Modes of Transport

The term ‘mode of transport’ refers to numerous methods of moving people, objects or both. The various modes of transportation include air, water and land transportation, which includes trains, highways and off-road travel. There are also other modes, including pipelines, cable transmission and space flight etc. The different modes of transport are shown below.

Fig.1 Modes of Transport

Advantage and Disadvantage Different Modes of Transport

1) Road Transport

Advantages	Disadvantages
Less capital outlay	Seasonal nature
Door to door service	Accidents and breakdowns
Service in rural areas	Unsuitable for long distance and bulky traffic
Flexible service	Slow speed
Suitable for short distance	Lack of organization
Lesser risk of damage in transit
Saving in packing cost
Rapid speed
Less cost
Private owned vehicles
Feeder to other modes of transport

2) Railway Transport

Advantages	Disadvantages
Dependable	Huge capital outlay
Better organized	Lack of flexibility
High speed over long distances	Lack of door to door service
Suitable for bulky and heavy goods	Monopoly
Cheaper transport	Unsuitable for short distance and small loads
Larger capacity	Booking formalities
Public welfare	No rural service
Administrative facilities of Government	Under-utilized capacity
Employment opportunities	Centralized administration
Safety

3) Air Transport

Advantages	Disadvantages
High speed	Very costly
Comfortable and quick services	Small carrying capacity
No investment in construction of track	Uncertain and unreliable
No physical barriers	Breakdowns and accidents
Easy access	Large investment
Emergency services	Specialized skill
Quick clearance	Unsuitable for cheap and bulky goods
Most suitable for carrying light goods of       high value	Legal restrictions
National defense
Space exploration

Elements of Transport

The movement of goods or passenger traffic, through rail, sea, air or road transport requires adequate infrastructure facilities for the free flow from the place of origin to the place of destination. Irrespective of modes, every transport system has some common elements. These elements influence the effectiveness of different modes of transport and their utility to users.

1) Vehicle or Carrier to Carry Passenger or Goods

The dimension of vehicles, its capacity and type are some of the factors, which influence the selection of a transport system for movement of goods from one place to the other.

2) Route or Path for Movement of Carriers

Routes play an important role in movement of carriers from one point to another point. It may be surface roads, navigable waterways and roadways. Availability of well-designed and planned routes without any obstacle for movement of transport vehicles in specific routes, is a vital necessity for smooth flow of traffic.

3) Terminal Facilities for Loading and Unloading of Goods and Passengers from Carriers

The objective of transportation can’t be fulfilled unless proper facilities are available for loading and unloading of goods or entry and exit of passengers from carrier. Terminal facilities are to be provided for loading and unloading of trucks, wagons etc. on a continuous basis.

4) Prime Mover

The power utilized for moving of vehicles for transportation of cargo from one place to another is another important aspect of the total movement system.

5) Transit Time and Cost

Transportation involve time and cost. The time element is a valid factor for determining the effectiveness of a particular mode of transport. The transit time of available system of transportation largely determines production and consumption pattern of perishable goods in an economy.

6) Cargo

Transportation basically involves movement of cargo from one place to another. Hence, nature and size of cargo constitute the basis of any goods transport system.

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Phases of Structural Engineering Projects

Structural engineering is the science and art of planning, designing and constructing safe and economical structures that will serve their intended purposes. Structural analysis is an integral part of any structural engineering project, its function being the prediction of the performance of the proposed structure. A flowchart showing the various phases of a typical structural engineering project is presented in Fig. 1.

Fig. 1 Phases of a Typical Structural Engineering Project

The process is an iterative one, and it generally consists of the following steps.

1) Planning Phase

The planning phase usually involves the establishment of the functional requirements of the proposed structure, the general layout and dimensions of the structure, consideration of the possible types of structures (e.g. rigid frame or truss) that may be feasible and the types of materials to be used (e.g., structural steel or reinforced concrete). This phase may also involve consideration of non-structural factors, such as aesthetics, environmental impact of the structure etc.

The outcome of this phase is usually a structural system that meets the functional requirements and is expected to be the most economical. This phase is perhaps the most crucial one of the entire project and requires experience and knowledge of construction practices in addition to a thorough understanding of the behavior of structures.

2) Preliminary Structural Design

In the preliminary structural design phase, the sizes of the various members of the structural system selected in the planning phase are estimated based on approximate analysis, past experience and code requirements. The member sizes thus selected are used in the next phase to estimate the weight of the structure.

3) Estimation of Loads

Estimation of loads involves determination of all the loads that can be expected to act on the structure.

4) Structural Analysis

In structural analysis, the values of the loads are used to carry out an analysis of the structure in order to determine the stresses or stress resultants in the members and the deflections at various points of the structure.

5) Safety and Serviceability Checks

The results of the analysis are used to determine whether or not the structure satisfies the safety and serviceability requirements of the design codes. If these requirements are satisfied, then the design drawings and the construction specifications are prepared, and the construction phase begins.

6) Revised Structural Design

If the code requirements are not satisfied, then the member sizes are revised and phases 3 through are repeated until all the safety and serviceability requirements are satisfied.

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Volumetric Strain (εv)

When a member is subjected to stresses, it undergoes deformation in all directions. Hence, there will be change in volume. The ratio of the change in volume to original volume is called volumetric strain.

Thus,

Where,

           εv = Volumetric strain 

          δV = Change in volume

           V = Original volume

It can be shown that volumetric strain is sum of strains in three mutually perpendicular directions.

For example consider a bar of length L, breadth b and depth d as shown in Fig. 1.

Fig. 1 Rectangular Bar

Now,

Volume, V = L b d

Since volume is function of L, b and d, by using product rule (The derivative of the product of two differentiable functions is equal to the addition of the first function multiplied by the derivative of the second, and the second function multiplied by the derivative of the first function.) we may write as;

Consider a circular rod of length ‘L’ and diameter ‘d’ as shown in Fig. 2.

Fig. 2 Circular Rod

Volume of the bar

                                                                                         (Since V is function of d and L)

Dividing the equation by V

In general for any shape volumetric strain may be taken as sum of strains in three mutually perpendicular directions.

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Hooke’s Law and Poisson’s Ratio

Hooke’s Law

Robert Hooke, an English mathematician conducted several experiments and concluded that stress is proportional to strain up to elastic limit. This is called Hooke’s law. Thus Hooke’s law states that ‘stress is proportional to strain up to elastic limit.’

σ ∝ ϵ

where ′σ′ is stress and ′ϵ′ is strain

Hence,

σ = E ϵ

Where ‘E’ is the constant of proportionality of the material, known as Modulus of Elasticity or Young’s modulus, named after the English scientist Thomas Young (1773–1829). 

The present day sophisticated experiments have shown that for mild steel the Hooke’s law holds good up to the proportionality limit which is very close to the elastic limit. For other materials, Hooke’s law does not hold good. However, in the range of working stresses, assuming Hooke’s law to hold good, the relationship does not deviate considerably from actual behaviour. Accepting Hooke’s law to hold good, simplifies the analysis and design procedure considerably. Hence Hooke’s law is widely accepted. The analysis procedure accepting Hooke’s law is known as Linear Analysis and the design procedure is known as the working stress method.

Poisson’s Ratio (μ)

When a material undergoes changes in length, it undergoes changes of opposite nature in lateral directions. For example, if a rectangular bar is subjected to direct tension in its axial direction it elongates and at the same time its sides contract as shown in Fig.1.

Fig. 1 Changes in Axial and Lateral Directions due to Tensile Force

If we define the ratio of change in axial direction to original length as linear strain and change in lateral direction to the original lateral dimension as lateral strain, it is found that within elastic limit there is a constant ratio between lateral strain and linear strain. This constant ratio is called Poisson’s ratio. 

Thus,

It is denoted by 1/m or μ.

For most of metals its value is between 0.25 to 0.33. Its value for steel is 0.3 and for concrete 0.15.

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Basic Terminologies in Mechanics

1) Mass (m)

The quantity of the matter possessed by a body is called mass. The mass of a body will not change unless the body is damaged and part of it is physically separated. If the body is taken out in a space craft, the mass will not change but its weight may change due to the change in gravitational force. The body may even become weightless when gravitational force vanishes but the mass remain the same.

2) Weight (w)

Weight of a body is the force with which the body is attracted towards the centre of the earth. The weight of the body is equal to the product of mass and the acceleration due to gravity. This quantity of a body varies from place to place on the surface of the earth.

Mathematically,

w=mg

Where ‘w’ is the weight of the body, ‘m’ is the mass of the body and ‘g’ is the acceleration due to gravity.

Table 1 Difference between Mass and Weight

Mass	Weight
Mass is the total quantity of matter contained in a body.	Weight of a body is the force with which the body is attracted towards the centre of the earth.
Mass is a scalar quantity, because it has only magnitude and no direction.	Weight is a vector quantity, because it has both magnitude and direction.
Mass of a body remains the same at all places. Mass of a body will be the same whether the body is taken to the centre of the earth or to the moon.	Weight of body varies from place to place due to variation of ‘g’ (i.e., acceleration due to gravity.
Mass resists motion in a body.	Weight produces motion in a body.
Mass of a body can never be zero.	Weight of a body can be zero.
Using an ordinary balance (beam balance), the mass can be determined.	Using a spring balance, the weight of the body can be measured.
The SI unit of the mass is the kilogram (kg).	The SI unit of the weight is Newton (N).

3) Time

The time is the measure of succession of events. The successive event selected is the rotation of earth about its own axis and this is called a day. To have convenient units for various activities, a day is divided into 24 hours, an hour into 60 minutes and a minute into 60 seconds. Clocks are the instruments developed to measure time. To overcome difficulties due to irregularities in the earth’s rotation, the unit of time is taken as second, which is defined as the duration of 9192631770 period of radiation of the cesium-133 atom.

4) Space

The geometric region in which study of body is involved is called space. A point in the space may be referred with respect to a predetermined point by a set of linear and angular measurements. The reference point is called the origin and the set of measurements as coordinates. If the coordinates involved are only in mutually perpendicular directions, they are known as cartesian coordination. If the coordinates involve angles as well as the distances, it is termed as Polar Coordinate System.

5) Length

It is a concept to measure linear distances. Meter is the unit of length. However depending upon the sizes involved micro, milli or kilo meter units are used for measurements. A meter is defined as length of the standard bar of platinum-iridium kept at the International Bureau of weights and measures. To overcome the difficulties of accessibility and reproduction now meter is defined as 1690763.73 wavelength of krypton-86 atom.

5) Continuum

A body consists of several matters. It is a well known fact that each particle can be subdivided into molecules, atoms and electrons. It is not possible to solve any engineering problem by treating a body as conglomeration of such discrete particles. The body is assumed to be a continuous distribution of matter. In other words the body is treated as continuum.

6) Particle

A particle may be defined as an object which has only mass and no size. Theoretically speaking, such a body cannot exist. However in dealing with problems involving distances considerably larger compared to the size of the body, the body may be treated as a particle, without sacrificing accuracy.

For example:

• A bomber aeroplane is a particle for a gunner operating from the ground.
• A ship in mid sea is a particle in the study of its relative motion from a control tower.
• In the study of movement of the earth in celestial sphere, earth is treated as a particle.

7) Rigid Body

A body is said to be rigid, if the relative positions of any two particles do not change under the action of the forces acting on it i.e., the distances between different points of the body remain constant. No body is perfectly rigid. Rigid body is ideal body.

Fig. 1 Rigid Body due to the action force F

8) Deformable Body

When a body deforms due to a force or a torque it is said deformable body. Material generates stresses against deformation. All bodies are more or less elastic.

Fig. 2 Deformable Body due to the action force F

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Force and System of Forces

Force is that which changes or tends to change the state of rest of uniform motion of a body along a straight line. It may also deform a body by changing its dimensions. The force may be broadly defined as an agent which produces or tends to produce, destroys or tends to destroy motion. It has a magnitude and direction.

Mathematically,

                                                            Force = Mass× Acceleration

                                                                   F = m a

Where, F - Force

            m - Mass

             a - Acceleration

Characteristics of Force

1) Magnitude: Magnitude of force indicates the amount of force (expressed as N or kN) that will be exerted on another body

2) Direction: The direction in which it acts

3) Nature: The nature of force may be tensile or compressive

4) Point of Application: The point at which the force acts on the body is called point of application

Units of Force

1) In C.GS. System

In this system, there are two units of force: (i) Dyne and (ii) Gram force (gmf). Dyne is the absolute unit of force in the C.G.S. system. One dyne is that force which acting on a mass of one gram produces in it an acceleration of one centimeter per second².

2) In M.K.S. System

In this system, unit of force is kilogram force (kgf). One kilogram force is that force which acting on a mass of one kilogram produces in it an acceleration of 9.81 m/ sec².

3) In S.I. Unit

In this system, unit of force is Newton (N). One Newton is that force which acting on a mass of one kilogram produces in it an acceleration of one m /sec².

                                                                1 Newton = 10⁵ Dyne

Effect of Force

A force may produce the following effects in a body, on which it acts.

1. It may change the motion of a body. i.e. if a body is at rest, the force may start its motion and if the body is already in motion, the force may accelerate or decelerate it.
2. It may retard the forces, already acting on a body, thus bringing it to rest or in equilibrium.
3. It may give rise to the internal stresses in the body, on which it acts.
4. A force can change the direction of a moving object.
5. A force can change the shape and size of an object

Principle of Physical Independence of Forces

It states, “If a number of forces are simultaneously acting on a particle, then the resultant of these forces will have the same effect as produced by all the forces.”

System of Forces

When two or more forces act on a body, they are called to form a system of forces. Force system is basically classified into the following types.

1) Coplanar Forces

The forces, whose lines of action lie on the same plane, are known as coplanar forces.

Fig. 1 Coplanar Forces

2) Collinear Forces

The forces, whose lines of action lie on the same line, are known as collinear forces.

Fig. 2 Collinear Forces

3) Concurrent Forces

The forces, which meet at one point, are known as concurrent forces. The concurrent forces may or may not be collinear.

Fig. 3 Concurrent Forces

4) Coplanar Concurrent Forces

The forces, which meet at one point and their line of action also lay on the same plane, are known as coplanar concurrent forces.

Fig. 4 Coplanar Concurrent Forces

5) Coplanar Non-Concurrent Forces

The forces, which do not meet at one point, but their lines of action lie on the same plane, are known as coplanar non-concurrent forces.

Fig. 5 Coplanar Non-Concurrent  Forces

6) Non-Coplanar Concurrent Forces

The forces, which meet at one point, but their lines of action do not lie on the same plane, are known as non-coplanar concurrent forces.

Fig. 6 Non-Coplanar Concurrent  Forces

7) Non-Coplanar Non-Concurrent Forces

The forces, which do not meet at one point and their lines of action do not lie on the same plane, are called non-coplanar non-concurrent forces.

Fig. 7 Non-Coplanar Non-Concurrent  Forces

8) Parallel Forces

The forces, whose lines of action are parallel to each other, are known as parallel forces.

Fig. 8 Parallel Forces

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Workability of Fresh Concrete

Workability of concrete is defined in ASTM C125 as “the property determining the effort required to manipulate a freshly mixed quantity of concrete with minimum loss of homogeneity (uniform)”. The term manipulate includes the early age operations of placing, compacting and finishing. Another definition of workability of fresh concrete is “the amount of mechanical work or energy, required to produce full compaction of the concrete without segregation.” Road Research laboratory, U.K., who has extensively studied the field of compaction and workability, defined workability as “the property of concrete which determines the amount of useful internal work necessary to produce full compaction.” Another definition is that the “ease with which concrete can be compacted hundred per cent having regard to mode of compaction and place of deposition.”

Workability is a parameter in which a mix designer is required to specify in the mix design process, with full understanding of the type of work, distance of transport, loss of slump, method of placing and many other parameters involved. Assumption of right workability with proper understanding backed by experience will make the concreting operation economical and durable. The effort required to place a concrete mixture is determined largely by the overall work needed to initiate and maintain flow, which depends on the rheological properties of the cement paste and the internal friction between the aggregate particles. Workability is completely depending upon the properties and quantity of various ingredients of concrete. The properties of fresh concrete affect the choices of handling, consolidation and construction sequence. They may also affect the properties of the hardened concrete.

The properties of fresh concrete are short term requirements in nature and should satisfy the following requirements.

• It must be easily mixed and transported.
• It must be uniform throughout a given batch and between batches.
• It must keep its fluidity during the transportation period.
• It should have flow properties such that it is capable of completely filling the forms.
• It must have the ability to be fully compacted without segregation.
• It must set in a reasonable period of time.
• It must be capable of being finished properly, either against the forms or by means of trowel or other surface treatment.

Workability of fresh concrete consists of two aspects: consistency and cohesiveness. Consistency describes how easily fresh concrete flows, while cohesiveness describes the ability of fresh concrete to hold all the ingredients together uniformly. Traditionally, consistency can be measured by a slump cone test, the compaction factor or a ball penetration compaction factor test as a simple index for fluidity of fresh concrete. Cohesiveness can be characterized by a Vee-Bee test as an index of both the water holding capacity (the opposite of bleeding) and the coarse aggregate holding capacity (the opposite of segregation) of a plastic concrete mixture. The flowability of fresh concrete influences the effort required to compact concrete. The easier the flow, the less work is needed for compaction. A liquid like self compacting concrete can completely eliminate the need for compaction. However, such a concrete has to be cohesive enough to hold all the constituents, especially the coarse aggregates in a uniform distribution during the process of placing.

A concrete which has high consistency and which is more mobile, need not be of right workability for a particular job. Every job requires a particular workability. A concrete which is considered workable for mass concrete foundation is not workable for concrete to be used in roof construction. Concrete, which is considered workable when vibrator is used, is not workable when concrete is to be compacted by hand. Similarly a concrete considered workable when used in thick section is not workable when required to be used in thin sections. Therefore, the word workability assumes full significance of the type of work, thickness of section, extent of reinforcement and mode of compaction. Workability is not a fundamental property of concrete and it must be related to the type of construction and methods of placing, compacting and finishing.

Hundred per cent compaction of concrete is an important parameter for contributing to the maximum strength. Lack of compaction will result in air voids whose damaging effect on strength and durability is equally or more predominant than the presence of capillary cavities. To enable the concrete to be fully compacted with given efforts, normally a higher water/cement ratio than that calculated by theoretical considerations may be required. That is to say the function of water is also to lubricate the concrete so that the concrete can be compacted with specified effort forthcoming at the site of work. Compaction plays an important role in ensuring the long term properties of the hardened concrete, as proper compaction is vital in removing air from concrete and in achieving a dense concrete structure. Subsequently, the compressive strength of concrete can increase with an increase in the density. Traditionally, compaction is carried out using a vibrator. Nowadays, the newly developed self compacting concrete can reach a dense structure by its self weight without any vibration.

Factors Affecting Workability

The factors helping concrete to have more lubricating effect to reduce internal friction for helping easy compaction are given below.

1) Water Content and Water-Cement Ratio

Water-cement ratio is one of the most important factors which influence the concrete workability. Generally, a water cement ratio of 0.45 to 0.6 is used for good workable concrete without the use of any admixture. Higher the water/cement ratio, higher will be the water content per volume of concrete and concrete will be more workable. Higher water-cement ratio is generally used for manual concrete mixing to make the mixing process easier. For machine mixing, the water/cement ratio can be reduced. This generalized method of using water content per volume of concrete is used only for nominal mixes. For designed mix concrete, the strength and durability of concrete is of utmost importance and hence water cement ratio is mentioned with the design. Generally designed concrete uses low water-cement ratio so that desired strength and durability of concrete can be achieved.

2) Mix Proportions

Aggregate-cement ratio is an important factor influencing workability. Higher the aggregate-cement ratio, the leaner is the concrete. In lean concrete, less quantity of paste is available for providing lubrication, per unit surface area of aggregate and hence the mobility of aggregate is restrained. On the other hand, in case of rich concrete with lower aggregate-cement ratio, more paste is available to make the mix cohesive and fatty to give better workability. The more cement is used, concrete becomes richer and aggregates will have proper lubrication for easy mobility or flow of aggregates. The low quantity of cement with respect to aggregates will make the less paste available for aggregates and mobility of aggregates is restrained.

3) Size of Aggregate

The bigger the size of the aggregate, the less is the surface area and hence less amount of water is required for wetting the surface and less matrix or paste is required for lubricating the surface to reduce internal friction. For a given quantity of water and paste, bigger size of aggregates will give higher workability. Surface area of aggregates depends on the size of aggregates. For a unit volume of aggregates with large size, the surface area is less compared to same volume of aggregates with small sizes. When the surface area increases, the requirement of cement quantity also increases to cover up the entire surface of aggregates with paste. This will make more use of water to lubricate each aggregate. Hence, lower sizes of aggregates with same water content are less workable than the large size aggregates.

4) Shape of Aggregates

The shape of aggregates influences workability. Angular, elongated or flaky aggregate makes the concrete very harsh when compared to rounded aggregates or cubical shaped aggregates. Contribution to better workability of rounded aggregate will come from the fact that for the given volume or weight it will have less surface area and less voids than angular or flaky aggregate. Being round in shape, the frictional resistance is also greatly reduced. The river sand and gravel provide greater workability to concrete than crushed sand and aggregate.

5) Surface Texture

The influence of surface texture on workability is due to the total surface area of rough textured aggregate is more than the surface area of smooth rounded aggregate of same volume. It can be seen that rough textured aggregate will show poor workability and smooth or glassy textured aggregate will give better workability. A reduction of inter particle frictional resistance offered by smooth aggregates also contributes to higher workability.

6) Grading of Aggregates

This is one of the factors which will have maximum influence on workability. A well graded aggregate is the one which has least amount of voids in a given volume. Other factors being constant, when the total voids are less, excess paste is available to give better lubricating effect. With excess amount of paste, the mixture becomes cohesive and fatty which prevents segregation of particles. Aggregate particles will slide past each other with the least amount of compacting efforts. Well graded aggregates have all sizes in required percentages and low water cement ratio is sufficient for properly graded aggregates.

7) Use of Admixtures

There are many types of admixtures used in concrete for enhancing its properties. There are some workability enhancer admixtures such as plasticizers and superplasticizers which increase the workability of concrete even with low water-cement ratio. They are also called as water reducing concrete admixtures. They reduce the quantity of water required for same value of slump. Air entraining concrete admixtures is used in concrete to increase its workability. This admixture reduces the friction between aggregates by the use of small air bubbles which acts as the ball bearings between the aggregate particles. Similarly, the fine glassy pozzolanic materials, increases the surface area and offer better lubricating effects for giving better workability.

8) Cement Content of Concrete

Cement content affects the workability of concrete in good measure. More the quantity of cement, the more will be the paste available to coat the surface of aggregates and fill the voids between them. This will help to reduce the friction between aggregates and smooth movement of aggregates during mixing, transporting, placing and compacting of concrete. Also, for a given water-cement ratio, the increase in the cement content will also increase the water content per unit volume of concrete increasing the workability of concrete. Thus, increase in cement content of concrete also increases the workability of concrete.

9) Ambient Temperature

In hot weather, if temperature increases, the evaporation rate of mixing water also increases and hence fluid viscosity increases. This phenomenon affects the flowability of concrete and due to fast hydration of concrete; it will gain strength earlier which decreases the workability of fresh concrete.

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