BH9

DEPARTMENT OF COMMERCE

WASHINGTON

ELIMINATION OF WASTE SERIES

RECOMMENDED

BUILDING CODE REQUIREMENTS

FOR WORKING STRESSES IN

BUILDING MATERIALS

REPORT

OF

BUILDING CODE COMMITTEE

KI-.

BUREAU OF STANDARDS

Wm

WASHINGTON

GOVERNMENT PRINTING OFFICE

1926
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UNITED STATES DEPARTMENT OF COMMERCE

HERBERT HOOVER, SECRETARY ELIMINATION OF WASTE SERIES

RECOMMENDED

BUILDING CODE REQUIREMENTS

FOR WORKING STRESSES IN

BUILDING MATERIALS

REPORT OF

BUILDING CODE COMMITTEE

JUNE 1, 1926

IRA H. WOOLSON, Chairman

EDWIN H. BROWN RUDOLPH P. MILLER

WILLIAM K. HATT JOHN A. NEWLIN

ALBERT KAHN JOSEPH R. WORCESTER

FRANK P. CARTWRIGHT, Technical Secretary

JOHN Me CRIES, Chief, Division of Building and Housing Bureau of Standards, Department of Commerce

PRICE, 10 CENTS

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C
MEMBERSHIP OF THE BUILDING CODE COMMITTEE OF

THE DEPARTMENT OF COMMERCE

Ira H. Woolson, Chairman, Consulting Engineer, National Board of Fire Underwriters, New York City; Member, American Society of Mechanical

Engineers, American Society for Testing Materials, National Fire Protection Association, American Concrete Institute. Edwun H. Brown, Architect, Minneapolis, Minn.; Secretary, American Insti- tute of Architects; Member, American Institute of Architects. William K. Hatt, Professor of Civil Engineering, Purdue University; Member,

American Society of Civil Engineers, American Concrete Institute. Albert Kahn, Architect, Detroit, Mich.; Fellow, American Institute of Architects. Rudolph P. Miller, Consulting Engineer, New York City; Past President,

Building Officials Conference; Member, American Society of Civil Engineers,

American Institute of Consulting Engineers, American Society for Testing

Materials; President, National Fire Protection Association. John A. Newlin, In Charge of Section of Timber Mechanics, Forest Products

Laboratory, Forest Service, United States Department of Agriculture,

Madison, Wis.; Member, American Society for Testing Materials, American

Society of Civil Engineers.

Joseph R. Worcester, Consulting Engineer, Boston, Mass.; Member, American Society of Civil Engineers, American Institute of Consulting Engineers.

Frank P. Cartwright, Technical Secretary.

II
CONTENTS

Part L—INTRODUCTION

Page

Diversity of present stress requirements 1 Recent stress practice more economical 1 Scope and purpose of report 2 Procedure of committee 2 Building inspection 3 Acknowledgment 4

Part n—RECOMMENDED CODE REQUIREMENTS

Sec. 1. General requirements 5

Sec. 2. Working stresses in reinforced concrete 5

Sec. 3. Cast iron 7

Sec. 4. Working stresses in structural steel shapes 7

Sec. 5. Working stresses in wood members 8

Part m.-APPENDIX

Par. 1. Purpose 11

Par. 2. Sources of information 11

Par. 3. Influence of building inspection 12

Par. 4. Variations in building code requirements 14

Par. 5. Working stresses in plain concrete 17

Par. 6. Working stresses in reinforced concrete 17

Par. 7. Design of web reinforcement 23

Par. 8. Cast-iron columns 27

Par. 9. Working stresses in structural steel 28

Par. 10. Factors involved in determining safe stresses for timber 36

Par. 11. Variables affecting strength of clear wood 43

Par. 12. Influence of defects on strength of timber 45

Par. 13. Moisture conditions of service 45

Par. 14. Conditions of loading 46

Par. 15. Working stresses for timber 46

Par. 16. Factors of safety for timber 47

Par. 17. Bending—stress in extreme fiber

^ 48

Par. 18. Bending—horizontal shearing stresses 49

Par. 19. Bending—stiffness 49

Par. 20. Compression parallel to the grain—short, intermediate, and

long columns 50

Par. 21. Compression perpendicular to grain.. 51

Par. 22. Composite columns 52

Par. 23. Welded connections 52

Par. 24. Construction resistive to earthquakes 52 m

Q
rv CONTENTS

TABLES

Page

1. Assumed strength of concrete mixtures 5

2. Maximum working stresses in structural steel 8

3. Maximum working stresses for timber (species locally used) 9

4. Safe stresses for square and rectangular wooden columns 10

5. Summary of analysis of wood stress requirements of 117 codes 15

6. Analysis of concrete stress requirements of 58 building codes from data

furnished by the Portland Cement Association, September, 1925 16

7. Working stresses for timber 39

8. Strength of columns of intermediate length in per cent of strength of short columns 40 FIGURES

1. Value of web reinforcement in resisting tensile stresses set up by shear, _ 24

2. Allowable values of shear for formulas (31) and (32) 26

3. Diagrams comparing recommended column reduction formulas with

thoiSe in common use 35

4. Variability in modulus of rupture of clear wood 44
LETTER OF SUBMITTAL

Washington, D. C., June 29, 1926.

Hon. Hekbert Hoover,

Secretary of Commerce, Washington, D. C. Dear Sir: I send you the inclosed report, entitled ^‘Recommended

Building Code Requirements for Working Stresses in Building

Materials.’’ It constitutes the sixth in the series thus far completed.

The stresses which are assumed in the design of buildings constitute a fundamental basis for establishing the structural safety of

such buildings; hence the necessity for exercising care in striking a

balance between employment of low stress assumptions affording

high safety factors and the economy which might accrue from use of

high stress values. Stresses must also be considered along with load

assumptions.

The fact that these recommendations are to be generally distributed

throughout the country, often in localities where there is little or no

control over quality of materials employed, is justification for the

moderately conservative attitude assumed by your committee on

this subject.

A tentative draft of the report was distributed to technical experts

and criticisms solicited. The suggestions received have been included

in the report as now presented in as far as it seemed advisable to do so. If the report meets with your approval, the committee recommends

that it be printed for public distribution.

Yours very truly.

Ira H. Woolson,

Chairman, Building Code Committee,

Department oj Commerce.
LETTER OF ACCEPTANCE

Department of Commerce,

Office of the Secretary,

Washington, July 13, 1926.

Mr. Ira H. Woolson,

Chairman, Building Code Committee,

Department of Commerce, Washington, D. C. Dear Mr. Woolson: I have received the Recommended Building

Code Requirements for Working Stresses in Building Materials pre- pared by the Building Code Committee. A great amount of painstaking work has been done for which the committee deserves much

credit. The need for undertaking such a task is made evident by the

variations in existing stress requirements in different localities. In

many instances improvements in building materials and the advance

of engineering knowledge appear to justify more economical practice.

Your committee has given thoughtful attention to this. At the same time it has surrounded its recommendations with adequate

safeguards.

Yours faithfully.

VI Herbert Hoover.
RECOMMENDED BUILDING CODE REQUIREMENTS FOR WORKING

STRESSES IN BUILDING MATERIALS

This report is in three parts. The first describes the organization

and purposes of the Building Code Committee; the second presents

requirements recommended for adoption in building codes; and the

third explains briefly the grounds upon which the recommendations

of Part II are based, and discusses the conditions by which they

are limited. Some of this material, such as stress tables, is quite

suitable for inclusion in city building codes. Part III also contains

various references to good building practice and other information

helpful to building code committees.

Part I.—INTRODUCTION

The Building Code Committee of the Department of Commerce

was organized early in 1921, to meet a generally expressed public

demand for greater uniformity and economy in building code requirements. Its first work concerned small dwelling construction, and

a report on that subject was published in January, 1923. Subsequent

reports issued by the department have presented minimum requirements for plumbing installations, for the erection of masonry walls

of all types, for the live loads to be assumed as a basis for building

design, and a recommended form for code arrangement.^

Diversity of Present Stress Requirements.

It is well known that existing building codes differ as to the working

stresses allowable for construction materials. These variations are

considerable, as will be seen from tables presented in the Appendix,

paragraph 4. Differences in the code requirements . of different

cities and in the manner of their statement, with resulting possibilities of misunderstanding, are reported to cause much inconvenience to

architects and engineers responsible for building design. The opinion

also was freely expressed, in connection with the earlier draft of this

report, that uniform code requirements for working stress limits

would greatly facilitate the work of contractors and would operate

to reduce building costs. Recent Stress Practice More Economical.

Considerable reduction in construction costs should result from

adoption of code stress requirements more in keeping with recent

1 See back cover page for list of department publications on building and building regulation. Q 1
2 EECOMMENDED BUILDING CODE. EEQUIEEMENTS

improvements in materials, design, and construction. There is reported a general trend in the building industry toward more accurate grading of materials, more careful analysis of loads and stresses,

and better control of construction operations, making possible,

according to the consensus of opinion, the use of higher design stresses where such improvements obtain. Building codes have been slow to recognize this tendency, with the result that prevailing stress limits are considerably lower than are recommended by the major professional groups in the industry. This often involves an unnecessary

additional building cost. It is only where high stresses are used by

those not fully competent to conduct building operations that danger

may result with more advanced stress provisions, and it is questionable in such cases if the most conservative requirements could be

regarded as safe. Stress limits for one material often are more conservative, from a

safety viewpoint, than those for others. Such unjustifiable departures in code requirements at times result in cost differentials sufla- cient to handicap one material or construction type seriously in

competition with others. While public safety is the primary object

of building codes it is desirable that public economy be promoted

also, so far as possible, and it is hoped that this report, by presenting recommendations based primarily on safety considerations, will help to prevent uneconomic situations due to disregard of such

considerations.

The differences between the working stress requirements and the

standards generally accepted are offset in some building codes by

compensating departures in live load assumptions or other influential

factors, with the result that construction is safe and under the cir- cumstances reasonably economical. In other instances this is not the

case. The Building Code Committee in previous reports has already

treated some of those related factors, such as live load assumptions

and the quality of masonry materials. The recommendations of

this report are predicated on those of previous reports and should be

used with caution unless the committee's findings on other related

questions are also adopted.

Scope and Purpose of Report.

This report presents recommended working stress requirements for

reinforced concrete, cast iron, steel, and timber, with an Appendix

explaining the reasons for certain of the recommendations and giving

in condensed form information bearing on some of the limits suggested. Wrought iron no longer is used to any extent for structural

purposes and, therefore, has been omitted from consideration.

Procedure of Committee.

The report in an earlier form was submitted widely for critical review by those whose interest and experience qualified them to dis-
WORKING STRESSES IN BUILDING MATERIALS 3

cuss its subject matter with authority. Over 250 reviews were

received, representing the opinions of a much larger number of

architects, engineers, contractors, building officials, steel manufacturers, and steel fabricators. The summary of professional opinion

thus obtained was found to justify important revisions of the

committee's tentative recommendations and was used extensively

for reference in preparing the final report.

The committee’s recommendations for concrete are based largely on those of the Joint Committee on Standard Specifications for

Concrete and Reinforced Concrete;^ those for timber on work of the

United States Forest Products Laboratory; and those for steel on work of the American Institute of Steel Construction and of the

American Society of Civil Engineers. These organizations are believed to have been instrumental more than any others in ascertaining and codifying good practice in their respective fields. Their

findings already represent a very wide consensus of professional

opinion, and in experienced hands should assure safe, economical

construction. Such variations as may be found in these recommendations from those advocated by the authorities named have

resulted from careful consideration, and are believed to be justified

by the present status of building regulation.

Building Inspection.

The committee’s recommendations represent in nearly all cases

relaxations from the prevailing limitations imposed by building

codes. To this extent they should effect economies in building construction. In order to be structurally safe, on the other hand,

buildings utilizing these increased stresses must be planned and constructed by those technically trained and experienced; and both

materials and workmanship must be of uniform, dependable grades.

Both the Joint Committee on Standard Specifications for Concrete

and Reinforced Concrete and the American Institute of Steel Construction have codified the requirements for materials and workmanship on which their stress recommendations are based, and while

publications of the latter have given somewhat less attention than

those of the former to th© factor of design, it is understood that

material bearing on this phase of the subject is in preparation.

The American Society of Civil Engineers’ Special Committee on

Stresses in Structural Steel, after long investigation, also has predicated its recommendation of higher steel stresses on thoroughly competent design and construction. The lumber industry also has done

much to safeguard the use of its products. Under the leadership of

* Representing the American Society of Civil Engineers; American Society for Testing Materials; American Railway Engineering Association; American Concrete Institute; and Portland Cement

Association. 9070°—26f 2

c
4 EECOMMENDED BUILDING CODE EEQUIREMENTS

the Forest Products Laboratory and the National Lumber Manufacturers^ Association carefully defined lumber grades have been

developed, and much information on the proper use of wood in construction has been published. The discussion of timber grades, fac- tors influencing strength, and directions for design presented in this

report are believed to afford at least a partial basis, in competent

hands, for use of the timber stresses recommended.

Discussions of this report in tentative form disclosed a prevailing

opinion that safety is not so much influenced by stress limits as by

the competency of those employing them. Eelatively few building

codes at present provide any machinery for controlling the class of men responsible for building design and construction. City inspection departments do not and should not assume the full responsibility

for safe construction. There undoubtedly is a strong trend toward

the employment of more competent talent to design and supervise

the construction of buildings. The registration of architects and

engineers in several States provides at least partial foundation for

control, and is utilized to that end in some cities. Nevertheless

there is, in general, no method, except the hurried and necessarily

casual check by building officials, of insuring that building design or construction will not be attempted by those utterly inexperienced or incompetent.

No very promising procedure has been proposed to remedy this

situation, and it is probable that safety in building must continue

for a time to depend on the personal standards of those in the industry, supplemented by the vigilance of public building officials. It, therefore, behooves cities adopting the higher stresses herein recom- mended to provide the necessary personnel and facilities for an effec- tive check on the conduct of building operations. The expenditures

involved are minor in comparison with the economies possible under

such a regime. It can readily be shown, for example, that the saving

in cost on one or two large buildings would pay the annual cost of providing adequate building inspection for a medium-sized city.

Acknowledgment.

The committee desires to acknowledge its indebtedness to the

great number of architects, engineers, builders, building officials, and

others who assisted in the preparation and review of the tentative

draft of this report, and whose advice has been helpful in making the

final draft representative as nearly as possible of the average opinion

in the building industry.
Part IL—RECOMMENDED CODE REQUIREMENTS

Section 1. General Requirements.

All members shall be so framed, anchored, tied, and braced together

as to develop the maximum strength and rigidity consistent with the

purposes for which they may be used or to which they are likely to be subjected, and the stresses hereinafter recommended are based on

the assumption that the details and connections used are fully as strong as the members connected. (See Appendix, par. 3.)

Workm^anship in fabrication, preparation, and installation of

material shall conform throughout to good engineering practice.

(See Appendix, par. 3.)

Section 2. Working Stresses in Reinforced Concrete.

1. The direct compressive working stress on plain concrete used

in reinforced concrete construction shall not exceed 0.25 of the

assumed ultimate compressive strength of the concrete. (See

Appendix, par. 5.)

2. The stress in the extreme fiber of the compression face of a

reinforced concrete beam shall not exceed 0.40 of the assumed ulti- mate compressive strength of the concrete. (See Appendix, par. 6-5.)

3. The working stress in shear on plain concrete in reinforced concrete construction shall not exceed 0.02 of the ultimate compressive

strength of the concrete. (See Appendix, par. 6-7.)

4. The working stress in bond between concrete and steel shall not

exceed 0.04 of the ultimate compressive strength of the concrete in

the case of plain bars and 0.05 in the case of deformed bars.

5. The temperature of freshly placed concrete shall be maintained

at not less than 50° F. for at least five days after placing. (See

Appendix, par. 6-5.)

6. Concrete in reinforced concrete construction, when mixed in the

following proportions, stated by volume, shall be assumed to develop

a laboratory cylinder compressive strength at 28 days as follows:

(See Appendix, par. 6-3.)

Table 1.

Assumed strength of concrete mixtures PLASTIC MASS CONCRETE (SLUMP 1 TO 3 INCHES)

Approximate mix: Volume of Portland cement to sum of separate volumes of fine and coarse aggregate

Water- cement

ratio,

1

U. S. gal- lons per 94-pound

sack of cement

Assumed

ultimate

strength

at 28 days

1-6 7M

6^

5M

Lbs.lin.^ 2,000

2, 500

3, 000

1-5.. 1-4

^ Water or moisture contained in the aggregate must be included in computing the water-cement ratio. 5 c
6 BECOMMENDED BUILDING CODE BEQUIEEMENTS

Table 1. —Assumed strength of concrete mixtures—Continued

MODERATELY WET CONCRETE (SLUMP 6 TO 8 INCHES)

Approximate mix: Volume of Portland cement to sum of separate volumes of fine and coarse aggregate

Water- cement

ratio, U. S. gal- lons per 94-pound

sack of cement

Assumed

ultimate

strength

at 28 days

1-G 8 Lbs-jin^

1, 600

2, 000

2, 500 3,000

1-5 1-4)^ . . . 63/2 1-3)4 VERY WET CONCRETE (SLUMP 10 INCHES OR MORE) «

1-5 8 1, 600

1-4- : 734 2,000

l-3)i 634 2, 500

1-23^ 5M 3, 000

2 Concrete of this consistency, while frequently used, should be avoided, as it will be porous and of low

resistance to weathering. In no case shall concrete for any assumed strength be placed with

a v/ater-cement ratio exceeding that shown. Where the aggregates

are such that the mixes shown do not produce proper workability

with the given water-cement ratios, the mixes shall be changed, but

not the water-cement ratios. The graded sizes of the combined aggregate shall be such that

when separated on a No. 4 standard sieve the weight passing shall not be less than one-half or more than two-thirds of the total. (See

Appendix, par. 6-5.)

7. If and when it is shown by evidence of tests made by competent

authorities satisfactory to the building official that concrete of a higher strength than that specified in the preceding paragraph will be used and that competent field control of mixing and placing is assured; or that concrete of less strength will be employed; the

stresses shall be proportionately modified, provided that in no case

shall a strength of more than 3,000 pounds per square inch be

assumed. (See Appendix, par. 6-5.)

8. The tensile stress in steel reinforcement shall not exceed 16,000

pounds per square inch, provided that this stress may be increased to 18,000 pounds per square inch when assurance is furnished the

building official that the material conforms to the American Society

for Testing Materials Standard Specifications for Billet Steel Concrete Reinforcement Bars, serial designation A 15-14, or for RailSteel Concrete Reinforcement Bars, serial designation A 16-14.

The tensile stress in cold-drawn steel wire meeting the tentative

specifications A 82-2 IT of the American Society for Testing Materials
WOEKING STRESSES IN BUILDING MATERIALS 7

shall not exceed 18,000 pounds per square inch. Concrete shall not

be assumed to resist direct tensional stress. (See Appendix, par. 6-8 .) 9. In the absence of rules adopted by the building official, and

except as otherwise specifically provided in this code, the assumed

formulas and specifications of the latest report of the Joint Committee

on Standard Specifications for Concrete and Reinforced Concrete

shall be assumed in calculating the strength of slabs, beams, columns,

and other elements of reinforced concrete structures, and designs

shall conform thereto. (See Appendix, par. 2.) Sec. 3. Cast Iron.

1. Compressive stresses in hollow cast-iron columns shall not ex- ceed values determined by the formula

2

equals 9,000-

in which

2

equals compression in pounds per square inch,

L equals length of the column in inches, and

r equals minimum radius of gyration of the column.

2. The maximum allowable ratio of Z to r shall not exceed 90,

except that when allowable working stresses computed by the above

formula are reduced one-third the ratio of Z to r may be increased,

but shall not exceed 120. (See Appendix, par. 8-2.)

3. Cast-iron columns shall not be used in any case where the load

is so eccentric as to cause tension in the cast iron; nor shall they be

used for parts of the structural frame of buildings which are required

to resist stress due to wind. Tensile stresses in the extreme fiber of

cast-iron lintels or elsewhere, except in columns, shall not exceed

3,000 pounds per square inch.

4. The material and workmanship of cast-iron columns shall be

equal in all respects to that described in the American Society for

Testing Materials Standard Specifications for Cast-Iron Pipe and

Special Castings, serial designation A 44-04. (See Appendix, par.

8-6.) All columns resting on or supporting other columns shall have

their ends machine faced to a plane surface perpendicular to the axis. Sec. 4. ¥/orking Stresses in Structural Steel Shapes.

1. For steel acceptable to the building official, but of which the

origin and physical characteristics are not definitely determined, the

maximum working stresses shall not exceed those given in column

(a) of the following tabic. (See Appendix, par. 9.)

2. When assurance satisfactory to the building official is furnished

that the steel to be used conforms to the A. S. T. M. Standard

c
8 KECOMMENDED BUILDING CODE EEQUIREMENTS

Specification for Structural Steel for Buildings, serial designation

A 9-24, the maximum working stresses shall not exceed those given

in column (5) of the following table. Table 2.

Maximum working stresses in structural steel

(a) Acceptable

steel

(b) Standard

steel

Lbsjin.i Lhs.linA Direct axial tension on net section 16, 000 18, 000 Direct axial compression, maximum for short columns. 12, 500 14,000 Compression in columns 1 16, 000-60p 1 18, 000-70^

Fiber stress in flexure, in tension, or in compression when the unsupported

length {L) is not more than 15 times the breadth (5) 16,000 18, 000 Compressive fiber stress in flexure for values between 15 and 40... 19, 600-210'^- 22, 000-270^

Fiber stress in pins 24,000

24,000

27, 000 Bearing on plane faced or rolled surfaces 27, 000 Shear in gross section of webs of girders and rolled shapes in which (d) the unsupported depth between flanges or the distance between stiffeners, if less, divided by \t) (the thickness of web) does not exceed 43 10, 700 12, 000 Shear when j- exceeds 43 13, 300-62^ 15, 000-70|- Shear in power-driven rivets or in pins Shear in hand-driven rivets or in rough bolts

12, 000 13, 500 9,000 10, 000 Bearing upon power-driven rivets or in pins subjeicted to single shear on one

side of the bearing in question 24,000 24, 000 Bearing upon power-driven rivets or on pins when the bearing metal lies between two planes of shear of opposite character immediately adjacent 30, 000 30,000 Bearing upon hand-driven rivets or on rough bolts subjected to single shear on one side of the bearing in question 16, 000 16, 000 Bearing upon hand-driven rivets or on rough bolts when the bearing metal

lies between two planes of shear of opposite character immediately adja- cent 20,000 20,000

1 Compression stresses in columns, computed by the formulas for column design, may not exceed in any case the maximum for direct axial compression short columns. L equals length of column; r equals

least radius of gyration.

3. Columns shall be limited in slenderness to a value of — equals

160. Compression flanges of beams and girders shall not exceed in

length between lateral supports 40 times their width. By the term

“lateral supports is meant points where definite resistance to

lateral deflection is provided of sufficient strength to prevent buckling.

Combined stress due to flexure and axial stress shall not exceed that

allowed for flexure. The axial stress alone, if compression, shall not

exceed that allowed in columns.

4. For stresses either direct or flexural produced by wind loads, or by a combination of wind loads and dead and live loads, the working

stresses allowed in paragraphs (1) and (2) may be increased by

25 per cent, provided the resulting sections are not less than those

required for the dead and live loads alone.

Section 5. ¥/orking Stresses in Wood Members.

1. All wooden structural members shall be of sufficient size to carry the load safely without exceeding the allowable working stress of the material specified in Table 3 below. The strength of such

members shall be determined from actual dimensions of the pieces

and not from nominal dimensions.
WORKING STRESSES IN BUILDING MATERIALS 9

2. Stress due to dead and live loads acting singly or in combination,

without wind load, shall not exceed the allowable stress specified in

Table 3, below. For stresses produced by wind loads or by a combination of wind loads and dead and live loads the working stresses allowed

below may be increased by 50 per cent, provided the resulting sec- tions are not less than those required for dead and live loads alone.

(See Appendix, par. 14.)

3. Stress in compression perpendicular to the grain may be

increased by 50 per cent above that specified in Table 4 below in the

case of joists supported on a ribbon board and spiked to the studding

rather than resting upon or in masonry. (See Appendix, par. 10.)

4. The restrictions and limitations of the two preceding paragraphs

apply to all timber structures in which the lumber is in a dry location

and not exposed to the weather. Timbers exposed to the weather

shall be designed on a basis of working stresses 25 per cent ^ lower

than those recommended in Table 3. Table 3

(A table of allowable working stresses similar to that given in the Appendix,

par. 10, Table 7, is recommended to be inserted at this point. It is not expected,

however, that each code will include all the species listed in Table 7. It should

include the species locally used and careful consideration should be given to all of the species listed in Table 7 since under present lumbering and transportation

conditions many species are available which grow in remote sections of the

country. Frequently such species are of secondary importance from the standpoint of lumber produced, but the very fact that they are little used is often

reflected in a lower price. Their inclusion in a code would not infrequently lead

to cheaper construction without loss in quality.)

5. Working stresses in compression parallel to grain for columns

shall not exceed those prescribed in Table 4 for the respective species

and ratio of unsupported length to least dimension. The ratio of

unsupported length of columns to least dimension shall not exceed 50. When the amount of timber construction exposed to weather justifies a scale of stresses, the reductions advocated by the Forest Products Laboratory in U. S. Department of Agriculture Circular No. 295 should

be followed. These reductions range from 12H to 33H per cent of the stresses recommended in paragraph

10 of the Appendix, depending upon the strength property under consideration.

Q
10 EECOMMENDED BUILDING CODE HEQUIREMENTS

Table 4.—Safe stresses for square and rectangular wooden columns

Species Grade

Aspen

Basswood

Cedar, western red. Cottonwood

Fir, balsam

Fir, commercial white Hemlock, eastern Poplar, yellow Chestnut Maple, red and silver. Pine, white, sugar

Pine, western white. .. Pine, western yellow. Douglas fir (Rocky

Mountain) Elm, slippery and white

.

Gum, red, black, cotton..

Pine, Norway

Spruce, red, white,

Sitka Hemlock, western. Ash, white Oak, red and white . Douglas fir (Coast type), dense

Pine, southern yellow,

I Select

I Common.

Select Common.

i Select [Common.

Select Common.

(Select (Common.

I Select Common.

Beech...

Birch, yellow and sweet.. Douglas fir (Coast type) . Pine, southern yellow... Maple, sugar and black.

.

[Select [Common.

Select Common.

Ratio of length to least dimension

11 or

Lhs.t

in.^ 700

560

700

660

750

600

800

640

900

720

1, 000 800

1, 285

1,060

1, 175 880

14

Lbs./

in.^ 668

644

678

549

718

583

774 627

872

706

967

783

1,222

1,025

1, 127

861

17

Lbs./

in.^ 630

624

653

536

680

564

742 610

839

688

927 762

1,147 985

1,070

837

20

Lbs./

in.i 566 492

611

515

617

532

688

582

783 660

728

1,023 913

975

796

23

Lbs./

in.^ 466 440

26 29 32 35

Lbs./

in.^ ^365

446

430

405

486

476

567

549

Lbs./

in.^ 293

^358

326

391

456

521

^521

Lbs./

in.^ 241

294

321

375

401

428

428

40 60

Lbs./

in.^ 201

224

268

313

336

358

358

Lbs./

in.^ 154

188

171

206

240

257

274

274

Lbs./

In.^99

121

110

132

153

164

175

175

Note.—The values in this table are obtained from the values given in Table 7, using the formulas for intermediate and long columns given in Part HI. The grouping of the species, in order to save space and make the table simple, has resulted in somewhat lower stresses for some of the little-used species than can

be safely applied. This table may be shortened by omitting some of the species, or more detail may be added if desired. There are available at the Forest Products Laboratory, Madison, Wis., tables which

give the stresses based on the factors of safety approved by the committee for each species and each ratio

of ^from 11 to 50. Source: IJ. S. Department of Agriculture, Forest Service, Forest Products Laboratory, Madison, Wis.,

Jan. 16, 1926. Note:—Masonry stresses: Requirements for quality of masonry materials,

workmanship, and the allowable stresses in masonry structures have already

been approved by the Building Code Committee. They appear in the committee's report on Recommended Minimum Requirements for Masonry Wall

Construction,
Part III.—APPENDIX

Par. 1. Purpose.

The Appendix consists of explanatory matter referring to Part II, and is a vital part of this report. The Building Code Committee

believes that every building code -should be printed with an appen»

dix containing sufficient explanation of the code requirements to make them easily understandable, and to assist building officials in

their enforcement. Such information on good practice as can not

be obtained elsewhere in concise form also may be presented. The

Appendix to this report is intended to clarify certain questions which

may be raised concerning the recommended requirements and to

assist in their adaptation for local use. Par. 2. Sources of Information.

The committee has endeavored in all cases to base its recommendations on the latest and most authoritative information. Where refer- ences are made in Part II to the requirements or standards of national

professional societies these references apply to the latest published

form of such requirements and standards and it is expected that those

writing or revising building codes will refer in similar manner to the

latest material then available.

Part II requirements for timber are based on investigations by the

U. S. Forest Products Laboratory, Forest Service, Department of

Agriculture, supplemented and checked by various national and local

associations concerned with the utilization of timber. The Forest

Products Laboratory was established in 1910. Since then it has

become the best equipped and most dependable source of information on the use of wood for building and other purposes. It conducts annually, in accordance with a carefully considered program,

a great number of experiments with timber, to develop the full structural possibilities of this material.

In preparing recommended stress requirements for steel the Building Code Committee has studied, among other sources of information, the report of a committee of architects and engineers retained

by the American Institute of Steel Construction to investigate and

recommend steel construction requirements for building codes, and

the report, submitted at the January, 1925, meeting of the American

Society of Civil Engineers by a Special Committee on Stresses in

Structural Steel, appointed in August, 1922. Careful consideration

also has been given to the specifications for railway and highway

9070°—26f ^3 11
12 KECOMMEITDED BUILDING CODE EEQUIREMENTS

bridges of the American Eailway Engineering Association and the

American Society of Civil Engineers as well as to other standards in

common use. The comparison with specification for bridges is believed entirely proper considering that the stresses therein specified are for equivalent static forces, full allowance having been made for

impact. No two specifications of those authorities are in precise

agreement and the committee can not accept in its entirety any one. Most of the worldng stresses recommended are to be found in one

or more existing specifications and where new suggestions are offered

they are for the purpose of simplification.

The recommended stresses for reinforced concrete are based mainly on the 1924 report of the Joint Committee on Standard Specifications

for Concrete and Keinforced Concrete. This report is the outcome

of a long series of investigations under the auspices of interested

national organizations. The first joint committee, representing the

American Society of Civil Engineers, the American Society for Testing Materials, the American Railway Engineering Association, and

the Portland Cement Association, was organized in 1904, to prepare

a code of practice for concrete and reinforced concrete. The American Concrete Institute became a member in 1915. Progress reports were made in 1909 and 1912 and a final report in 1916.

The present joint committee, representing the same organizations, was formed in February, 1920, and presented a report under date of

August 14, 1924.^ Its membership, and the standing of the agencies

concerned not only insured access to every valuable source of information on the subject, but also made it possible to institute

further investigations of questions not well developed, and to test the practicability of its recommendations in the field before their

adoption.

Par, 3. Influence of Building Inspection.

Throughout the report the quality of materials to be used has been

considered in conjunction with the working stresses allowed for such

materials. There are, however, many other factors influencing the

safety of structures, which are partly controllable by code requirements, but are in great part dependent on the knowledge and integrity of those in charge of construction.

Skill in design and construction is much more important with

the new types of skeleton-framed buildings than in those with

masonry bearing walls. Masonry buildings have only a moderate

area of openings and loads are readily and uniformly distributed

down and through the walls. In steel and reinforced concrete

buildings, on the other hand, modern practice permits excessively

1 Copies of the report may be obtained from the American Society for Testing Materials, 1315 Spruce

Street, Philadelphia, Pa., or the American Concrete Institute, 1807 East Grand Boulevard, Detroit, Mich.
APPENDIX 13

large wall openings with resulting slender supporting members, the

economical design of which introduces complicated problems. The

new structures do not depend on gravity alone for stability and

rigidity, but also upon interaction of beams, girders, and columns,

and the attachment of these last to footings which must be specially

adapted to support excessive foundation loads. Success in designing

such buildings demands extensive and intimate knowledge of

materials, of the mathematics by which the sizes of columns and

beams are computed, and the effective use of reinforcement in concrete. Years of training and preliminary experience under competent direction are essential before the responsibility for design and

construction of the modern large building can safely be undertaken.

A general increase in working stresses, which narrow the margins of

protection for buildings poorly designed or built, makes competency

even more necessary.

Popular opinion has been slow to recognize that the new types of

building construction are work for specialists. Building officials,

recognizing the need for greater care, have extended the scope of

their efforts and have done much to improve building construction

and promote safety. With present enforcement methods, however,

and with the facilities usually provided for municipal inspection, the

possibilities of such inspection are limited. It is questionable, fur- thermore, whether the extension of inspection service would materially improve the situation, since it would only tend to transfer

responsibility from those executing the work to the supervising

authorities.

Discussions of the tentative report reflect very strongly the opinion

that building officials should be given authority to distinguish in some manner whether applicants for permits are generally competent

to plan and conduct building operations, and to refuse permits to those judged incompetent. Such an arrangement might be, and

probably would be, subject to abuse in some cases, but the same

objection could be raised to practically all forms of licensing and

regulation of professional and trade activities. There is legal precedent in full measure for such an arrangement, and it is understood

in fact that measures partially effective in fixing responsibility are already in force in some cities. A full discussion of the subject is without the scope of this report.

It is strongly recommended, however, that in connection with increase

of stresses measures be taken by which owners will be compelled to employ competent architectural or engineering service throughout

each important building operation.

Such measures, compelling careful design and construction, would

bear dividends not only in increased safety but in economy. The

issue between 16,000 and 18,000 pounds per square inch stresses for

•c
14 EECOMMENDED BUILDIETG CODE REQUIEEMENTS

steel, with its possible marked economy in the use of material, is not

so much a question of what the material will safely bear, as whether

the designer is able to predict accurately the stresses which will actually occur and build the structure to resist them. Except for cases where ability to do this can be at least presumptively established, it seems unwise to suggest any increases over the customary

stresses. Competent supervision should also control workmanship

and quality of materials. This is particularly necessary with

reinforced concrete construction, since the major material of the

combination is made in the afield from a number of constituents

variable in quality and quantity, and often by workmen who are more

concerned with rapid production and easy handling than v/ith

accuracy of proportions and care in mixing and placing.

It is recommended that building officials, in supervising the

activities of privately employed inspectors, utilize the standards for

materials and workmanship outlined in the 1924 Report of the Joint

Committee on Standard Specifications for Concrete and Reinforced

Concrete, and the American Society for Testing Materials Tentative

Rules for Inspection of Concrete and Reinforced Concrete Work,

serial designation C 44-22 T. The American Concrete Institute also has prepared a report on building regulations for concrete and reinforced concrete construction which should assist considerably in

establishing standards of workmanship consistent with the stresses recommended in Part II. Directions for good workmanship on steel frame buildings may be

found in the code of standard practice of the American Institute of Steel Construction, and much valuable information on the proper use

of lumber in the publications of the National Lumber Manufacturers^

Association.

It also should be remembered that the Building Code Committee,

in its Report on Minimum Live Loads Allowable for Use in Design of

Buildings, has considerably reduced the prevailing live load assumptions with a view to making them approximately the maximum

reasonably probable in the projected use of a building and confining the factor of safety to the choice of stresses. If this policy is coupled with an increase in design stresses great care is necessary in

every department of the work to secure safety. Par. 4, Variations in Building Code Requirements.

Tabulations summarizing the requirements of several existing

building codes for working stresses in timber and reinforced concrete have been prepared by the National Lumber Manufacturers’ Association and the Portland Cement Association, respectively. These

are presented here to illustrate the variations in present practice and

the need of greater uniformity in code requirements.
APPENDIX 15

Table 5.—Summary of analysis of wood stress requirements of 117 codes, National

Lumber Manufacturers’ Association BENDING

Extreme fiber stress Horizontal shear stress

Species of timber Codes

giving

stresses Codes

with no ' stresses Range of require- ments * Codes

giving

stresses Codes

with no

stresses Range of require- ments

Douglas fir: Dense 61 56

Lbs.lin.^ 1,800- 800 60 57 i6s./m.2

750-80 Sound 71 96 1, 600-1, 000 18 99 350-85 Hemlock:

Eastern 68 49 1,300- 600 62 55 250-40 Western 30 87 1,500- 600 32 85 160-40 Oak 96 21 1,800- 600 96 21 240-80 Norway pine 46 ‘ 71 1,250- 700 43 74 150-40 Southern yellow pine: Dense.. 100 17 1,800- 500 101 16 400-70 Sound 59 58 1,500- 900 58 59 350-70 Spruce 85 32 1,350- 250 86 31 500-50 Tamarack 9 106 1,200- 900 9 108 170-95 White pine 81 36 1,500- 250 77 40 500-40 COMPRESSION

Parallel to grain “short columns” Perpendicular to grain

Species of timber Codes

giving

stresses

Codes with no

stresses Range of require- ments

Codes

giving

stresses Codes

with no

stresses Range of require- ments

Douglas fir: Dense - 58 59 Lbs.linJ

1, 600-100 60 57

Lbs.lin.^ 800-200 Sound - 15 102 1, 500-100 18 99 400-200 Hemlock:

Eastern 62 55 1, 200- 80 70 47 1, 000-150 Western 31 86 1, 500- 80 26 91 500-150 Oak 99 18 1, 500-100 99 18 1, 000-250 Norway pine 67 50 1, 000-100 43 74 400-100 Southern yellow pine: Dense 98 19 1, 800-820 102 15 1, 500-250 Sound 57 60 1, 200-750 62 55 800-170 Spruce 83 34 3, 000-600 88 29 1, 000-180 Tamarack 9 108 1, 000-750 9 108 350-220 White pine 81 36 1, 100- 80 80 37 1, 000-150 Note.—Dense and sound in the foregoing includes values for No. 1 and No. 2 structural, longleaf, shortleaf, etc., respectively.

c
Table

6.

—

Analysis

of

concrete

stress

requirements

of

58

building

codes

from

data

furnished

by

the

Portland

Cement

Association,

September,

1925

16 EECOMMENDBD BUILDING CODE HEQUIBEMENTS

Bond

Deformed bars

41

6.67 150 5.0 50

5.67

98.00 5.0

Plain

bars 48 4.0 90

3.33 50 3.

83

73. 97 4.0

Shear

Punching shear

21 6.0 150 5.0 100 5.

00

120. 00

Diagonal

tension

on

beams

and

slabs With

full

web

reinforcement

33 6.0 150 6.0 65

5.67

124.

83

6.0 to

12.

0

Without

web

reinforcement

50 2.5 75 2.0 30

2.

06

45.24

2.0 to

3.0

Compression

in

extreme

fiber

Under- side

of

beams over

support 16

37.5 85037.5 650 37. 50

750.00 45.0

Ordinary case }

52^

37.5 80032.5 500 34. 05

661.

11

40.0

Direct

compression

on

columns With

spiral

reinforcement

Addi- tional credit given

for

spirals

CO * It III 1 1 o; o It III 1 t

! I 1 ! ! ! ! lO II III 1 1 4 1 1

On concrete alone

44

35.5 900

25.0 500 32. 87

Variable.

Without spiral rein- force- ment: on

con- crete alone

47

29.0 650

22.5 400

24.00 502.

30

20.0

2'C

.t) c3 OQ>0 r-^ O . O (N 00 o cs . . M <N CO <M

<D

: p<

I fl

" ca ca.9 ® o3 O+i c3 w o' >,_ O' •sI^ iio.|i§i P O Q. ^ Di Q,^ o acsfl-isflflo !^a)ccooa>-. a -qOPlOOg

!2; S p

II§1 : .t

Sirt ta ® §.§ °i®O

tiCQ =3 S-^'a ^.9§
APPENDIX 17

Until recently code requirements for structural steel stresses have

been uniformly based on a maximum of 16,000 pounds per square

inch in tension and compression, though the stresses allowed for

rivets varied considerably and a number of column formulas v/ere in

use. Efforts by those interested in more economical utilization of

steel have changed this somewhat, and existing practice among the

larger cities is divided between the older scale of stresses and a scale

based on a maximum stress of 18,000 pounds per square inch. Par. 5. Working Stresses in Plain Concrete.

It will be noted that the percentages of ultimate strength recom- mended in this report for plain concrete used in reinforced concrete

building operations contemplate working stresses somewhat larger

than are provided for in the Committee’s Keport on Masonry Wall

Construction. This is in consideration of the better control over

materials, proportioning, and workmanship usually obtaining in

operations involving reinforced concrete. The working stress on the

extreme fiber of the compressive face of a concrete beam is a theoretical stress. It has not the same significance as a direct axial

stress. Failure of concrete beams in compression is rare. In the Committee’s Keport on Kecommended Minimum Requirements for Masonry Wall Construction, plain concrete was defined as concrete containing not more than two-tenths of 1 per cent of

reinforcement.

Par. 6. Working Stresses in Reinforced Concrete.

1. The method of making and testing concrete test pieces is described in Standards C31-21 and C39-25 of the American Society

for Testing Materials. The test specimens are cylinders 6 inches in

diameter and 12 inches high, aged 28 days. The average strength

of five test pieces is required.

2. Concrete materials vary so widely in the United States that it

is inexpedient to fix stresses by exact values. Where coarse and

fine aggregates are prevailingly uniform in quality, experimentation

will determine quite closely the strengths obtainable with different

mixes. In such cases the requirements of Part II, section 2, should

be replaced by the strengths determined by tests on the locally used

materials.

This procedure recently was followed by the District of Columbia

Building Code Committee in revising certain portions of the District

Building Code. Tests were made for the committee by the Bureau

of Standards. Two consistencies of concrete were used; that is, (1)

concrete defined by a cone slump of 6 inches, and (2) concrete defined

by a cone slump of 9 inches. Samples of the local materials, which

are quite uniform in nature, were mixed in several proportions in

these consistencies and the compressive strength determined at the
18 RECOMMENDED BUILDING CODE REQUIREMENTS

age of 28 days. The District of Columbia Committee then cited a

table of assumed ultimate compressive strengths at 28 days for concrete of these two consistencies and indicated the mixtures which

would be accepted as giving these strengths. To provide for field conditions the assumed values in the code were fixed at 60 per cent

of the ultimate strengths shown by laboratory tests. Such a procedure determines the quality and makes possible safe

and efficient designs, the stresses for which will certainly vary between the washed and accurately prepared aggregates of the larger centers and the poor local materials in the less favored regions.

Those expecting to invest large sums in important buildings should

secure skilled designers and make preliminary tests, and should sur- round the construction with technical control in manufacture and

placing of concrete.

3. The descriptive expressions “plastic mass concrete’^ and “reinforced concrete mixed moderately wet,’^ upon which the stresses in Part II, section 2-6 are based, are best interpreted in terms of slump.

The slump test determines the relative plasticity of fresh concrete by measuring its subsidence from the height of a truncated 12-inch cone after removal of the surrounding form. A standard procedure

for making the slump test is described in the Tentative Test for

Consistency of Portland-Cement Concrete for Pavements or Pavement Base, serial designation: D 138-25T, of the American Society

for Testing Materials. In general, plastic mass concrete will have

a slump of not more than 3 to 5 inches, and reinforced concrete mixed moderately wet not more than 6 to 8 inches.

4. Columns and beams, etc., in construction are larger and less carefully made than test pieces and will show lower strengths.

5. The average strength of three concrete test cylinders should

not fall below a minimum of at least 1,600 pounds per square inch.

The maximum to be expected from rich mixes carefully made will

not usually exceed 3,500 and the committee, for building-code purposes, has limited the assumed maximum strength to 3,000 pounds

per square inch. This implies a range of working stresses from 400

to 875 pounds per square inch for plain concrete, and 640 to 1,200

pounds per square inch in the extreme fiber of compression faces. These stresses suggested in the tentative report were criticized as somewhat too liberal. Careful investigation, however, discloses no

cases in which reinforced concrete beams have failed by compression

in the extreme fiber at the top of the beam.

High stresses for concrete are to be used only when tests demonstrate the increased strength of the material. Such strength results

from clean and tested materials. Concrete should be mixed at least one minute in a batch mixer with devices for water control and time

control. The concrete should be placed in uniform horizontal layers,
APPENDIX 19

sliced to avoid segregation, and protected, beginning after placing,

and lasting for a time depending upon exposure (at least seven days

when exposed to weather). The wetness of the mix necessary for

various classes of construction will exert a preponderating influence on the strength.

It is well established that concrete of the best quality in respect

to strength and durability is produced with the minimum amount

of mixing water that is practicable. The table of assumed compressive strength in Part II, section 2-6, indicates the minimum

ultimate strength in compression which may be expected at 28 days,

with The amounts of water specified, and when cured and tested as

specified by the joint committee’s report. When mixing water is increased the cement content should also be increased to preserve a constant water-cement ratio appropriate for the desired strength.

Water or moisture contained in the aggregates must be included

in determining the ratio of water to cement.

The Committee on Building Laws and Specifications of the American Concrete Institute has adopted the water-cement ratio as a means of controlling the strength of concrete. No specification of

mixtures and aggregates is made.

The proportions in Part II, section 2-6, are specified in terms of

unit volume of packed cement to the sum of the separate volumes of

fine and coarse aggregate. The volume of the mixed aggregate is approximately 0.8 of the sum of the volumes of the separated aggregates. Thus, in the case of premixed aggregate, it must be remembered that a mix of 1:2:4—that is, 1:6 expressed as the sum of the

volumes of separated aggregates—is approximately only a ratio of

1:4:8 expressed as the value of the premixed aggregate. The amount

of coarse material should be limited to avoid harshness in placing or honeycombing in the structure. High strengths in laboratory tests are sometimes obtained by high proportions of cement to aggregate

that does not contain enough fine material.

Variations occur in cement and aggregates. Fine aggregate is affected greatly by moisture; damp sand may increase in volume 30

per cent over dry and will produce a too wet mix or a nonuniform

concrete. Fine soluble dust in the aggregate causes a porous concrete

that shrinks; such concrete will absorb water and will be likely to deteriorate when exposed.

Fine material, such as silt or dust, even though inorganic, v/hen in concrete mixed with an excess of water, will rise and form fill planes

in a structure where subsequent disintegration will likely take place

because of porous concrete, especially after the action of freezing and

thawing. A recent survey of disintegrated concrete structures charges the causes of these defects to excess water in the mix. A

recent rule on heavy construction is that the mix shall be of such

9070°—26t 4

c-
20 EECOMMENDED BUILDING CODE EEQUIEEMENTS

consistency that workmen will not sink below their ankles in the

freshly placed concrete. The water-cement ratio specified should, of course, include the

amount of water present in the aggregate. Sand and gravel coming

directly from sand and gravel washing plants will contain an amount

of water which, if not accounted for, would upset the required watercement ratio. The varying amount of water in the sand also affects the volume occupied by a given weight of sand. A fine sand may

bulk to increase of 40 per cent in the presence of 8 per cent of water;

medium sand, 28 per cent; coarse sand, 24 per cent. The amount of

water in the sand can be determined by a test in advance. Recent

advanced practice measures the sand in an excess of water; that is, by inundation in order to obtain a more uniform concrete. Concrete shrinks mostly during the first 10 hours. Therefore,

protection against drying or freezing should begin as soon as possible

after placing so that the concrete may attain strength to withstand

shrinkage strains before these begin. The requirement in Part II, section 2-5, that the temperature of freshly placed concrete be

maintained at not less than 50° F. for at least five days, is to prevent damage by frost and to allov/ the concrete to attain sufficient

strength to withstand construction loads safely. Costly experience

with failures of concrete structures has demonstrated that such a requirement is essential for safety. Tests . —All tests' of materials called for in Part II, or ordered by the

authorities, should be made in accordance with the standard method

of testing covering the particular material under consideration

prescribed by the American Society for Testing Materials.

Tests should be made on all materials entering into concrete or

reinforced concrete construction when in the opinion of the building

official there is any doubt as to its suitability for the purpose.

The building official or his authorized representative should have

the right to require tests of the concrete from time to time during the

progress of the work to determine whether the materials and methods

in use are such as to produce concrete of the necessary quality, or

at any other time when in their opinion there is any doubt as to the

quality of the concrete being produced. Specimens for such tests should be taken at the place where concrete is being deposited, and

cured and tested in accordance with the American Society for Testing Materials Standards as required above.

All such tests should be made by competent persons approved by

the building official, and copies of the results kept on file in the office of the building official for a period of at least two years after the

construction work is completed.

If for any reason, in the opinion of the building official or his

authorized representative, the testing of a completed structure is
APPENDIX 21

necessary, the member or portion of the structure should be given a superimposed load equal to one and one-half times the live load plus

one-half the dead load. This load should be left in position for a period of 24 hours before removal. The structure should be considered to have failed under the test if within the 24 hours after the

removal of the load the floor system fails to recover 75 per cent of

the maximum deflection shown during the 24 hours while under

load. In cases where failure is declared the building official should

have the authority to order the defective construction removed.

If permission is sought to use systems of construction or reinforcement, the safety of which can not be demonstrated mathematically,

the question should be settled by testing the completed structure in

the manner described above.

6. Reinforced Columns.—When reinforcement is used to in- crease compressive strength of concrete, the allowable stress specified

for plain concrete may be increased by a factor to be applied to col- umns whose length between supports does not exceed 10 times the

least dimension. This increase is defined in the joint committee

report, previously mentioned, and is provided for in Part II, section

2-9. (See also Appendix, par. 2.)

Part II specifies working stresses for materials. Methods of

calculating the strength of materials in combination, such as slabs,

beams, flat slabs, columns, etc., are to be adopted as defined in the

Report of the Joint Committee on Standard Specifications for

Concrete and Reinforced Concrete, representing the most acceptable

practice. Stresses on the concrete cores of reinforced columns should be

considered in the light of test information. Spirals raise the ulti- mate crushing resistance and increase the toughness of columns but

they have no substantial effect in increasing the elastic limit. That

is to say the concrete in the core under vertical stress is not materially

restrained from flowing sidewise. Therefore, the elastic limit of the

column should be the basis of judgment, especially because there is a

plastic or time flow of the concrete under stress, by which the concrete flows away from the load and throws added stress on the steel. It will be noted that the assumed strength of concrete in compression is limited in Part II, section 2-7, to 3,000 pounds per square

inch.

The proportional elastic limit of concrete—that is, the stress beyond

which deformation shows a marked increase under load—will vary

with the age and stiffness of the concrete. While the available

scientific information is conflicting, reliable tests show a range of

elastic limit from one-third to two-thirds the ultimate strength in

the case of well made and aged concrete.

7. A plain concrete beam with no shear reinforcement fails (when

it fails in shear) by formation of cracks at approximately 45° with

c
22 RECOMMENDED BUILDING CODE REQUIREMENTS

the axis of the beam, caused by tensional stress on an internal in- clined plane. When this tensile stress exceeds 0.02 of the ultimate

compressive strength, the factor of safety is too greatly reduced, and

steel reinforcement is necessary. The rules of the joint committee

provide for such reinforcement.

Practice has fixed the custom of allowing a stress of 40 pounds

per square inch on concrete in shear and the remainder on the shear

reinforcement.

Inclined rods tend to retard the formation of cracks. The shearing

stress may then be increased. The greater the number of inclined

rods the greater the toughness of the beam.

8. The joint committee report allowed 18,000 pounds per square

inch on high-carbon steels. In this it follows the practice of a score

of large cities, in 10 of which a tensile stress of 20,000 pounds per

square inch is allowed in the reinforcement in the case of hard grade

steel. In indoor structures, the steel stresses may be raised to recognize

the greater factor of safety against exceeding the elastic limit of

high-carbon steel, provided, however, that allowable bond stresses are not exceeded. A favorable consideration is the fact that over- loads which would produce a permanent open crack in structures

reinforced with mild steel will not do so when reinforced with highcarbon steel, because the steel, still elastic, draws the crack together.

Keinforcing bars rolled from either billet steel or steel rails are suitable

reinforcement. Some question has been raised in past years as to the dependability of bars from the latter source and in 1922 extensive

tests were made at the Bureau of Standards to determine the suitability of this material for general reinforcement purposes. The committee, after careful consideration of these and other recent tests and of experience with their use, is convinced that if such material

meets the American Society for Testing Materials Standard Specifications for Rail Steel Concrete Reinforcement Bars, serial designation: A 16-14, it is suitable for concrete reinforcement. The joint

committee report also supports this conclusion. No question of public safety is involved.

9. The Building Code Committee’s recommendations for stresses

differ in certain respects from a report on recommended code requirements for concrete and reinforced concrete construction, now under

discussion by the American Concrete Institute.

1. The latter report allows only 16,000 pounds per square inch

for reinforcing bars of structural grade, whereas the Building

Code Committee has recommended that 18,000 be allowed for

all grades of steel passing standard specifications.

2. The American Concrete Institute report would allow rail steel reinforcing bars larger than three-fourths inch to be used
APPENDIX 23

only where bars are not to be bent. The Building Code Committee does not distinguish in size.

3. The American Concrete Institute Eeport, section D-1,

fixes the amount of water in United States gallons per sack of

cement against the expected ultimate strength in design as fol- lows:

, U. S. gallons Ultimate strength: per sack of cement 1.500 8H

2.000 7M

2.500 QH

3.000 5M

The Building Code Committee has adopted the water-cement

ratio as a simple and reliable method for controlling the strength of concrete. The American Concrete Institute report is practically in agreement

as to stresses and design formulas with the report of the joint committee. Except for the variations necessary because of the wide

range of conditions to which building code requirements apply, the

requirements of Part II reflect the recommended practice of both

these organizations.

Par. 7. Design of Web Reinforcement.^

The equations for designing web reinforcement as given in the 1924

report of the joint committee are

/ A

V equals 0.02 /'« plus ^

for angles a of 45° to 90^ and

f A

V equals 0.02 /'c plus^^^j^ (sin a plus cos a) for angles

a less than 45°

(31)

(32)

where

V equals shearing stress, fc equals compressive strength of concrete,

/v equals tensile stress in web reinforcement.

Ay equals sectional area of bent-up bar or stirrup,

h equals thickness of web,,

s equals distance from stirrup to stirrup or from support to first stirrup or bent-up bar. s is measured in the direction of the

axis of the beam,

a equals angle of web member with longitudinal axis. Although these equations seem unfamilar they are in fact quite

similar to those in ordinary use. The similarity and the differences

may be brought out by reducing them to a form which involves the

ratio of the sectional area (or volume) of web reinforcement to the

sectional area (or volume) of the concrete which is reinforced by it. The term s sin a of equation (31) is the distance from stirrup to stir2 Discussion supplied by Dr. W. A. Slater, of the Bureau of Standards. c
24 EECOMMENDED BUILDING CODE EEQUIEEMENTS

rup measured at right angles to the direction of the stirrup. The

A . term 7 — — of equation (31) then becomes the ratio of web reinos sm a

^

forcement. Call this ratio r. Equation (31) then becomes

V equals 0.02/'c plus r fy (31')

and equation 32 becomes

V equals 0.02/'c plus rjy (sin^ a plus sin a cos a) (32')

Figure 1 has been prepared for the purpose of showing relative

values of a given quantity of web reinforcement in resisting the tensile stresses set up by shear as stated by these equations.
APPENDIX 25

Equation (31) and Figure 1 bring out the fact that, as specified, a given quantity of steel is equally effective, in reinforcing for shear

for all angles from 45° to 90°. This equality is not peculiar to these specifications. Figure 1 shows that by the former joint committee’s recommendations nearly

the same percentage of reinforcement is required if the stirrups are placed at 45° as if they are placed vertically.

That a given percentage of steel was equally effective as web reinforcement whether placed at 45° or 90° was shown clearly by the

tests made in the concrete ship investigations of the Emergency

Fleet Corporation. Other angles were not used in these tests. Equation (32) is a general equation for shear, in terms of the angle a and the other variables given. It is correct for all angles a in so

far as the assumptions are correct on which the equation is based.

For angles of 45° and 90° the values of sheering stress v given by the

term

(sin a plus cos d)

are the same as are found by solving for v the familiar expression

that is

^ .vs 1 vl)id8

• V 6(JU8>ls G(][119.1s

V equals

.Ay/v hs For these two angles the tests made in the concrete ship investigation confirmed the results of the analyses with considerable accuracy,

but test results were not available for angles less than 45° or between

45° and 90°. For angles less than 45° the values given by equation

(32) are both conservative and reasonable and the equation is recom- mended for use up to that angle. For angles between 45° and 90°

equation (31) is recommended because the less conservative results

there indicated by equation (32) have not been checked by test results and because it is simpler to use. Except for the greater simplicity of equation (31) it will probably make little difference which

equation is used, since web reinforcement will seldom be used with

angles between 45° and 90°, and since for these two angles the two

equations give identical results. Figure 2 shows the relation of these two equations v/hen r equals

0.005 (0.5 per cent web reinforcement), /V equals 16,000 pounds

per square inch, and f'c equals 2,000 pounds per square inch.

A technologic paper of the Bureau of Standards which is under

preparation gives the derivation of the equations and gives the

results of extensive tests of the shearing strength of reinforced concrete beams.

Q
26 EECOMMENDED BUILDING CODE EEQUIEEMENTS

The formulas already stated give the area of reinforcement required

without reference to the spacings. Neither formula includes a limit

for the spacing, but such a limit is necessary in order to avoid having

the web members placed so far apart as to permit diagonal cracks to

rise nearly to the top of the beam without crossing a web member.

The formula for maximum permissible spacing, s equals , seems

to give reasonable values. Where shearing stresses greater than

0.06 f'c are used it seems well to limit the maximum spacing to twothirds of these values.
APPENDIX 27

Par. 8. Cast-Iron Columns.

There is evidence that prevailing code requirements governing

the use of cast-iron columns have failed to keep pace with developments in the material and its handling, and that as a result this

type of column either is unduly expensive to users or fails of its full economic utilization.

1. There is, as in the case of other materials, considerable variety

in code restrictions. At least seven column formulas are in use, some based on a relation of length to radius of gyration and some

on the ratio of length to least dimension.

The formula recommended in Part II is that most used. It gives

values about midway of the maximum range, and the allowable

stresses decrease somewhat more slowly with increasing y. The

few test data available indicate a factor of safety of two or more for columns exceeding the prescribed limits of

2. Many codes do not give limits for Others set maximum

limits of from 60 to 95, these values applying in both cases to the same formula recommended in Part II. In view of the general

improvement in casting practice in recent years and the quality of metal required by Part II, it is believed the increase to 120 is suffi- ciently conservative. It should permit considerable extensions in

the use of such columns for high top stories or where loads are nominal.

3. All cast-iron columns should have three-eighths inch test holes

drilled in the shaft 2 feet from each end, so that the thickness of the

walls may easily be determined.

4. A number of codes limit the total height of buildings in which

cast-iron columns are used, presumably on the grounds that neither

the columns nor their connections are particularly adapted to meet

the stresses caused in high buildings by wind pressure. The limits of

height prescribed vary from 6 stories, 75 feet, or one and threefourths of the least width of the building, to 10 stories, 100 feet, or

three times the least width of the building. It is believed no eco- nomic hardships will result if the limit is placed at 100 feet, or twice

the least width of the building. Cast-iron columns usually are

associated with masonry bearing walls, and since these latter have

little inherent stability a certain amount of rigidity is necessary in

interior construction.

5. It is quite customary to require that cast-iron columns have a minimum lateral dimension of 5 inches and a minimum wall thick- ness of three-fourths inch. Strictly enforced, this prevents the use

of modern cast-iron pipe machinery and practice for the production
28 EECOMMENDED BUILDING CODE KEQUIEEMENTS

of columns, and by making columns more or less special as compared

to the volume of pipe production increases their cost. Pipe machinery is usually adjusted to a wall thickness in 4-inch sizes of from

0.4 to 0.5 inch, and overall diameters are thus from 4.8 to 5.0 inches

plus. It is obvious that if the same amount of quality of metal is present in a given column, that having the thinner wall will have

the greatest radius of gyration and, within reasonable limits, the

greatest stability. A minimum wall thickness of 0.4 inch, or onetwelfth of minimum diameter, and a minimum lateral dimension of

4.75 inches, therefore, are suggested as combining safety with

economy.

6. The few codes which specify a tolerance in wall thickness put

it at one-eighth inch to one-fourth the wall thickness. In view of

improvements in casting practice such a wide tolerance is no longer

necessary, and in consideration of the recommended reduction in

wall thickness requirements it is undesirable, especially since variation on one side of a column often is accompanied by a corresponding

variation on the opposite side and the member, therefore, becomes

doubly eccentric in its reaction to stress. A tolerance of not more

than 0.08 inch is believed advisable. It is not too strict for columns

cast vertically or by the centrifugal processes. In view of standards suggested in other respects the acceptance

of imperfect columns to be used under slightly lov/er stresses is not

recommended. The extent and effect of imperfections can be determined only approximately, and there is the possibility that confusion will arise in the field as to the use of such columns.

Columns should not be coated or painted previous to their in- spection, as this is apt to prevent detection of defects. The American Society for Testing Materials Standard Specification for Cast-Iron Pipe and Special Castings, serial designation:

A 44-04, requires that they shall be made of cast iron of good quality

and of such character as shall make the metal of the castings strong,

tough, and of even grain, and soft enough to satisfactorily admit of

drilling and cutting. The metal shall be made without any admixture of cinder iron or other inferior metal, and shall be remelted in a cupola or air furnace. The castings shall be smooth, free from scales,

lumps, blisters, sand holes and defects of every nature which unfit

them for the use for which they are intended. No plugging or fill- ing will be allowed. Par. 9. Working Stresses in Structural Steel. The question of the proper working stresses for steel to be used in

the design of buildings resolves itself into two independent considerations: (1) I^Tiat is the proper basic stress, with reference to the

strength of the steel in common use? (2) What are the relative

values of the different forms of stress?
APPENDIX 29

(1) Basic Stress.—The basic stress is generally considered to be

a direct axial stress in tension. To decide upon this it is necessary

only to determine what grade of metal will actually be used and what

factor of safety should be applied to the specified strength of this

material.

Quality qf steel . —With the first introduction of rolled steel on a commercial basis specifications commonly called for a “medium

grade” having an ultimate strength in test specimens of from 60,000

to 70,000 pounds per square inch. During the last 20 years the use

of this grade has given place almost entirely to the grade known as

“structural,” having an ultimate strength between 55,000 and

65,000 pounds per square inch. This strength is now called for by

the standard specifications of the American Society for Testing

Materials, the American Kailway Engineering Association, and the

American Society of Civil Engineers.

The American Societ}^ for Testing Materials recognizes two grades

of steel which may be used for building purposes. The first, termed

“structural steel for bridges,” does not include metal made by the

Bessemer process. The second, termed “structural steel for buildings,” admits Bessemer steel on a par with open-hearth steel if the

physical requirements for test specimens are met. It is understood that in this country the use of Bessemer steel for structural

shapes has been practically discontinued; that the mills are accus- tomed to furnish and engineers and fabricators to require only openhearth steel; and that as a consequence practically all steel supplied

under specifications complies with the American Society for Testing

Materials standard specification for bridge steel. In deciding upon

working stresses, therefore, it is logical to assume that when steel is ordered to specification this is the grade which will be used.

The original choice of 16,000 pounds per square inch as a basic

stress appears to have been predicated on the idea that this gave a “factor of safety” of four, the ultimate strength of steel then in use averaging about 64,000 pounds per square inch. If so, its original

significance has long since disappeared through the general acceptance

of lower ultimate limits. The unanimity with which the 16,000

pounds per square inch has been accepted, not only in this country

but in Canada and Australia, and the persistence with which it has

been retained are remarkable. Up to the last three or four years

there has been little inclination to increase stresses for building

construction above this base, but recently there has been a move- ment to secure the adoption of higher stresses in building codes. One

of the arguments advanced for this increase is, that when the 16,000

pounds per square inch stress was first introduced the use of Bessemer

steel was quite general, but that now the open-hearth product has

practically superseded Bessemer in the rolled material used for

Q
30 RECOMMENDED BUILDING CODE REQUIREMENTS

structural work, and that the open-hearth product is much more

uniform and reliable. Structural steel according to the American Society for Testing

Materials Specifications must have a yield point of at least 50 per

cent of the ultimate strength, but in no case less than 30,000 pounds

per square inch. If a maximum tensile stress of 18,000 pounds per

square inch is adopted, as recommended in Part II, it affords a

safety factor approaching 2.0 based upon the average yield point

for test pieces as determined by recent extensive investigations by the

American Society of Civil Engineers;^ or a little over 3.0 referred to the

specified minimum ultimate strength. Factor of safety. —Neither of these factors, however, has a very

definite meaning. The yield point is the intensity of stress at which

the metal begins to undergo an appreciable permanent change in

shape, but it is not by any means a point of failure in tension. In

fact, since the yielding is in the direction of greatest stress, it tends

to Readjust the structure to its loads and, by correcting imperfections

of workmanship or fit, to bring parts under less stress to the aid of

those which have reached the yield point.

Neither is the ultimate tensile strength of small test specimens a

true criterion of the limiting strength of a structure. Full size fabricated material can not be expected to develop the same strength

per unit area as concentrically-loaded, parallel-edge specimens.

The exact point of failure of tension members as actually used is not

very clearly established. It is well above the yield point but below

the ultimate of tensile specimens; that is, it lies between 30,000 and

65,000 pounds per square inch. An assumption of 40,000 pounds

per square inch is thought to be on the safe side and not ultra- conservative. It gives a safety factor for full-sized members of 2.2 with a working stress of 18,000 pounds per square inch.

In considering the necessity of allowing so high a factor as this it must be remembered that close investigation shows the usual data on the tensile strength of small test specimens are not necessarily a

close indication of the actual strength of the material represented.

The customary method of acceptance testing, for example, specifies

yield point determinations. Those familiar with mill testing methods

know that it is difficult to get check results by different operators.

The testing conditions, speed of operation of testing machine, and

personal equation of the operator all seriously affect the results

obtained. Mill tests rarely agree with those conducted more carefully in standard testing laboratories.

There is the further question of how well test specimens really

represent the material supplied. Commonly in channels and I beams these are taken from the webs, the most worked, thinnest.

3 See p. 7 of Report of Special Committee on Stresses in Structural Steel; January, 1925,
APPENDIX 31

and therefore quickest cooled portion of the section. In plates, test pieces generally come from the edges. In the latter it is a rare thing

to call for transverse specimens; that is, taken perpendicular to the

direction of rolling. Careful tests on rolled sections show material

at the fiUet or root portions of the section of decidedly lower strength.

Transverse test specimens also show lower strengths.

Again, yield point determinations do not give, except in a general

way, information of the true behavior of a steel under stress. For

similar grade that material having the higher yield point will possess

the higher proportional limit. The relative value of the proportional limit can not be determined, however, from yield point tests of steel. In fact, it is doubtful whether any two specimens taken

from the same grade of steel will show like stress-strain characteristics, and the most marked differences will come in the evaluation

of the proportional limit stress. The proportional limit stress is at present practically ignored in acceptance testing. It is, therefore,

given slight consideration in the determination of working stresses. Whether this procedure is safe may be questioned. For example,

it has been proven by test that if two specimens cut from the same

sample of metal in adjacent positions are tested in compression, one without any other strain than compression, the other having been

subjected to an overstrain in tension before being tested in compression, the former will show an entirely different stress-strain curve from the latter. The overstrain in tension will almost completely

destroy the elasticity of the metal in compression.

The question of safety factors and maximum allowable stresses

for design of steel structures has been much agitated in the last few

years. A committee of architects and engineers retained by the

American Institute of Steel Construction has recommended for general practice the basic stress in tension of 18,000 pounds per square

inch, v/ith other stresses in proportion. This schedule of stresses has been quite widely adopted, both in building codes and in private

practice where codes do not control. Its more or less complete

acceptance in over 40 cities, after considerable investigation and the

canvassing of local technical opinion in numerous cases, may be

accepted as indicating general approval by those experienced in

construction.

A special committee also was appointed by the American Society

of Civil Engineers in August, 1922, to consider working stresses for

structural steel in buildings. The report of this committee, presented

at the January, 1925, meeting of the society, recognizes the possibility of increasing the basic structural steel stress, due to static

loads, not only to 18,000 but to 20,000 pounds per square inch. The

precautions with which the American Society of Civil Engineers

committee’s approval of this latter stress is qualified, however, are

c
32 RECOMMENDED BUILDING CODE REQUIREMENTS

believed such as to make its use unwise at present under the diversity

of circumstances comprehended by a building code.

Undoubtedly, under present conditions, a somewhat closer evaluation of the relation between strength of test specimens and working

stresses employed in structural design is possible than that upon

which the 16,000 pound per square inch tensile stress originally was

based. There is general agreement that present-day manufacturing

processes result in a more uniform grade of material; that there is less danger, both by reason of the quality of material and the perfection of fabricating processes, that structural shapes will be injured

in fabrication or erection; and that the knowledge of design by which

the stresses in steel are accurately determined is more complete and

widespread than during the earlier years in which the 16,000 pounds

per square inch basic stress was generally used. The weight of engineering opinion, as obtained through submission of the committee’s tentative report, favors the basic stress of 18,000 pounds per

square inch for steel, the quality of which is definitely established.

In the absence of experimental results to the contrary and in view of the conclusions arrived at by other recent investigations this con- sensus is accepted as authoritative.

Where evidence is lacking that the material meets the American

Society for Testing Materials standard specifications for steel for buildings it is necessary to consider what working stresses may be

allowed. Building codes should provide for this contingency, since

in a large proportion of buildings of types in which structural steel

is not an important factor, material is commonly purchased from the

most available source, regardless of specification. It has been argued

that under such circumstances it is unsafe to allow any definite

schedule of stresses. On the other hand, there does not appear to be sufficient reason for not allowing the stresses generally in use for many years and which have produced satisfactory results.

(2) Kelative Stresses.—In building codes, as well as in specifications for design, it is common to find an elaborate list of different

working stresses for varying conditions of stress. The important

ones are: (1) Direct tension, (2) direct compression; that is, axial

compression in columns of moderate length in proportion to their

stiffness, (3) fiber stress in beams under flexure, (4) shear in webs,

(5) shear in rivets and pins, and (6) bearing upon rivets and pins.

The last two are generally subdivided according to the way the

rivets are driven, and often other working stresses are given, such

as fiber stress in pins and rivets, and stresses upon bolts. In comparing these tables in codes and specifications it appears

that there is substantial unanimity of practice with two exceptions

the working stress for compression in short columns and rivet and

bolt stresses.
APPENDIX 33

Much experimental work has been done upon columns, but not

enough to settle definitely the proper relation between tension and

compression stresses. It is generally accepted, however, that the

ultimate strength of columns of which the ratio of length to least

radius of gyration does not exceed 60 is very close to the yield point

of the metal in compression ; and that this yield point is approximately

the same as the yield point of tensile specimens. . There is a very

important difference, however, between the way tension members

and compression members fail. When a tension member reaches

its elastic limit it remains straight and stretches, but with the increase

in length there is a gain in resistance. When a compression member,

on the other hand, reaches its elastic limit, or very shortly thereafter,

it buckles and gives way suddenly. For this reason, it is safe to consider the point of absolute failure of tension members as considerably above the yield point of the steel, while for compression members

the yield point must be assumed as the point at which failure is likely to ensue. In order, therefore, that compression members may be as safe,

relatively, as tension members, with reference to their probable

points of failure, it is necessary that the maximum compressive

stress be less than that permitted in tension. In the more modern

specifications are found the ratios of 75, 78, 83, and 90 per cent for

this relation. In view of the uncertainty as to whether the factor

of 2.2 in tension is actually obtained and considering also the weight

of qualified opinion, the committee has adopted a maximum stress of 14,000 pounds per square inch in compression. This represents a ratio of about 78 per cent between maximum compression and

tension, and approximates the ratio of 78 per cent used by the

American Railway Engineering Association.

As to values for rivets and bolts, working stresses are given in codes

for a variety of conditions, such as shop driven, power driven, hand

driven, inclosed bearing, uniiiclosed bearing, turned bolts, and rough

bolts. While all these conditions have shades of difference, there

does not appear to be any necessity for specifying different working

stresses for all. The proportion of hand-driven rivets is decreasing

rapidly and where inspection is careful the difference between rivets

power driven in shop and field is negligible.

Column reduction formula. —When the slenderness ratio, —» becomes

greater than about 60, a reduction in the stress should be made on

account of the increasing possibility of bending. For this reason

it is universal practice to set a limit of slenderness beyond which no

stress shall be allowed. This limit is frequently placed at ~ equals

120—sometimes at 120 for main members and at 160 for secondary

members. Theoretically this limit might be much higher, but on

Q
34 EECOMMENDED BUILDING CODE EEQUIEEMENTS

account of the increasing danger of the column being accidentally

bent out of direct line a restriction is usually considered desirable.

The general opinion among engineers reviewing the tentative

report was inclined to favor the traditional limit of about 120 for ~ of main members, with higher limits in most cases for bracing and

other “secondary’’ members. With the reduction in stress imposed

by the formula in Part II, however, and with proper care to insure

alignment, the higher limit is believed safe for main members.

Columns more than usually slender are more subject to accidental

bending, and should be shipped and handled with care. Continuous

columns should be given preference wherever possible over those one

story in height. In designing inclined or horizontal compression

members of extreme slenderness, allowance should be made for stress due to weight of the member.

Fabrication and erection should be in accordance vdth the Code of

Standard Practice adopted by the American Institute of Steel Construction which requires that compression members shall not have a

lateral variation greater than 1 to 1,000 of the axial length between

the points which are to be laterally supported.

At a limit of ^equals 160 it is shown by tests and by the consensus

of specifications that the working stress may be conservatively

placed at one-half that used for short columns. Between the values

of ^equals 60 and ^equals 160 the reduction may be made proportional, which is the case when a “straight line” formula is used; or

it may be made by means of a curve after the pattern of the Rankine

formula. Both methods have their strenuous advocates, but the

difference between the results obtained by the two methods is insignificant compared with the variation between the different formulas

recommended by various authorities (see fig. 3). The straight line formula 18,000 — 70— gives a stress of 14,000 pounds per square inch

L ^ when — equals 574- 8-^4 a stress of 6,800 pounds per square inch when

L

^ — equals 160. The Rankine type formula which coincides at these

two points is / equals

16564

1 plus

U

17828

The greatest divergence between these two formulas is at about the

value of ^

equals 110, where it amounts to 437 pounds per square

inch or about 434 per cent of the stress.
APPENDIX 35

The formula / equals 18,000 — 70— is very simple, easily remembered, and is sufficiently above the average of those now generally

in use to be consistent with the increase in tensile and other stresses.

Formulas giving higher results are not recommendedc Those

plotted in Figure 3, the curves of which lie below that recommended

by the committee, will give safe results, but in the interests of uniform

c
36 EECOMMEITDED BUILDING CODE KEQUIKEMENTS

code practice and of economy the committee urges that recommended

in Part II bo adopted.

Eccentric loading . —Any eccentricity of column loading should be

taken into consideration, as the working stresses for columns are based upon the assumption of a purely axial stress. The effect of an

eccentric loading is to produce a combination of flexure and direct

stress. In case the maximum fiber stress from the combination is within the value allowed for the bending stress alone, however, the

column section need not be increased. The reason for limiting

column stresses to a figure lower than the bending fiber stress is to allow for a hypothetical eccentricity which might increase the fiber

stress to the bending limit. Where the eccentricity is known and

allowed for, the hypothetical assumption is unnecessary.

Par, 10. Factors Involved in Determining Safe Stresses for Timber.

The determination of working stresses for timber which will be

safe under all circumstances and comparable in usefulness to working

stresses for other materials is complicated by the variability of

strength of clear v/ood, the influence of defects, and the differing

strength of wood under various conditions of service and loading.

Defects have the most pronounced influence on strength, variations

due to service and loading conditions come next, and variations in

strength of clear wood are least important.

Information obtained by research during the last 20 years has made

it possible to account for many of the strength variations of struc- tural timbers and to allov/ for them or control them. Data are

available on the variations to be expected in the strength of clear

wood, the decrease in strength due to defects, the effects of moisture,

stress duration, and the average strengths of individual species,

together with their strength characteristics.

In view of the fact that woods used for structural purposes in

different parts of the country are of quite different species and that

Part II is in form recommended for State or city adoption, it was

thought best not to include in Part II a general table of working

stresses for all species. Kecommended stresses for the principal

species suitable for structural use are given in Table 7, and it is expected that those revising building codes will select from this

table the species used pr which may possibly be used in their respective localities and place such a table in the code proper. The Forest

Products Laboratory, Madison, Wis., has compiled information on numerous other species, and in practically all cases recommended

working stresses for woods not included in Table 7 can be secured

from the laboratory. When this is not possible, information should

be obtained on locally used woods so that they may be classified in

accordance with similar woods listed in the table.
APPENDIX 37

Certain differences will be observed between the stresses here

recommended and those specified in section 23 of the Building Code

Committee’s report on “Recommended Minimum Requirements for

Small Dwelling Construction.” These differences are primarily due

to a change in grade, but in a few instances additional research has

caused minor changes.

The structural grading rules published by the United States

Department of Commerce in revised Simplified Practice Recommendation No. 16, “Lumber,” ^ have been substituted for those of the

Southern Pine Association as representing a broader and more

general agreement of the lumber industry.

A greater stress in compression perpendicular to the grain is allowed

when the joists rests upon a ribbon or ledger board than when the

ends rest upon or in masonry. This is because the stress is not at the extreme end of the joist and in the case of a slight overload they

will not yield so much; the joists are spiked to the studding, thus

gaining some support through the nails, and they are more certain

of being dry and are consequently stronger.

Compression stresses parallel to grain for short columns are of

considerable importance in most formulas for calculating the strength

of columns of the intermediate-length class. A study of such for- mulas is being made in connection with a series of tests on long,

short, and intermediate-length columns now under way at the Forest

Products Laboratory. The tests indicate that a fourth-power parabola tangent to the Euler curve is a conservative representation of

the law controlling the strength of columns of intermediate length.

Its equation is

where

p^

= allowable column stress, in pounds per square inch,

(7= safe stress for the material in compression parallel to grain,

in pounds per square inch, for short columns,

L = unsupported length of column, in inches,

d = least dimension of column, in inches,

£’= modulus of elasticity (see Table 7),

A’=a constant dependent on the modulus of elasticity and the

maximum crushing strength parallel to grain, which in

turn vary with the species and grade. (See Table 7

for K.)

‘ U. S. Department of Commerce, Elimination of Waste Series, Simplified Practice Recommendation

No. 16, “Lumber” (revised May 1, 1925), can be obtained from the Superintendent of Documents,. Govern- ment Printing Office, Washington, D, C., for 16 cents, cash or money order.

c
38 KECOMMENDED BUILDING CODE KEQUIREMENTS

That is, from the short block to the long column whose strength is dependent on the stiffness, there is a gradual falling off in the ultimate

load. The falling off follows a smooth curve which is very flat at

first and which curves sharply to meet the Euler curve at two-thirds

of the ultimate compressive strength of the species in question.

K in the formula represents the value of j

at the point of tangency.

Investigations have shown that the strength of square-end columns

is practically identical with that of carefully centered pin-end columns

up to a limit of

L

d

equals 11; and that from that value onward any

increase in strength in square-end columns over pin-ends is in very

large measure dependent upon the condition of the bearing surface.

On the whole, therefore, the information available as to the influence

of square-end conditions on the strength of wooden columns does not,

in the committee’s opinion, justify increasing the stress over that for carefully centered pin-end columns.

The investigation of the strength of columns indicates that the

Euler formula is quite accurate for long wooden columns with pinend connections and that the maximum load which they will carry

is dependent upon their stiffness. The Euler formula for rectangular

columns with the factor of safety of 3 included reduces to P 0.274E'

A /ZV
APPENDIX 39

Table 7.— Working stresses for timber

[Select grade,1 beams and stringers, posts, and square timbers, joists and rafters for use under shelter in dry locations] 2

Species

Allowable stress Compression

parallel to grain, intermediate columns; values

of Average

for 6 modulus

of elasticity. All grades

Bending Compression perpendicular

to grain. All grades

Compression

parallel to grain; short columns.

Select grade

In ex- treme

fiber.2 Select grade

Horizontal

shear.i Select grade

Select grade Common

grade

L5s./m.2 L6s./m2 Lbs.lin. 2 Lbs.lin. 2 Lbs.lin.'^ Lbs.lin.^ Lbs.lin.'^ Ash, black.. 1, 000 90 300 650 26.4 29.5 1, 100, 000 Ash, commercial white 1, 400 125 500 1,100 23.7 26.5 1, 500, 000 Aspen and large-tooth aspen 800 80 150 700 23.0 25.7 900, 000 Basswood 800 80 150 700 23.0 25.7 900, 000 Beech 1, 500 125 500 1,200 23.4 26.2 1, 600, 000

Birch, paper... 900 80 200 650 25.2 28.1 1, 000, 000 Birch, yellow and sweet 1, 500 125 500 1, 200 23.4 26.2 1, 600, 000 Cedar, Alaska. 1,100 90 250 800 24.8 27.8 1, 200, 000 Cedar, western red 900 80 200 700 24.2 27.1 1, 000, 000 Cedar, northern and southern white 750 70 175 550 24.5 27.3 800, 000 Cedar, Port Orford 1,100 90 250 900 23.4 26.2 1, 200, 000 Chestnut 950 90 300 800 22.7 25.3 1, 000, 000 Cottonwood, common and black 800 80 150 700 23.0 25.7 900, 000 Cypress, bald 1, 300 100 350 1, 100 21.2 23.7 1, 200, 000 Douglas fir (western Washington and

Oregon) 2 1,600 90 345 1, 175 23.7 27.3 1, 600, 000 Douglas fir (western Washington and

Oregon), dense 1, 750 105 380 1, 290 22.6 24.9 1, 600, 000 Douglas fir (Rocky Mountain type). .. 1, 100 85 275 800 24.8 27.8 1, 200, 000 Elm, cork 1, 500 125 500 1,200 21.1 23.6 1, 300, 000 Elm, slippery and white 1, 100 100 250 800 24.8 27.8 1, 200, 000

Fir, balsam 900 70 150 700 24.2 27.1 1, 000, 000

Fir, commercial white 1, 100 70 300 700 25.4 28.4 1, 100, 000 Gum, red, black, and cotton.. 1, 100 100 300 800 24.8 27.8 1, 200, 000 Hemlock, western 1, 300 75 300 900 25.3 28.3 1, 400, 000 Hemlock, eastern 1,100 70 300 700 25.4 28.4 1, 100, 000 Hickory (true and pecan) 1, 900 140 600 1, 500 22.2 24.8 1, 800, 000 Larch, western 1, 200 100 325 1, 100 22.0 24.6 1, 300, 000 Maple, sugar and black. 1, 500 125 500 1,200 23.4 26.2 1, 600, 000 Maple, red and silver. 1, 000 100 350 800 23.8 26.6 1, 100, 000 Oak, commercial red and white 1,400 125 500 1,000 24.8 27.8 1, 500, 000 Pine, southern yellow 2 . 1, 600 110 345 1, 175 23.7 27.3 1, 600, 000 Pine, southern yellow, dense 1, 750 120 380 1, 290 22.6 24.9 1, 600, 000 Pine, white, sugar, western white. western yellow 900 85 250 750 23.4 26.2 1, 000, 000 Pine, Norway 1,100 85 300 800 24.8 27.8 1, 200, 000 Poplar, yellow 1, 000 80 250 800 23.8 26.6 1, 100, 000 Redwood 1, 200 70 250 1,000 22.2 24.8 1, 200, 000 Spruce, red, white, Sitka 1, 100 85 250 800 24.8 27.8 1, 200, 000 Spruce, Englemann 750 70 175 600 23.4 26.2 800, 000 Sycamore 1, 100 80 300 800 24.8 27.8 1, 200, 000 Tamarack (eastern) 1, 200 95 300 1, 000 23.1 25.8 1, 300, 000

1 Specifications for the select and common grades were published in Simplified Practice Recommendation

No. 16 (Revised), May 1, 1925. Working stresses for the common grade of Douglas fir (western Washington

and Oregon), and southern yellow pine are 75 per cent of those given for the select grade (not dense) ; for other

species 80 per cent of the select grade, except in compression perpendicular to grain and modulus of elasticity, which are the same for all grades. The difierence in the percentage between the select and common grades

of Douglas fir and southern yellow pine as compared with other species is due to the fact that the rate of growth requirement for the select grade applies only to these two species.

2 See introductory paragraph. Part II.

2 Stress in tension along the grain. The stresses in extreme fiber in bending are identical with those recommended for tension parallel to grain.

^ Shearing stress for joint details. The shearing stress for joint details for all grades may be taken as 50 per

cent greater than the horizontal shear values given in the table.

is for use in Table 8 for determining the safe stress for columns of intermediate length. It is the

at which the F. P. L. fourth power parabola becomes tangent to the Euler curve.

2 Values for modulus of elasticity are average for species. In figuring safe loads for long columns a factor

of 3 must be applied to them.

2 If material of common grade of southern yellow pine or Douglas fir conforms to the density requirenaents

of the American Society for Testing Materials the stresses in compression (both parallel and perpendicular

to grain), in extreme fiber in bending and in horizonal shear may be increased by 15 per cent of the value

listed for the select grade. Source: U. S. Department of Agriculture, Forest Service, Forest Products Laboratory, Madison, Wis.

(Revised), January 20, 1926. Q
40 RECOMMENDED BUILDING CODE REQUIREMENTS

Table 8.—Strength of columns of intermediate length in 'per cent of strength of

short columns ^

Values

Ratio of length to least dimension in rectangular timbers 2 01 AT3

12 13 14 15 16 17 18 19 20 21 22 23 24 25

P.ct. P.ct. P.ct. P.ct. P.ct. P. ct. P. ct. P.ct. P.ct. P.ct. P. ct. P.ct. P.ct. P.ct. 21 96 95 93 91 89 86 82 78 73 67 22 97 96 95 93 91 88 85 81 77 72 67

23 98 97 95 94 92 90 87 84 81 77 72 67 24 98 97 96 95 93 92 89 87 84 80 76 72 67 25 98 98 97 96 94 93 91 89 86 83 80 76 72 67

26 99 98 97 96 95 93 92 91 89 86 83 80 76 72 27 .... 99 98 98 97 96 95 93 92 90 88 85 82 79 • 74 28 99 98 98 97 96 95 94 93 91 89 87 85 82 79

29 99 99 98 98 97 96 95 94 92 91 89 87 84 82

P.ct. P.ct. P.ct. P. ct.

C7

1 These percentages are values of the expression ^

1—^ ^

various values of Kand ^

* When the

r p amp,

— exceeds K, the column is in the Euler class and must be designed by the formula —=/ L\t

“ ^ {j)

2 Columns not rectangular. —This table can be used for other than rectangular columns, the ^

being equivalent to 0.289 — where r is the least radius of gyration of the section.

3 Values of Kare found in Table 7 opposite the species and under the grade. Source: U. S. Department of Agriculture, Forest Service, Forest Products Laboratory, Madison, Wis., January 18, 1926. Table 8 is given to simplify the use of the fourth-power parabolic

equation for determining the permissible load for columns of intermediate length. It is a tabulation of values for the expression

(>-Ki>)‘)

The values for this expression are given as percentages, which when

applied to the allowable stresses in compression parallel to grain for

short columns (Table 7) will give the permissible stresses for col- umns of intermediate length with the respective ratios of length to

least dimension.

Use of Table 8 to determine safe load on columns of intermediate length

To determine the load a given column will safely carry:

First. Obtain from Table 7 the value of K for the species and

grade of timber used;

Second

length divided by its least dimension ; ^ Determine its or slenderness ratio;

d

' that is, its

' If the is found to be 11 or less, the column is considered as “short’' and the load it may support is determined by multiplying its area by the allowable stress for short columns. (See Table 7.) If, for a column of given size and length, the exceeds K, then the column is in the Euler formula class, which means its carrying capacity is dependent on it stiffness, and its strength must be determined by the formula „ 0.274
APPENDIX 41

Third. At the side of Table 8 find the horizontal line beginning with the value of K just determined;

Fourth. In this line and vertically below the ^

of the column

imder consideration will be found the percentage which, when

applied to the allowable compression stress for short columns of

that species and grade (see Table 7) will result in the permissible

stress for the given column;

Fifth. Multiply the actual area of the column by the stress just found. This will give the total load the column will safely

support.

Example: Required to find the safe load for a Douglas fir or Southern yellow pine column, common grade, 8 by 8 inches in nominal section, and 14 feet long. The slenderness ratio should be

based on the actual size, which is 7.5 by 7.5 inches:

14X12_168

7.5 7.5“

In Table 7 the value K for the common grade of Douglas fir or Southern yellow pine is given as 27.3. In Table 8, in the line opposite K=27 and below the slenderness ratio of 22, we find the percentage 85. Referring again to Table 7, the allowable stress for short

columns of select grade is found to be 1,175 pounds; the stress for

common grade being 75 per cent of this, or 880. The permissible

stress for the intermediate column with ^

equal to 22, is, therefore,

880 X 85 per cent, or 748 pounds per square inch. This is the allowable working stress, and multiplied by the column’s cross-section

area, 7.5 by 7.5, or 56.25 square inches, gives the safe total load for

the column

42,200 pounds.

Use of Table 8 to determine the required size of a column of given intermediate

length to support a given load The following procedure affords a close approximation of the

necessary size : ® First. Determine the cross section of a square timber necessary

to sustain the load if the timber were a short column of the

intended species and grade.

Second. Determine the slenderness ratio of that size of column

for the given length.

® Except when is less than 11, the solution of a quadratic equation is necessary, even with a straight line formula, to determine directly and accurately the required column size for a given load and length. A

similar method would have to be pursued in the solution of the problem by the fourth power parabola

equation. It is consequently easier to resort to the trial method for solution.
42 EECOMMENDED BUILDING CODE REQUIREMENTS

Third. From Table 7 ascertain the proper K for the species

and grade.

Fourth. In Table 8 find the percentage for this opposite

the proper E for the species and grade. The area found for

the short column divided by this percentage gives the necessary

cross-sectional area for the required column (column of given

length).

It is understood, of course, that the least dimension of the required

column must be equal to the least side of the trial short square

column in order to leave the slenderness ratio unchanged. A square

column of the adjusted area will carry slightly more than the given

load. The dimensions of a square column of the adjusted area will

equal the square root of the area. For rectangular columns, one

dimension will be the same as the side of the trial column and the

other will equal the required area divided by that dimension.

Example. —Kequired to find the necessary cross-sectional size for a 14-foot Douglas fir column, common grade, to carry a 50,000

pound load. The allowable stress for a short column of that species

and grade is 880 pounds per square inch (75 per cent of the stress for

select grade in Table 7).

First. Load 50,000

Stress 880 = 56.80 square inches cross-sectional area necessary for a short (trial) column.

Second. VS6.80 = 7.5 = side of the short square trial column.

Trial given length a side ol trial square 7.5 7.5 Third. A=27.3 from Table 7, Fourth. If ^

=22 and A=27 then from Table 8 the working

stress is 85 per cent of that allowable for the short trial column

and the adjusted area of the required column is or 66.8

U -oO square inches.

Fifth. Q- —=5^^=8.9; therefore, a rectangular

Side of trial column 7.5 ’ ’ ^ column that is 7.5 by 8.9 inches in size will carry the load.

The dimension of a square column of equivalent area equals

V66.8 or 8.16 which is slightly larger than necessary. The

next lower commercial size is 7.5 by 8.5 actual dimensions,

which as is evident will not carry the load without exceeding the

permitted stress. Hence, the nearest commercial sizes which

will carry the load safest are 8 by 10 (71^ by 91^ actual) or 9 by

9 (8^ by SV2 actual).

If the trial ^is greater than K, then determine the necessary

size of a short square column using two-thirds the safe working
APPENDIX 43

stress for that species and grade; that is, proceed through the same steps as were used in determining the original trial column

and trial ^ but with two-thirds the allowable stress used before. a

If the trial ^thus determined is still more than K, then the

column is definitely in the Euler class. If, however, this second

trial j

is less than K, then as a trial use the next larger ^

in

Table 8 and solve as an intermediate column.

The foregoing examples have been treated mathematically in

detail with the purpose of illustrating quite fully the application of the fourth power parabola and the Euler curve to the determination

of column sections. It is realized that for the purposes of building

design and building code enforcement a simpler and more direct

statement of requirements is necessary. Table 4 following section

5-5 in Part II, therefore, has been worked out on the basis of the

formulas heretofore discussed. In addition to the working stresses for short columns it gives the safe working stresses for intermediate

and long columns of various species grouped according to the values

of K and the respective working stresses for short columns. Safe

working stresses for columns of ^

intermediate between the values

given in Table 4 may be obtained by interpolation. Par. 11, Variables Affecting Strength of Clear Wood.

The clear wood from trees of different species varies in strength.

The part of the tree from which the wood is cut and the characteristics

of the tree also influence strength. Such variations, however,

especially differences in strength betvreen different species, are neither

so great nor so important in comparison with other factors as is generally believed. The differences in the strength of the various

species are usually taken care of by specifying working stresses for

individual species. The enormous amount of testing necessary to determine the average strength of the clear wood of the various native

species has been largely completed, and the results for 126 species

have been published.’^ Data on approximately 35 additional species

are available but not yet published. There is ample information as to the average strength of clear wood of species commonly used in

building.

Variations within a species, however, are such that it is not uncom- mon to find one piece twice as strong as another, although both are sound, clear, and straight grained, and at the same moisture content. Such a variation is usually associated with a difference in density

^ These data are given in XT. S. Department of Agriculture Bulletin 656, '‘Mechanical Properties of Woods Grown in the United States.”

c
44 EECOMMENDED BUILDING CODE KEQUIEEMENTS

and may be accounted for either by a difference in position in the

tree, tree characteristics, or a combination of both.

Figure 4 is a frequency curve for modulus of rupture of Sitka

spruce. It is based on the results of 1,374 tests of material from

seven different localities. This graph is representative of the normal

variation of most species, in which the most probable value is some

6 or 7 per cent below the arithmetic mean; 25 per cent of the material

is more than 14 per cent below the mean; and 25 per cent more than

12 per cent above the mean. These differences are usually associated

with differences in density.

Fig. 4 . —Variability in modulus of rupture of clear wood, green Sitka spruce

U. S. Department of Agriculture, Forest Service, Forest Products Laboratory. Although density is the best indication of the strength of clear

wood of all species, no satisfactory visual method of estimating it has been devised except for Douglas fir and southern yellow pine,

in which the percentage of summer wood present is a good criterion

of density. Specifications for high quality dense material of these

two species, which produce most of our structural timber, are published in the Department of Commerce Simplified Practice Kecommendation No. 16 (revised). The present specifications of the American Society for Testing Materials ® are being revised so that they

will be in absolute conformity. Percentage of summer wood is not

generally applicable to other species as a test of quality.

8 Given in A. S. T. M. “Tentative Standard 1920, serial D 23-20 T for Douglas fir; for Southern pine

in serial D 10-15 A. S. T. M. Standard 1921.” The latter standard applies to bridge and trestle timbers, but the principle may be used for other grades.
APPENDIX 45

Par. 12. Influence of Defects on Strength of Timber.

As already remarked, the greatest variation in the strength of timbers results from defects. Defects often occur which reduce the

strength of the piece to practically zero.® The control of defects in

accordance with their influence on strength is, therefore, essential

before safe and practical working stresses can be assigned to timber.

Data on the influence of defects on strength of ^ wood have made

possible the preparation of grading rules by which structural timbers

are classified in accordance with their strength and such control is accomplished. Unfortunately, most grading rules formerly in use

for structural timbers were very general in their application, and did

not limit defects in accordance with their effect on strength. As a

step toward improving this condition, the Forest Service has lately

prepared basis rules for grading structural timbers,^® embodying the

fundamental principles of strength grading. Various committees of

the American Society for Testing Materials and the American Eailway Engineering Association, as well as the Central Committee on Lumber Standards, have accepted the principles therein set forth,

together with certain additions agreed to by the Forest Service as the basis for strength grading. As a result of the work of these committees, the Central Committee on Lumber Standards has adopted

select and common grades for beams and stringers, and select and

common grades for joists and plank, the select grade to insure 75

per cent and common grade 60 per cent of the green strength of clear

wood. These rules are published in United States Department of

Commerce Elimination of Waste Series, Simplified Practice Recommendation No. 16, ^Ujumber,” revised May 1, 1925. Par. 13. Moisture Conditions of Service.

The strength of wood is influenced largely by its moisture content

and, therefore, by the moisture conditions of service, which have an

important bearing also on decay and checking.

The loss of moisture accompanying seasoning differs in its effect upon strength according to the size of the material. Clear wood in

small sizes increases in strength with loss of moisture, the increase

being much greater for some properties than for others. Thus,

compressive strength of pieces 2 by 2 inches in cross section may

increase 100 per cent or more from the green to the air-dry condition,

and bending strength 50 per cent or more, whereas impact and work

to rupture values seldom increase by more than 20 per cent and may

even show a loss in some species.

» Discussion of defects and their influence is given in the 1924 Proc. of the A. S. T. M. under the title “Structural Timbers,” by J. A. Newlin and R. P. A. Johnson. “ U. S. Department of Agriculture Circular 295, “Basic Grading Rules and Working Stresses for Structural Timber,” by J. A. Newlin and R. P. A. Johnson,

c
46 RECOMMENDED BUILDING CODE EEQUIEEMENTS

Studies of moisture effects on strength have shown large timbers

to be about as strong in a green as in an air-dry condition, and have

also shown the increase in strength with drying which can be ex- pected in higher grades of dimension and in small clear material.

With larger pieces, the checking which accompanies drying increases

in severity, especially as knots increase in number and size. Thus,

clear dimension material 2 to 4 inches thick increases in strength

about 25 per cent on drying, but material of common grade increases

only about 8 per cent. Owing to injury from checking, the lower

grades of large timbers and about 25 per cent of large timbers of

higher grades show no increase in strength with drying, although

the average strength of the latter is raised slightly.

Timber in wet or damp locations is subject to deterioration from

decay. Under such conditions lower stresses are recommended in

order to offset the effects of depreciation and to avoid the danger

which might result from failure to replace the member frequently. Par. 14. Conditions of Loading,

Variations in strength with different conditions of loading result

principally from difference in the duration of stress. The studies on the effect of duration of stress on strength of wood make is possible

to know the factor of safety under varying conditions from impacts

of a fraction of a second to quiescent loads of several years^ duration.

The variation in strength with duration of stress can be figured

approximately by the rule that a known modulus of rupture will

increase about 10 per cent if the duration of the stress is decreased

to one-tenth of the original period. Thus, a timber will carry 10

per cent more load for 1 hour than it will for 10 hours.

Loads which are carried without failure for a few seconds may,

therefore, cause failure if applied for a long period. For example,

a locomotive on a smooth track can pass safely over a bridge at 60

miles per hour, giving the ordinary impact for such a speed, when

the same locomotive might cause failure if left in a position to cause

a maximum stress for a considerable length of time. Par. 15. Working Stresseis for Timber.

The working stresses given in Table 7 are based on the general

conclusion of the Forest Service, drawn from tests of structural timbers, that defects, such as knots, shakes, checks, and cross grain, have

approximately the same influence on the strength of timbers from all species. Therefore, working stresses in a given case can be arrived

at by adjusting the strength of clear wood in accordance with the

known effect of permitted defects and the duration of stress, at the

11 This rule of thumb was obtained from H. S. Grenoble’s work on “Effect of Rate of Loading on Strength

of Wood,” and from dead load and impact tests. Some of this information is given in the 1909 Proceedings of the American Society for Testing Materials under the title “Some Results of Dead Load Bending

Tests of Timbers by Means of a Recording Deflectometer,” by H. D. Tiemann.
APPENDIX 47

same time making allowance for probable variations of individual

timbers from the average. As a matter of fact, characteristic dif- ferences in the nature and prevalence of defects in various species

have been taken into account both in the wording of the rules and

in the stresses permitted. Thus, for some species in which the limbs

are borne in whorls rather than alternately, the rules are so worded

as to exclude timbers in which whorls of knots would cause greater

weakening than the allowable single knot. Again, if one species runs characteristically lower in the grade than another, consideration has been given that fact in the assignment of working stresses. The differences in strength due to species are taken care of in the

table by giving separate stresses for each species ; difference in effect of seasoning on dimension and timbers of larger size is provided for by allowing slightly larger defects in dry locations for dimension

timbers 4 inches and under in thickness. The stresses in Table 7

apply only for timber used in dry inside locations. Where timbers

are exposed to moisture or to weather, design stresses should be

decreased. (See Appendix, par. 13.)

The stresses shown in Table 7 are for long-time loadings.

Except in case of compression perpendicular to the grain these

working stresses are based rather on ultimate than elastic limit

values, although both are considered. There has been considerable

discussion among engineers as to whether elastic limit or ultimate

values should be used as the basis for safe working stresses. There

is undoubtedly an element of safety in the stress which a timber is able to take after the elastic limit is passed. Those species which

show a great difference between the elastic limit and the ultimate

stress show also a large deflection before failure occurs, so that in one

species a load may be applied slightly beyond the elastic limit and

ample warning received before failure, while with another species

the same loading condition would cause failure without apparent

warning. When elastic limit values are used as a base no consideration is given to the differences in strength which exist in species

between elastic limit and ultimate strength. In addition to this

nonuniformity of behavior the elastic limit values are subject to more

numerous test errors, to a greater “personal equation’’ of those

operating testing machines, and to larger errors in calculation of

stress values than are ultimate strength determinations. These

facts are believed to justify the use of ultimate values as the primary

base for safe working stresses in timber.

Par. 16. Factors of Safety for Timber.

Much information is available on the character and magnitude of

factors which affect the strength and variability of timbers and the

likelihood of their failure under various loading conditions. These
48 RECOMMENDED BUILDING CODE REQUIREMENTS

factors will be discussed for bending, compression, and shear stresses under their respective headings. (Appendix, pars. 17 to 21, in- clusive.) This information, however, is not sufficient alone for

arriving at factors of safety or safe working stresses. At times the

loading assumed is the maximum which can occur and contains the

factor of safety, none being used in determining working stresses. At other times the loading assumed is of a magnitude that is likely to occur and is accompanied by a relatively low working stress. In the first instance the occupant of the building must keep within

the design load by an unknown percentage in order to be certain

that no failure will occur. Under the second assumption he can

place the design load on the member with perfect assurance. In the stresses shown in Table 7 it is assumed that small overloads

are likely to remain on the timbers for long periods of time, and that

loads two or three times the permitted load are likely to continue

only for short periods.

The working stresses for bending and compression given in Table 7

are for timbers which meet the requirements of the select grade of

the Central Committee on Lumber Standards. Considering that

timbers of the select grade develop 75 per cent of the strength of the

clear wood, the average timber has a factor of safety of under

long-time loading, 4 under 5-minute loads, and 6 under impact loads.

One timber in 100, the clear wood of which is more than 36 per cent

below the average bending strength for the species, has under longtime loading a factor of safety of about 13^.

The endeavor has been to establish in Table 7 a correct and fair relation between the species and to make the stresses safe under all conditions of loading and service. However, because of the number

and nature of variables included in the factor of safety for wood it is difficult to make a correct comparison with the factors of safety for

other materials. It is hoped that this discussion of these variables

will clarify the methods used in arriving at safe working stresses for

wood.

Par. 17. Bending

Stress in Extreme Fiber.

Below are given the factors of safety which exist under various

conditions with the working stresses for extreme fiber in bending as shown in Table 7. About 1 time in 100 a timber is found, the clear

wood of which is more than 36 per cent below the average bending

strength for the species. If this 1 timber in 100, because of defects,

develops only 75 per cent of the strength of the clear wood then it would be expected to break at one and one-half times the stress given

imder a load of approximately 10 years duration.

The average timber has a factor of safety of 234 on long-time

loading, considering that it develops only 75 per cent of the strength

of clear, green wood. Using the same stresses the factor of safety
APPENDIX 49

of the average timber under a 5-minute loading is about 4, while for

impact it runs up approximately to 6. Par. 18. Bending—Horizontal Shearing Stresses.

There are very wide variations in the shear stresses developed in

structural timbers which fail by horizontal shear. These variations

are due largely to the effect of checks and shakes which have a greater

influence than their appearance would indicate. This tends to

justify the use of a high factor of safety. On the other hand, the

timbers with highest quality of clear wood are the most subject to shakes and develop the checks most injurious to strength in shear;

after failing in shear, however, these timbers can usually carry their

design load with safety because of their high Bending strength.

The establishment of factors of safety and safe working stresses

for shear is further complicated by an error in the fundamental

assumptions used in developing the shear formula. When the

formula is applied to checked beams with the load approaching the

supports a large error results. In extreme cases the actual stress developed is only about one-third that indicated by the formula.

Under other conditions the formula is about right. This error in the

formula has led bridge engineers to question the shear stresses in

Table 7. However, since the working stresses are about correct for

uniform loading, it does not seem advisable to raise them even though they are too low when applied to concentrated moving loads.

It seems desirable rather to correct the formula or to make allowances

in the assumption as to loading. Before much can de done toward

this it will be necessary to make an extensive investigation, for while

it is relatively easy to see the error in the shear formula, to correct it

is quite complicated.

Par. 19. Bending—Stiffness.

The modulus of elasticity values shown in Table 7 are not safe

working stresses. They are approximately the average stiffness of the species when green and are intended for use in computing the

average deflection of beams. As defects have little effect on the

stiffness the modulus of elasticity values are applicable to all grades.

As timber dries out it becomes somewhat stiffer, and as a conse- quence the deflection wiU not be as great as indicated by the use

of the values given in the table. In long-time loading timbers will sag somewhat, receiving a permanent set. This set, which may be

considered as independent of the deflection, is usually about equal

in amount to the deflection except where timbers are overloaded,

when the sag becomes much more pronounced.

In cases where the deflection of beams is limited to a deflnite fraction of the span a simple relation exists between span and depth

which determines when stiffness is of importance and when stress in

Q
50 EECOMMENDED BUILDIETG CODE EEQUIREMENTS

extreme fiber is the important factor in load-carrying capacity. If, as is commonly the case, specifications require that the deflection

shall not exceed 1/360 of the span in uniformly loaded beams, and if it is assumed that the ratio of allowable stress in extreme fiber to modulus of elasticity is 1 to 1,100, which for the select grade is about correct for all of the more important species, then when the

span in feet is less than one and one-fourth times the depth in inches

the stress in extreme fiber will govern the design; when more the

stiffness will govern. With the common grade the divison between

where stress rules and stiffness rules is at a span in feet one and onehalf times the depth in inches. Par. 20. Compression Parallel to the Grain—Short, Intermediate,

and Long Columns.

The factors influencing determination of the so-called factor of safety for compression parallel to the grain are the same as those in

bending but differ in their effect on the strength. Knots and cross grain are about one-half as injurious to compressive as to tensile

strength and checks and shakes have very little effect on compressive strength. The elastic limit is closer to ultimate strength in compression than in bending and the loss of strength with increased duration of stress is considerably less. The variation in strength of clear

wood is about the same for compression as for bending.

Features of design, other than strength, usually call for posts considerably larger than are required to carry the actual loads. It is seldom, therefore, that timbers used as posts are subjected to their

maximum stress in permanent construction although it frequently

happens in false work and temporary construction. For loads with

a duration of stress of about five minutes the factors of safety should

be about the same as for bending. For impact loading the factors

of safety are somewhat less and for long-time loading somewhat more. The factor of safety of 3 used with the Euler formula is necessary

to care for the effect of crooks and defects and for overloading. It has been maintained that since the fiber stress at elastic limit is never

passed in going to the maximum load of long columns the factor of safety could be about one-half that for short columns. The modulus

of elasticity of timber is its most variable property and the one most

difficult to estimate visually. Tests of clear wood and the laws of probability indicate that 1 timber in 100 will have less than 56 per

cent of the average stiffness. It is believed, therefore, that a factor

of safety of 3 should be used for long columns.

The effect of eccentric loading on the strength of columns of long,

short, and intermediate lengths is about the same. The reduction

in the maximum load which a column will carry due to an eccentric

loading is not as great as might be expected.
APPENDIX 51

The importance of crooks and eccentric loading of columns has

generally been overestimated. Timber when subjected to combined

bending and compression, whether the bending is from eccentricity or otherwise, develops a higher stress at both elastic limit and max- imum load than when subjected to compression only.^^ For example,

in loading a short block between plates supported and loaded by

knife-edges, the load obtained when the line of action of the force was such as to give zero stress on one side was about three-fourths

the load obtained with a true axial loading and not one-half, as would

be the case if the stress under the two conditions was identical.

This example is not intended to convey the idea that unrestricted

crooks and eccentricities are permissible or that higher stresses are permissible in eccentrically-loaded columns, but is for the purpose of

relieving anxiety as to influence of imperfect end conditions and the

influence of crooks such as are common in most columns.

No definite recommendation can be made from the data available

as to the influence of defects on column strength. However, suffi- cient information has been obtained to show that the injurious effect of defects on long columns is much less than is generally assumed.

Defects, such as are permitted in the Central Committee on Lumber

Standards grade of select posts and square timbers, have little influence upon the strength of long columns.

Par. 21. Compression Perpendicular to Grain.

The compression stresses perpendicular to grain in Table 7 are based on elastic limit rather than ultimate strength. This is because

the nature of the materials is such that it is practically impossible

to obtain a maximum either in test or in service even though the whole

surface be covered.

Usually compressive loads perpendicular to grain are applied over a relatively small portion of the surface and the stresses are consequently very much distributed before they reach any considerable depth in the beam where most of the shakes and checks occur. The compressive strength perpendicular to grain of wood in and

around knots is greater than that of clear wood. For these reasons

the effect of defects on this property is not important and the same

stresses are given for all grades.

If the moisture content is high, near 20 per cent, about 1 timber in

100 when loaded up to the stress specified for dry locations would

be expected to mash under the bearing to an extent that in time would

necessitate replacement. The average timber, however, would carry

the permitted stresses for an indefinite time without mashing. It is

Tests showing this are presented in Report No. 188 of the National Advisory Committee for. Aeronautics, “The Influence of the Form of a Wooden Beam on Its Stiffness and Strength," by J. A. Newlin

and G. W. Trayer.
52 RECOMMENDED BUILDING CODE REQUIREMENTS

seldom that crushing across the grain endangers the safety of a

structure. Par. 22. Composite Columns.

A type of composite column that has come into general use in

several sections of the country, ordinarily referred to as the “Lally

column,” consists of a steel pipe filled with concrete. This column,

however, is entitled to recognition as a safe structural element in

building construction only when extreme care is taken in its manufacture to secure by proper shop work a complete filling of the pipe

with a densel:l3^:3 gravel or crushed rock concrete. Only

new standard heavy-weight mild-steel pipe should be used. Unless

such care has been exercised the column has no greater strength value

than an unfilled steel tube used as a column. In multi-story construction special connections must be designed and used. This

column offers considerable fire resistance without exterior protection,

but when encased in fireproofing materials it can be given a corre- spondingly greater fire resistance classification. The committee

recommends for the properly designed and fabricated column of this

type safe working loads in accordance with the formulas for composite

columns in the Report of the Joint Committee on Standard Specifications for Concrete and Reinforced Concrete.

Par. 23. Welded Connections.

The welding, either electrically or by the use of gas, of all kinds of

connections is being advocated by some engineers engaged in steel construction. The advantages claimed for this method over the

usual riveting process are economy in cost of erection, lighter weight

in the connections, less noise during construction, greater rigidity,

and increased strength. Although tests have been made and welded

connections have been used in some actual structures, the experience

and information thus far available are too limited to justify recommendations. An investigation of this subject is contemplated by one

of the national technical societies, the outcome of which should be

awaited before conclusions are drawn.

Par. 24. Construction Resistive to Earthquakes.

Suggestions have been received from people living in localities subject to earthquakes that the committee should recommend requirements for construction to resist such shocks. Efficient and rational

design of structures to resist earthquake shocks must be based upon

reasonably accurate assumptions as to the nature and magnitude of

the forces which are to be resisted. At best such assumptions for

any particular locality are based upon a guess as to whether past

experience may be repeated or how much greater severity of shock

may sometime occur. Knowledge of the forces produced by certain
APPENDIX 53

extents and periods of motion is accumulating, but no means has been

devised for predicting the extent of motion or the periodicity of future

quakes.

In localities where earth shocks occur frequently buildings should

be designed to resist them. The necessity for such provisions is not

so apparent in other parts of the country where the intervals between

serious shocks, as judged by past records, indicate periods frequently

several times greater than the ordinary life of a building. However,

accepting the judgment of seismologists that earthquakes are likely

to occur from time to time on the East Coast as v/ell as on the West

Coast, it is doubtless the part of wisdom to make provision in the design of at least all large or monumental buildings to resist such forces. The subject, however, is related more closely to loads, foundations,

and details of connections and bracing than to working stresses, and

therefore does not logically belong in this report. For this, and

other reasons previously mentioned, this committee refrains from

making recommendations for such design.

Committees of architects, engineers, and earthquake experts were

organized in California subsequent to the Santa Barbara earthquake

to study the problem of building construction in its relation to earth

shocks. They included men of recognized authority upon the subject, and they had for study the records of all important earth shocks

of recent years in this and other countries. The information derived

from the investigations of these committees furnished the data for

the principles of design to resist earthquake shock which are the basis

of special building ordinances adopted in the cities of Santa Barbara

and Palo Alto. These ordinances, while agreeing in conception as to what should be accomplished, are not entirely in harmony as to methods of calculation and design. They are recommended to those

interested in such regulations as the best available treatment of the

subject to date.

o

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DEPARTMENT OF COMMERCE

WASHINGTON

PUBLICATIONS IN EELATION TO HOUSING AND MUNICIPAL REGULATIQnI

^ 'I

[These publications may be obtained from the Superintendent of Documents, Government Priutinjt

OflBce, Washington, D. C payments to be made by money order or New York draft; currency at sender’s / risk. Postage stamps or foreign money not accepted.] BHl. Recommended Minimum Requirements for Small Dwelling Construction.

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100 illustrations, 250 pages. Government Printing Office, Washington.

Price, 35 cents.

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