Applications of Spread Footing and Soil Pressure Distribution

Table of Contents

1. Introduction

2. Definition of footings

3. Importance of Spread Footings

4. Mode of Distribution of Soil Pressure in a Spread Footing

5. Types of Spread Footings

1. Introduction

The size and weight of a building as well as the nature of the soil structure it is built on play a critical role in foundation design. Soil pressure distribution must be addressed to ensure a sound structure. Spread footing is a crucial structural component that provides strength for a building's foundation.

2. Definition of footings

Footing is a structural element that transfers a building's weight to the soil using columns, walls, and lateral loads from earth-retaining structures. Our PE Civil exam review course discusses footings and their physical characteristics for engineers preparing for the PE Civil exam.

3. Importance of Spread Footings

1. A spread footing foundation has a wider bottom portion compared to a load-bearing foundation; the wider bottom "spreads" the weight of the structure over a larger area for greater stability. 
2. While traditional spot footings only have a single point of contact with the foundation, spread footings extend support continuously across the entire building layout. 
3. Spread footings are used to support a foundation or set of piers below a building. 
4. To add additional support, spread footings are constructed with concrete and reinforced with steel. Since spread footing transfers the weight of the building over a large area, spread footings have little risk of failure compared to spot footers. 
5. Spread footing extends the life of a building by minimizing structural damage. Footings must be designed to carry the column loads and transmit them to the soil safely. 
6. Spread footings may be circular, square, or rectangular. 
7. Spread footings are common in residential construction.

4. Mode of Distribution of Soil Pressure in a Spread Footing

Column loads act at the center of the footing, creating a uniform surface for the soil underneath the footing area. The distribution of pressure depends on the composition of the soil and on the degree of flexibility of the footing. 

5. Types of Spread Footings:

(i) Isolated Footing

When columns are spaced far apart, isolated footings are used to support single columns. 

(ii) Combined Footing

When two columns are close to each other and their individual footings overlap, a combined footing is required. A combined footing supports two columns so that the load is evenly distributed. A combined footing may be rectangular or trapezoidal.

(iii) Strap Footing (Cantilever)

In strap footing, two isolated footings are connected with a structural strap (rigid beam) or lever. 

(iv) Mat Foundation (Raft)

A mat foundation is a large slab that supports several columns and walls under the entire structure. If several columns overlap each other, then a single footing for all columns is provided. This type of footing is known as mat footing. Mat foundations are used to reduce the differential settlements on non-homogeneous soils. 

Basic Principles and Classifications of Pile Foundations

Table of Contents

1. Introduction

2. Conditions where deep foundations are used

3. Classification of deep foundations

4. Pile foundations

5. Installation of pile foundations

6. Categories of piles

7. Types of piles based on shape and composition

1. Introduction

Shallow and deep foundations signify the relative depth of the soil on which buildings are founded. When the depth of a foundation is less than the width of the footing and is less than ten feet deep, it is a shallow foundation. Shallow foundations are used when surface soils are strong enough to support the imposed loads. If the depth of a foundation is more than the width of the building foundation, it is a deep foundation. Deep foundations are often used to transfer building loads deeper into the ground. 

2. Conditions where deep foundations are used

• Soil near the surface that has relatively weak bearing capacities (700 pounds per square foot or less)
• Soils near the surface that contain expansive clays (shrink/swell soils) 
• Surface soils that are vulnerable to being removed by erosion or scour

3. Classification of deep foundations 

Deep foundations are classified into three categories:

• Pile foundations
• Well foundations
• Caisson foundations

Types of foundations and basic mechanisms involved in the classification of deep foundations are reviewed in our FE Civil exam review course for those preparing to become an engineer in training.

4. Pile foundations

A pile foundation is defined as a series of columns constructed or inserted into the ground to transmit loads to a lower level of subsoil. A pile is a long cylinder made up of a strong material, such as concrete. Piles are pushed into the ground to act as a steady support for structures built on top of them. Piles transfer the loads from structures to hard strata, rocks, or soil with high bearing capacity. The piles support the structure by remaining solidly placed in the soil. As pile foundations are set in the soil, they are more tolerant to erosion and scour.

5. Installation of pile foundations 

Piles are first cast at ground level and then hammered or driven into the ground using a pile driver. A pile driver is a machine that holds the pile vertical and hammers it into the ground. Blows are repeated by lifting a heavy weight and dropping it on top of the pile. Piles should be hammered into the ground until the refusal point is reached, which is the point where a pile cannot be driven into the soil any farther. The method of installing a pile is a major consideration in the structural integrity of pile foundations. The driven-pile method is an ideal option because it least disturbs the supporting soil around the pile and results in the highest bearing capacity for each pile. Since every pile has a zone of influence on the soil around it, piles must be spaced far enough apart from each other so that the loads are distributed evenly.

6. Categories of piles

Depending on their function, piles are classified as bearing piles, friction piles, friction-cum-bearing piles, batter piles, guide piles, and sheet piles.

Based on the composition of materials, piles are classified as timber piles, concrete piles, sand piles, or steel piles. 

1)Bearing piles are driven into the ground until a hard stratum is reached. Bearing piles rest on hard strata and act as pillars to support the structure. Bearing piles allow vertical loads and transfer the building load to the hard stratum underneath. 

2)Friction piles are used when the soil is soft and there are no hard strata available. These piles are long, and the surfaces are roughened to increase surface area and increase frictional resistance. They bear on frictional resistance between their outer surface and the soil in contact. Friction piles do not rest on hard strata. 

3)Batter piles are driven inclined to resist inclined loads.

4)Guide piles are used in the formation of cofferdams to provide stable foundations for under-water construction.

Basic principles of pile foundations and their classifications are recommended topics to review prior to taking the FE Civil exam. 

7. Types of piles based on shape and composition

The Conduction Process and its Importance in Mechanical Engineering Applications

Table of Contents

1. Introduction

2. Definition of Conduction

3. Importance of Heat Transfer Conduction

4. Types of conduction

(i) Molecular Vibration

(ii) Free Electron Diffusion

5.Conduction in Metals

6. Fourier's Law of Heat Conduction

7. Applications of Conduction Phenomena in Engineering

1. Introduction

Regions with greater molecular kinetic energy pass their thermal energy into regions with less molecular energy through direct molecular collisions. This process is known as conduction. In metals, a significant portion of the transported thermal energy is carried by conduction-band electrons.

2. Definition of Conduction

Conduction is the transfer of thermal energy that does not have any flow of material medium and is the main process by which thermal energy is transferred from one solid to another. Our PE Mechanical course reviews the physical properties of heat.

3. Importance of Heat Transfer Conduction

Sticking a metal pole into a fire is an example of heat transfer conduction. Particles at the heated end vibrate vigorously. They collide with the neighboring particles and transfer their energy. Eventually, the particles at the cooler end are set into vigorous vibration, which causes the entire metal pole to become hot.

4. Types of conduction

There are two types of conduction:

1)Molecular vibration

2)Free electron diffusion

(i) Molecular Vibration

When heat is supplied to one end of an object, the molecules at that end start to vibrate vigorously. During this process, they bump into their neighboring molecules, which transfers some energy. The receiving neighbor molecule gains energy and starts to vibrate more vigorously. The cycle continues.

(ii) Free Electron Diffusion

This form of conduction takes place only in metals because only metals have free electrons. Electrons are freed from a molecule when heat is applied, which forces the electrons to travel toward the colder end of the metal. At the colder end, the electrons collide into many molecules, and therefore, pass energy to the molecules at the other side.

5. Conduction in Metals

In solids, thermal energy is transferred through the vibration and collision of particles. However, in metals, due to the presence of free electrons, thermal energy is spread through electron diffusion. Electrons gain kinetic energy and move rapidly and collide with the atoms in the cooler parts of the metal to pass on their energy.

The process of conduction in metals is important for engineers to understand when preparing to pass the PE Mechanical exam.

6. Fourier's Law of Heat Conduction

The law of heat conduction, or Fourier's law, states that the time rate of the heat transfer through the material is proportional to the negative gradient in the temperature and to the area.

Q = -kA(dT/dx)

'Q' - heat flow rate by conduction (W)

'k' - thermal conductivity of body material (W m-1 K-1)

'A'- cross-sectional area normal to direction of heat flow (m2) and 'dT/dx' is the temperature gradient (k-m-1)

The negative sign in Fourier's equation indicates that the heat flow is in the direction of negative gradient temperature, which makes the heat flow positive

The thermal conductivity "k" refers to the transport properties 

Thermal conductivity "k" provides indication of the rate at which heat energy is transferred through the medium by the conduction process 

7. Applications of Conduction Phenomena in Engineering

1. Mechanical Engineering Equipment
2. Home Appliances
3. Boilers

Types and Sources of Air Pollution

Air pollution is defined as the presence of any particle or gas found in the air that is not part of the original composition. Air pollution is a change in the physical, chemical, and biological characteristics of the air surrounding us. The substances that cause air pollution are called air pollutants, and they may be in the form of a gas, liquid, or solid.

Air pollutants are transboundary in nature as they travel and affect areas far away from their point of origin. Air pollution causes adverse effects on humans and other living organisms. Our PE Environmental exam review course thoroughly reviews the types and sources of air pollution for those preparing for the PE Environmental exam.

Air Quality Index

Air quality index (AQI) indicates whether pollutant levels in the air may cause health concerns. AQI ranges from 0 to 500, with a higher number meaning a lower air quality.

The table below provides the AQI limits for human health.

Air Quality Index
Air Quality	Air Quality Index Range
Good	0-50
Moderate	51-100
Unhealthy for sensitive groups	101-150
Unhealthy	151-200
Very unhealthy - ALERT	201-500

The air quality index table is a useful reference for environmental engineers preparing to take the PE exam.

Types of Air Pollutants

Air pollutants may be natural, such as wildfires, or may be synthetic (manmade). Air pollutants are classified as primary pollutants and secondary pollutants. 

(i) Primary air pollutants are emitted directly into the atmosphere by the original source
(ii) Secondary air pollutants are formed because of reactions between primary pollutants and other elements in the atmosphere, such as the ozone.

The common air pollutants are discussed below:

1. Carbon Monoxide - Carbon monoxide is a colorless, odorless gas. Carbon monoxide can be present in car exhaust and smoke. Carbon monoxide deprives humans of their oxygen supply, which causes headaches, fatigue, impaired vision, and even death.
2. Sulfur Dioxide - Sulfur dioxide is produced when coal and fuel oils are burned and is also present in power plant exhaust. Exposure to sulfur dioxide narrows the airways in the respiratory system, which causes wheezing and shortness of breath. 
3. Nitrogen Dioxide - Nitrogen dioxide is both a primary and secondary air pollutant. Nitrogen dioxide is created when nitrogen reacts with oxygen in the atmosphere. Nitrogen dioxide can cause respiratory infections and other respiratory problems.
4. Particulate Matter - Particulate matter contains particles of different sizes that are released into the atmosphere from various sources, including fossil fuels, dust, smoke, and fog. Particulate matter can accumulate in the respiratory system, which can aggravate the heart and lungs and increase the risk of respiratory infections.
5. Ground-Level Ozone - Ground-level ozone is formed from automobile, power, and chemical plant exhausts. Ground-level ozone irritates the respiratory system and causes asthma by reducing lung function.
6. Smog - Smog is the combination of gases with water vapor and dust and forms when heat and sunlight react with gases, which is known as photochemical smog.