Existing Lifelines - are these “safe-for-use”

Scope

The intent of this document is to create an awareness of the complexity of some of the issues and variables that play a role in the design and selection of horizontal lifelines.

Single user lifelines

Lifelines a.k.a. “Anchor device employing a horizontal flexible anchor line” called “Class C” anchors in SANS 50795:1996 as amended is limited to single users. This is confirmed in the later version of EN 795:2012. Therefore, any lifeline certified under that standard may only be used by one person at any one time. All markings on lifelines must clearly indicate this limitation.

Multi-user lifelines

A technical specification has been developed by the International Organisation for Standardisation (ISO) for flexible lifelines for use by up to three persons simultaneously: This standard is ISO 16024.

In this document the horizontal flexible anchor line is quite similar to that depicted in SANS 50795 under the heading of “Type C”. The nomenclature is also very similar but the real difference between these two lifelines is the load that they are designed to sustain safely.

The single user line is designed to withstand the line forces caused by a 6,0kN fall arrest load whereas ISO 16024 uses 12,0kN irrespective of there being two or three persons attached to it. The ISO document is silent on the dynamic load requirement and specifies 12kN (or an appropriate dead weight) for the static strength test in clause 6.1.1.

Based on the design force of 6kN as per SANS 50795, the tension in a line rigged between two anchors can be calculated if all the variables are known.

The tables below show how the forces increase exponentially as the sag angle decreases.

TABLE 1- forces in lifelines and anchors for single users (SANS 50795)

TABLE 2- forces in lifelines and anchors for multi-users (ISO 16024)

These tables are for information only as they do not consider any other factors such as actual “stretch” under load in the lifeline or deformation / elongation of energy dissipators. These factors will reduce the actual end anchor forces due to the increased sag angle resulting from their elongation, but such performance predications need to be verified by actual tests. All these complexities confirm the notion that lifelines must be designed by competent professional engineers

NOTE – The selection of anchors and lifelines must take into account the minimum factor of safety as required by the relevant standard

Example:- SANS 50795 section 4.3.3.1 states:-

“the minimum breaking strength of the rope shall be at least twice the maximum calculated line tension”.

Testing of existing Horizontal Lifelines (HLL)

All existing HLL should comply with the relevant standard i.e. must be designed by a Pr. Eng. and all components used must withstand the predicted forces with the required factors of safety.

Existing HLL that do not have design certification must be deemed to be unsafe until inspected, tested or proven safe by calculation.

Typically, the in-situ test (or calculation) must consider the following:

• The number of simultaneous users allowed on the line,

• The energy dissipaters (if any) fitted at the end anchors and the anticipated deformation which will require these to be replaced after the test.

• Design strength of the end anchors and connectors

• Load bearing capacity of the structures supporting the end anchors and intermediate support points, if fitted.

• Material and specification of the actual lifeline.

• Length of the HLL between end anchors

• Number and spacings of intermediate support points.

• Maximum allowable deflection under load.

• Any other issues affecting the HLL.

With this information, predicted forces can be calculated and then compared to the capacity of the hardware installed on the site. Should the predicted loads exceed the rated / design capacity of any part of the HLL system, the system should be declared unsafe and taken out of service.

If the rated / design capacities exceed the predicted forces with the necessary factors of safety and following the engineer’s instruction and requirements, a test might have to be carried out keeping in mind that all deforming / sacrificial (energy dissipating) components will have to be replaced after the test.

Any person that is responsible for an installed lifeline and is unsure of its compliance is welcome to contact Riggers Steeplejacks (Pty) Ltd.

Information supplied by Saiosh Corporate Member

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