Design of mat and combined footing

Design of mat and
combined footing
Er. Md Faiz Ali
Lightning & Construction
mdfaizali123@gmail.com

Learning Outcomes:
• After this students will be able to do the detailing ofAfter this students will be able to do the detailing of
different types of isolated footings through drawingdifferent types of isolated footings through drawing
and bar bending schedule.and bar bending schedule.
Detailing of Steel in FootingsDetailing of Steel in Footings

FootingsFootings
The function of a footing or a foundation is to transmit the
load form the structure to the underlying soil.
The choice of suitable type of footing depends on the depth at
which the bearing strata lies, the soil condition and the type of
superstructure.
Detailing of Steel in FootingsDetailing of Steel in Footings

Types of footingTypes 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 footingsIsolated footings
Isolated footings are provided under each column and may
be square, rectangular, or circular in plan. Footing may be
flat or tapered.

Combined footingsCombined footings
COMBINED
FOOTING
COMBINED
FOOTING
PROPERTY LINE
RECTANGULAR AND TRAPEZOIDAL TYPES

Combined footingsCombined footings
• Combined footings are provided to support two or more
column loads. These may be continuous with rectangular
or trapezoidal in plan as shown. Combined footings
become necessary under the following circumstances:
• when the isolated footings overlap.
• when the exterior column is close to the property line with
the result symmetrical isolated footing can not be
provided.

Strap footingStrap footing
• Strap footing is one of the types of combined types of
combined. It consists of an isolated footing of two
columns connected by a beam called strap beam. The
strap beam does not remain in contact with the soil and
thus does not transfer any load to the soil. This is
provided when one of the columns is on the property
line.
STRAP FOOTING
STRAP
BEAM
PROPERTY LINE

Mat foundationMat foundation
• The is provided when the soil is having
very low bearing capacity and or when
columns loads are heavy, the required
footing area becomes very large and
uneconomical.
MAT FOUNDATION

DetailingDetailing
• Size of footing
• Depth at footing
• Depth of footing edge
• Nominal and effective cover
• Development length
• Minimum and maximum steel
• Spacing of bars and stirrups
PLAN
BM1
OWS
TWS
d/2
d
BM2

Detailing of steel in rectangularDetailing of steel in rectangular
footing as per IS:456-2000footing as per IS:456-2000
Reinforcement
Long and short direction
In 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 = 47φs for M20 concrete and Fe 415 steel
=
−
−
directionshorttotalst
bandcentralst
A
A
,,
,
1)/(
2
+BL
bd
4
.φf
τ
ss

PROBLEMPROBLEM
An isolated footing is to be provided for a column of section
400 mm x 400 mm. The following details re given:
Height of the column =3m
Main reinforcement in column = 4 Nos. 16 mm diameter.
Transverse reinforcement = 6 mm at 220 mm c/c
Plan size of footing = 2.7 m x 2.7 m
Depth of footing at column face = 500 mm
Depth of footing at edge = 150 mm
Depth of foundation = 1000 mm

Isolated footingIsolated footing Contd..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:Solution:
Development length in tension = 47φ = 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

Square footingSquare footing
GROUND LEVEL
CROSS SECTION
SECTIONAL ELEVATION
LEVELLING COURSE
DEPTH OF
FOUNDATION
≥ 500 mm
Ldt L dc
400
Ldt
300 min.
150 min.
75
75
4- #16
#6@220
#20@250
4- #16
3000 mm
PL
AN
27
00
27
00
400

GROUND LEVEL
CROSS SECTION
SECTIONAL ELEVATION
LEVELLING COURSE
DEPTH OF
FOUNDATION
≥ 500 mm
Ldt L dc
Ldt
300 min.75
75
6- #20
#8@200
#16@200
300
75
PLAN
PLAN
400
Rectangular footingRectangular footing

Rectangular footingRectangular footing
GROUND LEVEL
CROSS SECTION
SECTIONAL ELEVATION
LEVELLING COURSE
DEPTH OF
FOUNDATION
≥ 500 mm
Ldt L dc
Ldt
300 min.75
75
6- #20
#8@200
#16@200
300
75
PLAN
320
0
220
0
CB=
2200
EB EB

≤ 75 > 75
≤ 48φtr
Column steel detailingColumn steel detailing

> 75
> 75
> 75> 75
≤ 48φtr > 48φtr
> 48φtr

≤ 48φtr
φtr
≤ 75
> 48φtr

Detailing with dowel bars
Pu/bD < 0.45fck √A1/A2
2:1
A1=bxD
A2= [b+4Df] x [D+4Df}
Df
2Df

SECTIONAL ELEVATION
LEVELLING COURSE
DEPTH OF
FOUNDATION
≥ 500 mm
Ldt L dc
Ldt
300 min.75
75
#16@200
75
Detailing with
dowel bars
LAP
Dowel bars
or starter
bars

Bar bending schedule
For exercise problem
For footing only
Member Mark
No. of
bars
Diameter
in mm
Length
in
m/piece
Total
length in
m
Bar details Remarks
#20
Footing
A 11 20 2.55 28.05 #20@250
B 11 20 2.55 28.05 #20@250
Total length in m/diameter 56.10
Weight 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 m
B=Breadth in m
D=Thickness in m
Apply for rectangular and trapezoidal part
Weight of concrete, kg = Volume x density

Design of mat and combined footing

Recommended

Recommended

More Related Content

What's hot

What's hot (20)

Similar to Design of mat and combined footing

Similar to Design of mat and combined footing (20)

Recently uploaded

Recently uploaded (20)

Design of mat and combined footing