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SWLs for Aerial Ropeways

by Alan Sheehan BE

Vertical Rescue is both technical, practical and potentially dangerous. This information is provided for trained and experienced vertical rescue operators. As the author has no control over the degree to which the information provided here is understood or applied, he (nor any organisation with which he is associated) can accept any responsibility for the results of application of this information, whether correct or otherwise. While every effort has been taken to ensure there are no errors in this information, this cannot be guaranteed, so it remains the reader's responsibility to check and verify this information before application.

DANGER! Vertical activities where rope is used for life support are dangerous. Serious injury and death may result from misapplication or errors.

Introduction

Aerial Ropeways (flying foxes, tyroleans, highlines - call them what you will...) are amongst the most critical rigs for vertical rescue. The geometry of an aerial ropeway results in loads on the span line and its anchors well in excess of the load being lifted. This multiplying effect makes it vitally important that the geometry and construction of the ropeway is such that these loads can be withstood.

Equation 1 below is the usual formula rescuers use to assess span line tension when constructing an aerial ropeway. This equation is sufficiently accurate for rescue purposes: vector analysis shows its accuracy decreases with increasing sag. For sags up to 25% of the span, the formula is accurate to within 3 per cent. (Note: for sags greater than 25%, a load sharing pair of mainlines or mainline / moveable redirection rig is logically safer in the event of, say, an anchor failure.)

                     Load x Span
Span Line Tension = -------------           (Equation 1)
                       4 x Sag

free body fiagram

force polygonNote how much more force in in the rope tension than the load being lifted.

Safety

The following safety points must be adhered to when rigging and operating aerial ropeways:

*   maintain life support ropes for that purpose only

*   use only proven and approved knots

*   NEVER tension the span line to have a sag of less than 5% of the span distance eg. the sag on a span of 100 metres must never be less than 5 metres.

*   select "bomb-proof" span line anchors - they will be required to hold several times the load being lifted!

*   check and double check everything.

*   monitor all anchors during operations.

*   if rigging a double rope span line, ensure both lines are tensioned equally.

Safety Factors

The following tables present safe working loads (weight that can be safely lifted) on an aerial ropeway. The Safety Factor used is 8: as per Australian Standard AS4142.1 - 1993 Fibre ropes Part 1: Care and Safe Usage and the New South Wales State Emergency Service's Vertical Rescue Training Package. Users are reminded to check the Safety Factor which applies under their legislation and organisation's standards. The safe working loads in the table apply to the span line rope only: it is the operator's responsibility to ensure that all anchors, karabiners and associated equipment are also strong enough to withstand the span line loads with the appropriate safety factor. The safe working loads in these tables only apply to nylon kernmantle static life rescue line (and Steel Wire Rope used only for life support purposes where noted).

Safe Working Loads (kgs) for Single Rope Span Lines

SWL's for a range of Sags as a percentage of Span Length

Rope

Minimum Breaking Load, kgf

Safe Working Load, kgf

5% Sag

10% Sag

15% Sag

20% Sag

25% Sag

11mm Kernmantle

3000

375

75

150

225

300

375

13mm Kernmantle

4000

500

100

200

300

400

500

16mm Kernmantle

5700

713

143

285

428

570

713

11mm SWR

 

1500

300

600

900

1200

1500

Safe Working Loads (kgs) for Double Rope Span Lines

SWL's for a range of Sags as a percentage of Span Length

Rope

Minimum Breaking Load, kgf

Safe Working Load, kgf

5% Sag

10% Sag

15% Sag

20% Sag

25% Sag

11mm Kernmantle

3000

375

150

300

450

600

750

13mm Kernmantle

4000

500

200

400

600

800

1000

16mm Kernmantle

5700

713

285

570

855

1140

1425

Written by Alan Sheehan BE
Land Search Instructor, State Emergency Service
Oberon Unit, New South Wales, Australia
email: als@ix.net.au