REINFORCING DETAILING OF R.C.C MEMBERS PRESENTED BY Er.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, who was an engineer before he became a politician, said:
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.
His acts …..step by step …are in hard substances.
He cannot bury his mistakes in the grave like the DOCTORS.
He cannot argue them into thin air…..or blame the judge…..like the LAWYERS.
He cannot, like the ARCHITECT, cover his figures with trees and vines.
He cannot, like the politicians, screen his shortcomings by blaming his opponents….and hope the people will forget. The ENGINEER simply cannot deny he did it.
If his works do not work……he is damned.
A design engineer’s responsibility should include assuring the structural safety of the design, details , checking shop drawing.
Detailing is as important as design since proper detailing of engineering designs is an essential link in the planning and engineering process as some of the most devasting collapses in history have been caused by defective connections or DETAILING. There are many examples explained in the book" DESIGN AND CONSTRUCTION FAILURES by Dov Kaminetzky.
Detailing is very important not only for the proper execution of the structures but for the safety of the structures.
Detailing is necessary not only for the steel structures but also for the RCC members as it is the translation of all the mathematical expression’s and equation’s results.
For the RCC members for most commonly used for buildings we can divide the detailing for
SLABS-WITH OR WITHOUT OPENINGS.(RECTANGULAR,CIRCULAR,NON-RECTANGULAR-PYRAMID SLAB,TRIANGULAR ETC)
-BALCONY SLAB,LOFT SLAB,CORNER SLAB etc
BEAMS- WITH OR WITHOUT OPENIGS.(SHALLOW & DEEP BEAMS)
CIRCULAR,OCTAGONAL,CROSS SHAPE etc)
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
Prepare drawings properly & accurately if possible label each bar and show its shape for clarity.
12” I ¼”dia@12”c/c Cross section of retaining wall which collapsed immediately after placing of soil backfill because ¼” rather than 1-1/4” dia. were used. Error occurred because Correct rebar dia. Was covered by a dimension line. 1- ¼”dia@12”c/c
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.
Deformed bars need not have hooks at their
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.
Indicate all expansion, construction and contraction joints on plans and provide details for such joints.
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:
Where splices are provided in bars, they shall be , as far as possible, away from the sections of maximum stresses and shall be staggered.
Were the depth of beams exceeds 750mm in case of beams without torsion and 450mm with torsion provide face rein. as per IS456-2000.
Deflection in slabs/beams may be reduced by providing compression reinforcement.
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.
To accommodate bottom bars, it is good practice to make secondary beams shallower than main beams, at least by 50mm.
A reinforced column shall have at least six bars of longitudinal reinforcement for using in transverse helical reinforcement.-for CIRCULAR sections.
A min four bars one at each corner of the column in the case of rectangular sections.
Keep outer dimensions of column constant, as far as possible , for reuse of forms.
Preferably avoid use of 2 grades of vertical bars in the same element.
Reinforcement shall not extend across an expansion joint and the break between the sections shall be complete.
Flexural reinforcement preferably shall not be terminated in a tension zone.
Bars larger than 36mm dia. Shall not be bundled.
Lap splices shall be not be used for bars larger than 36mm dia. Except where welded.
Where dowels are provided, their diameter shall not exceed the diameter of the column bars by more than 3mm.
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 CUT OR 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 buildings.
6”depression for OT & 9” for sunken slabs. Portico slab in elevation 19’-6” 9’-6”suare opening 5’wide corridor all round Portico and other rooms roof slab in plan Portico slab in plan
Minimum and max.reinforcement % in beams, slabs and columns as per codal provisions should be followed.
It is better to provide a max spacing of 200mm(8”) for main bars and 250mm(10”) in order to control the crack width and spacing.
A min. of 0.24% shall be used for the roof slabs since it is subjected to higher temperature. Variations than the floor slabs. This is required to take care of temp. differences.
It is advisable to not to use 6mm bars as main bars as this size available in the local market is of inferior not only with respect to size but also the quality since like TATA and SAIL are not producing this size of bar.
A min. of 0.2% is to be provided for the compression bars in order to take care of the deflection.
The stirrups shall be min.size of 8mm in the case of lateral load resistance .
D D CRACK INCORRECT NONPRISMATIC SECTION OF BEAM D D D CORRECT D CLOSE RING CLOSE RING
COLUMN INCORRECT CANTILEVER BEAM PROJECTING FROM COLUMN CORRECT NOT LESS THAN GREATER OF 0.5L OR Ld 50mm 0.25Ast NOT LESS THAN 0.5Ast COLUMN Ld Ld/3
CRACK SLOPING BEAM CORRECT Ld Ld
CRACK CRACK INCORRECT HAUNCH BEAMS CORRECT L/8 TO L/10 L/8 TO L/10 L L Ld Ld Ld Ld
STRESSES AT CORNERS C C-COMPRESSION T-TENSION t t C RESULTANT TENSILE STRESS FOR ACROSS CORNER(ONE PLANE) t c RESULTANT TENSILE STRESS FOR ACROSS CORNER(DIFFERENT PLANE) t c CRACK CRACK
SHEAR AND TORSION REIN. IN BEAMS INCORRECT Min 0.2%bd to control deflection as well as for seismic requ. b D 100 to 200mm d D-n>500mm D/5 Skin rein.10dia is required when depth exceeds 450mm(0.1% of web area distributed equally on two faces) Stirrups taken round outermost bars spacing<=x1 <=(x1+y1)/4 <=300mm x1 y1 D-n>500mm n CORRECT
CANTILEVER BEAM WITH POINT LOAD CORRECT INCORRECT 2/3d d Shear rein. Extra ties Ld Ld
opening CORRECT OPENING IN WEB OF BEAM crack crack opening INCORRECT OPENING IN WEB OF BEAM Ld d/2 d/2 Closed stps for d/2 Closed stps for d/2
BEAM COLUMN JUNCTION-EXTERIOR COLUMN CORRECT INCORRECT CLOSED STPS 2”max IN TENSION-Ld U TYPE BARS Ld
CORRECT SPLICE DETAIL FOR COLUMN INCORRECT COVER CLOSE TIES @S/2 S-SPACING SLOPE 1:6
REDUCTION COLUMN BOTH SIDES INCORRECT CORRECT 3NO.CLOSE TIES SPLICE CLOSE STPS SPACIN <=75mm SLOPE 1:8 FROM BEAM BOTTOM 3NO.CLOSE TIES
TERMINATION OF COLUMN BARS INSIDE BEAM INCORRECT CORRECT Ld
BEAM COL. JUNCTION-EQ REGION EQ REGION -BEAM-COL JN-EXTERIOR CORRECT BEAM COL. JUNCTION-EQ REGION INCORRECT END REGION END REGION COL. CORE* SPACING OF LATERAL TIES <=100mm SPACING OF LATERAL TIES <=d/2 SPACING OF LATERAL TIES <=d/2 SPACING OF LATERAL TIES <=d/2 * COL.CORE HAS TO BE CONFINED BY CIRCULAR OR RECTANGULAR TIES IN ACCORDANCE WITH END REGION
COLUMN DETAILS IN EQ REGIONS END REGION END REGION First stirrups 50mm from beam face Spacing of shear rein. In columns incorrect Spacing of shear rein. In columns correct END REGION-h/6 or D or 450MM whichever is greater END REGION h d D b
EQ-REGION -CONTINUOUS BEAM INCORRECT Ld A=L1/3 CONTINUOUS BARS NOT LESS THAN ¼ AREA OF BARS AT COL.FACE A=L1/3 A=L1/3 2d 50mmmax Stirrup spacing=d/4 or 100mm or 8dia which ever is the least 2d 2d 2d stp@maxd/2 stp@maxd/2 CORRECT
INCORRECT FOOTING-DETAILS(INDEPENDENT) CORRECT COVER TO STARTER STARTER BARS COLUMN BARS NATURAL G.L Ldt Lb 3” SIDE COVER Ldt COVER50mm IF p.c.c below or 75mm Min.300
TYPICAL REIN DETAILS OF HAMMER FOUNDATION BLOCK INCORRECT CORRECT Ld Ld
SECTION OF TRENCH CRACK INCORRECT INCORRECT CORRECT Ld Ld Ld
STAIRCASE-WITH WAIST SLAB INCORRECT CORRECT Ld(min) Extra bar Ld(min) Ld(min)
Main bar Dist. Main bar Alternate 1 SLABLESS STAIRCASE
Main bar Alternate 2 A=0.25L L=horizontal span SLABLESS STAIRCASE
DEVELOPMENT LENGTH OF BARS FOR A CONCRETE GRADE M20 &STEEL STRENGTH Fy=415 APPROXIMATELY USE 50Xdia FOR TENSION 1203.0 1504.0 32 9 1053.0 1316.0 28 8 940.0 1175.0 25 7 827.0 1034.0 22 6 752.0 940.0 20 5 602.0 752.0 16 4 451.0 564.0 12 3 376.0 470.0 10 2 301.0 376.0 8 1 REMARKS COMPRESSION TENSIONmm BAR DIA. SLNO
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 :
Staircases- helical staircase, central beam type, cantilever type etc.
Different types of foundations-raft, pile foundation, strap foundation etc.
Retaining wall structures,
Liquid retaining structures.
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.
HANDBOOK ON CONCRETE REINFORCEMENT AND DETAILING-SP:34(S&T)-1987.
MANUAL OF ENGINEERING & PLACING DRAWINGS FOR REINFORCED CONCRETE STRUCTURES-
MANUAL OF STANDARD PRACTICE –CONCRETE REINFORCING STEEL INSTITUTE.
TWARD BOARD MANUAL FOR RURAL WATER SUPPLY SCHEMES.
DESIGN PRINCIPLES AND DETAILING OF CONCRETE STRUCTURES. By D.S.PRAKASH RAO.
SIMPLIFIED DESIGN-RC BUILDINGS OF MODERATE SIZE AND HEIGHT-BY PORTLAND CEMENT ASSOCIATION,USA.
DESIGN AND CONSTRUCTION FAILURES BY DOV KAMINETZKY.
IS:2502-1963 CODE OF PRACTICE FOR BENDING AND FIXING OF BARS FOR CONCRETE REINFORCEMENT.