STABILITY ANALYSIS OF SPREAD FOOTINGS ON STABILITY ANALYSIS OF SPREAD FOOTINGS ON WEAK MASSIVE & LAYERED FLOOR STRATA WEAK MASSIVE & LAYERED FLOOR STRATA

USING FINITE ELEMENT MODELING(FEM)USING FINITE ELEMENT MODELING(FEM)

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Selection of the TopicSelection of the Topic

Powerful technique for finding “approximate” solution for “real-world” problems.

Widely accepted by scientific and industrial community.

Less time - maximum observationAbility to handle complex problems of

unusual shapes,sizes & loading condition.

Need of understanding the behavior of Need of understanding the behavior of foundation strata…foundation strata…

Very important for the safe as well as economic design and implementation of mega to mini engineering structures like dams etc.,

Objectives of this investigation:Objectives of this investigation:Settlement analysis of weak floor strata(massive&layered) by Finite Element Modeling.

Thereby finding the stability of spread footings on it and comparing it with field/ model plate load test data.

Pros and Cons of FEA of Pros and Cons of FEA of present topic.present topic.unusual shapes and/or unusual loading

conditions as well as in situations where the foundation rock is highly variable.

The primary advantage is that the method is able to handle complex conditions and also provides direct solutions for deformation/ settlement.

The primary disadvantage of the finite element method is that the method does not provide a direct solution for the ultimate bearing capacity.

What is FEM & Why FEM?What is FEM & Why FEM?FEM is a computational technique used to obtain

approximate solutions of boundary value problems in engineering.

The method utilizes an integral formulation to generate a system of algebraic equations.

Can be applied to irregular shaped objects composed of several different materials and having mixed boundary conditions.

Applicable to steady-state and time dependent problems as well as for problems involving non-linear material properties.

How FEA gets done?How FEA gets done?(1)Preprocessing:-Define the geometric domain of the problem.Define the element type(s) to be used.Define the material properties of the elements.Define the geometric properties of the

elements (length, area, and the like).Define the element connectivities (mesh the

model).Define the physical constraints (boundary

conditions).Define the loadings. (Conti…)

(Conti…)(Conti…)(2)Solution:- During the solution phase, finite element

software assembles the governing algebraic equations in matrix form and computes the unknown values of the primary field variable(s).

(3) Postprocessing :- Sort element stresses in order of magnitude. Check equilibrium. Plot deformed structural shape. Animate dynamic model behavior. Produce color-coded plots.

Stability analysis of spread footings:Stability analysis of spread footings:

FE modeling(ANSYS 11) for floor strata of various thickness(T) and for various sizes(B) and shapes of footing keeping B/T ratio constant(0.625 ) to varying ratios.

The input parameters obtained from existing literature for laboratory simulated weak floor strata and footing.

What to find?What to find?

Settlement of foundation(simulated floor) with regards to the size and shape effects (i.e, effect of size and shape of footing on stability floor strata).

Settlement of foundation containing weak layers of varying thickness.

Parameters used for FE AnalysisParameters used for FE Analysis

Parameters used for FE AnalysisParameters used for FE Analysis

Parameters used for FE AnalysisParameters used for FE AnalysisFor FEA ,the bearing strength of simulated floor

strata was taken as loading pressure which has to be applied for the observation of settlement of foundation.

The Non-linear analysis (stress-strain relation is non –linear) with “Drucker-Prager” criteria for non metal plasticity was followed as failure criteria.

FE ModelsFE ModelsFloor strata: Floor strata: Massive Massive Thickness of strata(T):Thickness of strata(T):4 cm 4 cm B/T ratio= B/T ratio= 0.6250.625Footing plate size(B):Footing plate size(B):2.5 cm 2.5 cm Footing plate shape:Footing plate shape:circularcircular

visualizevisualize what is happening -the biggest what is happening -the biggest advantageadvantage of FEA of FEA

FE ModelsFE ModelsFloor strata: Floor strata: Massive Massive Thickness of strata(T):Thickness of strata(T):12 cm 12 cm B/T ratio= B/T ratio= 0.6250.625Footing plate size(B):Footing plate size(B):7.5 cm 7.5 cm Footing plate shape:Footing plate shape:circularcircular

More models More models

FE Models of strata containing weak layer.FE Models of strata containing weak layer.

Results of FEAResults of FEAFEM result of the footing settlement (central FEM result of the footing settlement (central circular footing).circular footing).

FEA result of the footing settlement (central FEA result of the footing settlement (central square footing).square footing).

FEA result of the footing settlement (central FEA result of the footing settlement (central square footing in the presence of weak layer).square footing in the presence of weak layer).

Why the variation ??Why the variation ??

In FEM analysis it is always assumed that the model material is fully isotropic. But the material properties of physically simulated floor strata might not have been fully isotropic as they were prepared in the laboratory using the mixture of cement, sand and water.

In FEM the model floor strata were fully

confined using boundary constraints. Whereas in laboratory the physical model may face problems in confinement.

ConclusionsConclusions

The minimum and maximum deviation of results from lab data are about 0.01% and 40% respectively.

Tensile cracks are initiated at the rim of footing forcing a depression beneath the footing leading to a stable crack growth as load increases.

The maximum stress concentration extends to a distance 2 to 3 times the footing plate width in all direction.

It can be concluded that the result obtained by FEM analysis is closely matches with experimental results.

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