ACI-318.1-89

COPYRIGHT American Concrete InstituteLicensed by Information Handling ServicesCOPYRIGHT American Concrete InstituteLicensed by Information Handling Services

A C 1 3 L B . L / 3 L B = L R 92 0662949 0504705 990 First Printing, September 1992

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Building Code Requirements For Structural Plain Concrete

(AC1 31 8.1 =89)* (Revised 1992) and Commentary-AC1 31 8.1 R-89 (Revised 1992)

Reported By AC1 Committee 318

W. G. Corley Chairman

Basile G. Rabbat Secretary

Claude V. Baker Eugene H. Boeke, Jr. John E. Breen James R. Cagley Gregory P. Chacos George Chironis Paul F. Fratessa Clifford L. Freyermuth

Bijan O. Aalami Roger J. Becker Edward M. Frisbee Richard W. Furlong

Julio Cesar Caballero

Luis E. Garcia Richard E. Holguin James G. MacGregor Charles G. Salmon Richard D. Gaynor David A. Hunter, Jr. Robert F. Mast Chester P. Siess Jacob S. Grossman Francis J. Jacques Alan H. Mattock Robert J. Smith David P. Gustafson Daniel P. Jenny Walter P. Moore, Jr. Mete A. Sozen John M. Hanson James O. Jirsa Clarkson W. Pinkham Irwin J. Speyer James R. Harris James Lefter Richard A. Ramsey Dean E. Stephan C. Raymond Hays H. S. Lew Lawrence D. Reaveley Loring A. Wyllie, Jr. Edward S. Hoffman

Voting Subcommittee Members

S. K. Ghosh Phillip J. lverson Cameron Maclnnes Jack P. Moehle Roger Green Paul Klieger David T. Lashgari Donald R. Strand Philip G. Griffin Cary Kopczynski Peter Marti David A. Whiting James K. lverson Michael E. Kreger Denis Mitchell James K. Wight

Liaison Members

Martin Isaac D. Shunsuke Otani Rudiger Tewes Henry Thonier Augusto Carlos De Vasconcelos Luis Eduardo Laverde Robert Park George Somenrille Mireya Veloz Harold P. lsaacs Peter Lenkei Horacio Ramirez de Alba Bai Shengxian Habib M. Zein Al-Abidien

AC1 318.1-89 (Revised 1992) was adopted as a standard of the American Concrete Institute July 1, 1992 to supersede AC1 31 8.1 -89 in accordance with the Institute's standardization procedure.

Vertical lines in the margins indicate the 1992 changes.

'Acomplete metric companion to AC1 318.11318.1R has been developed, 318.1M/318.1RM; therefore no metric equivalents are included in this document. tACl Committee Reports, Guides, Standard Practices, and Commentaries are intended for guidance in designing, planning, executing, or inspecting construction, and in preparing specifications. Reference to these documents shall not be

made in the Project Documents. If items found in these documents are desired to be part of the Project Documents they should be phrased in mandatory lan- guage and incorporated into the Project Documents.

Copyright O 1992 American Concrete Institute. All rights reserved including rights of reproduction and use in any form or by

any means, including the making of copies by any photo process, or by any elec- tronic or mechanical device, printed or written or oral, or recording for sound or visual reproduction or for use in any knowledge or retrieval system or device, unless permission in writing is obtained from the copyright proprietors.

318.1/318.1R-l


m Obb29V9 050V707 763 m 318.1I318.1R-2 MANUAL OF CONCRETE PRACTICE

The 1992 AC1 Building Code for Plain Concrete and Commentary are presented in a side-by-side column format, with code text placed in the left column and the corresponding commentary text aligned in the right column. To

further distinguish the Code from the Commentary, the Code has been printed in Helvetica, the same type face in which this pargraph is set. Vertical lines in the margins indicate changes from 31 8.1 -89.

This paragraph is set in Times Roman, all portions of the text exclusive to the Commentary are printed in this type face. Commentary section numbers are preceded by an “R” to further distinguish them from Code section numbers.

CONTENTS Chapter l-General requirements. .318.1-3 i .1 -scope 1.2-Limitations 1.3-Permits and Drawings 1.4-Inspection

Chapter 2-Definitions .......... .318.1-5

Chapter 3-Materials. ........... .318.1-6 3.1 -Materials for Concrete 3.2-Metal Reinforcement 3.3-Tests of Materials 3.4-Storage of Materials

Chapter 4-Concrete quality. .... .318.1-6 4.0-Notation 4.1 -General 4.2-Minimum Strength 4.3-Selection of Concrete Proportions 4.4-Mixing and Placing Concrete 4.5-Evaluation and Acceptance of Concrete

Chapter 5-Formwork and joints . .318.1-7 5.1-Formwork 5.2-Joints

Chapter 6”Analysis and design . .318.1-8 6.0-Notation 6.1 -Design Method 6.2-Permissible Stresses 6.3-Design

Chapter 7- Plain concrete members.. ....... 318.1-11 7.0-Notation 7.1 “walls 7.2-Footings 7.3-Pedestals 7.4-Precast Members

This code covers the proper design and construction of structural members of plain concrete, and is written in such form that it may be adopted by reference in a general building code. This code supplements AC1 Standard 318 “Building Code Requirements for Reinforced Concrete. ”

Among the subjects covered are: permits and drawings; inspection; materials; concrete quality; formwork; control joints; analysis and design (permissible stresses); and structural members (walls, footing, and pedestals).

Keywords: building codes; compressive strength; control joints; flexural strength; footings; inspection; plain concrete; precast concrete; shear strength; specifications: stresses; structural design; walls.


0662949 0504708 6 T T PLAIN CONCRETE CODE AND COMMENTARY 318.11318.1R-3

CHAPTER I-GENERAL REQUIREMENTS

CODE 1 .l-scope 1.1.1-This code provides minimum requirements for design and construction of structural plain concrete members (cast-in-place or precast) of any structure erected under requirements of the legally adopted general building code of which this code forms a part.

In areas without a legally adopted building code, this code defines minimum acceptable standards of design and construction practice.

1.1.2-This code supplements the general building code and “Building Code Requirements for Reinforced I Concrete (AC1 31 8-89) (Revised 1992)”* and shall govern in all matters pertaining to structural plain concrete design and construction except wherever this code is in conflict with requirements of the legally adopted general building code. Requirements of AC1 318.1-89 (Revised 1992) should govern where in conflict with requirements of AC1 31 8-89 (Revised 1992).

1.1.3-All applicable provisions of AC1 318 not in conflict with provisions of this code shall apply to plain concrete.

1.1.4-This code shall govern in all matters pertaining to design, construction, and material properties wherever this code is in conflict with requirements contained in other standards referenced in this code or recom- mended practices referenced in this code.

1.1.5-For special structures, such as arches, under- ground utility structures, gravity walls, and shielding walls, provisions of this code shall govern’where applicable.

1.2-Limitations

1.2.1-Provisions of this code shall apply for design of plain concrete members, defined as either unreinforced or containing less reinforcement than the minimum amount specified in AC1 318for reinforced concrete. See section 2.1

1.2.2-Use of plain concrete shall be limted to (a) members that are continuously supported by soil or supported by other structural members capable of providing continuous vertical support; (b) members for which

I

‘Published by American Concrete Institute, Detroit, Michigan. Hereafter ce- ferted to as AC1 318.

COMMENTARY R1.l-Scope The American Concrete Institute “Building Code Require- ments for Structural Plain Concrete (AC1 318.1)” provides minimum requirements for any structural plain concrete design and construction that is regulated by a legally adopted general building code of which it forms a part. AC1 3 18.1 is intended as a supplement to the general building code and the AC1 318 code for reinforced concrete, and is intended to govern for plain concrete when in conflict with the requirements in those codes.

Earlier editions of AC1 3 18 included design provisions for some uses of plain concrete, such as plain concrete footings; those provisions have been deleted from AC1 3 18 and are now contained exclusively in AC1 3 18.1

The design provisions of AC1 318.1 are empirical, based on present practice and successful experience in the use of plain concrete and unreinforced masonry for residential and light commercial buildings. Three types of plain concrete structural members are specifically included in Chapter 7: walls (Section 7.1), footings (Section 7.2), and pedestals (Section 7.3).

R1.1.2-The American Concrete Institute recommends that the code be adopted in its entirety; however, it is recognized that when this code is made a part of a legally adopted general building code that general building code may modify some provisions of this code.

R1.2-Limitations R1.2.1-By code definition, concrete that is either unreinforced or contains less reinforcement than the minimum amount specified for reinforced concrete is classified as plain concrete for design considerations. See definition of reinforced concrete in Section 2.1 of AC1 3 18.

R1.2.2 and Rl.2.3-Since the structural integrity of plain concrete members depends solely on the properties of the concrete, use of plain concrete structural members should be limited to: members that are primarily in a state of compres-


318.11318.1R-4 W 0bb2949 0504709 536 W

MANUAL OF CONCRETE PRACTICE

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CODE

arch action provides compression under all conditions of loading; or (c) cast-in-place concrete piles or piers, except in regions of high seismic risk, that have adequate lateral support for stability and where calculated compression occurs on the entire cross section under all conditions of loading.

1.2.3-Plain concrete shall not be used for structural members where special design considerations are required for earthquake or blast, unless explicitly permitted by the legally adopted general building code.

1.2.4-Plain concrete for compression members, other than arches and cast-in-place piles and piers permitted in Section 1.2.2, shall be limited to pedestals. See Section 7.3.

1.3-Permits and drawings

1.3.1-Copies of design drawings, typical details, and specifications for all structural plain concrete construction shall bear the seal of a registered engineer or architect. These drawings, details, and specifications shall show:

(a) Name and date of issue of code and supplement to which design conforms (b) Live load and other loads used in design (c) Specified strength of concrete at stated ages or stages of construction (d) Size and location of all structural members and any reinforcement (e) Details and location of all control joints

1.3.2-Calculations pertinent to design shall be filed with the drawings when required by the Building Official. When computer programs are used, design assump- tions and identified input a ld output data may be sub- mitted in lieu of calculations. Model analysis shall be permitted to supplement calculations.

1.3.3-Building Official means the officer or other desig- nated authority charged with the administration and en- forcement of this code, or his duly authorized representative.

COMMENTARY

sion; members that can tolerate random cracks without detri- ment to their structural integrity; and members where ductility is not an essential feature of design. The tensile strength of concrete can be utilized in design of members when the buildup of tensile stresses due to restraint from shrinkage or temperature are considered and sufficiently reduced by construction techniques to avoid uncontrolled cracks or when uncontrolled cracks due to such restraint effects can be antici- pated to occur in such a manner that will not induce structural failure or collapse.

It should be noted, however, that it is not within the scope of this code to provide serviceability requirements for nonstruc- turd members of plain concrete such as soil-supported slabs (slabs on grade).

The 1992 code was changed to specially include such structures as cast-in-place concrete piles and piers in ground or other material sufficiently stiff to provide adequate lateral support to prevent bucking.

R1.2.4-Since plain concrete lacks the necessary ductility that columns should possess and because a random crack in an unreinforced column will most likely endanger its structural integrity, the code does not permit use of plain concrete for columns. It does allow, however, its use for pedestals limited to a ratio of unsupported height to least lateral dimension of 3 or less (Section 7.3.2).

Plain concrete walls are permitted (see Section 7.1) without an absolute maximum height limitation. However, for multistory construction and other major structures, AC1 Commit- tee 318 strongly encourages the use of walls designed as reinforced concrete members in accordance with AC1 318. See Section R7.1.

R1.3-Permits and drawings R1.3.1-The provision for preparation of design drawings, specifications, and issuance of permits are, in general, con- sistent with those of most general building codes and are intended as supplements thereto.

The code lists some of the more important items of information that must be included in the design drawings, details, or specifications. The code does not imply an all-inclusive list, and additional items may be required by the Building Official.


PLAIN CONCRETE CODE AND COMMENTARY 318.11318.1R-5

CODE

1.4-Inspection 1.4.1-As a minimum, concrete construction shall be inspected as required by the legally adopted general building code. In the absence of such requirements, concrete construction shall be inspected throughout the various work stages by a competent engineer or architect, or by a competent representative responsible to that engineer or architect.

1.4.2-Inspector shall require compliance with design drawings and specifications.

Unless specified otherwise in the legally adopted general building code, inspection records shall include:

(a) Quality and proportions of concrete materials and strength of concrete (b) Construction and removal of forms and reshoring (c) Mixing, placing, and curing of concrete (d) Placing of any reinforcement (e) Any significant construction loadings on completed members, or walls (f) Sequence of erection and connection of precast members (9) General progress of work

1.4.3-When the ambient temperature falls below 40 F or rises above 95 F, a complete record shall be kept of concrete temperatures and of protection given to concrete during placement and curing.

1.4.4-Records of inspection required in Sections 1.4.2 and 1.4.3 shall be preserved by the inspecting agency, engineer, or architect for 2 years or longer after completion of the project.

COMMENTARY

R1.4-Inspection See Section R1.3 of AC1 318 for detailed discussion of in- Q

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CHAPTER 2-DEFINITIONS

2.1-The following terms are defined for general use in this code. For other terms used in this code, reference is made to AC1 318 where such terms have already been defined. Specialized definitions appear in individual chapters.

Control joint-Construction joint or partial joint (minimum 25 percent reduction of member thickness) used for the purpose of reducing buildup of internal stresses caused by restraint to movements due to creep, shrinkage, or temperature effects.

Plain concrete-Concrete that is either unreinforced or contains less reinforcement than the minimum amount specified in AC1 318 for reinforced concrete.

Precast concrete-Concrete member cast elsewhere than its final position in the structure.

Structural plain concrete-Plain concrete used for structural purposes.

R2.1-The definitions given are for use in application of this code only and do not always correspond to ordinary usage.

By definition, plain concrete is concrete that contains less than the minimum reinforcement required by the AC1 318 code for reinforced concrete.

The definition of control joint in this code is meant to serve the purpose of plain concrete construction only. See Section R5.2 for detailed discussion of jointing and the importance of jointing in plain concrete construction.

Soil-supported slabs, such as slabs on grade, are not considered to be structural slabs within the context of the definition for “structural” plain concrete, unless they transmit vertical loads from other parts of structúre to the soil.


318.1/318.1R-6 MANUAL OF CONCRETE PRACTICE 06627119 050117Ll 194

CHAPTER 3-MATERIALS

COMMENTARY

See commentary sections of AC1 318 for detailed discussion of applicable material requirements.

CODE 3.1-Materials for concrete

All materials for concrete (cement, aggregate, water, and admixtures when used) shall conform to Chapter 3 of AC1 318.

3.2-Metal reinforcement

Reinforcement, if used, shall conform to Section 3.5 of AGI 318.

3.3-Tests of materials

3.3.1-Building Official shall have the right to order testing of any materials used in plain concrete construction to determine if materials are of quality specified.

3.3.2-Tests of materials and of concrete shall be made in accordance with standards of the American Society for Testing and Materials, listed in Section 3.8 of AC1 318.

3.3.3-A complete record of tests of materials and of concrete shall be made available for inspection during progress of work and for 2 years after completion of the project, and shall be preserved by inspecting engineer or architect for that purpose.

3.4-Storage of materials

3.4.1-Cement and aggregates shall be stored in such manner as to prevent deterioration or contamination from foreign matter.

3.4.2-Any material that has deteriorated or has been contaminated shall not be used for concrete.

CHAPTER 4-CONCRETE QUALITY 4.0-Notation

c= specified compressive strength of concrete, psi

4.1-General

4.1.1-Concrete shall be proportioned to provide an average compressive strength as prescribed in Section 5.3.2 of AC1 318. Concrete shall be produced to mini- mize frequency of strengths below fias prescribed in Section 5.6.2.3 of AC1 318.

4.1.2-Requirements for c shall be based on tests of cylinders made and tested as prescribed in Section 4.6.2 of AC1 318.

4.1.3-Unless otherwise specified, c shall be based on 28-day tests. If other than 28 days, test age for eshall be as indicated in design drawings or specifications.

4.1.4-Design drawings shall show specified compressive strength of concrete cfor which each plain concrete member is designed.

Quality control requirements for plain concrete are the same as for reinforced concrete; this code, however, imposes a minimum concrete strength for plain concrete construction (2500 psi) for reasons explained in Section R4.2. See applicable commentary sections of AC1 318 for detailed discussion of concrete quality requirements.


8 ,

I PLAIN CONCRETE CODE AND COMMENTARY 318.11318.1R-7

CODE

4.2-Minimum strength

Specified compressive strength cof plain concrete to be used for structural purposes shall be not less than 2500 psi.

COMMENTARY

R4.2 Minimum strength

A minimum strength requirement for plain concrete construction is considered necessary because safety is based solely on strength and quality of concrete treated as a homogeneous material. Lean concrete mixtures may not produce adequately homogeneous material or well formed surfaces.

4.3-Selection of concrete proportions

Selection of concrete proportions shall conform to Sec- tion 5.2 of AC1 318.

4.4-Mixing and placing concrete

Mixing and placing of concrete shall conform to Chapter 5 of AC1 318.

4.5-Evaluation and acceptance of concrete

Evaluation and acceptance of concrete shall conform to Section 5.6 of AC1 318.

CHAPTER 5-FORMWORK AND JOINTS 5.1-Formwork

Design of formwork and removal of forms and shores shall conform to Chapter 6 of AC1 318.

5.2-Joints

5.2.1-ln plain concrete construction, control joints shall be provided to divide a structural member into flex- urally discontinuous elements. Size of each element shall be limited to control buildup of excessive internal stresses within each element caused by restraint to movements from creep, shrinkage, and temperature effects.

5.2.2-ln determining the number and location of control joints, consideration shall be given to: influence of climatic conditions; selection and proportioning of materials; mixing, placing, and curing of concrete; degree of restraint to movement; stresses due to loads to which an element is subject; and construction techniques.

5.2.3-Locations of control joints shall be indicated on the drawings or in the specifications. See Section 1.3.1.

5.2.4-Any reinforcement provided in a plain ,concrete member shall be terminated not less than 3 in. from a joint.

5.2.5-Interruptions of concrete placement shall be made only at joints.

5.2.6-Walls and similar members shall be keyed or dowelled to other intersecting members as required for lateral stability.

R5.2- Joints Joints in plain concrete construction are an important design consideration. In reinforced concrete, reinforcement is provided to absorb the stresses due to restraint of creep, shrinkage, and temperature effects. In plain concrete, joints are the only design means of controlling and thereby reliev- ing the buildup of such tensile stresses. A plain concrete member, therefore, must be small enough or divided into smaller elements by joints to control the buildup of the internal stresses. The joints may be a construction joint, or a control joint. The jointing must be such that no axial tension or flexural tension can be developed across a joint, a condition referred to by the code as flexural discontinuity.

No exact rules for the number and location of joints can be made. Each construction must be studied individually to determine where joints should be located, taking into account the requirements of the structural design. Where random cracking due to creep, shrinkage, and temperature effects will not affect the structural integrity, and is otherwise acceptable, such as transverse cracks in a continuous wall footing, transverse controljoints are not necessary. Control joints may be provided at intermediate locations between outside edges and construction joints to subdivide a large plain concrete member into smaller elements. Numerous ways have been devised for forming control joints depending on the type of construction. Control joints may be made with sheet metal or sheet plastic inserts, waterstop type rubber inserts or, formed, sawed or tooled grooves in the concrete surface to cause cracking at the predetermined location. The depth or thickness of the concrete section at these inserts or at formed, sawed, or tooled grooves should be reduced at least 25 per-


318.11318.1R-8 m 0662747 0504733 Tb7

CODE COMMENTARY


cent to make the control joint effective with enough remain- ing section for some degree of aggregate interlock to hold the adjacent elements in line. Use of free sliding dowels is considered an acceptable practice to augment the aggregate interlock with special attention to alignment of such dowels and to protection from corrosion (which satisfies the intent of Sec- tion 5.2.4).

CHAPTER 6"ANALYSIS AND DESIGN

6.0-Notation

A, = loaded area A2 = maximum area of the portion of the supporting

surface that is geometrically similar to and concentric with the loaded area

b = width of member, in. = specified compressive strength of concrete, psi,

See Chapter 4 = square root of specified compressive strength of

concrete, psi. f,, = average splitting tensile strength of lightweight

aggregate concrete psi. See Sections 5.1.4 and 5.1.5 of AC1 318

h = overall thickness of member, in. k = effective length factor. See Section 7.1.5.2 e, = vertical distance between supports, in. v,, = shear stress due to factored shear force at

V, = factored shear force at section ßc = ratio of long side to short side of concentrated

load or reaction area + = strength reduction factor. See Section 6.2.2

6.1-Design method

6.1.1-Plain concrete members shall be designed for adequate strength in accordance with provisions of this code, using load factors and permissible stresses.

6.1.2-Factored loads and forces shall be in such com- binations as specified in Section 9.2 of AC1 318.

6.1.3-Stresses due to factored loads and forces shall not exceed permissible stresses given in Section 6.2.

6.1.4-Where permissible stresses are to be exceeded, reinforcement shall be provided and the member designed as a reinforced concrete member in accordance with appropriate design requirements of AC1 318for reinforced concrete.

section

For plain concrete, the basic design concept that the member be proportioned to resist tensile stresses without the aid of reinforcement requires that an uncracked section be main- tained for all loading conditions. The permissible tensile stress is set sufficiently low to provide an uncracked section under factored loading conditions.

R6.1-Design methoc d

Plain concrete members are proportioned for adequate strength using factored loads and forces and keeping computed stresses within permissible stress limits. When computed stresses due to loads exceed the permissible stresses for the concrete strength specified, the section must be increased and/or the specified strength of concrete increased, or the member designed as a reinforced concrete member in accordance with AC1 318. The designer should note, however, that an increase in concrete section may have a detrimental effect; stress due to load will decrease while stresses due to creep, shrinkage, and temperature effects may increase.


CODE

PLAIN CONCRETE CODE AND COMMENTARY 318.11318.1R-9

COMMENTARY

6.2-Permissible stresses

6.2.1-Maximum fiber stresses in plain concrete due to factored loads and moments shall not exceed the following:

(a) Flexure Extreme fiber stress in compression. . . . . . . . . +c Extreme fiber stress in tension*. . . . . . . . .5$*

(b) Axial compression 0.60 + c l 1 - ($i$)‘]

(c) Shear* Beam action ....................... .2+*

Two-way action, . . . . . . . . . . . .

but not greater than 4 $C (d) Bearing on loaded areat . . . . . . . . . . .0.85 + c

‘Permissible shear and tension stresses apply for normal weight concrete: for lightweight aggregate concrete, one of the following modifications shall apply:

(a) When fn is specified and concrete is proportioned in accordancewith Section 5.2 of AGI 318, fJ6.7 shall be substituted for -but the value of fJ6.7 shall not exceed c (b) When f,, is not specified, the value of *hall be multiplied by 0.75 for “all- lightweight” concrete and by 0.85 for “sand-lightweight” concrete. Linear inter- polation is permitted when partial sand replacement is used.

When the supporting surface is wider on all sides than the loaded area, permissible bearing stress on the loaded area may be increased by -, , but not more than 2. When the supporting surface is sloped or stepped, A, may be taken as the area of the lower base of the largest frustum of a right pyramid or cone contained wholly within the support and having for its upper base the loaded area, and having side slopes of 1 vertical to 2 horizontal.

6.2.2-Strength reduction factor $ for flexure, compression, shear, and bearing of plain concrete shall be 0.65.

6.3-Design

6.3.1-Strength design of plain concrete members for flexure and axial loads shall be based on a linear stress- strain relationship in both tension and compression.

6.3.2-Tensile strength of concrete may be considered in design of plain concrete members when provisions of Sections 5.2.1 and 5.2.2 have been followed such that stresses will not exceed permissible stresses. See Sec- tion 6.2.

R6.2-Permissible stresses R6.2.1-The permissible stresses in flexural tension, shear, and bearing are the same as permitted for plain concrete pedestals and footings in earlier editions of the AC1 318 code. The permissible stress values for flexural and axial compression are new.

Application of the frustrum to find A, for permissible bearing strength in sloped or stepped supports is illustrated in Com- mentary Section 10.15 of AC1 318.

R6.2.2-The strength reduction factor $ for plain concrete design is made the same for all stress conditions. Since both flexural tension strength and shear strength for plain concrete depend on the tensile strength characteristics of the concrete, with no reserve strength or ductility possible due to the absence of reinforcement, equal understrength factors for both bending and shear are considered appropriate.

R6.3-Design R6.3.1”The code assumes that plain concrete is a homogeneous material capable of maintaining essentially a linear distribution of strains and stresses for the full range of loading conditions, even to ultimate. For design convenience, the code assumes a triangular stress distribution under flexure within the permissible fiber stresses. Stresses computed by the straight-line theory are not actual stresses; as a result, the permissible stresses are reduced to account for the difference in actual behavior versus assumed behavior.

R6.3.2-Flexural tension may be considered in design of plain concrete members to sustain loads, provided the computed stress does not exceed the permissible, and provided control joints are properly designed, spaced, and constructed to relieve the restraint and resulting tensile stresses due to creep, temperature, and shrinkage effects.


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6.3.3-No strength shall be assigned to metal reinforcement that may be present.

6.3.4-Tension shall not be transmitted through outside edges, construction joints, or control joints of an individual plain concrete element. No flexural continuity due to tension shall be assumed between adjacent plain concrete elements.

6.3.5-111 computing stresses due to flexure, combined flexure and axial load, and shear, the entire cross section of a member shall be considered in design, except for concrete cast against soil, overall thickness h shall be taken as 2 in. less that actual thickness.

6.3.6-Members subject to combined flexure and axial load shall be proportioned such that the sum of the ratios of all calculated to permissible stresses in compression given in Sections 6.2.l(a) and (b) shall be less than or equal to one. Tensile stress resulting from combined flexure and axial load shall not exceed permissible stress in tension given in Section 6.2.1 (a).

COMMENTARY R6.3.3-Concrete members containing less reinforcement than the minimum amount specified for reinforced concrete must be designed as plain concrete with strength based on the properties of the concrete alone. This assumption is not intended to apply to reinforcing used for the purpose of trans- femng an external force to a plain concrete element.

R6.3.4-Each element of plain concrete bounded by every outside edge or joint (construction or control joint) is considered as a separate structural element. Compressive and shear forces may be transferred to adjacent elements. Flexural continuity causing development of tensile stress between adjacent elements must be prevented.

R6.3.5-The reduced overall thickness h for concrete cast against earth is to allow for unevenness of excavation and for some contamination of the concrete adjacent to the soil.

R6.3.6-Plain concrete members subject to combined flexure and axial compressive load are proportioned such that on the compression face:

Calculated Calculated

6.3.7-Shear strength

6.3.7.1-Shear stress Y, for rectangular sections shall be computed by

n,,

v =- a v u

26h

where h is overall thickness of member. See Section 6.3.5.

and that on the tension face: Calculated - Calculated 5$)<

where the permissible stresses are as given in Section 6.2.

R6.3.7-Shear Strength

Proportions of plain concrete members will be controlled by tensile strength rather than shear strength for the usual plain concrete members of practical proportions. Shear stress (as a substitute for principal tensile stress) rarely will control. However, since it is difficult to foresee all possible conditions where shear may have to be investigated (e.g., shear keys), Committee 318 decided to maintain the investigation of this basic stress condition as a part of the code requirements. An experienced designer will soon recognize where shear is not critical for plain concrete members and will adjust his design procedure accordingly.

bending stress axial stress

R6.3.7.1-The shear requirements for plain concrete as- sume an uncracked section. Shear failure in plain concrete will be a diagonal tension failure, occurring when the principal tensile stress near the centroidal axis becomes equal to the tensile strength of the concrete. Since the major portion of the principal tensile stress comes from the shear, the code safeguards against tensile failure by limiting the permissible shear at the centroidal axis as calculated from the equation for a section of homogeneous material: Y = W h , where Y and V are the shear stress and shear force respectively at the section considered, O is the statical moment of the area outside the section being considered about centroidal axis of the


CODE

PLAIN CONCRETE CODE AND COMMENTARY 318.1/318.1R-l1

COMMENTARY

gross section, I is the moment of inertia of the gross section, and b is the width where shear stress is being computed.

This equation recognizes the more homogeneous nature of plain concrete and the concentration of shear stress near the centroidal axis. For a rectangular section, the computed shear stress will be about 50 percent greater than that computed for reinforced concrete. For sections other than rectangular, v= WQlIb should be used in place of W. (6-1). In special cases, investigation for principal tensile stresses in a homogeneous material may be appropriate.

6.3.7.2-Maximum shear stress Y.. shall be computed at a distance h from face of suppÕrt, and sections

5 located at a lesser distance may be designed for the same shear.

m 6.3.7.3-Shear stress v,, shall not exceed permissi- 2 ble shear stress for beam action given in Section

I4 a

6.2.1(~). Q il m CHAPTER 7-PLAIN CONCRETE MEMBERS

7.0-Notation Ag = gross area of section, sq in. bo = perimeter of critical section for shear in footings,

= specified compressive strength of concrete, psi.

-z

in.

See Chapter 4 h = overall thickness of member, in. k = effective length factor 4, = vertical distance between supports, in. P,, = nominal axial load strength of wall designed by

v,, = shear stress due to factored shear force at

V,, = factored shear force at section @ = strength reduction factor. See Section 6.2.2

Section 7.1.5

section

7.1-Walls

7.1.1-Plain concrete walls shall be continuously supported by soil or supported by footings, foundation walls, grade beams, or other structural members capable of providing continuous vertical support. See Section 1.2.2.

7.1.2-Plain concrete walls shall be designed for vertical, lateral, and other loads to which they are subjected.

7.1.3-Plain concrete walls may be designed in accordance with Section 6.3.6 provided the wall is designed for an eccentricity corresponding to the maximum moment that can accompany the axial load but not less than 0.10h. Otherwise, plain concrete walls shall be designed under provisions of Section 7.1 5.

7.1.4-Design for shear shall be in accordance with Section 6.3.7.

R7.1-Walls

Plain concrete walls are commonly used for basement wall construction for residential and light commercial buildings in low or nonseismic areas. Although the code imposes no absolute maximum height limitation on the use of plain concrete walls, designers are cautioned against extrapolating the experience with relatively minor structures and using plain concrete walls in multistory construction and other major structures where differential settlement, wind, earthquake, or other unforeseen loading conditions require the walls to possess some ductility and ability to maintain their integrity when cracked. For such conditions, AC1 Committee 318 strongly encourages the use of walls designed as reinforced concrete members in accordance with AC1 3 18 for reinforced concrete.

The provisions for plain concrete walls are applicable only for walls laterally supported in such a manner as to prohibit relative lateral displacement at top and bottom of individual wall elements (see Section 7.1.6.4). This code does not cover walls where there is no horizontal support to prohibit relative displacement at top and bottom of wall elements. Such later-


318.11318.1R-12 0bb2949 0504717 bo2


CODE

7.1.5-Empirical design method

7.1.5.1-Plain concrete walls of solid rectangular cross section may be designed by Eq. (7-1) if resultant of all factored loads is located within the middle-third of the overall thickness of wall.

7.1.5.2-Design axial load strength @ P,, of a plain concrete wall satisfying limitations of Section 7.1 S.1 shall be computed by

r 1

where + = 0.65 and effective length factor k shall be: For walls braced top and bottom against lateral transla- tion and

(a) restrained against rotation at one or both ends (top and/or bottom) . . . . . . . . . . . . . . . . . . . . . . . . .0.8 (b) unrestrained against rotation at both ends . . . 1 .O

7.1.6-Limitations

7.1.6.1-Unless demonstrated by a detailed analysis, horizontal length of wall to be considered effective for each vertical concentrated load shall not exceed center- to-center distance between loads, nor width of bearing plus 4 times the wall thickness.

7.1.6.2-Thickness of bearing walls shall not be less than 1 /24 the unsupported height or length, whichever is shorter, nor less than 5% in.

7.1.6.3-Thickness of exterior basement walls and foundation walls shall not be less than 7% in.

7.1.6.4-Walls shall be braced against lateral transla- tion. See Sections 5.2. and 6.3.4.

7.1.6.5-Not less than 2 # 5 bars shall be provided around all window and door openings. Such bars shall extend at least 24 in. beyond the corners of openings.

COMMENTARY

ally unsupported walls must be designed as reinforced concrete members in accordance with AC1 3 18.

Plain concrete walls as structural members are subject to the limitations of Section 1.2.2 and the jointing requirements of Section 5.2, which greatly affect their design. Plain concrete walls must be designed to resist all loads to which they are subjected, including eccentric axial loads and lateral forces. In general, the provisions apply to walls spanning vertically. Also, the empirical design method of Section 7.1.5 applies only to walls of solid rectangular cross sections; other shapes must be designed according to Section 6.3.6.

Plain concrete walls must be designed for combined flexure and axial load according to Section 6.3.6, considering the wall to be a compression member with flexure, unless meet- ing the requirements of Section 7.1.5. For some cases, shear strength may also need to be investigated.

R7.1.5-Empirical design method

When the resultant load falls within the middle third of the wall thickness (kern of wall section), plain concrete walls may be designed using the simplified Eq. (7-1). Eccentric loads and lateral forces are used to determine the total eccentricity of the factored load 4. If the eccentricity does not exceed h/6, Eq. (7-1) may be applied, and design performed considering e as a concentric load. The factored axial load e must be less than the design axial load strength @ew, computed by Eq. (7-1), or e G @ e,. Eq. (7-1) is presented to reflect the general range of braced and restrained end conditions encountered in wall design. The limitations of Section 7.1.6 apply whether the wall is proportioned by Section 6.3.6 or by the empirical method of Section 7.1.5.


I PLAIN CONCRETE CODE AND COMMENTARY 318.1/318.1R-13

I CODE

7.2-Footings 7.2.1-Plain concrete footings shall be designed for fac-

1 tored loads and induced reactions in accordance with appropriate design requirements of this code and as

3 o- provided in Section 7.2. Y

Q 7.2.2-Base area of footing shall be determined from

,=I unfactored forces and moments transmitted by footing to soil and permissible soil pressure selected through

o principles of soil mechanics.

T piles.

2 7.2.4-Thickness of plain concrete footings shall not be JI less than 8 in. See Section 6.3.5.

Y ‘ O 7.2.3-Plain concrete shall not be used for footings on

r

O

D

7.2.5-Moment in plain concrete footings

Maximum factored moment shall be computed at critical sections located as follows:

(a) At face of column, pedestal, or wall, for footing supporting a concrete column, pedestal, or wall. (b) Halfway between middle and edge of wall, for footing supporting a masonry wall. (c) Halfway between face of column and edge of steel base plate, for footing supporting a column with steel base plate.

7.2.6-Shear in plain concrete footings

7.2.6.1-Maximum factored shear shall be computed in accordance with Section 7.2.6.2, with location of critical section measured from face of column, pedestal, or wall for footing supporting a column, pedestal, or wall. For footing supporting a column with steel base plates, the critical section shall be measured from location defined in Section 7.2.5(c).

7.2.6.2-Shear strength of plain concrete footings in the vicinity of concentrated loads or reactions shall be governed by the more severe of two conditions:

(a) Beam action for footing, with a critical section ex- tending in a plane across the entire width and located at a distanceh from face of concentrated load or reaction area. For this condition, the footing shall be designed in accordance with Section 6.3.7. (b) Two-way action for footing, with a critical section perpendicular to plane of footing and located so that its perimeter bo is a minimum, but need not approach closer than h/2 to perimeter of concentrated load or reaction area. For this condition, the footing shall be designed in accordance with Sections 7.2.6.3. and 7.2.6.4.

COMMENTARY

R7.2-Footings

R7.2.4-Thickness of plain concrete footings will be controlled by flexural strength (extreme fiber stress in tension not greater than 5 G m r a t h e r than shear strength for the usual proportions of plain concrete footings. Shear rarely will control; see Section R6.3.7. For footings cast against soil, overall thickness h used for strength computations must be taken as 2 in. less than actual thickness to allow for unevenness of excavation and contamination of the concrete adjacent to soil as required by Section 6.3.5. Thus, for a minimum footing thickness of 8 in., calculations for flexural and shear stresses must be based on an overall thickness of 6 in.

R7.2.6-Shear in plain concrete footings


318.11318.1R-14 = 0bb29V9 0504739 V85 =


CODE

7.2.6.3-Shear stress v,, shall be computed by

v,=- 3 V" 2 4 4 (7-2)

where V,, and bo shall be taken at the critical section defined in Section 7.2.6.2(b) and h is overall thickness of footing. See Section 6.3.5.

7.2.6.4-Shear stress v, shall not exceed permissible shear stress for two-way action given in Section 6.2.l(c).

7.2.7-Circular or regular polygon shaped concrete columns or pedestals may be treated as square members with the same area for location of critical sections for moment and shear.

7.2.8-Bearing stress on concrete at contact surface between supporting and supported member shall not exceed permissible bearing stress for either surface as given in Section 6.2.l(d).

7.3-Pedestals

7.3.1-Plain concrete pedestals shall be designed for vertical, lateral, and other loads to which they are subjected.

7.3.2-Ratio of unsupported height to average least lateral dimension of plain concrete pedestals shall not exceed 3.

7.3.3-Maximum compressive stress in plain concrete pedestals shall not exceed permissible bearing stress given in Section 6.2.1 (d).

7.4-Precast members

7.4.1-Design of precast plain concrete members shall consider all loading conditions from initial fabrication to completion of the structure, including form removal, storage, transportation, and erection.

7.4.2-Limitations cited in Section 1.2 apply for precast members of structural plain concrete not only to the final condition but also during fabrication, transportation, and erection.

7.4.3-Precast members shall be connected securely, to transfer all lateral forces into a structural system capable of resisting such forces.

7.4.4-Precast members shall be adequately braced and supported during erection to insure proper alignment and structural integrity until permanent connec- tions are completed.

COMMENTARY

R7.2.6.3-As for beam action shear, shear stress for two- way action for plain concrete footings is calculated using the equation for a section of homogeneous material (v = VQ//b) since the critical principal tensile stress near mid-depth of the footing will approximate this value.

R7.3-Pedestals The height-thickness limitation for plain concrete pedestals does not apply for portions of pedestals embedded in soil capable of providing lateral restraint.

R7.4-Precast members Precast structural members are subject to all limitations and provisions for cast-in-place concrete contained in this code.

The approach to control joints is expected to be somewhat different than for cast-in-place concrete since the major portion of the internal stresses due to shrinkage takes place prior to erection. To assure stability, precast members should be connected to other members. Connection must be such that no tension will be transferred from one member to the other.


The AMERICAN CONCRETE INSTITUTE

was founded in 1905 as a nonprofit membership organization dedicated to public service and to representing user interests in the field of concrete. It gathers and distributes information on the improvement of design, construction, and maintenance of concrete products and structures. The work of the Institute is done by individual members and by volunteer committees.

The committees, as well as the Institute as a whole, operate under a consensus format, which assures all members the right to have their views considered. Committee activities include the development of building codes and specification standards; analysis of research and development results; presentation of construction and repair techniques; and education.

Anyone interested in the activities of the Institute is encouraged to seek membership. There are no educational or employment requirements. En- gineers, architects, scientists, constructors, and representatives from a va- riety of companies and organizations form the Institute membership.

All members are eligible and encouraged to participate in committee activities that relate to their specific areas of interest. Membership information, a publications catalog, and listings of educational activities are available.

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