Design Parameters and Constraints for Metal Framed Cable Railing
We will first address the issues encountered while designing a cable railing system with horizontal cables.
Horizontal cables used as an in-fill is acceptable in most jurisdictions. Climbability restrictions do exist in Canada and some cities. So the first step to take is to confirm if there is a climbability restriction in your local jurisdiction.
If you find you are unable to use horizontal cable railing, vertical cable railing is an option.
Cable is very strong in tensile strength and is a suitable in-fill material for a railing. There are many different types of constructions of cable (also referred to as wire rope).
Most cable is flexible for going over pulleys or for lifting/moving heavy loads. Other types of cable hold something in tension — such as a guy wire or a sailboat stay — and are less flexible.
For any diameter of cable, the trade-off for flexibility is strength. The inverse is also true. Cable that is capable of a higher degree of flexibility, will have less strength. Cable flexibility is an important consideration in designing a cable railing.
The I-Codes requirements for a guard note that no opening be large enough that a 4″ sphere can pass.
Having the rigidity to prevent deflection of cable infills is partly mitigated by the cable’s lack of flexibility. So, it is our initial preference to use the most rigid cable available when designing a cable railing system.
- The other factors include:
Let’s start with the span between intermediate members.
Cable can run long distances between terminating ends. 150 ft. or more — depending upon railing configuration. But, cable needs support between end posts to avoid cable deflection that would exceed permissible limits. These intermediate elements can be posts or cable braces. When using rigid, 1×19 cable, the spacing between intermediate posts and/or cable braces should not exceed 42″.
An intermediate post runs from the top rail to the mounting surface.
A cable brace is a non-structural element between posts. Its primary purpose is to support the cable and to restrict stretching of the cables.
The next variable is the diameter of the cable.
When tensioned and subjected to abuse, 1/8″ cable is too near the limits of the cable breaking strength and holding strength of the cable fittings.
We only recommend 1/8″ cable in areas that are unlikely to experience heavy pedestrian traffic. For most applications, we recommend 3/16″ diameter cable.
When the scale of the project is large, a larger diameter cable may be preferred from an aesthetics standpoint. For those situations, we offer systems using up to 3/8″ diameter cable.
Spacing of the cables is critical to assure compliance with the 4″ sphere limitation. We recommend spacing of 3″.
The last variable is the tension of the cables and the construction of your end posts.
Deflection of the end posts must be minimized. This is where we find the most mistakes made in the design of the railing framework.
An incredible amount of stress is on an end post when you have ten or more lines, each tensioned at 225 lbs. or more over a height of 36″ to 42″.
Designers and fabricators inexperienced in cable railings will not recognize the amount of the tension applied to the posts. The end result? All too often the end posts will bend as the cables once the cables are tensioned. Or the cables cannot be tensioned without an unacceptable amount of post deflection.
The posts to which hardware is mounted must be constructed so that they will not deflect perceptively as the cables are tensioned to loads of 225 lbs., or more.
These elements work together to permit the installation of a cable railing system that meets the opening limitations of the building codes.
Now, we will discuss issues encountered in designing a railing using vertical cables as in-fill.
Top and bottom rails are necessary in a vertical cable railing using. This is because mounting and tensioning hardware is attached to the top and bottom rails instead of end posts.
We recommend schedule 80 pipe or 2″ x 2″ x 1/4″ square tubing for both the top and bottom rail. But, the amount of force that applied to a vertical cable is generally less than is applied to a horizontal cable rail.
The result is less force applied to the mounting and tensioning fittings. So, you may consider using 1/8″ diameter cable with a vertical system, where you may not want to use it in a horizontal system.