Design of mat and combined footing

mat/raft footing, combined footing, details of steel in construction, footing, foundation, building construction, lightning and construction, ppt on design of construction, shallow foundation, deep foundation, strap footing, pair foundation, pile foundation, well foundation, squire foundation, depth of foundation, advantage of foundation

1. Design of mat and
combined footing
Er. Md Faiz Ali
Lightning & Construction
[email protected]

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

3. 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

4. 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

5. 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.

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

7. 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.

8. 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

9. 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

10. 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

11. Detailing of steel in rectangular Detailing 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= L
dt
=
L
dt
= 47φ
s
for M20 concrete and Fe 415 steel
=
-
-
directionshorttotalst
bandcentralst
A
A
,,
,
1)/(
2
+BL
bd
4
.φf
τ
ss

12. 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

13. Isolated footing Isolated 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

14. Solution:Solution:
Development length in tension = 47φ = 47 x 16= 752 mm
Depth at junction = 500 mm
Leg length available for column bars = L
dt
– 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.

15. 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

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

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

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

19. ≤ 75 > 75
≤ 48φ
tr
Column steel detailingColumn steel detailing

20. Column steel detailingColumn steel detailing
> 75
> 75
> 75> 75
≤ 48φ
tr
> 48φ
tr
> 48φ
tr

21. Column steel detailingColumn steel detailing
≤ 48φ
tr
φ
tr
≤ 75
> 48φtr

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

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

24. Bar bending schedule
For exercise problem
For footing only
MemberMark
No. of
bars
Diameter
in mm
Length
in
m/piece
Total
length in
m
Bar detailsRemarks
#20
Footing
A 11 20 2.5528.05 #20@250
B11 20 2.5528.05 #20@250
Total length in m/diameter56.10
Weight in kg/m 2.45
Total weight in kg/diameter137.45

25. 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

26. THANK YO
U