hemant kumar bhaskar roll no:-0610421. types of foundations shallow foundations versus deep...

Hemant Kumar BhaskarRoll No:-0610421

Types of FoundationsShallow Foundations versus Deep Foundations

Foundations

Shallow Foundations

Deep Foundations

Spread Footings

Mat Foundations

Driven Piles

Drilled Shafts

Auger Cast Piles

04/18/23 2CE-533 Advanced Foundation Engineering

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.


04/18/23 CE-533 Advanced Foundation Engineering 4

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.

Mat/raft foundation04/18/23 6CE-533 Advanced Foundation Engineering

Types of Mat Foundations


To Design Mat Foundation: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.


Bearing capacity of the Foundation Bearing Capacity Analysis follows the same

approach as for spread footings

Factor of Safety (Das, 2004): Under normal Dead loads = 3.0(Min)

Under extreme loads = 1.75-2.0(Min)

dsBNdsNdsNcq qqqzDcccult 5.0

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2). 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.

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

By IS Code – 2950 (Part-1)

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Foundation type

Expected maximum settlement, mm

Expected differential settlement, mm

Spread 25 20

Mat 50 20

Design of Mat Foundations

Differential Settlement of Mat Foundations (American Concrete Institute Committee 336, 1988)

3BE

IEK

s

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

12

Rigidity Factor,

Design of Mat Foundations

Differential Settlement of Mat Foundations (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

a

h

height

flexural rigidity of the mat foundationFE I

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

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Structural Design of Mat FoundationsApproximate MethodFlexible MethodFinite Difference MethodFinite Element Method


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.


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.


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)]:


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.


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.


FINITE ELEMENT METHOD:


In the finite-element analysis, element continuity is maintained through use of displacement functions. The displacement function is of the form

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

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 PlanLoading ConditionsEnvironmental FactorsGeotechnical InformationLimiting Value of Angular distortion and differential settlementRigidity of foundation and Super structure


METHODS OF ANALYSIS Rigidity of Superstructure And Foundation


3BE

IEK

s

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

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.


44

kB

EcI

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"


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.


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)

REFERENCES:


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