ACI-318.1-89

3LB.L/3LB=LR

92

0662949 0504705

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F i r s t P r i n ti n g , S e p t e m b e r

1992

AC1 Manual of Concrete Practice

Most 1 Standards and committee reports reports in the general areas of materials and aregathered together in the annually revisedproperites of concrete, construction practices AC1 Manual of Concrete Practice. The several and inspection, pavements and slabs, strucvolumes are arrange to group related material tural design and analysis, structural specificatogether and may be purchased individually or tions, and special products and processes. in sets. A complete catalog all AC1 publications is AC1 Committeespreparestandardsand availablewithout charge. American Concrete Institute ox 19150, Redford Station Detroit, Mich. 48219-0150

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Enhancement ofAC1 Documents

The technical committees responsible forC1 committee reports and standards striveavoid ambiguities, omissions, and errorsn these documents. In spite o these efforts, the users f 1 documents occasionally find informationor requirements that may be subject to more than one interpretation may be incomplete ing demand in the industry for certified workers: incorrect. To assist in the effort for accuracy and clarity, the Concrete Flatwork Finisher Technical Activities Committee solicits the helpf indiConcrete Field Testing Technician-Grade viduals using 1 reports and standard n identifyin Concrete Laboratory Testing Technician-Grade I and eliminating problems that may be associated with ConcreteLaboratory Testing Technician-Grade II their use. Concrete Construction Inspector-ln-Training Users who have suggestions for the improvement Concrete Construction Inspector-Level II ofAC1 documents are requeste to contact the AC1 Engineering Department n writing, with the following This document may already contain reference these information: 1 certification programs, which can be incorporate into project specifications or quality control procedures. 1. Title and number f the document containin he If not, suggested guide specifications are available onproblem and specific section in the document; 2. Concise descriptionof the problem; request from the AC1Certification Department. 3. If possible, suggested revisions for mitigating the problem. The Institute’s Engineering Staff will review and take appropriate action onll comments and suggestions received. Members as well as nonmembers of the Institute are encouraged assist in enhancing the accuracy and usefulness of 1 documents.

The final quality of a concrete structure depends on qualified people to construct it. A 1 certification programs identify craftsmen, technicians and inspectors who have demonstrated their qualifications. The follow-

COPYRIGHT American Concrete Institute

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 C1 Committee W.G.Corley

Basile G. Rabbat

Chairman

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

Secretary

Luis E. Garcia ichard olguin ames acGregor harles almon Richard aynor avid unter, oberthester iess Jacob S. Grossman rancis Jacquesattock obert mith David ustafsonaniel enny Walter oore,ozen John M. Hanson ames O. Jirsa larkson James R. Harris ames efter ichard amsey aymond eaveley oring yllie, S. awrence Edward S. Hoffman

W. inkham rwin peyer

E. Stephan

Voting Subcommittee Members Bijan O. Aalami Roger J. Becker Edward M. Frisbee Richard Furlong

S. K. Ghosh Roger Green Philip G . Griffin James K. lverson

Phillip J. lverson Paul Klieger Cary Kopczynski Michael E. Kreger

Cameron Maclnnes David Lashgari Peter Marti Denis Mitchell

Jack P. Moehle Donald R. Strand David A. W hiting James K. Wight

Liaison Members hunsuke udiger honier ewes enry tani Martin saac Julio Cesar Caballero Augusto Carlos De Vasconcelos Luis Eduardo Laverde Peter Lenkei Harold P. lsaacs

Robert Park Horacio Ramirez de Alba

AC1 318.1-89 (Revised 1992) was adopted as a standard of the American Concrete Institute July 1, 1992 o supersede AC1 31 1 -89 in accordance with the Institute's standardization procedure. Vertical lines n the margins indicate the 1992 changes. 'Acomp lete metric companion to AC1 318.11318.1Rhas been developed, 318.1M/318.1RM; therefore o metric equivalents are included n his document. tACl Committee Reports, Guides, Standard Practices, and Commentaries are intended for guidance n designing, planning, executing, or inspecting construction, and n preparing specifications. Reference o these documents shall not

Mireya Veloz Habib M. Zein Al-Abidien

made in he Project Documents.items foundn hese documents are desired be part of the Project Documents hey s hould be phrased n mandatory anguage and incorporated into the Project Documents. Copyright 1992 American Concrete Institute. All rights reserved including rights reproduction and usen any form or by any means,ncluding he making of copies by any photo process, or by any electronic or mechanical device, printed or written r oral, or recording for sound or visual reproduction or foruse in any knowledge or retrieval system r device, unless permissionn writing is obtained from the copyright proprietors.

318.1/318.1R-l COPYRIGHT American Concrete Institute Licensed by Information Handling Services

George Somenrille Bai Shengxian

318.1I318.1R-2 ANUAL

RACTICE CONCRETE

Obb29V9 0 5 0 V 7 0 7

763

Th 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. further distinguish the Code from the Commentary, the Code has been printed n Helvetica, the sameype face in which this pargraphs set. Vertical linesn the margins indicate changes from 31 89. 8.1 This paragraph is set in Times Roman, all portions Commentary section numbers are preceded by “R

the text exclusive the Commentary are printed in this type face. further distinguish them from Codee ction numbers.

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

.......... 318.1-5

Chapter 2-Definitions

........... 318.1-6

Chapter 3-Materials. 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 5.1-Formwork 5.2-Joints

318.1-7

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 concretemembers.. 7.0-Notation 1“w al ls 7.2-Footings 7.3-Pedestals 7.4-Precast Members COPYRIGHT American Concrete Institute Licensed by Information Handling Services

.......318.1-11

This code covers the properdesign and construction structural members ofplain concrete, and is ritten in such form thatt may be adoptedy referencen a general building code. Thiscode supplements A 1 Standard 31 “Building Code Requirements for Reinforced Concrete. Among the ubjects coveredre: permits and drawings; inontrol oints; spection; materials; concrete quality; formwork; analysis and design (permissible stresses); and structural members (walls, footing, and pedestals). Keywords: building codes; ompressive strength; control joints; flexural strength; footings; inspection;plain concrete; precast concrete; shear strength; specifications: stresses; structural design;walls.

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PLAIN CONCRETE CODE AND COMMENTARY

318.11318.1R-3

CHAPTER I-GENERAL REQUIREMENTS l-scope

COMMENTARY

CODE R1.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 his code forms a part. In areas without a legally adopted building code, this code defines minimum acceptable standardsf design and construction practice.

The American Concrete Institute “Building Code Requirements for Structural Plain Concrete (AC 318.1)” provides minimum requirements for structural plain concrete design and construction that is regulated by a legally adopted general building code of which it forms a part. AC 3 18.1 is intended as a supplement o the general building code and the AC 318 code for reinforced concrete, and is intended to govern for plain concrete when in conflict with the requirements in hose codes. Earlier editions of AC13 18 included design provisionsor some uses plain concrete, such as plain concrete footings those provisions haveeen deleted from AC 3 18 and are no contained exclusively in C1 18.1 The design provisions f AC1 318.1 empirical, based on present practice and successful experience in the useof plain concrete andunreinforced masonry for residential and light commercial buildings. Three types of plain concrete structural members are specifically included in Chapter walls (Section 7.1), footings (Section 7.2), and pedestals (Section 7.3).

1.1.2-Thiscodesupplements hegeneralbuilding R1.1.2-The American Concrete Institute recommends that code and “Building Code Requirements for Reinforced the codebe adopted in its ntirety; however, it is recognized Concrete (AC1 31 8-89) (Revised992)”*and shall govthat whenhis code is made a part of a legally dopted general ern in all matters pertaining to structural plain concrete building code that general building code may modify some design and construction except wherever this code is provisions in this code. conflict with requirementsf the legally adopted general building code. Requirements of AC1 318.1-89 (Revised 1992) should govern where in conflict with requirements of AC1 318-89 (Revised 1992). 1.1.3-All applicable provision 318 not n conflic with provisionsf 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 in conflictwith requirementscontained other standards referenced in this code or recommended practices referenced n 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. R1.2-Limitations

1.2-Limitations

y code definition, concrete that is either unrein1.2.1-Provisions of this code shall apply for design of plain concrete members, defined as either unreinforced forced or contains less reinforcement than the minimum amount specifiedor reinforced concrete is classified as plain or containing less reinforcement than theinimum f reinamount specifiedn 1 318for reinforced concrete. See concrete for design considerations. See definition forced concrete in Section 2. AC 18. section 2.1

1.2.2-Use of plain concrete shall be limted to (a) mem- R1.2.2 an Rl.2.3-Since the structural integrity of plain concrete members depends solely on the propertiesthe bers hat are continuouslysupported by soilor supported by other structural members capable of providingconcrete, use of plain concrete structural members should limited to: members thatre primarily in a tate of comprescontinuousverticalsupport; (b) members orwhich ‘Published by American Concrete Institute, Detroit, Michigan. Hereafter as 318.

ferted


ce-

MANUAL OF CONCRETE PRACTICE

318.11318.1R-4

COMMENTARY

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 tructural members where special design considerations are required for earthquake or blast, unless explicitly permitted by the legally adopted general building code.

0bb2949 0504709

sion; members thatan tolerate random racks without detriment to their structural integrity; and members whereductility isnot 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 echniques o avoid uncontrolledracks or when uncontrolled cracks due to such restraint effects can be anticipated to occur in such a manner that will not induce structural failure or collapse. It should be noted, however, that it is not within thescope of this code to provide serviceabilityrequirements for nonstructurd 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 1.2.4-Plain concrete for compression members, other than arches and cast-in-place piles and piers permitted that columns should possess and because a random crack in an unreinforcedcolumn will most likely endanger its strucin Section 1.2.2, shall be limited to pedestals. See Section tural integrity, he code does not permit us of plain concrete 7.3. for columns. It does allow, however, its use for pedestals limited to a ratio f unsupported height to least lateral dimension or less (Section 7.3.2).

1.3-Permits and drawings 1.3.1-Copies of design drawings, typical details, and specifications fo all structural plain concrete construction shall bear the seal f a registered engineer architect. These drawings, etails, 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 n design (c) Specified strength f concrete at stated ages or stages construction (d) Size and location ofll structural members and any reinforcement (e) Details and locationf all control joints

Plain concrete walls are permitted (see Section 7.1) without an absolute maximum heightimitation. However, for multistory construction and ther major structures, AC1 Committee 318 strongly encourages the use f walls designed s 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, consistent with those of most general building codesand are intended as upplementsthereto. The code lists some of the more important items of information that must included in the designdrawings, details, specifications. The code does not implyan all-inclusive list, and additional items may be requ ired by the Buildin Official.

1.3.2-Calculations pertinent to design shallbe filed with the drawings when required by the Building Official. When computer programs are used, design assumptions and identified input d output data may be sub mitted in lieu of calculations. Model analysis shall e permitted to supplement calculations. 1.3.3-Building Official means the officer other designated authority charged with the administration and enforcement of this code, or his duly author ized representative.

COPYRIGHT American Concrete Institute Licensed by Information Handling Services

OMMENTARY 18.11318.1R-5 NDONCRETE ODE PLAIN

COMMENTARY

CODE

1.4-Inspection

R1.4-Inspection

See Section R1.3 of AC1 31 for detailed discussion of ins a minimum, concrete construction shall be inesponsibility. spected as requiredby the legally adopted general spection 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 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) Sequenceof erection and connection f precast members (9) General progress of work

0-

1.4.3-When the ambient temperature falls below 40 or rises above 95 F, a complete record shall be kept of concrete temperatures andf protectiongiven to concrete during placement and curing. 1.4.4-Records of inspection requiredn 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.

CHAPTER 2-DEFINITIONS 2.1-The following terms are defined for general use this code. For other terms used n this code, reference made to AC1 where such terms have already been defined. Specialized definitions appear in individual chapters. Control oint-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.

R2.1-The definitions given re for use in application of this code only and o not always correspond to ordinary usage. By definition, plain concrete is concrete hat contains less than the minimum reinforcement requiredby the AC1 31 code for reinforced concrete. The definition control joint in this code is meant to serve the purposeof plain concrete construction only. See Section R5.2 for detailed discussion of jointing and themportance jointing in plain concrete construction.

Plain concrete-Concrete that is either unreinforced or Soil-supported slabs, such as slabs on grade, are not considered to be structural slabs within thecontext of the definition contains less reinforcement than the minimum amount for “structural” plain concrete, unless they transmit vertical specified in AC1 318 for reinforced concrete Precast concrete-Concrete member cast elsewhere than its final positionn the structure. Str uctur al plai n oncrete-Plain concrete used for structural purposes.


F-

loads from other parts

structúre to the soil.

MANUAL

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CONCRETE PRACTICE

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CHAPTER 3-MATERIALS COMMENTARY

CODE 3.1-Materials for concrete Al materials for concrete (cement, aggregate,water, and admixtures when used) shall conform to Chapter of AC 318.

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

3.2-Metal reinforcement Reinforcement, if used, shall conform to Section 318.

of

of materials

3.3-Tests

3.3.1-Building Official shallave the right to order esting of any materialsused in plain concrete construction to determine materials are of quality specified. 3.3.2-Tests of materials andf concrete shall be mad in accordance with standards of the American Society for Testing and Materials,isted n Section 3. f AC1 318.

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

3.4-Storage

materials

3.4.1-Cement and aggregates shall be stored n such manner as to preventdeterioratio contamination from foreign matter. 3.4.2-Any material that has deteriorated or has been contaminated shall not used for concrete.

CHAPTER 4-CONCRETE QUALITY 4.0-Notation

c= specified

compressive strengthf concrete, ps

4.1-General 4.1.1-Concrete shall be proportioned to provide an average compressive strength prescribed n Section 5.3.2of AC1 318.Concrete shall be produced to minimize frequency f strengths below s prescribed n Section 5.6.2.3 318. 4.1.2-Requirements for shall be based on ests of cylinders made and tested as prescribed in Section 4.6.2 of AC1318. 4.1.3-Unless otherwise specified, shall be based on 28-day tests. other than days, test age forshall as ndicated in design drawings or specifications. 4.1.4-Design drawings shallshow specified compressive strength of oncrete cfor which each plain concrete member is designed.


Quality control requirements for plain concrete are the same as for reinforced concrete; this cod e, how ever, imposes minimum concrete strength for plain concrete construction (250 0 psi) for reasons explained in Section R4.2. See applicable commentary ection s of AC 318 for detailed discussion of concrete quality requirements.

OMMENTARY 18.11318.1R-7 ND ONCRETE ODE PLAIN

COMMENTARY

CODE

R4.2 Minimum strength minimum strength requirement forplain concrete conSpecified compressive strengthof plain concrete to be struction is considered necessary because safety is ased used for structural purposes shall be not less than 2500 solely on trength and quality of concrete treated as a homogpsi. eneous material. Lean concrete mixturesmay not produce adequately homogeneous material or well formed surfaces

4.3-Selection

concrete proportions

Selection of concrete proportions shall conform to Sec318. tion 5.2

4.4-Mixing and placing concrete Mixing and placing of concrete hall conform to Chapter of

318.

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

CHAPTER 5-FORMWORK

AND JOINTS

5.1-Formwork Design f formwork and removalf forms and shores shall conform to Chapterof AC1 318.

5.2-Joints

R5.2- Joints

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

Joints in plain concrete construction are an important design consideration. In reinforced concrete, reinforcement is provided to absorb the stresses due to restraint f cr eep, shrinkage, and temperature effects. In plain concrete, joints the only designmeans of controlling and thereby relieving the buildup of such tensile stresses. 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 oint, or control joint. The jointing must be such that noxial tension or flexural tension can be developed across a joint, a condition referred to by the code as flexural discontinuity.

5.2.2-ln determining the number and locationf control joints, consideration shall be given to: influence of climatic conditions; selection and proportioning materials; mixing, placing, and uring of concrete; degree of restraint to movement; stresses due to loads which an element is subject; and construction techniques. 5.2.3-Locations of control joints shall be indicated on the drawings orn the specifications. See Section.3.1. 5.2.4-Any reinforcement provided in a plain ,concrete member shall be terminated not less than in. from a joint. 5.2.5-Interruptions of made only atoints.

concrete placement shall be

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


No exact rules for the number and locationof joints can be made. Eachconstruction must be studied individually o determine where joints should be located, taking into account the requirements of the structural design. Where random cracking due tocreep, shrinkage, and temperature effects will not affect the structural integrity,nd i s otherwise acceptable, such as transverse cracks in a continuous wall footing, transverse controljoints ar not necessary. Controloints may be provided at intermediate locations between outside edges andconstruction oints to subdivide a large plain concrete member into smaller elements. Numerousays have been devised for forming control joints depending n 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 theoncrete surface to cause cracking at the predetermined location. The depth or thickness of the concrete section athese inserts at formed, sawed, or tooled grooves should reduced at least 25 per-

MANUAL OF CONCRETE PRACTICE

318.11318.1R-8

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COMMENTARY

CODE

cent to make thecontrol joint effective with enough remaining section or some degree aggregate interlock to hold the adjacent elements in line. Us of free sliding dowels is considered an acceptablepractice to augment the ggregate nterlock with specialttention to alignment of such dowels and protection from corrosion (which satisfies the intentof Section 5.2.4).

CHAPTER 6"ANALYSIS AND DESIGN 6.0-Notation A, A2

f,,

e,

v,, V, ßc

For plainconcrete, the basic designconcept that the member loadedarea be proportioned to resist tensile stresses without the aid of maximum area of the portion of the supporting reinforcement requires that an uncracked section be mainsurface that s geometrically similaro and contained for all loading conditions. The permissible tensile centric with the loaded area stress is set sufficiently lowo provide an uncracked section widthofmember,n. specified compressive strength of concrete, psi, under factored loading onditions. See Chapter square root of specified compressive strength of concrete, psi. averagesplitting ensilestrengthof ightweight .1.4 and aggregate concrete psi. See Sections 5.1.5 of AC1 318 overall hicknessofmember, n. effective ength actor.Se Section7.1.5.2 verticaldistancebetweensupports, n. shear stress due t o factored shear force at section factored shear orceat section ratio of ong side to short side of concentrated load or reaction area strength eduction actor.Se Section6.2.2

6.1-Design method 6.1.1-Plain concrete members hall be designed for adequate strength n accordance with provisions of this code, using load factors and permissible stresses.

R6.1-Design

methoc

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 6.1.2-Factored loads and forces shall be n such comthe concrete trength specified, the section must be increased binations as specifiedn Section 9.2 of 318. and/or the specified strength concrete increased,or the member designed s a reinforced concrete member in accor6.1.3-Stresses due to factored loads and forces shall dance withAC1 318. The designer should note, however, that not exceed permissible stresses given n Section 6.2. an increase in concrete section may havea detrimental effect; 6.1.4-Where permissible stresses are to be exceeded,stress due to load will decrease while stresses due to creep, reinforcement shall be provided and the member de- shrinkage, and temperatureeffects may increase.

signed a reinforced concrete member n accordance with appropriate design requirements 318for reinforced concrete.


OMMENTARY 18.11318.1R-9 NDONCRETE OD PLAIN

COMMENTARY

CODE 6.2-Permissible stresses

R6.2-Permissible stresses

6.2.1-Maximum fiber stresses in plain concrete due factored loads and moments shall not exceed the following: (a) Flexure Extreme fiber stressn compression. . . . . . . . . +c

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

Extreme fiber stressn tension*. . . . . . . . 5$* (b) Axial compression0.60 (c) Shear* Beam action

+cl1

($i$)‘]

.......................

Applicationof the frustrum to find A, for permissible bearing strength in sloped or stepped supports is illustrated in Commentary Section 10.15 of AC1318.

.2+*

Two-wayaction, . . . . . . . . . . . . but not greater than (d) Bearing on loaded areat . . . . . . . . . . .0.85 ‘Permissible shear and tension stresses apply for normal weight concrete: for lightweight aggregate concrete, onethe following modifications shall apply: (a) When s specified and concrete sproportioned accordancewith Section the value of fJ6.7 shall 5.2 of AGI 318, J6.7 shall be substituted for but not exceed

(b) When is not specified, the value of *hall be multiplied by0.75 for “alllightweight” concrete and 0.85 or “sand-lightweight” concrete. Linear interpolation is permitted when partial sand replacement s used. Wh en the supporting surfaces wider on all sides than the loaded area, permissible bearing stress on the loaded area may be increased by but not more than When thesupporting surface is sloped or stepped,may be taken as he area of the lower base the largest frustum f a right pyramid or cone contained whollywithin the support and having forts upper base the loaded area, and having side orizontal. vertca to lopes

6.2.2-Strength reduction factor for flexure, compres- R6.2.2-The sion, shear, and bearing of plain concrete shall be 0.65.

strength reduction factor

for plain concrete

flexural tension trength and shear strength for plain concrete depend on he 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 andshear are considered appropriate.

6.3-Design

R6.3-Design

R6.3.1”The code assumes that plainconcrete is a homoge6.3.1-Strength design of plain concrete members for neous material capable of maintaining essentially a linear flexure and axial loads shall be based on a linear stressdistribution of strains and stresses for the full range of loadstrain relationshipn both tension and compression. ing conditions, even to ultimate. For design convenience, the code assumes a triangular stress distributionnder flexure within the permissible fiber stresses. Stresses computed by the straight-line heory 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 6.3.2-Tensile strength of concrete may be considere in design of plain concrete members when provisions plain concrete members to sustain loads, provided the computed stress does not exceed the permissible, and provided Sections 5.2.1 and 5.2.2 have been followed such that control oints are properly designed, spaced, and constructed stresses will not exceed permissible stresses. See Secto relieve the restraint and resulting tensile stresses due to tion 6.2. creep, temperature, and shrinkage effects.


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COMMENTARY

CODE 6.3.3-No strength shall be assigned metal reinforcement that maybe present.

R6.3.3-Concrete members containing less reinforcement than the minimumamount specified for reinforced concrete must designed as plain concrete with strength based on the properties of the concrete alone. This assumption is not intended to apply to reinforcing used or the purpose of transfemng an external orce to a plainconcrete element.

6.3.4-Tension shall not be transmitted through outside edges, construction oints, or control oints of an individual plain concrete element. flexural continuity due to tension shall be assumed between adjacentplain concrete elements.

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

1 computing stresses du flexure, combined flexure and axial load, and shear, the entire cross section of a member shall be consideredn design, except for concrete cast against soil, overall thickness shall be taken 2 in. less that actual thickness.

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

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 stressesn compression given in Sections.2.l(a) and (b) hall be less alculated than or equalo one. Tensile stress resulting from bined flexure andaxial load shall ot exceed permissible stressn tension given in Section 6.2.1 (a).

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

and that on the tension face: Calculated Calculated bending stress axial stress

5$)<

where the permissible stresses are as given inSection 6 . 2 .

6.3.7-Shear

R6.3.7-Shear

strength

Strength

Proportions plain concrete members will be ontrolledby tensile strength ather than shear strength for the usual plain concrete members practical proportions. Shear stress (as a substitute for principal tensile stress) rarelywill 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 investigationof this basic stress condition as a part the code requirements. An experienced designer will soon recognize where shear is not critical for lain concrete members and willdjust his design procedure accordingly.

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

=-

where 6.3.5.

6h

is overall thickness of member. See Section


R6.3.7.1-The shear requirements for plain concrete assume an uncracked section. Shear failure in plain concrete will be a diagonal tension failure, occurring when the principal tensilestress near the centroidal axis becomes equal to the tensile strength of the concrete. Since the major portion f the principal ensile stress comes from the shear, the ode safeguards against tensile failure by limiting the permissible shear at theentroidal axis as calculated fromhe equation for a sectionof homogeneous material: where and are theshear stress and shear force respectively at the section considered, is the statical momen of the area outside the section being considered about centroidal axisof the

OMMENTARY ND 18.1/318.1R-l1 ONCRETE PLAIN OD

COMMENTARY

CODE

gross section, is the moment of inertia of the gross section, an is the widthwhere shear stress is being computed. This equation recognizes the ore 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. or sections other han rectangular, 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. Y..

puted ata distance from face of suppÕrt, and sections located at a lesser distance may be designed for the sameshear. I4

v,, shallnotexceedpermissi6.3.7.3-Shearstress ble shear stress for beam action given in Section 6.2.1(~).

CHAPTER -PLAIN ONCRETE EMBERS 7.0-Notation Ag bo

v,, V,,

gross area of section, sq in perimeter of critical section for shearn ootings, in specified compressive strengthf concrete, psi. See Chapter overall hickness ofmember,n. effectiveengthactor verticaldistancebetweensupports, n. nominal axial load strength f wall designed by Section 7.1.5 shear stress due t o factored shear force at section factored shear orce at section strength eduction actor.Se Section6.2.2

R7.1-Walls

7.1-Walls 7.1.1-Plain concrete walls shall be continuously supported by soil or supported by footings, foundation alls, grade beams, or othertructural members capable providing continuous vertical support.See Section 1.2.2.

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 7.1.2-Plain concrete walls hall be designed or verconcrete walls in multistory constructionand other major tical, lateral, and other loads to which they are structures where differential settlement, wind, earthquake, subjected. or other unforeseen loading conditions require the walls to possess some ductility and ability to maintain their integrity 7.1.3-Plain concrete walls may be designed n accorwhen cracked. For such conditions, AC1 Committee 318 dance with Section 6.3.6 provided thewall is designed for an eccentricity corresponding to the maximum mo- strongly encourages the use of walls designed as reinforced concrete members inccordance with AC1 18 for reinforced ment that can accompany the axial load but not less than 0.10h.Otherwise, plain concret alls shall be de- concrete.

signed under provisionsf Section 7. 7.1.4-Design for Section 6.3.7.

shear shall be in accordance wit


The provisions for plain concrete walls are applicable only for walls laterallysupported in such a manner as to prohibit relative lateraldisplacement 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 op and bottom wall elements. Such later-

MANUAL

318.11318.1R-12

CONCRETE PRACTICE

0 b b 2 9 4 9 0504717

COMMENTARY

CODE

ally unsupported walls mustbe designed as reinforced concrete members inccordance with AC1 8. Plain concrete walls as structural members are subject to the limitations of Section 1.2.2 and the ointing requirements of Section 5.2, which greatly ffect their design. Plain concrete walls must be designed to resist all loads to which they are subjected, including eccentric axial loads and lateral orces. In general, the provisions pply to walls spanning vertically. Also, the empirical design method of Section 7.1.5 applies only to walls solid rectangular cross sections; other shapes must be designed accordingo Section 6.3.6. Plain concrete walls must bedesigned for combined flexure and axial load according to Section 6.3.6, onsidering the wall to be a compression member with lexure, unless meeting the equirements of Section 7.1.5.For some ases, shear strength may also need to be investigated.

7.1.5-Empirical

design method

7.1.5.1-Plain concrete walls of solid rectangula cross section may be designed q.by(7-1) if resultant of all factored loads is located within the middle-third of overall thickness wall. 7.1.5.2-Design axial load strength f a plain concrete wallsatisfying limitations of Section 7.1 shall be computed by

R7.1.5-Empirical

When the resultant load falls within the middle third of the wall thickness (kernof wall section), plain concrete walls may be designed using the simplified Eq. (7-1). ccentric the loads and lateralorces ar used to determine the total eccentricity of the factored oad 4. If the eccentricity does not exceed h/6, q. (7-1) ay be applied, and design performed considering as a concentric load. The factored axial load must be less than the design axial load strength @ew, omEq (7-1)s presented to puted by Eq. (7-1), reflect the general ange of braced and restrained end conditions encountered in wall esign. The limitations of Section 7.1.6 pply whetherhe wall is proportioned by Section 6.3.6 or bythe empirical method Section 7.1.5.

where 0.65 and effective length factorshall be: For walls braced top and bottom against lateral translation and (a) restrained against rotation t one or both ends (top and/or bottom) . . . . . . . . . . . . . . .. . .0.8 (b) unrestrained against rotation at both ends. 7.1.6-Limitations

7.1.6.1-Unless demonstrated by a detailed analysis, horizontal ength of wall to be considered effective for each vertical concentrated load shall not exceed centerto-center distance between loads, nor widthf bearing plus times the wall thickness. 7.1.6.2-Thickness of bearing walls shall not be less than 1 the unsupported height or length, whichever is shorter, nor less than in. 7.1.6.3-Thickness of exterior basement walls and foundation walls shall not be less than% n 7.1.6.4-Walls shall be braced against lateral translation. See Sections5.2. an 6.3.4. 7.1.6.5-Not less than bars shall be provided around all window and door openings. Such bars shall extend at least in. beyond the corners of openings.


design method

OMMENTARY 18.1/318.1R-13 ND ONCRETE ODE PLAIN

COMMENTARY

CODE

7.2-Footings

R7.2-Footings

7.2.1-Plain concrete footings shall bedesigned or facored loads andnduced reactions n accordancewith appropriate design requirements of this code and as provided in Section 7.2. 7.2.2-Base area of footing shall be determined from unfactored forces and moments transmitted by footing to soil and permissible soil pressure selected hrough principles of soil mechanics. 7.2.3-Plain concrete shall not be used for footings on piles. JI

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

7.2.5-Moment

in plain concrete footings

R7.2.4-Thickness of plain concrete footings will be controlled by flexural strength (extreme fiber stress in tension not greater than r than shear strength fo the usual proportions of plain concre teootin gs. Shear rarelywill control; see Section R 6 . 3 . 7 . For footings cast against oil, overall thickness used for strength computations must aken as in. less than actual thickness to allow for unevenness excavation and contamination f the concrete adjacent soil as required by Sect ion 6.3.5. hus, for a minimum footing thickness of in ., calculations for flexuralnd shear stresses must based on an overall thickness in

Maximum factored moment shall be computed at critical sections located ollows: (a) At face ofcolumn, pedestal, or wall, for footing supporting a concrete column, pedestal, or wall. (b) Halfway between middle and edge of wall, for footing supportinga masonry wall. (c) Halfway between face of column and edge of steel base plate, for footing supporting column with steel base plate. 7.2.6-Shear

in plain concrete footings

R7.2.6-Shear

7.2.6.1-Maximum factored shear shall be computed in accordance with Section 7.2.6.2, with location f critical section measured from facef column, pedestal, or wall for footing supporting a column, pedestal, or wall. For footing supportinga 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 he more severe of two conditions: (a) Beam action for footing, with critical section extending in a plane across the entire width and located at a distanceh rom face ofoncentrated loadr 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 planeof footing and located that its perimeter s a minimum, but need not approach closer thanh/2 to perimeterof concentrated load or reaction area. Forhis condition, the footing shall be designed in accordance with Sections.2.6.3. and 7.2.6.4. COPYRIGHT American Concrete Institute Licensed by Information Handling Services

in plain concrete ootings

MANUAL

318.11318.1R-14

CONCRETE PRACTICE

COMMENTARY

CODE 7.2.6.3-Shear

stress v,, shall be computed by v,=-

4

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

0bb29V9 0504739 V8

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

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 ocation of critical sections for moment and shear. 7.2.8-Bearing stress on concrete at contact surface betweensupporting and supported member shall not exceed permissible bearing stress for either surface given in Section 6.2.l(d).

7.3-Pedestals

R7.3-Pedestals

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

The height-thickness limitation for plain concrete pedestals does not apply for portions of pedestals embedded inoil capable providing lateral restraint.

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

R7.4-Precast members

all 7.4.1-Design of precast plain concrete members shall consider all loading conditions from initial fabrication to provisions for cast-in-place concrete contained in this code. completion of he structure, including form removal, storThe approach to control joints is expected to be somewhat age, transportation, and erection.

different than or cast-in-place concrete since th major por7.4.2-Limitations cited in Section 1.2 apply for precast tion of the internal stresses due to shrinkage takes place prior erection. assure stability, precast members should be members of structural plain concrete not only to the tofinal condition but also during fabrication, transportation, andconnected to other members. Connection must be such that no tension will beransferred from one member to the other. erection.

7.4.3-Precast members shall be connected ecurely, 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 tructural ntegrity until ermanent connections are completed.


The AMERICAN CON CRETE INSTITUTE was ounded in 1905 as a nonprofit membership organization dedicated to 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 he Institute as a whole, operate under 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 oremployment requirements. Engineers, architects, scientists, constructors, and representatives from a variety 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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ACI-318.1-89

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