DESIGN OF RAFT FOUNDATIONSAS OF (AMERICAN CONCRETE INSTITUTE COMMITTEE 336, 1988)

Prepared by : Eng.Hatim Mohammed Ahmed

TYPES OF FOUNDATIONS

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Shallow Strip Edge beams for lightly loaded buildingsPad To support internal columns

of buildingsRaft To keep movements to a

tolerable amountDeep Driven piles Significant depth to

competent layerBored piles Large capacity required

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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Raft foundation is generally suggested in the following situations:

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(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.

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(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.

GENERAL NOTES

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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.

Raft foundationRA F T F O U N D AT I O N S 8

TYPES OF RAFT FOUNDATIONS

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TO DESIGN RAFT FOUNDATIONS:1. Determine the capacity of the foundation2. Determine the settlement of foundation3. Determine the differential settlement 4. Determine the stress distribution beneath the

foundation5. Design the structural component of the mat

foundation using the stress distribution obtain from 4.

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DESIGN APPROACH ANDCONSIDERATIONS

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Basically two approaches have been suggested for analyzing the behavior of raft foundation:

A. Rigid foundation approach

B. Flexible foundation approach

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DESIGN OF COMBINED FOOTINGS AND MATS ACI COMMITTEE 336The 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

THE COMMITTEE SUGGESTS PROCEDURE TO BE FOLLOWED:

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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.

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

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SETTLEMENT OF FOUNDATIONThe 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

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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 approachIn rigid rafts, differential settlements are comparatively low but bending

moment and shear forces to which raft is subjected are considerably high.

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Bridging effects attributable toa . Raft rigidity.b. Contribution of superstructure rigidity to the

raft.

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Foundation type

Expected maximum settlement, mm

Expected differential settlement, mm

Spread 25 20Mat 50 20

RIGIDITY OF SUPERSTRUCTURE AND FOUNDATION (CONT ):As of (American Concrete Institute Committee 336,

1988)

12

3ahIIEIE 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

ah

heightflexural rigidity of the mat foundationFE I

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d

d

d d

If 0.5, then mat can be treated as rigid i.e. ( / ) 0If 0.5, then ( / ) 0.1If 0, then ( / ) 0.35(square mats) and ( / ) 0.5(long mats)

r

r

r

KKK

RIGIDITY OF SUPERSTRUCTURE AND FOUNDATION As of (American Concrete Institute Committee

336, 1988):

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3BEIEKs

br

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,

Determination Of Critical Column SpacingEvaluation 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.

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44kBEcI

FLEXIBLE APPROACHIn 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

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 FLEXIBLE METHOD:

1. Compute the plate rigidity D2. 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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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"

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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:

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التوفيق بالله و

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