During the last decade, tensioned wire balustrade systems have been increasing in popularity within the residential and commercial sectors. Compared to glass balustrades, tensioned wire systems provide a cost-effective solution while maintaining comparable unobstructed sightline features. Because the wire acts as the main structural component, it is necessary to understand the details of the specification, as well as the installation and the associated risks, which are not immediately obvious when viewing the system.

The majority of the wire systems used in balustrade systems employ stainless steel. For external balustrades, the most common specification is grade 316 stainless steel, and for internal applications, grade 304 is the most common. Because grade 316 has molybdenum as an alloy, it has enhanced resistance to corrosion from chloride, which is important for coastal installations or instances when road salt or cleaning agents are used. It is common practice to specify grade 304 for external balustrades in coastal or high-chloride environments, but in doing so, results in corrosion of the balustrade system in a very short time. Based on performance, the specification should be given careful consideration and not simply default to the lowest contractor bid.

Balustrades generally use wire with diameters between 3 mm and 5 mm, with 4 mm being the most common. Required tension and stiffness directly impacts the diameter and the visual weight of the installation. Horizontally arranged wire balustrades are required to withstand a horizontally applied load. According to the Building Regulations Approved Document K, most commercial and residential barriers are required to withstand a load of 0.74 kN/m for residential barriers and between 0.74 kN/m and 3.0 kN/m for commercial barriers depending on the occupancy classification of the building. The load in question is transferred to the balustrade assembly through the posts, end fittings, and the tensioning system, and the balustrade system as a whole, and not solely the wires. Balustrade system design must consider wire diameter, tension, and spacing to determine compliance.

According to Part K of the Building Regulations, the 100 mm sphere rule also governs wire spacing in balustrades. This means a 100 mm diameter sphere must not be able to pass through any opening. Horizontally arranged wires are allowed a vertical spacing of up to 100 mm. This spacing is measured at maximum sag. Wires also deflect mid-span. This must also be accounted for in the sag of a tensioned wire system, and will not be measured at a post. Balustrade systems using vertical wire spacing do not have a sag issue, but do have design challenges at the wire terminations.

The greatest variation among systems from different suppliers is in the tensioning hardware. The most common fitting in commercial systems is the swaged terminal, which provides a secure and permanent termination of the wire. Adjustable fittings are tensioned using a threaded barrel, which is an important feature because wire tension should be adjusted as the system settles. Tensioning hardware should never be adjusted from the design tension, which is specified by the manufacturer or a structural engineer. Beyond a prescribed tension, it will place excessive load on the posts and fixing points. On the other side of the spectrum, an insufficient tension will allow it to deflect and potentially place the system in violation of Part K.

Most environments require little maintenance for stainless steel wire, which is a true advantage compared to painted steel. In most environments, especially the aggressive marine, high chloride and urban environments, it should be cleaned routinely using a mild detergent and surface contaminants removed. If this is not done, crevice corrosion will occur at termination fitting. Deterioration is first noticed at the fittings and not the wire. Because of this, the fittings should be inspected annually for installed wire used externally.

When balustrading wire is routed through intermediate posts, a sleeve or grommet at the point of contact should be non-abrasive non-reactive and should not corrode stainless steel. In a humid environment, and especially with the presence of moisture, certain metals should not be in contact with stainless steel, or a form of galvanic corrosion will occur. The standard approach for this is to use nylon or PTFE, and should be specified and not left to the installer to source and procure.