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Fiberglass and Aluminum Ladder Performance Fiberglass and Aluminum Ladder Performance Under Dynamic Loading ConditionsUnder Dynamic Loading Conditions
James GlanceyJack Vinson
University of Delaware
G. A. SnyderNational Forensic Engineers
Our Work . . .Analysis of Accidents
Examinations of Failed Ladders
Engineering Analysis of LaddersCharacterize structural failures and identify modes of failureModel the ladder structure
Buckling analysis of the side railsFEA models
Estimate factors of safety for current ladder designs
Ladder TestingInstrumented Ladders used to Measure StressesDynamic LoadsMuch, Much more to do . . .
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Motivation
•The authors (primarily Vinson and Snyder) have been involved with many ladder accident cases in litigation.
•Failures include step, extension, and articulated ladders.
Ladder Standards
ANSI 14.x, OSHA 1926.1053, BSI 2037. In general, used to qualify new designs.Quality control and user tests limited at best.Not required to be used in a formal quality control program to assess manufacturing and material variations.
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Terminology
ANSI 14Type II – 225 lbsType III – 200 lbs Dimensions
in inches
Forensics of Accidents/Failures
Climber FallsLost his or her balance; improper setup; etc.Landed on the ladder during the fall, thus inducing mechanical damageOften the theory proposed by the ladder manufacturer
Ladder DeficienciesDesign defect – inherently poor designMaterial strength defectsManufacturing defects
RivetsImproper Lay-up (Composites)Front and rear side rails
Curvature Eccentric loading
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Side Rail ModelP
e
a
Curved Side rail
l
z
Dimensions in inches
• New ladders often have curved, eccentrically loaded siderails
• Artifact of ladder company’s poor design & manufacturing practices
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Measuring Dynamic Loads and Stresses
Standard 6 ft aluminum Type III step ladder30 rosettes/linear gaugesDynamic stresses measured for a variety of users and tasks
y
xy
x
yxz
IcM
IcM
AP
++=σ
Typical Gauge Orientation
x
y
6
7
0
500
1000
1500
2000
2500
0 0.5 1 1.5 2 2.5 3
Time (s)
Dyn
amic
Loa
d (N
)
Test SubjectWeight = 800 N
Stepping down onto the 2nd
step
0
0.5
1
1.5
2
2.5
3
0 0.5 1 1.5 2 2.5 3
Time (s)
DLF
(Ver
ticle
For
ce/W
eigh
t of C
limbe
r)
u
z
WFDLF =
Stepping onto the 2nd step . . .
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-150
-100
-50
0
50
100
150
0 1 2 3 4 5 6
Time (s)
Nor
mal
Str
ess
(MPa
)
Point APoint BPoint CPoint D
A C
B D
Peak Stress Situation - Repetitive Task
-80
-60
-40
-20
0
20
40
60
0 1 2 3 4
Time (s)
Norm
al S
tres
s (M
Pa)
Point APoint BPoint CPoint D
A C
B D
Dynamic Stresses - Descending
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-50
50
150
250
350
450
550
0 0.5 1 1.5 2 2.5 3 3.5 4
Time (s)
Forc
e (N
)
FxFyFz
-50
50
150
250
350
450
550
0 0.5 1 1.5 2 2.5 3 3.5 4
Time (s)
Forc
e (N
)
FxFyFz
Right RailLeft Rail
Effect of a 2.5% Slope
Descending while ladder is leaning to the left
Experimental SummaryTest Results
Small Supporting Slopes = Big EffectsPeak Dynamic Load Factor ~ 3Peak Stress = 140 MPa.Yield Strength (6061-T6) = 240 MPaBuckling Strength:
Curvature = 150 MPaEccentric Loading = 175 MPa
Apparent Factor of Safety = less than 2
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Experimental SummaryObservations
Loads rapidly change in magnitude AND direction.Complex loading on the siderails.Under normal use, step ladders become three-legged structures.Stability and balance require 3 points of contact.Forces exerted by climber can be briefly transmitted to 2 or 1 siderails.
Implications for Ladder Designs
ANSI requires factor of safety of 4.Actual dynamic load compared to the rated load accounts for a factor of almost 3.Remaining factor must account for ladder setup, material properties, geometry, manufacturing and assembly variations.This situation has most likely led to the structural failure of some ladders.
ANSI testing protocol is not sufficient to properly evaluateladder designs and loading conditions.
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Back to ANSI . . . Revised to incorporate some findings from the CPSC study conducted by Fox (mid 70’s)Loads are static in natureUniformly distributed loadsUni-directional loadsNo compound loadsLarge limits (e.g. deflection) acceptableTests design to generate reproducible resultsCan repeat tests that fail using statistics“Ladder Use Survey” leads to one of two conclusions regarding accidents: abuse or misuse.
Some Components of a New/Revised Standard . . .
Design requirements for structural members.Component qualification as well as ladder assembly qualification.Compound, dynamic loading.Limits
Siderail curvaturesEccentric loadsMaterial propertiesRealistic deflectionsMaterial properties
These changes will be costly and met with significant oppositionby the ladder industry.
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Questions?
Comments?
The U of D Human Performance LabThe U of D Human Performance Lab
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On-Going Work . . .
Combined Effects of Curvature, Eccentric Loading, and Bending on the Side Rail StrengthOther Ladder TypesFEA Animation of FailuresAlternate Designs
Experimental Procedure . . .
Additional testing on 6 degree of freedom force plates.
Measured dynamic reactions (x,y,z) at the bottom of the side rail.
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Finite Element Modeling
0
50
100
150
200
250
300
0 10 20 30 40 50 60 70 80 90 100
l/r
Buc
klin
g Fa
ilure
Str
ess
(MPa
)
a/s = 1.0
Euler Curve
Tangent Modulus Curve
a/s = 0.2
a/s = 0.4 a/s = 0.8a/s = 0.6
Alcoa
Effects of Curvature on Buckling Strength
Slenderness Ratio (l/r)
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-400
-200
0
200
400
600
800
1000
1200
1400
1600
0 1 2 3 4
Time (s)
Axia
l Loa
d (N
)
MeasuredPredicted
Dynamic Vertical Reaction
Failure Statistics
On average, about 157,000 people make emergency room visits due to ladder mishaps each year. Underwriters Laboratories Inc., (2000).
Annually, accidents involving ladders cause an estimated 300 deaths and 130,000 injuries requiring emergency medical attention. National Safety Council. (1996)
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Presentation Overview
Overview of Ladder AccidentsForensics of FailuresStructural AnalysisHuman Testing and Dynamic LoadingImplications Regarding Current Ladder DesignsSummary
Concept:A Polymer Capped Chisel
θ
Cap
CapHolder
Work
Hex ShankSteelChisel
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Reducing Vibration Transmitted to the Hand From Struck Tools
Introduce a material with poor transmission characteristics
Polymer Cap
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Automated Hammer
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Prototype Designs
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