable to excessive in service vibrations. Fatigue performance or the “cross-nicking” effect associated with the round wires on the adjacent strand layers because of the wire contact points is greatly reduced on FLC strand. The Z shaped wires provide flat elongated contact points between wire layers resulting excellent fatigue performance.

Circular Outer Shape

Superior clamping properties. The smoother outer surface of a FLC gives a greater surface area for friction clamps to attached when compared to the ribbed outer surface of a PSSC. A FLC is more resistant to lateral clamping forces meaning it retains its circular shape rather than elliptical out of roundness.

Variety of Socket Anchorage Options

No serrated mechanical grips are used when attaching the FLC strand to the socket anchorages, thus there is no mechanical damage to the outer wires of the strand or tendons. Instead a single solid cone develops in excess of 100% of the strand Minimum Breaking Strength. A wide variety of socket anchorage shapes and types enables the most appropriate cable option to be detailed to suit aesthetic considerations and bridge construction requirements,
i.e. FLC are not limited to cylindrical socket type anchorages.

Redaelli Sockets

Corrosion Protection

FLC has a vastly Improved corrosion resistance. A combination of the closed design construction of a FLC cable, with Galfan coated outer Z shaped wires and an internal zinc rich paste blocking compound that Redaelli cables use result in a finished cable that has several stages of corrosion resistance. We would also recommend a further additional corrosion coating of a product called Tensocoat which is essentially a paint system containing micro flakes of Aluminium which, when applied to the outer surface of the cable, become the primary sacrificial corrosion protection mechanism for the cables. It is possible to supply FLC cables with a factory controlled vacuum extruded HDPE sheathing to further increase resistance to corrosion.

Manufactured Ready for Installation On Site

Ease & safety of installation. As the cables are prefabricated in the factory they arrive ready for installation as an pre-assembled component. FLC cable acts as a single structural member and therefore requires only single element tensioning rather than the multiple pc strand tendon tensioning. FLC cables are therefore easily and safely tensioned.

Easy Service Inspection and Maintenance

Ease of in service inspection. Because there is no plastic ducts or HDPE coating on the cables, the FLC cables can be readily visually inspected at all points throughout the cable service life. It is also possible to run a FLC through a non-destructive examination, which is essentially a NDE magnetic head that provides a continuous trace showing any internal imperfections or wire breaks.

Why choose PSSC over FLC?

In the interest of transparency there are some advantages with PSSC over FLC.  

  • Flexibility: Strand can be curved on reduced curvature radius. Cable doesn’t need to be coiled on large diameters, to be accounted for transport
  • Modularity: The number of strands is adjusted to the force in the cable. Is it possible to achieve big diameters size
  • Individual corrosion protection for each strand
  • External HDPE pipe

Comparison Chart

Redaelli FLC v PSSC

 

As always for more information please feel free to contact the sales team at sales@arcuswire.com.

 

 

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