# Design of Mat Foundations

## Presentation on theme: "Design of Mat Foundations"— Presentation transcript:

Design of Mat Foundations

Types of Foundations Shallow Foundations versus Deep Foundations
Spread Footings Mat Foundations Driven Piles Drilled Shafts Auger Cast Piles 4/13/2017 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. 4/13/2017 CE-533 Advanced Foundation Engineering

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. 4/13/2017 CE-533 Advanced Foundation Engineering

Mat/raft foundation 4/13/2017 CE-533 Advanced Foundation Engineering

Types of Mat Foundations

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 Design the structural component of the mat foundation using the stress distribution obtain from 4. 4/13/2017 CE-533 Advanced Foundation Engineering

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 = (Min)

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.

Bridging effects attributable to
a. 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 Spread 25 20 Mat 50

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

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

Structural Design of Mat Foundations
Approximate Method Flexible Method Finite Difference Method Finite Element Method 4/13/2017 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. 4/13/2017 CE-533 Advanced Foundation Engineering

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. 4/13/2017 CE-533 Advanced Foundation Engineering

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)]: 4/13/2017 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. 4/13/2017 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. 4/13/2017 CE-533 Advanced Foundation Engineering

FINITE ELEMENT METHOD:
In the finite-element analysis, element continuity is maintained through use of displacement functions. The displacement function is of the form 4/13/2017 CE-533 Advanced Foundation Engineering

REINFORCEMENT DETAILS
OF MAT FOUNDATION 4/13/2017 CE-533 Advanced Foundation Engineering

MAT FOUNDATION WITH REINFORCED BARS 4/13/2017

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 4/13/2017 CE-533 Advanced Foundation Engineering

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 4/13/2017 CE-533 Advanced Foundation Engineering

METHODS OF ANALYSIS Rigidity of Superstructure And Foundation
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 4/13/2017 CE-533 Advanced Foundation Engineering

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. 4/13/2017 CE-533 Advanced Foundation Engineering

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' < " " " " " ” " " " 4/13/2017 CE-533 Advanced Foundation Engineering

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. 4/13/2017 CE-533 Advanced Foundation Engineering

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) REFERENCES: 4/13/2017 CE-533 Advanced Foundation Engineering

THANK YOU 4/13/2017 CE-533 Advanced Foundation Engineering