REINFORCING DETAILING OF R.C.C MEMBERS PRESENTED BY
,
Er.T.RANGARAJAN B.E,M.Sc(struct.engg),F.I.E,F ACCE,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 1.
SLABS-WITH OR WITHOUT OPENINGS. (RECTANGULAR,CIRCULAR,NON-RECTANGULAR-PYRAMID SLAB,TRIANGULAR ETC) -BALCONY SLAB,LOFT SLAB,CORNER SLAB etc
2.
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-GENERAL 1.
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/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.
2. 3. 4. 5. 6. 7. 8.
Prepare bar-bending schedule , if necessary. Indicate proper cover-clear cover, nominal cover or effective cover to reinforcement. Decide detailed location of opening/hole and supply adequate details for reinforcements around the openings. Use commonly available size of bars and spirals. For a single structural member the number of different sizes of bars shall be kept minimum. The grade of the steel shall be clearly stated in the drawing. Deformed bars need not have hooks at their ends. 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. 4. 5.
6.
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 6”depression for OT & buildings. 9” for sunken slabs.
Portico slab in elevation
19’-6” 5’wide corridor all
Portico slab in plan
9’-6”suare opening Portico and other rooms roof slab in plan
round
Minimum and max.reinforcement % in beams, slabs and columns as per codal provisions should be followed. SLABS: 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. BEAMS: 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 . The hooks shall be bent to 135 degree .
CANTILEVER BEAM
crack
INCORRECT Ldt
CLOSE STIRRUPS
Ldt/2
Ld/2
Ld/2 Ld
CORRECT
NON PRISMATIC BEAM crack
INCORRECT Ldt
CLOSE STIRRUPS
Ldt/2
Ld/2
Ld/2 Ld
CORRECT
GRID BEAM GRID BEAM
INCORRECT
Close rings 1.5d
1.5d 300
300
d
2#extra bars Slope 1:10
Hanger bars CORRECT
Details of Main & Secondary beams Secondary beam
Main beam INCORRECT Secondary beam Close rings 1.5d 300
1.5d 300 d 60degree
Hanger bars CORRECT
Main beam
BEAM d
INCORRECT
1/4OR 1/5 SPAN
Ld
Ld
LINE OF CRACK
d/2 t
1.5d
t
d/2
d/2+d/2Cot(t)
CORRECT
Continuous beam • continuous beam
Span/4
Span/4
Span/4
Span/4
Span/4
incorrect
Span/4
1.5d
1.5d
correct
Span/4
1.5d
CONTINUOUS BEAM SPAN/4
SPAN/4
.08L1
100% CRACK
SPAN/4
CRACK .08L1
L1
100%
CRACK
L2
.08L2
INCORRECT
L1/4
0.1L1
100% 20%
L1/4
L2/4
20%
100%
100%
L1 CORRECT
.15L1
.15L2
L2
NONPRISMATIC SECTION OF BEAM D
CRACK
D
D
INCORRECT
CLOSE RING
D
D
CLOSE RING
D
CORRECT
CANTILEVER BEAM PROJECTING FROM COLUMN
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
HAUNCH BEAMS
CRACK
CRACK
L INCORRECT Ld
Ld
CORRECT Ld
L/8 TO L/10
L
L/8 TO L/10
Ld
C-COMPRESSION
STRESSES AT CORNERS
T-TENSION C t C
CRACK t RESULTANT TENSILE STRESS FOR ACROSS CORNER(ONE PLANE)
t c
RESULTANT TENSILE STRESS FOR ACROSS CORNER(DIFFERENT PLANE) CRACK 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 Skin rein.10dia is required when depth exceeds 450mm(0.1% of web area distributed equally on two faces)
y1
D 100 to 200mm
D-n>500mm
D/5 x1 b
CORRECT
CANTILEVER BEAM WITH POINT LOAD
Shear rein. INCORRECT
d
2/3d
Ld
Ld
CORRECT Extra ties
INCORRECT opening
crack
crack
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 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
INCORRECT
CORRECT
REDUCTION COLUMN BOTH SIDES
INCORRECT
SLOPE 1:8 FROM BEAM BOTTOM
SPLICE
3NO.CLOSE TIES
3NO.CLOSE TIES
CLOSE STPS SPACIN <=75mm
CORRECT
TERMINATION OF COLUMN BARS INSIDE BEAM
INCORRECT
Ld
CORRECT
*COL.CORE HAS TO EQ REGION-BEAM-COL JN-EXTERIOR
BE CONFINED BY CIRCULAR OR RECTANGULAR TIES IN ACCORDANCE WITH END REGION
SPACING OF LATERAL TIES <=d/2 END REGION
COL. CORE*
SPACING OF LATERAL TIES <=100mm END REGION
BEAM COL. JUNCTION-EQ REGION
INCORRECT
SPACING OF LATERAL TIES <=d/2
CORRECT BEAM COL. JUNCTION-EQ REGION
COLUMN DETAILS IN EQ REGIONS incorrect
First stirrups 50mm from beam face
END REGION
correct
END REGION-h/6 or D or 450MM whichever is greater
h
b
d D
END REGION
Spacing of shear rein. In columns
END REGION
Spacing of shear rein. In columns
EQ-REGION-CONTINUOUS BEAM INCORRECT
50mmmax
CONTINUOUS BARS NOT LESS THAN ¼ AREA OF BARS AT COL.FACE
A=L1/3
A=L1/3
CORRECT A=L1/3
Ld
stp@maxd/2
2d
Stirrup spacing=d/4 or 100mm or 8dia which ever is the least
2d
2d
stp@maxd/2
FOOTING-DETAILS(INDEPENDENT)
INCORRECT
COLUMN BARS
STARTER BARS
NATURAL G.L
COVER TO STARTER
Lb 3” SIDE COVER
Ldt
Ldt
CORRECT
Min.300
COVER50mm IF p.c.c below or 75mm
INCORRECT
TYPICAL REIN DETAILS OF HAMMER FOUNDATION BLOCK
CORRECT
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 STAIRCASE
Dist.
Alternate 1 Main bar
Main bar
SLABLESS STAIRCASE
Main bar L=horizontal span
Alternate 2 A=0.25L
DEVELOPMENT LENGTH OF BARS FOR A CONCRETE GRADE M20 &STEEL STRENGTH Fy=415 SLNO
BAR DIA. TENSIONm COMPRESSION m
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
REMARKS
APPROXIMATELY USE 50Xdia FOR TENSION
CONCLUSION: Before concluding I will show some more details drawn in autocad exported in wwf format
1. 2. 3. 4. 5. 6. 7.
As there is no time to elaborately explaining ,the following topics are not covered : Flat slabs, Folded plates, shell structures-cylindrical shells, silos, 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. Deep beams. 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. 2.
3. 4. 5. 6. 7.
HANDBOOK ON CONCRETE REINFORCEMENT AND DETAILING-SP:34(S&T)-1987. MANUAL OF ENGINEERING & PLACING DRAWINGS FOR REINFORCED CONCRETE STRUCTURES(ACI 315-80 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.
8. 9. 10. 11. 12.
IS:2502-1963 CODE OF PRACTICE FOR BENDING AND FIXING OF BARS FOR CONCRETE REINFORCEMENT. IS:1893:2000. IS:4326. IS:456:2000 REINFORCED HAND BOOK BY REYNOLD.
THANK YOU.
