Zip Line Terminations

Purpose

The purpose of this safety alert is to inform zip line owners, operators, engineers and riggers of the risk of failure of zip line terminations and to provide guidance to help prevent the failure of zip lines.

A zip line consists of a steel wire rope that is suspended between two points, along which one or more people travel while suspended from trolleys. Zip lines are also known as flying foxes.

This safety alert provides information on zip line terminations and does not apply to other suspended rope applications.

Background

In October 2019, an 86-metre zip line termination failed and two patrons dropped to the ground, causing fatal injuries to one and serious injuries to the other. The two patrons were travelling along the zip line, one behind the other, with a rope link between them.

Rope terminations on zip lines generally use either of two basic types of rope termination:

• pressed ferrule or swage type rope terminations (refer Photograph 1)
• wire rope grips (e.g. bulldog grips) (refer Photograph 2).

Both pressed ferrule terminations and wire rope grips rely on the application of pressure between two parallel sections of wire rope, the rope tail and the loaded section. The applied pressure creates frictional resistance to avoid failure of the termination by preventing relative movement.

Over time the wire rope characteristics may change (e.g. the rope diameter may reduce) so that a termination which previously withstood applied loads may suddenly slip and cause failure of the zip line.

Visual inspection of rope terminations may not be adequate on its own as a means for verifying the ongoing adequacy of terminations.

The rope tension in zip lines can be extremely high and includes:

• pre-tension applied to the rope during set up (to reduce sag, i.e. ‘catenary’)
• tree movement
• the additional loads applied by patrons using the zip line.

It is important to note that loads applied by patrons to the zip line and its end anchorages will be many times their self-weight due to the triangulation effect of the load. For example if a 100 kg patron uses a zip line, the additional tension applied to the zip line and its anchorages could be more than five times greater (i.e. potentially in excess of 500 kg force).

Contributing factors

The incident is being investigated. It appears that the rope grip type termination at the top end of the zip line failed, allowing the zip line to disengage suddenly and causing both patrons to hit the ground.

Action required

All zip lines, including the zip line terminations and anchorage systems, should be designed or verified by a suitably qualified Registered Professional Engineer of Queensland (RPEQ). The engineer should verify all components of the zip line including the methods used to install the hardware. The engineer should provide written instructions on periodical inspection methods for the zip line along with any limitations on its use (e.g. loading, wind).

Periodic load testing of every zip line, with a safety factor of 2, and post-test inspection, should be carried out at intervals as specified by the design engineer, but should not exceed 12 months, unless specifically justified by the engineer

Topics include:

• Design Loads
• Test Loads
• Calculation of proof test load
• Pressed ferrule rope terminations
• Wire rope grips

Source
Safety Alert
Workplace Health & Safety