NBSIR 85-3271

Perimeter Safety Net Projection

Requirements

C.W.C. Yancey

N.J. Carino

M. Sansalone

U S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Center for Building Technology

Gaithersburg, MD 20899

November 1985

Issued May 1 986

QC

100

.U56

NO. 85-3271

1986 £.<2

U S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

NBSIR 85-3271

PERIMETER SAFETY NET PROJECTION

REQUIREMENTS

C.W.C. Yancey

N.J. Carino

M. Sansalone

U S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Center for Building Technology

Gaithersburg, MD 20899

November 1985

Issued May 1986

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

Perimeter Safety Net Projection Requirements

C. W. C. Yancey, N. J. Carino, and M. Sansalone

ARfiEB&CI

Current construction-site safety net regulations set limitations

on the minimum horizontal projection of perimeter nets and the

maximum vertical distance between an elevated working surface and

the net below. These limitations were arbitrarily established as

no actual or simulated fall data existed. The adequacy of these

requirements in ensuring construction worker safety has been

questioned. Thus, a test program was carried out to determine

the adequacy of existing regulations.

Simulated fall tests were conducted using anthropomorphic dummies

to represent falling workers. The dummies fell from a 30-foot

(9.1 m) high platform and their trajectories were recorded

photographically. The photographs were used to reconstruct the

dummies' trajectories and to determine the horizontal distance

between the face of the platform and the center of gravity of the

dummy. Results are presented to show the trajectory of the

falling body and the maximum horizontal distance in the final

landing position. An analytical model was developed to simulate

a falling worker. The model can be used to predict trajectories

for a given set of initial conditions including worker height and

weight, departure horizontal velocity and fall height. Guidelines

are presented for revising existing regulations pertaining to the

dimensional requirements for perimeter nets.

Key words: Construction safety; falling bodies, fall trajectory;

horizontal distance; safety nets; simulated falls.

iii
TABLE OF CONTENTS

Page

ABSTRACT iii

LIST OF TABLES V

LIST OF FIGURES vi

1. INTRODUCTION . 1

1.1 Perimeter Net Applications 1

1.2 Current Requirements for Perimeter Nets 1

1.3 Possible Fall Conditions 3

1.4 Problem Statement and Test Objective 5

1.4.1 Problem Statement 5

1.4.2 Test Objective 6

2. SIMULATED FALL TESTING 6

2.1 Selection of the Fall Sequences to

be Simulated 6

2.2 Description of Test Setup and Test Dummies ...... 6

2.2.1 Test Setup 6

2.2.2 Description of Test Dummies 8

2.3 Measurement of Trolley Horizontal Velocity 11

2.4 Photographic Recording of Falls 14

2.4.1 Procedure for Observing the Falls

Trajectories 14

2.4.2 Correction for Vertical Parallax

in Photographs 16

2.5 Summary of Test Procedure 16

3. PRESENTATION OF TEST RESULTS 16

3.1 Horizontal Displacement of Body at Landing 16

3.2 Trajectory of the Falling Body 19

4. ANALYSIS OF FALL TRAJECTORY 23

4.1 Introduction 23

4.2 Analytical Model 23

4.2.1 Initial Angular Velocity 23

4.2.2 Rotation About Fixed End 25

4.2.3 Fall Trajectory 27

4.2.4 Parametric Study 28

4.2.5 Comparison of Measured and Computed

Trajectories 35

4.2.6 Summary 44

5. DESIGN CONSIDERATIONS 44

6. SUMMARY 50

7. REFERENCES 52

iv
LIST OF TABLES

Page

Table 2.1 Height and Weight Measurements of Test

Dummies 10

Table 2.2 Measured Trolley Speeds for Various

Pressure/Weight Combinations 12

Table 3.1 Summary of Landing Distance Data 21

v
LIST OF FIGURES

Page

Figure 1.1 Typical Perimeter Net Installation 2

Figure 1.2 Possible Conditions Leading to Accidental

Falls 4

Figure 2.1 Elevation View of Test Scaffold and Padded

Mat 7

Figure 2.2 Side View of Trolley/Track Assembly 9

Figure 2.3 Pertinent Dimensions of Test Dummies 10

Figure 2.4 Velocity Measurements of the Trolley 13

Figure 2.5 Elevation View of Test and Camera Scaffolds ... 15

Figure 2.6 Correction for Vertical Parallax 17

Figure 3.1 Distribution of Simulated Fall Tests 18

Figure 3.2 Description of Measured Landing Distance 21

Figure 3.3 Typical Trajectory of Test Dummy 22

Figure 4.1 Analytical Rod Model 24

Figure 4.2 Effect of Coefficient of Friction on Computed

Fall Trajectories 29

Figure 4.3 Effect of Height of Center of Gravity on

Computed Fall Trajectories 30

Figure 4.4 Conditions at Start of Free-Fall 31

Figure 4.5 a) Fall Trajectory Under "Lift-Off"

Conditions 33

b) Fall Trajectory With "Lift-Off" and

Reduced Height of Center of Gravity 33

Figure 4.6 Effect of Initial Horizontal Velocity on

Computed Fall Trajectories 34

Figure 4.7 Measured Locations of Center of Gravity for

Series K and L Compared With Computed

Trajectory 36

Figure 4.8 Measured Locations of Center of Gravity for

Series I and J Compared With Computed

Trajectory 37

Figure 4.9 Measured Locations of Center of Gravity for

Series E, F, G and H Compared With Computed

Trajectory 39

Figure 4.10 Measured Locations of Center of Gravity for

Series M Compared With Computed Trajectory .... 40

Figure 4.11 Range of Measured Trajectories for 50th

Percentile Dummy And Computed Trajectory 41

Figure 4.12 Measured Locations of Center of Gravity for

Series A and B Compared With Computed

Trajectory 42

Figure 4.13 Position of Dummy During Test 11-7-4-1 43

Figure 4.14 Measured Locations of Center of Gravity for

Series C and D Compared With Computed

Trajectory 45

Figure 5.1 Location of Center of Gravity of 50th Percentile

Dummy for Various Initial Horizontal

Velocities 47

vi
LIST OF FIGURES cont '

d

E&se

Figure 5.2 Location of Center of Gravity of 95th Percentile

Dummy for Various Initial Horizontal

Velocities 48

Figure 5.3 Effect of Angular Orientation of Body on

Landing Position 49

vii

1. INTRODUCTION

1.1 PERIMT-E£-^ET . AP-ELJXATJ.QNS

During the construction of a variety of structures such as

bridges, high rise buildings and towers, safety nets are used as

fall arrestors. The nets are usually assembled on the ground and

then hung from the completed portion of the structure at a level

specified by regulations. A net which projects from one edge of

the structure is referred to as a perimeter or outrigged net. In

a typical perimeter net installation the two sides of the net that

are parallel to the supporting edge of the structure are attached

mechanically to wire cables. The cable supporting the far edge

of the net is in turn attached to several tubes, pipes, or

cables. The near side cable may be attached to the outriggers,

and/or directly to the structure. The outriggers are usually

oriented in a diagonal position, extending from the far edge of

the net to the supporting member (i.e., beam or column) of the

structure. Generally, the connection between one end of the

outrigger and the supporting member features a swivel device to

permit the assembly to pivot, both during installation and upon

impact of a falling object. A typical perimeter net installation

is shown in figure 1.1.

1.2 CURRENT REQUIREMENTS FOR PERIMETER NETS

Briefly, an effective perimeter safety net installation must:

1) be wide enough to capture and retain the falling body; 2) be

strong enough to withstand the stresses created by the impact of

the falling body; 3) be flexible enough to arrest the downward

motion without causing serious injury; 4) be placed far enough

above rigid or sharp surfaces to prevent "bottoming out"; and

5) optimize the allowable fall distance so that the assembly does

not have to be moved too many times. Regulatory requirements for

perimeter nets must address these attributes. This report is

intended to provide a technical basis for Occupational Safety and

Health Administration (OSHA) safety standards for minimum

permissible width of net.

In light of the above mentioned objectives, it is helpful to

review the current safety net regulations as they pertain to the

aforementioned dimensional requirements. The technical provisions

of existing safety net standards are tabulated in reference [1].

The list of domestic safety net standards includes: 1) OSHA

Part 1926.105; 2) ANSI A10. 11-79; 3) U. S. Army Corps of Engineers,

07. D; and 4) California/OSHA Standard, Article 24. Two British

documents, CP93-72 and BS3913-73 are also cited in reference

[1]

.

Following is a summary of the regulations related to

perimeter nets. For a more comprehensive review of safety net

standards the reader should consult reference [1]

.

1
Figure 1.1 Typical periir.eter net installation 1 /

1/ Photograph courtesy of Sinco Products Inc.

2
The minimum horizontal projection of the safety net beyond the

edge of the structure is specified as 8 feet (2.4 m) by all four

of the U. S. safety net standards. British Code of Practice

CP93-72 specifies that outrigged nets should project horizontally

a minimum of (2 + H) meters beyond the edge of the working

surface, where H is the vertical distance, in meters, between the

net and the outermost working point above. The standards differ

in their requirements for the maximum allowable distance between

the working surface level and the net level. The California/OSHA

requirement is the most restrictive? it specifies between working

surface level and net level a maximum allowable distance of 10

feet (3.3 m) , except in the case of steel erection where the

maximum distance is increased to 25 feet (7.7 -m) . A 25-foot

(7.7-m) limitation is cited in OSHA 1926.105 and in the U. S.

Corps of Engineers Standard. British Code of Practice CP93 sets

the maximum limit at 6 m (19.8 feet), except in special situations

where the maximum distance is increased to 12 m (39.6 feet). The

30 foot (9.1 m) vertical distance permitted by ANSI A10. 11-79 is

the most liberal regulation. None of the above-mentioned codes

are accompanied by a commentary that explains the bases for the

limitations that are set on the vertical and horizontal dimensions.

This lack of technical basis, coupled with a lack of description

of the ways in which construction workers accidentally fall from

elevated working surfaces, make it difficult to establish limits

for the elevation and projection of perimeter safety nets.

1.3 EQSfilBLB . FALL-.g-Qmiy.LQNS

Typically a construction worker can fall from an elevated working

surface by: 1) falling head first from a vertical standstill

position, 2) falling head first with some initial horizontal

velocity (i.e., walking off the edge), 3) falling head first from

a kneeling position, 4) tipping backwards from a crouched

position, 5) slipping sideways while walking parallel to the edge

of the working surface, 6) being pushed forward or backwards over

the edge and 7) walking, tripping or stumbling over an obstruction

and being propelled over the edge. These possible fall events

are shown schematically in figure 1.2. While there are other

possible conditions that could lead to accidental falls, it is

felt that those listed above sufficiently define the range of

horizontal landing distances that can occur at any given fall

height. Because no systematic comparison of the trajectories of

construction workers falling from elevated working surfaces have

been made, it is not known which sequence of pre-fall events

causes the largest horizontal excursion. However, it seems

reasonable to assume that, for a given vertical fall height, a sequence including horizontal velocity toward the unguarded edge

would lead to a greater horizontal excursion at landing than one

with no horizontal velocity toward that edge. Following this line

of reasoning, it is deduced that of the events described above,

3
( 2) V

*

( 1 )V - 0 1

S >0 0+

(3)

'

( 6)

Figure 1.2 Possible conditions leading to accidental falls

4
number 7 would cause a falling worker to land the greatest

distance from the edge.

Two data bases (NTIS and COMPENDEX) were queried in search of

data compiled on trajectories and maximum landing distances of

falling construction workers. Several reports prepared for the

National Institutes for Occupational Safety and Health (NIOSH)

and OSHA were identified as possible sources of information.

However, a review of these reports revealed that they contain

statistical data on such areas as the nature of injuries sustained

by various categories of construction worker, age distribution of

injured workers, geographic comparisons and seasonal variations

in falls. There was practically no data relative to fall

trajectories. In addition to performing the above mentioned

literature searches, phone contact was made with the National

Safety Council, the National Fire Protection Association, NIOSH,

OSHA, the Stuntmens Association of California, several local fire

departments in major metropolitan areas and two U. S. manufacturers

of safety nets. None of these contacts produced any evidence that

a systematic comparison of the trajectories of construction

workers falling from elevated working surfaces has been made.

A survey of published literature showed that there are no data

from ergonomic studies of construction workers on elevated

working surfaces from which to ascertain a practical range of

walking speeds. The authors conducted a limited study to determine

the order of magnitude of "normal" and "fast" walking speeds. Four

trial walks were performed by each of three persons, at what was

considered to be normal and fast paces. The trials were conducted

in an unobstructed corridor that was 363.5 ft (110.8 m) long. A

stop watch was used to measure the time required to walk the

length of the corridor. The average walking speeds obtained from

the trials were: normal, 4.9 fps (1.5 m/s); fast, 6.4 fps (2.0

m/s). In an article in the March 1985'issue of Psychology Today

[2] , authors R. Levine and E. Wolff report on the results of

observations of pedestrians in two American cities. At least 100

people were observed, walking alone, in each city and the times

required to walk 100 ft (30.5 m)were measured. The average time

required to cover the distance was 22.5 seconds, yielding a rate

of 4.4 fps (1.4 m/s). In the simulated fall test study, horizontal velocities of about one-half the normal walking speed determined

by Levine and Wolff were used based on the assumption that

walking speeds at construction sites on elevated surfaces would

be significantly less than normal. In the analytical study,

velocities ranging from those used in the fall tests up to those

associated with a fast walking pace were used to predict fall

trajectories.

1.4 PROBLEM STATEMENT AND TEST OBJECTIVES

1.4.1 Problem Statement

As mentioned above, current U. S. safety net regulations require

5
a minimum horizontal projection of 8 feet (2.4 m) for perimeter

safety net systems. It is reasonable to question the adequacy of

the projection requirement for installed perimeter nets for the

current limit was established with no technical basis. Moreover,

if the current minimum horizontal projection is found to be

insufficient the question as to what should be the new minimum

limit must be answered.

1.4.2 Test Objective

The first phase of the Safety Nets Standards Study at the

National Bureau of Standards was intended to evaluate the existing

horizontal projection criteria for perimeter safety nets and to

propose new dimensional limits if necessary. Thus, a number of

simulated fall tests were conducted using anthropomorphic dummies

and a computer model was developed to examine minimum allowable

horizontal projection requirements.

2. SIMULATED FALL TESTING

2.1 SELECTION OF THE FALL SEQUENCES TO BE SIMULATED

The objective of the laboratory fall testing was to simulate the

trajectories of falling construction workers. It was decided that

the testing procedure would focus on creating the effect of a worker who, while walking - either forward or backward - stumbles

over an object and falls over an unbarricaded edge of the working

surface. Of interest was the descent pattern for a range of fall

heights, up to the current maximum of 30 feet(9.1 m) . By starting

with simulated falls from the 30-ft elevation and filming the

sequential positions of the falling body, it was possible to

acquire intermediate fall-height data during the course of a

single fall test.

2.2 DESCRIPTION QF. TEST SETUP AND TEST DUMMIES

2.2.1 Test Setup

The simulated fall tests were performed from a 30-ft (9.1-m)high

wooden platform. The elevated platform was attached to a

rectangular scaffold made of aluminum tubing. The erected frame

measured approximately 40 feet (12.2m) in height and was 8 ft by

4 ft (2.4 m by 1.2 m) in cross section. A 2 ft-6 in (0.8 m) thick

padded mat was placed adjacent to the base of the scaffold to

provide a landing surface for the falling dummies. The test

scaffold and padded mat are shown in figure 2.1. The underside of

the 30-ft high wooden platform is shown in the upper left corner

of the photograph.

6
Figure 2.1 Elevation view of test scaffold and padded mat

7
The dummy was put into motion through the use of an aluminum

trolley-track assembly which was installed atop the 30-ft high

platform. A side view of the 6-wheeled trolley is shown in

figure 2.2. The frame supporting the rails was 5 ft-6 in

(1.7 m) long and ran parallel to the 4-ft sides of the wooden

platform. Thus, the track extended over the edge of the platform

by approximately 18 in (0.5 m) . The movement of the trolley was

controlled by a pneumatically-actuated cylinder. Attached to the

front end of the track frame was a 3/8-in (9.5 mm) aluminum plate

which stopped the forward motion of the trolley. The dummy, that

was standing near the front edge of the trolley, was thus caused

to fall over the unguarded edge of the scaffold in the manner of

a worker who stumbles over an obstacle on the working surface.

To aid the photographic recording of the dummies' trajectories, a grid was hung adjacent to one side of the scaffold (see figure

2.1). One vertical edge of the 11 by 30 ft (3.4 by 9.1 m) grid

was touching the nearest leg of the scaffold. The grid pattern

was 2 ft (0.6 m) vertical by 1 ft (0.3 m) horizontal.

2.2.2 Description of Test Dummies

Two anthropomorphic dummies, with the physical features of 95th

and 50th percentile American adult males, were used for making the

simulated falls. Their pertinent anthropometric measurements are

shown in figure 2.3 and are listed in table 2.1. The 95th percentile

dummy used for these tests was in poor mechanical condition

compared with the 50th percentile. The 95th percentile dummy had

been used in a previous research study on guard rails at the NBS,

as well as by the U. S. Department of Transportation for automobile

crash tests. As a result of this test history, some of its joints

became excessively flexible and had to be retightened after each

fall test. On the other hand, the 50th percentile dummy was brand

new. Both dummies were used in the erect standing position with

the feet squarely planted on the top surface of the trolley and

arms down at their sides.

Safety boots were put on the 95th percentile dummy to help

stabilize the dummy in the standing position. Furthermore, an

upright wooden box was placed at the dummy's back to restrict the

tendency to pitch to the side while the trolley was in motion.

The box was 5 ft (1.5 m)high and was partially filled with a foam

plastic packing material which conformed to the contour of

the dummy's back. Thus, prior to each fall test, the dummy was

brought in contact with this "mold” and forced into a virtually

upright position. The box was permanently attached to the

trolley, so that it remained on the trolley as the dummy was

propelled over the edge of the scaffold platform.

8
(1 Inch = 25.4 mm)

Figure 2.2 Side view of trolley/track assembly

9
Table 2.1 Height and weight measurements of test dummies

PERCENTILE WEIGHT fi Z 1/

50th 166 lb 68.3 in 36.9 in

75.3

kg 1.73 m 0.94 m

95th 217 lb

98.4

kg

72.8 in

1.85 m

Z was measured while dummy was wearing boots.

40.5

in

1.03 m

Figure 2.3 Pertinent dimensions of test dummies

10
Because it had experienced no loss of rigidity in the joints due

to previous impacts, the 50th percentile dummy was able to remain

in an upright position during the motion of the trolley without

the use of boots or a supporting box.

2.3 MEASUREMENT OF TROLLEY HORIZONTAL VELOCITY

To measure the instantaneous horizontal velocity of the loaded

trolley immediately prior to impact, the trolley/track/cylinder

assembly was set up on the laboratory floor and subjected to a number of trial runs while loaded with dead weights. The dead

weights were added in increments of 50 lb (22.7 kg) to effect

loads ranging from 50 to 2501b (22.7 to 113.6 kg).

Air pressure for moving the trolley was supplied by the NBS

'

central compressor, which was the same source of air pressure for

the simulated fall tests. Also, the same pressure regulator was

used during both the velocity measurement and fall tests. The

regulator was preset for various line pressures in increments of

10, ranging from 20 to 80 psi (138 to 552 kPa) . For a given load

magnitude, there were three trolley runs made for each pressure

setting

.

The procedure for determining the velocity of the trolley involved

the measuring of the time required for two closely-spaced points

on the trolley to past a beam of light. The light beam was

emitted by a photovoltaic reflective scanner. The scanner was

positioned adjacent to the end of the trolley track and its

longitudinal axis was directed perpendicular to one side of the

track. A light-reflective target was attached to the side of the

trolley facing the scanner. The target consisted of a series of

black and white strips that were parallel and spaced a known

distance apart. Each strip was 1/8 inch (3.1 mm) wide. As the

alternate black and white strips of the target passed the

scanner, the light beam was reflected to the sensor of the

scanner with alternating high and low intensities. The scanner

converted these peaks and valleys of light energy into electronic

signals which were transmitted to a digital processing oscilloscope

From the known distance between any two strips of the same color

(i.e., 0.25 in)and the measured elapsed time between the correspond

ing peaks or valleys of the recorded waveform, the instantaneous

velocity of the trolley was calculated.

There were six samples of time data obtained for each waveform.

Thus, for each combination of air pressure and imposed weight

there were eighteen velocity values calculated. The average

velocity, standard deviation, and coefficient of variation were

calculated for each combination. Table 2.2 summarizes the

results of the velocity measurements in units of inches per

second. Figure 2.4 shows a family of line graphs of velocity

versus air pressure for the specified range of dead weights and

for the empty trolley.

11
Table 2,2 Measured trolley speeds for various

pressure/weight combinations

PRESS, PSI

AVERAGE SPEED (in/sec)

STAND DEVIATION (in/sec.)

COEF. OF VARIATION (%)

IMPOSED WEIGHT (lb)

0 50 100 150 200 250

12 .2 11.2 10.82 10 .10 9.53 8.85

20 0.13 0.16 0.19 0.12 0.10 0.08

1.07 1.43 1.7 6 1.19 1.05 0.90

19 .54 18 .02 16 .96 16.06 15.39 14.81

30 1.43 0.13 0.12 0.17 0.13 0.12

7.32 0.7 2 0.71 1.06 0.84 0.81

22.94 21.71 22.17 21 .64 20.16 20.35

40 0.18 0.16 0.26 0.15 0.35 0.13

0.78 0.74 1.17 0.69 1.74 0.64

24.03 24.31 23.63 23.61 23.39 22.81

60 0.16 0.16 0.38 0.25 0.11 0.12

0.67 0.66 1.61 1 .06 0.47 0.53

24.22 24.22 24.49 24.47 23.66 23.53

80 0.10 0.29 0.30 0.31 0.19 0.33

0.41 1.20 1.22 1.27 0.80 1.40

1 psi = 6.894 kPa

1 in/sec = 25,4 mm/sec

1 lbm = 0 .454 kg

12
(*/U|) JULI0013A

Figure 2.4 Velocity measurements of the trolley

13

AIR

PRESSURE

(pst)
Referring to figure 2.4, there are several noteworthy observations

about the velocity measurements of the trolley. There was a

nonlinear relationship between the pressure and the velocity of

the trolley. Compare, for example, the velocity magnitudes at 40

and 80 psi (276 and 552 kPa) . At pressures of 20, 30 and 40 psi

(138, 207 and 276 kPa) , the lesser the imposed weight the greater

the horizontal velocity. Beginning with 60 psi (414 kPa) , the

effect of increased weight on the velocity magnitudes was

diminished. In fact, at 80 psi (552 kPa) there was only a difference of about 1 in/sec (25 mm/sec) between the velocities

of the empty and heavily loaded (i.e. 250 lb) trolley.

2.4 PHOTOGRAPHIC RECORDING OF FALLS

2.4.1 Procedure for Observing the Fall Trajectories

One of the principal parts of the test procedure was the photographic

recording of the trajectories of the dummies, including the point

of impact with the padded mat. The total time of fall from the

test platform was computed to be about 1.5 seconds. In order to

photographically record four or five discrete positions of the

body's center of gravity, both the times of the camera's shutter

opening and closing and the film advancing had to be less than

300 milliseconds (1500/5). In fact, because the dummy's body

would be accelerating vertically downward, it was concluded that

the camera system had to be capable of photographing an event at

least once every 200 milliseconds.

The technique used to photographically record fall trajectories

involved the use of a 35-mm single lens reflex camera with a motor drive for advancing the film. The motor drive unit was

capable of continuous film advance at the maximum rate of 5 frames

per second. The motor drive's shutter release button was

depressed at the same time as the trolley was set in motion. The

filming was continuous until the dummy came to rest on the padded

mat. After some preliminary experimentation, it was concluded

that a shutter speed setting of 1/250 sec produced the best

photographic results. The camera was equipped with a wide-angle

lens (28 mm) and ASA 400 film. The camera was placed on a tripod

and located on a scaffold platform such that the center of the

lens was at the same elevation as the center of the grid pattern

shown in figure 2.1. The camera platform was moved sufficiently

far away from the test scaffold to allow the entire vertical

distance between the test platform and padded mat to be viewed

through the camera's lens. In addition, the platform was squared

with respect to the plan of the gridded cloth backdrop and

positioned such that a line segment from the center of the grid

to the center of the lens was perpendicular to both the backdrop

and the camera body. An elevation view of the above mentioned

setup is shown in figure 2.5.

14
5

' 4

- 1 / 2

"

Figure 2.5 Elevation view of test and camera scaffolds

1 - 1 /2

"

O'O"

15
2.4.2 Correction for Vertical Parallax in Photographs

Due to the changing vertical position of the test dummies

relative to the position of the camera, there was a built-in

distortion (parallax) in the photographs. As illustrated in

figure 2.6, the lens of the camera was positioned at approximately

the same elevation as the center height of the grid. Thus, when

a point on the falling body (e.g. the c.g.) was at any elevation

other than that of the grid's center point, it appeared on the

photograph to be higher or lower than its actual elevation.

Because of this vertical parallax, the measurements of vertical

position obtained from photographs, had to be corrected. Figure

2.6 shows the geometric relationship between the actual and

apparent positions of the falling body. The derivation of the

correction equation is also given.

2.5 SUMMARY OF TEST PROCEDURE

Prior to each test, the dummy was hoisted into an upright

position near the leading edge of the trolley using an overhead

crane. Once the dummy was stabilized atop the trolley the

cylinder's piston was fully retracted to its starting position.

Then the pressure regulator was set for the desired air pressure.

A countdown was initiated by the operator of the cylinder's

solenoid switch to synchronize the initial movement of the

trolley with the starting of the camera's motor drive. The

trolley assembly rolled along the 20-in (0.5-m) long track until

it hit the metal stop and the dummy was propelled over the

unguarded edge of the platform. Discrete points along the

dummy's trajectory were photographically recorded. Once the

dummy had come to rest on the mat, the horizontal distance from

the edge of the scaffold to the farthest point on the dummy's

body (usually at the bottom of the feet) was measured and

recorded.

3. PRESENTATION OF TEST RESULTS

3.1 HORIZONTAL DISPLACEMENT OF BODY AT LANDING

A total of 48 simulated fall tests was performed from the

elevated platform to obtain data on the maximum horizontal

movement of falling bodies. There were four tests variables:

1) size of dummy - 95th and 50th percentile, 2) orientation of

dummy before falling (i.e., facing unguarded edge, back to unguarded edge and side to unguarded edge, 3) air pressure in the

cylinder - 80 and 40 psi, and 4) vertical position of the pneumatic

cylinder (i.e., at ankle level and at the approximate level of

the center of gravity of the 95th percentile dummy) . The distribution of the tests across these variables is shown graphically

in figure 3.1.

16
A - 6 = h - 6 d X

From Points Above the :

A - 6 = d (P-^-) = E

From Field Measurements:

d = 5.375’

h = 15.79’

X = 38.60'

E = (15.79 - 6) = (0.1391) (15.79 - 6)

A=6+E=S+ 0.139 (15.79 - 6)

A = (1 - 0.139)6 + 2.195

A = 0.8616 + 2.195

This correction equation can be used for body positions above or below

the <3= . Figure 2,6 Correction for vertical parallax

17
2

X

a

ui

o

o

m

<

Ui

hiw

>-2

2

2

UJ

QC

Figure 3.1 Distribution of simulated fall tests

18

SERIES

CODE
It should be noted that the two different pressures were selected

to obtain two significantly different horizontal velocities (2.5

and 1.25 fps) of the dummies at the time of departure from the

platform. However, as was discussed in section 2.4, the ratio of

the measured velocities was considerably smaller than the 2 to 1

ratio of cylinder pressures. The measured departure velocities

were 2.0 and 1.7 fps (0.61 and 0.52 m/s).

The horizontal distance of the dummies in the final landing

positions are summarized in table 3.1. The individual test and

series average measurements are given in columns (5) and (6)

respectively. Where there were three or more tests performed for

a series, a standard deviation value is given in the last

column. Figure 3.2 illustrates the landing distance to which

reference is made in table 3.1. With two exceptions (Series J

and L) , the average values of the distance from the end of the

track to the farthest point on the dummy's body (usually the

bottom of the feet) equaled or exceeded 10 ft (3.04 m) . Moreover,

on eight falls the distance to the farthest point exceeded 12 ft

(3.66 m)

.

3.2 TRAJECTORY QF THE FALLING BODY

Figure 3.3 presents a typical sequence of photographs of the

trajectory of a test dummy. The photographs show the descent

pattern of the 95th percentile dummy during one of the Series

A tests. The photograph in the upper left corner of figure 3.3

was taken just after the trolley had made impact with the metal

stop plate. As is illustrated in the upper middle photograph,

the dummies first pivoted about their feet until they had fallen

to a nearly horizontal position. Then, as shown in the upper

right photograph, they were propelled horizontally away from the

face of the scaffold. As shown in the remaining photographs of

the series, the dummies rotated in air while translating horizontally. In general, they rotated about three-fourths of a full

revolution before coming to rest in either a prone or supine

position. On several occasions, the dummies underwent almost a

full somersault, thereby touching the mat feet first in an almost

standing position. They would then rotate back to the mat and

come to rest as described above. By observing the lower right

photograph in figure 3.3, one can discern that the dummy came to

rest in a supine position with the feet farthest from the

scaffold.

Figure 3.3 also provides a case-in-point for illustrating the

graphical procedure used to determine discrete positions of each

dummy's center of gravity during descent. Enlargements were made

of each negative frame containing the test dummy in descent and

the dummy's center of gravity was identified on each photograph.

It was observed in obtaining the measurements listed in table

2.1 that along the length of each dummy's body the center of

gravity fell on an imaginary transverse line passing through the

19
Table 3.1 Summary of Landing Distance Data

(1) (2) (3) (4) (5) (6) (7)

SERIES

CODE

TEST

NUMBER1 DUMMYDESCRIPTION

DEPARTURE

VELOCITY

(fps)

LANDING2 DISTANCE

(ft)

AVERAGE

LANDING

DISTANCE (ft)

STANDARD

DEVIATION

(ft)

A

3-27-4-3

3-29-4-1

3-30-4-1

3-30-4-2

3-30-4-3

3-30-4-4

12-12-3-1

10-16-4-1

95th 1.7

11.92

12.17

12.25

12.42

12.75

12.25

10.67

10.00

11.80 0.89

B

12-9-3-1

12-9-3-2

12-9-3-3

12-9-3-4

3-4-4-1

3-5-4-1

3-27-4-1

3-27-4-2

10-22-4-1

95th 2.0

10.92

10.58

10.33

11.00

10.83

10.42

12.00

10.42

12.00

10.94 0.64

C

3-29-4-2

4-4-4-1

4-4-4-

3

4-4-4-4

4-5-4-1

3-30-4-5

11-7-4-2

95th 1.7

11.25

12.25

10.83

10.83

10.58

9.67

9.67

10.75 0.87

D

4-6-4-1

4-6-4-2

11-1-4-1

11-7-4-1

95th 2.0

10.83

11.17

9.00

10.00

10.25 0.96

E 11-8-4-4 50th 1.7 10.67 10.67

F 11-7-4-3

11-8-4-1

50th 2.0 11.75

9.92

10.83 -

G 11-8-4-5 50 th 1.7 10.33 10.33 -

1. The test number are coded according to the date on which the number of tests run on that date. For example test no. 11-7-4-4

refers to the 4th test run on Nov. 7, 1984.

2. Refer to figure 3.2,

20
Table 3.1 Summary of Landing Distance Data (Continued)

SERIES

CODE

TEST

NUMBER 1 DUMMYDESCRIPTION

DEPARTURE

VELOCITY

(fps)

LANDING2 DISTANCE

(ft)

AVERAGE

LANDING

DISTANCE (ft)

STANDARD

DEVIATION

(ft)

H 11-7-4-4 50th 2.0 11.42 11.62

11-8-4-3 11.83

12-4-4-4 8.83

I 12-5-4-2 50th 1.7 9.83 10.08 1.39

12-5-4-3 11.58

J 12-3-4-1 50th 2.0 8.83 8.75 _ 12-3-4-3 8.67

K 12-5-4-1 50th 1.7 10.75 10.96 _ 12-5-4-4 11.17

12-4-4-3 10.75

12-3-4-2 9.25

L 12-4-4-1 50 th 2.0 9.17 9.56 0.79

12-4-4-2 9.08

12-5-4-6 1.7 8.17

M 11-8-4-2 50th 2.0 11.92 10.42 1.98

11-8-4-6 1.7 11.17

The test numbers are coded according to the date on which the number of

tests run on that date. For example test no. 11-7-4-4 refers to the 4th

test run on Nov. 7, 1984.

2 Refer to figure 3.2.

21
Figure 3.3 Typical trajectory of test dummy

22
middle of the buttocks. This observation became the basis for

the guideline for locating the center gravity on the side of a dummy in each photograph.

The respective center of gravity positions were referenced to the

grid pattern in the background of the photograph and the coordinates

were recorded. Corrections were made to the vertical measurements

to compensate for the parallax due to the relative elevations of

the falling body and the lens of the camera as described in

section 2.5. Also, corrections were made to the horizontal

excursion measurements to account for horizontal parallax.

4. ANALYSIS OF FALL TRAJECTORY

4.1 INTRODUCTION

Analysis of the motion of the dummy during the tests can be

separated into 5 stages:

1. Horizontal translation with constant initial velocity,

2. Transformation from translational motion to rotational motion

when trolley impacts the rigid stop.

3. Rotation about its feet.

4. Departure from platform.

5. Free fall.

In order to perform a parametric study of fall trajectories, an

analytical model was developed for predicting fall trajectories

for different initial conditions. This chapter describes the

analytical model and compares predicted fall trajectories with

measured values.

4.2 ANALYTICAL MODEL

The human body is a complex assemblage of rigid members, such as

arms and legs, and it would be difficult to develop an accurate

kinematic model to describe the motion of the center of gravity

and of the various members. For the purpose of this study a simple

model was used to predict the motion of the center of gravity of

the dummy. It is assumed that the dummy can be represented by a long slender rod of weight W and length . The equations

describing the motion of the rod during the stages discussed in the

previous section are given below.

4.2.1 Initial Angular Velocity

The rod is assumed to be moving with a constant initial horizontal

velocity of (figure 4.1 (a)). The translational motion of

the rod is stopped by a small rigid stop. This is to simulate the

sudden stopping of the trolley when it impacts the rigid plate

at the end of the track. The impact causes the rod to change its

23
ro|<v

Vhi

(_ 2

777777777

P

/

/

/

/

/

a) Translation

Figure 4.1 Analytical rod model

24
motion from pure translation to rotation about its bottom

end. The angular velocity at the instant after impact may be

computed by applying the principle of conservation of angular

momentum about point C (see figure 4.1.b). First, the moment

about 0 of the linear momentum H^, is computed.

H

i “ mvhi \

^

After the impact the rod has an angular velocity a^ and the

angular momentum about 0, E

Q , is:

H0

= I 0 Wi (4.2)

where I Q = mass moment of inertia about point 0 {= 1/3 m

By equating (4.1) and (4.2), the initial angular velocity wp is:

w- mvhi4

(4.3a)

u> ?Ihi

IT

( 4.3b)

4.2.2 Rotation About Fixed End

After the forward motion of the trolley was stopped, the dummies

rotated about their feet. During the course of the rotational

motion, the dummies slid off the platform. Sliding occurred when

the external and inertial forces surpassed the sliding resistance

beneath the dummies' feet. Thus, the forces acting on the rod

model as a function of the angle of rotation must be calculated.

Figure 4.1 (c) is a freebody diagram of the rod showing the

external forces as solid lines and the inertial forces as dashed

lines. The inertial forces arise from the normal and tangential

accelerations of the center of gravity. By equating forces in the

horizontal and vertical directions, the following expressions for

the normal (N) and horizontal (R) reactions acting at point 0

are obtained.

8 9 N = W - m - (asin 0 + cos 0 ) (4.4)

JL o R = m

^

(w sin© - a cos 0 ) (4.5)

where, W = weight

g = acceleration due to gravity

a = angular acceleration

25
Sliding occurs when the horizontal force, R, equals the sliding

resistance; that is.

R = pN

where, H = coefficient of friction

(4.6)

To determine the angular position of the rod when sliding occurs,

it is necessary to determine the angular velocity and angular

acceleration as functions of the rotation angle. The angular

acceleration is determined by equating the moment of W about

point 0 to the time rate of change of the angular momentum about

the same point.

W * sin 0 = I 0a (4.7a)

a =

| |

sin e (4.7b)

The angular velocity is determined by integration of Eq. (4.7b)

and applying the boundary condition that for 0=0° the angular

velocity has the initial value given by Eq. (4.3b)

.

Alternatively,

the angular velocity may be computed by using the principle of

conservation of energy. The latter approach is used. When the

rotation angle is 0°, the rod has a rotational kinetic energy

given by

KE i = \

I Q w? (4.8)

As the rod rotates to an angle 0, there is a decrease in the

potential energy of the center of gravity, which is given by

APE = mg ^

(1 - cos 0) (4.9)

The decrease in potential energy causes an increase in rotational

kinetic energy. Thus, the kinetic energy is

1

2 + mg 4

z

26

(1 - COS 0

)

(4.10)
Therefore, the angular velocity becomes

(4.11)

By substituting Eqs 4.7 (b) and 4.11 into Eqs. 4.4 and 4.5 and by

applying the condition given by Eq. 4.6, the angular orientation

of the rod can be determined when sliding occurs. By substituting

this angle into Equation 4.11, the angular velocity at sliding

is determined.

4.2.3 Fall Trajectory

It is assumed that when sliding occurs, the rod leaves the

platform and undergoes free-fall. The initial velocity, Vf

, at

the start of free-fall is

Where is the angular velocity when sliding occurs. The

velocity Vf is directed at the angle 0£, so that the center of

gravity has vertical and horizontal velocity components (see

figure 4.1 (d) )

:

The position (H,V) of the center of gravity may now computed.

Vf -

5 «£ (4.12)

V

v

= Vf sin 0f (4.13)

vh

= vf cos ef (4.14)

(4.15)

(4.16)

where, h = height of platform

t = time from the start of sliding

27
The vertical position is referenced with respect to ground level

and the horizontal position is referenced with respect to the

edge of the platform. The angular orientation of the rod, during

free-fall can be determined by adding the product x t to the

angle 0f

.

4.2.4 Parametric Study

The analytical model requires the following input parameters:

initial velocity, Vi^; length of rod, ; weight of rod, W ; and

coefficient of friction, |i. Of these, only the coefficient is not

known. Hence, the effects of the coefficient of friction on the

computed fall trajectory were studied. The governing equations

given in the previous section were programmed into a computer and

the trajectories were computed for different values of coefficient

of friction. Figure 4.2 shows the positions of the rod at 0.1

-second increments for coefficients of friction varying from 0.1

to 0.9. Time is measured from the instant of sliding. The

quantities "Height" and "Distance" represent the location of the

center of gravity at the last time step. The "take-off angle" is

the angular position of the rod when sliding occurs (i.e., 0^ ) . It is seen that a wide variation in the coefficient of friction

results in a negligible effect on the trajectory.

Fall trajectories were computed for different rod lengths (approximately equivalent to heights of people) to determine the

sensitivity of trajectory to different heights of people. Figure

4.3 shows the rod positions at different time steps as the rod

length was varied from 6.5 ft (2.0 m) to 4.5 ft (1.4 m) . The

quantity "CG" is the distance from the lower end of the rod to

its center of gravity, which in this case is one-half the rod

length. It is seen that a reduction in rod length has a significant

effect on the fall trajectory; the horizontal distance from the

edge of the platform to the center of gravity of the rod at rest

decreases with decreasing rod length while the cumulative angular

rotation at impact increases. The results of the CG Parameter

Study can be used to qualitatively account for the possible shift

in the position of the center of gravity during a dummy's

approach to the edge of the platform. The location of the center

of gravity can be practically measured only while a dummy is in

the erect stationary orientation. However, it is possible that

the relative positions of the limbs and torso can change prior

to the start of sliding and hence the vertical position of the

center of gravity can change. The qualitative effect of this

shifting center of gravity can be inferred from the results of the

CG Parameter Study.

So far in the analysis, it has been assumed that free-fall starts

when sliding occurs. Physically, this means that the rod is at

the edge of the platform when sliding starts (see figure 4.4).

However, it is possible to stop the horizontal translation of the

28
Figure 4.2 Effect of coefficient of friction on computed fall

trajectories.

29
INITIAL UELOCI TV= 2 FT/S

UEIGHT= 166 LBS CG= 2.25 NU= .1

HEICHTr 2 . 71 FT

PISTANCE= 6.64 FT

TAKE-OFF ANGLE= 48

1 1 I III I 111 1 1 1 lUJ-LUFigure 4.3 Effect of height of center of gravity on computed fall

trajectories

30
a) Sliding R = nN b) Lift-off N = 0

Figure 4.4 Conditions at start of free-fall

31
rod at a point before the platform edge. In this case the start

of sliding does not result in free-fall until either; 1) the rod

slides off the platform or 2) the rod lifts-off the platform.

The rod can lift off the platform because the normal force given

by Eq. 4.4 can become equal to zero as the angle 0 increases.

This lift-off condition is analogous to a diver lifting off from

a diving board. Rigorous analysis of these two conditions is

rather difficult as it would be necessary to consider energy

losses due to sliding friction. For this study a simplified

approximation was used. It was assumed that a limiting condition

would be to consider the case of lift-off without sliding. This

simply requires determining the angle 0

n when the normal force,

N, as given by Eq. 4.4, equals zero. As shown in figure 4.4 (b)

the angle 0

Q

is greater than 0f, the angle for the start of

sliding. Figure 4.5 (a) shows the fall trajectory for the same

initial conditions as in figure 4.2, except that free-fall is

assumed to start when lift-off occurs. It is seen that the

distance is significantly reduced compared with the case of

sliding. In addition, the angular rotation at ground impact

increases. Figure 4.6 compares the fall trajectories for initial

horizontal velocities varying from 0 to 8 ft/s (2.4 m/s) . It is

seen that the horizontal fall distance increases with increasing

velocity, but the increase is not as dramatic as would be expected.

Note that zero initial velocity still produces a large horizontal

fall distance. The zero velocity case corresponds to a worker

standing at platform's edge, simply losing his balance and

falling over the edge (i.e condition 2 in figure 1.2).

Finally, the effect of the rod's weight was studied. Referring to

section 4.2.3, it is seen that the only governing equations that

involve the weight are those dealing with the start of sliding (or

lift-off), that is, Eqs 4.4 to 4.6. A closer examination shows

that when Eq. 4.6 is used (or N=0), the weight term can be

cancelled from the equation. Thus, it can be concluded that the

fall trajectory is independent of the rod weight.

In summary, these parametric studies show that the fall trajectories

of the rod model are affected significantly by the location of the

center of gravity and whether free-fall occurs under a "sliding"

or "lift-off" condition. The initial horizontal velocity has a lesser influence. When free-fall is considered to occur at the

start of sliding, the value of the coefficient of friction has a negligible effect. The fall trajectory is independent of weight.

With this understanding, the measured fall trajectories were

compared with the predictions of the rod model.

32
INITIAL UELOCI TV= 2 FT/S WEIGHT^ 166 LBS CG= 3.1 NO SLIDING

-y HEI CHT= 2 FT

<f DISTANCE= 7.03 FT

TAKE- OFF ANGLE= 66

eFigure 4.5 a) Fall trajectory under lift-off condition

INITIAL UELOCI T¥= 2 FT/S

Figure 4.5 b) Fall trajectory with lift-off and reduced

height of center of gravity

33
INITIAL UELOCITV= 0 FT/S

MEI GHT= 166 LBS CG= 3.1 MU= .3

Figure 4.6 Effect of initial horizontal velocity on computed

fall trajectories

34
4.2.5 Comparison of Measured and Computed Trajectories

Chapter 3 described the various testing conditions that were used.

As a review, the following variables were studied:

a) size of dummy;

b) air pressure in the loading cylinder;

c) orientation of the dummy; and

d) location of loading cylinder.

The trajectories of the centers of gravity of the dummies were

estimated from photographs taken during the free-fall. It was

assumed that the location of the center of gravity was at the

same point on the dummy, irrespective of the .orientation of the

legs and arms. However, the photographs indicated movement of the

arms and legs during free-fall; this would change the position of

the center of gravity. Hence it is estimated that the true Xand Y- coordinate of the center of gravity could lie within plus

or minus 1/2-foot (0.15 m) of the values estimated from the photographs.

The experimental results are compared with the predictions from

the analytical model to determine whether the model adequately

represents the measured trajectories. Since cylinder pressures

of 40 and 80 psi (276 and 552 kPa) produced small differences in

initial velocity, and since the analytical model indicates that

initial velocity does not have a strong effect on the trajectory,

the data for the two cylinder pressures were grouped. The

results with the. 50 percentile dummy are presented first.

Figure 4.7 shows the data for series K and L. The analytical

curve is based on the following conditions:

a) coefficient of friction = 0.3

b) initial velocity = 2 ft/s (0.6 m/s)

c) height of center of gravity = 3.1 ft (0.9 m)

Although the model appears to overestimate the horizontal

distance to the center of gravity over the upper half of the

trajectory, the analytical curve does give a reasonable fit to

the data. It should be noted that both the x and y coordinates

of the test data points were corrected for parallax.

Figure 4.8 shows the comparison for series I and J. The solid

curve is the analytical prediction for the same conditions as in

the previous figure. The fit at the lower-half of the trajectory

is reasonable; there is more scatter at the upper half. Note that

the date for test 12-3-4-3 (Series J) have significantly lower

horizontal distances than for the other tests. The dashed line is

the analytical solution for the "lift-off" condition. It is

seen that the analytical curve moves closer to the data for test

35
- 00

- <0

- -M-

- CM

in

10 oK) in

CM o

CM

in in

(«) HV0lia3A

Figure 4.7 Measured locations of center of gravity for series K

and L compared with computed trajectory

36

HORIZONTAL

(ft)
o

(u) nvoua3A

Figure 4.8 Measured locations of center of gravity for

and J compared with computed trajectory

series I

37

HORIZONTAL

(ft)
series 12-3-4-3, but it still over predicts the measured horizontal

distance by about 1/2 foot (0.15 m)

.

Comparing figures 4.7 and 4.8, it is concluded that the pre-fall

orientation of the dummy, that is facing forward or backward,

does not have a significant effect on the fall trajectory. For

these tests series (I through L) the movement of the trolley was

produced by pushing the dummy at the level of its center of

gravity. By raising the cylinder, the effect of a horizontal

force accidentally applied to the back or chest of a worker was

produced (Condition 6 in figure 1.2)

.

Figure 4.9 shows the results for series E,F,G, and H for which

the loading cylinder pushed directly on the trolley. The analytical

curve is for the same input parameters as in figures 4.7 and

4.8. Again the predicted trajectory is a reasonable fit to the

data. Thus, it is concluded that the location of the loading

cylinder has no significant effect on the fall trajectory.

In series M the dummy was positioned sideways. The test results

are shown in figure 4.10. The solid line is the computed trajectory

obtained with the same input parameters as in figures 4.7 through

4.9. The fit is similar to that obtained in the other test

series. Thus, it is concluded that placing the dummy sideways

did not affect the results significantly.

The dashed lines in figure 4.11 represent the ranges of

measured trajectories for the 50 percentile dummy. The data

from test 12-3-4-3 have been excluded as it did not fit the

overall trends. The solid line is the computed trajectory.

From this comparison it is concluded that the simple slender rod

model gives reasonable predictions for the fall trajectories of

the test with the 50 percentile dummy.

The 95th percentile dummy was in poor mechanical condition

compared with that of the 50th percentile dummy. As was mentioned

in section 2.2, it was difficult to tighten all of the joints and

stabilize the dummy. As a result, the dummy was excessively

flexible, and there was some difficulty in performing the tests

and obtaining repeatable fall patterns.

Figure 4.12 shows two test results from series A and B in which

the 95th dummy was facing forward. The analytical trajectory was

based on the same input parameters as for the 50th percentile

dummy except that the center of gravity level was 3.4 ft

(1.04 m) . For this limited data the fit of the calculated

trajectory is similar to that obtained with the 50th percentile

dummy

.

The final comparison is for the 95th percentile dummy facing

backward. Figure 4.13 shows the dummy just before departing from

the platform during test 11-7-4-1. The dummy was not in an erect

38
AO

10

CN

(u) 1V0lld3A

Figure 4.9 Measured locations of center of gravity for series E,

F, G, and H compared with computed trajectory

39

HORIZONTAL

(ft)
(u) "IVOIliGA

Figure 4.10 Measured locations of center of gravity for series M

compared with computed trajectory

40

HORIZONTAL

(ft)
o

00 *1V0I1H3A

Figure 4.11 Range of measured trajectories for 50th percentile

dummy and computed trajectory

41

HORIZONTAL

(ft)
Figure

- 00

- <0

- ^

- CM

m

hi o

H> m

CM O

CM in m

(U) IVOIldBA

4.12 Measured locations of center of gravity for series A

and B compared with computed trajectory

42

HORIZONTAL

(ft)
Figure 4.13 Position of dummy during test 11-7-4-1

43
position. As a result the center of gravity was closer to its

feet than if the dummy were in the erect position. According to

the parametric study, it is expected that a lower center of

gravity would reduce the horizontal distance of the fall trajectory

Figure 4.14 shows the measured trajectories for series C and D.

It is seen that the data are consistent and the horizontal

distances are significantly lower than for the forward facing

test. The analytical model was used to compute the fall trajectory

assuming a "lift-off" condition and assuming a height of the

center of gravity of 2.5 feet (0.8 m) . The computed trajectory

is shown in figure 4.14. There is a good fit of the computed

trajectory with the data points.

4.2.6 Summary

An analytical model was developed to predict the fall trajectories

assuming the laboratory test conditions. The model assumes that

the dummy behaves as a slender rod. While this might appear to

be a crude approximation, comparisons of predicted trajectories

with measured values showed that the predictions are generally

satisfactory.

The analytical model was used to study the effects of the

following parameters affecting the free-fall trajectories:

a) initial horizontal velocity;

b) height of the center of gravity;

c) coefficient of friction; and

d) "lift-off" as opposed to "sliding" at the start of

free-fall.

The coefficient of friction had negligible effect on the trajectory

The initial horizontal velocity, as expected, did not have a strong effect on the trajectory. Decreasing the height of the

center of gravity, reduced the horizontal distance. Likewise,

changing from a "sliding" to "lift-off" as defining the start of

free-fall causes a reduction in the horizontal distance.

5. DESIGN CONSIDERATIONS

The parametric study discussed in the previous chapter showed

that for the fall conditions used in the testing program, the

greatest horizontal fall distance occurs when the dummy, in an

erect position, "slides" off of the platform. Physically this

condition means that the center of gravity of the body is at its

maximum distance from the feet, and that the pivot point of the

body is at the edge of the platform. This fall condition results

in the most conservative estimate of horizontal fall distances.

44
o

(«) 1V0lia3A

Figure 4.14 Measured locations of center of gravity for series C

and D compared with computed trajectory

45

HORIZONTAL

(ft)
Figures 5.1 and 5.2 show the calculated trajectory of the center

of gravity of the slender rod model as a function of the initial

horizontal velocity. The vertical axis represents the distance

below the working platform; the horizontal axis represents the

horizontal distance from the edge of the working platform.

Figure 5.1 was constructed using a value of 3.1 ft (0.95 m) for

the center of gravity, which represents the value for an erect

50th percentile man. Figure 5.2 was constructed using a value of

3.4 ft (1.0 m)for the center of gravity which represents the

corresponding value for an erect 95th percentile man.

These curves may be used to arrive at a rational answer to the

question: "What should be the net projection distance so that a falling workman will be caught?" To answer this question, decisions

must be made as to the size of the body that is to be caught and

the maximum horizontal velocity with which this body will leave

the working platform. In addition, the angular orientation of

the falling body must be considered. Figure 5.3 shows two

different angular positions of a body just before impact with the

net. The dashed line represents the trajectory of the center of

gravity of the body. In figure 5.3 (a) the body is nearly

vertical at impact. The extreme part of the body (i.e. the head)

will come to rest approximately a full body length away from the

point indicated by the center of gravity. In figure 5.3 (b) the

body is inclined at impact. The final position of the head in this

case will be approximately half a body length away from where

the center of gravity of the body hits the net. Thus, if the net

is to catch the entire body, an additional distance must be added

to the center of gravity distance obtained from figures 5.1 and 5.2.

As an example, suppose one wants to design a net projection to

catch a body with proportions similar to the 95th percentile

dummy. Using a conservative approach, assume the body falls with

an initial horizontal velocity of 4 ft/s (1.2 m/s) (i.e., normal

walking speed on the ground but a relatively fast speed on an

elevated working surface) . The net is to be hung 30 ft (9.1 m)

below the working platform. From figure 5.2, the predicted

location of the center of gravity of the body at impact is about

9 ft (2.7 m) . If the body hits the net in an inclined position then

an additional 3 ft (0.9 m) needs to be added to the 9-ft (2.7 m)

distance if the net is to catch the entire body. If the body

hits the net in a nearly vertical position, an additional 6 ft

(1.8 m) needs to be added. This last case represents the worst

condition - a required net projection of 15 ft. Taking a somewhat

less conservative approach, using the 50th percentile dummy,

falling 30 ft (9.1 m) with an initial horizontal velocity of

2 ft/s(0.6 m/s), figure 5.1 gives a predicted projection of the

center of gravity of about 8.3 ft (2.5m). The height of the 50th

percentile dummy is 5.7 ft(1.7 m) . Thus, the required net

projection to catch the entire body would be between 11.1 ft

(3.4 m) and 14.0ft (4.3 m) , depending on the inclination of the

46
ON^tOOON^lflDON^iODO | | | |T-^^-r-r-MNNMCS»0

I I I I I I I I I I I

(«) T1VJ "IV0I1H3A

Figure 5.1 Location of center of gravity of 50th percentil

dummy for various initial horizontal velocities

HORIZONTAL

DISTANCE

(ft)
(w) nvj !V0lld3A

Figure 5.2 Location of center of gravity of 95th percentile

dummy for various initial horizontal velocities

48

HORIZONTAL

DISTANCE

(ft)
Trajectory of ^

Trajectory of\

center of \ gravity

Figure 5.3 Effect of angular orientation of body on landing

position

49
body at the time it first hits the net.

Referring to the two examples above, if the maximum fall height is

limited to 25ft (7.6 m) figures 5.1 and 5.2 indicate that the

required net projection is reduced by about 0.5 ft (0.15 m)

.

Thus, for the 95th percentile person with an initial horizontal

velocity of 4 ft/s (1.2 m/s) the required net projection ranges

between 11.5 ft (3.5 m) and 14.5 ft (4.4 m) depending on the body's

angle of inclination at impact. Likewise, the 50th percentile

person falling with an initial horizontal velocity of 2 ft/s

(0.6 m/s) would require between 10.6 ft (3.2 m) and 13.5 ft (4.1

m) of net projection. In this manner, figures 5.1 and 5.2 can be

used to establish design requirements.

6 . SUMMARY

Of the several requirements applicable to perimeter safety nets,

this study concentrated on the two dimensional requirements

currently specified in OSHA Safety Standards: 1) minimum

horizontal projection and 2) maximum fall height. The experimental

program included 48 simulated fall tests with 50th and 95th

percentile adult male dummies from a 30-foot-high platform. The

investigative approach was to first identify the range of

possible accidental fall events and to select for laboratory

simulation the event that seemed most likely to cause the

greatest horizontal displacement of a falling worker. The fall

events selected involved walking, tripping, or stumbling over an

obstacle which resulted in a worker falling from the edge of the

working surface.

Eased on the information in reference [2] regarding walking

speeds and limited test data, it was concluded that "normal"

walking speed is approximately 4.5 fps (2.0 m/s). Given that

construction workers on elevated surfaces are not usually walking

at even normal speed, it was felt that tests should be run with

the dummies moving at a somewhat slower than normal walking

speed. All of the tests were run with the dummies moving with a horizontal velocity of either 1.7 or 2.0 fps (0.5 or 0.6 m/s).

The tripping event was simulated by stopping the trolley with a metal plate attached to the end of the track while permitting the

continuance of the horizontal motion of the dummy's body. After

undergoing rigid body rotation about the feet, the dummy either

slid off or lifted off the trolley and underwent free fall until

landing on the padded surface located 30 feet (9.1 m) below. The

trajectory of the dummy's body was recorded photographically and

its landing position was measured relative to the end of the

trolley track. In addition to the slight variation in horizontal

velocity and the difference in dummy sizes, the orientation of

the dummy with respect to the edge of the platform was varied.

Falls were conducted with the dummies facing forward, backward

and sideways.

50
A long, slender rod model was developed to predict the trajectory

of the body's center of gravity, given the fall event considered

in the laboratory experiments. The analytical model was used to

compute the angular velocity at the start of free fall, the

horizontal and vertical positions of the center of gravity as it

falls and the angular position of the rigid body. This model

v/as used to conduct a study of the effects of varying several

parameters: 1) initial horizontal velocity (i.e., walking

speed), 2) length of the rod (i.e., worker's height), 3) weight

of the rod (i.e., worker's weight), and 4) the coefficient of

friction between the bottom of the feet and the working surface. In

general, the analytical model gave good agreement with the

experimental trajectory data. Where there was significant

disagreement, the analytical model yielded slightly non-conservative

trajectory predictions.

Considering all the experimental data (see table 3.1) without

regard to the aforementioned variables, the calculated average

landing distance for the 30-ft fall tests is 10.5 ft (3.2 m)

.

Assuming that normal probability law statistics (i.e., a bellshaped probability curve) can be applied to the fall test data,

it would be expected that one-half of the landing distances

exceed 10.5 ft (3.2 m) from the edge of the work platform. This

distance to the extreme part of the body is significantly greater

than the current minimum allowable perimeter net projection of 8

ft (2.4 m)

.

In fact, when 3.1 ft (0.95 m) is subtracted from the

average landing distance, the experimental results indicate a landing distance of 7.4 ft (2.3 m) for the center of gravity.

Thus, for the fall event used in the laboratory tests, the

location of the center of gravity, on average, was approximately

6 in (150 mm) from the edge of a hypothetical perimeter net that

satisfies the current minimum requirements for horizontal projection.

It is interesting to investigate the distribution of the test

data above the sample average. The computed standard deviation for

the entire set of test data is 1.2 ft (0.4 m)

.

Applying the

properties of a normal density function to the data, it is found

that 75% of the horizontal landing distances would be less than

11.2 ft (3.4 m)

.

In a like manner, 95% of the horizontal landing

distances would be less than 12.4 ft (3.8 m)

.

By subtracting

3.1 ft ((0.95 m) from the latter value, it is seen that the 95%

limit for the center of gravity's landing distance is 9.3 ft

(2.8 m)

.

The parametric studies show that the height of the center of

gravity and whether free-fall occurs under a sliding or lift

off condition are the two factors having the greatest influence

on the trajectory. In the range of 0 to 4 fps (0 to 1.2 m/s) for

the initial horizontal velocity, there is a narrow band of landing

distances (see figures 5.1 and 5.2). Both the analytical and

experimental results indicate that the fall trajectory is

independent of weight.

It is emphasized that the experimental results and the companion

51
analytical model apply to the selected event of walking toward

the edge, accidentally tripping over an obstacle, and falling

from the edge of the work surface. It was felt that this

represented a realistic set of worst-case conditions. It is

recommended that these assumptions be critically evaluated by a

panel representative of the construction community. To establish

a consensus set of design requirements, the panel should recommend:

a set of pre-fall conditions, a departure horizontal velocity,

the statistical size of the worker (i.e., 50th or 95th percentile)

and whether lift-off or sliding should be considered. Contingent

upon the panel's recommendations, the analytical model may be

refined and the envelope of predicted horizontal displacements

modified to reflect the revised initial conditions. If the panel

confirms the set of conditions that were simulated as a reasonable

worst case, then there is a possibility of a worker's center of

gravity projecting at least 7.5 ft (2.3 m) from the edge of the

working surface when he falls 30 ft.

7. REFERENCES

1. Pielert, J. H., Status of Safety Net Standards for Construction

and Research Needs, NBSIR 83-2709, National Bureau of Standards,

Washington, D.C., Septemberl983 , 35 pp.

2. Levine, R. and Wolff, E., "Social Time: The Heartbeat of

Culture," Psychology Today . Volume 19, Number 3, pp. 28-35,

March 1985.

52
NBS-I14A (REV. 2-8CI U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET (See instructions)

1. PUBLICATION OR

REPORT NO. NBSIRt-85/3271

2. Performing Organ. Report No. 3. Publication Date

4. TITLE AND SUBTITLE

Perimeter Safety Net Projection Requirements

5. AUTHOR(S)

C. W. C. Yancey, N. Carino, and M. Sansalone

6. PERFORMING ORGANIZATION (If joint or other than NBS, see instructions)

national bureau of standards

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

7. Contract/Grant No.

8. Type of Report & Period Covered

NBSIR

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City. State, ZIP)

Occupational Safety and Health Administration

200 Constitution Avenue, N.W.

Washington, D.C. 20210

10.

SUPPLEMENTARY NOTES

] Document describes a computer program; SF-185, FIPS Software Summary, is attached.

11.

ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bi bl iography or literature survey, mention it here) Current construction-site safety net regulations set limitations on the minimum horizontal projection of perimeter nets and the maximum vertical distance between an elevated working surface and the net below. These limitations were arbitrarily

established as no actual or simulated fall data existed. The adequacy of these

requirements in ensuring construction worker safety has been questioned. Thus, a test program was carried out to determine the adequacy of existing regulations.

Simulated fall tests were conducted using anthropomorphic dummies to represent falling

workers. The dummies fell from a 30-foot high platform and their trajectories were

recorded photographically. The photographs were used to reconstruct the dummies’

trajectories and to determine the horizontal distance between the face of the platform and the farthest point on a dummy's body. Results are presented to show the

trajectory of the falling body and the maximum horizontal distance in the final

landing position. An anlytical model was developed to simulate a falling worker.

The model can be used to predict trajectories for a given set of initial conditions

including worker height and weight, departure horizontal velocity and fall height.

Guidelines are presented for revising existing regulations pertaining to the dimensional requirements for perimeter nets.

12.

KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolon s > construction safety; falling bodies; fall trajectory; horizontal distance; safety

nets; simulated falls.

13.

AVAILABILITY

[]K] Unlimited

| | For Official Distribution. Do Not Release to NTIS

Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C.

20402. Q^Order From National Technical Information Service (NTIS), Springfield, VA. 22161

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PRINTED PAGES

15. Price
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