Safety Science 43 (2005) 469–483

www.elsevier.com/locate/ssci

The eVects of straight ladder setup and usage on ground reaction forces and friction requirements

during ascending and descending

Chien-Chi Chang ¤, Wen-Ruey Chang, Simon Matz

Liberty Mutual Research Institute for Safety, 71 Frankland Road, Hopkinton, MA 01748, USA

Abstract

Many ladder accidents occur despite standards and regulations. The causes of these inci-dences may often be related to the setup and usage of the ladders. Slippage at the ladder base isone of the most common sources of accidents involving straight ladders. In a previous paper, wereported the coeYcient of friction requirements at the bottom of the ladder under several ladderclimbing conditions during ladder ascending. In this paper, a comprehensive analysis was per-formed to further investigate these eVects with the addition of ladder descending. The normaland shear forces at the bottom of the ladder were also compared as well as additional factorsincluding climbing direction and climbing heights on the ladder. The results indicated thatregardless of climbing direction, the ladder inclined angle was the most critical factor in the fric-tion requirements among the factors evaluated. Within the tested conditions, a reduction of theladder inclined angle from 75° to 65° resulted in an average increase of 73% in friction require-ment between the base of the ladder and the Xoor. The climbing height also had a signiWcanteVect on the required coeYcient of friction, followed by the climbing speed. The potential of aslip incident increased when the subject climbed higher or faster on the ladder. The data alsoshowed that there existed a statistically signiWcant diVerence between ladder ascending anddescending for the normalized maximum normal and shear forces and the required COF. Thesecritical risk factors should be considered carefully when developing guidelines for preventivemeasures that workers can take during straight ladder setup and usage. 2005 Elsevier Ltd. All rights reserved.

* Corresponding author. Tel.: +1 508 497 0260; fax: +1 508 435 8136.E-mail address: chien-chi.chang@libertymutual.com (C.-C. Chang).

0925-7535/$ - see front matter 2005 Elsevier Ltd. All rights reserved.doi:10.1016/j.ssci.2005.08.002

470 C.-C. Chang et al. / Safety Science 43 (2005) 469–483

Keywords: Straight ladder; Friction requirement; Ground reaction force

1. Introduction

In workplaces, falls often result in costly injuries and are often signiWcant contrib-utors to fatal injuries (Hunting et al., 1991, 1994). Fall accidents not only causeburdens to injured workers and their families, but also result in signiWcant economicloss among many industries every year with lost work time and compensation costs(Leamon and Murphy, 1995; Courtney et al., 2001). According to the Liberty MutualWorkplace Safety Index report (2003), falls are the leading cause of serious work-related injuries, second only to overexertion. The Census of Fatal Occupational Injuries(1998–2002) reported by the Bureau of Labor Statistics, based on the data collectedfrom years 1998–2002, showed that 88% of fatal workplace falls were falls to a lowerlevel. Of the falls to a lower level, falls from ladders were second in frequency only tofalls from the roof.

Over the years, standards and regulations have been developed on ladder safety.For example, the American National Standards Institutes (ANSI) and AmericanLadder Institute (ALI) have partnered to create the ANSI A14 Committee whichsupervises subcommittees in the writing of a set of A14 standards. These A14 stan-dards prescribe rules that govern the safe construction, design, testing, care, and useof ladders, which include portable wooden ladders (ANSI, 2000a), portable metalladders (ANSI, 2000b), Wxed ladders (ANSI, 2002), etc. However, many ladder acci-dents still occur. The accidents may more often be due to the setup and usage ofladders than to the strength of the ladder structures, which are likely to be well-speciWed and designed. Among all falls, falls from ladders are likely more serioussince they frequently involve falls from heights. Lipscomb et al. (2003) indicatedthat ladders were involved in 21% of falls from heights among union carpentersinvolved in their study. The average height of falls reported in the literature wasbetween 2.3 and 3 m (Häkkinen et al., 1988; Björnstig and Johnson, 1992). At suchheights, it can cause an enormous impact force acting on the victim’s body and mayresult in a substantial injury or even a fatality.

Straight ladders appear to be involved in the majority of portable ladder accidents(e.g.: Häkkinen et al., 1988; Björnstig and Johnson, 1992; Axelsson and Carter, 1995).Slipping at the ladder bases appears to be one of the common causes among thestraight ladder accidents. Pesonen and Häkkinen (1988) indicated that more than33% of the accidents involving straight ladders originated from slipping of the ladderbases. Tsipouras et al. (2001) reported that the ladder sliding from its positionaccounted for 23% of the accidents. This percentage did not include sideway tilting ofthe ladder. Other studies (Dewar, 1977; Björnstig and Johnson, 1992; Axelsson andCarter, 1995) also indicated that ladder sliding at the base was the most commonsource of accidents involving straight ladders.

To understand the importance of the appropriate ladder setup in preventing acci-dents and to improve guidelines for operation, an in-depth investigation of the inter-

C.-C. Chang et al. / Safety Science 43 (2005) 469–483 471

action at the ladder bottom and Xoor surface was needed. The eVects of critical riskfactors that might potentially cause a slide at the bottom of the ladder while climbingon the ladders needed to be examined. Several previous studies (e.g., Häkkinen et al.,1988; Pesonen and Häkkinen, 1988; Lee and Tung, 1992) investigated the contactinterface between ladder shoes (surface component of ladder support that is in con-tact with the Xoor surface) and Xoor surfaces. However, applications of the resultsfrom most of these studies could be limited due to the small numbers of factors con-sidered, as well as the small numbers of subjects. The eVects of the ladder setup, lad-der usage and climbing conditions on the normal and shear forces, and the frictionrequirements were not quantiWed in detail. Although these studies have addressedsome important issues related to the safety of ladder usage, a more comprehensiveapproach is needed.

As reported in a previous paper (Chang et al., 2004), an experiment was designedto systematically investigate the friction requirements under various ladder setup andclimbing conditions. The results focused on the coeYcient of friction (COF) require-ments at the bottom of the ladder and the Wndings were based on the average of themaximum COF recorded for the top three rungs during ladder ascent only. TheeVects of climbing direction and climbing height on COF and their interactions withother factors such as ladder inclined angle, climbing speed, ladder type, support attop of the ladder, and subject weight were not investigated. In this paper, a morecomprehensive analysis was performed to investigate these factors in further detail inaddition to the results from the ladder descent. Besides quantifying the COF require-ments for diVerent climbing conditions, the normal and shear forces at the bottom ofthe ladder were also compared.

2. Methods

2.1. Experiment design

A force platform system was set up that allowed the collection of contact forces atthe bottom of the ladder when subjects climbed on the ladder under various test con-ditions. Fig. 1 illustrates the experimental setup with a subject climbing on the ladder.

To investigate whether diVerent ladder types will aVect ground reaction forces andrequired COF, two commonly used commercially available ladders were tested. Ofthe two straight ladders used in this study, one was made of aluminum (weight: 14.5kg, length: 4.88 m, width: 38.7 cm) and the other was made of Wberglass (weight:16.3 kg, length of 4.93 m, width: 38.7 cm). The space between adjacent rungs wasapproximately 30.5 cm for both ladders. The upper end of each ladder was coupled toa pair of “retractable” rollers to simulate two diVerent contact conditions at the topof the ladder: the “normal” (original conditions—plastic cap at the upper end) vs.“reduced friction” (using the rollers). The bottom of the ladder was placed on a forceplate (Model 4060A; Bertec Corporation, OH, USA) and the ladder was inclinedagainst a wall at an angle. The movement of the ladder was constrained at the bot-tom to the force plate with a block attached to the force plate. The base of the force

472 C.-C. Chang et al. / Safety Science 43 (2005) 469–483

plate was mounted to a concrete Xoor at two diVerent pre-set positions for the twodiVerent ladder inclined angles tested in this experiment, 65° and 75°. A harnesssystem was used to prevent subjects from accidentally falling from the ladder. AU-shaped safety hook was placed around the highest rung of the ladder, without con-tacting the ladder itself, to limit the movement of slippage.

2.2. Protocol and subjects

Seventeen subjects were recruited from local newspaper advertisements to partici-pate in this study. Experience in ladder climbing was not in the selection criteria of

Fig. 1. Experimental setup.

C.-C. Chang et al. / Safety Science 43 (2005) 469–483 473

subject recruitment. The experimental procedures were explained to each subject. Allsubjects gave written informed consent, Wlled out a brief medical history and werescreened to assure they had no active musculoskeletal disorder. The protocol wasapproved by an institutional review committee for the protection of human subjects.Several anomalies were found in the data recorded during ladder descent with onesubject and the data of that subject was excluded from further analyses. The datafrom sixteen subjects were reported in the current paper. To examine the eVects ofbody weight, the subjects’ weight information was also collected and considered inthe statistical analysis. Due to a limited number of participants and to simplify theanalysis, the subjects were divided into three categories according to their weight(light, intermediate, and heavy), with an equal or similar number of subjects in eachgroup. The light weight category included Wve subjects with an average weight andstandard deviation of 62.6 § 8.9 kg, and an average height and standard deviation of169 § 12 cm. The intermediate group included six subjects with an average weightand standard deviation of 84.6 § 5.9 kg, and an average height and standard devia-tion of 180 § 5 cm. The heavy weight category included Wve subjects with an averageweight and standard deviation of 108.7 § 12.5 kg, and an average height and standarddeviation of 185.7 § 6.6 cm.

The subjects were asked to climb up a total of 10 steps on the ladder and thendown to the Xoor for each trial. The climbing heights of the 10 steps were approxi-mately 2.7 m and 2.9 m at 65° and 75° inclined angles, respectively. The climbingmethod was limited to one foot only on each rung, with the subject facing the ladder.A total of 16 diVerent climbing conditions were tested for each subject in a randomorder. These conditions included combinations of two climbing speeds (slow vs. fastat 55 and 75 steps/min), two ladder inclined angles (65° vs. 75°), two contact condi-tions at the upper end of the ladders (reduced friction vs. normal), and two laddertypes (aluminum vs. Wberglass). For each condition, the subjects repeated the ascend-ing and descending tasks Wve times. A metronome was used to control the climbingspeed. The force plate measured the normal and shear ground reaction forces at thebottom of the ladders during subject climbing.

To prevent undue fatigue of the subjects, the data collection period for each sub-ject was divided into four sessions over an 11-day period (Day 1, Day 4, Day 8, andDay 11). After several practices, a total of 80 trials were collected from each subject.

2.3. Data analysis

The independent variables considered in the study included: climbing direction,climbing height (rung), body weight category, climbing speed, ladder inclined angle,ladder type, and type of support at the ladder top. The points of interest duringclimbing cycles were extracted from the data for each trial at each step to investigatethe eVects of independent variables on the normal and shear forces, and the frictionrequirements at the interface between the Xoor surface and ladder shoes during lad-der climbing activities. The deWnition of a step used in this study for data extractionand analysis was the phase starting with a foot stepping on a rung until the oppositefoot steps on the next rung. Rung 1 represents the highest full step on the ladder, rung 2

474 C.-C. Chang et al. / Safety Science 43 (2005) 469–483

represents a step below rung 1, and rung 3 represents 2 steps below rung 1. The dataof normal and shear forces were normalized with respect to the sum of each subject’sweight and ladder weight. The instantaneous COF values were calculated by dividingthe shear force by the normal force at the same instant based on the collected rawdata from the laboratory experiment. A customized computer program was used toidentify the maximum points of these dependent variables in each step to representthe maximum normal and shear forces, and the required COF for that step.

A mixed linear module (models with both Wxed and random eVects) with analysisof variance (ANOVA) split-plot repeated measures design was used to analyze thedata (WolWnger et al., 1991). The independent variables were regarded as Wxed fac-tors. The “subject” was treated as a random factor. The Wve repeats for each climbingcondition were considered as repeated measures. �D 0.05 was used as a signiWcancelevel. The statistical analyses were performed using SAS® version 8.0 software. Theassumptions included linear structure for the mixed model, the error component tobe additive and normality of error distribution. In using the mixed module in thestudy, it was assumed that observations in the repeated measures on the same subjectcorrelated for diVerent climbing conditions and could have diVerent variability insiderepeated measures for the same climbing conditions across diVerent subjects. Themixed procedure also was selected due to its advantages over other methods inemploying a more general covariance structure approach (WolWnger and Chang,1995). The main factors and the two-way interactions between factors were includedin the model.

3. Results

Fig. 2 shows a typical example of the proWles of the normal and shear forces, andthe calculated COF during a trial of ladder climbing in this study. The Wgure illus-trates that the shear force and COF increased as the subject climbed higher to the topof the ladder and decreased as the subject descended back to the Xoor.

During ladder ascending, the normal force reached the maximum (point 2) of thatstep shortly after the subject stepped on a new rung starting at point 1. After thispeak, the normal force decreased to a local minimum (point 3) and rebounded to thesecond peak (point 4). The value of the second peak (point 4) is usually less than thevalue of the Wrst peak (point 2). After the second peak (point 4), the normal forcedecreased continually until it reached the starting point of the next step (point 5).

During ladder descending, the normal force appeared to increase gently at thebeginning of each step when the subject Wrst stepped on a new rung starting at point6. After the normal force reached its initial peak (point 7), the value reduced a little toa local minimum (point 8) and then quickly raised to its second peak (point 9), whichwas usually the maximum of that step. The normal force decreased after point 9 untilthe beginning of the next step at point 10. The normal force proWle of ladder descend-ing seems to be a mirror image of the ladder ascending proWle.

The maximum values in each step of these proWles of the normal and shear forces,and the calculated COF for each trial and each subject were identiWed. The extracted

C.-C. Chang et al. / Safety Science 43 (2005) 469–483 475

maximum normal and shear forces data were normalized with respect to the sum ofeach subject’s weight and ladder weight. Tables 1–3 show the averages of normalizedmaximum normal and shear forces, and required COF, respectively, for each of theexperimental treatments which included climbing direction, climbing height (rung),body weight category, climbing speed, ladder inclined angle, ladder type, and type ofsupport at the ladder top. The required COF for a particular rung was deWned as themaximum COF during the step on that particular rung. The results presented herewere calculated using the data from the last three full steps for ascending and the Wrstthree steps for descending. When presenting the diVerences in one variable, except theweight category, the data from all climbs were pooled together and then divided intotwo or three groups based on the levels of that variable. The means and standarddeviations for each subject and level were Wrst calculated from this pool of data foreach experiment treatment. The averages of means and standard deviations for allsubjects were then calculated for each level. For the weight category, the means andstandard deviations from all climbs for each subject were Wrst calculated. Then theaverages of means and standard deviations for all subjects in the same weight cate-gory were calculated.

Fig. 2. A typical example of the normal and shear forces, and the COF proWle during ladder ascending anddescending. (a) and (b) represent enlarged views of one of the steps during ladder ascending (points 1–5)and descending (points 6–10), respectively.

-100

100

300

500

700

900

1100

1300

1500

1700

1900

-0.1

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

-100

100

300

500

700

900

1100

1300

1500

1700

1900

-0.1

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

-100

100

300

500

700

900

1100

1300

1500

1700

1900

0 2 4 6 8 10 12 14 16 18 20 22

Time (s)

-0.1

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

Normal Shear COF

15

4

3

2

610

9

8

7

-100

100

300

500

700

900

1100

1300

1500

1700

1900

-0.1

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

Ascending Normal Forces, Shear Forces, and Coefficient of Friction

Normal Forces, Shear Forces, and Coefficient of Friction

Descending Normal Forces, Shear Forces, and Coefficient of Friction

-100

100

300

500

700

900

1100

1300

1500

1700

1900

No

rmal

an

d S

hea

r F

orc

es (

N)

No

rmal

an

d S

hea

r F

orc

es (

N)

No

rmal

an

d S

hea

r F

orc

es (

N)

-0.1

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

Co

effi

cien

t o

f F

rict

ion

Co

effi

cien

t o

f F

rict

ion

-100

100

300

500

700

900

1100

1300

1500

1700

1900

0 2 4 6 8 10 12 14 16 18 20 22

Time (s)

-0.1

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

15

4

3

2

610

9

8

7

(a) (b)

Ascent Descent

476 C.-C. Chang et al. / Safety Science 43 (2005) 469–483

Table 1The mean, standard deviation, and maximum of normalized maximum normal force for all experimentconditions

Mean Standard deviation Maximum

Climbing direction Ascending 1.289 0.083 1.853Descending 1.245 0.076 1.876

Climbing height Rung 1 1.249 0.085 1.876Rung 2 1.273 0.085 1.705Rung 3 1.279 0.089 1.853

Weight category Light 1.283 0.125 1.876Intermediate 1.267 0.107 1.648Heavy 1.251 0.084 1.560

Climbing speed Fast 1.308 0.085 1.853Slow 1.226 0.068 1.876

Inclined angle 65° 1.266 0.089 1.85375° 1.268 0.085 1.876

Ladder type Aluminum 1.263 0.086 1.876Fiberglass 1.271 0.089 1.853

Top support Reduced friction 1.252 0.081 1.876Normal friction 1.282 0.091 1.853

Table 2The mean, standard deviation, and maximum of normalized maximum shear force for all experimentconditions

Mean Standard deviation Maximum

Climbing direction Ascending 0.218 0.071 0.456Descending 0.214 0.068 0.419

Climbing height Rung 1 0.236 0.073 0.456Rung 2 0.216 0.068 0.402Rung 3 0.195 0.064 0.378

Weight category Light 0.223 0.075 0.456Intermediate 0.215 0.070 0.389Heavy 0.210 0.070 0.378

Climbing speed Fast 0.222 0.072 0.456Slow 0.209 0.068 0.406

Inclined angle 65° 0.281 0.030 0.45675° 0.150 0.020 0.338

Ladder type Aluminum 0.214 0.069 0.421Fiberglass 0.217 0.072 0.456

Top support Reduced friction 0.213 0.068 0.414Normal friction 0.218 0.073 0.456

C.-C. Chang et al. / Safety Science 43 (2005) 469–483 477

The statistical analyses were performed to analyze these data and included allmain factors and two-way interactions between factors. Table 4 provides the resultsof statistical analyses of the normalized maximum normal and shear forces, and therequired COF, based on the mixed linear module with split-plot repeated measuresdesign mentioned earlier. These results showed that, for the normalized maximumnormal forces, all main factors led to statistically signiWcant diVerences except theladder inclined angle. For the normalized maximum shear forces and the requiredCOF, all factors resulted in statistically signiWcant diVerences. Further contrast testsrevealed that there were statistically signiWcant diVerences (p < 0.05) between eachclimbing height (rung) and between each weight category for all output measuresexcept that those between the light weight and intermediate weight categories had nosigniWcant diVerence (p D 0.77) for the required COF.

The statistical analysis results also indicated that there were several statisticallysigniWcant interactions between two factors for three output measures. For the nor-malized normal force, the data showed that the climbing direction had signiWcantinteractions with the speed, weight, and climbing height. Fig. 3 illustrates the interac-tions between the climbing direction and the climbing speed with a higher level ofstatistically signiWcant interaction (F(1,7636) D 107.54, p < 0.0001) for the normalizednormal force.

There also existed a higher level of signiWcant interaction (F(1,7636) D 105.99,p < 0.0001) between the ladder type and the top support for the normalized normalforce. The interaction is illustrated in Fig. 4.

Table 3The mean, standard deviation, and maximum of required COF for all experiment conditions

Mean Standard deviation Maximum

Climbing direction Ascending 0.208 0.061 0.376Descending 0.207 0.057 0.346

Climbing height Rung 1 0.222 0.062 0.376Rung 2 0.210 0.058 0.346Rung 3 0.189 0.053 0.344

Weight category Light 0.210 0.063 0.376Intermediate 0.210 0.058 0.342Heavy 0.201 0.060 0.322

Climbing speed Fast 0.213 0.059 0.376Slow 0.201 0.059 0.355

Inclined angle 65° 0.263 0.023 0.37675° 0.152 0.018 0.285

Ladder type Aluminum 0.208 0.060 0.376Fiberglass 0.206 0.058 0.357

Top support Reduced friction 0.209 0.060 0.371Normal friction 0.206 0.059 0.376

478 C.-C. Chang et al. / Safety Science 43 (2005) 469–483

The data also pointed out that the climbing direction had signiWcant interactionswith all other factors for the required COF except the climbing speed (p D 0.50) andladder type (p D 0.82). In addition, the climbing direction had a higher level of statis-tically signiWcant interaction (F(1,7636) D 133.48, p < 0.0001) with the ladder inclinedangle for the required COF. Fig. 5 illustrates the interaction between the climbingdirection and ladder inclined angle on COF.

4. Discussion

As shown in Fig. 2, the data proWles indicated that during ascending, when thenormal force is in proximity to its local minimums (points 1, 3, 5), the COF usuallyachieves a higher value. In general, the value of the COF approached its maximum

Table 4Statistical analysis results of p value for three output measures: normalized normal force, normalizedshear force and coeYcient of friction (COF)

Abbreviation Key: CD: climbing direction, CH: climbing height (rung), WC: weight category, CS: climb-ing speed, IA: inclined angle, LT: ladder type, TS: top support.

Variable name Normalizednormal force

Normalizedshear force

CoeYcient of friction(COF)

CD <.0001 <.0001 0.0176CH <.0001 <.0001 <.0001WC <.0001 <.0001 <.0001CS <.0001 <.0001 <.0001IA 0.5378 <.0001 <.0001LT <.0001 <.0001 <.0001TS <.0001 <.0001 <.0001CD¤CH <.0001 0.0649 <.0001CD¤WC <.0001 <.0001 <.0001CD¤CS <.0001 <.0001 0.4977CD¤IA 0.8155 <.0001 <.0001CD¤LT 0.7128 0.9312 0.8213CD¤TS 0.2913 0.9706 0.0113CH¤WC 0.0001 0.0962 <.0001CH¤CS 0.0005 0.0134 <.0001CH¤IA 0.0005 <.0001 <.0001CH¤LT 0.4108 0.508 0.5064CH¤TS 0.0419 0.0252 0.0148WC¤CS 0.0237 0.2827 0.3707WC¤IA <.0001 0.0001 <.0001WC¤LT 0.9316 0.0394 0.0256WC¤TS <.0001 0.0002 0.6102CS¤IA 0.4769 <.0001 0.8065CS¤LT 0.5919 0.1518 0.0043CS¤TS 0.0005 0.0004 <.0001IA¤LT <.0001 <.0001 <.0001IA¤TS 0.0002 <.0001 0.047LT¤TS <.0001 <.0001 0.8177

C.-C. Chang et al. / Safety Science 43 (2005) 469–483 479

when the normal force reached a local minimum at point 3. Shear force was close toone of its peaks around this point. During descending, the scenario seemed to be a lit-tle diVerent from the ascending data proWle. Peaks in shear force usually occurredafter the normal force already passed its local minimum at point 8. Although theremay exist a peak of the COF around this instant, its value usually did not reach themaximum of a step. Instead, the maximum COF during descending often occurredeither at the beginning of the step or at the end of the step, where the normal forceoften reached its minimum.

Fig. 3. Interaction between climbing speed and climbing direction on normalized normal force.

Fig. 4. Interaction between ladder type and top support on normalized normal force.

480 C.-C. Chang et al. / Safety Science 43 (2005) 469–483

The results shown in Table 1 indicated that the maximum normal forces can reach1.88 times the sum of body and ladder weights due to the dynamics of climbing.Among the test factors, the climbing speed was the most signiWcant factor that con-tributed to the diVerence of normalized maximum normal forces, followed by climb-ing direction. The faster climbing speed resulted in an average increase ofapproximately 7% in the maximum normalized normal force as compared to theslower speed. The ascending data showed an average increase of 3% compared to thedescending data. The interaction between the climbing speed and direction, as shownin Fig. 3, indicated that when the climbing speed was fast, the diVerence of the aver-age normalized normal force between ascending and descending was approximately5.1%. The diVerence decreased to 1.8% when the climbing speed was slow. A fasterclimbing speed produced a signiWcant but small increase in the normalized normalforce due to diVerent climbing directions than a slower speed. The interactionbetween the top support and ladder type shown in Fig. 4 indicated that under thenormal top support condition, the average normal force for the Wberglass ladder washigher than that for the aluminum ladder by approximately 2.4%. However, underthe conditions of reduced friction at the top support, the average normal force for theWberglass ladder was approximately 1% lower than that for the aluminum ladder.The average normal force of the aluminum ladder was less inXuenced by the type oftop support. In contrast, the Wberglass ladder was more sensitive to the contact con-dition at the ladder top compared to the aluminum ladder.

The inclined angle was the most signiWcant inXuence on the normalized maximumshear forces. According to the data shown in Table 2, a change of the inclined anglefrom 75° to 65° resulted in an approximately 87% increase in the average mean valueof the normalized maximum shear force. The climbing height (rung) was the secondmost signiWcant eVect. The value of shear force increased about 10% when subjectspositioned themselves on the next higher rung.

Fig. 5. Interaction between climbing direction and ladder inclined angle on coeYcient of friction.

C.-C. Chang et al. / Safety Science 43 (2005) 469–483 481

The inclined angle was the most signiWcant contributor to the diVerences in therequired COF value among the factors tested. Table 3 indicates that a change of theinclined angle from 75° to 65° yielded an approximately 73% rise in the average meanvalue of the required COF. It appears that, regardless of the climbing direction, theladder inclined angle was the most critical factor in the friction requirements amongthe factors evaluated. The climbing height also had a signiWcant eVect on the requiredCOF, followed by the climbing speed. The potential of a slip incident increased whenthe subject climbed higher or faster on the ladder due to a higher required COF. Therequired COF increased an average of 9% as subjects positioned themselves a runghigher. The fast climbing speed used in this experiment led to approximately a 6.5%increase in the required COF in comparison to the slow climbing speed. Subjects ofthe light weight category did not have a signiWcant diVerence in the required COFcompared with the intermediate weight category. However, the heavy weight cate-gory had an approximately 5% decrease in the required COF as compared to that forthe other two weight categories. The factors of climbing direction, ladder type, andtype of support at the top of the ladder provided signiWcant but small diVerences inthe required COF as compared to the other test factors.

Young and Wogalter (2000) reported on the variation in human perception of theangle of inclination of a ladder. Participants were asked to set up a straight ladderinclined at an angle of 75.5°, as recommended by the ANSI (2000a,b). The resultsshowed that the actual angle without using an angle measurement device had a meanof 71.8° and standard deviation of 4.48°. This implies that the accuracy of humanperception in the setup of an intended ladder inclined angle could easily yield consid-erable variation. Although such deviation might result in only a few degrees diVer-ence in inclined angle setup, the data from the current study show that this could leadto a considerable change of required COF in both ladder ascending and descending.When the ladder shoe and Xoor interface becomes less than ideal (e.g. shoe worn orsurface contaminated with oil, water, soil, etc.), the eVects of setup deviation couldbecome especially critical. The potential risks for ladder climbing could multiply dueto a substantial increase in required COF along with the reduction in the availablefriction.

The interaction between climbing direction and ladder inclined angle presented inFig. 5 indicates that when the ladder inclined angle was set at 65°, the required COFfor ascending is approximately 2.1% higher than that for descending. When the lad-der inclined angle was set up at 75°, the required COF for ascending became approxi-mately 2.2% lower than that for descending. Although the change in the requiredCOF due to the climbing direction was smaller than the change due to the ladderinclined angle, it appears that, as the ladder inclined angle increases, the requiredCOF for both directions would decrease. However, the required COF for ascendingwould decrease slightly faster than that for descending.

The results in Tables 1 and 2 indicate that the mean values of the maximum nor-mal and shear forces in ascending were 3% and 0.9% higher, respectively, than thosein descending. The larger increase of the normal force than the shear force in ascend-ing compared to descending seems to suggest that the required COF for the ascend-ing should be lower than that for the descending. Nevertheless, the analysis results

482 C.-C. Chang et al. / Safety Science 43 (2005) 469–483

proved to be the contrary. The required COF in ascending were actually 0.5% higherthan the descending, although the amount is small. Since the COF was an instanta-neous measure, the maximum COF requirement could occur at any instant wherethere is a relatively higher shear force and/or a lower normal force or anywhere inbetween within a step as shown in Fig. 2a and b. The ratio of the mean maximumshear force over the mean maximum normal force should not be used to predict themaximum COF requirement.

From the results of the current study, it is clear that the subject’s dynamic move-ments on the ladder inXuence the interface between the bottom of the ladder and theXoor surface. These movements could change the ground reaction force proWles thatare one of several important factors of concern to guard the ladder from slipping atthe base. A previous study (Hammer and Schmalz, 1992) had reported that diVer-ences existed in human movement behavior such as climbing time, hand-foot dis-tance, motion patterns and rhythm when subjects climb a ladder with variousinclined angle setups and at diVerent climbing directions. However, our current studydid not directly investigate the correlation between the body segment movement onthe ladder and ground reaction force proWles at the base of ladder. How much thesediVerent movement behaviors may aVect the ground reaction proWle was not investi-gated. Also, the current experiment setup did not monitor the contact movement oftop of the ladder, which might have an eVect on the measured data. An additionallimitation in the current study is that the division of the subjects into three weightcategories might result in an imperfection due to the fact that body weight is a para-metric variable. Some information in terms of statistics might be lost. Further studiesare needed to explore these in detail.

5. Conclusion

Slippage between the bottom of the ladder and the ground surface is one of themajor causes of serious accidents in straight ladder usage. To ensure safety in ladderclimbing, one of several important steps is to understand the eVects of factors thatcould inXuence the slippage at the bottom of the ladder. In this study, we investigatedthe climbing direction, climbing height, weight category, climbing speed, ladderinclined angle, ladder type, and type of ladder top support on the ground reactionforces at the base of the ladder. The results indicated that, in either ascending ordescending tasks on a ladder, the inclined angle of the ladder appears to be the mostcritical factor in friction requirements among the factors evaluated, followed by theclimbing height, and then climbing speed. These factors should be taken into consid-eration for proper straight ladder setup and usage under various working conditions.In addition, while the standards and common practices (such as using an inclinedangle of 75° or a 4–1 ratio in lengths) specify proper ladder setup and have been inexistence for a long time, most ladder users comply with these guidelines subjectively.Given the limitations of subjective compliance, more objective methods (i.e., the useof a simple device for proper angle measurements instead of estimates based on per-ception) should be employed to better assist users in preventing ladder accidents.

C.-C. Chang et al. / Safety Science 43 (2005) 469–483 483

References

American National Standards Institute (ANSI), 2000a. A14-1: American national standard for ladders—portable wood—safety requirements, American Ladder Institute, Chicago.

American National Standards Institute (ANSI), 2000b. A14-2: American national standard for ladders—portable metal—safety requirements, American Ladder Institute, Chicago.

American National Standards Institute (ANSI), 2002. A14-3: American national standard for ladders—Wxed—safety requirements, American Ladder Institute, Chicago.

Axelsson, P.O., Carter, N., 1995. Measures to prevent portable ladder accidents in the construction indus-try. Ergonomics 38 (2), 250–259.

Björnstig, U., Johnson, J., 1992. Ladder injuries mechanisms, injuries and consequences. Journal of SafetyResearch 23, 9–18.

Census of Fatal Occupational Injuries (CFOI), 1998–2002. Bureau of Labor Statistics, US Department ofLabor. Available from: <http://www.bls.gov/iif/oshcfoi1.htm>.

Chang, W.R., Chang, C.C., Son, D.H., 2004. Friction requirements for diVerent climbing conditions instraight ladder ascending. Safety Science 42, 791–805.

Courtney, T.K., Sorock, G.S., Manning, D.P., Collins, J.W., Holbein-Jenny, M.A., 2001. Occupational slips,trips, and fall related injuries—can the contribution of slipperiness be isolated? Ergonomics 44, 1118–1137.

Dewar, M.E., 1977. Body movements in climbing a ladder. Ergonomics 20 (1), 67–86.Häkkinen, K.K., Pesonen, J.P., Rajamäki, E., 1988. Experiments on safety in the use of portable ladders.

Journal of Occupational Accidents 10, 1–9.Hammer, W., Schmalz, U., 1992. Human behavior when climbing ladders with varying inclinations. Safety

Science 15, 21–38.Hunting, K.L., Nessel-Stephens, L., Stanford, S.M., Shesser, R., Welch, L.S., 1994. Surveillance of construc-

tion worker injuries through an urban emergency department. Journal of Occupational Medicine 36(3), 356–364.

Hunting, K.L., Matanoski, G.M., Larson, M., Wolford, R., 1991. Solvent exposure and the risk of slips,trips, and falls among painters. American Journal of Industrial Medicine 20 (3), 353–370.

Leamon, T.B., Murphy, P., 1995. Occupational slips and falls: more than a trivial problem. Ergonomics 38,487–498.

Lee, Y.H., Tung, E.K., 1992. Body and ladder mechanical stress analysis in a climbing strike. In: Kumar, S.(Ed.), Advances in Industrial Ergonomics and Safety IV. Taylor & Francis, London.

Liberty Mutual Research Institute for Safety, 2003. Liberty Mutual Workplace Safety Index Fall 2003,Boston, MA: Liberty Mutual Group.

Lipscomb, H.J., Li, L., Dement, J.M., 2003. Falls among union carpenters. American Journal of IndustrialMedicine 44, 148–156.

Pesonen, J.P., Häkkinen, K.K., 1988. Evaluation of safety margin against slipping in a straight aluminumladder. Hazard Prevention 24, 6–13.

Tsipouras, S., Hendrie, J.M., Silvapulle, M.J., 2001. Ladder: accidents waiting to happen. Medical Journalof Australia 174 (10), 516–519.

WolWnger, R.D., Tobias, R.D., Sall, J., 1991. Mixed models: A future direction. In: Proceedings of the Six-teenth Annual SAS Users Group Conference, SAS Institute Inc., Cary, NC, pp. 1380–1388.

WolWnger, R.D., Chang, M., 1995. Comparing the SAS GLM and MIXED procedures for repeated mea-sures. In: Proceedings of the Twentieth Annual SAS Users Group Conference, SAS Institute Inc., Cary,NC, p. 1554.

Young, S.L., Wogalter, M.S., 2000. On improving set-up angle accuracy for extension ladders. In: Proceed-ings of the XIVth Triennial Congress of the International Ergonomics Association and 44th AnnualMeeting of the Human Factors and Ergonomics Society, San Diego, California, vol. 4, pp. 111–114.