Foundation: Types, Use and Objectives

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Foundation: Types, Use, Objectives

Foundations

Definition & Introduction

Foundation are structural elements, which transfer loads to the soil from columns, walls or lateral loads from earth retaining structures.

A structure essentially consists of two parts, namely the super structure which is above the plinth level and the substructure which is below the plinth level. Substructure is otherwise known as the foundation and this forms the base for any structure. Generally about 30% of the total construction cost is spent on the foundation.The soil on which the foundation rests is called the “foundation soil”.Shallow FootingDeep Foundation

Types of Foundation

The two main types of foundation are :

Shallow foundation/Footings
Deep foundation

Shallow Foundation

Shallow Foundation are usually located no more than 6 ft below the lowest finished floor OR Depth (D) of foundation is less than or equal to its width (B). When the soil bearing capacity of soil upto low depth is sufficient to take the structure load then it is provided.

Shallow Footing

Types of Shallow foundation

Isolated spread footing
Combined footing
Cantilever or strap footings
Wall footings
Raft or Mat foundation

Use of Shallow Foundation

A shallow foundation system generally used when

The soil close the ground surface has sufficient bearing capacity
Underlying weaker strata do not result in undue settlement. The shallow foundations are commonly used most economical foundation systems.

bearing-capacity-failure-mechanism-in-soil-under-a-rough-rigid-continuous-foundation — Isolated Footing

Deep Foundations

The shallow foundations may not be economical or even possible when the soil bearing capacity near the surface is too low. In those cases deep foundations are used to transfer loads to a stronger layer, which may be located at a significant depth below the ground surface. The load is transferred through skin friction and end bearing.Deep Foundation

Types of Deep foundation

Pile foundation
Pier foundation
Types of Pile foundation :
Friction pile
Load bearing pile

Foundation Load Mechanism — Pile Cap. Pile. Imposed Load (P) Skin Friction (Pf) End bearing (Pb) P = Pf + Pb.

Objectives of a foundation

To distribute the total load coming on the structure on a larger area.
To support the structures.
To give enough stability to the structures against various disturbing
forces, such as wind and rain.
To prepare a level surface for concreting and masonry work.

Machine Foundations

Foundations provided for machines are called machine foundations.

These foundations have to be specially designed taking into account the impact and vibration characteristics of the load and the properties of soil under dynamic conditions. Thus the design of foundations of Turbines, Motors, Generators, Compressors, Forge hammer and other machines having a rythmic application of unbalanced forces require special knowledge of theory of harmonic vibrations.

All the above consideration are made in the design of machine foundations because inertial forces of rotating elements of machine contribute dynamic loads in addition to their static loads. Moreover, the machinery vibration influences adversely the foundation supporting soil by densifying it which may result differential settlement of the foundation.

Design Requirements

Machine foundations must fulfil the following design requirements.

A machine foundation should be safe against shear failure.
It should not settle excessively under static loads.
There should be no resonance due to dynamic force i.e the natural frequency of the foundation soil system should not be coincide with the operating frequency of the machine.
The amplitude at operational frequency of the foundation system must be within telerable limits.
The vibrations of the foundation soil system must not annoying to the workers working in that area.
It should not create bad effect on the other precision machines and instruments.

Types of Machines Foundations

Machine foundations are broadly classified into the following three types, depending upon the type of machines for which they are provided:

Reciprocating type Machines Foundations.
Centrifugal type Machines Foundations.
Impact Type Machines Foundations.

Foundations for Heavy Crane, Compressor & Cooler in MOl Oil & Gas Co.

Soil Penetrometer Test

Penetrometer is a fantastic little invention which geotechnical engineers and technologists find very handy. It is a small handheld gauge which contains a telescoping rod which can be pushed into the soil. The distance the rod goes into the soil corresponds to a compressive strength on the dial.

Measurement of Soil

The pocket penetrometer measures the compressive strength of the soil. Most penetrometers available today contain units of tons/ft² or kg/cm², and the compressive strength is read directly from the gauge. Some common conversions are:

1 ton/ft²= 2000 psf = 13.9 psi

1 kg/cm²= 98.1 kPa

Limitations

A pocket penetrometer is a primative instrument that is subject to many errors such as non-uniform soil. As a minimum, you should take a series of measurements in one area and average them. The penetrometer should not replace laboratory testing or field analysis, or be used to produce foundation design data.

Factor of Safety (FoS) and Margin of Safety (MoS)

In this blog you will learn about the Factor of Safety (FoS) and Margin of Safety (MoS) that is used in the design of Civil Engineering Structures e.g Buildings, Bridges, Over-head Tanks etc.

Factor of Safety (FoS)

It is the load carrying capacity of any structure or system beyond the expected or actual loads.

Factor of Safety (FoS) used in the design of any civil engineering structures e.g Buildings, Bridges, Over-Head tanks and other RCC, Steel, Wood and composite structures.

The Term Ultimate Stress and Allowable Stress is can be expressed as:

Ultimate Stress = Ultimate Load, Failure Load

Allowable Stress = Allowable load, Design Load

Purpose of FoS

The main purpose of FoS to take some extra loads or to resist some accidental loads by the structure that may occur during the life of that structure. So the structure designer intentionally built the structure stronger then the allowable load. The accidental loads may be due to Earthquakes, Wind, Snow or any other dead or live load that is unexpected in the structure.

Mathematical Representation of FoS

Factor of Safety (FoS) = (Ultimate Stress)/(Allowable Stress)

FoS= ðus/ðas Eq (1)

Example

ðus= 600 N/mm2, ðas= 300 N/mm2

Put the values of ðus and ðas in the above Eq (1)

FoS = (600)/(300) = 2

FoS = 2 , mean that the structure can still take load twice of Allowable Load.

You can derive any value from Eq (1) if there is any two of them are known.

Margin of Safety (MoS)

MoS can be simply defined as it is the margin/gap provided from the allowable stress to ultimate stress. Margin of Safety (MoS) totally depends onFactor of Safety (FoS).

Purpose of MoS

The main purpose of MoS to add some extra margin to adjust or resist an accidental loads by the structure that may occur due to any reason in the structure.Through Margin of Safety (MoS) the Structural designer store some reserve capacity in the structure to take extra loads beyond the allowable load. Those Structures whose margin of safety (MoS) is equal or greater than the value one are more resistant to accidental loads. The MoS value adds the margin to structure to take some additional loads.

Mathematical Representation of MoS

Margin of Safety (MoS) = (Failure Load)/( Design Load) – 1

FoS = (Failure Load)/( Design Load)

Mos = FoS – 1

if FoS = 2

MoS = 2-1 = 1

So MoS = 1 , mean that the structure has still reserve the load carrying capacity upto 100% more than the allowable load.

relationship-between-factor-of-safety-fos-and-margin-of-safety-mos

Types of Soil & Suitable Soil for Foundations Bed

In this article you will learn about different types of Soil and Suitable soil for any structure’s foundation bed.

Soil

Soil is sediments or other unconsolidated accumulations of solid particles produced by the chemical and physical disintegration of rocks which may or may not contain organic matter.

Types of Soil

The soil on which the foundations of various types of structure is to be constructed are classified into:

(1) Sand

The grain size of Sand varies between 0.075mm to 2mm. The shape of the grains may be angular, rounded or irregular and silica is the major constituents of sand.

Image result for sand for construction — Sand

Properties of Sand

It doesn’t shrink when dry.
It doesn’t swell when wet.
It is cohesionless.
It doesn’t affected by the action of frost.
It doesn’t allow water to rise up by capillary action.

Suitability

Coarse sand provides a good foundation bed. Since it is angular shape that’s why it prevent the structures from slipping and also from escaping from under surface of the foundation concrete. Fine and saturated sand are not suitable for foundation use.

(2) Gravel and Shingle

This type of soil consists of mostly big size particles of coarse material resulting from the disintegration of rocks and often transported by water from their original source. Size of the particles varies from 3 mm to 200 mm. The stone particles having size more than 200 mm are termed as boulders.

Image result for gravel and shingle in construction — Shingle

Properties of Gravel and Shingle

It is not affected by freezing of water.
It doesn’t swell when wet.
It doesn’t shrink when dry.
It has great power and strength of load bearing.
It doesn’t settle over the load.

Suitability

Gravel and Shingle provides a good foundation bed and is suitable for foundation of almost all types of structures.

(3) Clay

It consists of particles having grain less than 0.002. It is composed of microscopic and sub-microscopic particles of weathered rocks. It consists of particle having grain size less than 0.002 mm.

Properties of Clay

Clay can retain it shape vertically when hard but flows down when wet and exert pressures.
It consolidates under load and may cause settlement of the structure.
It is cohesive type of soil.
It shrinks and cracks when dry.
It swells and heaves when wet.

Suitability

It is suitable for foundation of ordinary and light structures. But when heavy structure is to be constructed then must check through various laboratory tests because it is can settle down when saturated. It is also difficult to excavate when dry or when heavily saturated.

(4) Silt

It is finer variety of soil having grain size of 0.002 mm to 0.6 mm.

Properties of Silt

It has slight tendency towards swelling and shrinkage.
It is relatively impervious.
It is not as superior is sand.
It is generally found in beds of river, canals and reservoirs.

Suitability

It is not considered as a good foundation material.

(5) Alluvial Soil

This soil is transported by water forces and mixed with soils of different origin. When velocity of water reduced then large size particles are start settling down. On further reduction of the velocity of water, still smaller fraction separates out. Thus the alluvial soils are deposited according to the grains sizes.

Properties of Alluvial Soil

It is a cohesive soil.
It is plastic but consolidate under load.
It Cracks on Drying.

Suitability

This type of soil are suitable for light Structures.

(6) Black Cotton Soil

This type of soil is inorganic in nature. It is also called peat and bungum.

Properties of Black Cotton Soil

It swell when confined between walls.
It can withstand a high pressure in dry state.
It becomes so soft during rains that even mam cannot walk through it.
It is dark, grey or black in color.

Suitability

It is the most unreliable soil for foundation bed.

(7) Reclaimed soil

This is also known as made-up soil. This types of soil consist of ballast or brick bats, ashes, old iron pieces etc used for filling the low lying areas or back filling

Properties of Reclaimed Soil

Its bearing strength is very low.
It is usually porous in nature.

Suitability

This type of soil is not suitable for laying or constructing structure over it.

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Civil Engineering

Definition

Civil Engineering is a branch of Engineering deals with Plan, Analyze, Design, Build & Maintain facilities i.e Bridges, Buildings, Dams, Culverts, Irrigation Channels and Steel Structures etc for the people.

Career

The civil engineer has the challenge to satisfy the vital needs of society by designing, building, managing and maintaining complex infrastructure projects with a global approach, taking into account socio-economical and environmental interactions.
He is a generalist with high competences in various fields such as structures, hydraulic schemes and energy, geotechnics and tunnelling, transportation infrastructures and systems, management, legal and economical aspects as well as environmental issues. Civil engineers work in multidisciplinary and very often multicultural teams.

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Objectives

Civil engineering will give the students the required knowledge to face a big challenge: create the necessary infrastructures for economic prosperity, hence guaranteeing the whole population an adequate standard of living.
High priority is given to a transdisciplinary teaching in the ENAC School by bringing together civil engineers, architects and environmental engineers, through the program “Design and build together”.
An EPFL Master of Science in Civil Engineering will lead the young graduate to a stimulating career in an engineering consulting firm, a construction company or a government agency, or allow him/her to open his/her own civil engineering consulting firm.

Area Moment of Inertia vs Mass Moment of Inertia

The term “moment of inertia” is used very frequently Strength of Materials (SoM). There are two different types of moment of inertia: Mass moment of inertia and Area moment of inertia. Sometimes this creates confusion about which moment of inertia is to be used in which place. In this article you will learn and understand the concept of the mass moment of inertia and the area moment of inertia.

(1) Mass Moment of Inertia

Mass moment of inertia (sometimes called just “moment of inertia”) is responsible for providing resistance against changing the rotational speed of a rotating body. The mass moment of inertia is represented by “I” and used in structural design calculations where angular moment involved.

The units of the mass moment of inertia are Kg-M²

The general formula for calculating the mass moment of inertia can be given as:

I = ∫ r² dm

Where,

I – Mass moment of inertia.

dm – A very small mass parallel to the desired axis.

r – Distance of the small area from the axis.

However, you need not to use this equation most of the time as mass moment of inertia values for standard geometries are readily available.

The mass moment of inertia is the rotational analog of mass. That means, in all the rotational equations of angular momentum, angular kinetic energy, force etc. the mass moment of inertia (I) should be used.

(2) Area Moment of Inertia

Area moment of inertia or second moment of area or second moment of inertia is used in beam equations for the design of structures. Area moment of inertia is the property of a section. Like mass moment of inertia, area moment of inertia is also represented by “I” but the units of the area moment of inertia are different than that of the mass moment of inertia. The units of the area moment of inertia are m4, mm4, inch4, etc.

The general formula for calculating the area moment of inertia can be given as:

Ixx = ∫ y² dA

Where,

Ixx – Area moment of inertia about X axis.

dA – A very small area parallel to the X axis.

y – Distance of the small area from the X axis.

However, you do not need to use this equation most of the time as area moment of inertia values for standard geometries are readily available.

Uses

The mass moment of inertia and area moment of inertia both are represented by I. Sometimes it may be confusing, but you have to figure it out by the application. The mass moment of inertia is used as a rotational analog of mass, and the area moment of inertia is used mainly for beam equations. The other difference is the units used in both the moments of inertia are different.

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Structural Analysis & Methods

Structural Analysis

It is a method or tool by which we find out how a structure or a member of a structure behaves when subjected to certain loads.

In other words finding out internal forces (axial force, shear force, moment), stress, strain, deflection etc in a structure under applied load conditions.

Types, Assumptions and Fundamental Approaches of Structural Analysis | Engineersdaily | Free engineering database — Structural analysis and Design

Whenever design any structure first of all we Analyze the structure either manually or through software or may be both. So there is number of methods of Structural Analysis. Mainly these methods are used to analyze indeterminate structures ( no. of reactions more than equilibrium equations) In this article we will discuss just the name of methods.

Methods of Analysis

There are two main Methods which have further divided into different methods. There is some limitations while analysing any structure that’s why you have to chose the most appropriate method while analysing any structure.

Exact Methods
Approximate Methods

(1) Exact Methods

It is the most accurate methods while analyzing any indeterminate structures. It is also called accurate method. In this method the material property called Modulus of Elasticity (E) and geometric property called Area Moment of Inertia (I) of the materials are required.

This method is further subdivided into two methods. We will discuss each methods in detail in the next article.

Force Methods
Displacement Methods

1. Force Methods

Three Moment Theorem
Flexibility Matrix
Virtual Work/Unit Load Method
Strain Energy Method
Methods of Consistent deformation
Column Analogy
Minimum Potential Energy Method
Castigliano’s Method etc

2. Displacement Methods

Moment Distribution Method
Stiffness Matrix Method
Slope Deflection Method
Kani’s Method etc

(2) Approximate Methods

In this method the material property called Modulus of Elasticity (E) and geometric property called Moment of Inertia (I) of the materials are not required. In this method indeterminate structures first convert to determinate structures and then analysed using equation of equilibrium.

The following three methods normally used in analysing.