REINFORCING DETAILINGOF R.C.C MEMBERS PRESENTED BYEr.T.RANGARAJAN,B.E,M.Sc(struct.engg),F.I.E, FACCE,LACI,LISSE,LIASE
WHO IS AN ENGINEER?According to USA President Herbert Hoover, whowas an engineer before he became a politician, said:The great liability of the engineer …compared to menof other professions……is that his works are out inthe open where all can see them.His acts …..step by step …are in hard substances.He cannot bury his mistakes in the grave like theDOCTORS.He cannot argue them into thin air…..or blame thejudge…..like the LAWYERS.He cannot, like the ARCHITECT, cover his figureswith trees and vines.He cannot, like the politicians, screen hisshortcomings by blaming his opponents….and hopethe people will forget. The ENGINEER simply cannotdeny he did it.If his works do not work……he is damned.
A design engineer’s responsibility should includeassuring the structural safety of the design,details, checking shop drawing.Detailing is as important as design since properdetailing of engineering designs is an essentiallink in the planning and engineering process assome of the most devasting collapses in historyhave been caused by defective connections orDETAILING. There are many examples explainedin the book" DESIGN AND CONSTRUCTIONFAILURES by Dov Kaminetzky.Detailing is very important not only for theproper execution of the structures but for thesafety of the structures.Detailing is necessary not only for the steelstructures but also for the RCC members as it isthe translation of all the mathematicalexpression’s and equation’s results.
For the RCC members for most commonly used for buildings we can divide the detailing for1. SLABS-WITH OR WITHOUT OPENINGS.(RECTANGULAR,CIRCULAR,NON- RECTANGULAR-PYRAMID SLAB,TRIANGULAR ETC) -BALCONY SLAB,LOFT SLAB,CORNER SLAB etc2. BEAMS- WITH OR WITHOUT OPENIGS.(SHALLOW & DEEP BEAMS)3. COLUMNS.(RECTANGULAR,L-SHAPE,T-SHAPE, CIRCULAR,OCTAGONAL,CROSS SHAPE etc)4. FOUNDATIONS. 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. We will now dwell on the DETAILING OF MEMBERS FOR THE GRAVITY AND SOME CODAL DETAILINGS AS PER IS CODE IS 13920 AND IS 4326 AS REQUIRED FOR SEISMIC FORCES.
DO’S & DONOT’S FOR DETAILING DO’S-GENERAL1. Prepare drawings properly & accurately if possible label each bar and show its shape for clarity. 12” I ¼”dia@12”c/c 1- ¼”dia@12”c/cCross section of retaining wall which collapsed immediately after placing of soilbackfill because ¼” rather than 1-1/4” dia. were used. Error occurred becauseCorrect rebar dia. Was covered by a dimension line.
2. Prepare bar-bending schedule , if necessary.3. Indicate proper cover-clear cover, nominal cover or effective cover to reinforcement.4. Decide detailed location of opening/hole and supply adequate details for reinforcements around the openings.5. Use commonly available size of bars and spirals. For a single structural member the number of different sizes of bars shall be kept minimum.6. The grade of the steel shall be clearly stated in the drawing.7. Deformed bars need not have hooks at their ends.8. Show enlarged details at corners, intersections of walls, beams and column joint and at similar situations.
9. Congestion of bars should be avoided at points where members intersect and make certain that all rein. Can be properly placed.10. In the case of bundled bars, lapped splice of bundled bars shall be made by splicing one bar at a time; such individual splices within the bundle shall be staggered.11. Make sure that hooked and bent up bars can be placed and have adequate concrete protection.
12. Indicate all expansion, construction and contraction joints on plans and provide details for such joints.13. The location of construction joints shall be at the point of minimum shear approximately at mid or near the mid points. It shall be formed vertically and not in a sloped manner.DO’S – BEAMS & SLABS:1. Where splices are provided in bars, they shall be , as far as possible, away from the sections of maximum stresses and shall be staggered.2. Were the depth of beams exceeds 750mm in case of beams without torsion and 450mm with torsion provide face rein. as per IS456-2000.3. Deflection in slabs/beams may be reduced by providing compression reinforcement.4. Only closed stirrups shall be used for transverse rein. For members subjected to torsion and for members likely to be subjected to reversal of stresses as in Seismic forces.
5. To accommodate bottom bars, it is good practice to make secondary beams shallower than main beams, at least by 50mm.Do’s –COLUMNS.1. A reinforced column shall have at least six bars of longitudinal reinforcement for using in transverse helical reinforcement.-for CIRCULAR sections.2. A min four bars one at each corner of the column in the case of rectangular sections.3. Keep outer dimensions of column constant, as far as possible , for reuse of forms.4. Preferably avoid use of 2 grades of vertical bars in the same element. DONOT’S-GENERAL:1. Reinforcement shall not extend across an expansion joint and the break between the sections shall be complete.2. Flexural reinforcement preferably shall not be terminated in a tension zone.
3. Bars larger than 36mm dia. Shall not be bundled.4. Lap splices shall be not be used for bars larger than 36mm dia. Except where welded.5. Where dowels are provided, their diameter shall not exceed the diameter of the column bars by more than 3mm.6. Where bent up bars are provided, their contribution towards shear resistance shall not be more than 50% of the total shear to be resisted. USE OF SINGEL BENT UP BARS(CRANKED) ARE NOT ALLOWED IN THE CASE OF EARTHQUAKE RESISTANCE STRUCTURES.
DETAILING OF SLABS WITHOUT ANY CUTOR OPENINGS. The building plan DX-3 shows the slabs in different levels for the purpose of eliminating the inflow of rainwater into the room from the open terrace and also the sunken slab for toilet in first floor. The building plan DX-A3 is one in which the client asked the architect to provide opening all round. Refer the 3d elevation
Different shapes of slabs used in the 6”depression for OT & buildings. 9” for sunken slabs. Portico slab in elevation 19’-6” 5’wide corridor all round 9’-6”suarePortico slab in plan opening Portico and other rooms roof slab in plan
Minimum and max.reinforcement % in beams, slabs andcolumns as per codal provisions should be followed.SLABS:It is better to provide a max spacing of 200mm(8”) for mainbars and 250mm(10”) in order to control the crack width andspacing.A min. of 0.24% shall be used for the roof slabs since it issubjected to higher temperature. Variations than the floorslabs. This is required to take care of temp. differences.It is advisable to not to use 6mm bars as main bars as thissize available in the local market is of inferior not only withrespect to size but also the quality since like TATA and SAILare not producing this size of bar.BEAMS:A min. of 0.2% is to be provided for the compression bars inorder to take care of the deflection.
The stirrups shall be min.size of8mm in the case of lateral loadresistance .The hooks shall be bent to 135degree .
HAUNCH BEAMS CRACK CRACK L INCORRECT Ld Ld CORRECTLd L/8 TO Ld L/10 L/8 TO L L/10
STRESSES AT CORNERS C-COMPRESSION T-TENSION C t C CRACK t RESULTANT TENSILE STRESS FOR ACROSS CORNER(ONE PLANE) RESULTANT TENSILE STRESS FOR t ACROSS CORNER(DIFFERENT PLANE) CRACK c t c
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 y1Skin rein.10dia is Drequired when depth 100 to D-n>500mmexceeds 450mm(0.1% 200mmof web area distributedequally on two faces) D/5 x1 CORRECT b
CANTILEVER BEAM WITH POINT LOAD Shear rein.INCORRECT 2/3d d Ld CORRECT Ld Extra ties
INCORRECT opening crack crack OPENING IN WEB OF BEAM d/2 opening d/2Closed stps Closed stpsfor d/2 for d/2 Ld OPENING IN WEB OF BEAM CORRECT
BEAM COLUMN JUNCTION-EXTERIOR COLUMNINCORRECT 2”max CLOSED STPS U TYPE BARS Ld IN TENSION-Ld CORRECT
SPLICE DETAIL FOR COLUMN COVER CLOSE TIES @S/2 SLOPE 1:6 S-SPACING CORRECTINCORRECT
REDUCTIONCOLUMN BOTHSIDES INCORRECT SPLICE SLOPE 1:8 FROM BEAM BOTTOM 3NO.CLOSE TIES CLOSE STPS SPACIN <=75mm 3NO.CLOSE TIES CORRECT
TERMINATION OF COLUMN BARS INSIDE BEAMINCORRECT LdCORRECT
*COL.CORE HAS TO BE CONFINED BY EQ REGION-BEAM-COL JN-EXTERIOR CIRCULAR OR RECTANGULAR TIES IN ACCORDANCE WITH END REGION SPACING OF LATERAL TIES <=d/2 COL. END REGION CORE* SPACING OF LATERAL TIES <=100mm END REGIONBEAM COL. JUNCTION-EQREGIONINCORRECT CORRECT SPACING OF LATERAL TIES <=d/2 BEAM COL. JUNCTION-EQ REGION
COLUMN DETAILS IN EQ REGIONS incorrect First stirrups correct 50mm from beam faceENDREGION END REGION-h/6 or D or 450MM whichever is greater h b d DENDREGION 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=L1/3Ld stp@maxd/2 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 NATURAL BARS G.L COVER TO STARTER Lb 3” SIDE COVER Ldt Min.300 COVER50mm Ldt IF p.c.c below CORRECT or 75mm
TYPICAL REIN DETAILS OF INCORRECT HAMMER FOUNDATION BLOCKCORRECT Ld Ld
SECTION OF TRENCH INCORRECT CRACK CORRECT Ld Ld Ld
STAIRCASE-WITH WAIST SLAB INCORRECT Extra bar Ld(min) Ld(min) Ld(min) CORRECT
SLABLESS STAIRCASEDist. Alternate 1 Main barMain bar
SLABLESS STAIRCASEMain bar L=horizontal span Alternate 2 A=0.25L
DEVELOPMENT LENGTH OF BARS FOR A CONCRETE GRADE M20 &STEEL STRENGTH Fy=415 SLNO BAR TENSION COMPRESSION REMARKS DIA. mm 1 8 376.0 301.0 2 10 470.0 376.0 3 12 564.0 451.0 4 16 752.0 602.0 5 20 940.0 752.0 6 22 1034.0 827.0 7 25 1175.0 940.0 8 28 1316.0 1053.0 9 32 1504.0 1203.0 APPROXIMATELY USE 50Xdia FOR TENSION
CONCLUSION: Before concluding I will show some more details drawn in autocad exported in wwf format As there is no time to elaborately explaining ,the following topics are not covered :1. Flat slabs, Folded plates, shell structures-cylindrical shells, silos,2. Staircases- helical staircase, central beam type, cantilever type etc.3. Different types of foundations-raft, pile foundation, strap foundation etc.4. Retaining wall structures,5. Liquid retaining structures.6. Deep beams.7. Shear wall, walls. Hope that I have enlighten some of the detailing technique for the most commonly encountered RCC members in buildings. In the above statements if my senior colleagues and ACCE members can find different method or any new detailing system it will be of immense help not only for me but to other young engineers who should learn in wright ways and not wrong lessons.
REFERENCES:1. HANDBOOK ON CONCRETE REINFORCEMENT AND DETAILING-SP:34(S&T)-1987.2. MANUAL OF ENGINEERING & PLACING DRAWINGS FOR REINFORCED CONCRETE STRUCTURES- (ACI 315-803. MANUAL OF STANDARD PRACTICE –CONCRETE REINFORCING STEEL INSTITUTE.4. TWARD BOARD MANUAL FOR RURAL WATER SUPPLY SCHEMES.5. DESIGN PRINCIPLES AND DETAILING OF CONCRETE STRUCTURES. By D.S.PRAKASH RAO.6. SIMPLIFIED DESIGN-RC BUILDINGS OF MODERATE SIZE AND HEIGHT-BY PORTLAND CEMENT ASSOCIATION,USA.7. DESIGN AND CONSTRUCTION FAILURES BY DOV KAMINETZKY.
8. IS:2502-1963 CODE OF PRACTICE FOR BENDING AND FIXING OF BARS FOR CONCRETE REINFORCEMENT.9. IS:1893:2000.10. IS:4326.11. IS:456:200012. REINFORCED HAND BOOK BY REYNOLD.