Verification of the strength of the anchor point of a fall arresting and positioning system for reinforcing steel erectors Presented by
Andre Lan, P. Eng. Research and Expertise Support Department IRSST 505, de Maisonneuve blvd., West Montreal, Quebec H3A 3C2
International Society for Fall Protection Symposium Seattle, Washington, June 14-15, 2006
Plan
Context
Research Project - Objectives
Fall arrest equipment selected
Limit States Design Method
Dynamic tests
Tests results
Discussion and conclusion
ISFP Symposium, Seattle, Washington, June 14-15, 2006
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Context
During the erection of steel reinforcement of large dimension walls, fall protection is problematic. The reinforcing steel erector climbs and has to anchor himself on the reinforcement bars with a lineman’s belt with two lateral Drings and a connecting assembly to position and to protect himself from falls from heights. Work Positioning System. Reinforcing steel erector with work positioning system
ISFP Symposium, Seattle, Washington, June 14-15, 2006
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Context
Theses conditions generate severe fall injuries risks: congested floor and rebar protruding from the floor. Two problems are identified:
The selection of a harness, positioning and fall arresting equipment; An anchor point, from the reinforcement bars, sufficiently strong for the personal fall arrest system.
ISFP Symposium, Seattle, Washington, June 14-15, 2006
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Context
The revised articles on fall protection have been implemented in the Quebec Safety Code on the 15th of February 2001. In the context of this revision, fall protection for reinforcing steel erectors as discussed above is the only item whose implementation is still problematic. There is a strong willingness and consensus between unions and employers of the review committee of the Quebec Safety Code to study this problem. At their unanimous request, IRSST has initiated a research project ‘Selection of a fall arresting and positioning system for reinforcing steel erectors and verification of the strength of the anchor point to study fall protection for reinforcing steel erectors.
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Research Project - Objectives
The project consists of two parts:
The first part is carried out by École de Technologie Supérieure (ÉTS). It’s objective is to select a fall arresting system for reinforcing steel erectors by identifying a harness, the most appropriate for the task of reinforcing steel erectors on a vertical wall, a connecting subsystem (lanyard with an energy absorber or a self-retracting lanyard with an integrated energy absorber, 7’ to 8’ long) and the location of the attachment on the harness (dorsal or sternal); The second part is carried out by IRSST. It’s objective is to verify the strength of an anchor point from the reinforcement bars in actual work situations for the fall arrest system selected in part I of the study.
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Literature review
Literature review yields few information on fall protection of reinforcing steel erectors. Most relevant document is ‘Fall Protection Rebar and Concrete Formwork’ (Oregon OSHA). According to Oregon OSHA:
Positioning-device systems are the most appropriate type of personal fall-protection for working on and placing rebar. A positioning-device system enables one person to work on a vertical surface with both hands free and it limits free-fall distance to two feet or less. The difference between a positioning-device system and a personal fall-arrest system is that a positioning-device system supports a worker on an elevated surface and limits a fall to two feet. A personal fall arrest system, on the other hand, prevents a worker from free falling more than six feet.
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Literature review
In our opinion, there is confusion between systems and methods which ensure positioning and those which arrest the fall. The positioning-device system is a primary mean of suspension which maintains the worker at the desired height and enables him to have his hands free to perform his tasks. A positioning-device system is always under tension and the live load applied is the weight of the worker. A fall arrest device activates only when there is an accidental fall; it stops the fall before the worker hits the floor; it is independent from the positioning-device system and thus works in parallel to the positioning device and ensures redundancy.
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Literature review
According to Oregon OSHA, a positioning-device system is simultaneously a positioning-device and a fall arrest system. First, a system cannot be at the same time the positioningdevice system (primary system) and the fall arrest system (secondary system) because there is no more redundancy, thus absence of complete protection. Moreover a 2 feet free fall distance arrested by a positioning belt and strap or a connecting chain can generate a large fall arrest force and cause serious injuries (Crawford measured a fall arrest force of 8.87 kN in tests with 2 ft free fall distance for the Electricity Association, U.K).
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Fall arrest equipment selected
After a literature review, observations on site, ÉTS has selected the following equipment to be evaluated: Harnesses : Lineman’s belt (reference), lineman’s belt with leg connectors, harness + belt and harness + sub-pelvic straps + comfort pad for lumbar support. Lanyards : Lanyard with energy absorber (DBI Sala, Model EZ-Stop II, 5’ long (1.52 m)) complying with CAN/CSAZ259.11-M92-Energy absorbers and lanyards and/or a SRL (Talon, DBI Sala, 8 ft) meeting or exceeding OSHA and ANSI industrial standards. Attachment on harness : dorsal or sternal?
ISFP Symposium, Seattle, Washington, June 14-15, 2006
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Fall arrest equipment selected
For the verification of the anchor point on the reinforcement bars, the harness the most appreciated in part I of the study will be used. When ÉTS was ready to evaluate the FAS, no reinforcing steel erectors were available because of a construction boom in Montréal. The project was already late and it was decided to go ahead with the verification of the strength of the anchor point. Whether the SRL with an integrated energy absorber or the lanyard with an energy absorber is used, both generate a maximum arrest force of 4 kN. As the lanyard generates the greater free fall distance, it was used for the dynamic tests.
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Fall arrest equipment selected
ÉTS recommended to do the tests with the prototype harness + sub-pelvic straps + comfort pad for lumbar support corresponding to ADP class which, in their opinion, is more likely to be appreciated by reinforcing steel erectors.
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FAS used in anchor tests
ADP Harness mounted on wooden torso – Sternal view
ADP Harness mounted on wooden torso – Dorsal view
ISFP Symposium, Seattle, Washington, June 14-15, 2006
Lanyard with energy absorber, 1.5 m (5’ (5’)
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Method to test anchor points
The reinforcement wall is complex to modelize to apply classical methods of structural analysis to verify its strength. The simplest and fastest method is to carry out dynamic fall tests in situ. Because dynamic tests are destructive and dangerous to be done in situ, it was decided to reconstruct a steel reinforcement wall in the laboratory at the Centre de formation des métiers de l’Acier (CFMA).
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Limit States Design Method
Commonly used for structural design. Philosophy of this method : Two basic requirements:
Not collapse under the loads for which it was designed (strength requirement); Fulfill the function for which it was intended (behaviour or performance requirement).
From these two requirements, two limit states are defined:
Ultimate limit state;
Serviceability limit state.
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Limit states
Ultimate limit state corresponds to safety, such as exceedence of load-carrying capacity, overturning, sliding and fracture due to fatigue or other causes. It corresponds to structural ruin or failure and it is thus a strength requirement and verified with factored loads. Serviceability limit state relates to the planned use of the structure and involves deflection, vibration, permanent deformation and cracking and it is thus a performance requirement and verified with specified (actual) loads applied.
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CSA Z259.16-04 Design of active fallprotection Systems
According to CSA Z259.16-04 Design of Active Fall-Protection Systems, the limit state design method is expressed by: λ λ
ν ν
R≥F R : factored resistance of the component or subsystem F : the worst-case factored effect of the applied loads on the component or subsystem
R = ØU Ø = the reduction capacity factor, usually ≤ 1. U = the ultimate strength of the component or subsystem.
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Limit Stats Design Method ν
F = αDD + ψ(αAA + αLL + αQQ + αTT) λ
D : dead loads;
λ
A : fall-arrest loads;
λ
L : Live loads;
λ
Q : wind, earthquake or other loads;
λ
T : influences resulting from temperature changes, shrinkage or creep.
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Limit States Design Design ν
α = load factor for the specific type of loading, as defined in CSA Z259.16-04.
λ
αD = 1.25 or when the dead load opposes the effect of A, 0.85; αA = 1.5;
λ
αL = 1.5 or, when the live load opposes the effect of A, 0;
λ
αQ = 1.5;
λ
λ
αT = 1.25 or, when thermal stress change the HLL pretension, 1.5.
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Limit state design method ν
ψ = the load combination factor, as follows: λ
ψ = 1, when only A is applied;
λ
ψ = 0.7, If A acts in combination with either L or Q;
λ
ψ = 0.6, If A acts in combination with both L and Q.
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Dynamic tests
In applying the concepts of the limit states design method to dynamic tests, two types of tests are defined:
1) Dynamic strength test corresponding to the ultimate limit states calculated with factored loads and are therefore carried out in order to maximize the impact forces in the fall arrest system. Thus, the test is performed using a rigid and compact mass to which a test wire rope lanyard is attached. 2) Dynamic performance test corresponding to the serviceability limit states calculated with actual load and are therefore carried out by complying as much as possible with the actual serviceability conditions. This test is performed with the fall arrest system. Only the person is replaced with a 100 kg rigid wooden torso. The main criteria is the deformation of the fall arrest system to satisfy clearance.
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Anchorage – Strength requirements REGULATIONS OSHA
ARTICLE
STRENGTH REQUIREMENTS
1926.104 (b)
24 kN (5400 lb)
1926.500 ANSI Z359.1
Quebec Safety Code
22.2 kN (5000 lb) 16 kN (3600 lb) with certification
2.10.12.(2)(a)
22.2 kN (5000 lb) without certification 18 kN (4000 lb)
CSA Z259.15
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Anchor point – Strength requirement with CSA Z259.16-04
With an E4 energy absorber complying with CSA Z259.11-05 Energy absorbers and lanyards, the maximum arrest force in ambient dry is F = 4 kN; frozen, F = 6 kN. According to CSA Z 259.16-04:
Ff (factored load) = αAA = 1.5 x 4 = 6 kN (ambient dry)
Ff (factored load) = αAA = 1.5 x 6 = 9 kN (frozen)
Any anchor point with a factored resistance of: R ≥ 6 kN (ambient dry) λ
R ≥ 9 kN (frozen)
is satisfactory.
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Reconstructed wall at CFMA
Wall 2
Wall 1
Rear and front layers Typical vertical layers 15M at 300 mm c/c Rear and front layers Typical horizontal layers 15M at 300 mm c/c
Wall thickness 250 mm approximately
Reconstructed wall, L shaped
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Reconstructed wall at CFMA
Typical section of wall reinforcement : 9 m (29.5’) long by 4.5 m (14.75’) high and 250 mm (10’’) thick reconstructed at CFMA (Centre de formation des métiers de l’acier). Wall reinforcement consisted of one rear and one front layers made up of vertical and horizontal 15M bars. Vertical bars spacing = 300 mm (12’’) c/c and horizontal bars spacing = 250 mm (10’’) c/c. Typical wall reinforcement built according to well established practices with regards to bars binding with no 16 wire to tie vertical and horizontal bars. No particular care in order to have a typical on-site wall reinforcement which can be reproduced.
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Anchor points on reinforcement bars
3 anchor points representing typical anchor points used by reinforcing steel erectors on site were tested.
Performance A
Performance B
Performance C
Anchor point : vertical bar, rear layers
Anchor point : horizontal bar, front layers
Anchor point : vertical bar, front layers
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Dynamic performance tests
As defined before:
Dynamic performance tests simulate the serviceability states by generating testing conditions that are as similar as possible to reality. They are carried out with the fall arrest system which the reinforcing steel erector uses (ADP harness, a lanyard with an energy absorber or a SRL with an integrated energy absorber); only the worker is replaced by a 100 kg wooden torso.
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Criteria for dynamic performance tests
No pull-out or start of pull-out of the anchor point. No rupture, breaking, or start of rupture/breaking of any component in the anchor point. Plastic deformation permitted, but never release of the load. Total fall distance such that the energy absorber does not bottom out.
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Observations and measurements for dynamic performance tests
Observation of the general behaviour of the FAS.
Measurement of the force in the lanyard.
Measurement of the displacement of the mass, hand and electronic measurements with a displacement sensor (slide-wire potentiometer). Measurement of total fall distance, hand and electronic measurements. Respect of clearance to ensure the mass does not hit the floor.
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Tests description – Performance tests
Description of FAS:
Harness (Sécurité Landry), Class ADP, Model Project 220 complying with CSA Z259.10 – Full Body Harness, ANSI Z359.1 & A10.14 and meets OSHA. Lanyard with energy absorber (DBI Sala, Model EZ-Stop II), 5’ long (1.52 m) complying with CAN/CSA-Z259.11M92-Energy absorbers and lanyards. 100 kg wooden Torso complying with CSA Z259.10-Full Body Harness and EN 364-1992 Personal protective equipment against falls from heights-Test Methods.
Free fall distance : 1.8 m (6’), free fall distance in the certification test for an E4 energy absorber complying with CAN/CSA-Z259.11-M92-Energy absorbers and lanyards.
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Steps 1 and 2
Step 3
slipping
Anchorage
level
Final
distance
Total fall
distance
Release level Free fall
Reference level
Measurement parameters
Steps 4 and 5
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Video – Performance test
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Typical view of a performance test
Release of wooden torso
Arrest of wooden torso by FAS
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Deployment of energy absorber after the fall test
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Charge (kN (kN)) or
Total fall distance
Test result – Performance A
Time (sec)
ISFP Symposium, Seattle, Washington, June 14-15, 2006
Anchorage : vertical bar, rear layers
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Charge (kN (kN)) or
Total fall distance (m)
Test result – Performance B
Time (sec)
ISFP Symposium, Seattle, Washington, June 14-15, 2006
Anchorage : horizontal bar, front layers
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Charge (kN (kN)) or
Total fall distance (m)
Test result – Performance C
Time (sec) Anchorage : vertical bar, front layers
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Table 1 – Tests results of performance tests Test No. Anchor point Bar orientation Position Pre-test hand measurement Initial length of lanyard Reference level Release level Free Fall Distance Post-test hand measurement Final length of lanyard Deployment of lanyard (energy absorber) Anchorage slip Final level Total Fall Distance Electronic measurements Pmax (lanyard) Distance to Pmax Maximal distance Final distance
Performance A
Performance B
Performance C
Vertical External, rear layer
Horizontal Internal, Front layer
Vertical External, Front layer
1524 2255 4155 1900
1535 2170 4215 2045
1550 2025 3825 1800
mm mm mm mm
2400 876
2405 870
2580 1030
mm mm mm
14 1030 3125
152 620 3595
5 575 3250
mm mm
3.698 2260 3183 3134
3.149 2610 3636 3565
2.950 2365 3247 3194
kN mm mm mm
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Units
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Observations – Performance tests
In all performance tests, partial or complete rip off of dorsal straps by tear off of dorsal support at the strap junction. In Performance B, the leather belt was incorrectly installed and this caused the complete tear off of the dorsal straps. It is recommended to study the load transfer within the harness straps so as to improve the design of this particular part of the harness. This prototype harness is complex and hence there are some risks of errors when wearing it, so training is very important.
ISFP Symposium, Seattle, Washington, June 14-15, 2006
Typical view of tear off at dorsal support
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Observations – Performance tests
Performance B: Plastic flexural deformation of anchor bar (inner horizontal bar, front wall) on a 1.5 m span (slipping without binding wire rupture)
Deformation of the support bar observed during performance test B
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Observations – Performance tests
Performance C: The jamming of the snap hook against the reinforcement bars stopped the wooden torso fall causing a plastic deformation of the snap hook.
Deformation of snap hook during performance test C
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Dynamic strength tests
As defined before:
Dynamic strength tests simulate the ultimate limit states calculated with factored loads and are carried out in order to maximize the impact forces in the fall arrest system. They are carried out with a rigid and compact 100 kg mass to which a test wire rope lanyard is attached.
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Criteria for dynamic strength tests
No pull-out or start of pull-out of the anchorage. No rupture, breaking, or start of rupture/breaking of any component in the anchorage. Plastic deformation permitted, but never release of the load.
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Observations and measurements for dynamic strength tests
Observation of the possible failure mode of the FAS. Measurement of the force in the lanyard. Measurement of the displacement of the mass, hand and electronic measurements with a displacement sensor (slide-wire potentiometer). Measurement of total fall distance, hand and electronic measurements.
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Tests description – Strength tests
100 kg rigid mass. Wire rope lanyard, 10 mm (3/8’’) diameter, 1.5 m long. Free fall distance : 1.8 m (6’). Wire rope lanyard
100 kg mass
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Video – Strength test
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Typical view of a strength test
Release of 100 kg mass
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Arrest of 100 kg mass
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Test results – Strength A
Total fall distance (m)
Charge (kN (kN)) ou
Strength A
Anchorage : vertical bar, rear layers Time (sec)
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Test result – Strength B
Total fall distance (m)
Charge (kN (kN)) ou
Strength B
Time (sec)
ISFP Symposium, Seattle, Washington, June 14-15, 2006
Anchorage : horizontal bar, front layers
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Test result – Strength C
Total fall distance (m)
Charge (kN (kN)) or
Strength C
Note : The 100 kg mass hit the floor
Time (sec)
ISFP Symposium, Seattle, Washington, June 14-15, 2006
Anchorage : vertical bar, front layers
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Table 2 – Tests results of strength tests Test No. Anchor point Bar orientation Position
Strength A
Strength B
Strength C
Vertical External, rear layer
Horizontal Internal, Front layer
Vertical External, Front layer
Pre-test hand measurement Reference level Release level Free Fall Distance
2455 4255 1800
2405 4205 1800
2165 3965 1800
mm mm mm
Post-test hand measurement Anchorage slip Final level Total Fall Distance
282 2110 2145
684 1720 approx. 2485 approx.
Not defined, > 2000 0 3965
mm mm mm
Electronic measurements Pmax (lanyard) Distance to Pmax Maximum distance Final distance
11.31 2057 2146 2118
6.72 2270 2535 2496
2.950 -------
kN mm mm mm
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Units
Observation – Strength test A
Strength test A: Large flexural deflection of the support bar (vertical, rear) as well as the displacement of the horizontal bar in the front layer. Deformation of the reinforcing bar in Strength test A
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Observation – Strength test B
Strength test B: Flexural plastic deformation of longitudinal bars on 4 levels. Wire bindings of the support bar have yielded on 2200 mm. Rupture of the gate of the snap hook.
Deformation of the reinforcing bars in strength B
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Observation – Strength test C
Strength test C: The test mass fell to the floor, ripping all binding wires on it’s fall.
The test mass fell to the floor, ripping all binding wires
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Discussion and conclusion
The dynamic tests on a reconstructed wall at the CFMA allowed the behaviour of the fall arrest system and the strength of the anchor points from the reinforcing bars to be studied and evaluated under typical on-site conditions. During the tests even if the FAS and anchor points were subjected to severe loadings conditions that will be seldom reached in regular reinforcing steel jobs, the behaviour of the FAS and the anchor points were excellent. The recorded measurements of the FAS were consistent with the requirements of the corresponding CSA standards. The recorded measurements of total fall distance ensured that the worker will not hit the floor during an accidental fall. The free fall distance was 1.8 m for all tests (CSA certification test for an E4 energy absorber). In real work situations, the free fall distance is usually much less than 1.8 m.
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Discussion and conclusion
Based on the test results, we can conclude:
The FAS and anchor points tested succeeded the dynamic performance tests. The reinforcing bars as erected in large walls with wire bindings at every second bar have succeeded the dynamic performance tests; thus they are sufficiently strong to be an anchor point for the lanyard of reinforcing steel erectors. Note: Reinforcing steel erectors say they always put extra wire ties on reinforcement bars they use as anchor points. The maximum flexural deformations of the fall arrest system measured during performance tests ensure that the worker will not hit the floor.
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Discussion and conclusion
The strength tests with a wire rope lanyard are too severe. We would recommend to carry out the strength tests with a lanyard without the energy absorber. The results of this project will be the subject of recommendations to the review committee for inclusion in articles relating to fall protection of reinforcing steel erectors in the Quebec Safety Code. Reinforcing steel erectors will have a fall arrest system adapted to their tasks on a vertical wall and appropriate anchor points verified and validated by dynamic fall tests.
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END.
Thank you.
Questions?
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