METAL CABLE RAILING FRAMEWORK

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 designed to be flexible for going over pulleys or for lifting/moving heavy loads. Other constructions of cable are designed to hold something in tension, such as guy wire or a sailboat stay, and are less flexible. For any particular diameter of cable, the tradeoff for flexibility is strength. The opposite is also true. You compromise strength when you require a construction of cable that is capable of a higher degree of flexibility.

Cable flexibility is an important consideration in designing a cable railing. The IRC and IBC require that a 4″ sphere shall not pass through any portion of railing/stair rail. Having the rigidity to prevent deflection of a horizontally run cable that is subjected to a vertical load is partly mitigated by the cable’s lack of flexibility. Therefore, it is our preference to use the most rigid of cable constructions possible when designing a railing using cable. The other factors are the tension of the cable, the span between supporting intermediate members, the diameter of the cable, and the vertical spacing of the cables on center.

Let’s start with the spacing of your intermediate members, which are posts and/or braces, which will support the cable as it passes through the walls of the railing frame. (An intermediate post runs from the top rail to the mounting surface. A brace is a lighter weight material placed between posts, its primary purpose being to support the cable.) Cable can be run quite long distances between terminating ends (60 ft. or more, depending upon railing configuration), but it needs to be supported at intervals between end posts, to avoid cable deflection in excess of that permitted by building codes. When a rigid cable construction is used, such as 1×19, the spacing between posts and/or braces should not exceed 48″.

The next variable is the diameter of the cable. While 1/8″ is the cable diameter most often used for residential applications, we recommend it be used only in an area that is unlikely to experience heavy pedestrian traffic. For most applications, we suggest 3/16″ diameter cable. When the scale of the project is large, a larger diameter cable may be preferred from an aesthetics standpoint. We offer systems using up to 3/8″ diameter cable.

Spacing of the cables vertically is critical to minimize deflection of the cables under a vertical load. Our specifications provide recommended vertical spacing of 3-1/8″ on center for cables when they are installed.

The last variable is the tension of the cables and the construction of posts to which mounting and tensioning hardware is attached. Deflection of the end posts must be minimized, and this is where we have found the most mistakes made in the design of the railing framework. An incredible amount of force is placed on an end post when you have ten or more lines, each tensioned to a minimum of 225 lbs. over a height of 36″ to 42″. Often, 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 is end posts which will bend considerably as the cables are being tensioned…or with a railing where the cables cannot be properly 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.