Design of RCC structures Detailing of Reinforcement (IS456-2000, IS13920) Pradip Paudel (M.Sc. in Structural Engineering)

WHO IS AN ENGINEER? The great liability of the engineer …compared to men of other professions……is that his works are out in the open where all can see them. He cannot bury his mistakes in the grave like the DOCTORS. He cannot, like the ARCHITECT, cover his figures with trees A design engineer’s responsibility should include assuring the structural safety of the design, details, checking shop drawing. Detailing is as important as design

Requirements of Good Detailing Should be simple for fabrication and placing Check spacing for crack control – maximum spacing Minimum spacing for concrete placing Check for splicing requirements and development length Special care for section of varying depth and sloped slabs Care for corner joints, beam column junction, openings etc.

Cont.… Detailing for gravity loads is different from the lateral loads specially for the SEISMIC FORCES. Apart from the detailing for the above there is a different detailing required for the rehabilitation and strengthening of damaged structures. Detailing codes SP16 and IS456-2000 EQ code is 13920 as required for seismic forces.

Minimum Nominal Cover Clear cover shall be provided To develop the desired bond strength through out the perimeter of steel To protect against corrosion and fire Slab 20mm Beam 25mm Column 40mm Footing 60mm For detail see Table-16 IS 456-2000

Spacing of reinforcement The horizontal distance between two parallel main reinforcing bars shall not be less than The diameter of the bar if the bars are of equal diameter The diameter of the larger bar 5mm + nominal size of aggregate Maximum spacing ----as per design Further reading Page 167 A. K. Jain Clause 26.3.2, IS456-2000

Limits for reinforcement Beam Min As/bd = 0.85/Fy Maximum = 4% of bD Compression = 0.2% Web reinforcement = 0.1% of web area Vertical Shear bars not exceeding 0.75d or 300mm whichever is less. Minimum shear bars = cl 26.5.1.6 Slab = min 0.12% for Fe415 and 0.15% for Fe250 Clause 26.5, IS456-2000

The stirrups shall be minimum size of 8mm in the case of lateral load resistance . The hooks shall be bent to 135 degree . In column size of bar not less than 12 mm Spacing of bars not greater than 300mm Arrangement of lateral ties as per cl 26.5.3.2

DEVELOPMENT LENGTH OF BARS FOR A CONCRETE GRADE M20 &STEEL STRENGTH Fy=415 SLNO BAR DIA. TENSIONmm COMPRESSION REMARKS 1 8 376.0 301.0 2 10 470.0 3 12 564.0 451.0 4 16 752.0 602.0 5 20 940.0 6 22 1034.0 827.0 7 25 1175.0 28 1316.0 1053.0 9 32 1504.0 1203.0 APPROXIMATELY USE 50Xdia FOR TENSION

Lap splicing -clause 26.2.5.1 IS456-2000

Lap splicing as per clause 26.2.5.1 IS456-2000 Lap splices should not be used for bar larger than 36mm Larger than 36mm bar may be welded Lap length including anchorage value of hooks in flexural tension is development length (Ld)or 30ϕ whichever is greater. Lap length including anchorage value of hooks in direct tension is 2Ld or 30ϕ whichever is greater. The straight lap length should not be less than 200mm or 15ϕ whichever is greater.

Lap splicing Cont.…. Lap splices are considered as staggered if the c/c distance of the splices is not less than 1.3 times the lap length. The lap length in compression is equal to the development length in compression but not less than 24ϕ. Lap length is calculated on the basis of diameter of smaller bar when bars of different diameters are to be spliced.

DO’S For Detailing Prepare drawing properly and accurately Prepare bar bending schedule, if necessary Indicate proper cover to the reinforcement Decide location of the openings/hole and supply adequate details for the reinforcement around openings. Commonly available size of bars and spirals shall be used for reinforcement.

For a single structural member the number of different sizes of reinforcement bar should be minimum. The grade of reinforcement bars should be clearly mention in the structural drawings When reinforcement is left exposed for future construction, it should be adequately protected from corrosion and weathering. Congestion of the reinforcement should be avoided at points where members intersect and make certain that all the reinforcement shown can be properly placed. Show enlarged details at the corners, beam and column joint and at similar special situations.

Do Not's for Detailing Flexure reinforcement shall not be terminated in a tension zone. Lap splices should not be used for bars larger than 36 mm dia. Different types of reinforcing bars such as deformed bars and plain bars and various grades should not be used side by side as this practice would lead to confusion at site.

Do’s for Columns A reinforced column should have min 4 bars for rectangular or square column and minimum 6 bars for circular columns. Keep outer dimensions of the column constant, as far as possible, for re-use of forms. Preferably avoid use of two grades of vertical bars in the same element.

Do’s for Beams and Slabs Where splices are provided in the reinforcing bars, they should be staggered, and away from the sections of maximum stress. Where the depth of the beam exceeds 750 mm in case of beams without torsion and 450 mm with torsion side face reinforcement shall be provided. All spacing shall be c/c spacing of the bars.

Deflection in beams/slabs may be reduced by providing compression reinforcement. At beam column intersection ,ensure that the main beam bars avoid the main column bars. At beam column intersections , main reinforcement may be so arranged that layers in mutually perpendicular beams are at different levels. To accommodate bottom bars, it is good practice to make secondary beams shallower than the main beams at least by 50 mm.

Curtailment of reinforcement   Clause26.2.3 IS456

Cont.…  

Cont.… Positive moment reinforcement, Cl26.2.3.3 Negative moment reinforcement,Cl26.2.3.4 Curtailment of bundled bars,Cl26.2.3.5 Further reading A. K. Jain p 172 Simplified rules of curtailment as per BS 8110-1985

Bar Bending Schedule Bar Bending Schedule should include: Identification of structural member Position of each bar in the member Bar marks and diameter of each bar Number of bars Shape and bending dimension of each bar Length of each bar Remark, if any Typical example for slab, beam and column: see page 180 (A.K. Jain)

IS13920:1993,Ductile Detailing of RCC Structures Subjected to Seismic Forces This code applies to all RCC structures which satisfy one of the four conditions- The structure is located – In zone IV or V In zone III and I> 1.0 In zone III and industrial structure In zone III and is more than 5 story high

What is EQ Resistant Design? The acceptable response levels of the structure under design earthquake. Designer should exercise some degree of control on magnitude and distribution of stiffness, mass and relative strength of member and their ductility to achieved desired results.

Seismic Design Criteria(IS 1893) Earthquake Desired Behavior Controlling parameters Minor No damage to non-structural components Control deflection by providing stiffness Moderate No significant structural damage, minor cracks in beam and columns, Response should be predominantly elastic Avoid yielding of members or permanent damage by providing strength Severe, Catastrophic No collapse of the system which could cause loss of life. Allow structure to enter into inelastic range and absorb energy by providing ductility

If elastic strength of structural elements exceeds the greatest imposed load upon that structure there can be no significant damage. In severe earthquake some of the resisting elements will be loaded to their full strength. If they are brittle, they will fail, throwing their share of the load on the remaining elements. If they are ductile, they can continue to participate in resisting the lateral force up to their full strength after they yield.

DUCTILITY A ductile material is the one that can undergo large strains while resisting loads Ductility implies the ability to sustain significant inelastic deformations prior to collapse. Brittle material is one that fails suddenly upon attaining its maximum load

Brittle and Ductile force-deformation behavior Δy Δu Deformation Brittle

 

Significance of Ductility It can be expected to adapt to unexpected overloads, impact and structural movements due to foundation settlements and volume changes. Occupants will have sufficient warning of the impending failure thus reducing the probability of loss of life in the events of collapse. All joints and splices must be able to withstand forces and deformations corresponding to yielding of the reinforcement.

Design for Ductility Structural layout should be simple and regular. Amount of tension reinforcement in beams should be restricted and more compression reinforcement should be provided.

Cont.…  

Cont.… The shear reinforcement should be adequate to ensure that the strength in shear exceeds the strength in flexure and thus, prevent a non-ductile shear failure before the fully reversible flexural strength of a member has been developed. See on clause 6.3.3 in IS13920

BEAMS At least two bars should be provided continuously both at top and bottom. The positive moment resistance at the face of the joint should not be less than one –half of the negative moment resistance provided at that face of the joint. Neither the negative nor the positive resistance at any section along the member length should be less than one-fourth of the moment resistance provided at the face of the either joint Clause 6, IS13920

BEAM REINFORCEMENT MIN 2 BARS FOR FULL LENGTH ALONG TOP AND BOTTOM FACE AS > MIN. Bd AS < MAX Bd 50 mm max 50 mm max d db 2d 2d HOOP SPACING > d /2 HOOP SPACING < d/4 and 8 db B = BREADTH OF BEAM db = DIAMETER OF LONGITUDINAL BAR BEAM REINFORCEMENT

Spacing of hoops over a length of 2d at either end of the beam shall not exceed- 8 times the diameter of the smallest longitudinal bar, need not be less than 100 mm Elsewhere, the beam shall have vertical hoops at a spacing not exceeding d/2. Clause 6.3.5, IS13920

COLUMN  

Cont.… The special confining reinforcement shall be provided above and below the beam connections, in a length of the column at each end which is largest of the following- 1/6 of the clear height of the column Larger dimension of the column 450 mm When a column terminates into a footing, special confining reinforcement shall extend at least 300 mm into the footing

PROVISION OF SPECIAL CONFINING REINFORCEMENT IN FOOTINGS > 300 mm PROVISION OF SPECIAL CONFINING REINFORCEMENT IN FOOTINGS

COLUMN AND JOINT DETAILING lO JOINT REINFORCEMENT AS PER 8.1 lO > hc / 4 SPECIAL CONFINING REINFORCEMENT AS PER 7.4.1 TRANSVERSE REINFORCEENT AS PER 7.3.3 SPLICE TRANSVERSE REINFORCEMENT AS PER 7.2.1 hC > hc / 4 lO CONFINED JOINT WITH BEAMS FRAMING INTO ALL FOUR SIDES CONFINING REINFORCEMENT AS PER 8.2 COLUMN AND JOINT DETAILING

The spacing of hoops used as special confining reinforcement shall not exceed – ¼ of the minimum member dimension need not be less than 75 mm nor more than 100 mm For further information see on Clause 7.4.6 and 7.4.7 in IS 13920

FIG. 1: ANCHORAGE OF BEAM BARS IN AN EXTERNAL JOINT. Ld + 10db db Ld + 10db Ld = DEVELOPMENT LENGTH IN TENSION Db = BAR DIAMETER FIG. 1: ANCHORAGE OF BEAM BARS IN AN EXTERNAL JOINT.

LAP SPLICES IN BEAM Not more than 50 % of the bars shall be spliced at one section The longitudinal bars shall be spliced , only if hoops are provided over the entire splice length, at a spacing not exceeding 150 mm The lap length shall not be less than the bar development length in tension. Clause 6.2.6 IS13920

Cont.… Lap splices shall not be provided- within a distance of 2d from the joint face within a joint within a quarter length of the member where flexural yielding may generally occur under the effect of earthquake forces. Use of welded splices and mechanical connections may also be made as per IS456-1978 Clause 6.2.6 IS13920

Ld db FIG. 2: LAP, SPLICE IN BEAM 150 mm Ld = DEVELOPMENT LENGTH IN TENSION db = BAR DIAMETER FIG. 2: LAP, SPLICE IN BEAM

Web Reinforcement   Clause 6.3 IS13920

Cont.….

CANTILEVER BEAM Ld/2 Ld/2 Ld INCORRECT CLOSE STIRRUPS CORRECT Ldt crack INCORRECT Ldt CLOSE STIRRUPS Ldt/2 Ld/2 Ld/2 CORRECT Ld

NON PRISMATIC BEAM Ld/2 Ld/2 Ld INCORRECT CLOSE STIRRUPS CORRECT Ldt crack INCORRECT Ldt CLOSE STIRRUPS Ldt/2 Ld/2 Ld/2 CORRECT Ld

GRID BEAM INCORRECT Hanger bars CORRECT Close rings 1.5d 1.5d 1.5d 300 2#extra bars Slope 1:10 CORRECT

Details of Main & Secondary beams Main beam INCORRECT Secondary beam Close rings 1.5d 1.5d 1.5d 300 300 d 60degree Hanger bars Main beam CORRECT

d INCORRECT d/2 d/2 CORRECT BEAM 1/4OR 1/5 SPAN d/2+d/2Cot(t) 1.5d Ld LINE OF CRACK d/2 t d/2 t d/2+d/2Cot(t) 1.5d CORRECT

Continuous Beam incorrect correct Span/4 Span/4 Span/4 Span/4 Span/4 1.5d 1.5d 1.5d correct

CONTINUOUS BEAM L1 L2 INCORRECT L1 L2 CORRECT SPAN/4 SPAN/4 SPAN/4 100% CRACK 100% CRACK CRACK L1 .08L1 .08L1 L2 .08L2 INCORRECT 100% L1/4 L2/4 L1/4 20% 20% 0.1L1 100% 100% L1 .15L1 L2 .15L2 CORRECT

NONPRISMATIC SECTION OF BEAM D CRACK D D INCORRECT CLOSE RING D D CORRECT D CLOSE RING

CANTILEVER BEAM PROJECTING FROM COLUMN INCORRECT NOT LESS THAN 0.5Ast NOT LESS THAN GREATER OF 0.5L OR Ld 50mm Ld 0.25Ast COLUMN CORRECT Ld/3

SLOPING BEAM CRACK Ld Ld CORRECT

L INCORRECT CORRECT L HAUNCH BEAMS Ld Ld Ld Ld CRACK CRACK L/8 TO L/10

C-COMPRESSION T-TENSION STRESSES AT CORNERS C t C CRACK t RESULTANT TENSILE STRESS FOR ACROSS CORNER(ONE PLANE) RESULTANT TENSILE STRESS FOR ACROSS CORNER(DIFFERENT PLANE) t CRACK c t c

Stirrups taken round outermost bars spacing<=x1 SHEAR AND TORSION REIN. IN BEAMS Stirrups taken round outermost bars spacing<=x1 <=(x1+y1)/4 <=300mm Min 0.2%bd to control deflection as well as for seismic requ. INCORRECT n d y1 Skin rein.10dia is required when depth exceeds 450mm(0.1% of web area distributed equally on two faces) D 100 to 200mm D-n>500mm D-n>500mm D/5 x1 CORRECT b

Shear rein. INCORRECT 2/3d d Ld CORRECT Ld Extra ties CANTILEVER BEAM WITH POINT LOAD Shear rein. INCORRECT 2/3d d Ld CORRECT Ld Extra ties

opening crack crack INCORRECT OPENING IN WEB OF BEAM d/2 opening d/2 Closed stps for d/2 Closed stps for d/2 Ld OPENING IN WEB OF BEAM CORRECT

BEAM COLUMN JUNCTION-EXTERIOR COLUMN INCORRECT CLOSED STPS 2”max U TYPE BARS Ld IN TENSION-Ld CORRECT

SPLICE DETAIL FOR COLUMN COVER CLOSE TIES @S/2 SLOPE 1:6 S-SPACING CORRECT INCORRECT

REDUCTION COLUMN BOTH SIDES INCORRECT SPLICE SLOPE 1:8 FROM BEAM BOTTOM 3NO.CLOSE TIES CLOSE STPS SPACIN <=75mm 3NO.CLOSE TIES CORRECT

INCORRECT Ld CORRECT TERMINATION OF COLUMN BARS INSIDE BEAM

EQ REGION-BEAM-COL JN-EXTERIOR *COL.CORE HAS TO BE CONFINED BY CIRCULAR OR RECTANGULAR TIES IN ACCORDANCE WITH END REGION SPACING OF LATERAL TIES <=d/2 COL. CORE* END REGION SPACING OF LATERAL TIES <=100mm END REGION BEAM COL. JUNCTION-EQ REGION INCORRECT CORRECT SPACING OF LATERAL TIES <=d/2 SPACING OF LATERAL TIES <=d/2 BEAM COL. JUNCTION-EQ REGION EQ REGION-BEAM-COL JN-EXTERIOR

First stirrups 50mm from beam face correct incorrect First stirrups 50mm from beam face correct END REGION END REGION-h/6 or D or 450MM whichever is greater COLUMN DETAILS IN EQ REGIONS h b d D END REGION END REGION Spacing of shear rein. In columns Spacing of shear rein. In columns

EQ-REGION-CONTINUOUS BEAM INCORRECT 50mmmax CONTINUOUS BARS NOT LESS THAN ¼ AREA OF BARS AT COL.FACE CORRECT A=L1/3 A=L1/3 A=L1/3 Ld stp@maxd/2 2d 2d 2d stp@maxd/2 2d Stirrup spacing=d/4 or 100mm or 8dia which ever is the least

FOOTING-DETAILS(INDEPENDENT) INCORRECT COLUMN BARS STARTER BARS NATURAL G.L COVER TO STARTER Lb 3” SIDE COVER Ldt Min.300 COVER50mm IF p.c.c below or 75mm Ldt CORRECT

INCORRECT INCORRECT CRACK CORRECT Ld Ld Ld SECTION OF TRENCH

INCORRECT Extra bar Ld(min) Ld(min) Ld(min) CORRECT STAIRCASE-WITH WAIST SLAB

Dist. Alternate 1 Main bar Main bar SLABLESS STAIRCASE

SLABLESS STAIRCASE Main bar L=horizontal span Alternate 2 A=0.25L