CSA Z259.16 Standardına göre Esnek Yatay Yaşam Hatlarının tasarım ve hesabı

Fatih Özcan

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Designing and calculating for flexible Horizontal Lifelines based on design code CSA Z259.16
CSA Z259.16 Standardına göre Esnek Yatay Yaşam Hatlarının tasarım ve hesabı

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).

1.PNG

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, theWorkplace 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
2.PNG
The following information are given in Figure 2:
3.PNG

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,
4.PNG

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

5.PNG

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

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). Thismidpoint 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 PEAwill 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 themidpoint sagging by guessing an arbitrary value for s, then iterating s until F= Fmax.
7.PNG
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.

8.PNG

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 clearancemargin (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),
9.PNG

Thus, the fall clearance required (measured from the platform),
10.PNG
Let us review the analysis results against the essential design criteria for an effective HLL.

11.PNG

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 inWorked 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
12.PNG

Applying the lumping factor of 1.75 for 2 falling users, the following parameters and assumptions are adjusted as follows.
13.PNG

We now use the above adjusted values to analyse for a 2-users fall.We iterate s until F = adjusted Fmax of 10.5kN
14.PNG

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

16.PNG

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 ropemodulus 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 andminimize 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 MaximumArrest 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 -
    Bağlantıyı görmek için üye girişi yapmalısınız.
  • 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
 
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