twf anchors and fall arrest-finalr0 · 2020. 11. 21. · breaking strength of 25kn when tested in...

Anchors and Fall Arrest System

A guide to good practice

Publication

Published by HK Temporary Works Forum (HK-TWf)

Published October 2020

HK-TWf2017: WG3_Draft

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Contents

Contents ............................................................................................................................................................. 1

Foreword ............................................................................................................................................................ 3

Acknowledgement .......................................................................................................................................... 3

References ...................................................................................................................................................... 3

1. FALL ARREST FORCES .................................................................................................................................. 4

1.1 Fall Arrest ............................................................................................................................................ 4

1.2 Fall-Restraint Lanyard / Non-Shock-Absorbing Lanyard ..................................................................... 4

1.3 Shock-Absorbing Lanyards .................................................................................................................. 5

1.4 Self-Retracting Inertia Fall Arrestors ................................................................................................... 6

1.5 Self-Retracting Lanyards ..................................................................................................................... 6

3 VERTICAL LIFE LINES .................................................................................................................................... 7

3.1 Vertical Life Lines ................................................................................................................................ 7

3.2 Rope Knots .......................................................................................................................................... 8

4 ANCHORS .................................................................................................................................................... 9

4.1 Temporary Anchors ............................................................................................................................ 9

4.2 Permanent Works Anchors ............................................................................................................... 10

5 PERMANENT FALL ARREST & RESTRAINT SYSTEMS .................................................................................. 11

5.1 Permanent Fall Arrest & Restraint Systems ...................................................................................... 11

6 HORIZONTAL LIFE LINES ............................................................................................................................ 12

6.1 Horizontal Life Lines – Fall Arrest ...................................................................................................... 12

6.2 Horizontal Life Lines – Fall-Arrest Forces .......................................................................................... 13

7 INFORMATIVE ........................................................................................................................................... 14

ANCHORS AND FALL-ARREST SYSTEM

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7.1 Standard 20kN Fall-Arrest Bracket .................................................................................................... 14

7.2 Horizontal Life Lines – Typical Assembly Details ............................................................................... 15

7.3 Vertical Life Lines – Typical Fixing Details ......................................................................................... 16

7.4 Typical Fall Arrest Anchor Details ..................................................................................................... 17

7.5 Typical Fall Restraint Detail ............................................................................................................... 18

7.6 Typical Fall Arrest Detail ................................................................................................................... 19

7.7 Typical Fall Restraint Detail ............................................................................................................... 20

8 REFERENCES: HONG KONG AND BS EN CODES AND GUIDES ................................................................... 21


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Foreword

Fall-arrest systems do not prevent falls and they should only be used when no other practical means of access is available. This guidance document highlights the two fall-arrest systems commonly used in Hong Kong: 1. The traditional non-shock-absorbing lanyard is used by most workers in Hong Kong and should only

be connected to a 20kN anchor and appropriately designed systems. 2. Most new buildings in Hong Kong have permanent 6kN anchors and fall arrest systems that comply

with international codes and are only compatible with shock-absorbing lanyards. Workers should never hook onto permanent works fall arrest anchors and systems without EM approval and appropriate lanyards.

These systems are not interchangeable, and non-shock absorbing lanyards should never be connected to anchors (systems) designed for use with shock absorbing lanyards.

This guidance document should be read together with relevant Hong Kong and International codes, and for ease of reference, only HK and BS EN codes are referred to. The anchor loads stated in this document are unfactored, and appropriate safety factors should be applied when designing these systems.

Iain Mowatt Chairman HK TWf

Acknowledgement

The Hong Kong Temporary Works Forum (HK-TWf) gratefully acknowledges the contribution made by members of the working group in the preparation of this guidance:

Working Group Chair Robinette, William Lambeth Associates Working Group Chan, Tommy Tsz Fung MTRC Limited Cheng, Chris Lambeth Associates

References

1. Guidance Notes on Classification and Use of Safety Belts and their Anchorage Systems, Labour Department, HKSAR

2. BS 8437:2005+A1: 2012 Code of Practice for selection, use and maintenance of personal fall protection systems and equipment for use in the workplace

3. BS EN 795:2012 Personal fall protection equipment – Anchor devices 4. PDS/05 – Temporary Works Design & Construction Control 5. A technical guide to the selection and use of fall prevention and arrest equipment – HSE (UK)


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1. FALL ARREST FORCES

1.1 Fall Arrest

The forces generated when arresting a fall are dependent on the height of the fall, and the system arresting the fall.

A system made up of rigid components will impart the largest arresting forces (and shortest fall distance). While a system made up of elastic components will impart the lowest arresting forces.

For example, rock climbers experience relatively low arrest forces, because they use dynamic (elastic) ropes that are designed to stretch and slow a fall over a relatively long distance.

Equally, the arrest forces generated when a worker falls 4.0m while attached to a rigid anchor and non-shock-absorbing lanyard (fall restraint lanyard) are relatively high – and the worker should immediately be taken to hospital for a check-up.

1.2 Fall-Restraint Lanyard / Non-Shock-Absorbing Lanyard

When a 100kg worker is connected to a 2.0m fall restraint / non-shock-absorbing lanyard, and falls 4.0m (fall factor 2), the impact force on arrest of the fall will be 18kN to 20kN (reference: BS 8437:2005+A1:2012: 12.8.1).

An example of a 4.0m fall would be a worker erecting handrails on the edge of a building, while hooked on at foot level.

The minimum clear fall height when using a 2.0m non-shock-absorbing / fall-restraint lanyard is 4.0m + 1.0m free space = 5.0m.

The fall distance and arresting forces can be significantly reduced by hooking on above waist level, and preferably above shoulder level.

The fall restraint / non-shock-absorbing lanyard is the most commonly used lanyard in Hong Kong, and all anchor systems should be capable of safely supporting a minimum of 20kN when connected to a single lanyard or vertical life line.

4.0M

2.0M


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Note that arrest forces induced in anchors for horizontal life lines must be calculated for every system and can be > 20kN.

Restraint lanyards should be removed from service and inspected by a competent person after any fall-arrest incident.

1.3 Shock-Absorbing Lanyards

When a 100kg worker is connected to a 2.0m shock-absorbing lanyard and falls, the maximum arrest force is limited to 6.0kN by allowing the lanyard to stretch and slow the fall over a maximum of 1.75m (reference: BS EN 355:2002).

It is important this 1.75m stretch is considered when specifying these systems, as the fall distance for a “fall factor 2” increases from 4.0m to 5.75m when compared to a non-shock-absorbing lanyard.

The minimum clear fall height when using a shock-absorbing lanyard is 5.75m + 1.0m free space = 6.75m.

Shock-absorbing lanyards should never be used for fall restraint (they will activate / stretch when loaded beyond 2kN).

BS EN 795 specifies a 12kN (2:1 Safety Factor) anchor for use with these systems. However, where there is a risk of workers clipping on with a non-shock-absorbing lanyard, anchors should be designed for a 20kN arrested fall when connected to a single lanyard or vertical life line.

Shock-absorbing lanyards must be inspected regularly and should never be re-used after any fall-arrest incident.


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1.4 Self-Retracting Inertia Fall Arrestors

When a 100kg worker is connected to a self-retracting inertia fall arrestor and falls, the maximum arrest force is limited to 6.0kN by locking the internal mechanism the instant a fall (or trip) is detected. The system works on a similar principle to a car seat belt – and when installed and operated correctly, can limit fall heights to < 100mm (reference: BS EN 360:2002).

The minimum clear fall height when using an inertia fall arrestor is dependent on the application and range of movement of the operator. When necessary, load tests should be conducted to confirm fall heights / lateral swing distance.

Whenever possible, inertia fall arrestors should be fixed directly above the worker, and BS 8437:2005+A1:2012: recommends not > 14° from vertical.

The fall-arrestor life line (and lanyard if connected) must be free of obstructions and must remain taught (engaged) at all times. Refer to sections 7.6 and 7.7.

Fall arrestors should NOT be fixed below head height unless fitted with an additional energy absorber and authorized by the EM.

Inertia fall arrestors must be inspected daily and in accordance with the manufacturer’s instructions. The operation of the inertia lock must be tested before use – and if not satisfactory, the fall arrestor will be withdrawn from use.

1.5 Self-Retracting Lanyards

Self-retracting lanyards should only be used for fall restraint. They are most useful when a non-standard length restraint lanyard is required – Section 7.5 details a typical fall restraint system on a slab edge.

Self-retracting lanyards should not be used for fall arrest.

Self-retracting lanyards should be inspected daily before use. Damaged and suspect units should be removed from site.


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3 VERTICAL LIFE LINES

3.1 Vertical Life Lines

Vertical life lines should comply with the Hong Kong Guidance Notes. A separate life line must be provided for every worker, and workers must use an approved fall-arrest device to clip on.

Wire ropes shall have a minimum diameter of 8mm, and have a minimum breaking strength of 25kN when tested in accordance with BS EN 12385-1:2002 + A1: 2008.

Fibre ropes shall have a minimum diameter of 16mm and have a minimum breaking strength of 22kN. Refer to BS EN 1891:1998 / low-stretch Kernmantel ropes / Grade A.

Wire and fibre ropes should be terminated with eyes and fixed with shackles to suitable permanent works / anchor points.

Anchor points should be designed to support the applied loads and must be certified by a Registered Professional Engineer as noted in the HK Guidance Notes.

• Do not allow unprotected life lines to run over sharp edges. • Do not use knots to join life lines end to end. • Do not fix life lines to handrails, pipework, window frames. • Do not fix life lines to scaffolding unless it is safe to do so / approved by EM. • Do use 2 No ropes / worker where ropes are the primary means of support (slope work etc).


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3.2 Rope Knots

Knots should never be used to hook on / join ropes together or provide ad-hoc connections for life lines and anchorages.

The breaking strength of a rope can be reduced by 45% when knotted (reference: HSE (UK) Contract Research Report 364/2001 Sec. 3.4.3), and a 50% reduction in breaking strength is recommended.

Workers hooking onto knotted ropes may not be protected in the event of a fall from any height, and the practice should be prohibited for any fall arrest / restraint system.

Knotted ropes and ropes exposed to any fall arrest incident should NOT be re-used, and should be removed from site.

Approved fall arrestors must be used to hook onto vertical life lines in accordance with the Hong Kong Guidance Notes.


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4 ANCHORS

4.1 Temporary Anchors

Work should be planned sufficiently in advance to ensure that the installation of ad-hoc anchors is minimized, and the use of the permanent works for attaching life lines and anchor systems is maximized.

Good examples are: columns / holes in slabs and walls / exposed re-bar in walls and slabs that provide workers with engineered and correctly positioned anchor points.

All anchor points shall be certified by a Registered Professional Engineer as noted in the HK Guidance Notes.

All anchor points shall be clearly marked with their safe working load / lanyard type / maximum no of workers permitted to use the anchor system.

Anchor points shall be designed to suit the applied load / but NOT less than 20kN when used for fall arrest, and with a minimum 2:1 safety factor. The Hong Kong Guidance Notes also require that all anchor points shall be designed for a minimum pull out load of 5kN.

Safe use and operator instructions - In addition to the information provided by the anchor / system designer, formal training must be provided for all users and will include: safe use, number of workers who can use the system, type of lanyards, rescue and supervision.


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4.2 Permanent Works Anchors

Permanent works anchors will normally be designed in accordance with international codes and regulations that limit fall arrest forces to 6kN.

Workers should NOT be permitted to use these systems unless the systems have valid test certificates and manufacturers / designers user information (refer BS EN 795:2012 Annex A).

• The test certificates and the user information should confirm:

• The maximum load / number of workers permitted to use each anchor.

• The type of lanyard and fall distance

o Shock absorbing lanyards. o Fall restraint lanyards. o Retractable / inertia fall arrestors. o Length of lanyard. o Fall distance.

• For example, a worker attaching a non–shock-

absorbing lanyard to a permanent anchor could overload the anchor by a factor of 3. Equally, a worker attaching a shock-absorbing lanyard to a permanent anchor may impact the ground before the fall is arrested.

The HKCIC has published a useful guidance document for the design / maintenance / inspection of permanent works anchors in new buildings (www.hkcic.org). These anchor points are designed for use with shock-absorbing lanyards in accordance with BS EN 355:2002.


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5 PERMANENT FALL ARREST & RESTRAINT SYSTEMS

5.1 Permanent Fall Arrest & Restraint Systems

Permanent works fall arrest / restraint systems will normally be designed in accordance with international codes and regulations that limit fall arrest forces to 6kN.

Workers should NOT be permitted to use these systems unless the systems have valid manufactures test certificates and user information.

User information should confirm:

• The maximum number of workers permitted to use each system. The number of users can vary between systems and locations within the same building.

• The maximum length of lanyard permitted on each system - lanyard

lengths can vary between systems and locations within the same building.

• The type of connection to the system – it may be unsafe to connect

lanyards directly to horizontal life lines / systems.

• The type of lanyard to be fixed to the system – this can vary between systems and locations within the same building.

o Fall-restraint lanyard. o Fall-arrest lanyard. o Retractable / inertia fall arrestors.

• Fall Distance - this can vary between systems and locations within

the same building. Where fall heights are restricted by new permanent works / temporary works / vehicles / workers at a lower level – it may be unsafe to operate the system.

• Safe use and operator instructions - in addition to the information

provided by the designer / manufacturer, formal training must be provided for all users and will include: safe use, rescue and supervision.

• Systems – systems are supplier, code and location specific. Do not

mix components and accessories between systems. Do not allow workers to move between systems unless approved by EM.


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6 HORIZONTAL LIFE LINES

6.1 Horizontal Life Lines – Fall Arrest

The forces generated when arresting a fall with a horizontal life line are dependent on the height of the fall (A), and the deflection (V) of the life line from horizontal.

Typically, horizontal life lines with short spans, and / or heavy wire rope will generate the highest anchor loads – and the shortest stopping distance (wire deflection and stretch).

Equally, horizontal life lines with long spans, and / or light wire rope will generate the lowest anchor loads – and the longest stopping distance (wire deflection and stretch).

The combined deflection of the horizontal life line, and the fall height of the connected lanyard, must be considered when specifying these systems. The minimum free space below a suspended worker (C) is 1.0m, and when necessary, the free space should be confirmed with load tests.

Note that the free space must be maintained for the full length of the horizontal line, and an additional allowance may be necessary for any swing induced by the arrested fall.

When connected to a retractable / inertia fall arrestor and arresting a fall, the vertical movement of the life line may cause the inertia fall arrestor to release and engage a number of times, resulting in an increased fall distance. When necessary, fall distances should be confirmed by load tests.


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6.2 Horizontal Life Lines – Fall-Arrest Forces

The vertical loads imposed on a horizontal life line / system are similar to those imposed on a vertical system – but the resultant loads in the horizontal life line and anchors must be determined for each system.

Where there is a risk of workers clipping on with a non-shock-absorbing lanyard - the system will be designed for a minimum static load of 20kn / worker.

(The Hong Kong Guidance Notes refer to US Code: ANSI.Z.359.1.1992 and a load of 5000lb / worker applied anywhere along the line)

Fall Arrest Unfactored Static Vertical Load on System

Reference

1 No Worker

2 No Workers

Each Additional

Worker

PD CEN/TS 16415:2013

Shock Absorbing Lanyard 6kN 12kN 1kN BS EN 355:2002

Inertia Fall Arrestor 6kN 12kN 1kN BS EN 360:2002

Fall Restraint Lanyard 20kN 40kN 1kN BS 8437:2005

Note that the Hong Kong Guidance Notes, Section V3, recommends 1No Worker / Horizontal Life Line.

Anchors will be subject to a combination of horizontal and vertical loads and their design must include a pre-tension load of 0.75kN – 1.0kN to take up the initial stretch and deflection within the wire rope / system. *Note that a fall can occur at any point on a horizontal line and a worst-case combination of shear and tension should be considered.

Horizontal Life Line - Anchor Reactions (Excluding 1.0kN Pre-Load)

Final Deflected Angle α

(T) Tension in Deflected Life Line

(P) Horizontal Reaction in Anchor

150° 1.9W 1.9W

140° 1.4W 1.5W

130° 1.07W 1.2W

120° 0.86W 1.0W

α W*

W

P T


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7 INFORMATIVE

Fall arrest / fall restraint / standard detail and drawings.

7.1 Standard 20kN Fall-Arrest Bracket

• Attach life lines with suitable eyes, clips and shackles.


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7.2 Horizontal Life Lines – Typical Assembly Details

• Always provide suitable protection to wire ropes / chains / slings • Never use open-ended turnbuckles • Minimum 0.75kN -1.0kN pre-tension to take up the initial sag and stretch in the system. • Fall arrest: Minimum 20kN each worker. • Fall restraint: Minimum 200kg each worker. • Do not use fibre ropes for horizontal life lines.


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7.3 Vertical Life Lines – Typical Fixing Details

• Do not use lightweight 1mm blue water hose for edge protection. • Do not join or terminate life lines with knots. • Always use eyes and shackles to connect life lines to permanent works / anchor points. • Always use approved fall arrestor to clip onto rope life lines. • Minimum 1No life line for each worker. • Fall arrest: Minimum 20kN each worker. • Fall restraint: Minimum 200kg each worker.


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7.4 Typical Fall Arrest Anchor Details

• Worker not prevented from moving to slab edge / high risk of fall. • Fall Arrest: Minimum 20kN each worker (non-shock-absorbing / fall-restraint lanyard).


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7.5 Typical Fall Restraint Detail

• Worker restrained by lanyard, and physically prevented from moving to slab edge. • Fall restraint: Minimum 200kg each worker. • Do not use fibre ropes for horizontal life lines.

RESTRAINT LANYARD OR SELF-RETRACTING LANYARD


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7.6 Typical Fall Arrest Detail

• Worker not prevented from moving to slab edge / high risk of fall. • Fall arrest: Minimum 6kN each worker (self-retracting inertia fall arrestor). • Never use a fall arrestor where a worker can fall a significant distance before the fall arrestor will engage

– the initial fall can overload the system / anchor / fall arrestor.


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7.7 Typical Fall Restraint Detail

• Worker not prevented from moving to slab edge / high risk of fall. • Fall arrest: Maximum 6kN each worker (self-retracting inertia fall arrestor). • Consider connecting the fall arrestor directly to the harness D hook to minimize fall distance. • Always use a fall arrestor + shock absorber when the worker can fall a significant distance before the fall

arrestor will engage. (Not preferred / requires EM approval.)

LIFE LINE < 14° FROM VERTICAL

ANCHOR BELOW HEAD HEIGHT. WORKER CAN FALL >2M BEFORE SYSTEM WILL ENGAGE. SYSTEM MUST INCLUDE SHOCK ABSORBER. REQUIRES EM APPROVAL


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8 REFERENCES: HONG KONG AND BS EN CODES AND GUIDES

HKCIC Guidelines on the Design, Installation and Maintenance of Cast-in Anchors at External Walls of New Buildings (2017).

HKOSHC Guidance Notes on Classification and Use of Safety Belts and their Anchorage Systems

(2018) PD1645:2013 Anchor Devices for use by more than one person simultaneously BS 7883:2019 Code of Practice for Anchor Systems – System Design, Installation, and Inspection.

BS 8437:2005 +A1:2012 Code of Practice for Selection, Use and Maintenance of Personal Fall Protection Systems

& Equipment for Use in the Workplace. BS 7985:2013 Code of Practice for the use of Rope Access Methods for Industrial Purposes. BSEN 354:2010 Personal Protective Equipment against Falls Form Height – Lanyards BSEN 355:2002 Personal Protective Equipment against Falls Form Height – Energy Absorbers BSEN 358:2018 Personal Protective Equipment for Work Positioning & Prevention of Falls from a Height

– Belts for work positioning and restraint and work positioning lanyards. BSEN 360:2002 Personal Protective Equipment against Falls Form Height – Retractable Type Fall

Arrestors. BSEN 361:2002 Personal Protective Equipment against Falls Form Height – Full Body Harnesses. BSEN 363:2018 Personal Protective Equipment against Falls Form Height – Personal Fall Systems.

twf anchors and fall arrest-finalr0 · 2020. 11. 21. · breaking strength of 25kn when tested in...

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