REINFORCING DETAILING

OF R.C.C MEMBERS

S.Aravindan

Dexter Engg Colsultant

ksaravind@gmail.com

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 1compared to men of other professions11is that his works are out in the open where all can see them.

His acts 1..step by step 1are in hard substances.

He cannot bury his mistakes in the grave like the DOCTORS.

He cannot argue them into thin air1..or blame the judge1..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 opponents1.and hope the people will forget. The ENGINEER simply cannot deny he did it.

If his works do not work11he 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

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

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 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.

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 .

Ld/2 Ld/2

Ld

crack

INCORRECT

CORRECT

CLOSE

STIRRUPS

Ldt

Ldt/2

CANTILEVER BEAM

NON PRISMATIC BEAM

Ld/2 Ld/2 Ld

INCORRECT

CORRECT

CLOSE

STIRRUPS

crack

Ldt/2

Ldt

GRID BEAM

GRID BEAM

INCORRECT

1.5d 1.5d 1.5d

Close rings

Hanger bars

Slope 1:10

2#extra bars

300 300

d

CORRECT

Details of Main & Secondary beams

INCORRECT

1.5d 1.5d 1.5d

Close rings

Hanger bars

300 300

d

CORRECT

Secondary beam

Main beam

Secondary beam

60degree

Main beam

d

1/4OR

1/5 SPAN

1.5d

CORRECT

INCORRECT

d/2+d/2Cot(t)

t t

Ld

d/2

d/2

Ld

LINE OF CRACK

BEAM

Continuous beam

• continuous beam Span/4

Span/4 Span/4

Span/4

incorrect

Span/4

Span/4

1.5d 1.5d

Span/4

correct

1.5d

100%

L1

100% CRACK CRACK

.08L2 .08L1 L2

SPAN/4 SPAN/4

CONTINUOUS BEAM

INCORRECT

.08L1

.08L1

SPAN/4

CRACK

100%

L1

100%

.15L2 L2

L1/4 L2/4

CORRECT

0.1L1

.15L1

L1/4 100%

20% 20%

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

SLNO BAR

DIA.

TENSION

mm

COMPRESSION REMARKS

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

FOR A CONCRETE GRADE M20 &STEEL STRENGTH Fy=415

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 315-80

3. 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:456:2000

12. REINFORCED HAND BOOK BY REYNOLD.

THANK YOU.