Designing and calculating for flexible Horizontal Lifelines based on design code CSA Z259.16 By 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) 2)
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 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) (ii) (iii)
system components and its anchorages are of adequate strength to withstand the Maximum Arrest Load (MAL) or Maximum Arrest Force (MAF) to prevent failure; Maximum Arrest Force (MAF) experienced by the user(s) is within acceptable limits to minimize the probability of injuries; 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 fall Figure 2: Common example of HLL
The following information are given in Figure 2: HLL Rope Properties
HLL Configuration
Rope diameter
d
10mm
Nominal Rope Elastic Modulus
E
55.8GPa
Nominal Rope Unit Weight
w
3.4N/m
Anchor-to-anchor span length
L
10m
Pretension Force
Ti
1kN
From the above, we can calculate: Cross-sectional area of HLL Initial cable sag due to its own weight
Initial cable length
A
=
si
=
li
4
wL
8T 1 −
=L 1+
= 3.142 wL 2T
8 s 3 L
=
10 4
(3.4)(10)
8(1 × 10 ) 1 −
= 10 1 +
(3.4)(10) 2(1 × 10 )
8 42.506 × 10 3 10
=
78.550mm2
= 42.506 × 10 m
=
10.0004818m
Unstressed HLL cable length
lo
=
l
T 1 + AE
=
10.0004818 1 × 10 1+ (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 =
Figure 3: Stage 1 of Fall
√10.0004818 − 10 = (49.082155 × 10 )m
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
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 Favg = 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
Lanyard length PEA: Personal Energy Absorber
Ly
2m
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 which the PEA is certified to.
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 -- (Given) --
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 potential energy 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.
Midpoint sag (m) Cable length for given sag (m) HLL elongation (m) Tension in cable (kN) Force in Lanyard (kN)
s= l=
(l − L )
L − 4s
=
T k
F = 4T
s l
x
T = kx
1
2
0.3
0.5
Iterate s until F = Fmax = 6kN 3 4 5
0.55
0.551 0.5515
10.018 10.050 10.060 10.061 10.061 0.020
0.052
0.062
0.062
0.062
8.67
22.65
27.23
27.32
27.37
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 (hTFD), Uw and
the initial energy stored in the HLL at cusp sag, UHLLo 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 = Energy Stored in HLL at cusp sag Energy absorbed by HLL elongation
AE l
=
UHLLo =
k
s
=
UHLL =
k
x
=
(78.55 × 10 )(55.8 × 10 ) 9.998200715
=
438.388kN/m
=
0.00kN-m
1 (438.388)(0.054) 2
=
0.64 kN-m
1 (438.388)(49.082155 × 10−3 ) 2
For clearance calculations, the PEA average deployment force, Favg should now be used instead as per CSA Z259.16 Clause 7.3.3.2.
PEA extension (m) Total Fall Distance (m) Energy generated by falling user (kN-m) Energy absorbed by PEA extension (kN-m) Remaining energy (kN-m)
x
h
= FF − s + s + x
U
= F
U = Wh
U =U +U
x
=F
−U
x
−U
Iterate xPEA until Uk = 0 3 4
1
2
0.3
0.6
0.55
0.56
3.352
3.652
3.602
3.612
3.29
3.58
3.53
3.54
1.44
2.88
2.64
2.69
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 “bottomedout”.)
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 =
10.5kN
PEA 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 F max of 10.5kN
Midpoint sag (m) Cable length for given sag (m) HLL elongation (m) Tension in cable (kN) Force in Lanyard (kN)
s= l=
(l − L )
L − 4s
=
T k
F = 4T
s l
x
T = kx
Iterate s until F = Fmax = 10.5kN 4 5 6
1
2
3
0.5
0.8
0.6
0.66
0.667
0.6675
10.05
10.127
10.072
10.087
10.089
10.089
0.052
0.129
0.074
0.089
0.090
0.091
22.65
56.54
32.24
38.81
39.62
39.68
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. 1
PEA extension (m) Total Fall Distance (m) Energy generated by falling user (kN-m) Energy absorbed by PEA extension (kN-m) Remaining energy (kN-m)
x
h
= FF − s + s + x
U
= F
U = Wh
U =U +U
x
=F
−U
x
−U
Iterate xPEA until Uk = 0 2 3
0.5
0.55
0.545
3.668
3.718
3.713
6.30
6.38
6.37
4.20
4.62
4.58
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
Free fall experienced by user (m)
2.549
Maximum Arrest Force, MAF (kN) (experienced by the user)
6
3-users
4-users
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/OSHDAR2012/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
