Non Engineered Reinforced Concrete Buildings
NON ENGINEERED REINFORCED CONCRETE BUILDINGSSChapter 8NON ENGINEERED REINFORCEDCONCRETE BUILDINGS8.1 INTRODUCTIONcolumns are supposed to resist vertical asWith the spread of reinforced concrete con-well as horizontal seismic loads and thefiller walls are assumed to be neither loadstruction to semi-urban and rural area invarious countries, often buildings are constructed using reinforced concrete columnsand beams, without proper engineeringdesign, based on the experience of localmasons and petty contractors. Use of isolated columns in parallel with load bearing walls for supporting long internalbeams or those in verandahs and porchesis becoming quite common. In most cases,such constructions suffer from deficienciesfrom the seismic view point since no consideration is given for the effect of seismiclateral loads and the connection details areusually such that no moment carrying capacity can be relied upon. Beams simplybearing nor taking part in the lateral resistance of the building. Large halls for gymnasia, assembly halls, etc., having a floorarea more than 60 m2 or beam spans morethan 7 m must be designed by an engineer.8.2 TYPICAL DAMAGE ANDCOLLAPSE OF RC BUILDINGSThe following types of damage are quitecommon in reinforced concrete buildings:(a) Sliding of roofs off supportsWhere the beams simply rest on walls orcolumns, they are bound to slide, when theearthquake intensity exceeds the frictionalrest on top of columns and mostly held inposition through friction.resistance and many times leave the support and fall down, particularly if the bear-The other serious deficiency is in concrete quality in respect of mixing, compact-(b) Falling of infill wallsing and curing. The aim of this chapter isto provide working guidelines for suchlow-rise, (upto three storeys) small buildings in RC frame constructions in whiching length is small.The infill panel walls in between reinforcedconcrete columns overturn outside theframework if not held tight or connectedwith the frames.79
IAEE MANUAL(d) Short column effectWhen infill walls with wide openings areattached to the columns, the portions of thecolumns, that will deform under lateral seismic loads, become very short as comparedto their normal height. Such short columnsbecome much stiffer than other columnsand attract much larger shear forces underwhich they get severe diagonal tensionwhich may lead to failure of the column,Fig 8.2.(e) Diagonal cracking in columnsColumns are subjected to diagonal crackFig 8.1 Crushing of concrete at ends of column(c) Crushing of column ends andvirtual hingingthe ‘building also occurs, the cracks maytake spiral form reducing the load capacityDuring severe shaking, the column ends areof the columns severely.subjected to heavy eccentric compressivestresses under which the concrete getscrushed and spalled off from the outer surfaces. In repeated cycles the damageprogresses inwards, thus effective sectiongets very much reduced, both column endsvirtually start behaving as pins and thewhole framework collapses like a mechanism, Fig 8.1.Fig 8.2 Shear failure of short column80ing due to large seismic shears caused under severe ground shaking. If twisting of(f) Diagonal cracking of columnbeam jointMany times diagonal cracking occursthrough the junction of the column with thebeam which seriously impairs the strengthof the frame.(g) Pulling out of reinforcing barsWhere the anchor length of column bars or
NON ENGINEERED REINFORCED CONCRETE BUILDINGSSoverlaps between the longitudinal bars arewooden box with handles having a volumenot adequate for developing full tensilestrength of the bar, they are often pulledequal to one sack of cement will be mostaccurate as well as convenient to use. Theout due to tensions caused in the columnunder severe reversal of stresses.measurements of such a box are shown inFig 8.3. Such a box can also be made of steel(h) Collapse of gable framessheets.Reinforced concrete gable frames, oftenused for school workshops, gymnasia and(b) Mixing materialsassembly halls, or cinema halls, have a tendency of spreading out with no secondaryusing a power driven mixer, it should bedone on an impervious platform, say, us-resistance available once a joint fails. Theseare often found to fail and collapse (as wasing iron sheets or cemented floor. For making a mix of 1:2:4, four boxes of aggregateshown in Fig 6.6 for wooden gable frames),unless very carefully designed and de-should first be measured and flattened onthe platform, then two boxes of sand shouldtailed.be spread on the aggregate and finally onefull sack of cement opened on top. The ma-(i) Foundation sinking and tiltingWhere mixing is done manually withoutSinking or tilting of foundations of columnsterial should first be mixed thoroughly indry state so as to obtain uniform colour anddue to seismic shaking occurs in loose softsoils and can lead to severe cracking of thethen water added. The quantity of watershould be enough to make a soft ball of thesuperstructure and even collapse.mixed concrete in hand. A little wetter mixis better for hand compaction and drier mix8.3 CARE IN CONCRETECONSTRUCTIONIn reinforced concrete work, the most important requirement for good behavior isgood quality of concrete, which is not usually achieved in non-engineered construc-where vibrator is used for compaction.(c) FormworkThe quality of not only the concrete surfacebut also the strength of concrete dependson the surface of the formwork and its im-tion. Here simple guidelines are given formaking concrete of adequate strength.(a) Measuring materialsIn non-engineered reinforced concrete constructions, the proportions of concrete mixare usually kept 1:2:4 by volume ofcement:sand:aggregate. The aggregate maybe in the form of river shingle, or crushedstone, of maximum 20-mm size. A 50 kg cement sack has a nominal volume of0.0317 m3. It will be best to make the concrete mixture using whole bags of cement.For measuring sand and aggregate, aFig 8.3 Measuring box81
IAEE MANUALFig 8.4 Use of cement brickets for coverperviousness to the leakage or oozing outof the water and cement through the joints.25 mm to bars in beams and columns. In large columns, say 450 mmWooden formwork with well-formed surface and joints between planks should bein thickness, the cover should be 40mm. For achieving proper cover, aused. Use of water resistant plywood forthe skin of the formwork will give very goodsimple and effective method is tomake cement mortar brickets of re-surface of the concrete.quired size and install them betweenthe bars and formwork. Tying withd. Placing of reinforcementWhile placing reinforcing bars, the follow-bars with thin binding wire will ensure the proper cover, see Fig 8.4.ing points must be taken care of, otherwisethe structure will get into undefined weak- Tying of longitudinal bars withnesses: Minimum clear cover to the reinforcement: l5 mm to the bars in slabs,transverse bars and stirrups andlinks at each crossing with bindingwire. Minimum overlap in bars: 45 timesthe diameter of the bar for plain mildsteel and 60 times the diameter forhigh strength deformed bar. The overlapping portion should preferably bewound with binding wire. Shape of links and stirrups: The endsof bars should be hooked by bending through 180º in mild-steel barsand 135º in deformed bars, see Fig8.5.(e) Casting and compacting concreteFig 8.5 Hooks at ends of bars82The concrete should normally be cast in
NON ENGINEERED REINFORCED CONCRETE BUILDINGSSone continuous operation so as to avoidpoint, in which case the joints in the gird-discontinuity of more than one hour. Mixedconcrete should not be allowed to stay oners shall be offset a distance equal to twicethe width of the beam. Provision of keysthe platform by more than 45 minutes andmust be placed in the forms and compactedshould be made for transfer of shearthrough the construction joint.continually. Hand compaction must bedone by rodding through the freshly placedconcrete. Simply leveling the surface withtrowels will leave voids in the mass. It may8.4 TYPICAL MATERIALPROPERTIESConcrete is made to have the desiredbe mentioned that lack of compaction results in large reduction in concrete strength,strength for the required use. The strengthis defined on the basis of 28 days age, cubehence utmost attention should be given tothis factor. For rodding, good results willcrushing strength or cylinder crushingstrength. For use in buildings, the cubebe obtained by using 16 mm diameter rodsabout 50 cm long.strength Fc between 15 to 20 N/mm2, willbe adequate for RC work. The concrete mix(f) Curing of concreteis accordingly designed but ordinarily cement: sand:coarse aggregate mix proportionConcrete work requires water-curing for aminimum of 14 days so as to gain itsstrength, otherwise the gain of strength islow and concrete becomes brittle. Concreteslabs may be kept under water by pondingof water over it by making earthen barriersall around the edges. Columns should bekept covered with wet empty gunny bags.Keeping the side forms intact on the beamwebs will prevent the evaporation of waterfrom the concrete and help in curing. Covering any concrete surface with polythlenesheets after wetting the surface will helpretain the moisture.(g) Construction jointsWhere a joint is to be made, the surface ofthe concrete shall be thoroughly cleanedand all laitance removed. The surface shallbe thoroughly wetted, and covered with acoat of neat cement slurry immediately before placing of new concrete. Constructionjoints in floors shall be located near themiddle of the spans of slabs, beams or girders, unless a beam intersects a girder at thistaken as 1:2:4 or 1:1½:3 by volume.The mass density of RC is about2400 kg/m3 and modulus of elasticity isvariously related with the concrete strength.Since the stress-strain characteristics arenon-linear, the value of modulus of elasticity is ambiguous. Similarly the allowablestresses are differently specified by codesof practice in relation to Fc. Each countryhas its own standards for allowablestresses and load factors which may be referred to in this regard. A factor of safety ofabout 3 is used in determining the allowable stress in axial compression relative tothe 28-day cube strength. Under seismiccondition these allowable values may beincreased by 33.33 percent and the load factors may be decreased by 25 percent unlessspecified otherwise in national standards.It is important to know that the tensilestrength of concrete is only about one-tenthof the compressive strength. The diagonaltension caused by seismic shear forces, ifnot throughly protected by well designed83
IAEE MANUALFig 8.6 Critical section in RC framestirrups or ties, can lead to wide crackingand failure.rups or spiral is required are shown shadedin Fig 8.6 and explained below:Concrete is a brittle material and weakagainst impact shock and vibrations. Duc-1 Ends of beam upto a length of abouttwice the depth of the beam wheretility is imparted to it by the reinforcing steelonly. The compressive strength as well aslarge negative moments and shearsdevelop are likely locations for plas-straining capacity can be greatly increasedby using closely spaced lateral stirrup tiestic hinges. Here shear and momentreversal is possible under large seis-or spiral reinforcement. This is an important characteristic for improving the earth-mic forces.quake resistance of reinforced concrete columns and frames.The critical zones in reinforced concreteframes where ductility of sections and confinement of concrete by closely spaced stirTable 8.1 Recommended limits on steel area ratio in beamConcreteSteelPmaxPmin2 Ends of columns where maximummoments develop due to lateralforces. Values of maximum columnmoments closely approaching plastic moment capacity can be expectedand these moments are likely to undergo full reversal. High lateralshears can be developed based onmoments of opposite sign at the col-1:2:4 (Fc 15 MPa)MS (Fy 250 MPa)HSD (Fy 415 MPa)0.0110.0070.00350.0022umn ends and these shears can undergo full reversal. The length of such1:1½:3 (Fc 20 MPa)MS (Fy 250 MPa)HSD (Fy 415 MPa)0.0150.0090.00480.0029zones is about one-sixth of the clearheight of the column between floorsNotes:Fc 28-day crushing strength of 150-mm cubesFy Yield strength of reinforcementMS mild steel, HSD high-strength deformed barsP AS/ bhAs bh PmaxbhmaxAs84min Pmin b hor the dimension of the column section in the plane of the frame3 Joint regions between beams and columns undergo very high localshears, their full reversal is likely,
NON ENGINEERED REINFORCED CONCRETE BUILDINGSSFig 8.7 Detailing of beam reinforcementshears, their full reversal is likely,diagonal cracking and local deformation may cause significant part ofrotation at joint increasing the lateral displacement of frame.8.5 DETAILING OF BEAMS(a) Longitudinal steelBeams should be reinforced on both top andbottom face throughout. Where reinforcement is required by calculation, the percentage should correspond to ductile behavior.The recommended limits on steel area areshown in Table 8.1. Minimum steel shouldconsist of two bars of 12 mm diameter incase of mild steel (MS) and l0 mm diameterFig 8.8 Column reinforcement7
IAEE MANUALFig 8.9 (a) Connection between beam and girder (b) Connection between floor slab and beam (c) Connection betweenroof beam and exterior column86
NON ENGINEERED REINFORCED CONCRETE BUILDINGSS(b) Splicing of steel(c) Transverse steel stirrupsAll longitudinal bars should be an-The ultimate shear strength of the beamchored or spliced for full strength development. All splices should be containedshould be designed to be more than its ultimate flexural strength, Fig 8.7. Verticalwithin at least two stirrups at each endof the splice so as to avoid spalling ofshear stirrups should be closely spaced atnot more than one-fourth of effective depthcover concrete, Fig 8.7in end 2h-length of spans of the beams. InFig 8.9 (d) Connection between floor beam and interior column (e) Interior joint between haunched beam and column87
IAEE MANUALhkAcA1bkhbs5hbb1s4hk2h1/6 Hk45 cms6IIIA1hIIIbbhk1/6 Hk45 cmhbA13s521 - Floor beam2 - Exterior column3 - Take largest ofthe three valuesIII - IIIs3s2s43A1(f)bkAbkhb hs5A’1 - Tie beam / plinth beam2 - Exterior column3 - Take the largest of the three values4 - Individual footing in soft soilbb2VII - VII1/6 Hk45 cms62h1VII s2s3ld3s4hkldhVIIhbbb4(g)Fig 8.9 (f) Connection between floor beam and exterior (g) Column footing and foundation/plinth beam88
NON ENGINEERED REINFORCED CONCRETE BUILDINGSSthe remaining length spacing should hot8.7 CONNECTIONexceed h/2.The beam and column bars must be wellanchored in the compression zone so as to8.6 DETAILING OF COLUMNSachieve their full strength.(a) Column sectionIn view of earthquake force acting in alldirections, square section of columns is better than rectangular.(b) Longitudinal steelWhere the joint is not confined by beamson all four sides, it will be necessary to placethe closely spaced ties in the columnthroughout the height of the joint as well.Vertical reinforcement should be distrib-8.8 ILLUSTRATE SKETCHESuted on all the faces of the columns. Use of8 vertical bars is preferable to four bars ofFig 8.9 (a) to (g) give typical recommendedequal area; minimum diameter of bars, 12mm.(c) Lateral reinforcementConcrete confined within the spirals isstronger as well as much more ductile ascompared with plain concrete or that containing widely spaced stirrups.The behavior of columns can be muchimproved by using the ties with adequateanchorage at ends in the form of suitablehooks as shown in Fig 8.5 and Fig 8.8 atclose spacing.In a length of about 450 mm near theends of columns, a spacing not more than100 mm may be adopted for achieving ductility there.(d) Corner columnThe corner columns of buildings arestressed more than any other column dueto biaxial bending and must therefore havesteel distributed on all faces and closelyspaced lateral ties.details of connections in earthquake resistant frames in which the following dimensions are to be used (all in mm):s2: maximum value h/4 or 16d whichever smaller where d bar diameterof beam reinforcement.s3: maximum value h/2.s4: value 75 mm to 100 mm.s5: maximum value bk/2 or 200 mmwhichever smaller .s6: optimum value 50 mm.lo : length of overlap to develop full tensile strength 55d including bendsor hooks.ld : anchorage length to develop full tensile strength 55d including bendsor hooks.Use diameter of stirrup bar in beam andcolumn:Minimum 6 mm,preferable 8 mm. 89
Chapter 8 NON ENGINEERED REINFORCED CONCRETE BUILDINGS 8.1 INTRODUCTION With the spread of reinforced concrete con-struction to semi-urban and rural area in various countries, often buildings are con-structed using reinforced concrete columns and beams, without proper engineering design, based on the experience of local masons and petty .
vary the overall capacity of the reinforced concrete and as well as the type of interaction it experiences whether for it to be either over reinforced or under reinforced. 2.2.2.1 Under Reinforced Fig. 3. Under Reinforced Case Figure 3.2 shows the process in determining if the concrete beam is under reinforced. The
reinforced concrete for pavement applications. However, Merta et al., (2011) studied wheat straw reinforced concrete for building material applications. They concluded that there is an increase (i.e. 2%) in fracture energy of wheat straw reinforced concrete. Thus, wheat straw reinforced concrete needs to be investigated for rigid pavements.
Recommended Practice for Glass Fiber Reinforced Concrete Panels - Fourth Edition, 2001. Manual for Quality Control for Plants and Production of Glass Fiber Reinforced Concrete Products, 1991. ACI 549.2R-04 Thin Reinforced Cementitious Products. Report by ACI Committee 549 ACI 549.XR. Glass Fiber Reinforced Concrete premix. Report by ACI .
experimental flexural behavior of concrete beams reinforced with glass fiber reinforced polymers bars" is done. D.Modeling . ANSYS Workbench 16.1 is used to model the concrete beams and 28 different models are considered. Concrete beams reinforced with reinforced with steel bars of circular cross
Keywords: RC Shear walls; Single layer reinforced; Double layer reinforced; In-plane shear strength; Ductility 1 Introduction Reinforced concrete shear walls (RCSW) are key lateral load resisting elements in concrete buildings; adequate capacity and ductility of these walls are central to the safety of the shear wall dominated buildings.
Concrete Beams 9 Lecture 21 Elements of Architectural Structures ARCH 614 S2007abn Reinforced Concrete - stress/strain Concrete Beams 10 Lecture 21 Elements of Architectural Structures ARCH 614 S2007abn Reinforced Concrete Analysis for stress calculations steel is transformed to concrete concrete is in compression above n.a. and
REINFORCED CONCRETE BUILDINGS This report discusses construction, contraction and expansion joints in reinforced concrete buildings. The report addresses the purpose of each type of joint and emphasizes the selection of joint locations and joint spacings. Some aspects of joint configuration and construction are also covered.
This publication was developed through a consultative process led by the International Telecommunication Union (ITU) and UNICEF and benefited from the expertise of a wide range of contributors from leading institutions active in the information and communications technologies (ICT) sector and on child online safety issues. UNICEF Corporate Social Responsibility Unit: Amaya Gorostiaga, Eija .
