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Notes by M E. H A B E R S H O N OBEMEng MlS tructE MICE,W H U N T E RR O S E MSc MlStructE MICE1 FGSand 0 A KERENSKY CBE BSc MlS truc tE MICE Memberof Council)

CP 114 The Structural Use of Reinforced Concrete nBuildings 1965 Edition)The revision of the 1957 edition of CP 114 was firstsuggested to the Institution of Structu ral Engineers, theconvening institution, by the British Standards Institu-tion in November 1961. This request followed representa-tions made to th e Codes of Practice for Building Com-mittee by the Reinforced Concrete Association and theCement and Concrete Association.

Initially it was thought desirable to defer such a eviewunt il he impending eport of the European ConcreteCommittee had been published and consideration givento th e est method of unifying th e common requirements

of all the st ruc tu ral codes. Th e Reinforced ConcreteAssociation and he Cement and Concrete Associationdid not concur with this approach and as an alternativesuggested a limited evision of th e present 1957 documentwhich could be dealt with in a comparatively short time.This modified approach was accepted by the Institutionof S tru ctura l Engineers in October 1962.

CP 114 Committee was then reconvened and held itsfirst meeting on 27 November 1962, Mr. M E. Habershonbeing the Chairman and Dr. F. G Thomas the Vice-Chairman. So far as practicable the other members werethose who had served on the committ ee when it last met.

The Committees terms of reference consisted of th erapi d and limited revision of those p ar ts of t he Codereferred t o them y he nstitution of StructuralEngineers. The main items related to:--

1) concrete mixes and permissible stresses in concrete;

2) permissible stresses in reinforcement;

3) bond and anchorage;

4) resistance to shear;5) th e use of high-alumina-cement concrete;6) lightweight concrete;

7) ire resistance.

New clauses have been introduced for DesignedConcrete Mixes and hese replace the old clauses for

Special Concrete Mixes . Special attention is given toquality control equirements ncluding the tatist icalreview of both tria l mix and works cube tests. Provisionis also made for standard mixes where the making ofspecial tri al mixes is no t justified. The maximum allowedworks cube st ren gth for hese designed concrete mixes israised to 7500 lbf/in2.

Consideration was given to the omission of NominalConcrete Mixes but t hey were retained as meeting theneeds of small obs. N o variation was made n th estrength requirements f or these mixes.

A new clause has been included dealing with durability.This supplements th e cover requirements and stressesth e need in cases of severe exposure in industrial areasand the like for a higher grade of concrete and greater

st ren gth tha n would be justified solely from struct uralconsiderations.The permissive concrete stress in bending for designed

concrete mixes is raised by 10 per cent of th e specifiedworks cube trength o 1/2 -73 of this tren gth . Noincrease is allowed for the nominal concrete mixes.

The permissive tensile stress n steel einforcementconsisting of high-bond bars and high-yield wire meshhaving guaran teed yield or proof stress has beenraised to 0 - 5 5 of such guaranteed yield or proof stressbut not more than 33,000 lbf/in2 for bars not exceeding8 in effective diamete r nd 30,000 lbf/inZ for barsexceeding 8 in effective diameter. Similarly th e permis-sive tensile stress in shear reinforcement nd the permis-sive compressive stress have both been raised to 0 55 ofthe guarant eed yield or proof stress bu t not more tha n25,000 lbf/in2.

N o change has been made in th e permissive stressesfor mild-steel ba rs to BS 785.

Modifications have been introduced into the Stiffnessof Members

clause to restrict he former permissiblevalues of spa n/depth rati o f beams and slabs t o memberswith teel tresses not more th an 20,000 lbf/in2 andconcrete tresses otmore than 1500 lbf/inz. Formembers with greater steel stresses O R greater concretestresses the span/dept h ratio s not to exceed 90 per centof the tab le values and for members with greater steelstresses A N D greater concrete stresses this ratio is notto exceed 85 per cent of the tab le values.

Dimensions are now specified for hooks in high-yieldbars.

A new clause allows th e bond stress in high-bond barsto be increased by 40 per cent provided the bond strengthexceeds th at of a plain ba r by 40 per cent.

A new clause has been included imiting memberswithout hear einforcement to those of secondaryimportance nd even then dopting a onservativeapproach n calculating th e shear esistance of suchmembers. This is a mat te r which will require amplifica-tion when the next revision of t he Code is made, andmore research dat a are available. I n the meantime, itwas considered desirable to give this warning.

For main beams equired now t o have shear rein-forcement even though th e shear stresses are less tha nth e permissible values for the concrete, the amount ofreinforcement is not specified. I n general it need not besufficient to ake he whole shear as is necessary formain eams of lightweight-aggregate oncrete, see

Clause 344) but should probably be not less than about0 -15 per cent of th e horizontal area of th e concrete,and should increase as th e shear stress approaches th epermissible value for the concrete. This is in accordancewith normal good practice bu t, with t he present confusedposition with egard to shear esistance, no specificrecommendation could be included in Amendment o 1.

Where th e load-factor method of design is used, beamsand slab may e designed to hav e a load-factor generallyof 1 8 n place of 2 O as at present. Similarly for columnsa load-factor generally of 1 .8 may be used.

A special section has been introduced for reinforcedlightweight-aggregate concrete. I n general hese havebeen based on th e Building Research Sta tion Studies.

A new clause has been added dealing with he bendingaside of steel reinforcement a t construction joints.

For bonds ests, details of t he reinforcement of thecubes for pull-out tests are specified.

The Fire Resistance clauses have been revised inaccordance with th e recommendations of the Join t FireResearch Organization.

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A special appendix dealing with high-alumina-cementconcret e has been included in th e Code. This covers thesame ground as the Report f the Institu tion f Structura lEngineers, These o High-Alumina Cement in.Strzcctural Engineering,but all the recommendations ofthe Report are not necessarily included n th e Code.One of th e main aims of this appendix is o warn designersof t he harmful effects of conversion, bu t it also codifiesgenera l information on the proper use of high-alumina-

cement concrete.Special care was take n to ensure that ther e was noserious conflict with the new BS Code of Practice CP 116:The Structural Use o Precast Concrete,which was beingfinalized a t t he same time as these revisions of CP 114.

The Committee had referred to it many items whichdid not come within its terms of reference for the limitedrevision agreed upon. Some of these have been referredto in hese notes but there are others and many may haveto be dealt with at an early date. It is assumed that thiswill be done as pa rt of t he new procedure for th e periodicrevision of all str uct ura l codes which h as been initiatedrecently by the Institution.

CP 116: The Structural Use of Precast ConcreteThe recently published British St an da rd Code of Practicefor precast concrete is th e result of four years work bya committee convened by the Inst itut ion of StructuralEngineers. The Chairman was Mr. W. Hun ter Rose,MSc, MIStructE, MICEI, FGS. who represented theInstitu tion on the Committee.

Throug hout he Code it is assumed th at th e designand manufactur e of th e precast units will be under theoverall control of a Charter ed Structu ral or Civil Engi-neer experienced in concrete technology, and it is or hisguidance-as well as for th at of architec ts and consul-tants-that th e Code is written.

In all, t he Code has seven main sections, which coverpermitt ed materials, design considerations, standar ds ofworkmanship, inspection and testing requirements, fireresistance gradings, high-alumina cement concrete andautoclaved aerated concrete. In addition there are threeappendixes, dealing respectively with statistical calcula-tions and quality control, movement oints and bondtest s. These are all subjects of great import ance to theprecast structural concrete indust ry and their inclusionin the Code is intended to make it self-contained andself-explanatory.

Much of the informat ion given on materials has neverbeen included in st ruc tur al codes before. High-aluminacement s ntroduced for the first time as a permittedcement for use in precast concrete an d a separate sectionof th e Code gives detailed guidance on it s use. I t isrecognized tha t var iat ions in the str eng th of mixes areunavoidable, but reatmportance is attached oreducing these variations as much as possible, by properstatistical quality control methods, o ensure the pro-duction of concrete of consistent quali ty.

One of t he main features of t he materials section isth e new concept of relating the amount of concrete coverto the steel to ( a ) the environment in which th e concreteunit will be placed and ( b ) th e grade of concrete. Theaim is to secure maximum durability in relation to theconditions of exposure. The minimum concrete coverto all steel is in (for internal sites only), and this maybe extended to 2 in or more in outdoor positions wherethe concrete may be exposed to sea water, chemicalatt ack or other corrosive atmospheres.

The design section follows fairly closely that ofC P 114 for n-situ reinforced concrete, but some of ithas been brought up to date a nd a number of points ofspecial inte rest o he precast concrete industry havebeen brought out n greater detail. Although or hemoment his section of the new Code is fairly closely

related to CP 114 and 115 it is emphasized that thereis ample scope for hange and progress. The designsection covers reinforced concrete eams and slabs,columns, walls and bases, together withprestressedstru ctu res and composite (in-situ precast) constructions.I t also deals with lightweight-aggregate concrete.

Many pa rt s of th e workmanship section have neverbeen codified before, notably the information given onsurface finishes and on dimensional olerances. It is

hoped that th e inclusion of t hese points in the Code willhelp to settle any dispute s which may arise betweenmanufacturers and architects. The subject of curing-and he factors influencing the choice of curing treat -ment-is dealt with in detai l.

Several of the tes ting methods suggested have neverappeared n a code or stan dard before, although heyhave been used successfully in the indus try. The testsare designed to ensure that finished units have reachedthe required stand ard s of finish, dimensional accuracyand strength.

Where a manufacturer uses quality-control methodscovering the entire manufacturing process, and keepsrecords-certified by th e engineer-which show tha t heis regularly producing units of a qual ity not less th anth at required by he Code, these m;ly be accepted asconfirming tha t th e required quality has been reached.

The drafting committee had a good deal of help fromth e Joint Fire Research Organization in preparing therequirements for precast nitsequired to be fireresistant.

The section on high-alumina cement is completely upto dat e and fully in line with the recent Report of t heIns titu tion of S truc tura l Engineers on this subject, whileth e section on autoclaved high-pressure team-cured)aerated concrete is a useful first step towards giving thisnew material official recognition.

CP 117: Composite Construction in Structural Steel and

Concrete, Part l-Simply Supported Beams for BuildingsWhen, n June 1961, aCommittee was set up by theBSI nd he nstitut ion of Structur al Engineers t oproduce a Code of Practice for Composite Constructionin Str uctu ral Steel and Concrete, it probably came as asurprise to all concerned that so few factual data aboutthis form of const ruction were available in Great Britain,and that so many problems still remained unsolved.

The Committee, meeting under th e Chairmanship ofMr. 0 A . Kerensky, CBE, BSc, MIStruct E, MICE(Member of Council), began it s work by a detailed surveyof existing composite struct ure s, foreign regulations andresearch carried out to date . The results of th is surveywere published in March 1964 as an Institution Report.

Part 1 of t he Code, dealing with simply supported beamsin buildings, i.e. beams subjected to the loads specifiedin Chapter V of CP 3, took another year of ha rd work.Parts 2 and 3, which will deal with simply supportedand continuous beams n bridges and with columns,should be ready early in 1966.

In general the Code deals in detail only with mattersconcerned with composite action. Materials, workman-ship and the design of the steel section and of the con-crete slab should be n accordance with he elevantSpecifications and Codes of Pract ice.

Par t stip ulat es hat he composite section can bedesigned on the elastic theory (using M = 15) or, alterna-tively, on u ltimate streng th by the load factor method.Wi th th e recommended load factor of 1.75 for the steel

section it will be found that he att er method will,almost always, produce a more economic solution. Toallow for the greater variability of concrete, its com-pressive strength is taken as 8 of the nominal slab crush-ing strength, so th at th e load factor against failure ofthe concrete element s 3 / 2 x 1-75, .e. approximately 2 6

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This however, hould hardly ever be he controllingfactor. Using these factors may result in high workingstresses in either of th e elements. The Code stipulatesth at these should not exceed 0.9 of th e guaranteed yieldstress of steel and Q of th e crushing str engt h of th econcrete lab, respectively. Deflexions under workingloads should also be checked and must comply with therequirements of BS 449.

The only special element in composite construction

is he shear connector. This s a structu ral device tointerconnect the steel beam and the reinforced concreteslab in such a way that they act compositely, withoutsignificant slip or separation, throughout the life of t hestructure, independently of an y natural bond betweenthe two elements.

For both elastic and load fact or designs the shearconnectors are to be designed by the load factor methodthus ensuring that the connectors are fully effective upto he po int of failure. Ultima te safe loads for a fewtypes of connectors are tabulat ed. For any other typethe value can be obtained by standard tests, as laid ownin the Code.

Useful ppendixes re dded llustrating he oadfactor design of t he composite section and the calcula-tion of th e required number of shear connectors.

This first part of the Code has been intentionally madeas simple as possible. The various complex phenomenaassociated with omposite onstruction, uch as theeffects of shrinkage, creep, va riat ions of the las ticmodulus of concrete, elasticity of shear connectors andthe presence of other stresses in he slab due o heimposed oads, have been considered and, where notspecifically mentioned, can bedeemed to have beenprovided for. All the above matters will be dealt with

more fully i n Par t 2 (Beamsor

Bridges . However, ifapprec iable difference of t emperature between the steeland concrete elements can arise, its effect, especially ondeflexions, should be allowed for in the normal way.

There is no doubt ha t by ensuring a composite actionbetween the steel beam and the concret e slab apprecia blesaving in the weight of steel, and a saving in cost, canbe achieved, combined, generally, with greater rigidi tyof const ruction and an almost infinite varie ty of form.Furthermore, the overall saving in weight and depth offloors should result in additional consequential saving inthe cost of the structu re.

It is hoped that CP 117 will help to make the best useof the two mate rial s of our t ime, steel and concrete, andwill bring closer toget her the two vit al sect ors of thestructural industry.

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London, 1964.

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