RC Frame Design of Buildings November 2012 SKJ

8/11/2019 RC Frame Design of Buildings November 2012 SKJ

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Seismic Detailing of RC

Structures (IS:13920-1993)

Sudhir K JainIndian Institute of Technology Gandhinagar

November 2012


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Outline

.This lecture covers:.Covers important clauses of IS13920.With particular emphasis on Buildings.Many importantclauses applicableto buildings may not bediscussed in this lectureindetail.Sudhir K. Jain, IITGNSlide 2

Seismic Design of Buildings / November 2012


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How to ensure ductility How to ensure ductility.Correct collapse mechanism.Adequate ductility at locations likely to formhinge in collapse mechanism.Need sufficient member ductility to ensureadequate structural ductility..Prevent brittle failure mechanisms to take placeprior to ductile yieldingSudhir K. Jain, IITGNSlide 3



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Collapse Mechanism Collapse Mechanism.Storey Mechanism.Columns require too much ductility.Columns are difficult to make ductileSudhir K. Jain, IITGN Slide 4



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Collapse Mechanism Collapse Mechanism.Beam Hinge Mechanism (Sway Mechanism).Preferred mechanism.Ensure that beams yield before columns do.Strong Column Weak Beam DesignSudhir K. Jain, IITGN Slide 5



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R C Members

Bond Failure: BrittleShear Failure: BrittleFlexural FailureBrittle: if over-reinforced section (compressionfailure)Ductile: if under-reinforced section (tensionfailure)Hence, Ensure thatBond failure does not take placeShear failure does not precede flexural yieldingBeam is under-reinforced.

Sudhir K. Jain, IITGN Slide 6



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Failure of RC Section Failure of RC Section.Yielding of tension bars.Ductile.Tension failure.Under-reinforced section.Crushing of compression concrete.Brittle.Compression failure.Over-reinforced sectionSudhir K. Jain, IITGN Slide 7



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R C Section

.Tension failure more likely if:.Less tension reinforcement.More compression reinforcement.Higher grade of concrete.Lower grade of steel.Lower value of axial compressionSudhir K. Jain, IITGN Slide 8



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Section ductility increases as Section ductility increases as.Grade of concrete improves.Grade of steel reduces.Tension steel reduces.Compression steel increases.Axial compression force reduces.Generally, columns are less ductile than beamsSudhir K. Jain, IITGN Slide 9



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Capacity Design ConceptBrittleLinkDuctileLinkCapacity Design ConceptBrittleLinkDuctileLink.The chain has both ductile and brittle elements..To ensure ductile failure, we must ensure thatthe ductile link yields before any of the brittlelinks fails.Sudhir K. Jain, IITGNSlide 10



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Capacity Design Concept (contd)

.For instance, in a RC member.Shear failure is brittle.Flexural failure can be made ductile.Element must yield in flexure and not fail in shearSudhir K. Jain, IITGN Slide 12



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Capacity Design of Frames

Choose yield mechanismLocate desirable hinge locationsEstimate reasonable design seismic force on thebuildingDesign the members at hinge locations(upper bound type)Assess the member forces at other locationsunder the action of capacityforce

Design other locations for that force; need notdetail these for high ductilitySudhir K. Jain, IITGN Slide 13



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Materials in RC Members Materials in RC MembersConcrete and steel have very differentcharacteristicsSteel ductile: strain capacity: ~12% to 25%Concrete brittle: strain capacity: ~0.35%HYSD

Mild Steel

0.35%

20-25%




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Confinement of concrete

.Considerably improves its strain capacityStress-strain relationship for concrete proposed by Saatcioglu andRazvi, (1992)




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Confinement of Column SectionsFig. from Paulayand Priestley, 1992Confinement of Column SectionsFig. from Paulayand Priestley, 1992Sudhir K. Jain, IITGN Slide 16



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Main Steps

Weak Girder Strong Column PhilosophyShear Failure Prevented by Special Calculations(Capacity Design Method)Good Development LengthRegions Likely to have Hinges Confined withClosely-spaced and Closed StirrupsSudhir K. Jain, IITGN Slide 17



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Applicability of Code (Cl. 1.1.1)

Originally, this code was applicable for:All structures in zones IV or VStructures in zone III with I > 1.0Industrial structures in zone IIIMore than 5-storey structures in zone IIIAfter the Bhuj earthquake, the code madeapplicable to all structures in zones III, IV andV.Even though the code title says structures, itwas written primarily for buildings.




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Background Materials Background Materials.The code emerged from the following. These alsoprovide commentary:Medhekar M S, Jain S K and Arya A S, "Proposed Draft forIS:4326 on Ductile Detailing of Reinforced ConcreteStructures," Bulletin of the Indian Society of EarthquakeTechnology, Vol 29, No. 3, September 1992, 15 -35.Medhekar M S and Jain S K, "Seismic Behaviour, Design,and Detailing of R.C. Shear Walls, Part I: Behaviour andStrength," The Indian Concrete Journal, Vol. 67, No. 7, July1993, 311-318.Medhekar M S and Jain S K, "Seismic Behaviour, Design,and Detailing of R.C. Shear Walls, Part II: Design andDetailing," The Indian Concrete Journal, Vol. 67, No. 8,September 1993, 451-457.Sudhir K. Jain, IITGNSlide 19



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Concrete Grade

.Originally, as per Cl.5.2: buildings more than 3storeys high, minimum concrete grade shallpreferably be M20..Now, word preferablyhas been dropped..Most codes specify higher grade of concrete forseismic regions than that for non-seismicconstructions. Examples:.ACI allows M20 for ordinary constructions, but aminimum of M25 for aseismic constructions..Euro code allows M15 for non seismic, butrequires a min grade of M20 for low-seismic andM25 for medium and high seismic regions.Sudhir K. Jain, IITGNSlide 20



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Steel Grade (Cl. 5.3) Steel Grade (Cl. 5.3).Originally, the code required that steelreinforcement of grade Fe415 or less only beused..Higher grade of steel reduces ductility. Hence,there is usually an upper limit on grade of steelrequired.Sudhir K. Jain, IITGNSlide 21



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Steel Grade (Contd)

.Recently, the code relaxed this requirement.Cl.5.3 now reads as5.3 Steel reinforcements of grade Fe415 (see IS 1786:1985) or lessonly shall be used.However, high strength deformed steel bars, produced bythe thermo-mechanical treatment process, of gradesFe500 and Fe550, having elongation more than 14.5percent and conforming to other requirements of IS1786:1985 may also be used for the reinforcement.

.Thus, higher grades of steel are now allowed inthe Indian code subject to the above restrictionson ductility of bars.Sudhir K. Jain, IITGNSlide 22



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Steel Grade (Contd)

.ACI has two additional requirements on steelreinforcement:.Actual yield strength must not exceed specifiedyield strength by more than 120 MPa..The shear or bond failuremay precede the flexural hingeformation..If the differenceis very high,thecapacitydesign conceptwill not work..Ratio of actual ultimate strength to actual yieldstrength should be at least 1.25..To develop inelasticrotationcapacity, need adequate lengthof yield regionalong axis ofthe member. This attempts toensurethat.Sudhir K. Jain, IITGN

Slide 23



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Flexural Members




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Positive Reinforcement

.At a joint face, positive reinforcement should be at least50% of the negative reinf.Negative steel (At)

Negative steel (At)

Positive steel (Ab

0.5At)

Positive steel (Ab

0.5At)

.Two reasons:.

Need adequate compression reinforcement to ensureductility..Seismic moments are reversible..Seenextslide.Sudhir K. Jain, IITGNSlide 25



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Reinforcement Elsewhere (Cl. 6.2.4) Reinforcement Elsewhere (Cl. 6.2.4).Steel at top and bottom face anywhere shouldbe at least 25% of max negative moment steelat face of either joint.8 Nos 20

12 Nos 20

Min 3 Nos 20Min4 Nos 20

Min6 Nos 20

Sudhir K. Jain, IITGNSlide 26



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Reinforcement (Contd)

.Reasons:.Actual moments away from joint may be higherthan the design moment..We do not want to reduce large amount of steelabruptly away from the joint.Sudhir K. Jain, IITGNSlide 27



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External Joint of Beam with Column External Joint of Beam with Column.Very important toensure adequateanchorage ofbeam bars in thecolumnSudhir K. Jain, IITGNSlide 28



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External Joint (Contd)

.Notice the top bar of beam is shown to go intocolumn well below soffit of the beam..This is a problem in the construction..One would cast the columns up to beam soffitlevel before fixing the beam reinforcement..Problem arises since Indian code does notrequire minimum column width..If column is wide enough, this will not be aproblem..Seismic codes generally require column width tobe at least 20 times the largest beam bar dia..More on column width later in the section onjoints.Sudhir K. Jain, IITGN

Slide 29



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Lap Splice (Cl. 6.2.6).Lap length development length in tensionLap Splice (Cl. 6.2.6).Lap length development length in tension.Due to reversal of seismic loads, the bar couldbe in compression or tension..Lap splice not to be provided.Within a joint.Within a distance of 2d from joint face.Within a quarter length of member whereyielding may occur due to seismic forces..Lap splices are not reliable under cyclicinelasticdeformations and hencenot to be provided in thecriticalregions.Sudhir K. Jain, IITGNSlide 30



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Lap Splice (Contd)

.Wherever longitudinal bar splices are provided:.Hoops @ not more than 150 mm c/c should beprovided over the entire splice lengthLd = development length in tensiondb = bar diameter




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Web Reinforcement Web Reinforcement.Most important requirement in seismic regionsSudhir K. Jain, IITGN Slide 32



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Web Reinforcement (Contd)

.Several actions by web reinforcement:.Shear force capacity.Confinement of concrete.Lateral support to compression reinforcementbarsSudhir K. Jain, IITGNSlide 33



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.Vertical hoops.Shear direction may reverse during earthquakeshaking.Hence, inclined bars not effective..Closed stirrups.Open stirrups cannot confine concrete.135 degree hooks.As against normal 90 degree hooks.Provides good anchorage to stirrups.10 dia extension ( 75 mm).As against 4 dia extension

.Provides good anchorage.Sudhir K. Jain, IITGNSlide 34



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.Two pieces allowed:.U-stirrup and a cross tie.Both with 135 degree hooks at either end..This is more conservative than the ACI Code.See next slide for ACI provision.Sudhir K. Jain, IITGN Slide 35



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Hoops as perACI318Hoops as perACI318Sudhir K. Jain, IITGN Slide 36



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Spacing of Hoops Spacing of Hoops.Hoop spacing over 2d length at either end ofbeam not to exceed.d/4.8 times dia of smallest longitudinal bar2d 2d 2dSpacing >d/4>8dbSudhir K. Jain, IITGNSlide 37



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Spacing of Hoops (Contd)

.But, hoop spacing need not be less than 100mm.To ensure space for needle vibrator..Also, close spacing of hoops over 2d on eitherside of any other location where flexural yieldingis likely.Elsewhere, hoop spacing to not exceed d/2.As against 3d/4 permitted by IS:456.First hoop should be placed within 50 mm of thejoint face.Sudhir K. Jain, IITGNSlide 38



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Shear Des ign Shear Des ign.Shear reinforcement to be designed for:.Factored shear forces as per calculations forapplied design loads..Shear forces that will develop when flexuralyielding takes place at either end of the beam.Capacity design conceptto ensure shear failure(brittlefailure) will not precedetheflexural yielding.Sudhir K. Jain, IITGNSlide 39



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Capacity Design for Shear Capacity Design for Shear.Cantilever Beam Example100kN (Factored Design Load)

.Factored design load 100 kN,.Height of 5m.Design moment at base =100x 5=500 kNm.Design for this moment.5m.Generally, the actualreinforcement may besomewhat higher thancalculated..Say the moment capacityofthesection is 600kNm (instead of500

kNm).Sudhir K. Jain, IITGNSlide 40



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Cantilever example (Contd)

.Design assumes steel stress as 0.87fy (due topartial safety factor of 1.15).But, steel can take upto say 1.25fy (due to strainhardening)..Hence, section can take moment upto about 860kNm (= 600x1.25/0.87)..When moment at base is 860 kNm, the shearforce must be 172 kN (= 860/5)..Hence, to prevent shear failure prior to flexuralyielding, design shear force is 172 kN.As against 100 kN factored shear force!Sudhir K. Jain, IITGNSlide 41



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Capacity Design (Contd)

.Ratio 1.25 / 0.87 = 1.44 has been rounded offto 1.4 in the code (Cl. 6.33)Sudhir K. Jain, IITGN Slide 42



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Capacity Design for Shear Capacity Design for Shear.Consider beam part of a frame.EQ Force

Sagging Hogging

EQ ForceHogging Sagging

.Flexural yielding will be in sagging at one endand hogging at the other end, and vice versa.Sudhir K. Jain, IITGN Slide 43



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Capacity Design for Shear (Contd) Capacity Design for Shear (Contd)MSA MHBL+ MHBMSA

Shear force =

L

MHA MSBL+ MSBMHA

Shear force =

L




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Capacity Design for Shear (Contd)




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Example ExamplekNLDVVLDbLDa5.6122.1231'paM295'pbM'

M'

Mpa

pb

105L

(V= 61.5-105=-45.5kN

a)min

(Vb)max = 61.5 + 105 = 166.5 kN




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Example (Contd) Example (Contd)102LMM'mbpa303Mpa209M'pb(Va)max= 61.5 + 102 = 163.5 kN= 61.5-102 = 40.5 kN

(Vb)min

Design shear reinforcement for these shearforce values as usual.




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Detailing Reqmnts for Beams Detailing Reqmnts for BeamsSudhir K. Jain, IITGN Slide 48



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Columns




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Location of Lap Splices Location of Lap SplicesAll laps should be only in the central half of thecolumn height.Seismic moments are maximum in columns justabove and just below the beam: hence,reinforcement must not change at thoselocations.Seismic moments minimum in the central half ofthe column height.Hence, reinforcement should be specified frommid-storey-height to next mid-storey-height.Sudhir K. Jain, IITGN Slide 50



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Locations of Laps in ColumnsRegion forlap splicesLocations of Laps in ColumnsRegion forlap splicesBending Moment Diagram




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Lap Splices

Should be proportioned as tension splices.Columns may develop substantial moments.The moments are reversible in direction.Hence, all bars are liable to go under tension.Sudhir K. Jain, IITGN Slide 52



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No of bars to be lapped No of bars to be lappedCode does not allow more than 50% of the barsto lapped at the same location.For buildings of normal proportions, it means:Half the bars to be spliced in one storey, and theother half in the next storey.Construction difficulties.The clause appears to be very harsh.It should allow all bars to be lapped at the samelocation but with a penalty on the lap length.Sudhir K. Jain, IITGN Slide 53



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Detailing at Lap Locations Detailing at Lap LocationsHoops to be provided over entire splice lengthat spacing not exceeding 150 c/c.Sudhir K. Jain, IITGN Slide 54



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Transverse Reinforcement

A hoop must be (Cl. 7.3.1):Closed stirrupHave 135 degree hookHave 10 dia extension (but not less than 75mm)at each end which is embedded in coreconcrete.10 dia extension: difficulties in constructionACI now allows 6 dia extension (subject to aminimum of 75 mm).Sudhir K. Jain, IITGN Slide 55



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Transverse Reinforcement

If length of any side of hoop exceeds 300mm,cross tie to be provided (Cl. 7.3.2)Sudhir K. Jain, IITGN Slide 56



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Transverse Reinforcement (Contd)




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As per ACI318




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Spacing of Hoops (Cl. 7.3.3) Spacing of Hoops (Cl. 7.3.3)Spacing of hoops anywhere not to exceed halfthe least lateral dimension of the column.Except where confinement reinforcement isneeded: closer spacing will be needed there.Sudhir K. Jain, IITGN Slide 59



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Shear Design Shear DesignColumn to be designed for larger ofCalculated factored shear force.Shear force by capacity design conceptassuming plastic hinge forms at the beams oneither side.It is assumed in this clause that the columns will not yieldbefore the beams do (Strong Column Weak BeamDesign)However, recall that our code does not have the clause forstrong column weak beam design.Sudhir K. Jain, IITGN Slide 60



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Design Shear Force for Column




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Special Confining Reinf. Special Confining Reinf.Must be provided over a length lo from eachjoint face. Length lo must be larger of:Larger lateral dimension of the column1/6 of the clear span of member450mmSudhir K. Jain, IITGN Slide 62



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Special Confining Reinf. (Contd)

.If point of contraflexure not within middle halfof the member clear height:.Special confining reinforcement should beprovided over full column height.Sudhir K. Jain, IITGNSlide 63



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Column End at Footing Column End at FootingSudhir K. Jain, IITGN Slide 64



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Spacing of Special Conf. Reinf.

Spacing of hoops for special confinementreinforcementNot to exceed of minimum column dimension.But need not be less than 75mm nor more than100 mm.The above spacing is really for buildings.For large bridge piers, may allow larger spacingAASHTO: minimum spacing of 100mmJapanese code: minimum spacing of 150mmIndian code needs to incorporate this.Sudhir K. Jain, IITGN Slide 65



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Confinement Reinf. Area

Area of cross section of circular hoops or spiralsto be not less than:0.109.0kgyckkshAAffSDASudhir K. Jain, IITGN Slide 66



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Example:

Column dia: 300 mmM20 concrete, Fe415 reinforcementSpacing of confinement reinforcement should notexceed 300/4 = 75, or 100mm and cannot be lessthan75mm.Hence, spacing of confinement reinf. = 75 mmAssuming clear cover of 40mm:Core dia (Dk) is 220mm; Ak=38,000 sq.mOverall dia = 300mm; A=70,700 sq.mg

Ash = 0.09 x 75 x 220 x (20/415) x [(300/220)2 -1] = 61.5 sq.mmHence, 10 mm dia bars are needed.




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Another Example: Another Example:Same as earlier: change column dia to 200mm.Stirrup spacing will still be 75mm.Core dia is 120mmAsh = 0.09 x 75 x 120 x (20/415) x [(200/120)2 -1] = 69.4 sq.mmNeed 10 mm stirrups.

Same as earlier: change column dia to 150mm.Stirrup spacing will still be 75mm.Core dia is 70mm= 0.09 x 75 x 70 x (20/415) x [(150/70)2 -1] = 81.8 sq.mmAsh

Need 12 mm dia stirrups!!




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Confinement Reinforcement

The last term in bracket tends to increase as thecolumn size reduces.For very small sections, you will get larger diabars.Can be a problem in the detailing of boundaryelements of shear walls.Sudhir K. Jain, IITGN Slide 69



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More Example More ExampleSame as earlier: change column dia to 2000mm.Stirrup spacing will now be 100mm.Core dia is 1920mmAsh = 0.09 x 100 x 1920 x (20/415) x [(2000/1920)2 -1]= 70.84 sq.mmNeed 10 mm stirrups!! Clearly, too small for 2 mdia column.




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Confinement Reinforcement

For very large diameters, the last term inbracket tends to be very small.This leads to under-design of largediameter bridge piers.Sudhir K. Jain, IITGN Slide 71



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Rectangular Hoops Rectangular Hoops0.118.0kgyckshAAffShASudhir K. Jain, IITGN Slide 72



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Confinement Hoops Confinement HoopsThus, equations of Cl. 7.4.7 and Cl. 7.4.8break down for very large sections andvery small sections.

This needs to be fixed in the code. IRC draftunder discussion provides additionalrequirements on this.




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Beam Column Joints




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Joints in RC Frames Joints in RC Frames.Moment resisting frame has three components.Beams.Columns.Rigid joint between beams and columns..Joint is a very important element..Earlier, joint was often ignored in RCconstructions, even though in steel constructionsadequate attention was always paid to the joint.Sudhir K. Jain, IITGNSlide 75



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Codal Provisions

.Provisions in IS:13920 on joints are very weak..Considerable improvements are needed in thenext edition..Partly, this is because IS:456 lacks generalframework for joint calculations.Sudhir K. Jain, IITGNSlide 76



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Reinforcement in Joint

.Joint too needs to have stirrups like columns do..In most constructions in our country, joints arenot provided with stirrups..It is often tedious to provide stirrups in joint due tocongestion..In gravity design, there was a practice thatbottom beam bars need not be continuousthrough the joint..It is simply not acceptable when building has tocarry lateral loads.Sudhir K. Jain, IITGNSlide 77



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RC Detailing Handbook of BISIncorrectPracticeRC Detailing Handbook of BISIncorrectPracticeSudhir K. Jain, IITGN Slide 78



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Issues

.Serviceability.Cracks should not occur due to.Diagonal compressionm.Joint shear.Strength.Should be more than that in adjacent members.Ductility.Not needed for gravity loads.Needed for seismic loads.Ease of Construction

.Should not be congested.Sudhir K. Jain, IITGN Slide 79



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Cracks in Joint Region Cracks in Joint RegionSudhir K. Jain, IITGN Slide 80



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Type of Joints Type of JointsSudhir K. Jain, IITGN Slide 81



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Geometric Description of Joints




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Moment Strength Ratio Moment Strength Ratio.Moment strength ratio to ensure StrongColumn Weak Beam.Columns should have higher moment capacitythan the beams0.1MM)beams(n)cols(n.Normally, the codes require this ratio to be atleast 1.2Sudhir K. Jain, IITGNSlide 83



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Moment Strength Ratio (Contd)

.Our code does not have this requirement..Notice that the original draft contained inMedhekars paper had this clause

.This clause requires much larger column sizesthan prevalent in India..It was felt that this may not be followed inpractice and hence it should be deferred for thetime being..It is perhaps time to think of bringing this clausein the code.Sudhir K. Jain, IITGNSlide 84



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Confinement of Concrete Core Confinement of Concrete Core.Core concrete acts as compression strut, and.It carries shear force.shellcoreSudhir K. Jain, IITGN Slide 85



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Compression Strut Compression StrutCompression StrutMomentMoment




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Confinement Confinement.Provided by the beams (and slabs) around thejoint, andCol.Plan.By the reinforcement:.Longitudinal bars (from beams and columns,passing through the joint), and.Transverse reinforcementSudhir K. Jain, IITGNSlide 87



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Confinement (Contd)

.Better to provide more number of smaller dialongitudinal bars in beams and columns..Requirements on transverse reinforcementreduced if joint is confined by beams on allfaces.Sudhir K. Jain, IITGNSlide 88



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IS:13920

.Unless the joint is confined by beams, specialconfinement reinforcement provided in thecolumns to also be provided in joint..If beams frame on all four faces of the joint, thejoint may be provided half the reinforcementgiven above. This is provided:.Beam widths are at least column width..Spacing of hoops in the joint region not toexceed 150 mm.Sudhir K. Jain, IITGNSlide 89



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Shear Force in Joint Shear Force in JointSudhir K. Jain, IITGN Slide 90



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Shear Force in Joint (Contd) Shear Force in Joint (Contd)Sudhir K. Jain, IITGN Slide 91



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Shear Strength Shear Strength.Indian code does not require shear strength ofjoint to be checked..This should be introduced..ACI and other codes provide a formal method tocheck shear stress within the joint region.Sudhir K. Jain, IITGNSlide 92



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Anchorage for Longitudinal Bars

.Joints should be capable of providing anchorageto beam and column bars.Sudhir K. Jain, IITGN Slide 93



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External Joints External Joints.ACI has standard hooks. Hence, the columnwidth is checked to ensure anchorage.lcbydhfdfl'65Sudhir K. Jain, IITGN Slide 94



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Bar Stresses

Under Lateral

Gravity Loads Lateral Loads

Loads




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Internal Joints Internal JointsSeismic Codes usually require that20DiameterBar BeamthColumn WidSudhir K. Jain, IITGN Slide 96



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RC Frame Design of Buildings November 2012 SKJ

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