Summary SheetSession Number :

Date :

Subject Expert :

2

13.03.2007

Dr. M.C. NatarajaProfessor

Department of Civil Engineering,

S.J. College of Engineering,

Mysore 570 006.

Phone:0821-2343521, 9880447742

E-mail: nataraja96@yahoo.com

Learning Outcomes:

After this students will be able to do the detailing ofdifferent types of isolated footings through drawingand bar bending schedule.

Detailing of Steel in Footings

Footings

The function of a footing or a foundation is to transmit theload form the structure to the underlying soil.

The choice of suitable type of footing depends on the depth atwhich the bearing strata lies, the soil condition and the type ofsuperstructure.

Detailing of Steel in Footings

http://www.tekniksipil.org/civil-engineering/

Types of footing

Following are the different types of footing used for concrete structure

Isolated footing Combined footings Strap footing Mat or raft foundation Pile foundation

Isolated footings

Isolated footings are provided under each column and may

be square, rectangular, or circular in plan. Footing may be

flat or tapered.

Combined footings

COMBINEDFOOTING

COMBINED FOOTING

PROPERTY LINE

RECTANGULAR AND TRAPEZOIDAL TYPES

Combined footings

Combined footings are provided to support two or morecolumn loads. These may be continuous with rectangularor trapezoidal in plan as shown. Combined footingsbecome necessary under the following circumstances:

when the isolated footings overlap.

when the exterior column is close to the property line withthe result symmetrical isolated footing can not beprovided.

Strap footing Strap footing is one of the types of combined types of

combined. It consists of an isolated footing of twocolumns connected by a beam called strap beam. Thestrap beam does not remain in contact with the soil andthus does not transfer any load to the soil. This isprovided when one of the columns is on the propertyline.

STRAP FOOTING

STRAP BEAM

PROPERTY LINE

Mat foundation

The is provided when the soil is havingvery low bearing capacity and or whencolumns loads are heavy, the requiredfooting area becomes very large anduneconomical.

MAT FOUNDATION

Detailing Size of footing

Depth at footing

Depth of footing edge

Nominal and effective cover

Development length

Minimum and maximum steel

Spacing of bars and stirrupsPLAN

BM1OWS

TWS

d/2

d

BM2

Detailing of steel in rectangularfooting as per IS:456-2000ReinforcementLong and short directionIn short direction larger steel area is needed in the central portion and is given by

Reminder of the steel in end bands

Development length= Ldt=

Ldt = 47s for M20 concrete and Fe 415 steel

=

d i r e c t is h o r tt o t a ls t

b a n dc e n t r a ls t

AA

,,

,

1)/(2+BL

bd4.f

ss

PROBLEMAn isolated footing is to be provided for a column of section400 mm x 400 mm. The following details re given:

Height of the column =3mMain reinforcement in column = 4 Nos. 16 mm diameter.Transverse reinforcement = 6 mm at 220 mm c/cPlan size of footing = 2.7 m x 2.7 mDepth of footing at column face = 500 mmDepth of footing at edge = 150 mmDepth of foundation = 1000 mm

Isolated footing Contd..Footing reinforcement = a mesh of 20 mm diameter steel at

250 mm c/c.

Grade of concrete= M20

Grade of steel = Fe415

Draw to a suitable scale the following:

Plan

Sectional Elevation

Prepare bar bending schedule 20 Marks

Solution:Development length in tension = 47 = 47 x 16= 752 mm

Depth at junction = 500 mm

Leg length available for column bars = Ldt 500 = 252 mm < 300 mm.

Provide 300 mm minimum.

Development length of 20 mm bars in footing = 47 x 20 = 940 mm

This is provided by the horizontal projection of bars. Bars need not be bent at ends in to the depth of footing.

Number of bars in footing = 11 Numbers at 200 mm c/c with a

side cover of 100 mm.

Width of footing= 10 x 250 + 100 x 2 =2700 mm

Follow all specification of SP 34 and prepare the drawing

Detailing with dowel bars

Pu/bD < 0.45fck A1/A2

2:1

A1=bxD

A2= [b+4Df] x [D+4Df}

Df

2Df

Bar bending scheduleFor exercise problem

For footing only

Member Mark No. of barsDiameter

in mm

Lengthin

m/piece

Total length in

mBar details Remarks

#20

FootingA 11 20 2.55 28.05 #20@250B 11 20 2.55 28.05 #20@250

Total length in m/diameter 56.10Weight in kg/m 2.45

Total weight in kg/diameter 137.45

Estimation of quantity of concrete

Volume of concrete = L x B x D

L= length in mB=Breadth in mD=Thickness in m Apply for rectangular and trapezoidal part

Weight of concrete, kg = Volume x density

Slide Number 1Slide Number 2Slide Number 3Slide Number 4Isolated footingsCombined footingsCombined footingsStrap footingMat foundationDetailingDetailing of steel in rectangular footing as per IS:456-2000PROBLEMIsolated footing Contd..Slide Number 14Slide Number 15Slide Number 16Slide Number 17Slide Number 18Slide Number 19Slide Number 20Slide Number 21Detailing with dowel barsSlide Number 23Slide Number 24Estimation of quantity of concrete