Using wire barriers to secure worksite

Wire barrier construction

For clarity, the following terms are defined in relation to wire barriers: 

• Continuous — the wire spans over three or more supports 
• Non-continuous — the wire is supported over two supports 
• Pulley block — a device consisting of a wheel in which a wire runs around to change its direction 
• Permissible deflection — is the allowable bending of the wire 
• Support rails — are the horizontal components of the barrier system that span across the top and bottom to provide structural support. 

Openings in barriers (including decorative barriers) must be constructed so that any opening does not permit a 125mm diameter solid cone to pass through it. For stairs, the space is measured above the nosing line. 

To comply with this requirement, spacing of support posts (post or rail spacing), wire tension, deflection and lay of wire (number of strands by the number of individual wires in each strand) are specified within the National Construction Code (NCC). 

Wire tension 

The installer needs to ensure that the required wire tension is achieved and maximum permissible deflections are not exceeded. The tension can be measured using a strain indicator, if available, or the deflection can be measured using a 2kg mass suspended at mid-span on the wire between support posts. 

The tension and deflection measurements will depend on the type of wire used, support post or rail spacing, wire diameter and lay and wire spacing. Refer to the tables below from NCC Volume Two 2019. 

• Table 3.9.2.1 gives the required wire tension for stainless steel horizontal wire systems and non-continuous vertical systems. 
• Table 3.9.2.2 gives the widest spacing between wires (in mm) and the required wire tension (N) for continuous vertical wire systems or near vertical sloped wire systems 
• Table 3.9.2.3 gives the maximum permissible deflection for stainless steel wires.

It is important to read the notes to the tables as they provide additional information on their application to horizontal, vertical and near-vertical wire barriers. 

Wire spacing 

In horizontal systems and non-continuous vertical wire barriers, the maximum spacing is 100mm and for continuous vertical wire systems 110mm. 

Other provisions 

Other provisions include: 

• For vertical wires the use of two pulley blocks to each 180° change of direction in the wire 
• The maximum spacing for support rails in vertical systems is 900mm to ensure that the rails do not deflect and decrease the tension in the wires 

Care and maintenance are necessary to ensure that the wire tension will be maintained during the life of the barrier. In some situations, it is necessary to install 'locking oft' devices to prevent loosening of wires. 

NCC Volume Two Table 3.9.2.1 Wire barrier construction — Required tension for stainless steel horizontal wires

Notes to Table 3.9.2.1

1. Lay = number of strands by the number of individual wires in each strand. For example a lay of 7x19 consists of 7 strands with 19 individual wires in each strand.
2. Where a change of direction is made in a run of wire, the tensioning device is to be placed at the end of the longest span. 
3. If a 3.2mm wire is used the tension figures for 3.0mm wire are applied. 
4. This table may also be used for a set of non-continuous (single) vertical wires forming a balustrade using the appropriate clear distance between posts as the vertical clear distance between the rails. 
5. X = Not allowed because the required tension would exceed the safe load of the wire.
6. Tension measured with a strain indicator. 

Table 3.9.2.2 Continuous wire barrier construction — Required tension for vertical or near vertical stainless steel wires 

Notes to Table 3.9.2.2 

1. Lay = number of strands by the number of individual wires in each strand. For example a lay of 7x19 consists of 7 strands with 19 individual wires in each strand. 
2. Vertical wires require two pulley blocks to each 180° change of direction in the wire. 
3. Near vertical wires may only require one pulley block for each change of direction. 
4. Tension measured with a strain indicator. 
5. The table only includes 7x7 and 7x19 wires due to other wires not having sufficient flexibility to make the necessary turns. 

Table 3.9.2.3 Continuous wire barrier construction — Maximum permissible deflection for stainless steel wires 

Notes to Table 3.9.2.3 

1. Where a change of direction is made in a run of wire the 2kg mass must be placed at the middle of the longest span. 
2. If a 3.2mm wire is used the deflection figures for 3.0mm wire are applied. 
3. This table may also be used for a set of non-continuous (single) vertical wires forming a balustrade using the appropriate clear distance between posts as the vertical clear distance between the rails. The deflection (offset) is measured by hooking a standard spring scale to the mid span of each wire and pulling it horizontally until a force of 19.6 N is applied. 
4. X = Not allowed because the required tension would exceed the safe load of the wire. 
5. This table has been limited to 60mm and 80mm spaces for 2.5mm, 3mm and 4mm diameter wires because the required wire tensions at greater spacings would require the tension to be beyond the wire safe load limit, or the allowed deflection would be impractical to measure. 

You can download new editions of the NCC from the ABCB website. 

To find out more, contact HIA’s Building Services team.

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