Hemant Kumar Bhaskar
Roll No:-0610421
Types of Foundations
Shallow Foundations versus Deep Foundations
Foundations
Shallow
Foundations
Spread
Footings
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Mat
Foundations
Deep
Foundations
Driven
Piles
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Drilled
Shafts
Auger Cast
Piles
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Mat/Raft Foundation?
A foundation system in which essentially the entire
building is placed on a large continuous footing.
Usually large concrete slab supporting many columns.
Commonly used as foundation for silos, chimneys, large
machinery.
It is a flat concrete slab, heavily reinforced with steel,
which carries the downward loads of the individual
columns or walls.
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Mat Foundation often considered to be used
when dealing with the following conditions:
The spread footings cover over 50% of the foundation
area because of large column loads.
The soil is soft with a low bearing capacity.
Hydrostatic uplift resistance is needed etc.
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Mat/raft foundation
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Types of Mat Foundations
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To Design Mat Foundation:
Determine the capacity of the foundation
Determine the settlement of foundation
Determine the differential settlement
Determine the stress distribution beneath the
foundation
5. Design the structural component of the mat
foundation using the stress distribution obtain from 4.
1.
2.
3.
4.
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Bearing capacity of the Foundation
 Bearing Capacity Analysis follows the same approach as
for spread footings
qult  cN c sc d c   zD N q sq d q  0.5 BN  s d 
 Factor of Safety (Das, 2004):
Under normal Dead loads = 3.0(Min)
Under extreme loads = 1.75-2.0(Min)
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2). Settlement of foundation
The settlement tends to be controlled via the
following:
Use of a larger foundation to produce lower soil
contact pressures.
Displaced volume of soil (flotation effect);
theoretically if the weight of excavation equals the
combined weight of the structure and mat, the
system "floats" in the soil mass and no settlement
occurs.
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Bridging effects attributable to
 a. Mat rigidity.
 b. Contribution of superstructure rigidity to the mat.
Foundation type
Expected
maximum
settlement, mm
Expected
differential
settlement, mm
Spread
25
20
Mat
50
20
By IS Code – 2950 (Part-1)
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Design of Mat Foundations

Differential Settlement of Mat Foundations (American
Concrete Institute Committee 336, 1988)
Modulus of Elasticity of Material used in
Structure
Rigidity Factor,
E I b
Kr 
Es B 3
Modulus of Elasticity of Soil
Moment of inertia of
structure per unit length
at right angles to B
Width of raft
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Design of Mat Foundations
 Differential Settlement of Mat Foundations (American
Concrete Institute Committee 336, 1988)

ah3 

EI b  E I F   I b  
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

E I b  flexural rigidity of the Superstructure and Mat
 EI   flexural rigidity of the framed members at right angles to B
  Eah /12   flexural rigidity of shear walls
b
3
a  shear wall thickness
h  shear wall height
E I F  flexural rigidity of the mat foundation
If K r  0.5, then mat can be treated as rigid i.e. ( d /  )  0
If K r  0.5, then ( d /  )  0.1
If K r  0, then ( d /  )  0.35(square mats) and ( d /  )  0.5(long mats)
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Structural Design of Mat Foundations
 Approximate Method
 Flexible Method
 Finite Difference Method
 Finite Element Method
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AN APPROXIMATE METHOD:
 The mat is divided into strips loaded by a line of columns and resisted
by soil pressure.
 This strip is then analyzed as a combined footing. (This method can be
used where the mat is very rigid and the column pattern is fairly
uniform in both spacing and loads.)
 This method is not recommended at present because of the substantial
amount of approximations and the wide availability of computer
programs that are relatively easy to use.
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FLEXIBLE METHOD:
1. Compute the plate rigidity D
2. Compute the radius of effective stiffness L (Note: the
approximate zone of any column influence is ~ 4L).
3. Compute the radial and tangential moments, the shear,
and deflection.
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FINITE DIFFERENCE METHOD
The finite-difference method uses the fourth-order differential equation based on the theory of
plates and shells [Timoshenko and Woinowsky-Krieger (1959)]:
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The finite-difference method has several advantages:
 It has been widely used (and should be used as a check on alternative
methods where it is practical).
 It is reliable if the mat can be modelled using a finite-difference grid.
 It is rapid since the input data are minimal compared with any other
discrete method, and the computations to build the stiffness array are
not so extensive as other methods. Usually only three to five lines of
input data are needed compared with up to several hundred for the
other methods.
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There are also a number of disadvantages:
 It is extremely difficult to model boundary conditions of
column fixity.
It is very difficult to model notches, holes, or re-entrant
corners.
It is difficult to apply a concentrated moment (as from a
column) since the difference model uses moment/unit of
width.
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FINITE ELEMENT METHOD:
In the finite-element analysis, element continuity is maintained through
use of displacement functions. The displacement function is of the form
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DESIGN OF MAT FOUNDATION USING SOFTWARES:
 Many types of software are available to design mat foundation some are like
ABAQUS V6.8, STAAD FOUNDATION,RISA FOUNDATION, ANSYS, etc.
MODELLING OF MAT
FOUNDATION
Using
STAAD Foundation 2006
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Some Highlight points of IS-2950 Part-1 (DESIGN AND CONSTRUCTION OF
RAFT FOUNDATIONS )
For satisfactory design and construction of a raft
foundation, the following information is necessary:
Site Plan
Loading Conditions
Environmental Factors
Geotechnical Information
Limiting Value of Angular distortion and differential settlement
Rigidity of foundation and Super structure
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METHODS OF ANALYSIS
 Rigidity of Superstructure And Foundation
Modulus of Elasticity of Material used in
Structure
E I b
Kr 
Es B 3
Modulus of Elasticity of Soil
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Moment of inertia of
structure per unit length
at right angles to B
Width of raft
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Determination Of Critical Column Spacing
Evaluation of the characteristics γ is made as follows:
 4
kB
4 EcI
Where,
k = modulus of subgrade reaction in KN/m3
B = width of raft in cm
Ec = modulus of elasticity of concrete in MPa
Z = moment of inertia of the raft in m4
Depth of Foundation :
 The depth of foundation shall generally be not less than 1 m.
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Construction Practices Applicable to the Design of Mats.
 Thickness T is determined from two-way shear (punching
shear);
 Typical mat thickness T:
 Stories
B=45'
B=90'
<5
24"
31"
5 - 10
35"
47"
10 - 20
59"
78"
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B=120'
39"
59”
98"
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Conclusions:
 The primary objective of this report is to discuss various methods of
design of mat foundation with structural reinforcement view has been
done starting from simple analysis to methods in research with rigorous
analysis.
 The use of software's to design mat foundation had been introduced
to track the current era of computers.
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REFERENCES:
Bowles,J.E. (2000) Foundation Analysis and Design.
A.C.I.Committee 336, Suggested analysis and design procedures for combined footings
and mats, ACI Struct J (1988), pp. 304–324.
 S.N. Shukla, A simplified method for design of mats on elastic foundations, ACI J (1984),
pp. 469–475. View Record in Scopus | Cited By in Scopus (2)
 O.C. Zienkiewicz, The finite element method (3rd ed.), McGraw-Hill, UK (1977).
G. Bézine, A new boundary element method for bending of plate on elastic foundations,
Int J Solids Struct 24 (1988), pp. 557–565. Abstract | PDF (589 K) | View Record in Scopus
| Cited By in Scopus (0)
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