HLL Basic Calculation

7/24/2019 HLL Basic Calculation

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Designing and calculating for flexible

Horizontal Lifelines based on design code

CSA Z259.16By HOE Yee Pin and Dr. GOH Yang Miang

Introduction

This article uses two worked examples to illustrate the fundamental design approach and calculations

of a Horizontal Lifeline (HLL) systems based on the design code Canadian Standards Association

Z259.16. The upcoming Singapore Standard on “Specification for Design of Active Fall Protection

Systems” is based on the CSA Z259.16. The authors are members of the Working Group for this

upcoming Singapore Standard.

Horizontal Lifelines (HLL) are commonly used to protect users in a fall

Falling from heights is the leading cause of workplace fatalities in Singapore (Ministry of Manpower

2013). Efforts to mitigate this risk has resulted in increased use of fall protection systems. One of the

fall protection systems commonly used in the construction and maintenance industries is horizontal

Lifelines (HLLs).

A HLL is a component that extends horizontally from one end anchorage to another and consists of a

flexible line made from wire, fibre rope, wire rope, or rod, complete with end terminations (Canadian

Standards Association 2004). It provides a continuous anchorage line to which users can attach their

lanyards and other fall arrest equipment (Figure 1).

Figure 1: A typical Horizontal Lifeline (HLL) System and its parameters



Non-manufactured HLL systems more widely used than Manufactured HLL systems

HLLs can be permanent or temporary, and either a manufactured or non-manufactured system.

Manufactured fall arrest system refers to a complete system designed by a manufacturer. In contrast,

non-manufactured system refers to a system that is not designed by a manufacturer but may or may

not be designed by a Professional Engineer.

Non-manufactured systems are usually assembled from separate fall arrest system components and

can be from different manufacturers.

Non-manufactured systems are more commonly used than manufactured systems in the Singapore

construction industry (Hoe, Goh, et al. 2012). However, non-manufactured systems are more

vulnerable to component incompatibility and require more considerations to ensure effectiveness of

the system.

Critical for Engineers to properly design non-manufactured HLL systems

In Singapore, it is common practice to mitigate this risk by engaging a Professional Engineer (PE) to

design the HLLs. Based on a study by Hoe et al. (2012), 3 out of 5 fall arrest systems sampled from the

construction industry were designed and endorsed by PEs. Since non-manufactured HLLs are

prevalent and PE design usually comes with the HLLs, it is imperative that PEs properly design HLL

systems to function effectively.

A properly designed HLL protects users and complies with the legal

requirements

Common design mistakes

The purpose of a HLL (or any other fall arrest system) is to minimize injury to the users in the event of

a fall. Two common mistakes designers make are

1) only considering the strength aspects of the anchorages and the HLL components but

neglecting to evaluate the effects on the user(s) e.g. Maximum Arrest Force (MAF), and

2) using static analysis that ignored the dynamic force component generated in a fall.

These mistakes had led to strength requirements being grossly underestimated and critical safety

factors being neglected in the design (Wang, Hoe, et al. 2014).

The essential design criteria for an effective HLL

With reference to Figure 1, for a HLL system to be effective in protecting user(s), the following criteria

have to be met:

(i) system components and its anchorages are of adequate strength to withstand the

Maximum Arrest Load (MAL) or Maximum Arrest Force (MAF) to prevent failure;

(ii) Maximum Arrest Force (MAF) experienced by the user(s) is within acceptable limits to

minimize the probability of injuries;

(iii) clearance height required in a fall is less than clearance available to prevent the user(s)from hitting the ground or an obstruction in the fall path.



Compliance with legal requirements

At the same time, the Workplace Safety and Health (Work At Heights) Regulations 2013 Regulation 11

requires that a fall arrest system

(a) is of good construction, sound material and adequate strength,

(b) incorporates a suitable means of absorbing energy and limiting the forces on a user’s body,

and

(c) in the event of a fall, there is enough fall clearance available to prevent the user from hitting

an object, the ground or other surfaces.

Worked Example 1: Single span HLL single user

Figure 2 shows a common design and setup of a HLL with a wire rope attached to anchor posts at both

ends.

Before a fallFigure 2: Common example of HLL

The following information are given in Figure 2:

HLL Rope Properties HLL Configuration

Rope diameter d 10mm Anchor-to-anchor span length L 10m

Nominal Rope Elastic Modulus E 55.8GPa Pretension Force Ti 1kN

Nominal Rope Unit Weight w 3.4N/m

From the above, we can calculate:

Cross-sectional

area of HLL A =

4 = 3.142

104

= 78.550mm2

Initial cable sag

due to its own

weight

si =wL

8T 1 − wL2T

=(3.4)(10)

8(1 × 10) 1 − (3.4)(10)

2(1 × 10)

= 42.506 × 10m

Initial cable length li = L 1 +8

3s

L = 10 1 +

8

342.506 × 10

10 = 10.0004818m



Unstressed HLL

cable length lo

=l

1 + TAE

=10.0004818

1 + 1 × 10(78.55 × 10)(55.8 × 10)

= 9.998200715m

Stage 1 of fall arrest: onset

Assume the user fell at mid-span of the HLL. The HLL will sag to “cusp sag” before it begins to provide

significant deceleration force to stop the fall (Figure 3). Cusp sag is the state where the initial length

of the cable, at essentially its pretension force, pulled into two essentially straight lines extending from

one anchorage, to the point of fall arrest load application, to the next adjacent anchorage (Canadian

Standards Association 2004) i.e. li pulled into two straight lines. Hence, using Pythagoras Theorem,

Cusp sag, sc = l − L =

√ 10.0004818 − 10 = (49.082155 × 10)m

Figure 3: Stage 1 of Fall

To continue our analysis, we have to make some assumptions on the personal fall arrest system.

Personal Fall Arrest System Basis of assumption made

PEA Max Deployment Force Fmax 6kN Without more accurate information on the

manufacturer and model, these values are assumed as

they are the maximum allowed in SS 528:Part 2 to whichthe PEA is certified to.

PEA Max Extension xmax 1.75m

PEA Average Deployment Force Favg 4.8kN CSA Z259.16 Clause 7.3.3.2 where Favg = 0.8 Fmax since SS

528:Part 2 is similar to CSA Z259.11. Note: We can also

take F avg = 3.2kN (Goh 2014)

User(s) weight W 100kg Using the maximum user weight allowed in SS 528.

D-ring height above HLL anchorage hD 0.5m Assuming harness D-ring is an average of 1.5m from

user’s feet.

hD = 1.5 – (height of HLL anchor post) = 1.5 – 1 = 0.5m

Lanyard length Ly 2m -- (Given) --

PEA: Personal Energy Absorber



We can now calculate the Free Fall (FF) experienced by the user,

FF = h + s − + L = 0.5 + (49.082 × 10) + 2 = 2.549m

Stage 2 of fall arrest: energy absorption

The user’s fall is now being arrested. Energy analysis is used as per CSA Z259.16 Clause 9.3.3.

Stage 2.1: Kinetic energy generated in the fall will be absorbed by the elongation or sagging of the HLL

cable (beyond cusp sag). This midpoint sagging, s, will continue until the force in the lanyard, F, reaches

the deployment force of the lanyard’s Personal Energy Absorber (PEA).

Stage 2.2: At the PEA’s deployment force, the PEA will deploy and is assumed to be solely responsible

for the absorption of the energy generated by the falling user (HLL assumed to stop extending in this

stage).

Stage 3 of fall arrest: Energy is dissipated and fall is arrested

The PEA will continue to extend until the potentialenergy is totally absorbed and the remaining energy

Uk is zero. The fall is arrested and the user comes to a stop.

Analysing the fall using energy balance method

One approach is to balance the energy generated and absorbed for Stage 2 then Stage 3.

Stage 2: The fall energy generated is absorbed by the sagging of the HLL cable. We find the value of

the midpoint sagging s at which the force in the lanyard, F, is equal to the PEA deployment force. For

strength calculations, the PEA maximum deployment force should be used as per CSA Z259.16 Clause

7.3.3.1. Thus, we find the midpoint sagging by guessing an arbitrary value for s, then iterating s until F

= Fmax.

Iterate s until F = Fmax = 6kN

1 2 3 4 5

Midpoint sag (m) s = (l − L) 0.3 0.5 0.55 0.551 0.5515

Cable length for given

sag (m) l = L − 4s 10.018 10.050 10.060 10.061 10.061

HLL elongation (m) x =T

k 0.020 0.052 0.062 0.062 0.062

Tension in cable (kN) T = kx 8.67 22.65 27.23 27.32 27.37

Force in Lanyard (kN) F = 4Ts

l 1.04 4.51 5.95 5.99 6.00

Stage 3: When F reaches the PEA deployment force, the PEA deploys. The sagging of the HLL has

already absorbed UHLL and the PEA will absorb UPEA as it extends xPEA.

However, as the PEA is extending, energy is also being generated in addition to the energy generated

during the free fall. This energy generated by the falling user over the total fall distance (h TFD), Uw and



the initial energy stored in the HLL at cusp sag, U HLLo has to be completely absorbed for the user to

come to a stop.

To analyse this, we start with an arbitrary value for xPEA then iterate xPEA until the remaining fall energy

Uk = 0. Before we do that, we have to calculate the following parameters.

HLL Rope Modulus kHLL =AE

l =(78.55 × 10)(55.8 × 10)

9.998200715= 438.388kN/m

Energy Stored in

HLL at cusp sag UHLLo =

k s =1

2(438.388)(49.082155 × 10−3) = 0.00kN-m

Energy absorbed

by HLL elongation UHLL =

k x =1

2(438.388)(0.054) = 0.64 kN-m

For clearance calculations, the PEA average deployment force, Favg should now be used instead as per

CSA Z259.16 Clause 7.3.3.2.

Iterate xPEA until Uk = 0

1 2 3 4

PEA extension (m) x 0.3 0.6 0.55 0.56

Total Fall Distance (m) h = FF − s + s + x 3.352 3.652 3.602 3.612

Energy generated by

falling user (kN-m) U

= Wh

3.29 3.58 3.53 3.54

Energy absorbed by PEA

extension (kN-m)U = Fx = Fx 1.44 2.88 2.64 2.69

Remaining energy (kN-m) U = U + U − U − U 0.99 -0.15 0.04 0.0

The fall energy has been fully absorbed by the HLL and PEA and the fall is now been completely

arrested. (Note: A situation can arise when there is fall energy remaining even after the PEA has

extended to its maximum length i.e. the capacity of the PEA is exceeded and the PEA has “bottomed-

out”.)

Results of Analysis

Workplace Safety and Health (Work At Heights) Regulation 11(2)(b) requires the fall arrest system to

have “enough fall clearance available to prevent the user from hitting an object, the ground or other

surfaces”.

This fall clearance includes the harness and D-ring slide during the fall, xw and a clearance margin (also

known as safety distance), E. We will assume xw to be 0.3m for a harness using normal webbing.

The clearance margin (as per CSA Z259.16 8.2.6.2),

E= 0.6 + 0.1(s − s) = 0.6 + 0.1(0.5515 − 49.082155 × 10−3) = 0.650m



Thus, the fall clearance required (measured from the platform),

C = h + x + E = 3.612 + 0.3 + 0.650 = 4.562m

Let us review the analysis results against the essential design criteria for an effective HLL.

Summary of Results Remarks

Fall clearance required, Cp = 4.562m For the HLL to be effective, an assessment of the site where

the HLL is to be installed should be carried out to verify that

there is at least 4.562m of clearance available.

Maximum Arrest Force, MAF

(to the user)

= Fmax = 6kN Since the capacity of the PEA was not exceeded in this fall,

the forces on the user is limited (as required by WSH WAH

Reg 11(2)(a)) to an acceptable 6kN as specified in CSA

Z259.16 Clause 6.4.2.2.

Maximum Arrest Load, MAL

(to the anchors and wire rope)

= T = 27.37kN The anchorages and wire rope will need to be able to

withstand this MAL with an additional safety factor of 1.5

as per CSA Z259.16 Clause 6.2.3 i.e. 41.06kN.

Worked Example 2: Single span HLL multiple users

For both safety and productivity reasons, a HLL should be designed for at least 2 users. Using the

same parameters in Worked Example 1 above, we now analyse the HLL for the effect of 2-user fall

using the equivalent lumped mass approach as per CSA Z259.16 Clause 7.2.7.2.

Lumping factor, M, for flexible anchorage systems

Number of users falling Systems using PEAs

2 1.75

3 2.25

4 2.75

Applying the lumping factor of 1.75 for 2 falling users, the following parameters and assumptions are

adjusted as follows.

Personal Fall Arrest Systems

User(s) weight W 100kg x 1.75 = 175Kg

PEA Max Deployment Force Fmax 6kN x 1.75 = 10.5kNPEA Average Deployment Force Favg 4.8kN x 1.75 = 8.4kN



We now use the above adjusted values to analyse for a 2-users fall. We iterate s until F = adjusted Fmax

of 10.5kN

Iterate s until F = Fmax = 10.5kN

1 2 3 4 5 6

Midpoint sag (m) s = (l − L) 0.5 0.8 0.6 0.66 0.667 0.6675

Cable length for given

sag (m) l = L − 4s 10.05 10.127 10.072 10.087 10.089 10.089

HLL elongation (m) x =T

k 0.052 0.129 0.074 0.089 0.090 0.091

Tension in cable (kN) T = kx 22.65 56.54 32.24 38.81 39.62 39.68

Force in Lanyard (kN) F = 4T

s

l

4.51 17.87 7.68 10.16 10.48 10.50

Again, we now iterate for xPEA until the fall energy is totally absorbed i.e. Uk = 0.

Iterate xPEA until Uk = 0

1 2 3

PEA extension (m) x 0.5 0.55 0.545

Total Fall Distance (m) h = FF − s + s + x 3.668 3.718 3.713

Energy generated by

falling user (kN-m) U = Wh 6.30 6.38 6.37

Energy absorbed by PEA

extension (kN-m)U = Fx = Fx 4.20 4.62 4.58

Remaining energy (kN-m) U = U + U − U − U 0.30 -0.03 0.00

The adjusted clearance margin is now

E = 0.6 + 0.1(0.6675 − 49.082155 × 10−3) = 0.662m

The clearance for the equivalent lumped mass

C = h + x + E = 3.713 + 0.3 + 0.662 = 4.675m

The clearance required for the last user to fall (as per CSA Z259.16 Clause 8.2.7)

C = 1.6C − 0.6C = (1.6 × 4.675) − (0.6 × 4.562) = 4.743m

Using the same methodology as above and applying different lumping factors, 3 and 4-user falls can

also be analysed. The results are summarized as follows.



Comparison of Results 1-user 2-users 3-users 4-users

Free fall experienced by user (m) 2.549

Maximum Arrest Force, MAF (kN)

(experienced by the user)

6

Fall clearance required, Cp (m) 4.562 4.743 4.834 4.913

Maximum Arrest Load, MAL (kN)

(to the anchors and wire rope)

27.37 39.68 46.90 53.62

Minimum tensile strength required

for anchorages and wire rope (kN)

41.06 59.52 70.35 80.43

Other scenarios for consideration

The above two examples are simplified to illustrate the fundamental design parameters. HLLs

deployed in the real world can be more complicated requiring sophisticated analysis. Such real-world

HLL scenarios can include:

Energy absorbers incorporated in-line with the HLL where balance sag analysis will apply.

Multiple-span HLLs where the slack from the other spans will be pulled into the span where

the user fell before the HLL begins to tension up, affecting the cusp sag. The rope modulus will

also decrease with the longer length of wire rope used.

Pre-tension forces in the HLL changing due to temperature effects.

HLLs are anchored to flexible end anchorages instead of rigid end anchorages.

Conclusion

HLLs are commonly used to protect workers and minimize injuries to users in a fall. However, strength

requirements were often grossly underestimated and critical safety factors were neglected due to

common design mistakes.

A properly designed HLL needs to minimize injury to the user and to comply with the relevant legal

requirements. Thus the design criterion need to consider the Maximum Arrest Force (MAF) to the user,

the Maximum Arrest Load (MAL) to the anchors and the clearance height required.

This article demonstrated using energy balance approach to evaluate the above-mentioned design

criterion for a single-span HLL system based on the design code CSA Z259.16. A 1-user fall was first

analysed followed by a 2-user fall.

It is hoped that this article can raise awareness of the various parameters that designers should take

into consideration in their design and evaluation of horizontal lifeline systems.

Acknowledgement

The authors have attended the Qualified Fall Protection Engineer course by Engineer Greg Small and

his co-trainers in North America. The calculations described herein are based on an Excel template

created by Er. Small.



References

Ministry of Manpower (2013) Occupational Safety and Health Division Annual Report 2012

http://mom.gov.sg/Documents/safety-health/reports-stats/OSHD-

AR2012/OSHD_AR2012_part1.pdf

Canadian Standards Association (2004) Z259.16-04 Design of Active Fall-Protection Systems Ontario:Canadian Standards Association

Goh, Y.M., 2014. An Empirical Investigation of the Average Deployment Force of Personal Fall Arrest

Energy Absorbers. J. Constr. Eng. and Manage. - Am. Soc. of Civ. Eng. (published online).

Hoe, Y. P., Goh, Y. M., Sim, S. Y. (2012) Design of Fall Arrest Systems: A Review of the Current Issues in

the Singapore Construction Industry. “CIB W099 International Conference on Modelling and

Building Health and Safety” 10-11 September 2012, Singapore

Wang, Q., Hoe, Y. P., Goh, Y. M. (2014) Evaluating the Inadequacies in Horizontal Lifeline Designs: Case

Studies in Singapore. “CIB W099 International Conference on Achieving Sustainable

Construction Health and Safety”, 2-3 June 2014, Lund, Sweden