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Author: Perner, Judd, J Title: Effects of Lanyard Leg Variation on Maximum Average Arrest Force in
Energy Absorbing Lanyards Used for Fall Protection The accompanying research report is submitted to the University of Wisconsin-Stout, Graduate School in partial
completion of the requirements for the
Graduate Degree/ Major: MS Manufacturing Engineering
Research Adviser: John Dzissah, Ph.D.
Submission Term/Year: Summer, 2013
Number of Pages: 71
Style Manual Used: American Psychological Association, 6th edition
I understand that this research report must be officially approved by the Graduate School and that an electronic copy of the approved version will be made available through the University Library website
I attest that the research report is my original work (that any copyrightable materials have been used with the permission of the original authors), and as such, it is automatically protected by the laws, rules, and regulations of the U.S. Copyright Office.
My research adviser has approved the content and quality of this paper. STUDENT:
NAME Judd Perner DATE: 08/02/13
ADVISER: (Committee Chair if MS Plan A or EdS Thesis or Field Project/Problem):
NAME John Dzissah DATE: 08/02/13
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Perner, Judd J. Effects of Lanyard Leg Variation on Maximum Average Arrest Force in
Energy Absorbing Lanyards Used for Fall Protection
Abstract
Industrial fall protection is an ever expanding category of occupational health and safety. Unlike
other forms of hazardous energy such as electrical or mechanical, gravitational potential energy
is universal and affects every organization. To combat the dangers associated with working at
height, fall protection equipment manufacturers have developed a plethora of devices to safely
arrest a worker in the event of a fall. Although there are many different types of fall protection
devices, some of the most common are energy absorbing lanyards. Energy absorbing lanyards
are tested to national standards to ensure they meet all applicable performance requirements.
When dynamically tested, the critical performance measurement for energy absorbing lanyards is
the Maximum Average Arrest Force (MAAF). It has been observed that when tested using
different lanyard leg materials, energy absorbing lanyard produce different MAAF values. To
grow the body of knowledge related to the effects of lanyard leg materials on MAAF results, this
study performed a variety of dynamic and static tests to evaluate this relationship. Descriptive
and inferential statistical techniques were employed to analyze the experimental data. The
statistical data obtained during this study suggests that there is a strong correlation between
lanyard leg material properties and MAAF measurements.
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Table of Contents
............................................................................................................................................. Page
Abstract ...................................................................................................................................... 2
List of Tables .............................................................................................................................. 5
List of Figures ............................................................................................................................. 6
Chapter I: Introduction ............................................................................................................... 7
Statement of the Problem ................................................................................................. 8
Purpose of the Study ........................................................................................................ 9
Assumptions of the Study ................................................................................................ 9
Definition of Terms ......................................................................................................... 9
Limitations of the Study ................................................................................................ 10
Methodology ................................................................................................................. 10
Chapter II: Literature Review .................................................................................................... 12
Industrial Fall Protection Can Save Lives ...................................................................... 13
Types of Fall Protection ................................................................................................. 14
Common Fall Protection Products, Past and Present ...................................................... 16
ANSI Z359 Committee and Related Standards .............................................................. 19
Testing Products to ANSI Z359 Standards ..................................................................... 22
Energy Absorbing Lanyards Designed to Meet the ANSI Z359.13 Standard .................. 23
Summary ....................................................................................................................... 25
Chapter III: Methodology .......................................................................................................... 27
Lanyard Selection and Description ................................................................................ 28
Instrumentation and Test Equipment .............................................................................. 29
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Data Collection Procedures ............................................................................................ 29
Determination of Lanyard Leg Stiffness ........................................................................ 30
Strain Rate Analysis ...................................................................................................... 31
Descriptive Statistical Analysis ...................................................................................... 32
Inferential Statistical Analysis ....................................................................................... 32
Limitations .................................................................................................................... 32
Assumptions of the Study .............................................................................................. 33
Summary ....................................................................................................................... 33
Chapter IV: Results ................................................................................................................... 34
Descriptive Statistical Analysis ..................................................................................... 34
Inferential Statistical Analysis ...................................................................................... 37
Tear Webbing Velocity Response Analysis .................................................................. 45
Chapter V: Discussion ............................................................................................................... 49
Discussion .................................................................................................................... 50
Limitations ................................................................................................................... 51
Conclusions ................................................................................................................... 52
Recommendations ......................................................................................................... 53
References ................................................................................................................................ 55
Appendix A: Descriptive Statistics ............................................................................................ 57
Appendix B: Graphical Data and Images ................................................................................... 59
Appendix C: Collected Data ...................................................................................................... 65
Appendix D: Inferential Statistics .............................................................................................. 68
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List of Tables
Table 1: Published ANSI/ASSE Z359 Standards ....................................................................... 21
Table 2: Descriptive Statistics - Ambient Condition .................................................................. 36
Table 3: ANOVA: Two-Factor With Replication ...................................................................... 38
Table 4: Single Factor ANOVA - Ambient Condition ............................................................... 40
Table 5: Tukey’s Range Test – Hot Dry Condition Single Factor ANOVA................................ 41
Table 6: Lanyard Leg Stiffness Comparison Tests ..................................................................... 42
Table 7: Regression Analysis of Mean MAAF versus Strain – Hot Dry Condition .................... 45
Table 8: Tear Web Strain Rate Analysis .................................................................................... 47
Table 9: Regression Analysis of Average Force versus Velocity ............................................... 48
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List of Figures
Figure 1: Example of force versus time graph for steel cable leg EAL ....................................... 35
Figure 2: Bar chart of mean MAAF results for lanyard leg materials in the ambient condition ... 37
Figure 3: Mean MAAF versus strain in the hot dry condition .................................................... 43
Figure 4: MAAF versus impact velocity .................................................................................... 46
Figure 5: Average force versus velocity..................................................................................... 47
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Chapter I: Introduction
In the natural world there are fundamental forces that govern all things and impact human
life at every level. Although some of these forces are easier to recognize than others, the
universally understood force is that of gravity. As civilization continues to build taller buildings,
bridges, and structures, the dangers associated with gravity become more prevalent to those
working at height. When a fall hazard cannot be eliminated through engineering controls, a
common safety practice is the use of fall protection equipment. If not properly controlled the
forces applied to the human body during a fall can cause serious injury or death. To combat
these forces, industrial fall protection products have been developed to gradually decelerate
workers and reduce their exposure to bodily harm.
In general industry a typical product used as a deceleration device is the energy absorbing
lanyard (EAL). This type of fall protection system is used to connect a user’s full body harness
to an appropriate anchor point capable of withstanding the forces generated during a fall. The
full body harness and energy absorbing lanyard are examples of personal protective equipment
(PPE). Historically, EALs are manufactured from synthetic woven products such as polyester or
nylon webbing but can be produced from a variety of different materials. The fundamental
components of an EAL consist of an elongating energy absorber, a fixed length lanyard leg, and
a connector at each end. Although every manufacturer produces energy absorbers to their own
specifications, the two most common forms of energy absorber are the tear web style and
Partially Oriented Yarn (POY) style. Due to varying customer requirements, EAL manufacturers
produce lanyards with multiple leg material options ranging from steel cable to polyester
webbing.
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Prior to marketing and selling energy absorbing lanyards, manufacturers must subject the
devices to rigorous performance criteria to ensure they meet or exceed all applicable standards.
In the United States the most common fall protection standards are developed by the
Occupational Safety and Health Administration (OSHA) and the American National Standards
Institute (ANSI) Z359 committee. Historically, the OSHA and ANSI test requirements for EALs
included dynamic tests which measured maximum arrest forces (MAF) generated by dropping a
specified weight a specified distance.
In recent years, the ANSI committee has determined that the maximum arrest force
method is not satisfactory for evaluating EALs and has switched to a maximum average arrest
force (MAAF) test criterion. The MAAF method differs greatly from the MAF method as it
measures the average force over an extended period of time while the MAF method only
measures peak forces which are often presented in milliseconds. The ANSI Z359 committee’s
decision to adopt MAAF test criteria was due to research on the ergonomic effects of impulse
loading on the human body. Since the MAAF method has been adopted, there has been little
research done in regards to how different lanyard leg materials or constructions affect final
MAAF test results.
Statement of the Problem
Manufacturers that design, test, and certify EALs to standards which include MAAF test
criteria have observed correlations between lanyard leg material characteristics and MAAF
variance. Energy absorbers that respond differently to dissimilar leg materials pose to increase
EAL development costs due to additional testing requirements. MAAF variance also reduces
product interchangeability and predictability when combined with other fall protection devices.
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Purpose of the Study
The purpose of this study is to determine the relationship between EAL leg material
characteristics and MAAFs when tested in accordance with ANSI Z359 requirements. All
observed correlations will be used to improve product consistency, reduce costs associated with
testing, and add to the body of knowledge relating to personal fall protection equipment.
Assumptions of the Study
Due to the minor variability in test set up and measurement devices, this study will
assume that all tests were conducted without error and or inconsistency. In addition to test setup,
it will also be assumed that all tests were conducted using the same calibrated test equipment.
Also, all EAL samples tested during this study will be assumed to have zero manufacturing
related defects that influence test results.
Definition of Terms
Aramid. “Any of a group of lightweight but very strong heat-resistant synthetic aromatic
polyamide materials that are fashioned into fibers, filaments, or sheets and used especially in
textiles and plastics” (“Merriam-Webster,” para. 1, 2013).
Arrest force. Force generated when decelerating a human body or test weight from free
fall to a complete stop.
Constructional stretch.
When a load is applied to a wire rope, the helically laid wires and strands react in a
constricting manner, compressing the core and bringing all of the elements of the rope
into closer contact. The result is a slight reduction in diameter and an accompanying
lengthening of the rope. (“Wirerope Works, Inc.,” para. 2, 2013)
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Deceleration device. A component within a fall protection system that limits the forces
applied to the body during a fall by controlling the rate of deceleration.
Tear element. A component of tear webbing comprised of synthetic yarn that joins two
plies of webbing together and absorbs energy through plastic deformation during separation.
Tear web. A two ply webbing that is specifically woven to separate at a predetermined
load and maintain consistent resistance to separation while in tension.
Partially oriented yarn. “A continuous synthetic filament made by extruding a polymer
so that a substantial degree of molecular orientation is present in the resulting filaments, but
further substantial molecular orientation is still possible” (“Encyclo,” para. 2, 2013).
Webbing. “A strong narrow closely woven fabric designed for bearing weight and used
especially for straps and upholstery” (“Merriam-Webster,” para. 1, 2012).
Limitations of the Study
Because there are many fall protection equipment manufacturers, each producing a
variety of EALs, the effects of all lanyard leg materials cannot be practically studied. The scope
of lanyard leg materials will be limited to: polyester webbing, nylon webbing, steel cable, aramid
webbing, and nylon/polyester rope. Additionally, the style of energy absorber will be limited to
one product family and based on a tear web style lanyard. A simplified drawing of tear webbing
showing a two ply construction woven together with tear elements is shown in Appendix B.
POY style lanyards will be excluded from this study as the POY energy absorber acts
independently of the tubular lanyard leg.
Methodology
To provide background information on energy absorbing lanyards, lanyard testing, and
fall protection systems, a literature review has been included within this document. The
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literature review section describes the purpose of industrial fall protection products and how they
are tested to meet OSHA and ANSI Z359 standards. Following the literature review is a detailed
report on how the study was conducted and all methodologies related to testing, data collection,
and analysis. The results of this study are discussed in chapter IV and include detailed statistical
analysis. A final recommendation follows chapter IV and includes discussions, conclusions, and
recommendations based on the data obtained during this study.
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Chapter II: Literature Review
Although hazards can be found in almost any work environment, hazards associated with
falls are some of the most common due to the universal constant of gravity. Whether building a
skyscraper or climbing stairs in a factory, the risks associated with working at height are the
same and can be fatal if not controlled properly. To aid the workforce in the continuous goal of
working safely, manufacturers of safety equipment have developed new and innovative products
to protect individuals working at height. As materials and technology progresses so do the
devices that are used to protect individuals working at height. Some of the most common forms
of fall protection devices include energy absorbing lanyards that are used to tether a worker to an
anchorage and safely arrest them in the event of a fall.
During the development of new energy absorbing lanyards, manufactures rigorously test
designs to comply with national standards. Although OSHA standards are the only group of
standards required by law in the United States, ANSI standards have become increasingly
popular and often required by organizations. One of the main differences between OSHA and
ANSI energy absorbing lanyard test requirements is the adoption of maximum average arrest
force measurements (MAAF) by ANSI compared to the maximum arrest force measurement
(MAF) method used by OSHA. The MAAF method was implemented by the ANSI Z359
committee in 2009 and is currently used during the qualification testing of all EALs designated
as ANSI Z359 compliant. There has been little research conducted on the correlation between
lanyard leg material characteristics and their effect on MAAFs when tested using ANSI
requirements. This deficiency in technical information requires fall protection equipment
manufactures to perform a greater amount of research and development tests.
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While energy absorbing lanyards are the focal point of this study, they are only a
subsystem within larger fall protection systems. Other components that typically make up a fall
protection system include an anchorage, full body harness, connectors such as EALs or self-
retracting lifelines, and rescue or controlled decent devices. Each of these product categories
have been developed to address challenges associated with fall protection. The information
presented in this literature review will provide a detailed view of the importance, history, and
progression of fall protection. This background information will be related to OSHA and ANSI
standards which have helped shape the fall protection industry. In addition to giving background
information on fall protection, this literature review will also describe the different types of fall
protection products and how they are tested in accordance with national standards.
Industrial Fall Protection Can Save Lives
Industrial fall protection equipment can be found in nearly all industrial settings because
of OSHA regulation. Although fall protection PPE is easily accessible and commercially
available in every state, workers still fall from height every day. According to Dunhamel (2012)
the bureau of Labor Statics reported that 605 workers were fatally injured in 2009 and
approximately 212,760 workers were involved in serious accidents due to falling one level or
less. Fall related statistics are published yearly by OSHA and show the severity of occupational
hazards associated with working at height. Falls were the leading cause of fatalities in the
construction industry during 2011 and accounted for 35% of all work related deaths (OSHA,
Construction’s “Fatal Four”, para. 4). In addition to OSHA, other government agencies
periodically publish statistics related to falls. Bickrest (2009) explains that according to the U.S.
Department of Labor (DOL), falls are related to 8% of all occupational deaths from trauma in the
United States and that this number makes falls a primary cause of occupational fatalities. The
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Department of Labor also published reports that indicated at least 442 construction worker
fatalities were attributed to falls in 2007 (Bickrest, 2009). Bickrest (2009) also explains that
roofs are some of the most dangerous fall hazards in the construction industry and were related to
686 deaths between 2003 and 2007. Due to the importance of fall protection on worker safety,
industrial fall protection is key topic among employers, regulators, and end users.
Types of Fall Protection
Fall protection is needed wherever a fall hazard exists. OSHA regulation requires the use
of fall protection when working at 4 feet above a walking/working surface in general industry or
6 ft and higher in construction (Epp, 2007). The best approach is to eliminate the fall hazard by
using passive methods such as guardrails or covers over holes (Denis, 2010). In addition to their
simplicity, passive fall protection systems are also preferred due to their low training
requirements. Torres (2007) explains that:
Railing systems are popular because the worker doesn't have to go through any type of
training in order to stay protected. Once the guard rail is erected, usually on rooftops, the
workers are automatically safer. The only way that a worker is in danger is if he climbs
over the guard rail. (p. 35)
Sometimes passive protection is too costly or cannot be justified given the small amount
of exposure or frequency of fall hazards (Denis, 2010). When passive fall protection cannot be
implemented the use of fall restraint PPE is often the second best option (Denis, 2010). When
fall protection is needed there are multiple types of fall protection systems (FAS) that can be
used depending on the situation. Fall restraint is a type of FPS that does not allow the worker to
fall at all by keeping them away from the fall hazard (Epp, 2007). If work must be done near a
fall hazard, fall arrest and work positioning systems are often needed. Feldstein (2007) describes
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a work positioning system as a subsystem within a fall arrest system that is utilized to stop a fall
from taking place. Work positioning systems must be used in combination with fall arrest
systems to ensure the worker is always protected from a fall in the event that the work
positioning system fails. Fall or work positioning systems are required to limit a workers free
fall to two feet or less (Epp, 2012). According to Denis (2010) “If fall restraint is infeasible, fall
arrest is the next preferred method of fall protection” (p. 48). If a worker falls while using the
appropriate fall arrest equipment, the fall will be arrested so that force and clearance
requirements are maintained (Denis, 2010). As the need for better and more innovative fall
protection equipment increases, so does the need for proper training, planning, and testing.
Torres (2007) states:
Despite all the innovations in technology in the past few years, experts point out that fall
arrest systems and equipment, while important, only are part of the solution to reducing
fall-related injuries and deaths. Employers and workers share the misconception that just
having the right fall protection equipment is the best solution to keep workers safe. (p.
32)
It is not uncommon for employers that have fall hazards to provide formal fall protection
training in addition to providing the required fall PPE (Denis, 2010). There are times when
providing PPE and training is still not enough to protect workers at height. Denis (2010)
explains that an important factor in any safety program is that the fall hazards are continuously
surveyed and the program is updated to reflect a changing work environment. If an organization
does not have the expertise or knowledge to implement a fall protection program, they can look
to the ANSI Z359.2 standard for direction (Denis, 2010). Another important factor in any fall
protection plan is the amount of clearance required for the specific equipment being used. Self-
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retracting lanyards require less fall clearance than shock-absorbing lanyards and therefor can be
used in different settings. The typical required clearance for a six foot shock-absorbing lanyard
is 17.5 ft and includes: free fall, deceleration distance, D-Ring height, harness stretch, and a
safety factor (Epp, 2007).
Rescue is also an important category of fall protection because it is common for
individuals to become incapacitated during or after a fall. Being unconscious while hanging in
fall protection equipment increases the likelihood of suspension related injury. Because of this,
it is imperative that rescue efforts are carried out as soon as possible. Epp (2007) states that
“The final misconception is that calling 9-1-1 automatically fulfills the employer’s responsibility
for rescue planning. Depending on the situation calling 9-1-1 may or may not work. Hang time,
height and available equipment are the true determining factors” (p. 33). Even when all of the
components associated with a fall protection system perform as intended the user can still
become fatally injury due to suspension trauma (Epp, 2007, p. 34). Not only is it important to
use fall protection equipment, it is also important to plan and train for rescue situations.
Common Fall Protection Products, Past and Present
Looking back 50 years, it was difficult to find fall protection equipment and often
workers would resort to crudely made rope lanyards tied around the waist. While this practice
might have stopped workers from hitting the ground, it caused extreme bodily harm in the
process. For this reason, the technology surrounding fall protection has perpetually improved
and continues to reduce workers exposure to injury. Today there are thousands of fall protection
products but many can be categorized into the three categories of anchorages, body support, and
connecting devices. Dunhamel (2012) explains that a simple way to remember the components
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of a fall arrest system is to think of the ABCs where A is anchorage, B is body support, and C is
connecting device.
Anchorages need to be rated to 5,000 lbs per person to be in compliance with OSHA for a
fall arrest system (Epp, 2007). Ellis (2012) describes that an anchorage can be used to support
many fall protection devices ranging from safety nets to EALs and should be designed to
withstand the loads associated with a fall. If a suitable anchorage is not available, the use of fall
protection equipment such as energy absorbing lanyards will not protect the worker. Bickrest
(2009) describes that “Selecting inadequate anchorages is a major problem. The best harness
with the best lanyard or lifeline cannot arrest a fall if unsuitable anchorages are selected” (p. 37).
Another important factor when selecting an anchorage is its location and nearby obstacles.
When investigating a potential anchorage, the qualified person should choose one that is
positioned overhead, will reduce the possibility of swing falls, and allow rescue if needed
(Bickrest, 2009). It is recommended that any anchorage used for fall protection be reviewed and
approved by an engineer with fall protection knowledge (Ellis, 2002).
A full-body harness can be defined as “a body support device that distributes fall arrest
forces across the shoulders, thighs and pelvis and has a center back fall arrest attachment for
connection to the connecting device” (Dunhamel, 2012, p. 44). Although the variety of
harnesses on the fall protection market is similar to the vast number of lanyards styles, the
common theme involves a back D-Ring located between the shoulder blades. Ellis (2012)
describes that the back D-Ring is often allowed to slide and is popular because it distributes the
arrest forces across the harness straps, permits the user to breath comfortably when suspended,
and reduces the possibility of whiplash related injury. Even when harnesses are designed for
specific applications they are still required by ANSI to have a back connection point such as a D-
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Ring. A properly designed D-Ring should be circular and include geometry that will inhibit the
accidental disconnection of snap hooks (Ellis, 2002). It is not uncommon for harness
manufactures to produce various styles for different applications that can include welding, rope
access work, and rescue applications. Ellis (2002) explains that full body harnesses are
traditionally tested for a six foot free fall and must be adjusted properly to ensure the user does
not fall out in the event of a fall.
Connecting devices can take on a variety of forms but are typically self-retracting
lanyards (SRLs) or shock-absorbing lanyards (Dunhamel, 2012). Self-retracting lanyards are
different from EALs as they arrest falls more rapidly (Ellis, 2012). Bickrest (2012) explains that
“One common example is that many contractors buy shock-absorbing lanyards and use them in
areas with inadequate fall clearance. A retractable lifeline or a fall limiter should be used in
certain circumstances” (p. 36). SRLs typically contain a lifeline that is wound on a drum and can
be unwound when slight tension is applied to the device. During a fall the SRL will
automatically lock in a similar fashion to a car seatbelt. Once the SRL is locked, it will arrest the
fall in under 3.5 feet to comply with OSHA and ANSI standards (Duhamel, 2012).
Energy absorbing lanyards do not typically self-retract and are often manufactured from
synthetic materials such as polyester. Lanyards can be described as “a short, flexible rope or
strap webbing that connects a worker’s safety belt to the anchorage point or the grabbing device
on a lifeline” (Ellis, 2002, p. 43). Spotts (1998) describes that EALs are used to increase the
deceleration distance of a fall and subsequently are able to reduce the forces generated during a
fall by 65 to 80 percent which is below the threshold for injury as specified by OSHA.
Two common types of EAL include models with the energy absorber inside a tubular
webbing cover and models based on tear webbing that tears apart in a controlled manner during a
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fall. The tear web based energy absorbers can be added to lanyards that typically are not used
for fall arrest (Spott, 1998). Energy absorbing lanyards are often produced in user specified
lengths which can cause safety concerns if the user does not understand the products limitations.
Wingfield (2010) explains that certain industries such as wind energy and aerospace create
hazards that require the use of a 12 ft free fall EAL which has historically not fallen within the
scope of ANSI. It is not uncommon for users to misuse EALs designed for six foot free falls in
ways that allow a 12 ft free fall (Ellis, 2012). Allowable fall clearance is also a topic of concern
when employees utilize 12 ft free fall EALs. Because of this type of issue in the marketplace,
ANSI has introduced testing criteria for 12 ft free fall lanyards to standardize their construction
and performance characteristics (Wingfield, 2012).
Another common type of fall protection device is the lifeline which can be used
horizontally or vertically (Ellis, 2002). Both horizontal and vertical lifelines can be constructed
of similar materials but their applications can be very different. Ellis (2002) describes vertical
lifelines as “a flexible line for connection to an anchorage point at one end, to hang vertically; it
serves as a means of connecting other components of a personal fall arrest system to the
anchorage” and horizontal lifelines as “an engineered rail, rope, wire or synthetic cable installed
horizontally and used for attaching a worker’s lanyard or lifeline device while moving
horizontally; it is used to control dangerous, pendulum-like swing falls” (p. 44). As with all fall
protection equipment, lifelines are tested and qualified to meet the requirements of their intended
application in accordance with OSHA and ANSI Z359 standards. A component that is regularly
used with vertical lifelines is the rope grab. As their name implies, rope grabs are a type of fall
arresting devices that are connected to vertical lifelines and grab the rope in the event of a fall
(Ellis, 2002).
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ANSI Z359 Committee and Related Standards
The ANSI Z359 committee is a group of safety professions that create standards relating
to fall protection under the provision of the American Nation Standards Institute. The Z359
committee was chartered in 1988 and published the Z359.1 standard four years later in 1992
(Tech Safety Lines, 2009, para. 2). The standard remained essentially unchanged from 1992
until 2007 when multiple revisions were made to improve the performance of fall protection
products baring the Z359 stamp. Since 2007, numerous ANSI Z359 standards have been
published to address specific areas of fall protection. If overlap occurs between the base Z359.1
standard and newly published standards, the new standards will “supersede sections detailed in
the current Z359.1” (Tech Safety Lines, 2009, p. 2). The American National Standards Institute
(2012, code package) has compiled a list of active ANSI/ASSE standards that can be purchased
for use by end users or fall protection equipment manufactures. Table 1 includes all of the ANSI
standards currently being used today (American National Standards Institute, 2012, code
package).
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Table 1
Published ANSI/ASSE Z359 Standards
Title Year Published Description
Z359.0 2007 Definitions and Nomenclature Used for Fall Protection and Fall Arrest
Z359.1 2007 Personal Fall Arrest Systems, Subsystems, Components
Z359.2 2007 Minimum Requirements for a Comprehensive Managed Fall Protection Program
Z359.3 2007 Positioning and Travel Restraint Systems
Z359.4 2007 Assisted-Rescue and Self-Rescue Systems, Subsystems and Components
Z359.6 2009 Specifications and Design Requirements for Active Fall Protection Systems
Z359.7 2011 Qualification and Verification Testing of Fall Protection Products
Z359.12 2009 Connecting Components for Personal Fall Arrest Systems
Z359.13 2009 Personal Energy Absorbers and Energy Absorbing Lanyards
Z359.14 2012 Safety Requirements for Self-Retracting Devices For Personal Fall Arrest and Rescue Systems
Some of the major differences between the ANSI standards and OSHA requirements
include a connector gate face strength increase from 220 pounds of force to 3,600 pounds of
force, an increased lanyard test weight, and rescue systems (Tech Safety Lines, para. 5). Also,
unlike OSHA standards which are required per federal law, the Z359 standards are non-
mandatory. Other differences are pointed out when (Firl & Wolner, 2008) describe “Rescue and
Self-Rescue Systems, Subsystems and Components (ANSI/ASSE Z359.4-2007). These topics
are not addressed in detail in any OSHA standard. Therefore, when people seek guidance on
positioning, travel restraint or rescue, the code provides assistance” (p. 50). One reason that the
ANSI Z359 series standards are becoming popular in the United States is that they are
22
continually updated to reflect current work practices and technologies. Epp (2007) describes that
although OSHA standards are often questioned or confused which has led to over 365 letters of
interpretation by OSHA.
The latest ANSI Z359 standard in regards to energy absorbing lanyards was the Z359.13
standard which was published in 2009. This standard supersedes the original Z359.1 energy
absorbing lanyard requirements and includes a broad range of improvements. The new standard
is tougher and provides specific requirements for EALs from performance criteria to labels and
instructions (Wingfield, 2007). The new standard also includes new testing requirements that are
aimed to improve the performance of fall protection products regardless of the application
(Wingfield, 2007). Overall the Z359.13 standard is a more stringent qualification standard that
will push manufacturers to produce better, more reliable, and safer products. Another important
aspect of the Z359.13 standard is the detailed testing criteria for twin leg or Y-Lanyards. To
reduce the possibility of product misuse, the Z359.13 standard has additional test requirements
for Y-Lanyards and specifies the use of special labeling if products do not meet the Y-Lanyard
tests (Wingfield, 2007). In addition to Y-Lanyard specific testing, the new standard also requires
abrasion testing of wrap around energy absorbers which was not included in previous ANSI
standards (Wingfield, 2007).
Testing Products to ANSI Z359 Standards
The testing required for fall protection equipment has changed dramatically in the past 20
years. A new qualification and testing standard was published in 2011 and is described by
Griffith (2012, p. 34) as “the minimum requirements for the test laboratory, whether that is a
third party or the manufacturer’s in-house lab, as well as the unique equipment to properly test
fall protection equipment covered by any ANSI/ASSE Z359 standard”. This type of standard
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will benefit the industry by ensuring all manufacturers who mark their product to the ANSI
standard have performed the appropriate testing. The new ANSI standard is titled ANSI Z359.7
and gives manufacturers and or test labs defined test protocols that must be in place prior to
marking a product as meeting ANSI (Griffith, 2012). Griffith (2012) goes on to explain that the
quality requirements of this standard are vital to the industry by keeping non-conforming
products out of circulation.
In addition to ensuring products are tested correctly, the new ANSI standard also requires
manufactures that test their own products to attain ISO 17025 accreditation. The main reason
behind requiring manufacturer test labs to attain ISO certification is to reduce the possibility of
biased test procedures (Griffith, 2012). For a test lab to become ISO 17025 accredited, various
measures must be taken. One of the first things that a test lab needs to do is “establish and
maintain a quality control management program which encompasses the product being tested”
(Griffith, 2012, p. 74). Griffith (2012) also explains that after a product has been qualified in a
17025 accredited test lab, it must be labeled with the appropriate Z359 standard as well as the
Z359.7 standard.
Energy Absorbing Lanyards Designed to Meet the ANSI Z359.13 Standard
Energy absorbing lanyards that have been designed to meet the ANSI Z359.13 standard
are evaluated for both their static and dynamic properties. Although the forces applied to the
human body during a fall cannot be definitively re-created, the Z359 standard has been designed
to simulate real life fall situations. To ensure that the forces recorded when dropping a steel test
weight are representational of the forces applied to the human body, a 1:1.1 test weight
conversion factor has been agreed upon by the Z359 committee. Wingfield (2010) describes that
“Using a more accurate conversion factor and, therefore, a greater weight will model energy
24
absorbers during testing that will more closely represent applications by the workings in the
field” (p. 75). Historically, the factor between a steel test weight and human body has been 1:1.4
but was changed in 2009 based on extensive research conducted by Gravitec Systems
(Wingfield, 2010). By applying the 1:1.1 factor to the maximum user weight recognized by
ANSI, a test weight of 282 lbs was determined. In addition to using a representative test weight,
the Z359.13 standard also specifies that EALs be tested in a variety of environmental conditions
to simulate real world work settings. Among the ambient dynamic tests, EALs are also
dynamically tested when wet, cold, and hot. After the EAL has been dynamically tested using the
282 lb test weight, the sample is then statically tested to ensure its structural integrity post
deployment. Various static tests are specified in the Z359.13 standard but the one constant is
that an energy absorbing lanyard shall be capable of withstanding a 5,000 lb static load for a
period of one minute. Although not new to the Z359.13 standard, the need for multiple static
tests is exemplified by:
Concerns over potential misuse of twin-leg shock-absorbing lanyards prompted
additional test requirements and warnings for these products, which were not mentioned
in the 1999 version. The new standard includes a 5,000 lb static test of the joint between
the two lanyard legs. (Feildstein, 2007, p. 47)
One goal of the final static requirement is to reduce the possibility that a lanyard will fail after it
has successfully arrested a fall. Another reason that the 5,000 lb static requirement exists is to
provide protection against falls that exceed the energy absorbing capacity of the EAL.
When comparing the new and old ANSI Z359 testing requirements, the most profound
changes are the use of a heavier test mass, an increase allowable energy absorber elongation,
different classes based on free fall allowance, and the use of MAAFs versus MAFs. Since its
25
first publication in 1992, the Z359.1 standard has specified that energy absorbing lanyards be
tested using a 220 lb test weight (Wingfield, 2010). This 220 lb test weight requirement
remained part of the standard for 17 years until the release of the Z359.13 standard in 2009.
Wingfield (2010) explains the importance of having two classes by stating “Until now, standards
for lanyards that were manufactured for a free fall greater than 6 ft did not exist” (p. 74). The
new 12 ft class has allowed manufacturers to produce ANSI approved products that are used in
this application. As the test weight increased so did the maximum allowable elongation from 42
inches to 48 inches for 6ft EALs and 60 inches for 12ft EALs which allowed arrest forces to
remain within acceptable ranges (Wingfield, 2010).
Lastly, the Z359.13 committee adopted the MAAF measurement method in place of the
traditional MAF method. Prior to 2009, all EALs were tested to a pass/fail criterion that only
allowed arrest forces up to 900 pounds force (lbf) regardless of duration. The Z359 committee
elected the MAAF method based on research that impulse forces below a particular duration
were not detrimental to the human body and sustained forces are the leading cause of injury
during fall arrest. Using the MAAF measurement method, the Z359 committee specified that the
MAAF shall not be greater than 900 lbf after a 6 ft free fall or 1,350 lbf after a 12 ft free fall.
Although the MAAF method is utilized in the Z359.13 standard, a MAF of 1,800 lb is specified
for both 6 ft and 12 ft free fall EALs.
Summary
Industrial fall protection is a continually growing category of occupational safety due to
the widespread exposure to fall hazards. Falling from height has historically proven to be one of
the leading cases of workplace fatalities in the U.S. and a major contributor to employee injuries.
Workplace accidents related to falling from height have caused an influx of interest in fall
26
protection from workers, employers, and government agencies. Over the past 50 years, fall
protection has become a large and diverse industry. This new industry has generated many new
practices and techniques to protect workers from falls. Fall restraint, fall arrest, work
positioning, and rescue systems are all examples of the perpetually evolving methods used to
ensure worker safety. In addition to various types of fall protection, there are also many different
types of fall protection products that are used for specific applications. Oftentimes the design,
function, and performance of fall protection devices are governed by national standards such as
OSHA and ANSI. These national standards impose strict guidelines on the qualification testing
of safety products to ensure reliability. Some of the most recent developments related to fall
protection standards include the ANSI Z359.13 standard which was published in 2009 and
relates to energy absorbing lanyards.
27
Chapter III: Methodology
When evaluating an industrial setting for workplace hazards it not uncommon to find
unguarded elevated work surfaces. If elevated work surfaces cannot be guarded or eliminated
through engineering controls, the use of personal protective equipment is a common solution to
protect workers at height. While there is an abundance of fall protection products available to
organizations around the world, some of the most common fall protection PPE devices are
energy absorbing lanyards used to connect a workers full body harness to an anchorage.
Although the construction of energy absorbing lanyards differs from manufacturer to
manufacturer, the fundamental principles of operation are similar due to standardized testing.
Historically, U.S. manufacturers have designed, built, and tested EALs to meet OSHA
requirements which specified that the maximum arresting force applied to the human body could
not exceed 1,800 lb. As PPE technology progresses and standards organizations such as ANSI
investigate how falls affect the human body, standards are ever changing to improve product
performance. Although not required by law the ANSI Z359.13 fall protection standard has
become widely adopted in the U.S. fall protection market. One of the most significant
differences between the ANSI Z359.13 fall protection code and OSHA regulations is the use of
maximum average arresting force criteria during dynamic testing of energy absorbing lanyards.
When EALs are tested using ANSI prescribed dynamic test methods, the MAAF results have
varied depending on the material and construction of the lanyard legs.
The topics described in the methodology portion of this study are closely related to the
testing procedures as required by the ANSI Z359.13 standard for EALs. These procedures
include test set up, equipment, measurement, and data acquisition. In addition to the testing of
28
EALs, the methodology used to interpreted, statistically analyze, and draw conclusions from test
data will also be described.
Lanyard Selection and Description
The energy absorbing lanyards selected for this study include 6 ft free fall models based
on ANSI Z359.13 requirements and exclude all 12 ft free fall models. While similar in
constriction to the 6 ft free fall lanyards, the 12 ft free fall models include additional webbing
within the energy absorber for greater energy absorbing capacity. Higher capacity models were
excluded due to their construction and use being similar to 6 ft free fall models. A drawing that
describes a typical single web leg energy absorbing lanyard can be found in Appendix B. All
test samples were built in a production setting from computer generated work orders to improve
product consistency. The specific lanyard leg materials chosen for this study included all of the
materials commonly offered by fall protection equipment manufacturers. The following lanyard
leg materials were dynamically tested during this study:
Polyester webbing (8,000 lb tensile strength)
Polyurethane coated polyester webbing (9,800 lb tensile strength)
Polyester tubular webbing (6,600 lb tensile strength)
Nylon webbing (13,000 lb tensile strength)
Aramid webbing (9,800 lb tensile strength)
Nylon/polyester static kernmantle rope (10,000 lb tensile strength)
Galvanized Steel Cable (7,000 lb tensile strength)
All test lanyards consisted of one tear web style energy absorber, one lanyard leg, and two ANSI
qualified connectors. Dual leg lanyards were not tested during this study as they have a similar
construction to single leg lanyard with the exception of two legs versus one.
29
Instrumentation and Test Equipment
The instrumentation and test equipment utilized for this experiment included the
following: 282 lb test weight, 10,000 lb rated strain gauge load cell, Microsoft Windows based
data acquisition software program, calibrated measurement equipment per ISO 17025, electronic
quick release mechanism, and desktop computer. Auxiliary equipment such as data cables and
connecting carabiners was also used to couple the various components together. In addition to
the drop test equipment, an environmental conditioning chamber was utilized to condition the
samples to the ambient wet, cold dry, and hot dry environmental conditioning requirements in
accordance with ANSI Z359.13. Lastly, the structure used to perform the tests was built and
tested to meet the structural and frequency requirements specified in the ANSI Z359.13 standard.
Data Collection Procedures
The data collection procedure employed during this study involved taking measurements
from the load cell at a frequency no less than 1,000 Hz during each dynamic drop test. When
testing the 6 ft free fall EALs the 282 lb test weight was raised 6 ft and dropped using the quick
release mechanism. In total a series of 84 dynamic tests were conducted using twelve samples of
each lanyard leg material. Each group of twelve samples was divided into subgroups of three
and tested in accordance with the conditioning requirements of ANSI Z359.13. For each lanyard
leg type the following tests were performed: three ambient dry, three ambient wet, three cold dry,
and three hot dry. The measurements were then imported into the desktop computer via the data
acquisition software. Once the test was complete, the data acquisition software was used to
appropriately filter the data and convert it into a usable form. After data processing, the test
results were transferred into a formal test report document which includes values such as MAF,
MAAF, initial lanyard length, and final lanyard length. All of the procedures, documentation,
30
and test protocols utilized during this study met the requirements of an ISO 17025 accredited
testing laboratory.
Determination of Lanyard Leg Stiffness
For purposes of this study, the lanyard leg materials were treated as ideal springs where
all deformations are elastic and strain energy is recoverable in the form of mechanical work.
Under this assumption, stress and strain values of the lanyard legs are proportional up to the
material proportionality limit and follow Hooke’s law (Cutnell & Johnson, 1992). While some
lanyard leg deformation is not elastic, it is assumed to be related to constructional stretch of the
leg material and not due to plastic deformation. Hooke’s law can be described as:
(1)
where F is the force required to stretch or compress a spring, k is the spring constant or stiffness,
and x is elastic deformation. This relationship between force and elongation was used to
determine the stiffness of each lanyard leg material.
To benchmark the lanyard leg materials against each other, a standardized test was
developed to determine the stiffness of each material. The basis of this test was to gather force
versus elongation data by statically pulling each leg material at a rate of two inches per minute
and measure elastic deformation while under load. Seven test specimens were produced using
the applicable lanyard leg materials and included a 2 ft effective length between stitch patterns or
swage fittings. Although 6 ft free fall EALs typically utilize lanyard legs longer than 2 feet, this
length was used due to tensile test equipment height restrictions. It should be noted that material
stiffness is proportional to length and longer test specimens would result in lower spring
constants. The samples were measured before, during, and after being tensile tested to 1,800 lb
and constructional stretch was then subtracted from elongation measurements. To ensure the
31
samples were in tension prior to initiating the test, a preload of 20 lb was used to set initial
length.
Strain Rate Analysis
During dynamic testing of EALs, it is often observed that higher velocities produce
higher average arrest forces. This information is widely accepted in the fall protection industry
and is typically available through free fall versus MAAF charts in EAL user instructions. To
verify this concept, nine 6 ft free fall energy absorbers were tested using steel cable test lanyards.
Three samples were dynamically tested using 2 ft, 4 ft, and 6 ft free falls. The data collected
from these tests was then plotted versus impact velocity to determine any correlation. The
velocity at impact can be calculated using the general kinematic equation for constant
acceleration (Cutnell & Johnson, 1992):
(2)
which reduces to:
√ (3)
where is velocity at impact, is the acceleration due to gravity (32.2 ft/sec), and y is free fall
distance.
In addition to testing tear web in a dynamic context, tear webbing samples were also
tensile tested in a quasi-static setting at various rates to establish any relationship between
velocity and MAAF at low speeds. The test procedure included a tear web sample being tensile
tested in a tensile testing machine at velocities of 0.25, 1.00, 4.00, 10.00, and 20.00 inches per
minute. The duration of each test was modified according to the velocity to elongate one inch of
tear webbing only. Using this data, the average force required to elongate one inch of tear
webbing at a particular speed was calculated.
32
Descriptive Statistical Analysis
Standard descriptive statistical analysis techniques were used to organize the data
obtained during this experiment. The overall sample population was broken down into multiple
subpopulations so that descriptive statistical values could be calculated for individual lanyard leg
types and conditions. Environmental test conditions are a major factor in product performance
and therefore were used to separate the data. The values associated with each lanyard leg type
were tabulated and can be found in Appendix A. Bar charts and other graphical descriptions of
the data can be found in Appendix B.
Inferential Statistical Analysis
After the data was collected and organized, various inferential statistical analysis
techniques were used to determine if the relationship between lanyard leg materials and MAAFs
is statistically significant. Hypothesis testing was conducted where the null hypothesis states that
the lanyard leg material has no effect on MAAF measurements and the alternate hypothesis
states that the lanyard leg materials causes a statistically significant effect on MAAF
measurements. An alpha value (α) of 0.05 was consistently used during this study to obtain a
minimum of 95% confidence. Other inferential statistical analysis methods were utilized and
included: linear regression analysis, one-way Analysis of Variance (ANOVA), two-way
ANOVA with replication, Tukey’s range test, and normality tests. Various graphs, tables, and
plots relating to the inferential statistics performed during this study can be found in Appendix
D.
Limitations
Because of the wide variety of energy absorbing lanyard styles available, this study was
limited to one brand. This brand only utilizes one type of energy absorber which is a propriety
33
product supplied by one webbing manufacturer. Also, all EALs tested during this study were
designed to meet the 6 ft requirement of ANSI Z359.13. Test results from standards that do not
require MAAF measurements such as CSA and OSHA were not included with this study.
Lastly, due to the small sample sizes for each environmental condition and lanyard leg
combination, normality testing was limited.
Assumptions of the Study
Based on the requirements necessary to become an ISO 17025 Testing Laboratory, all
measurements taken during this study were assumed to be correct. Also, it was assumed that all
documentation and procedures were conducted accurately. Other sources of variation such as
manufacturing related defects and material variation will be assumed to be zero. Lastly, it was
assumed that all data sets obtained from testing are normally distributed.
Summary
The energy absorbing lanyards tested for the purposes of this study were chosen to
represent common fall protection equipment found in nearly all industries with workers at height.
Due to stringent testing procedures and requirements set forth by ANSI, this study is
representative of the testing required for a manufacturer to qualify products to ANSI Z359.13.
The statistical analysis resulting from this study will increase the body of knowledge surrounding
the testing of fall protection equipment.
34
Chapter IV: Results
The purpose of this study was to determine the effects of lanyard leg construction
variation on MAAF measurements when testing energy absorbing lanyards in accordance with
ANSI Z359 requirements. The information obtained from this study will be used as a reference
in the future development of energy absorbing lanyards used for fall protection. To simulate the
testing requirements of a new lanyard product line, eighty-four dynamic drop tests were
conducted. Seven lanyard leg materials were studied in combination with a specific 6 ft free fall
energy absorber. Each of the seven lanyard models was subjected to four different
environmental conditions prior to performing the dynamic drop tests. The data collected from
these tests was analyzed using descriptive and inferential statistical methods to determine if
lanyard leg materials have an effect on MAAF measurements.
Descriptive Statistical Analysis
As described in the methodology portion of this study, standard descriptive statistical
values were calculated using MAAF measurements taken during dynamic drop tests per ANSI
Z359.13. The MAAF measurement is obtained by dynamically dropping a test sample and
recording the force versus time data as shown in Figure 1.
35
Figure 1. Example of force versus time graph for steel cable leg EAL.
After the data has been collected, all data points above 500 lb are combined and divided by the
total number of data points to calculate the MAAF. Figure 1 is an example of a cable leg lanyard
dynamic drop test. When compared to the force versus time graphs of web and rope leg
lanyards, there is a noticeable difference in response curve shape. It is typical for lanyards with
synthetic leg materials to have higher force values near the end of the dynamic test. Examples of
polyester web and polyester/nylon rope leg lanyard force versus time graphs are shown in
Appendix B.
Because environmental conditioning can affect MAAF measurements, values for each leg
type were calculated four times to separate the different environmental test results. The
descriptive statistical values for the ambient test condition are shown below in table 2:
36
Table 2
Descriptive Statistics - Ambient Condition
Aramid Nylon Polyester Polyester, Tubular Polyester/TPU Steel Cable Polyester/Nylon
Mean 830.33 850.67 854.33 828.67 858.33 842.67 874.00
Standard Error 4.18 13.84 5.24 9.82 19.06 10.84 7.51
Median 834.00 864.00 853.00 828.00 859.00 853.00 867.00
Standard Deviation 7.23 23.97 9.07 17.01 33.01 18.77 13.00
Sample Variance 52.33 574.33 82.33 289.33 1089.33 352.33 169.00
Skewness -1.69 -1.73 0.65 0.18 -0.09 -1.73 1.72
Range 13.00 42.00 18.00 34.00 66.00 33.00 23.00
Minimum 822.00 823.00 846.00 812.00 825.00 821.00 866.00
Maximum 835.00 865.00 864.00 846.00 891.00 854.00 889.00
Count 3.00 3.00 3.00 3.00 3.00 3.00 3.00 Confidence Level (95.0%) 17.97 59.53 22.54 42.25 81.99 46.63 32.29
The lanyards with the highest mean MAAF were models that included lanyard legs constructed
from polyester/nylon rope. It should also be noted that the rope leg models accounted for the
second highest MAAF value following polyester/TPU lanyard leg models. The lanyards with the
lowest mean MAAF were those constructed with tubular polyester webbing legs. Models with
tubular polyester legs also produced the lowest MAAF values. A bar chart that graphically
describes the difference in leg type means is shown below in Figure 2.
37
Figure 2. Bar chart of mean MAAF results for lanyard leg materials in the ambient condition.
Bar charts for each of the environmental test conditions were created to visually represent how
lanyard leg materials affect MAAF measurements. Bar charts for the cold dry, hot dry, and
ambient wet environmental test condition can be found in Appendix B.
Inferential Statistical Analysis
To properly understand how lanyard leg materials affect MAAF measurements, the data
collected during this study was analyzed using standard inferential statistical techniques. The
first statistical test that was performed was the two-factor Analysis of Variance (ANOVA) with
replication test. This test was used to determine if the mean MAAF results were significantly
different between environmental test conditions, lanyard leg types, and to evaluate interaction
between both variables. Table 3 describes the calculated results from the two-factor ANOVA
with replication.
38
Table 3
ANOVA: Two-Factor With Replication
Summary Aramid Nylon Polyester Polyester, Tubular Polyester/TPU Steel Cable Polyester/Nylon Total
Ambient Count 3.00 3.00 3.00 3.00 3.00 3.00 3.00 21.00
Sum 2491.00 2552.00 2563.00 2486.00 2575.00 2528.00 2622.00 17817.00
Average 830.33 850.67 854.33 828.67 858.33 842.67 874.00 848.43
Variance 52.33 574.33 82.33 289.33 1089.33 352.33 169.00 492.36
Cold Dry Count 3.00 3.00 3.00 3.00 3.00 3.00 3.00 21.00
Sum 2765.00 2777.00 2708.00 2563.00 2614.00 2553.00 2658.00 18638.00
Average 921.67 925.67 902.67 854.33 871.33 851.00 886.00 887.52
Variance 345.33 2809.33 554.33 456.33 1952.33 721.00 589.00 1575.26
Hot Dry Count 3.00 3.00 3.00 3.00 3.00 3.00 3.00 21.00
Sum 2425.00 2584.00 2456.00 2498.00 2529.00 2326.00 2829.00 17647.00
Average 808.33 861.33 818.67 832.67 843.00 775.33 943.00 840.33
Variance 404.33 120.33 956.33 16.33 837.00 440.33 169.00 2809.23
Ambient Wet Count 3.00 3.00 3.00 3.00 3.00 3.00 3.00 21.00
Sum 2624.00 2480.00 2566.00 2480.00 2518.00 2548.00 2424.00 17640.00
Average 874.67 826.67 855.33 826.67 839.33 849.33 808.00 840.00
Variance 30.33 190.33 52.33 1166.33 80.33 460.33 76.00 641.20
Total Count 12.00 12.00 12.00 12.00 12.00 12.00 12.00 Sum 10305.00 10393.00 10293.00 10027.00 10236.00 9955.00 10533.00 Average 858.75 866.08 857.75 835.58 853.00 829.58 877.75 Variance 2213.48 2134.63 1270.75 483.54 897.45 1439.72 2692.75 ANOVA Source of Variation SS df MS F P-value F crit Sample 32290.52 3.00 10763.51 20.04 5.98E-09 2.77 Columns 20196.07 6.00 3366.01 6.27 4.40E-05 2.27 Interaction 60092.31 18.00 3338.46 6.22 6.05E-08 1.79 Within 30072.67 56.00 537.01 Total 142651.57 83.00
Using the probability values (P-values) from Table 3, it can be concluded that there is a
statistically significant difference in mean MAAF results between leg types, environmental test
39
conditions, and there is significant interaction between the two independent variables. The null
hypothesis (H0) that the lanyard leg mean MAAF results, environmental mean MAAF results,
and interaction between results are equal can be rejected and the alternate hypothesis can be
accepted. All P-values obtained from this test were below an alpha (α) value of 0.05 which
indicates a greater than 95% confidence in the analysis results.
The second statistical test that was performed was a single factor ANOVA. The single
factor ANOVA was used to determine if the difference between the MAAF results for each
lanyard leg type was statistically significant. This single factor ANOVA was performed for each
of the four environmental test conditions. Table 4 describes the results of the ambient condition
single factor ANOVA. The null hypothesis (H0) states that the mean MAAF values for each leg
type will be equal. The alternate hypothesis (HA) states that the mean MAAF values for leg type
are different. An alpha (α) value of 0.05 was used to test for 95% confidence.
40
Table 4
Single Factor ANOVA - Ambient Condition
Groups Count Sum Average Variance Aramid 3.00 2491.00 830.33 52.33 Nylon 3.00 2552.00 850.67 574.33 Polyester 3.00 2563.00 854.33 82.33 Polyester, Tubular 3.00 2486.00 828.67 289.33 Polyester/TPU 3.00 2575.00 858.33 1089.33 Steel Cable 3.00 2528.00 842.67 352.33 Polyester/Nylon 3.00 2622.00 874.00 169.00 ANOVA Source of Variation SS df MS F P-value F crit Between Groups 4629.14 6.00 771.52 2.07 0.12 2.85 Within Groups 5218.00 14.00 372.71 Total 9847.14 20.00
The calculated P-value for Table 4 was 0.12 which indicates that the null hypothesis can be
accepted and the data does not indicate a statistically significant difference in mean MAAF
measurements between lanyard leg types in the ambient condition. A single factor ANOVA was
performed on the cold dry, hot dry, and ambient wet environmental condition data sets and
produced P-values of 0.06, 4.12E-06, and 0.01, respectively. These P-values indicate that the
null hypothesis can be accepted for the cold dry condition and rejected for the hot dry and
ambient wet test conditions. This is evidence that during the hot dry and ambient wet condition
dynamic drop test, there is a statistically significant difference in MAAF measurement between
lanyard leg types. Single factor ANOVA results for the cold dry, hot dry, and ambient wet test
conditions can be found in appendix D.
Using the single factor ANOVA statistical test results, a Tukey’s range test was
performed on the ANOVA data from the hot dry and cold dry condition dynamic drop tests. The
purpose of the Tukey’s range test was to determine which leg materials were statistically
41
different from each other as the ANOVA test only described that there was a difference within
the group. The Tukey test statistic was calculated using the mean square (MS) within groups and
sample size (n) data from the hot dry and cold dry single factor ANOVA in combination with a
value from “a studentized range statistic (q) table” (“Duke,” para. 1, 2013). After the test
statistic was calculated, the difference between lanyard leg type means was determined and
compared against the Tukey statistic. The Tukey’s range test showed that polyester/nylon rope
lanyard leg mean MAAF results were significantly different than all of the other leg types in the
hot dry condition. The hot dry condition Tukey’s range test also showed that the steel cable
mean MAAF results were significantly different than the nylon, tubular polyester, and
polyester/TPU lanyard legs mean MAAF results. Table 5 describes the Tukey’s range test for
the hot dry condition single factor ANOVA and highlights the significantly different leg types.
Table 5
Tukey’s Range Test – Hot Dry Condition Single Factor ANOVA
q-value MS n Tukey Test Statistic
4.83 420.52 3.00 57.18 Groups Average 1st 2nd 3rd 4th 5th 6th
Aramid 808.33
Nylon 861.33 53.00
Polyester 818.67 10.33 42.67 Polyester, Tubular 832.67 24.33 28.67 14.00
Polyester/TPU 843.00 34.67 18.33 24.33 10.33
Steel Cable 775.33 33.00 86.00 43.33 57.33 67.67
Polyester/Nylon 943.00 134.67 81.67 124.33 110.33 100.00 167.67
42
A Tukey’s range test for data calculated from the cold dry condition single factor ANOVA can
be found in Appendix D.
As described in the methodology portion of this document, seven test specimens were
tensile tested to determine the lanyard leg stiffness characteristics. The values calculated from
the data obtained during these tests are related to elastic deformation, stiffness, and strain. Table
6 is shown below and describes the results obtained from the lanyard leg stiffness comparison
tests.
Table 6
Lanyard Leg Stiffness Comparison Tests
Initial Length
(Li) Final Length
(Lf) Elongation Under Load
Constructional Stretch
Elastic Deformation
(δ) Stiffness (k) Strain (ε)
in. in. in. in. in. lb/in. in./in.
Aramid Webbing 34.63 35.00 1.53 0.38 1.16 1558.44 0.03
Nylon Webbing 35.50 36.50 2.25 1.00 1.25 1440.00 0.04
Polyester Webbing 36.50 38.00 3.65 1.50 2.15 837.21 0.06
Polyester, Tubular Webbing
32.50 34.00 3.85 1.50 2.35 765.96 0.07
Polyester/TPU Webbing 35.00 35.25 2.20 0.25 1.95 923.08 0.06
Steel Cable 38.13 38.16 0.22 0.03 0.19 9442.62 0.00
Polyester/Nylon Rope 33.25 34.13 4.91 0.88 4.04 446.10 0.12
Based on the lanyard leg stiffness comparison tests, it is evident that the leg material with the
highest spring constant is steel cable. The material with the lowest spring constant was the
polyester/nylon rope with a k value of 446.10 lb/in. and a strain value of .12 or 12 percent.
Using the mean MAAF data obtained during dynamic drop tests and the stain data from Table 6,
43
a regression analysis was performed to evaluate the relationship between these variables. A
scatterplot of the mean MAAF versus strain relationship for the hot dry environmental condition
is shown below in Figure 3. In addition to the individual data points, a trend line and linear
equation have been added to the scatterplot to show the positive correlation between mean
MAAF and strain. The hot dry condition scatterplot was chosen due it having the highest
coefficient of determination (R2) value which was equal to 0.76. This R2 value indicates that
approximately 76 percent of the variability in the data set can be associated with the relationship
between mean MAAF and strain. Mean MAAF data from each of the environmental condition
was plotted versus strain and the R2 values for the ambient, ambient wet, and cold dry condition
were 33 percent, 45 percent, and 0.2 percent, respectively. This information suggests that during
the ambient and hot dry condition tests there is a direct relationship between mean MAAF and
strain. The ambient wet condition data suggests that there is an inverse relationship between
mean MAAF and strain. Based on the R2 value being nearly zero for the cold dry condition tests,
a relationship between mean MAAF and strain could not be determined.
Figure 3. Mean MAAF versus strain in the hot dry condition.
44
The fourth statistical analysis technique utilized to determine the effects of lanyard leg
variation on MAAF was linear regression analysis. Using the same data from the mean MAAF
versus strain scatterplots, a regression analysis was performed on the hot dry and ambient wet
data as they produced the highest R2 values. Table 7 shows the results from the hot dry condition
mean MAAF versus strain linear regression analysis.
45
Table 7
Regression Analysis of Mean MAAF versus Strain – Hot Dry Condition
Summary Regression Statistics Multiple R 0.87
R Square 0.76 Adjusted R Square 0.71 Standard Error 28.55 Observations 7.00 ANOVA df SS MS F Significance
F Regression 1.00 12690.08 12690.08 15.57 0.01 Residual 5.00 4075.70 815.14 Total 6.00 16765.78 Coefficients Standard
Error t Stat P-value Lower 95% Upper 95%
Lower 95.0%
Upper 95.0%
Intercept 771.96 20.41 37.81 0.00 719.48 824.43 719.48 824.43 Velocity 1253.48 317.69 3.95 0.01 436.84 2070.12 436.84 2070.12
Using the P-value from Table 7, the null hypothesis that there is no significant linear correlation
between variables can be rejected and the alternate hypothesis can be accepted. This indicates
that there is 95 percent confidence that the mean MAAF values will increase when lanyard leg
strain values increase during the hot dry condition tests. The ambient wet condition linear
regression analysis is shown in Appendix D and also includes a P-value of less than 0.05
indicating the null hypothesis can be rejected and there is an inverse relationship between mean
MAAF.
Tear Webbing Velocity Response Analysis
As described in the methodology portion of this study, the MAAF data obtained from
dynamic testing nine energy absorbers was plotted versus impact velocity which was determined
from free fall distance. Figure 4 describes the data using a scatterplot and also includes a trend
46
line with linear equation and R2 value of 0.54. Linear regression analysis was also performed on
this data set and can be found in Appendix D. The linear regression analysis showed a P-value
of 0.02 which indicates that the null hypothesis can be rejected and the alternate hypothesis that
there is a relationship between MAAF and impact velocity can be accepted.
Figure 4. MAAF versus impact velocity.
To verify if tear webbing MAAF results are related to velocity at low speeds, a tensile
test was developed as described in the methodology portion or this study. The tensile test
procedure was related to tensile testing tear webbing through a distance of one inch at five
different velocities. The test was performed three times and then averaged to accumulate the 5
data points as shown below in Table 8.
47
Table 8
Tear Web Strain Rate Analysis
Velocity Average Force Elongation Energy
(in./min) (lb) in. (ft·lb)
0.25 1313.12 1.00 109.43
1.00 1222.82 1.00 101.90
4.00 1097.86 1.00 91.49
10.00 1031.39 1.00 85.95
20.00 1046.82 1.00 87.24
The data points were then averaged and plotted against velocity as shown in Figure 5.
Figure 5. Average force versus velocity.
A trend line was fitted to the data and shows an inverse relationship between velocity at low
speeds and average force. The R2 value of 0.54 indicates that 54% of the variation can be
attributed to this relationship. In addition to the scatter plot with linear equation, a linear
48
regression analysis was performed on the data. Table 9 includes the regression analysis data for
average force versus velocity.
Table 9
Regression Analysis of Average Force versus Velocity
Summary Regression Statistics
Multiple R 0.73 R Square 0.54 Adjusted R Square 0.50 Standard Error 84.01 Observations 15.00 ANOVA df SS MS F Significance
F Regression 1.00 105752.98 105752.98 14.98 0.00 Residual 13.00 91754.99 7058.08 Total 14.00 197507.97 Coefficients Standard
Error t Stat P-value Lower 95% Upper 95%
Lower 95.0%
Upper 95.0%
Intercept 1223.18 30.10 40.64 0.00 1158.15 1288.20 1158.15 1288.20 Velocity -11.46 2.96 -3.87 0.002 -17.85 -5.06 -17.85 -5.06
The linear regression analysis for average force versus velocity produced a P-value of 0.002
which allows the null hypothesis to be rejected and the alternate hypothesis accepted. This gives
95% confidence that the average force to elongate one inch of tear webbing is related to the
velocity. To test the normality of error for this linear regression analysis, an Anderson-Darling
normality test was performed on the residual data. The normality test produced a P-value of 0.39
which indicates that the error within the analysis is normally distributed. A plot of the
Anderson-Darling normality test for residuals can be found in Appendix B.
49
Chapter V: Discussion
Energy absorbing lanyards have the potential to dramatically reduce the likelihood of
injury during a fall by limiting arrest forces. Historically, manufactures of energy absorbing
lanyards have reduced arrest forces by utilizing the elastic properties of rope. As fall protection
standards become more stringent, so do the tests associated with qualifying energy absorbing
lanyards, making rope lanyards obsolete. To ensure equipment compliance, manufacturers
typically use woven products such as polyester or nylon webbing in the construction of energy
absorbing lanyards. Because every work environment is different, so are the requirements of
individuals who use fall protection equipment. This variation in equipment requirements has
driven manufactures to produce energy absorbing lanyards in a multitude of configurations.
Because every material reacts differently under stress, the force generated from different lanyard
leg materials can affect test results, which causes variability within product lines. This
variability adds development cost to new EAL projects due to the additional research and
development testing needed to verify materials.
Based on the historical information presented in this study, it is apparent that testing of
fall protection device is an important and continually changing process. Fall protection
equipment manufactured in the United States is typically tested to OSHA standards which only
require MAF measurements as well as the Z359.13 standard for EALs that requires both MAF
MAAF measurements. The MAAF requirement has only been in use since 2009 and little data
has been published as to how different lanyard leg materials affect MAAF test results. The
testing and analysis of this study was related to MAAF measurements and lanyard leg variation
and employed a variety of statistical tests to determine the significance of the observed data. Test
results from this study compared MAAF data with lanyard leg types, environmental test
50
conditions, leg stiffness, and impact velocity. Each of these tests yields insight into how tear
webbing based energy absorbers react when tested using various leg materials.
Discussion
The data and analysis from this study suggest that there is a direct relationship between
MAAF measurements and lanyard leg stiffness when tested dynamically in accordance with the
ANSI Z359.13 standard. The seven lanyard leg materials that were tested ranged from steel
cable to polyester/nylon rope and had a wide range of spring constants. The first portion of the
analysis was to use descriptive statistics to organize the data for subsequent analysis. Once the
data was organized, descriptive statistical techniques such as ANOVA, linear regression,
Tukey’s range test, and normality tests were utilized. By using the one way and two-way
ANOVA tests, it was determined that there was a significant difference in MAAF forces between
leg types and test conditions. The ANOVA results also indicated that the only statistically
significant differences between leg materials occurred during the hot dry and ambient wet
condition tests. Although these tests are based on small data sets, they are beneficial in
determining how 6 ft free fall EALs will react when tested using different leg materials. By using
the Tukey’s range test, it was established that the polyester/nylon rope leg MAAF results were
the most significantly different from the other leg materials.
The environmental test condition that showed the highest correlation between lanyard leg
stiffness and MAAF was the hot dry condition. The linear regression analysis was based on
MAAF versus strain data which is the ratio of elastic deformation in relation to initial length.
This relationship was mirrored by the ambient condition tests but with less statistical confidence.
The ambient wet condition tests showed the opposite effect and displayed a negative correlation
between MAAF and strain. Data obtained from the cold dry tests did not show a significant
51
correlation between MAAF and lanyard leg material strain. The material the highest amount of
leg strain was the polyester/nylon rope and accounted for two of the four test conditions highest
MAAF results.
Another aspect of energy absorber performance that was analyzed during this study was
that of energy absorber response to strain rate or impact velocity. Two tests were set up to
collect quantitative data on the MAAF versus velocity relationship. The first test involved
dynamically drop testing lanyards from different heights and measuring the MAAF. It was
found that MAAF results show a positive correlation with impact velocity at high speeds. The
second force versus velocity test involved tensile testing tear webbing in a quasi-static state at
five different velocities. This series of tests produced the opposite effect as the dynamic MAAF
versus velocity tests and showed a negative correlation between strain rate and average force to
separate one inch of tear webbing.
Limitations
The major limitation of this study was sample sizes. Because each data set only included
three data points for each lanyard leg material/test condition combination, the normality of the
data could not be adequately verified. Small sample size also contributes to percent error as the
data cannot be fully representational of the greater population. Another limitation to this study
was the lanyard leg stiffness comparison tests only being conducted in the ambient condition and
with test samples approximately half the length of a typical 6 ft free fall lanyard leg. Due to
stiffness being directly proportional to length, the spring constant values obtained during this
study are conservative. Also, because the properties thermoplastic materials such as nylon and
polyester are greatly affected by temperature, the stiffness tests at room temperature are not fully
representative of the lanyard legs during environmental conditioning tests. Lastly, because the
52
majority of lanyard leg materials are based on woven products such as webbing or rope, the
mechanical analysis of these materials is limited and computer based simulation programs are
not sufficient to accurately model the internal geometry and fiber interactions of lanyard leg
materials.
Conclusions
In conclusion to this study, it has been observed that tear web style energy absorbing
lanyard MAAF test results are affected by different lanyard leg materials. When tested in the
ambient and hot dry condition, lanyards with legs comprised of high stretch materials such as
polyester/nylon rope typically exhibit higher MAAF forces. Lanyards constructed with stiff leg
materials such as steel cable are less susceptible to this issue. When tested in the ambient wet
condition, the opposite effect has been observed and lanyard leg stiffness is negatively correlated
with MAAF. A correlation between lanyard leg stiffness and MAAF was not observed when
tested in the cold dry condition. The literature review portion of this study described how testing
of EALs has evolved throughout the past 50 years and the importance of adequate testing on fall
protection equipment performance. The energy absorber was described as an integral part in
many fall arrest systems and understanding how EALs react during fall events is paramount to
safety and user wellbeing.
It has also been observed that at high speeds such as during a dynamic fall event, the
MAAF measurement will increase with velocity. Increasing free fall height yields this effect as
impact velocity is a function of free fall distance. When tested at speeds less than two feet per
minute, tear webbing shows signs of increased average force per unit distance as speed
decreases. This observation does not mirror the material properties when tested at high speeds.
By calculating the amount of energy required to elongate one inch of tear webbing at various
53
speeds, it is apparent that as speed decreases from impact velocity to zero, the amount of energy
absorbed per tear element increases. When combined with the lanyard leg stiffness correlations
and force versus time graphs of dynamic tests, the velocity versus average force at low speeds
results indicate a relationship between leg stiffness and strain rate of tear elements.
Collectively, all of the analysis and data from this study show that higher strain values for
lanyard legs will produce higher MAAFs when tested in the ambient and hot dry condition.
Because materials such as polyester webbing and polyester/nylon rope become less stiff when
heated, the hot dry condition amplifies this effect. The data suggests that during the initial
portion of energy absorption, each lanyard leg material will perform in a similar manner. As the
test weight velocity is decelerated, the relationship between dynamic event speed and high
MAAF is decreased and the effect of higher MAAFs at lower speeds ensues. When looking at
the last few inches of elongation at the tear element level, the leg materials connecting the energy
absorber to the weight are allowed to elastically deform and then recover as each tear element
plastically deforms. Upon loading the subsequent tear element, the lanyard leg material must
elongate which determines the speed in which tear elements are loaded. Stiff materials such as
steel cable or aramid, quickly load the tear element and the high MAAF at lower speeds
correlations is attenuated. High elongation materials such as polyester/nylon rope are less stiff
and require more time to transfer the load to the tear element. By increasing the amount of
energy required to elongate the tear webbing, the MAAF will increase. The specific mechanism
that causes tear webbing to absorb additional energy at lower speeds is unknown but has been
observed through empirical data.
Recommendations
54
Based on the analysis and observations of this study, it is recommended that more data
be captured in relation to the effects of lanyard leg variation on MAAF in energy absorbing
lanyard used for fall protection. Although multiple tests were conducted and showed statistically
significant evidence that a relationship exists between lanyard leg stiffness and increased MAAF
results, additional testing will improve the reliability of this study. Also, the elastic properties of
lanyard legs should be studied in greater detail and under all environmental test conditions. This
will enable better conclusions to be drawn based on MAAF versus stain linear regression tests.
Other testing procedures that include elements such as high speed photography and or
videography should be implemented to visually capture the dynamic free fall event. This will
provide better insight into what is happening as each tear element absorbs kinetic energy.
Further mathematical dynamic analysis should also be performed using material properties
obtained from lanyard leg material manufacturers. Lastly, it is important that manufactures of
fall protection equipment consistently verify that the products they produce meet all applicable
standards for quality, reliability, and safety. By diligently analyzing each component within
systems and how those components interact with the system as a whole, fall protection
equipment manufactures will continue to advance the products people depend their lives on.
55
References
American National Standards Institute. (2012). ANSI/ASSE Z359 fall protection code package.
Retrieved from http://webstore.ansi.org/RecordDetail.aspx?sku=ANSI/ASSE Z359 Fall
Protection Code Package
Bickrest, E. (2009). Fall protection: Failure is not an option. EHS Today, 2(3), 34.
Cutnell, J., & Johnson, K. (1992). Physics (2nd ed.). Hoboken, NJ: John Wiley & Sons, Inc.
Denis, K. (2010). Best practices in fall protection. Professional Safety, 55(11), 47-48.
Duhamel, K. (2012). Five things to consider before implementing a fall arrest system. EHS
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Duke University. (1998). The studentized range statistic (q)*. Retrieved from
http://www.stat.duke.edu/courses/Spring98/sta110c/qtable.html
Ellis, J. (2002). Personal fall arrest systems. Professional Safety, 47(12), 42.
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http://www.encyclo.co.uk/define/partially oriented yarn
Epp, R. J. (2007). Fall protection misconceptions & myths. Professional Safety, 52(9), 26-34.
Feldstein, J. (2007). ANSI/ASSE Z359 fall protection code. Professional Safety, 52(9), 47-51.
Firl, C., & Wolner, T. (2008). Standards developments. Professional Safety, 53(11), 50.
Griffith, R. (2012). Testing fall protection products. Professional Safety, 57(1), 34.
OSHA. (n.d.). Commonly used statistics. Retrieved from
http://www.osha.gov/oshstats/commonstats.html
Spotts, S. (1998). Critical link in fall protection. Occupational Hazards, 60(11), 77.
Tech Safety Lines. (2009, November). ANSI Z359 fall protection code. Retrieved from
http://techsafetylines.com/ANSI Z359 1-TSL 2009NOV.pdf
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Torres, K. (2007). New frontiers in fall protection equipment. Occupational Hazards, 69(6), 31.
Merriam-Webster.com. (2012). Webbing. Retrieved September 21, 2012, from
http://www.merriam-webster.com/dictionary/webbing
Wingfield, R. (2010). New standard for energy-absorbing lanyards what you need to know.
Professional Safety, 55(6), 74-75.
Wirerope Works, Inc. (2008). Elevator rope stretch. Retrieved from
http://www.wwwrope.com/product_pdfs/EL_TB_05.pdf
57
Appendix A: Descriptive Statistics
Table A1
Descriptive Statistics - Cold Dry Condition
Aramid Nylon Polyester Polyester, Tubular Polyester/TPU Steel Cable Polyester/Nylon
Mean 921.67 925.67 902.67 854.33 871.33 851.00 886.00
Standard Error 10.73 30.60 13.59 12.33 25.51 15.50 14.01
Median 927.00 949.00 901.00 863.00 876.00 866.00 893.00
Standard Deviation 18.58 53.00 23.54 21.36 44.19 26.85 24.27
Sample Variance 345.33 2809.33 554.33 456.33 1952.33 721.00 589.00
Skewness -1.19 -1.60 0.32 -1.53 -0.47 -1.73 -1.19
Range 36.00 98.00 47.00 40.00 88.00 47.00 47.00
Minimum 901.00 865.00 880.00 830.00 825.00 820.00 859.00
Maximum 937.00 963.00 927.00 870.00 913.00 867.00 906.00
Count 3.00 3.00 3.00 3.00 3.00 3.00 3.00 Confidence Level (95.0%) 46.16 131.67 58.49 53.07 109.76 66.70 60.29
Table A2
Descriptive Statistics - Hot Condition
Aramid Nylon Polyester Polyester, Tubular Polyester/TPU Steel Cable Polyester/Nylon
Mean 808.33 861.33 818.67 832.67 843.00 775.33 943.00
Standard Error 11.61 6.33 17.85 2.33 16.70 12.12 7.51
Median 814.00 855.00 810.00 832.00 831.00 768.00 950.00
Standard Deviation 20.11 10.97 30.92 4.04 28.93 20.98 13.00
Sample Variance 404.33 120.33 956.33 16.33 837.00 440.33 169.00
Skewness -1.17 1.73 1.16 0.72 1.55 1.38 -1.72
Range 39.00 19.00 60.00 8.00 54.00 40.00 23.00
Minimum 786.00 855.00 793.00 829.00 822.00 759.00 928.00
Maximum 825.00 874.00 853.00 837.00 876.00 799.00 951.00
Count 3.00 3.00 3.00 3.00 3.00 3.00 3.00 Confidence Level (95.0%) 49.95 27.25 76.82 10.04 71.87 52.13 32.29
58
Table A3
Descriptive Statistics – Ambient Wet Condition
Aramid Nylon Polyester Polyester, Tubular Polyester/TPU Steel Cable Polyester/Nylon
Mean 874.67 826.67 855.33 826.67 839.33 849.33 808.00
Standard Error 3.18 7.97 4.18 19.72 5.17 12.39 5.03
Median 875.00 832.00 859.00 819.00 844.00 839.00 812.00
Standard Deviation 5.51 13.80 7.23 34.15 8.96 21.46 8.72
Sample Variance 30.33 190.33 52.33 1166.33 80.33 460.33 76.00
Skewness -0.27 -1.48 -1.69 0.96 -1.71 1.66 -1.63
Range 11.00 26.00 13.00 67.00 16.00 39.00 16.00
Minimum 869.00 811.00 847.00 797.00 829.00 835.00 798.00
Maximum 880.00 837.00 860.00 864.00 845.00 874.00 814.00
Count 3.00 3.00 3.00 3.00 3.00 3.00 3.00 Confidence Level (95.0%) 13.68 34.27 17.97 84.84 22.27 53.30 21.66
59
Appendix B: Graphical Data and Images
Figure B1. Example of force versus time graph for polyester web leg EAL.
Figure B2. Example of force versus time graph for polyester/nylon rope leg EAL.
60
Figure B3. Bar chart of mean MAAF results for lanyard leg materials in the hot condition.
Figure B4. Bar chart of mean MAAFs for lanyard leg materials in the ambient wet condition.
61
Figure B5. Bar chart of mean MAAF results for lanyard leg materials in the cold dry condition.
Figure B6. Simplified drawing of tear webbing
62
Figure B7. Mean MAAF versus strain in the ambient condition.
Figure B8. Mean MAAF versus strain in the ambient wet condition.
63
Figure B9. Mean MAAF versus strain in the cold dry condition.
Figure B10. Example of a typical single web leg energy absorbing lanyard.
64
Figure B11. Anderson-Darling normality test for residuals.
65
Appendix C: Collected Data
Table C1
Collected Data
Leg Materials Free Fall (ft) Condition MAAF (lb) MAF (lb) Elongation (in.) Aramid 6 Ambient 834 1101 37.25 Aramid 6 Ambient 822 1060 37.75 Aramid 6 Ambient 835 1029 37.25 Aramid 6 Cold 901 1173 31 Aramid 6 Cold 927 1198 30 Aramid 6 Cold 937 1205 29.75 Aramid 6 Hot 786 1017 40 Aramid 6 Hot 814 1093 39 Aramid 6 Hot 825 1070 39 Aramid 6 Wet 869 1064 33.5 Aramid 6 Wet 880 1152 32.25 Aramid 6 Wet 875 1173 34.25 Nylon 6 Ambient 823 1149 38 Nylon 6 Ambient 865 1192 35.5 Nylon 6 Ambient 864 1236 35.5 Nylon 6 Cold 865 1092 33 Nylon 6 Cold 949 1203 30.25 Nylon 6 Cold 963 1376 26.25 Nylon 6 Hot 874 1312 36.5 Nylon 6 cold 855 1291 36.25 Nylon 6 Hot 855 1192 33.5 Nylon 6 Wet 832 1132 40 Nylon 6 Wet 811 1154 38.5 Nylon 6 Wet 837 1138 38
Polyester 6 Ambient 846 1083 36.75 Polyester 6 Ambient 864 1119 36.25 Polyester 6 Ambient 853 1058 36.25 Polyester 6 Cold 880 1226 32.5 Polyester 6 Cold 927 1096 31.5 Polyester 6 Cold 901 1081 31.5 Polyester 6 Hot 853 1153 37 Polyester 6 Hot 793 1131 41.5 Polyester 6 Hot 810 1118 38.25 Polyester 6 Wet 859 1091 35.5 Polyester 6 Wet 860 1078 34.25 Polyester 6 Wet 847 1112 31
66
Polyester, Tubular 6 Ambient 812 1245 34.25 Polyester, Tubular 6 Ambient 828 1310 30 Polyester, Tubular 6 Ambient 846 1303 26 Polyester, Tubular 6 Cold 830 1276 32 Polyester, Tubular 6 Cold 870 1312 29.25 Polyester, Tubular 6 Cold 863 1376 27.75 Polyester, Tubular 6 Hot 837 1333 32 Polyester, Tubular 6 Hot 829 1353 30.5 Polyester, Tubular 6 Hot 832 1350 29.5 Polyester, Tubular 6 Wet 797 1261 34.5 Polyester, Tubular 6 Wet 819 1223 33 Polyester, Tubular 6 Wet 864 1153 32.75 Polyester/Nylon 6 Ambient 866 1295 37 Polyester/Nylon 6 Ambient 867 1241 36.5 Polyester/Nylon 6 Ambient 889 1238 34.25 Polyester/Nylon 6 Cold 859 1142 34.5 Polyester/Nylon 6 Cold 893 1123 33.5 Polyester/Nylon 6 Cold 906 1265 33 Polyester/Nylon 6 Hot 928 1315 33 Polyester/Nylon 6 Hot 951 1240 33.25 Polyester/Nylon 6 Hot 950 1298 34.25 Polyester/Nylon 6 Wet 798 997 37.5 Polyester/Nylon 6 Wet 812 1089 37.25 Polyester/Nylon 6 Wet 814 1198 35.75 Polyester/TPU 6 Ambient 859 1098 35.5 Polyester/TPU 6 Ambient 825 1178 35 Polyester/TPU 6 Ambient 891 1214 34.25 Polyester/TPU 6 Cold 876 1106 31.5 Polyester/TPU 6 Cold 825 1137 30.75 Polyester/TPU 6 Cold 913 1220 28 Polyester/TPU 6 Hot 831 1160 37.5 Polyester/TPU 6 Hot 822 1128 36.5 Polyester/TPU 6 Hot 876 1154 34.5 Polyester/TPU 6 Wet 845 1189 37.5 Polyester/TPU 6 Wet 829 1184 35.25 Polyester/TPU 6 Wet 844 1190 34.5
Steel Cable 6 Ambient 854 1110 34 Steel Cable 6 Ambient 821 1140 34 Steel Cable 6 Ambient 853 1070 33.5 Steel Cable 6 Cold 866 1090 32.5 Steel Cable 6 Cold 867 1202 31 Steel Cable 6 Cold 820 1210 31 Steel Cable 6 Hot 768 1033 40
67
Steel Cable 6 Hot 759 965 39.5 Steel Cable 6 Hot 799 1037 36.75 Steel Cable 6 Wet 839 1127 34.5 Steel Cable 6 Wet 835 1232 34.5 Steel Cable 6 Wet 874 1253 31.5
68
Appendix D – Inferential Statistics
Table D1
Single Factor ANOVA – Cold Dry Condition
Groups Count Sum Average Variance Aramid 3.00 2765.00 921.67 345.33 Nylon 3.00 2777.00 925.67 2809.33 Polyester 3.00 2708.00 902.67 554.33 Polyester, Tubular 3.00 2563.00 854.33 456.33 Polyester/TPU 3.00 2614.00 871.33 1952.33 Steel Cable 3.00 2553.00 851.00 721.00 Polyester/Nylon 3.00 2658.00 886.00 589.00 ANOVA Source of Variation SS df MS F P-value F crit Between Groups 16649.90 6.00 2774.98 2.62 0.06 2.85 Within Groups 14855.33 14.00 1061.10 Total 31505.24 20.00 Table D2
Single Factor ANOVA – Hot Condition
Groups Count Sum Average Variance Aramid 3.00 2425.00 808.33 404.33 Nylon 3.00 2584.00 861.33 120.33 Polyester 3.00 2456.00 818.67 956.33 Polyester, Tubular 3.00 2498.00 832.67 16.33 Polyester/TPU 3.00 2529.00 843.00 837.00 Steel Cable 3.00 2326.00 775.33 440.33 Polyester/Nylon 3.00 2829.00 943.00 169.00 ANOVA Source of Variation SS df MS F P-value F crit Between Groups 50297.33 6.00 8382.89 19.93 4.12E-06 2.85 Within Groups 5887.33 14.00 420.52 Total 56184.67 20.00
69
Table D3
Single Factor ANOVA – Ambient Wet Condition
Groups Count Sum Average Variance Aramid 3.00 2624.00 874.67 30.33 Nylon 3.00 2480.00 826.67 190.33 Polyester 3.00 2566.00 855.33 52.33 Polyester, Tubular 3.00 2480.00 826.67 1166.33 Polyester/TPU 3.00 2518.00 839.33 80.33 Steel Cable 3.00 2548.00 849.33 460.33 Polyester/Nylon 3.00 2424.00 808.00 76.00 ANOVA Source of Variation SS df MS F P-value F crit Between Groups 8712.00 6.00 1452.00 4.94 0.01 2.85 Within Groups 4112.00 14.00 293.71 Total 12824.00 20.00
Table D4
Tukey’s Range Test – Cold Dry Condition Single Factor ANOVA
q-value MS n Tukey Test Statistic
4.83 293.71 3.00 47.49 Groups Average 1st 2nd 3rd 4th 5th 6th
Aramid 874.67
Nylon 826.67 48.00
Polyester 855.33 19.33 28.67 Polyester, Tubular 826.67 48.00 0.00 28.67
Polyester/TPU 839.33 35.33 12.67 16.00 12.67
Steel Cable 849.33 25.33 22.67 6.00 22.67 10.00
Polyester/Nylon 808.00 66.67 18.67 47.33 18.67 31.33 41.33
70
Table D5
Regression Analysis of Mean MAAF versus Strain – Ambient Wet Condition
Summary Regression Statistics
Multiple R 0.71 R Square 0.50 Adjusted R Square 0.38 Standard Error 13.65 Observations 6.00 ANOVA df SS MS F Significance
F Regression 1.00 756.73 756.73 4.06 0.11 Residual 4.00 745.20 186.30 Total 5.00 1501.93 Coefficients Standard
Error t Stat P-value Lower 95% Upper 95%
Lower 95.0%
Upper 95.0%
Intercept 852.61 10.69 79.76 0.00 822.93 882.29 822.93 882.29 Velocity -316.53 157.06 -2.02 0.11 -752.59 119.53 -752.59 119.53
71
Table D6
Regression Analysis of MAAF versus Impact Velocity
Summary Regression Statistics
Multiple R 0.73 R Square 0.54 Adjusted R Square 0.47 Standard Error 11.67 Observations 9.00 ANOVA df SS MS F Significance
F Regression 1.00 1100.22 1100.22 8.08 0.02 Residual 7.00 953.34 136.19 Total 8.00 2053.56 Coefficients Standard
Error t Stat P-value Lower 95% Upper 95%
Lower 95.0%
Upper 95.0%
Intercept 719.79 18.35 39.22 0.00 676.39 763.20 676.39 763.20 Velocity 3.25 1.14 2.84 0.02 0.55 5.95 0.55 5.95