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NBS TECHNICAL NOTE 833

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NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards 1 was established by an act of Congress March 3, 1901. The Bureau's overall goal is to strengthen and advance the Nation's science and technology

and facilitate their effective application for public benefit. To this end, the Bureau conducts

research and provides: (1) a basis for the Nation's physical measurement system, (2) scientific and technological services for industry and government, (3) a technical basis for equity in trade, and (4) technical services to promote public safety. The Bureau consists of the Institute for Basic Standards, the Institute for Materials Research, the Institute for Applied Technology,

the Institute for Computer Sciences and Technology, and the Office for Information Programs.

THE INSTITUTE FOR BASIC STANDARDS provides the central basis within the United

States of a complete and consistent system of physical measurement; coordinates that system

with measurement systems of other nations; and furnishes essential services leading to accurate and uniform physical measurements throughout the Nation's scientific community, industry, and commerce. The Institute consists of a Center for Radiation Research, an Office of Measurement Services and the following divisions: Applied Mathematics — Electricity — Mechanics — Heat — Optical Physics — Nuclear

Sciences 2 — Applied Radiation 2 — Quantum Electronics 3 — Electromagnetics * — Time

and Frequency 3 — Laboratory Astrophysics 3 — Cryogenics 3

. THE INSTITUTE FOR MATERIALS RESEARCH conducts materials research leading to improved methods of measurement, standards, and data on the properties of well-characterized

materials needed by industry, commerce, educational institutions, and Government; provides

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Radiation — Physical Chemistry.

THE INSTITUTE FOR APPLIED TECHNOLOGY provides technical services to promote

the use of available technology and to facilitate technological innovation in industry and

Government; cooperates with public and private organizations leading to the development of

technological standards (including mandatory safety standards), codes and methods of test; and provides technical advice and services to Government agencies upon request. The Institute consists of a Center for Building Technology and the following divisions and offices: Engineering and Product Standards — Weights and Measures — Invention and Innovation — Product Evaluation Technology — Electronic Technology — Technical Analysis — Measurement Engineering — Structures, Materials, and Life Safety 4 — Building

Environment * — Technical Evaluation and Application * — Fire Technology.

THE INSTITUTE FOR COMPUTER SCIENCES AND TECHNOLOGY conducts research and provides technical services designed to aid Government agencies in improving cost effec- tiveness in the conduct of their programs through the selection, acquisition, and effective

utilization of automatic data processing equipment; and serves as the principal focus within

the executive branch for the development of Federal standards for automatic data processing

equipment, techniques, and computer languages. The Institute consists of the following

divisions: Computer Services — Systems and Software — Computer Systems Engineering — Information Technology.

THE OFFICE FOR INFORMATION PROGRAMS promotes optimum dissemination and

accessibility of scientific information generated within NBS and other agencies of the Federal Government; promotes the development of the National Standard Reference Data System and

a system of information analysis centers dealing with the broader aspects of the National Measurement System; provides appropriate services to ensure that the NBS staff has optimum

accessibility to the scientific information of the world. The Office consists of the following

organizational units: Office of Standard Reference Data — Office of Information Activities — Office of Technical

Publications — Library — Office of International Relations.

1 Headquarters and Laboratories at Gaithersburg, Maryland, unless otherwise noted; mailing address Washington, D.C. 20234.

1 Part of the Center for Radiation Research.

3 Located at Boulder, Colorado 80302.

* Part of the Center for Building Technology.
Bureau of 6**»<»i#

1974

Fire Department Ground Ladders

3 Results of a Preliminary Study

?

Harvey P. Utech

Fire Technology Division

Institute for Applied Technology

National Bureau of Standards

Washington, D.C. 20234

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U.S. DEPARTMENT OF COMMERCE, Frederick B. Dent, Secretary

NATIONAL BUREAU OF STANDARDS, Richard W. Roberts, Director

Issued July 1974
Library of Congress Catalog Card Number: 74-600092

National Bureau of Standards Technical Note 833

Nat. Bur. Stand. (U.S.), Tech. Note 833, 82 pages (July 1974) CODEN: NBTNAE

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1974

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402 (Order by SD Catalog No. C13.46:833). Price $1.20
ACKNOWLEDGMENT

The following persons have contributed to the work reported in this document.

Of these, particular thanks are due to--

Mr. T. Robert Shives, Mechanical Properties Section, Metallurgy Division, who

developed the technical content of the metallurgical studies presented in the

appendices

Mr. Donald C. Robinson, Engineering Mechanics Section, Mechanics Division, who

performed the measurements of ladder deflection in response to load.

Dr. Donald B. Novotny, Fire Services Section, Fire Technology Division, who

carried out the tests to failure in horizontal bending.

Mr. Henry C. Warfield, Fire Services Section, Fire Technology Division, who

assisted in several aspects of the work, particularly the tests on resistance

to local impact.

Mr. Joseph M. McDonagh, Fire Service Extension Department, University of

Maryland, who explained to us how fire departments use ladders and the performance they expect from them.

The Bowie, Maryland Fire Department, who spent a day with us, demonstrating

ladder raises and carries.

Mr. William E. Clark, former Director, Department of Fire Protection, Prince

George's County, Maryland, who first called our attention to some of the

problems fire departments are having with ladders.

111

CONTENTS

Page

1. INTRODUCTION 1

2. DIMENSIONS 8 2.1. Existing Standards 8 2.2. Existing Ladder Performance 9 2.3. Recommendations 9

3. LOAD-CARRYING CAPACITY 10

3.1. Existing Standards 12

3.2. Existing Ladder Performance 15

3.3. Recommendations 16

4. HIGH TEMPERATURE STRENGTH 16

4.1. Existing Standards 18

4.2. Existing Ladder Performance 18

4.3. Recommendations 19

5. WEIGHT 21

5.1. Existing Standards 21

5.2. Existing Ladder Performance 21

5.3. Recommendation 23

6. STIFFNESS 23

6.1. Existing Standards 23

6.2. Existing Ladder Performance 25

6.3. Recommendations 25

7. RESISTANCE TO LOCAL IMPACT 25

7.1. Existing Standards 25

7.2. Existing Ladder Performance 28

7.3. Recommendations 29

8. RUNGS 29

8.1. Existing Standards 29

8.2. Existing Ladder Performance 30

8.3. Recommendations 31

9. HARDWARE 31

9.1. Existing Standards 31

9.2. Existing Ladder Performance 33

9.3. Recommendations 33

10. SUMMARY 34

APPENDIX A. LADDER FAILURE A 36

A.l. Visual and Macroscopic Examination 36

A. 2. Chemical Analysis 39

A. 3. Thickness Measurements 41

A. 4. Hardness Measurements 41

A. 5. Microscopic Examination 42

A. 6. Tensile Tests 45

A. 7. Summary 45
APPENDIX B. LADDER FAILURE B 46

B.l. Visual Examination 46

B.2. Fractographic Examination 46

B.3. Hardness Measurements 51

B.4. Tensile Properties 52

B.5. Thickness Measurements 53

B.6. Metallographic Examination 54

B.7. Discussion 57

B.8. Conclusions 58

APPENDIX C. LADDER FAILURE C 59

C.l. Hardness Measurements 59

C.2. Macroscopic Examination 61

C.3. Microscopic Examination 61

C.4. Temperature of Failure 69

C.5. Fasteners 69

C.6. Summary 69

APPENDIX D. CORRELATION OF TENSILE AND YIELD STRENGTHS WITH HARDNESS

FOR ALUMINUM ALLOY 6061-T6 7 REFERENCES 73

ILLUSTRATIONS

1 Using aerial ladders to direct hose-streams to best advantage . . 3

2 Accomplishing a rescue down a ladder 3

3 Ladders are often needed to provide access for ventilation. ... 4

4 Ladders sometimes provide the firefighter's only means of escape 4

5 Using a ground ladder to bring the nozzle closer to the seat of

the fire 5

6 A ladder provides a platform from which to accomplish overhauling 5

7 Ladder trucks not only provide a mount for the aerial ladder but

are used for stowing most ground ladders as well 6

8 A typical scene during a serious fire 6

9 Roof ladders are designed to lie flat on roof, held in place by

large top-mounted hooks 7

10 Sketch illustrating the simple loading of a ladder 10

11 When a ladder is supported unevenly at the top and then loaded,

it twists as illustrated here 11

12 Illustrating the difference between the NFPA test for loadcarrying capacity and the ANSI test 14

13 Fire department ladders are frequently subjected to radiant and

convective heating 17

14 Temperature distribution in flames emanating from a burning

building 17

15 Heat-treatment curves for 6061-T6 aluminum alloy, showing the

rapid loss of tensile strength at elevated temperatures 19

vi
16 The strength properties of aluminum ladders may be checked non- destructively 20

17 Comparison of the strength-to-weight ratio with temperature for

several common metals 22

18 Deflection of a three-section ladder with two men on it 24

19 Schematic representation of the American National Standard

Institute Deflection Test 24

20 Deflection vs. Bending Moment for several ground ladders .... 26

21 ' Transverse Deflection vs. Bending Moment for several ground

ladders 26

22 A falling building wall produced the damage shown to these

ladders . 27

23 Method of measuring resistance of ladder side rails to bending. . 28

24 Fireman demonstrating leg-lock on a ladder. .

.' 30

25 A ladder undergoing test about to give way at a guide 33

A-l Sketch of ladder immediately prior to failure 37

A-2 Failed portion of ladder immediately after failure 37

A-3a Ladder parts as received 38

A-3b Ladder parts as received 38

A-4 View of member A showing cracks at the edge of the flange near fracture 40

A-5 Cracks at the edge of the flange in member A adjacent to fracture 40

A-6 Fracture surface member A 40

A-7 Transverse section through member A which is typical of those

sections examined 42

A-8 Microstructure of transverse section through member A near fracture 43

A-9 Microstructure of transverse section through member B near

fracture 43

A-10 Longitudinal section through member A 44

A-ll Longitudinal section through member B 44

B-l Failed fire ladder as received 47

B-2 Cross section through one of the ladder rails showing the two

extruded parts 47

B-3 Parts of the deformed extruded members from location AF 48

B-4 Parts of the deformed extruded members from location BF 48

B-5 Deformed part of extruded member B outside from location BF . . . 49

vii
B-6 Scanning electron photomicrograph showing the fracture surface of

a crack at location BF inside 50

B-7 Scanning electron photomicrograph of fracture at location BF

inside showing ductile overload 50

B-8 Scanning electron photomicrograph of fracture at location BF

inside showing smearing on the compression side of the fracture . 51

B-9 Photomicrograph of a typical unetched cross section (longitudinal) showing precipitate particles 54

B-10 Microstructure of longitudinal cross section from location B3

inside 55

B-ll Microstructure of transverse cross section from location BF

inside 55

B-12 Microstructure of longitudinal cross section from location BF

inside 56

B-13 Microstructure of longitudinal cross section from location Al

inside 56

B-14 Microstructure of longitudinal cross section from location AF

showing fine grains near the center 57

C-l Fire ladder as received showing damaged area at right 60

C-2 Damaged portion of ladder as received 60

C-3 Macroetched sections of ladder side rail 62

C-4 Section of unaffected rail 63

C-5 Microstructure of unaffected rail 63

C-6 Section of rail from blackened, but undeformed area 64

C-7 Microstructure of blackened, but undeformed section of rail ... 64

C-8 Section of rail from twisted area 65

C-9 Microstructure of section of rail from twisted area 65

C-10 Section of rung unaffected by heat . 66

C-ll Microstructure of rung unaffected by heat 66

C-12 Section from damaged rung 67

C-13 Microstructure of damaged rung 6 7

C-14 Stock 6061-T6 68

C-15 Microstructure of stock 6061-T6 68

D-l Relationship of hardness to tensile strength and yield strength

for 6061-T6 aluminum alloy 72

viii
FIRE DEPARTMENT GROUND LADDERS - RESULTS OF A PRELIMINARY STUDY

Harvey P. Utech 1

The key performance requirements for fire department ground

ladders were determined. Existing ladder standards were reviewed

and found to be unnecessarily restrictive in some areas and inadequate and unrealistic in others. Included in the report are metallurgical studies of three ladders that failed in service as well as

a correlation of hardness with tensile and yield strength for

6061-T6 alloy.

Key words: Aluminum; fire department; ladders; performance

requirements; standards.

1 . INTRODUCTION

The National Bureau of Standards has conducted a preliminary study in

which the key performance requirements for fire department ground ladders have

been identified. The purpose of this report is to describe the results of that

study.

Fire departments most frequently use ladders for the following purposes:

1. To deliver fire men and their equipment across vertical and horizontal

distances in order to:

a. Accomplish firefighting operations (figure 1)

b. Rescue trapped occupants (figure 2) c. Permit building ventilation (figure 3), and

d. Provide alternate means of escape, should interior stairways be

unexpectedly blocked (figure 4).

2. To provide a platform from which firemen can:

a. Direct hose streams (figure 5), and

b. Perform mechanical work on the supporting structure (figure 6).

In addition, fire department ladders are occasionally used for horizontal

bridging and as battering rams.

To accomplish these purposes, fire departments have at their disposal two

types of ladders:

1. Aerial ladders, which are power-driven, up to about 100 feet in length,

and mounted on a specially designed chassis usually referred to as a ladder truck (figure 7) . Recently, there has been a trend toward

replacing aerial ladders with a bucket at the end of an articulated

or telescoping boom. Aerial ladders are generally used where a long

reach is necessary, as to the upper floors of a high-rise.

2. Ground ladders, which are hand-carried and raised manually. They are used for the lower levels of a building. Figure 8 shows a variety of

aerial and ground ladders in use.

Presently working as a consulting engineer in Washington, D, 1
Only ground ladders will be discussed in the remainder of this report;

however, many of the performance requirements for ground ladders apply qualitatively to aerial ladders and articulated booms as well.

Several types* of ground ladders are considered in this report; they are defined as follows:

1. Wall, straight, or single ladders - ladders consisting of one section;

these typically range from 12 to 24 feet in length.

2. Extension ladders - ladders with two or more sections; these range in

length from 2 4 to 55 feet extended.

3. Roof ladders - straight ladders equipped with folding hooks at one end

and designed to lie flat on the roof surface; these range from 12 to

2 feet in length (figure 9)

Other types of ground ladders, such as combination, folding, and pompier are infrequently used by fire departments and will not be specifically discussed in

this study. The reader interested in a more detailed review of fire department

ladder practice as well as an exposition of standard fire department carries

and raises is referred to reference [l]. 2

Virtually all ground ladders used in this country are made from either of

two materials:

1. Wood - the traditional material. Still favored by some departments

(e.g. Los Angeles F.D.) primarily for its low electrical conductivity.

Disadvantages of wood ladders are their relatively high weight and

susceptibility to deterioration with age.

2. Aluminum - much more widely used than wood. Preferred for its light

weight and low maintenance but its high electrical conductivity is a drawback.

There are two existing voluntary standards that appear to apply to fire

department ground ladders:

1. NFPA No. 19 3-19 72 - Standard on Fire Department Ladders - Ground and

Aerial [2]

2. American National Standard A14. 2-19 72 - Standard for Portable Metal

Ladders [3]

The first of these standards is published by the National Fire Protection

Association and specifically applies to fire department ground ladders. The

second standard appears to apply to fire department ground ladders as well,

since it states that it "is intended to prescribe rules and minimum requirements for the construction, care, and use of the common types of portable

metal ladders, in order to insure safety under normal conditions of usage"

(par 1.2). The standard goes on to say that so-called special-purpose ladders

are not covered by the standard but it is difficult to judge from the American

National Standards Institute (ANSI) definition of Special-Purpose ladders

(section 3) whether fire department ladders are in this category or not. At

any rate, as this report shows, the requirements of the ANSI standard are con- siderably more stringent than those of the NFPA standard and, for that reason,

the requirements of both have been included in this study. While ANSI A14.2

cites performance criteria for light, medium, and heavy duty ladders, only the

requirements for heavy duty properly apply to fire department ground ladders

and therefore, only those criteria will be cited in this report.

2 Figures in brackets indicate the literature references at the end of this paper.

2
Figure 1. Using aerial ladders to direct hose streams to best advantage. (photo

courtesy Chet Born, San

Francisco Fire Department)

Figure 2. Accomplishing a rescue

down a ladder. (photo

courtesy Chet Born, San

Francisco Fire Department)
Figure 3. Ladders are often needed

to provide access for

ventilation.

Figure 4. Ladders sometimes provide

the firefighter's only means of escape.
Figure 5. Using a ground

ladder to bring

the nozzle closer

to the seat of the

fire. Note the

leg-locks of both

men and the upper

man's leaning to

the left, (photo

courtesy of Joseph

M. McDonagh, Fire

Service Extension

Dept. , University

of Maryland)

Figure 6. A ladder provides a platform from which

to accomplish overhauling, (photo

courtesy of Joseph

M. McDonagh, Fire

Service Extension

Dept. , University of

Maryland)
Figure 7. Ladder trucks not

only provide a mount

for the aerial ladder

but are used for

stowing most ground

ladders as well.

Figure 8. A typical scene during

a serious fire.
Figure 9. Roof ladders are designed to lie flat on the roof,

held in place by large, top-mounted hooks.

Several recurrent themes will be illustrated at various points in this

report:

1. The firefighter, on a regular basis, makes more severe demands upon

his ladders than virtually any other ladder user. This stems largely

from the following unique characteristics of the fireman's job:

a. Emergency conditions, where time is of the essence,

b. Poor lighting, caused by darkness or smoke,

c. An uncontrolled working environment.

Because of these conditions, firemen cannot always observe the same precautions

(e.g. proper angle of set, firm footing) expected of other users.

2. The metal fire department ground ladders examined in this study have

not been designed to withstand the service conditions of f irefighting.

3. Some existing fire department ground ladders are demonstrably unsafe

under the severe conditions to which they may be exposed.

4. The existing voluntary consensus standard for fire department ladders

is inadequate.

The remaining sections of this report are devoted to a description of what

the author regards as reasonable performance requirements for fire department

ground ladders. Each section is organized in similar fashion. First, the need

for the performance requirement is explained. Next, the test methods and

requirements of existing standards are discussed. Then, the performance of

existing ladder designs is reviewed. Each section concludes with a brief set

of recommendations.
2 . DIMENSIONS

Ladders are generally classified by fire departments in terms of length.

A typical assortment of ladder sizes likely to be found on a ladder truck is

shown in table 1. The width of a ladder determines its lateral stability as well as the ease with which two men can pass on the ladder. Of considerable

importance too is stacking profile which determines whether a ladder will fit

into the available space on a ladder truck.

Table 1. Assortment of Ground Ladder Sizes

Typically Found on Ladder Trucks

Number

Number Size of Sections

1 50 foot or 40 foot 2 or 3 1 40 foot 2 2 35 foot 2 1 28 foot 1

1 20 foot 1

1 16 foot 1 1 14 foot 1

1 10 foot 1

2.1. Existing Standards

NFPA 19 3 has no definition for length and no requirement for width or stacking profile. It simply states "Ladder widths across the outside of the

ladder beams shall be as specified by the purchaser to insure proper ladder

nesting. The usable length shall be indicated on the ladder in a suitable

manner." (par 109) However, NFPA No. 19 standard on fire apparatus [4] does

carry a definition of ladder length: "Lengths shall be measured when extended

and shall not be the simple sum of the lengths of the sections" (par 9310)

This definition leaves unresolved the question of whether a fire department

ladder measuring 23' 6" is rated as 23-foot ladder or a 24-foot ladder. The

same NFPA 19 is less helpful on the subject of ladder widths; it says simply,

"Widths shall be consistent with lengths ..." (par 9310)

American National Standard A14.2 is considerably more explicit on both

length and width. It states that "the size of a single ladder is designated

by the overall length at the side rail, excluding any foot or end caps, with

a tolerance of ±1/2 inch" (par 5.2.3) . . . . "The size of an extension ladder

is designated by the sum of the lengths of one side rail of each section

measured along the side rails, excluding any foot or end caps. A tolerance of

±1/2 inch shall be allowed per section."

The American National Standard also specifies that for single ladders,

"The minimum clear width between side rails shall be not less than 12 inches

for ladders 10 feet and under and shall increase 1/8" for each additional foot

of length" (par 5.2.1), while for extension ladders (par 5.2.2), "The minimum

clear width between side rails at the bottom of the base section shall not be

less than

Ladder Size

Over 16', up to and including 28'

Over 28', up to and including 40"

Over 40', up to and including 72'

Minimum Clear Width

Between Side Rails

14"

15"

18"
2.2. Existing Ladder Performance

The designated lengths of four manufacturers' ladders are compared with

actual extended lengths and lengths defined in accordance with ANSI A14.2 and

NFPA 19 in table 2. The ladders supplied by manufacturers B and C did not

meet the length requirement of NFPA 19. One of the three ladders of manufacturer A also fails the requirement. On the other hand, manufacturer D's wooden

ladder exceeded the NFPA length requirement by 2 feet.

Table 2. Ground Ladder Sizes

Sum of Actual Actual

Manufacturer Designated No. of Section Extended Rail Width

Length (ft) Sections Lengths3 Length° at Basea A 24 2 27' 8 3/4" 23' 6" 15 7/8"

A 30 2 30' 10"

A 35 2 35' 6"

B 20 2 19' 10"

C 24 2 23' 2" 19' 11" 21 1/2"

C 32 2 30' 3" c 35 2 39' 2" 34' 9" 21 1/4"

D 35 2 37' 0"

a in accordance with definition of American National Standard A14.2 [3],

"in accordance with requirement of NFPA Standard 19 [4],

Three of the five ladders that failed the NFPA requirement were checked

against the ANSI "sum of section length" requirement. Two exceeded the ANSI

requirement by three and four feet (thus meriting a longer size designation,

were the ANSI definition being used) ; the third fell 10 inches short of even

that definition.

Based on the data in table 2, it is possible that a fire department

ordering a 24-foot, 2-section ladder could wind up with one as short as 19' 11" or as long as 23' 6" extended.

The three ladders checked in table 2 for width easily met the requirements

of ANSI A14.2. However, it is interesting to note the 35 percent difference in

width between the 24-foot, 2-section ladders of manufacturers A and C.

2.3. Recommendations

The ANSI definition of ladder size overstates usable length and is therefore judged inappropriate for fire department ground ladders. The definition

in NFPA 19 is basically a good one but it should utilize more explicit language

such as the following:

"The size of a single ladder is designated in feet by the overall

length at the side rail, excluding any foot or end caps. The size

of an extension ladder is designated in feet by overall length at

the side rail, excluding any foot or end caps, when the ladder is

fully extended. Fractions of a foot shall be disregarded. Example:

A ladder measuring 24' 10" shall be designated as a 24-foot size."

This definition should be incorporated in NFPA 19 3 and fire departments should

make this length requirement mandatory in their purchase specifications.

9
A study should be performed to determine appropriate width requirements

for ground ladders. Factors to be considered should include lateral stability,

ability of two men to pass on the ladder, and space available for ladders on apparatus. Pending results of that study, the minimum width requirements of

ANSI A14.2 should be adopted for fire department ladders.

The space available for ladder storage on fire trucks should also be

standardized, so that any existing manufacturer's ladder of given size and

number of sections will fit in the storage space allotted for that size ladder.

LOAD-CARRYING CAPACITY

The most important characteristic of a ladder is its ability to carry a load. When a ladder is positioned normally and loaded at one point with a weight as shown in figure 10, the load L can be resolved, for purposes of

analysis, into two mutually perpendicular forces: F , a force perpendicular

to the plane of the ladder and FA , an axial force parallel to the axis of the

ladder. The relative magnitudes of these forces are dependent upon the angle

6 at which the ladder is set. As decreases, F becomes larger and FA

smaller

until, when the ladder is horizontal, F_, is equal to the load L and F a is zero,

P A

Conversely, as 6 increases, FA becomes larger and F smaller until, when the

ladder is vertical, FAis equal to L and FD

is zero,

makes it possible to calculate F; p and F from a knowledge of L and

Thus, simple geometry

0.

F = L cos

FA

= L sin

(1)

(2)

Figure 10. Sketch illustrating the simple loading of a ladder.

10
Figure 11. When a ladder is supported "unevenly at the top and

then loaded, it twists as illustrated here.

11
There are limits to the amount of deformation any structure can withstand

before it buckles and fails. Were the loading of a ground ladder as simple in

practice as the sketch in figure 10 suggests, it would be a fairly simple

matter to devise a test method to evaluate the load-carrying capacity of a ladder. Unfortunately, the situation represented by the simple sketch in

figure 10 represents an idealized picture. The applied load is assumed in the

figure to be static, vertically downward, and evenly distributed between both

side rails. In actual practice, the loading of ladders will be far more complex. To begin with, loads will generally be applied at several points along

a ladder rather than concentrated at one point as shown in figure 10. The

ladder may be resting on uneven ground so that one side rail is bearing more

load than the other. The support at the top of the ladder may also be uneven,

resulting in the twist illustrated in figure 11. Rather than static, loading

will usually be dynamic as firemen move up and down the ladder. The kind of

work a fireman does on a ladder results in additional complex loadings. For

example, a fireman frequently has to push, pull, or reach from a ladder as illustrated by the photograph in figure 6 . He may often have to manipulate a hose from a perch on the ladder (figure 5) , forcing the ladder to absorb the

reaction force of the hose. These result in additional stresses which complicate the loading conditions far beyond the idealized conditions of the sketch

in figure 10.

3.1. Existing Standards

NFPA 19 3 gives the load-carrying capacity of various types of ladders as follows (par 132)

"ROOF LADDERS - when resting on roof, these ladders are satisfactory with a man on every other rung.

WALL LADDERS

a. For solid beam type up to 20-foot length, not over two men

on ladder.

b. For solid beam type over 2 feet, not over three men on ladder.

c. For trussed type up to 20 feet, not over three men on ladder.

d. For trussed type over 20 feet, not over four men on ladder.

EXTENSION LADDERS

a. For solid beam extension ladders from 16 to 26 feet, not over two men on ladder.

b. For solid beam ladders over 26 feet, not over three men on ladder.

c. For trussed ladders not over 26 feet, not over three men on ladder.

d. For trussed ladders 26 to 3 6 feet, not over four men on

ladder.

e. For trussed ladders 36 to 45 feet, not over five men on ladder.

f. For trussed ladders over 45 feet, not over six men on ladder."

The above load limit recommendations are based on the ladder supported at the top end against a building with the butt of the

ladder at a distance not to exceed one-third the ladder length

away from the building (approximately 55 degree angle) , and preferably with the butt of the ladder placed one-fourth of the ladder

length away from the building (approximately 65 degree angle)

These angles are measured from the horizontal with the ladder in

position. At angles less than these the load limit on the ladder

will decrease accordingly."

12
This specification of load-carrying capacity contains several weaknesses:

1. Defining load-carrying capacity in terms of number of men lacks precision since the weight of firemen could range from less than 200 to over 300 pounds (the latter case for a heavy-set fireman, fully

equipped, and carrying a load of hose). No indication is given of the

average weight assumed in setting the criteria in the standard.

2. The multiplicity of criteria for wall and extension ladders (length,

type, and rail construction) would be difficult to recall on the

f ireground

3. The angles stated in the specification are incorrect. The angles

55 and 65 degrees should be 70.7 and 75.5 degrees, respectively.

4. No guidance is given as to how much the maximum permissible load

changes with angle of ladder set.

ANSI A14.2 is briefer and more specific (section 3): "A heavy duty (Type I)

ladder shall be capable of supporting a 250 lb. load . . . These ratings are based on a safety factor of four [4], For non-self-supporting ladders, the

values are based on using the ladder at an angle of 75 1/2° from the horizontal." Here is a firm commitment to support a given load when the ladder is

used at a stated angle. In contrast to the NFPA specification, there is no dependence on such factors as length, type, and rail construction.

Both the ANSI and the NFPA standards use a horizontal bending test to

evaluate a ladder's load-carrying capacity. However, details of the test

method differ significantly between the two standards. In both procedures, the

ladder is supported in a horizontal position with vertical supports located

6 inches from either end. In both procedures, a load is applied at the center

of the ladder and the permanent set at the center after removal of the load may

not exceed certain limits. In the NFPA test (par 135) , sections of extension

ladders are tested individually, a 200 lb load is applied, and the permanent

set criterion is 1/8 inch. In the ANSI test (par 6.1.1), extension ladders

are tested as a fully extended unit, a 250 lb load is applied, and the permanent set criterion is 1/1000 of the effective span of the side rails.

The ANSI test is more stringent. The applied load is higher; the permanent set criterion is much lower; and, most important of all, extension ladders

must be tested as a fully extended unit. The significance of the latter point

is shown schematically in figure 12. The failure of a ladder is dependent on

the stress in the outer web of the rail section. That web stress is dependent

upon the bending moment applied to the ladder. The bending moment reaches

a maximum at the center of a simply supported horizontal ladder such as that

shown in figure 12a and is the product of one-half the applied load, L, and

the distance from the ladder support to the load, Y. Thus,

v - \

' Y (3)

When sections of a two-section extension ladder are tested individually,

the distance from the support point to the applied load is only slightly more

than half of what it is when the same load is applied at the center of the

fully extended ladder. The bending moment, and hence the maximum stress in

the outer web of the rail section, is similarly reduced by approximately onehalf. For three-section ladders, the figure is 1/3; for four-section ladders,

1/4, etc. This is illustrated graphically in figures 12b and 12c.

Thus, when a ladder passes the ANSI horizontal bending test (250 lbs.

applied in the center of the fully extended ladder) , the user can be justifiably assured that the same ladder can support the load capacity promised for

a heavy duty ladder, i.e. 250 lbs when the ladder is placed at an angle of

75 1/2° (although not necessarily with the promised safety factor of four, as

13
////*/>5

«^-

77m^7

BENDING

MOMENT

7777,777

^-S o.

(a.)

^7775' 3>

Figure 12. Illustrating the difference between the NFPA test

for load-carrying capacity (a) and the ANSI test

(b) . The latter produces the higher maximum bending moment as shown in (c) . However, the maximum

stress in each ladder section occurs at either end

of the overlap section as shown in (d)

14
discussed below) . However, the fireman whose ladder has passed the NFPA horizontal bending test has no such assurance. All he knows, for the reasons discussed in the preceding paragraph, is that his fully extended 2-section

ladder, placed at a 75 1/2° angle will support a little over 100 pounds. The

NFPA table for load-carrying capacity states that two, three, and even four men

can safely climb a two-section ladder . Assuming 200 pounds per man, that is

eight times the load for which the ladder has been tested. The comparison is worse for a three or four-section ladder. The NFPA 193 test procedure for

load-carrying capacity is therefore inadequate.

Both ANSI A14.2 and NFPA 193 state that the horizontal bending test provides a safety factor of 4 when the ladder is used at the recommended angle of

75 1/2°. However, this is not true if the following conditions are taken into

account

1. The axial load (FA

in figure 10) also contributes to the deflection

of the ladder.

2. Dynamic loading occurs as firemen climb, descend, and move about on ladders. It is a generally accepted rule of thumb that a dynamically

applied load increases the effective load by a factor of two.

3. In practice, other loads will be applied to ladders at the same time

as the simple axial and perpendicular loads shown in figure 10. For

example, the application of a twisting load about one of the side rails

during ladder use could significantly reduce its load-carrying capacity.

There is one additional requirement related to load-carrying capacity in

NFPA 193: "Structural members of ladders shall be constructed of aluminum

alloy having a minimum ultimate tensile strength of 35,000 pounds per square

inch. (Present aluminum alloys 6061-T6 and 6062-T6 satisfy the above requirements.)" (par 101) Such a requirement, while perhaps of some value as a quality control criterion, does not adequately define the load-carrying capacity

of the ladder, since that depends not only on the tensile strength of the

material but also on its cross-sectional area and on the ladder design.

3.2. Existing Ladder Performance

The preceding discussion indicates that NFPA 19 3 does not provide an adequate test of the load-carrying capacity of fire department ground ladders.

The National Bureau of Standards has investigated two recent accidents involving

ladders, both of which resulted in serious injury to the men on them. In both

cases, the ladders appeared to have passed all of the test requirements in NFPA

193 but failed in normal use with two men on them. Subsequent metallurgical

investigation of both failures at the National Bureau of Standards revealed

probable defects, which were judged sufficient in both cases to cause the

ladders to fail, but not sufficient to be detected by the methods of NFPA 193.

These two service failures are described in detail in Appendices A and B.

Three representative ground ladders have been tested to failure in horizontal bending at the National Bureau of Standards. The results are shown in

table 3. Each ladder reported in the table is a 2-section ladder of the truss

type. The tests were conducted by applying loads in 50-pound increments at the

center of each fully extended ladder. After application of each load increment,

the entire load was removed and the ladder checked for evidence of permanent set,

The load was increased beyond the onset of permanent set until the ladder failed

completely. Both the load to produce permanent set and the load at failure are reported in table 3

15
Table 3. Horizontal Bending Tests to Failure Performed on Two-Section Extension Ladders

Manufacturer

Nominal Permanent Seta Failure

Length Load Moment Load Moment

(ft) (lbs) (ft-lbs) (lbs) (ft-lbs)

30 750 5,500 1,200 8,900

24 400 1,900 470 2,200

35 350 3,000 365 3,100

A

C

C

aANSI criterion: span length/1000

The results show that all three ladders qualify under the ANSI criterion

for a "heavy duty (Type I) " ladder in that they are all capable of supporting

a 250-pound load. However, note that the ladder of manufacturer A does this

with a considerably wider margin of safety. Since the ladders are of different

lengths, their relative strengths can be more realistically compared by com- paring maximum bending moments (eq (3) above). These are also listed in table 3. Such comparison shows that the ladder of manufacturer A is of a substantially

stronger design than either of those of manufacturer C.

All of the ladders tested failed at the point where the overlapped portion

of the ladder ends, i.e. points F in figure 12b. (Both of the service failures

reported in Appendices A and B also failed at these points.) It is not difficult

to account for this observation.

As mentioned above, the failure of a ladder is dependent upon the stress

in the outer web of the rail section. That stress depends upon the bending

moment, as discussed above, and also upon the cross-sectional area of the ladder.

That area will be larger within that portion of the ladder where butt and fly

sections overlap and thus, the stress in that portion of the ladder will be

proportionately reduced. The result is shown qualitatively in figure 12d.

The stress is greatest at the ends of the overlap portion of the ladder.

3.3. Recommendations

Tests for load-carrying capacity are needed that more realistically

simulate the loads imposed during service than does the simple horizontal

bending test. What is needed is a detailed study of how ladders are loaded in

actual practice. The results of such a study must then be translated into a set of new tests that can be used to assess more realistically the ability of

a ladder to stand up under complex loading conditions.

However, as an interim measure, the requirements of ANSI A14.2 for the

load-carrying capacity of heavy-duty ladders should be incorporated into

NFPA 19 3.

4. HIGH TEMPERATURE STRENGTH

It is inevitable that ladders used in firefighting become exposed to heat.

This can happen in any number of ways. One is illustrated in figure 9 where

a roof ladder has been placed directly on a hot roof and appears about to be

subjected to direct flame contact. Another type of exposure is illustrated

by figure 13; here, hot gases and radiant heat from the fire itself and from

hot building parts serve to heat the ladder. The most severe type of exposure

occurs when the ladder is subjected to flames emanating from a burning window

of a flashed-over room. The temperatures of such flames are shown in the sketch

in figure 14. Such occurrences are by no means infrequent. One large Eastern

16
Figure 13. Fire department

ladders are frequently subjected to radiant

and convective

heating.

Figure 14. Temperature distribution

in flames emanating from

a burning building [6].

The term, "fire load"

refers to the weight of

fuel per unit floor area

of the involved structurf

3000

2500

u. 2000

LlT cr< 1500

cr

1000

500

COMPUTED TEMPERATURES

• MEASURED TEMPERATURES

a 5lb/ft FIRE LOAD

O 20 lb/ft

2 FIRE LOAD

TV

I*. "-Ok

\

\

\

\ S >-^^o

5 10 15 20 25 DISTANCE ALONG FLAME AXIS, feet30

17
fire department has told the author that it melts the tips off approximately 12

ladders per year.

4.1. Existing Standards

NFPA 19 3 has no requirement for high temperature performance.

ANSI A14.2 also has no requirement for high temperature performance.

However, it does make reference to fire exposure:

"Exposure to Fire - If ladders are exposed to excessive heat as in the case of fire, the strength may be reduced. After such

exposure, the ladder should be inspected visually for damage

and tested for deflection and strength characteristics. In

doubtful cases, refer to manufacturer." (par A. 2. 6. 2)

NFPA 19 3 requires that metal fire department ground ladders be made of

aluminum alloy, a requirement that severely limits possible design improvements to provide high temperature strength. ANSI A14.2 contains no such

restriction.

4.2. Existing Ladder Performance

Aluminum is desirable for light weight but its high temperature properties introduce an inherent limitation for fire department use. A volunteer

fire company discovered this two years ago while fighting a serious fire in

Maryland. An aluminum extension ladder was inadvertently exposed to direct

flame contact during a fire. Within a matter of seconds, the ladder collapsed

against the side of the building, unable to support even its own weight. A

detailed study of this failure was made by the Metallurgy Division of the

National Bureau of Standards. Their technical results are summarized in

Appendix C. That study confirms that ladders made of aluminum will not withstand even brief exposure to elevated temperatures.

Raising the temperature of the ladder so high that immediate collapse or melting occurs is only one of the ways in which heat can damage aluminum. A

second way is the much more subtle damage that can result from the phenomenon

known as overaging. The aluminum alloy generally used in fire ladders (6061-T6)

is a so-called age-hardenable alloy, which means that as a final step in its

manufacture, it is placed in a furnace at 350 to 400°F (aging temperature) for

approximately 10 hours. Such a heat treatment produces a structural change

(called age-hardening) in the alloy that increases its yield strength to its

final design value. However, the beneficial effect of this heat treatment can be dissipated by holding the alloy at its aging temperature too long or by

heating it above the aging temperature. The higher the temperature to which the

alloy is heated, the lower the yield and tensile strengths after it cools to room temperature; this is referred to as "overaging."

The effects of overaging are illustrated graphically in the heat treating

curves shown in figure 15. Note that tensile strength drops rapidly with

increasing exposure temperature and time. The decline is even more rapid at

temperatures above 500 °F, the highest temperature shown in the figure.

Thus properly heat-treated 6061-T6 aluminum has excellent properties at room temperature. But, as the above discussion of overaging shows, the room temperature properties of the alloy will be reduced after exposure to the kinds

of temperatures that prevail around fires. For example, a ladder could easily

be heated to temperatures around 400-600°F by exposure to radiant heat from a window or hot fire gases sweeping past it. This does happen in practice, since

firemen tell of conditions where ladders had to be wet down with a hose to cool

them. Under these conditions, the ladder may not show any visible signs of

18
50

45

5 40h

5 35

CO

30h

25

A

1 ^^ 250F

^

300F

\ 1 340F

1 4001

450F

/ 1 1

1 \ 500 F

i \ , i

0.01 0.1 1.0 10 100 10 10

DURATION OF HEAT TREATMENT, he.

Figure 15. Heat-treatment curves for 6061-T6 alumiaum alloy, showing the rapid loss

of tensile strength at elevated temperatures [7J

damage and may simply be cleaned up and placed back on the truck for use at

the next fire. There is no way of detecting visually that such a ladder has

been damaged and is now much weaker than before. This is a serious hazard and

one against which minimum standards for ground ladders ought to provide reasonable protection.

One convenient way of detecting the deterioration of mechanical properties

in the field is by the use of hardness tests. The use of a portable hardness

tester to check the properties of an aluminum ground ladder is illustrated in

figure 16. While it is widely recognized that hardness correlates roughly with

tensile and yield strength, exact correlation has to be established experimentally for each particular alloy. Such a correlation has been performed at

the National Bureau of Standards for the aluminum alloy 6061-T6 used in fire

department ground ladders. The results of that work appear as Appendix D to

this report.

4.3. Recommendations

Fire department ground ladders ought to perform satisfactorily under two

types of high temperature exposure:

1. During brief exposure to high radiant heat loads or direct flame contact, the ladder should be able to support a substantial fraction of

its design load.

2. After long term exposure to moderate heat levels, say, up to 600°F,

the ladder should retain all of its room temperature strength.

19
Figure 16. The strength properties of aluminum ladders may be

checked non-destructively in the field by the use

of a portable hardness tester, as demonstrated here.

Thus, existing minimum standards should be modified to incorporate two new performance tests. The exact nature of these tests requires further study. The

following descriptions are meant only to suggest their general outline.

1. High temperature strength test. The ladder is set up in the same fashion as for the horizontal bending test used to determine loadcarrying capacity. One half of the design load is placed at the midpoint of the fully extended ladder. A series of gas burners is placed

directly above or below the ladder such that an area at least thirtysix inches in length and the entire width of the ladder is bathed in

flame for one minute. In the case of single ladders, application of

the flame shall be at the midpoint of the span. In the case of extension ladders, the flame shall be applied such that at least six but no more than 12 inches of the heated length shall include the overlap

portion of the ladder, i.e. point of maximum stress (point F in figure

12b) . The deflection of the ladder from the horizontal shall be

measured before and after the application of the gas flame. If the

deflection after flame application is less than twice that before

flame application, the ladder will be considered to have passed the

test.

2. High temperature aging test. The entire ladder is placed inside a furnace at 60 0°F for 1 hour, then removed, and air-cooled. When it

has returned to room temperature, the load-carrying capacity, rigidity,

and rung tests are repeated. The ladder must still be capable of

passing the test for load-carrying capacity, the rigidity criterion

must not have changed by more than 10 percent in either direction,

and all rungs must pass the rung test criteria.

In addition, the requirement in NFPA 19 3 that fire ladders be made of

aluminum alloy should be eliminated. This is extremely important because

aluminum alloys are among the worst metals for high temperature strength.

20
Figure 17 shows the variation of tensile strength with temperature for several

common metals. The 2024-T6 aluminum alloy shown in the figure is the best high

temperature aluminum alloy available with high temperature performance considerably better than the 6061-T6 aluminum alloy of which most fire ladders are made. There are obviously a variety of materials with high temperature performance superior to that of aluminum.

Many fire service personnel erroneously believe that, to be light, a ladder must be made of aluminum or magnesium. This is not necessarily true

because good design can often overcome the weight penalty imposed by a denser

material.

5. WEIGHT

Weight is an important criterion in fire ladder selection. Since more manpower is required to erect a heavy ladder than a light one, volunteer

departments and those that are undermanned generally prefer the lighter ladder.

Of course, all fire departments benefit from having as light a ladder as possible to carry and set up.

5.1. Existing Standards

Neither NFPA 19 3 nor ANSI A14.2 make any mention of weight requirements.

5.2. Existing Ladder Performance

The total weights and weights per foot of extended length of several

representative fire ladders are listed in table 4 . The wooden ladder proves to

be the heaviest while the aluminum ladders of manufacturer C are seen to be

significantly lighter than those of manufacturer A.

Table 4 . Relative Weights of Fire Ladders

Material Manufacturer

Extended

Length

Total

Weight (lbs)

Weight per Foot of

Extended Length (lb/ft)

Aluminum A

Aluminum A

Aluminum A

Aluminum B Aluminum C

Aluminum C

Aluminum C

Wood

23' 6" 62

30' 10" 118

35' 6" 133

19' 10" 62

19' 11" 47

30' 3" 85

34* 9" 118

37' 0" 157

2.6

3.8

3.8

3.1

2.4

2.8

3.4

4.2

21
ile strength

Density

heat treated

urn beta alloy

I I die steel

4340 (mod) _

PH stainless

steels

Titanium

alloys

uminum alloy

(b)

400 800 1200

Temperature, F

Figure 17. Comparison of the strength-to-weight ratio with

temperature for several common metals \£8j . Note

the rapid drop in the aluminum alloy properties

above 200°F.

22
5.3 Recommendation

Several categories of weight classification could be incorporated into

a fire ladder standard. A suggested weight classification scheme follows:

Class

A

BC

D

E

F

Weight Range

(per foot of extended length)

Less than 2.0 lb

From 2.0 to 2.5 lb

From 2.5 to 3.0 lb

From 3.0 to 3.5 lb

From 3.5 to 4.0 lb

More than 4.0 lb

STIFFNESS

By stiffness is meant the extent to which a ladder deflects under load

(figure 18) . The less stiff the ladder, the more it tends to bounce and shake

in use. Too much bounce and shake can be unnerving and dangerous. Many fire

chiefs seem willing to tolerate an increase in weight in return for greater

stiffness

6.1. Existing Standards

NFPA 193 contains no requirements for stiffness.

ANSI A14.2 does include a stiffness requirement under the heading,

Deflection Test (par 6.1.6). The test arrangement is shown schematically in

figure 19. A 60 pound load is hung from one rail of the single ladder or

fully extended extension ladder at the center of the span. The resulting midspan deflection and the angle a (figure 19b) are measured and compared with

maximum permissible values. The latter are presented in a table for various

size ladders, some of which are listed below:

Extension Ladder Size (ft]

20

24

32

36

Max. Deflection (in) Max. ct (degrees)

3.1 3.6

3.8 4.7

5.6 5.7

6.8 6.1

The purpose behind measuring a is presumably to measure the extent to

which the ladder redistributes an eccentrically applied load to both side rails.

It thus provides some measure of the lateral stiffness of the ladder in the

presence of an unevenly applied load.

The extent to which a ladder resists the application of a load from the

side is also evaluated in ANSI A14.2 by the so-called "Side Sway Test"

(par 6.1.7). The ladder is placed on edge resting on level supports 6 inches

from each end. The sections of extension ladders are tested individually.

When a 60-pound load is applied at the mid-point of the lower rail, the maximum

deflection may not exceed certain maximum permissible values listed in a table, some of which are listed below:

Section Length (ft!

10

12

14

16

18

Maximum

Allowable Deflection (in)

25

50

75

00

25

23
Figure 18. Deflection of a three-section ladder with two men on it.

X

|60#

(a) SIDE VIEW

farrdl

(b) END VIEW

a

_i

"1

Figure 19. Schematic representation of the American National Standard

Institute Deflection Test.

24
6.2. Existing Ladder Performance

The National Bureau of Standards has tested the stiffness of several fire

department ground ladders using a scheme that differs from that utilized in

ANSI A14.2 in that the load was applied equally to both side rails. The

results of these tests are shown in figure 20. The results are plotted in

terms of bending moment which is simply the product of 1/2 the applied load

and 1/2 the span of the ladder (X in figure 19). For example, a 60 pound

load applied to the mid-point of a 24 foot ladder (3-foot overlap) produces

a bending moment of 300 ft-lbs. The steeper the slope in figure 20 for ladders

of a given size, the less stiff the ladder.

The National Bureau of Standards has also tested the transverse bending

of fully extended ground ladders. The results are shown in figure 21.

6.3. Recommendations

The deflection and side sway tests of ANSI A14.2 should be incorporated

into the minimum standard for fire ladders. However, instead of setting one minimum requirement for all ladders, several classes of performance could be

set up and ladders rated for stiffness according to the class in which they

fall. At least one class should specify stiffness in excess of that of the

stiffest ladder presently available.

The twist requirement (a factor) that is a part of the American National

Standard needs to be critically evaluated in light of fire department usage.

It is, for example, often not possible for fire departments to place ladders

such that both rails of the unloaded ladder are firmly supported at the top.

Under those circumstances, it is desirable that the ladder twist under relatively light loading so as to provide a firm footing for an ascending and

descending fireman. The kind of twisting that a ladder must undergo in these

circumstances is illustrated in figure 21. Thus it may be appropriate to

require a fire ladder to display some minimum a as well as the maximum a now

required. Further study and analysis is needed before such a range can be

specified.

7. RESISTANCE TO LOCAL IMPACT

Ladders used by fire departments typically undergo rough handling. They

may be accidentally struck by axes, falling objects, etc.; they may be dropped,

or they may be blown down by the wind. It is important that fire department

ladders be able to withstand such abuse , from the standpoint of both safety

and economics (i.e. the cost of replacement or repair). A bent side rail can drastically reduce the strength of a ladder.

An extreme example of impact damage to a fire ladder is shown in figure 22

The collapse of a wall of a heavily involved building resulted in the badly

bent ladders shown in the figure; several firemen were killed in the accident.

7.1. Existing Standards

NFPA 19 3 has no requirement for resistance to impact. It does, however,

acknowledge the fact that impact damage to a ladder can easily occur and that

it has serious consequences: "Ladders can be damaged during removal from the

truck as well as during replacement on the truck .... Sometimes this is done

carelessly, the shorter ladders being thrown to the ground or against sharpedged curbing or other objects which may cause serious damage. Use care in

removing ladders, placing those not needed immediately in a safe position where

they cannot be injured." (par 112)

25
V 24 FT TYPE A WITH "GUIDES'-

N

O 24 FT TYPE A WITH "GUIDES"- U

24 FT TYPE A WITH "SLIDES

" O 30 FT TYPE A > 35 FT TYPE A D 20 FT TYPE B 24 FT TYPE C T 30 FT TYPE C

•« 35 FT TYPE C A 35 FT TYPE D < 30 FT TYPE E Figure 20. Deflection vs. Bending

Moment for several ground

ladders.

400 800 BENDING MOMENT. FT-IBF

3.60 r

Figure 21. Transverse Deflection vs. Bending Moment for several

ground ladders.

1.80

V 24 FT TYPE A WITH "GUIDES "-N O 24 FT TYPE A WITH "GUIDES"-U O 24 FT TYPE A WITH "SLIDES" O 30 FT TYPE A > 35 FT TYPE A Q 20 FT TYPE B + 24 FT TYPE C Y 30 FT TYPE C 4 35 FT TYPE C A 35 FT TYPE D

300 600 900 BENDING MOMENT, FT-LBF

26
uH

CDow

CD00

ct) T3

CD

0)u3

T3O

60C3

00C

0)

CM

CM

CDu3

00

27
ANSI A14.2 also has no requirement for resistance to local impact. Like

the NFPA standard, it assigns to the user sole responsibility for avoiding this

kind of damage. "Ladders, like any tool, must be handled with care and not

subjected to unnecessary dropping, jarring, or misuse." (par A2.1) [After

tipping over], "inspect ladder for side rail dents or bends, or excessively

dented rungs . . . . " (par A2.6.1) . "Ladders having defects are to be marked

and taken out of service until repaired. Never straighten or attempt to use

a bent ladder." (par A2.7) "Never climb a damaged ladder." (par A3. 6)

7.2. Existing Ladder Performance

No systematic testing has, to the author's knowledge, ever been undertaken

to evaluate a ladder's resistance to local impact. However, the resistance of

side rails to deformation applied at low strain rates would be expected to be

proportional to their resistance at high strain rates and this property has

been measured in tests conducted at the National Bureau of Standards. A load was applied at a constant slow rate to the upper beam of a trussed ladder, midway between the rungs, as shown in figure 23. Load and deformation were con- tinuously recorded in order to determine the load at yield (0.2% offset) and

the maximum load sustained.

LOAD

777~77T77

( (0) (o)\

< \

Figure 23. Method of measuring resistance of ladder side rails to bending.

28
The results of tests on two manufacturers' ladders are shown in table 5. Clearly, there is a significant difference in their performance. Such a difference could also be expected, were the load to be applied at some higher strain

rate, more nearly characteristic of impact loading.

Table Ability of Ladder Rail to Sustain Load Between Rungs

Load at Yield Maximum

Manufacturer Type (0.2% offset) Load3

A 35-foot, 2-section 2,110 lbs 3,110 lbs

C 24-foot, 2-section 1,115 lbs 1,420 lbs

Each number represents an average of 2 tests on 2 rails of the same ladder.

7.3. Recommendations

Firemen, working under emergency conditions, cannot always exercise care

in handling ladders. Therefore, it is desirable that ladders be designed to

provide reasonable protection from local impact loading. A criterion for resistance to local impact should be included in a standard for fire department

ground ladders. A suitable test method using a pendulum or a dead-weight drop

test should be developed. A series of performance classes is preferable to a single minimum requirement and would be in keeping with similar schemes suggested in this report for other ladder properties.

8 . RUNGS

Rungs serve several purposes: (1) they support the climber, (2) they

provide a connection between the side rails, and (3) they permit a fireman to

establish a leg-lock (figure 24).

8.1. Existing Standards

NFPA 19 3 specifies that rungs be 14 inches apart (par 103) while ANSI

A14 . 2 specifies 12-inch centers ±1/8" (par 4.3). No reason for this difference

is apparent; possible explanations are that wider spacings on the fire ladder mean fewer rungs and therefore less weight or that the wider spacing is needed

to permit a fireman's leg-lock.

NFPA 19 3 specifies rung construction: "Rungs shall be constructed of

heavy duty corrugated aluminum not less than 1 1/4 inches in diameter" (par 103)

ANSI A14.2 contains no restrictions as to rung materials or dimensions and

states that rungs shall be either "corrugated, serrated, knurled, dimpled, or coated with skid-resistant material", (par 4.5)

Both standards include a twist test. In NFPA 193 (par 133), a 350 in-lb

torque is applied once to each rung by means of a clamped lever arm. Any relative motion between rung and side rail necessitates rung replacement. The ANSI

A14.2 twist test, called the Rung Torque Test (par 6.1.5), is far more demanding

A torque of 900 in-lb is applied 20 consecutive times in alternate directions

to each rung. Relative motion between rung and side rail disqualifies the rung.

Both standards also include a strength test. In NFPA 193, a 450 pound

load is applied along a 4-inch length at the center of the rung. Permanent

deflection at the center of the rung must not exceed 1/16 inch. Again ANSI

29
Figure 24. Fireman demonstrating leg-lock on a ladder.

A14.2 is considerably more demanding. For its Rung Bending Strength Test

(par 6.1.3) , a 1000 lb load is applied to a 3 1/2-inch wide portion of the

center of one rung. Permanent deformation in excess of 1/100 of the rung

length is not permitted.

ANSI A14.2 includes two additional tests not found in NFPA 19 3. The Rung

to Side Rail Shear Strength Test (par 6.1.4) is identical to the Bending

Strength Test except that the criterion for failure is "indication of failure

either in the fastening means attaching the rung or the side rail." The Side

Rail Cantilever Bending Test (par 6.1.8) applies a 200 pound axial load for one minute to one side rail while the other is held clamped- the criterion for

failure is "visible defects or permanent deformation."

2. Existing Ladder Performance

Some fire service

the appropriateness of

of human factors probl

have been observations

either too far apart o "The distance between

firemen into account,

it could be important

accommodate climbing o this design problem ha

personnel wi

the 14-inch

ems in firefi

made by the

r too close .

the rungs of

Since most 1 that the rung

r descending.

s ever been c

th whom the author has spoken have questioned

rung spacing. A Government-sponsored study

ghting equipment [5] pointed out that "there

firemen that the rungs on the ladder are Elsewhere, the same report suggests that

the ladders should take the size of the

adder operations are done by blind reach,

s of the ladders are correctly spaced to

" To the author's knowledge, no study of

onducted.

Rungs do not otherwise appear to be causing firemen any serious problems

However, the previously cited report [5] also points out that "the rungs on

the ladder can become slippery from water and oil, ice or other foreign

materials .

30
8.3. Recommendations

A systematic study of optimum rung spacing would be desirable.

The NFPA requirement specifying the construction material and diameter of

rungs is restrictive and should be deleted. As discussed previously, aluminum

has insufficient high temperature strength which limits its suitability as a material for fire ladders.

The ANSI Rung to Side Rail Shear Strength Test and Side Rail Cantilever

Bending Test should be included in a fire department ground ladder standard.

In addition, the present NFPA rung torque test and bending strength test

should be upgraded to the higher performance called for by the American

National Standard.

Finally, a study might be conducted of possible ways to combat the

slipperiness of rungs due to water, oil, ice, or other materials.

9 . HARDWARE

While neither ANSI A14.2 nor NFPA 193 define the term, "hardware," it is

used here to describe all components of a ladder other than rungs and side

rails. Thus, it includes the rivets used to hold the ladder together, rope

and pulley guides, and locking devices (frequently referred to as "dogs" or

"pawls") on extension ladders, safety shoes, staypoles, and hooks.

9.1. Existing Standards

NFPA 193 makes extensive references to various hardware items but the

requirements are for the most part qualitative and vague. NFPA hardware

requirements are listed below with the author's comments:

"Provisions shall be incorporated in the side members to prevent

ladder sections from becoming disengaged during use." (par 102)

Unfortunately, no test is provided for evaluating this characteristic .

"All riveting or welding should develop strength equal to the side

members." (par 103, sentence 3)

"Ladder locks or pawls on extension ladders shall be so fastened or secured to the beams that vibration and use will not cause loosening

of bolts and nuts. Pawls or ladder locks shall be so constructed

that the hook portion of the pawl shall have sufficient bearing

surface or area to prevent hook from cutting into rungs when en- gaged. Such hooks shall be properly finished to eliminate sharp

edges and points." (par 104)

"Staypoles shall be furnished on all extension ladders extending

over 36 feet. Staypoles will be furnished detachable only if

specified by the customer. Staypole spikes shall not project

beyond the end of the ladder when nested." (par 105) There is no mention of minimum strength requirements for staypoles nor is any

suitable test method specified in either the NFPA or the American

National Standard.

"Rope used for raising extension ladders shall be Class I quality

Manila rope. On three-section ladders the third section may be

extended by wire rope." (par 106)

31
"Roof ladders shall be provided with folding type hooks of sufficient strength to carry imposed loads. Hooks shall be securely

fastened to beams. Hooks shall support a load of 500 pounds."

(par 107) This requirement is inadequate for two reasons. First,

there is no statement as to how the load is to be applied, i.e. at

the point of the hook, at some other point along its length, or as

a distributed load along its length. Secondly, a 50 pound strength

requirement is too low for a ladder rated to support a man on every

other rung (see section 3 above)

.

"There shall be placed on the butt or base end of each beam of the

main ladder of extension ladders, and on the butt or base of each

beam of wall and roof ladders a metal reinforcement. This shall be

butt spurs or other suitable means to prevent ladders from slipping

when in place." (par 108)

ANSI A14.2 also makes numerous recommendations on hardware:

"Hardware shall meet strength requirements of the ladder's com- ponent parts, and shall be of a material that is protected against

corrosion unless inherently corrosion-resistant. Metals shall be so selected as to avoid excessive galvanic action." (par 4.6)

"All workmanship shall be free from burrs in excess of 1/64 inch. Bolt

and rivet holes are to be accurate and within tolerances of good com- mercial metal working practices. Rivets are to be set properly and are

to be free of structural defects. Welds are to be of commercially

accepted practices and free of defects." (par 4.7)

"Extension ladders shall be equipped in such a manner that the ladder can- not be used with an overlap less than the minimum specified in table 2."

(par 5.2.6)

"The extension locking device shall be designed to withstand all load

tests. Locks may be gravity or spring-action types to engage the rung of

the base section or the base section rung and the mating fly section rung.

A fly section incorporating stationary lock, or locks, which by their

location eliminate a rung in the section shall include a permanent marking

in letters not less than 1/8-inch high: "Caution - This Ladder Section is

Not Designed for Separate Use" or permanently attached stops shall be provided to prevent removal of the fly section. Permanently attached stops

are considered those that would require cutting or drilling for removal."

(par 5.2.7)

"Extension ladders may be equipped with a rope and pulley. The pulley

shall be attached to the ladder in such a manner as not to weaken either

the rungs or the side rails. The pulley shall be not less than 1 inch in

diameter measured at the base of the sheave." (par 5.2.8.1)

"The rope used with the pulley shall be not less than 1/4 inch diameter,

having a minimum breaking strength of 560 pounds, and shall be of sufficient length for the purpose intended. On three section ladders, where

cable is used in a rope and pulley hook up, the cable may be 3/16 inch in

diameter." (par 5.2.8.2)

In addition to these descriptive requirements, ANSI A14.2 includes a hard- ware test requirement for heavy duty extension ladders. (par 6.1.2.1) The

fly section is extended at least one rung and, with the ladder in a vertical position, a 1000 pound load is applied axially. The ladder must withstand this load without permanent deformation or other visible weakening.

This test would appear to be mainly a test of the extension locking device,

and not of any of the other hardware items.

32
9.2. Existing Ladder Performance

The author is unaware of any tests conducted specifically for the purpose

of evaluating ground ladder hardware. However, some of the horizontal bending

tests conducted by the National Bureau of Standards and described above (table - produced initial failure at the hardware rather than in a structural member. Ar example is shown in figure 25.

Figure 25. A ladder undergoing test about to give way at

a guide (arrow)

.

9.3. Recommendations

Generally speaking, it should not be necessary to specify special mechanical tests for hardware items. If suitable mechanical tests are developed for

the ladder as a whole, these tests will show up any shortcomings or deficiencies

in the hardware. The only exceptions will likely be the rope and pulley, extension ladder stop, and safety shoes.

The more stringent hardware requirements of ANSI A14.2 should be incorporated into the standard for fire department ground ladders. In addition, study

should be initiated to insure adequate test procedures for hardware items with

particular emphasis on roof ladder hooks, staypoles, and safety shoes.

33
10. SUMMARY

The performance of fire department ladders can only be improved if the performance requirements are clearly understood. An essential first step is the

upgrading of existing voluntary standards to reflect accurately the performance

demands likely to be made on these ladders.

The existing voluntary standard for fire department ground ladders (NFPA

No. 193-1972) can be upgraded quickly by incorporating in it many of the performance requirements or test procedures listed in American National Standard

A14 . 2 for heavy duty straight and extension ladders. Specifically, the following ANSI A14.2 requirements are recommended for inclusion in NFPA 193:

1. Width (par 5.2.1 and 5.2.2)

2. Load-carrying capacity, including heavy duty rating (sec. 3), and

horizontal bending test (par 6.1.1).

3. Stiffness, including deflection test (par 6.1.6) and side sway test

(par 6.1.7) .

4. Rungs, including rung bending strength test (par 6.1.3), rung to side

rail shear strength test (par 6.1.4), rung torque test (par 6.1.5),

and side rail cantilever bending test (par 6.1.8).

5. Hardware, including general requirements (par 4.6, 4.7, 5.2.6, 5.2.7,

5.2.8.1, and 5.2.8.2) and hardware test. (par 6.1.2.1)

Besides these additions, certain unnecessarily restrictive requirements

ought to be dropped from NFPA Standard 19 3. Principal among these is the

requirement that metal ladders be made out of aluminum alloy. Also, the sizing

requirement presently described in NFPA Standard 19 ought to be made more ex- plicit and incorporated in NFPA 193.

But even with such upgrading of the present standard, deficiencies remain.

These can, however, be corrected after further study and analysis have clarified the following needs

1. Width. Appropriate widths for fire ladders ought to be based on how

firemen use ladders, stability requirements, and weight and space

restrictions.

2. Load-carrying capacity. Test methods are needed that go beyond the

simple horizontal bending test in simulating the loads firemen place

on ladders.

3. High temperature tests. New test methods are needed to evaluate a ladder's resistance to exposure to the heat of the fire.

4. Weight. While an important criterion for ladder selection and an important consideration in manning and set-up time, weight is omitted

from present standards.

5. Stiffness. Better criteria are needed for evaluating the stiffness of

ladders in fire service use.

6. Resistance to local impact. This is an especially important consideration for ladders used under emergency conditions, particularly since a damaged ladder is usually an unsafe ladder.

7. Rung spacing. It should be optimized for ease of climbing, human

factors, and minimum weight.

34
8. Hardware. All ladder test methods should be designed, as far as practical, to test hardware and structural members simultaneously.

For such items as roof ladder hooks, staypoles , and safety shoes,

special test methods may be needed.

To the maximum extent possible, ladder test methods should utilize simple

equipment, likely to be available or readily obtainable in a fire station.

Where this is not possible, e.g. the high temperature tests, equipment and

procedures should still be kept as simple as possible.

This report has pointed out the shortcomings of fire department ground

ladders and applicable standards, while offering suggestions for improving the

latter. The following quotation from a letter sent to the author by one of

the major ladder manufacturers offers a fitting conclusion to this study:

"We, as manufacturers of aluminum fire ladders, welcome the institution

of and need a set of fire ladder standards which would apply industrywide and feel that you will concur with our statement that NFPA-193

is sadly lacking in useful information."

35
APPENDIX A. LADDER FAILURE A

A number of pieces from a 35 foot, two section, aluminum fire ladder, owned

by a fire department, which had failed in service were examined by Mr. T. Robert

Shives, metallurgist in the Mechanical Properties Section, Metallurgy Division

of the National Bureau of Standards to determine the following:

1. Whether the submitted material consisted of properly heat treated

aluminum alloy 6061-T6.

2. The origin of the small cracks along the edges of the member labeled

"top rt. rail, bed section."

3. The likely cause of failure.

4. Tensile strength of failed rail material.

According to representatives of the fire department, failure occurred

under the following circumstances. A fireman became incapacitated while

working at the top of a fully-extended 3 5 foot aluminum ground ladder. When

a second fireman ascending the ladder to assist him reached approximately the

tenth rung from the bottom, the ladder abruptly collapsed against the building

reportedly injuring seriously the fireman at the top of the ladder. The situation immediately prior to failure is shown schematically in figure A-l. There

appears to have been a hose line in use on the ladder at the time of failure;

this has not been shown in the sketch. A photograph of a portion of the failed

ladder taken at the scene of the accident is shown in figure A-2

.

Figures A-3a and A-3b show the pieces of the ladder as received by the

National Bureau of Standards. The T-members of the rails and the end cap are designated as follows:

NBS Designation Piece Description

A Top right rail, bed section

B Top left rail, bed section

C Lower right rail, bed section

D Lower right cap, fly section

E Lower left rail, 1 bed section

A.l. Visual and Macroscopic Examination

As shown in figure A-2, the bed section of the ladder failed just below

the fifth rung from the top. This was at the approximate location of the

bottom of the fly section. All T-members of the bed section except for E had

fractured. All four T-members had deformed to a considerable extent indicating

good ductility and yielding before fracture.

Examination of the fracture surfaces indicates failure from overload.

The ladder was being used within the load carrying limits specified in

NFPA 193 [2] . According to paragraph 132 of that standard, a trussed extension

ladder of length 26 to 35 feet can support up to four men when placed with

"The butt of the ladder at a distance not to exceed one-third the ladder length

away from the building .... At angles less than these the load limit on the

ladder will decrease accordingly. " Since the ladder was placed at a somewhat

longer distance from the building (see figure A-2) , it is necessary to calculate a reduced load limit from geometric considerations (see eqs (1) and (2) in

section 3). This works out to a 2.75-man load limit. Thus, the 2-man load

reportedly applied was within the rated capacity of the ladder, as defined in

NFPA 19 3.

36
Figure A - 1. Sketch of ladder

immediately prior

to failure, as reported by fire

department.

Figure A - 2. Failed portion of ladder immediately after failure. Photograph taken at scene.

37
' 4

"^T*

-*-.,

Figure A-3a. Ladder parts as received. X 1/2

Figure A-3b. Ladder parts as received. X 1/3
The submitted members were examined for cracking along the edges with the

following results:

Member Location Observations

adjacent to fracture

adjacent to upper

end cap

numerous cracks exist at

both edges of flange and at

edge of stem

cracks exist at both edges of

flange, but none observed on stem

about five rungs

below fracture

adjacent to fracture

adjacent to upper end

cap

adjacent to fracture

cracks observed at one flange

edge onlyno cracks

no cracks

cracks observed at both edges of

flange, but to a lesser extent

than in A; no cracking at edge of

stem

adjacent to upper end

cap

cracks observed on one flange

edge onlyadjacent to upper end

cap

no cracks

no cracks

The cracks mentioned above in piece A are shown in figures A-4 and A-5

.

The T-members are extruded. It appears most likely that the cracks in

pieces A and C occurred during the extrusion process rather than when the

ladder was in service. Cracking and even complete disintegration can occur

during extrusion of the aluminum alloys if the rate of extrusion is too high

resulting in excessive temperatures [9]. Some of the fractures went through

these cracks (figure A-6), but it does not appear that the fractures started

at these cracks .

A. 2. Chemical Analysis

A sample from piece C was analyzed by the NBS Analytical Chemistry

Division. The specified composition for aluminum alloy 6061 and the results

of the NBS analysis are given below.

Element

Si

Fe

Cu

Mn

Mg

Cr

Ni

Zn

Ti

Specification

0.40 - 0.8%

. 7 max

0.15 - 0.40

0.15 max

0.8 - 1.2

0.04 - 0.35

0.25 max

0.15 max

Analysis

0.53%

0.20

0.18

<0.02

0.88

0.08

<0.02

<0.02

0.05

39
Figure A-4. View of member

A showing cracks

at the edge of

the flange near

fracture X 7.

Figure A-5. Cracks at the edge of the flange in raembe

A adjacent to fracture. X 10

Figure A-6. Fracture surface member A. This fracture

went through extrusion crack (left) . x 10

40
The material meets chemical composition requirements for aluminum alloy

6061.

A. 3. Thickness Measurements

Thickness measurements were made on five ladder members close to the area

of fracture, but not where the section thickness had been affected by the deformation.

Member

A

B

C

D

E

Thickness

Flange (in) Stem (in)

124

127

123

129

124

.114

.116

.114

.121

.115

A.

4

Hardness Measurements

Rockwell B hardness measurements were made on the various members as follows. Each value reported is an average for three measurements.

Member

R

B Hardness

Location

adjacent to fracture

adjacent to upper end cap

about 5 rungs below fracture

adjacent to fracture

adjacent to upper end cap

adjacent to fracture

adjacent to upper end cap

adjacent to upper end cap

Flange Stem

30.8 30.0

20.5 23.7

30.3 31.8

55.3 52.7

47.2 51.5

49.2 50.7

48.3 50.5

54.0 55.3

19.7 25.0

The acceptable R hardness range for aluminum alloy 6061-T6 is 47-72 [10]

.

The hardness of members B, C, and D indicates that this material is in

the T6 temper condition. Members A and E are too soft to be in the T6 temper;

they are in the hardness range of the T4 temper (acceptable range approximately RB 19-4 5) but this hardness could have been developed either by tempering to T4 or by softening material in the T6 temper by heating it. If

the material is in the T4 temper, it could have a yield strength as low as one half the yield strength of material in the T6 temper.

A piece of the soft material from the ladder was solution treated and

artificially aged to produce the T6 temper. The hardness was Rg 62.5. A

piece of material in the T6 temper was solution treated and aged at room

temperature. After 7 days the hardness was RB

18 which is just about at the

T4 temper. This shows that if properly heat treated, the soft material will

develop a hardness in the T6 range. Also by reheat-treating a piece of the

material in the T6 temper and naturally aging it, the T4 temper can be obtained,

41
A. 5. Microscopic Examination

Metallographic examination of sections from the various members indicates

that the material is relatively free of inclusions. Figure A- 7 shows an un- etched transverse section from member A which is typical for the sections

examined. Figures A-8 and A-9 show the microstructure in transverse sections

through members A and B, respectively. There are a large number of very small

grains in the section through B, but there is very little difference in average

grain size among the four sections examined. The ASTM grain size is as follows:

Member ASTM Grain Size

A

B

C

E

Since members A and E, which are softer than the other members, have a grain

size no greater than members B and C, there has been no apparent grain growth

in A and E. This material, therefore probably has not been subjected to ex- cessive temperatures for any length of time.

Longitudinal sections through the flanges of the T-members show essentially

no grain elongation and no gross differences in grain size across the section,

but some variation in grain size among different areas of a given section.

Figures A-10 and A-ll show the structure in longitudinal sections through A and

B, respectively.

6 1/2

7

66 1/2

Figure A-7. Transverse section through member

A which is typical of those sections

examined. Unetched. X 100

42
Figure A-8. . Microstructure of transverse section

through member A near fracture.

Etch: 10% sodium hydroxide. X 100

Wv*" -f

Figure A-9 . Microstructure of transverse section

through member B near fracture.

Etch: 10% sodium hydroxide. X 100

43
Figure A-10 . Longitudinal section through member

A. Etch: 10% sodium hydroxide. X 40

Figure A-ll . Longitudinal section through member

B. Etch: 10% sodium hydroxide. X 40

44
A. 6. Tensile Tests

Two tensile specimens were taken from the top right rail of the bed section.

They were machined according to ASTiM Specification E8 for subsize specimens.

The specimens were pulled at a rate of 0.02 inch per minute. The results, to

the nearest 25 psi, are as follows:

Ultimate tensile Yield strength

strength 0.2% offset Elongation,

Specimen (psi) (psi) % in 1 inch

I 37,750 31,575 12.5

II 37,375 31,400 14.1

The 1970-1971 edition of Aluminum Standards and Data [11] published by the

Aluminum Association specifies the following minimum properties for aluminum

alloy 6061 in the T6 temper:

Ultimate tensile strength 38,000 psi

Yield strength, 0.2% offset 35,000 psi

Elongation, percent in 2 inches 8%

With respect to the above specification the results from the tensile tests

indicate that the ultimate tensile strength is slightly lower than acceptable,

and that the yield strength is about 3,5 00 psi below the minimum acceptable.

Because of differences in gauge length between the specification and specimens,

elongation is not directly comparable. However, the ultimate tensile strength

does meet the specified minimum of 35,000 psi listed in NFPA 193 [2],

A. 7. Summary

The ladder was being used within the load-carrying limits specified in NFPA

193 (par 132). Of those parts examined only members B, C, and D (top left rail,

bed section; lower right rail, bed section; lower right cap, fly section, re- spectively) were hard enough to be in the T6 temper. The T6 temper is obtained

by artificial aging at about 350°F for 7-9 hours after a solution treatment.

T-members A and E were softer than acceptable for the T6 temper; the hardness

values are in the range of the T4 temper which is produced by naturally aging

at room temperature. Tensile tests on material from member A, however, indicate

that the ultimate and yield strengths are lower than acceptable for aluminum

alloy 6061-T6, and higher than would normally be expected for 6061-T4.

Three areas of member A covering a span of about ten feet were examined

and all were soft. If this member had been softened by exposure to fire, it seems very unlikely that this length of material would have been affected while

the other T-member on the same side of the ladder was unaffected.

While only one area of member E was examined, it was much softer than the

corresponding area of the other T-member on the same side of the ladder. This

condition indicates that the soft members were probably improperly heat treated

initially rather than softened in a fire.

The cracks that appear on the edges of some of the members most likely

occurred during the extrusion process. Although some of the fractures went

through these cracks, they do not appear to have originated at the cracks. he

fractures appear to have been caused by overload.

The microstructure of the material examined appears quite satisfactory.

45
APPENDIX B. LADDER FAILURE B

The upper two sections of a failed 35 foot, three section, aluminum alloy

extension ladder used for fire fighting purposes were submitted to Mr. T. Robert

Shives of the Mechanical Properties Section, Metallurgy Division of the National

Bureau of Standards for examination. The ladder was reported to have been

fabricated from aluminum alloy 6061 in the T6 temper.

According to information furnished by the fire department, the ladder had

failed during a training exercise. At the time of failure, the ladder was extended to its full length and was leaning against a building. Two men were

on the ladder; one was at the top preparing to step onto the roof of the

building, and the second had just stepped onto the middle fly section when there was a loud cracking sound and the ladder began collapsing toward the building.

The ladder buckled and fell to the left.

B.l. Visual Examination

The two sections of the ladder submitted for examination are shown as received in figure B-l. The letter designations on the photograph indicate the

approximate locations in the rail members where hardness and thickness measurements were made and where tensile specimens were taken. The "A" rails were on

the right side of the ladder and the "B" rails were on the left side. Locations

1, 2, and 3 were in the middle fly section and locations 4, 5, F, and 6 were in

the top section. Each rail member of the ladder was comprised of two extrusions

riveted together as shown in figure B-2 . The two extrusions in each rail were designated as inside and outside, respectively, according to their location in

the ladder.

When the ladder failed, the top section buckled just above the top of the

middle fly section. None of the four extruded members in the top section

fractured completely, but all exhibited cracks or tears and considerable deformation as shown in figures B-3, B-4 and B-5. Much of the cracking was longitudinal (parallel to the extrusion direction and to the length of the ladder) and

occurred at the 90° angles in the transverse cross section of the extruded

members and at rivet holes. When the ladder buckled, the sizes of these angles

changed in the region of deformation, in some cases increasing to nearly 180°.

There was no visual evidence such as soot to indicate that the ladder had

been exposed to fire.

B.2. Fractographic Examination

Several of the partially fractured surfaces were examined optically, and one such surface was examined with the scanning electron microscope. Figure

B-6 is a scanning electron photomicrograph showing the entire width of the

fracture surface. Note that the left and right sides of the fracture as shown

in the figure have distinctly different features. Figure B-7 shows part of the

left side of the fracture at a higher magnification. The features indicate that

a ductile, overload fracture occurred. The right side of the fracture as it

appears in figure B-8 seems to have been badly smeared. This side was on the

compression side of the fracture and the smearing apparently was caused by the

opposing sides of the fracture moving one over the other. This crack, and the

others examined, appear to have been produced by a single event when the ladder

failed. Therefore, the cracks appear to have occurred as a result of failure,

rather than having contributed to it.

46
Figure B-l. Failed fire ladder as received. The number and

letter designations indicate locations of the

ladder rails which were examined. The "A" rails

are on the right side and the "B" rails are on the

left side.

Figure B-2. Cross section through one of the ladder rails

showing the two extruded parts. These parts were riveted together . The bolt shown in the photograph was used to attach a fitting to the ladder.

X 1

47
Figure B-3. Parts of the deformed extruded members from

location AF. AF outside is at the left and AF

inside is at the right. X 1/3

Figure B-4. Parts of the deformed extruded members from location

BF. BF outside is at the left and BF inside is at

the right. X 1/3

48
a) X4-1 4-J C > o •H

V4 oX ai

CO

N

—' u

QJ

PH 15 n

en e •H o

•H D.

4-1

O

n) 4-1 oo 0)

<~t

X4-4

BO 4-1 .H P n)

14-4

M

X

a) 01 -a 4J • •H c fi w a) 0)

4-1 <u D to O <D X <u PQ 4J xi

1-1 e c

01 •H nj X O

0) <u rH

W e O H -c u

T3 cd

0) <u ^

T)X

u 3 H M 01 4J 6 X • o

a) <r

co

14-1 o <U • u 4J

4-) 3 3 U 00 O

nj •H O, u-i -a

QJ -a C H

<u •H S 3 u 4-1 a o J3 m 00 13

<u •H <d n M -C m

tfi

0)M3

043

49
Figure B-6. Scanning electron photomicrograph showing the

fracture surface of a crack at location BF inside.

The features at the left indicate ductile overload,

The features on the right indicate smearing on the

compression side of the fracture. X 24

Figure B-7 . Scanning electron photomicrograph of fracture at

location BF inside showing ductile overload

(left side of figure 6). X 240

50
Figure B-8 . Scanning electron photomicrograph of fracture at

location BF inside showing smearing on the com- pression side of the fracture. X 23 5

B.3. Hardness Measurements

Rockwell 4 5T hardn

members at the location

at each location. The

Rockwell B equivalent h

considerable spread in

The acceptable range of

temper is 47-72 [10]

.

least one location in t suits that fell within

ess measurements were made on each of the extruded rail

s indicated in figure B-l. Five measurements were made

results of these measurements and the approximate

ardness numbers are given in table B-l. Because of the

the results, the hardness data are reported as ranges.

Rockwell B hardness for aluminum alloy 6061 in the T6

The hardness was below the acceptable minimum for at

ach of the eight extruded members, and many of the rethe acceptable range were near the lower limit.

51
Table B-l. Results of Hardness Measurements

Rockwell 45T Approximate Rockwell B Location Hardness Equivalent

Al inside 32.5-37.0 51-57

A2 inside • 29.0-33.0 46-52

A3 inside 26.5-29.5 43-47

A4 inside 18.0-20.0 32-34

AF inside 21.0-22.0 36-37

A5 inside 27.0-28.5 44-46

A6 inside 22.5-24.0 38-40

Al outside 28.5-30.0 46-48

A2 outside 27.0-32.0 44-50

A3 outside 21.0-23.5 36-39

A4 outside 21.0-24.5 36-40

AF outside 20.0-24.0 34-40

A5 outside 21.5-24.5 36-40

A6 outside 24.0-28.0 40-45

Bl inside 16.0-20.0 30-34

B2 inside 12.0-14.0 23-26

B3 inside 13.0-17.0 24-31

B4 inside 21.0-24.0 36-40

BF inside 22.5-23.5 38-39

B5 inside 31.0-32.0 49-50

B6 inside 30.0-31.0 48-49

Bl outside 27.5-30.0 44-48

B2 outside 28.5-30.5 46-48

B3 outside 27.5-30.5 44-48

B4 outside 29.0-31.0 46-49

BF outside 27.5-30.0 42-48

B5 outside 29.0-30.0 46-48

B6 outside 26.5-28.5 43-46

B.4. Tensile Properties

Tensile specimens were machine from several of the locations indicated

in figure B-l in accordance with ASTM Standard E8-68. The specimens were pulled at a cross head rate of 0.04 inches per minute. Ultimate tensile

strength, 0.2% offset yield strength, and, for those specimens which failed

within the gauge length, percent elongation are reported in table B-2 . The

specified minimum values [11] for mechanical properites for aluminum alloy

6061 in the T6 temper are as follows:

Ultimate tensile strength 38.0 ksi

Yield strength, 0.2% offset 35.0 ksi

Elongation, percent in two inches 8%

One of the 16 tensile specimens had an ultimate tensile strength lower

than the specified minimum, and the results from several others were close

to the specified minimum. The yield strength of six of the 16 specimens was below the specified minimum. Elongation for all specimens for which it could

be determined met specifications.

52
Table B-2 . Results of Tensile Tests

Specimen Ultimate Tensile3

Location Strength, ksi

Yield Strength, 3

0.2% Offset, ksi

Percent Elongation

in 2 inches

10.2

9.7

c

c 11.0

9.9

c 10.4

c 11.1

c 10.2

11.2

10.9

11.5

12.0

'

Al inside

A3 inside

A4 inside

A5 inside

Al outside

A3 outside

A4 outside

A5 outside

Bl inside

B3 inside

B4 inside

B5 inside

Bl outside

B3 outside

B4 outside

B5 outside

45.5

40.7

38.8

38.2

41.2

38.6

43.8

40.4

38.6

37.5

41.4

38.1

43.6

43.2

42.3

42.9

42.0

36.6

32.1

31.7

37.7

34.7

38.3

34.2

31.6

30.1

36.0

35.3

39

38

38

38

aGiven to the nearest 0.1 ksi.

"Given to the nearest 0.1 percent.

c Specimen broke outside gauge length,

..5, Thickness Measurements

Thickness measurements were made on the tensile specimens. The results

of these measurements are given in table B-3. The variation in thickness within any given extruded member was small, but the maximum thickness measured was about 20% greater than the minimum thickness measured..

Table B-3 Results of Thickness Measurements

Location Thickness (in)

Al inside

A3 inside

A4 inside

A5 inside

0.0723

0.0721

0.0611

0.0605

Al outside

A3 outside

A4 outside

A5 outside

0.0718

0.0714

0.0618

0.0616

Bl inside

B3 inside

B4 inside

B5 inside

0.0620

0.0616

0.0618

0.0617

Bl outside

B3 outside

B4 outside

B5 outside

0.0610

0.0612

0.0609

0.0609

53
B.6. Metallographic Examination

Unetched cross sections taken from both the region of buckling and away

from it for four different extruded members were examined metallographically.

The microstructure of the material appeared to be essentially the same in all cases. It was clean with a relatively uniform distribution of precipitate.

Figure B-9 shows a typical unetched cross section.

Etched specimens taken from various locations in the ladder revealed wide

variations in grain size and grain elongation, ranging from what appears to be

an "as extruded" microstructure at location B3 (figure B-10) to what appears

to be a nearly completely recrystallized microstructure as found at location

BF (figure B-ll) . This location was selected to show the tip of a longitudinal

crack at one of the 90° angles of the extrusion in the region of buckling. The

crack appears to be intergranular . The microstructure in an area at location

BF slightly removed from that shown in figure B-ll (figure B-12) exhibits more

of the larger precipitate particles than that shown in figure B-ll and what

appears to be an extrusion lap or seam. Etched microstructures in two other

areas are shown in figures B-13 and B-14. In area Al (figure B-13) , the grains were relatively fine and nearly equiaxed in a band along each surface. Outside

these bands (toward the interior of the cross section) the grains are elongated

in the longitudinal direction of the extrusion. Another extrusion lap or seam

can be seen. The microstructure in a longitudinal cross section at location AF

is shown in figure B-14. There are no bands of fine, equiaxed grains near the

surface, but there are some very fine grains near the center. It appears as though recrystallization had begun to take place, but was far from being complete.

Figure B-9. Photomicrograph of a typical unetched cross section

(longitudinal) showing precipitate particles. This

cross section is from location BF inside. X 60

54
Figure B-10 . Microstructure of longitudinal cross section from

location B3 inside. Grains are elongated in the

longitudinal direction of the ladder. The microstructure appears to be in the "as extruded"

condition. Etch: 10% NaOH. X 60

. * ^5

Figure B-ll. Microstructure of transverse cross section from

location BF inside. The microstructure appears

to be nearly completely recrystallized. There

is an intergranular longitudinal crack. Etch:

10% NaOH. X 60

55
Figure B-12. Microstructure of longitudinal cross section from

location BF inside. An extrusion lap or seam is

exhibited (running vertically) in the figure.

Etch: 10% NaOH. X 60

Figure B-13. Microstructure of longitudinal cross section from

location Al inside. There are fine grains near

the surfaces and elongated grains in the center.

An extrusion lap or seam can be seen. Etch:

10% NaOH. X 60

56
Figure B-14. Microstructure of longitudinal cross section

from location AF showing fine grains near the

center. There appears to have been incomplete

recrystallization and there is considerable

fine precipitate. Etch: 10% NaOH. X 60

B.7 Discussion

The results of both the tensile tests and the hardness measurements indicate

that certain parts of the extruded rail members of this fire fighting ladder were not in the T6 temper as specified. This coupled with a lack of evidence

that the ladder had been exposed to fire indicates that at least parts of some

of the extruded members had not been properly heat treated, if, indeed, they had

been heat treated at all.

The grains elongated in the longitudinal direction in some areas (as shown

in figure B-10) provide further evidence that either no heat treatment or an improper heat treatment was given to some of the material. Material from the

location where figure B-10 was taken had low hardness, low ultimate tensile

strength, and low yield strength compared to other areas examined. The best

mechanical properties were found in material which appears to have a partially

recrystallized microstructure. Thus, material which shows evidence of having

been heat treated exhibited better mechanical properties than the material

which appeared to be "as extruded"

.

There was a considerable variation in thickness among the various extruded

members. The thickest specimen measured was about 20% greater in thickness

than the thinnest specimen measured.

The buckling of the ladder occurred where the strength of the material was

low, although not the lowest found in the extruded members. The extruded

members which buckled were among the thinner members, although not the thinnest.

A stress analysis would be necessary to determine the point at which the load

concentration was greatest, but apparently a combination of low strength material, thin members, and high load concentration caused the ladder to fail where it

did.

57
The cracks found in the extruded members appear to have occurred at the

time of failure, and therefore probably did not contribute to the failure.

Extrusion laps or seams found in two of the areas examined, while considered

undesirable, apparently did not contribute to the failure.

B.8. Conclusions

1. This fire fighting ladder appears to have failed where it did due to

a combination of low strength materials, thin members, and a probable

high load concentration.

2. There was no evidence that the ladder had been exposed to fire or excessive heat.

3. No evidence was found that any cracks were present before the ladder

failed, although some cracks did form as the ladder buckled.

4. The mechanical properties varied considerably among the locations

examined.

5. The results of both hardness measurements and tensile tests indicated

that, in some regions, the mechanical properties of the extruded

members did not meet the minimum hardness and strength requirements

for aluminum alloy 6061 in the T6 temper.

6. In many areas where the mechanical properties did meet the minimum

specified values, they were close to the minimum.

7. Examination of the microstructures indicated that some parts of the

extruded members may have been in the "as extruded" condition, and

therefore were never properly heat treated, and that other parts were improperly heat treated.

8. There was an apparent correlation between the "as extruded" microstructure and poor mechanical properties.

9. Extrusion seams or laps were found in two areas examined, but they

apparently did not contribute to the failure.

10 . Thickness measurements indicated that some of the extruded members

were about 20% thicker than the thinnest members.

In summary, the most deleterious condition found in the ladder rail materi

al was the lack of a proper heat treatment which resulted in mechanical properties which do not meet specifications.

58
APPENDIX C. LADDER FAILURE C

The uppermost section from an aluminum fire extension ladder had failed in

service when subjected to a fire. According to the information provided, this

28-foot aluminum ladder, extended to a length of about 26 feet, was placed at

an angle of about 80 degrees into a window of the fully-involved third floor of

a three-story apartment building. The third floor suddenly gave way, blowing

out the second-floor window below that in which the ladder had been placed. As

the second floor was now also fully involved, the ladder was reportedly exposed

to direct flame contact for approximately 20 seconds before it was pulled away

from the building. No one was on the ladder at the time of the incident although a fireman was about to climb it. The failed ladder was submitted for

examination to Mr. T. Robert Shives, Metallurgist in the Mechanical Properties

Section, Metallurgy Division of the National Bureau of Standards.

The ladder was bent and twisted from about the third to the eighth rung,

counting from the top. The fifth, sixth, and seventh rungs from the top had

fractured and sections were missing. A black, sooty material covered the ladder

from the eighth rung from the top to the top. The deformed section of the ladder

is shown in figures C-l and C-2. The specified material for the ladder is

aluminum alloy 6061-T6 (an age hardening alloy)

.

Sections of the side rails and rungs from damaged and undamaged areas were selected for hardness measurements and macroscopic and microscopic examinations. In addition, a stock piece of 6061-T6 was examined for comparison.

C.l. Hardness Measurements

Hardness measurements were made on the Rockwell 30T scale. The R,„ scale was used instead of the Rockwell B scale because, for the softer specimens,

there was an insufficient thickness of material to support the load (100 kg)

employed when using the RB scale and some of the results on the softer material

were not within the range covered by the RB scale. R30T hardness values and

the approximate Rockwell B equivalent values are given below:

Part and Location R3 0T Approx RB equiv

Rail

Near bottom undamaged 5 8 54

Near top, slightly blackened

but not deformed 56 48

Blackened but not deformed

(between 4th and 5th rung) 48 30

Twisted and blackened 37 6 Rung

Undamaged (#12 from top) 53 42

Damaged (#7 from top) 29 off scale

Control piece 6061-T6 57 52

The acceptable Rg hardness range for aluminum alloy 6061-T6 is 47-72

[10]. The sections of undamaged-unblackened side rail, undeformed-blackened

side rail near the ladder top, and undamaged rung that were examined exhibited

hardness values within the acceptable range. The rail section in the area which was not deformed, but was near the deformed area had softened considerably. Sections from the twisted rail and the damaged rung had softened con- siderably more than the undeformed rail near the twisted section, but were

still much harder than they would have been in the annealed condition. Alumi- num 6061 in the annealed condition (6061-0) would have a Rockwell B hardness

approaching zero.

59
-Sag*

-

Figure C-l. Fire ladder as received showing damaged area at right. X 1/10

Figure C-2. Damaged portion of ladder as received. X 1/5

60
C.2. Macroscopic Examination

Three sections of rail were examined macroscopically as shown in figure C-3.

The unaffected area near the bottom of the ladder (figure C-3a) exhibits flow

lines caused by the extrusion process and shows no evidence of having been overheated. A blackened but undeformed section near the top of the ladder, which

had shown only a slightly reduced hardness compared to the bottom, exhibits a structure which had been affected by heat (figure C-3b) . The material had

apparently recrystallized. The section shown in figure C-3c is from the deformed part of the ladder. The material had recrystallized with some subsequent

grain growth

.

C.3. Microscopic Examination

A metallographic examination of side rail material indicates that the precipitate particles increase in size going from (1) the undamaged area to (2)

the blackened but undeformed area to (3) the twisted area (figures C-4, C-6, and

C-8, respectively). This is expected, based on the hardness measurements. The

larger precipitate particle size in the areas subjected to heat indicates overaging of the material.

The grain size of material from both undamaged and deformed areas was essentially the same (figures C-5 and C-9, respectively). The grain size of

material from the blackened but undeformed area (figure C-7) of the ladder was somewhat finer than that shown in figures C-5 and C-9. Recrystallization has

apparently taken place in the heated areas, as was indicated by the macroetched

specimens

.

The microstructure of the rung material (figures C-10 to C-13) indicates

an increase in grain size in the damaged material (figure C-13) compared to the

undamaged material (figure C-ll) . Unetched specimens (figures C-10 and C-12)

exhibited very little difference in the precipitate particles between the two

conditions

.

For comparison, the microstructure of a stock piece of 6061-T6 is shown in

figures C-14 and C-15.

The material adjacent to the fracture of rung #5 (counting from the top) was also examined metallographically . Two sections through the fracture were

selected for examination. The grain size did not appear to be significantly

different from that shown in figure C-13 (microstructure of damaged rung)

.

The fracture was definitely intergranular . There appeared to have been fusion

in the vicinity of the grain boundaries adjacent to the fracture. The grains

protruding at the fracture reached a high enough temperature so that there was

apparent surface melting of the exposed parts of the grains. The surface tension

of this melted portion of the grains caused rounding of the grains.

61
rH IX

CJ o

X! o 4J lO . 4-> 1-1 • • o CO CO rH en CM cu CU •H SB x! a

nj * U rH CJ S>. e

CJ CJ 43 o

CU 33 •H

XI

m XI 44 •H CJ CM CU CJ

co <J rH 4-1 CJ <U

CD

1-1 u-l #» cu

0) • ta y-i XI CO XI rg SB CM 0) cu -a cfl S (-4

cfl

M o 3 u CO rH d a 3

•H LH XI

M-t c o £ CU 0) o •H • o X) 4J

CO CM 4-1 3 CO

CO

u 44 3 •H

aC *.

O 3 o o ^f 43 4J 44 •H o o 3

44 CO

H 43 e

CJ

a CN a) o

0) o SB cu •*

u

0) •H 3 XI M-l 4J o 0) T3 3 CJ 3 c c

<1J rH o cu o

x:

O o •H A! •H

CJ CO • 44 CJ 4-1

4-> rH

O CO

O

ai CU rH CU o M CO m to u 43 CO o CJ O

cO 4J 2 • s W SB CO J3 o m

0)

>-. =100

62
Figure C-4 • Section of unaffected rail.

X 200

Unetched

,

rV

y:

Figure C-5 • Microstructure of unaffected rail,

Etch: 10% NaOH. X 200

63
Figure C-6 . Section of rail from blackened, but

undeformed area. Unetched. X 200

Figure C-7. Microstructure of blackened, but

undeformed section of rail. Etch

10% NaOH. X 200

64
Figure C-8 • Section of rail from twisted area,

Unetched. X 200

v • . ,,/

^ ;

ft. *

" .

'

i *&" ' £- ', *-

^

'*-- -" ' -.£ . m

-^ «J V.

/ • -«* - '.t\|

f

-. * i+ .. V' : '*% *,» • --- V« / "... **--.- - -'* ''*'*"*

z. * *t *

«. ~"~~ -' :

.'• •?*• r ***

"*- •» * _

• -i* ^ o - „ *m

>~~„ «*-•.. • s > ~^J*

a *-_"•

V

—-- -^ »' ', /»

. - f

~" t

"•- -

*i .'•

" « « • *

'

Figure C-9 . Microstructure of section of rail from

twisted area. Etch: 10% NaOH. X 200

65
Figure C-10 • Section of rung unaffected by heat.

Unetched. X 200

Figure C-ll. Microstructure of rung unaffected

by heat. Etch: 10% NaOH'. X 200

66
Figure C-12. Section from damaged rung. Unetched,

X 200

**.~

Figure C-13 . Hicrostructure of damaged rung,

10% NaOH. X 20

Etch

67
%;;.vxmm

Figure C-14- Stock 6061-T6. Unetched. X 200

*.Y -

• *

• - ',*** . V

• •

* >r :? »y •*•

i-4

,' < *

3^*«.v 5K

# > . '* 4' j-{»-< 1 . y ; —

> :

* *< -. . ", * ' ' / ^i o

* *• \i

_-

, *.

• i - y- .

- . ,°/. ^%i/ * -X »f • ^nK* ' "' "'"<*i

',# A * ' •" '- \» » i *©*,'/ *>i '"T-Sfc )WJ* .J>*

Figure C-15. Microstructure of stock 6061-T6

Etch: 10% NaOH. X 200

68
C.4. Temperature of Failure

In order to estimate the temperature to which the ladder was subjected at

failure, a number of one-half inch thick rail sections were cut from the ladder

and heated at selected temperatures in an air furnace with the air being circulated by a fan. After the furnace treatment, the sections were cooled in

still air. Hardness measurements (R^

n

) were then made.

Data for Heat Treated Rail Sections

Temperature (°F) Time (min)

1

R3 0T

58

Approx RB equiv

400 54

500 1 58 54

600 1 58 54

700 1 58 54

800 1 54 44

900 1 54 44

1000 1 47 28

1100 1 38 8 1200 1 38 8 1300 1 12 off scale

400 2 1/2 56 48

500 2 1/2 57 52

600 2 1/2 56 48

700 2 1/2 55 46

800 2 1/2 48 30

900 2 1/2 41 14

1000 2 1/2 20 off scale

1100 2 1/2 4 off scale

It was stated by witnesses that the ladder had been subjected to the fire

for a very short time. That is the reason for the short duration of the above

tests

.

It is not known how closely our tests approximated the actual conditions

leading to failure. Small sections were used in our tests rather than the

complete ladder itself, the cooler parts of which would have acted as a heat

sink during the fire. With these qualifications in mind, rough estimates of

temperatures to which the ladder was subjected are as follows:

1 minute exposure 1100-1200°F

2 1/2 minute exposure 850- 950°F

C.5. Fasteners

The fasteners in this ladder appear to have been made of an aluminum alloy

and not of plated steel.

C.6. Summary

The ladder appears to have been made of properly heat treated aluminum

alloy 6061-T6. There is no evidence that the lower, unblackened part of the

ladder had been overheated either in prior service or at the time of failure.

The damaged material appears to have been severely overaged, but not annealed,

during the fire. The fasteners appear to have been made of an aluminum alloy

and not of steel. A rough estimate of the temperature to which the ladder was

69
subjected is as follows, depending on length of exposure to the heat:

1 minute exposure

2 1/2 minute exposure

1100-1200°F

850- 950°F

APPENDIX D. CORRELATION OF TENSILE AND YIELD

STRENGTHS WITH HARDNESS FOR ALUMINUM ALLOY 6061-T6

A newly purchased 35-foot, 2-section fire department ground ladder was cut into sections for the purpose of obtaining hardness specimens. Specimens were ! taken from each rail member at the 3rd, 9th and 17th segment from the bottom

of both the bed and fly sections. The specimens were identified as follows:

Bed Sect

(Specimen

Lon

No. ) Segment from

bottom Top right

1

2

3

Bottom right

4

5

6

Top left

7

8

9

Bottom

10

11

12

left

3

9 17

Fly Section

(Specimen No .

)

Segment from

bottom

3

9 17

Top right

13

14

15

Bottom right

16

17

18

Top left

19

20

21

Bottom left

22

2 3 24

Specimens were machined according to ASTM Specification E8 for standard

specimens. Hardness measurements taken on the specimens gave the following

results

Specimen

1

2

34

5

6

7

52.7

53.9

56

57

56

54

57

53

Specimen

9 10

11

12

13

14

15

16

R B 52.0

51.3

52.6

52.8

55.2

55.1

54.5

54.4

Specimen

17

.18

19

20

21

22

23

24

Rt

54,

54,

56,

56

54,

55.0

55.0

54.5

Each hardness value is the average of 6 determinations. All of the above

values are well within the acceptable hardness range for 6061-T6.

In order to produce a range of hardness values and mechanical properties,

specimens 5 through 24 were overaged in an air furnace and air cooled prior to

tensile tests. Specimens given more than one heat treatment were cooled to

70
room temperature between treatments. Heat treatments are given below:

Specimens 5-8

9 -12

13 -16

17 -20

21 -24

30 min at 425°F + 30 min at 450°F

30 min at 500°F

15 min at 550°F

25 min at 550°F + 10 min at 575°F

four separate 5 min treatments at 575°F

30 min at 525°F + 30 min at 525°F

All specimens were tested on the tensile testing machine at a rate of 0.02

inch per minute. The results of the tensile tests and the hardness measurements

are given below. Ultimate tensile strength and yield strength are given to the

nearest 25 psi . Each hardness value is the average of six measurements.

Table D-l. Results of Hardness, Tensile Strength, and

Yield Strength Measurements on 6061-T6 Aluminum Specimen

Ultimate tensile Yield strength Elongation

Specimen Hardness strength 0.2% offset in 2 inches

R

B

(psi) (psi) - (%)

1 52.7 43,300 40,025 8.0

2 53.9 44,000 40,600 8.1

3 56.6 44,600 41,050 7.8

4 57.2 44,900 41,075 8.8

5 41.9 38,450 34,550 9.5

6 39.2 38.100 33,825 9.8

7 42.4 38,675 34,800 9.0

8 38.1 37,100 32,600 8.1

9 26.2 34,375 28,575 10.4

10 25.5 34,250 28,000 11.0

11 26.8 34,600 28,950 10.5

12 21.4 34,075 28,350 10.0

13 9.9 31,800 25,250 10.7

14 5.3 30,700 23,525 10.3

15 8.7 31,325 23,825 11.0

16 8.7 31,375 24,150 11.1

17 18.3 32,800 26,425 10.4

18 15.3 32,350 25,750 10.7

19 18.4 33,150 26,725 9.0

20 17.2 32,775 26,300 10.6

21 26.1 34,600 28,850 8.6

22 26.0 34,525 28,775 9.9

23 22.9 34,800 28,950 10.6

24 28.6 35,050 29,625 9.5

The above data are plotted in figure D-l. This 'figure may be used to convert

hardness measurements made on 6061-T6 aluminum alloys to the corresponding

tensile strength and yield strength.

71
CSl c

46

44

42

40

38

36

34

O

8 32

*(/> 30

28

26

24

22

20

6061 -T6 ALUMINUM SPECIMENS

A ULTIMATE TENSILE

© 0.2% OFFSET YIELD

4 8 12 16 20 24 28 32 36 40 44 48 52 56

HARDNESS, ROCKWELL B

Figure D-l. Relationship of hardness to tensile strength

and yield strength for 6061-T6 aluminum alloy,

72
REFERENCES

[1] "Fire Service Ladder Practices," validated by the International Fire

Service Training Association, published by the Department of Fire

Protection, Oklahoma State University (1969) .

[2] NFPA No. 19 3-19 72 "Standard on Fire Department Ladders - Ground and

Aerial," published by the National Fire Protection Association, Boston,

Mass. (19 72) .

[3] ANSI A14. 2-1972, "American National Standard Safety Requirements for

Portable Metal Ladders," published by American National Standards

Institute, Inc., New York, N. Y. (1972).

[4] NFPA No. 19-1973 "Standard for Automotive Fire Apparatus," published by

the National Fire Protection Association, Boston, Mass. (1973).

[5] Price, H. E., Crain, C. L. and Siciliani, F. A., "A Survey of Human

Factors Engineering Problems in Firefighting Equipment," Final report

on Office of Civil Defense (Dept. of Defense) Contract #OCD-PS-64-3

(Feb. 1964)

.

[6] Siegel, L. G. , "The Projection of Flames from Burning Buildings," Fire

Technology, 43-51, (Feb. 1969).

[7] Metals Handbook, 8th edition, American Society for Metals, Vol. 2, 276

(1964) .

[8] Metals Handbook, 8th edition, American Society for Metals, Vol. 1, 525

(1961) .

[9] Pearson, C. E. and Parkins, R. N. , The Extrusion of Metals, 2nd edition

(Wiley, New York, 1969) .

[10] Metals Handbook, 8th edition, American Society for Metals, Vol. 2 (1964)

[11] Aluminum Standards and Data, 2nd edition, The Aluminum Association

(1969) .

73

NBS-1UA (REV. 7-73)

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. NBS TN-8 3 32. Gov't Accession

No.

3. Recipient S Accession No.

4. TITLE AND SUBTITLE Fire Department Ground Ladders - Results

of a Preliminary Study

5. Publication Date July 1974

6. Performing Organization Code

7. AUTHOR(S)

Harvey P . Utech

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No. 4901133

11. Contract/Grant No. N/A

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP) same as No. 9

13. Type of Report & Period Covered Preliminary

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

Library of Congress Catalog Card Number: 74-600092

16. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant bibliography or literature survey, mention it here.)

The key performance requirements for fire department

ground ladders were determined. Existing ladder standards were reviewed and found to be unnecessarily restrictive in some areas and inadequate and unrealistic in others. Included

in the report are metallurgical studies of three ladders

that failed in service as well as a correlation of hardness

with tensile and yield strength for 6061-T6 alloy.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons)

Aluminum; fire department; ladders; performance requirements;

standards.

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