Cable Railing Systems

Designing for building code compliance, maximum aesthetic effect, and sustainability
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Sponsored by Atlantis Rail Systems
By Peter J. Arsenault, FAIA, NCARB, LEED AP

Corrosion Resistance

Something to keep in mind about these products is that they are “stainless,” not “stain proof.” There can still be some things in the environment that can stain or discolor the metal, particularly if the wrong type or grade of stainless steel is used. The best defense against corrosion is based on selecting the best material, finish, and treatment. For best results on cable railings and guards, using only austenitic 316L is recommended. Further, during installation, the cables should be immediately cleaned to remove any oils or free ions that may be present. Cleaning should only be done using mild detergent (car wash soap is recommended) and avoiding commercially available “stainless steel cleaners,” as most contain harsh chemicals.

Another common concern is galvanic corrosion, which is caused by using dissimilar metals in a corrosive environment without proper insulation or separation between those materials. Generally speaking, the most common galvanic corrosion on railings occurs between aluminum and stainless steel or steel in saltwater environments. Metals that are finished with anodic properties, such as anodized aluminum, tend to be sacrificed or even dissolve in these conditions when in contact with cathodic materials such as stainless steel. Hence, proper separation of any other materials needs to be detailed and installed correctly.

Metal Finishing

Stainless steel is often finished during manufacturing to create a superior chromium oxide layer and remove impurities (iron) from the metal surface. There are two ways to carry out this finishing:

  • Passivation: This is the application of citric acid or nitric acid to the metal, followed by a complete water rinse. It is usually done in a tank within a specific temperature range. Welders often field passivate their work using a citric acid paste. These welds are a potential place for future corrosion.
  • Electro-polishing (super passivation): This process adds an electrical current in a tank environment to further smooth the metal surface while achieving passivation. It is the ultimate metal treatment for corrosion resistance since the current and the acid work together to clean and smooth the metal surface, thus developing a superior and uniform chromium oxide layer. This is an excellent choice for any exterior condition, but in a salt-water environment in particular, electro-polished materials should always be used.

In electro-polishing, electrical current and acid work together to clean and smooth the surface of stainless steel, thus developing a superior and uniform chromium oxide layer.

Components of Cable Railing Systems

With a good understanding of the code, safety, and material aspects of cable guardrail systems, we can now take a closer look at the individual component parts that make up a full system.

Cables

A cable is simply the combination of multiple strands of wires. The strength and characteristics are therefore directly influenced by the thickness of the wires and the number of wires or clusters of wires that make up any given cable. Hence, cables are referred to by the number of wires in a cluster and the number of clusters in a cable. They are then classified by their overall thickness. Accordingly, a 1 x 19 cable is made up of one cluster of 19 single wires, while a 7 x 19 cable is composed of seven clusters but with 19 smaller wires in each cluster. Similarly, a 7 x 7 cable will have seven clusters of wire bound in sets of 7 inches each cluster. Any one of these options could be a total of 18 inch, 532 inch, 316 inch, or 14 inch thick based on the thickness of the individual wires used. The working load (WLL) and the cost of each type of cable are, of course, directly affected by the type of cable used and the size.

Cables used in railing systems will vary in cost and working load (WLL) capacity based on the type of clustering of wires used in the cable and the overall size of the cable.

Based on the above, the design question quickly becomes, what is the best cable to use? The common safe working load is two times the WLL of a cable, taking into account the number of cables used. For most residential and commercial guardrail situations, it is highly recommend to use only 1 x 19 cable, and generally 532 inch diameter is appropriate—two times its WLL will be 5,800 pounds and satisfy most guardrail loading conditions. One-eighth inch is the minimum diameter that may be used safely, but it has a working load 38 percent less than 532 inch, which is usually acceptable for residential use. Nonetheless, the cable will be the strongest part of the system. That is not usually the case for other types of guardrail systems, particularly if they use vertical, composite balusters, which will often break under as little as 250 pounds of applied pressure.

Cable Attachment Hardware

The cables need to be attached to vertical posts in some manner to be anchored and tensioned. There are two basic ways to do this. The first is with hardware that is designed to receive the cable and be mechanically attached and tensioned using specific hardware and common hand tools. This hardware is typically made of the same stainless steel as the cable and is sized to match the size of the cables used. Mechanical attachment of the cables is most common for surface-mount systems, particularly since hardware is available that is intended to be screw attached to wood or metal posts. The connection to the cable is done by tightening nuts or specialty hardware around the cable and tensioning as required.

Mechanical connection of cables to posts relies on specialty hardware and common hand tools and is often used for surface-mount systems.

The second means of attachment is to use different specialty hardware that is attached to the cable using a swage tool. Swaging is the practice of forming a metal receiver around wire cable in such a way that the two components become virtually one. Essentially, the swage hardware is hollow to receive the cable, and a swaging hand tool or machine can be used to compress the hardware around the cable. Swaging is the highest-strength cable connection available when done properly. Swaging is most commonly used for through-post cable attachment but can be used for surface-mount systems in some cases too. Tensioning of the cable is done on the rear side of the post, commonly using a nut or similar hardware tightened down on a stud that is swaged to the cable.

Swaging is the practice of forming a metal receiver around a cable using a hand tool or machine, such that the two components become virtually one.

 

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Originally published in Architectural Record
Originally published in February 2020

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