Raft foundations are used when buildings have heavy loads, compressible soil, or require minimal differential settlement. A raft foundation is a continuous concrete slab that supports all building columns. It can be designed using either a rigid or flexible approach. The rigid approach assumes the raft bridges soil variations, while the flexible approach models soil-structure interaction. Key considerations for raft design include bearing capacity, settlement, stress distribution, and structural component sizing.

1. DESIGN OF RAFT FOUNDATIONS
AS OF (AMERICAN CONCRETE INSTITUTE COMMITTEE 336, 1988)
Prepared by : Eng.Hatim Mohammed Ahmed

2. TYPES OF FOUNDATIONS
R A F T F O U N D A T I O N S 2
Shallow Strip Edge beams for lightly loaded buildings
Pad To support internal columns of
buildings
Raft To keep movements to a tolerable
amount
Deep Driven piles Significant depth to competent
layer
Bored piles Large capacity required

3. 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.
R A F T F O U N D A T I O N S 3

4. R A F T F O U N D A T I O N S 4

5. Raft foundation is generally suggested in the
following situations:
R A F T F O U N D A T I O N S 5
(a) Whenever building loads are so heavy or the allowable
pressure on soil so small that individual footings would cover
more than floor area.
(b) Whenever soil contains compressible lenses or the soil is
sufficiently erratic and it is difficult to define and assess the
extent of each of the weak pockets or cavities and, thus, estimate
the overall and differential settlement.
(c) When structures and equipment to be supported are very
sensitive to differential settlement.

6. R A F T F O U N D A T I O N S 6
(d) Where structures naturally lend themselves for the use of raft
foundation such as silos, chimneys, water towers, etc.
(e) Floating foundation cases wherein soil is having very poor
bearing capacity and the weight of the super-structure is
proposed to be balanced by the weight of the soil removed.
(f) Buildings where basements are to be provided or pits located
below ground water table.
(g) Buildings where individual foundation, if provided, will be
subjected to large widely varying bending moments which may
result in differential rotation and differential settlement of
individual footings causing distress in the building.

7. GENERAL NOTES
R A F T F O U N D A T I O N S 7
 Distribution of soil pressure by means of sub-grade reaction has been
suggested where sub-soils are of such character that the deformations are
localized in the general vicinity of the loads and when the maximum contact
pressure is smaller than about one and a half times the ultimate bearing
capacity.
 In case of rigid footings, it is suggested that uniform or linear distribution of
soil pressure can be assumed and the design based on statics.
 Flexible footing procedure is divided into 2 parts i.e. uniform condition and
general condition.
Uniform conditions are considered to be those where the variation in adjacent
column loads and spans is not greater than 20%. For cases where supporting
columns are at random location with varying intensities of loads detailed design
procedure based on plate theories has been recommended.

8. Raft foundation
R A F T F O U N D A T I O N S 8

9. TYPES OF RAFT FOUNDATIONS
R A F T F O U N D A T I O N S 9

10. TO DESIGN RAFT FOUNDATIONS:
1. Determine the capacity of the foundation
2. Determine the settlement of foundation
3. Determine the differential settlement
4. Determine the stress distribution beneath the
foundation
5. Design the structural component of the mat
foundation using the stress distribution obtain
from 4.
R A F T F O U N D A T I O N S 10

11. DESIGN APPROACH AND
CONSIDERATIONS
R A F T F O U N D A T I O N S 11
Basically two approaches have been suggested for analyzing the
behavior of raft foundation:
A. Rigid foundation approach
B. Flexible foundation approach

12. R A F T F O U N D A T I O N S 12
DESIGN OF COMBINED FOOTINGS AND MATS ACI COMMITTEE 336
The following factors should be considered while examining any
problem:
(1) Soil type immediately below the footing
(2) Soil type at the greater depth
(3) Size of footing
(4) Shape of footing
(5) Eccentricity of loading
(6) Rigidity of footing
(7) Rigidity of the super-structure
(8) Modulus of sub-grade reaction

13. THE COMMITTEE SUGGESTS PROCEDURE TO BE FOLLOWED:
R A F T F O U N D A T I O N S 13
 Linear soil pressure distribution is suggested for footings which can
be considered rigid to the extent that only very small relative
deformations result from the loading.
 The rigidity may result from the spacing of the columns on the
footing ,from the rigidity of the footing itself or the rigidity of the
super-structure.
 Limitations which must be fulfilled to make this assumption valid
have been discussed in the report.
 Distribution of soil pressure by means of sub-grade reaction has
been suggested.

14. BEARING CAPACITY OF THE FOUNDATION
 Bearing Capacity Analysis follows the same
approach as for spread footings
 Factor of Safety :
Under normal Dead loads = 3.0(Min)
Under extreme loads = 1.75-2.0(Min)
 dsBNdsNdsNcq qqqzDcccult
 5.0
14

15. 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.
 In general, the pressure causing settlement in a mat
analysis may be computed as:
Net pressure = {[Total (including mat) structure weight] -
Weight of excavated soil}/Mat area
15

16. RIGID APPROACH:
In rigid foundation approach, it is presumed that raft is rigid enough to
bridge over non-uniformities of soil structure. Pressure distribution is
considered to be either uniform or varying linearly.
Design of rigid raft follows conventional methods where again following two
approaches have been suggested:
(a) Inverted floor system
(b) Combined footing approach
In rigid rafts, differential settlements are comparatively low but bending
moment and shear forces to which raft is subjected are considerably
high.
R A F T F O U N D A T I O N S 16

17. Bridging effects attributable to
a . Raft rigidity.
b. Contribution of superstructure rigidity to the raft.
17
Foundation type Expected
maximum
settlement, mm
Expected
differential
settlement, mm
Spread 25 20
Mat 50 20

18. RIGIDITY OF SUPERSTRUCTURE AND
FOUNDATION (CONT ):
As of (American Concrete Institute Committee 336, 1988)






  
12
3
ah
IIEIE bFb
 3
flexural rigidity of the Superstructure and Mat
flexural rigidity of the framed members at right angles to B
/12 flexural rigidity of shear walls
shear wall thickness
shear wall
b
b
E I
E I
E ah
a
h
 
  
 




height
flexural rigidity of the mat foundationFE I 
18
d
d
d d
If 0.5, then mat can be treated as rigid i.e. ( / ) 0
If 0.5, then ( / ) 0.1
If 0, then ( / ) 0.35(square mats) and ( / ) 0.5(long mats)
r
r
r
K
K
K
 
 
   
 
 
  

19. RIGIDITY OF SUPERSTRUCTURE AND
FOUNDATION
As of (American Concrete Institute Committee 336, 1988):
19
3
BE
IE
K
s
b
r


Width of raft
Moment of inertia of
structure per unit length
at right angles to B
Modulus of Elasticity of Soil
Modulus of Elasticity of Material used in
Structure
Rigidity Factor,

20. Determination Of Critical Column Spacing
Evaluation of the characteristics γ is made as follows:
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.
R A F T F O U N D A T I O N S 20
4
4
kB
EcI
 

21. FLEXIBLE APPROACH
In flexible foundation approach, raft is considered to distribute load in the
area immediately surrounding the column depending upon the soil
characteristics. In this approach differential settlements are
comparatively larger but bending moments and shear forces to which
the raft is subjected are comparatively low.
Analysis is suggested basically on two theories
(a) Flexible plate supported on elastic foundation, i.e., Hetenyi's Theory
(b) Foundation supported on bed of uniformly distributed elastic springs
with a spring constant determined using coefficient of sub-grade
reaction. Each spring is presumed to behave independently, i.e.,
Winklers's foundation
R A F T F O U N D A T I O N S 21

22. 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.
R A F T F O U N D A T I O N S 22

23. R A F T F O U N D A T I O N S 23

24. 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' B=120'
< 5 24" 31" 39"
5 - 10 35" 47" 59”
10 - 20 59" 78" 98"
R A F T F O U N D A T I O N S 24

25. Burt G. Look , Handbook of Geotechnical Investigation and Design Tables, 2007 Taylor &
Francis Group, London, UK
ACI 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)
 SHARAT CHANDRA CUPTA ,Raft Foundations Design and Analysis with a Practical
Approach, 1997 New Age International (P) Limited, Publishers.
REFERENCES:
R A F T F O U N D A T I O N S 25

26. R A F T F O U N D A T I O N S 26