Design of Mat Foundations

Hemant Kumar BhaskarRoll No:-0610421

Types of FoundationsShallow Foundations versus Deep Foundations**CE-533 Advanced Foundation Engineering

CE-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.**CE-533 Advanced Foundation Engineering


*CE-533 Advanced Foundation Engineering*

Mat Foundation often considered to be used when dealing with the following conditions:

*CE-533 Advanced Foundation Engineering*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 foundation**CE-533 Advanced Foundation Engineering


Types of Mat Foundations**CE-533 Advanced Foundation Engineering


To Design Mat Foundation:Determine the capacity of the foundationDetermine the settlement of foundationDetermine the differential settlement Determine the stress distribution beneath the foundationDesign the structural component of the mat foundation using the stress distribution obtain from 4.

**CE-533 Advanced Foundation Engineering


Bearing capacity of the FoundationBearing 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)

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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)*

Foundation type Expected maximum settlement, mm Expected differential settlement, mm

Spread2520Mat5020

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

*Rigidity Factor,

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

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Structural Design of Mat FoundationsApproximate MethodFlexible MethodFinite Difference MethodFinite Element Method**CE-533 Advanced Foundation Engineering


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.*CE-533 Advanced Foundation Engineering*


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 METHODThe finite-difference method uses the fourth-order differential equation based on the theory of plates and shells [Timoshenko and Woinowsky-Krieger (1959)]:**CE-533 Advanced Foundation Engineering


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.*CE-533 Advanced Foundation Engineering*


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.*CE-533 Advanced Foundation Engineering*


FINITE ELEMENT METHOD:**CE-533 Advanced Foundation EngineeringIn the finite-element analysis, element continuity is maintained through use of displacement functions. The displacement function is of the form


REINFORCEMENT DETAILSOF MAT FOUNDATION**CE-533 Advanced Foundation Engineering

MAT FOUNDATIONWITH REINFORCED BARS**CE-533 Advanced Foundation Engineering

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



Determination Of Critical Column SpacingEvaluation of the characteristics is made as follows:

Where,k = modulus of subgrade reaction in KN/m3B = 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.



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. 304324.

S.N. Shukla, A simplified method for design of mats on elastic foundations, ACI J (1984), pp. 469475. View Record in Scopus | Cited By in Scopus (2)

O.C. Zienkiewicz, The finite element method (3rd ed.), McGraw-Hill, UK (1977).

G. Bzine, A new boundary element method for bending of plate on elastic foundations, Int J Solids Struct 24 (1988), pp. 557565. Abstract | PDF (589 K) | View Record in Scopus | Cited By in Scopus (0)

REFERENCES:**CE-533 Advanced Foundation Engineering




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Design of Mat Foundations

Documents

foundation system

larger foundation

foundation area

foundation type

mat rigidity

maximum settlement

large column loads

large concrete slab