Designing and calculating for flexible Horizontal Lifelines based on design code CSA Z By HOE Yee Pin and Dr. GOH Yang Miang

Transcription

1 Designing and calculating for flexible Horizontal Lifelines based on design code CSA Z 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 Z The upcoming Singapore Standard on Specification for Design of Active Fall Protection Systems is based on the CSA Z 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

2 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) (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.

3 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 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 = = mm 2 Initial cable sag due to its own weight si = wl 8T 1 wl 2T = (3.4)(10) 8(1 10 ) 1 (3.4)(10) 2(1 10 ) = m Initial cable length li = L s L = = m

4 Unstressed HLL cable length lo = l 1 + T AE = ( )( ) = m 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. l i pulled into two straight lines. Hence, using Pythagoras Theorem, Cusp sag, s c = l L = = ( )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 PEA Max Extension xmax 1.75m 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. PEA Average Deployment Force Favg 4.8kN CSA Z Clause where Favg = 0.8 Fmax since SS 528:Part 2 is similar to CSA Z 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 Assuming harness D-ring is an average of 1.5m from user s feet. hd = 1.5 (height of HLL anchor post) = = 0.5m Lanyard length Ly 2m -- (Given) -- PEA: Personal Energy Absorber

5 We can now calculate the Free Fall (FF) experienced by the user, FF = h + s + L = ( ) + 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 Z Clause 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 U k 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 Z Clause Thus, we find the midpoint sagging by guessing an arbitrary value for s, then iterating s until F = F max. Iterate s until F = F max = 6kN Midpoint sag (m) s = (l L ) Cable length for given sag (m) l = L 4s HLL elongation (m) x = T k Tension in cable (kn) T = kx Force in Lanyard (kn) F = 4T s l Stage 3: When F reaches the PEA deployment force, the PEA deploys. The sagging of the HLL has already absorbed U HLL and the PEA will absorb U PEA as it extends x PEA. 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), U w and

6 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 x PEA then iterate x PEA until the remaining fall energy U k = 0. Before we do that, we have to calculate the following parameters. HLL Rope Modulus khll = AE l = ( )( ) = kN/m Energy Stored in HLL at cusp sag UHLLo = k s = 1 2 ( )( ) = 0.00kN-m Energy absorbed by HLL elongation UHLL = k x = 1 ( )(0.054) = 0.64 kn-m 2 For clearance calculations, the PEA average deployment force, F avg should now be used instead as per CSA Z Clause Iterate x PEA until U k = PEA extension (m) x Total Fall Distance (m) h = FF s + s + x Energy generated by falling user (kn-m) Energy absorbed by PEA extension (kn-m) U = Wh U = F x = F x Remaining energy (kn-m) U = U + U U U 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, x w and a clearance margin (also known as safety distance), E. We will assume x w to be 0.3m for a harness using normal webbing. The clearance margin (as per CSA Z ), E= (s s ) = ( ) = 0.650m

7 Thus, the fall clearance required (measured from the platform), C = h + x + E = = 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) Maximum Arrest Load, MAL (to the anchors and wire rope) = 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 Z Clause = 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 Z Clause i.e kN. 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 Z Clause Lumping factor, M, for flexible anchorage systems Number of users falling Systems using PEAs 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

8 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 Iterate s until F = Fmax = 10.5kN Midpoint sag (m) s = (l L ) Cable length for given sag (m) l = L 4s HLL elongation (m) x = T k Tension in cable (kn) T = kx Force in Lanyard (kn) F = 4T s l Again, we now iterate for x PEA until the fall energy is totally absorbed i.e. U k = 0. Iterate x PEA until U k = PEA extension (m) x Total Fall Distance (m) h = FF s + s + x Energy generated by falling user (kn-m) Energy absorbed by PEA extension (kn-m) U = Wh U = F x = F x Remaining energy (kn-m) U = U + U U U The adjusted clearance margin is now E = ( ) = 0.662m The clearance for the equivalent lumped mass C = h + x + E = = 4.675m The clearance required for the last user to fall (as per CSA Z Clause 8.2.7) C = 1.6C 0.6C = ( ) ( ) = 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.

9 Comparison of Results 1-user 2-users 3-users 4-users Free fall experienced by user (m) Maximum Arrest Force, MAF (kn) (experienced by the user) 6 Fall clearance required, Cp (m) Maximum Arrest Load, MAL (kn) (to the anchors and wire rope) Minimum tensile strength required for anchorages and wire rope (kn) 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 Z 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.

10 References Ministry of Manpower (2013) Occupational Safety and Health Division Annual Report AR2012/OSHD_AR2012_part1.pdf Canadian Standards Association (2004) Z Design of Active Fall-Protection Systems Ontario: Canadian Standards Association Goh, Y.M., 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 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