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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

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Reducing Vibration Transmitted to the Hand From Struck Tools

Introduce a material with poor transmission characteristics

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Prototype Designs

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