What is Structural Engineering? All the Basics You Should Know

Structural Engineering is a branch of civil engineering, which deals with the design and analysis of structures for their strength and stability of the structures. Thus, it can be said that the structural engineering involves designing of bones and muscles of the structure forming a framework for the structure.

Structural engineering involves calculation of stability, strength, rigidity, earthquake resistance of the structure.

Structure- Basic Meaning of the Term

In general, structure means a building or an object consisting of numerous parts. In other words, structure is a system built for specific purpose. For instance

Building is a structure solving the purpose of habitation
Bridge is a structure solving the purpose of transportation
Dam is a structure fulfilling the purpose of storage, flood control, etc

Technically, structure is the framework of a number of members/elements connected together at joints or supports to make the structure functional i.e. help in fulfilling the purpose for which the structure is designed. The elements are so placed and connected in design that they can transmit the loads coming on it to the lowermost part of the structure, foundation, safely without considerable deflection.

Structure and the Surroundings

Structural design is dependent on various environmental constraints, which include the following:

Sound and structure
Natural light

Sound and structure

Dome– Concentrates the sound
Dish roof– Diffuses the sound

Natural light

Flat roof– Lacks sufficient lighting
Bearing and shear wall– Blocks the opening for daylight
Frame design– Allows more light

Structural Engineering

Structural engineering requires the pre-requisite of the following concepts:

Applied Mechanics
Material Science
Applied Mathematics

Besides these, a good knowledge of design codes, techniques of structural analysis and corrosion resistance of the structure is also required.

Concept of Structural Engineering

In structural engineering design of following structures is provided:

Architectural structures
Civil structures
Aerospace structures
Mechanical structures
Naval structures

Architectural structures– Design of buildings, commercial complexes, factories, houses is provided by a structural engineer with the co-operation of an architect.

Civil structures– These structures are designed by a structural engineer along with the consultation of transportation, hydraulic, nuclear and other engineers.

Bridge, Road, Railways
Dam/ Reservoirs
Pipelines
Offshore structures
Tunnels
Waterways
Power stations
Retaining structures
Water and waste water infrastructure

Aerospace structures– These structures require structural engineer to verify its structural safety. In the design of aerospace structures, thin plates are provided stiffeners on external surface to increase its strength. Frames provide support to shape. Fasteners like weld, rivet, screw, and bolts are used at joints of the members.

Spacecraft
Aeroplane

Mechanical structures– These structures require structural engineer to verify its structural safety.

Boilers
Pressure vessels
Crane
Elevators and Escalators
Marine vessels

Naval structures– These structures require structural engineer to verify its structural safety.

Submarine

Structural Engineering in Daily Life

The concepts of structural engineering are not only limited to engineering applications but also apply to our daily life instances.

You all might have seen the aluminium container for carrying food parcels. Do you find vertical ribs on its surface? These ribs are not for aesthetic purpose but they impart stiffness to the container, which prevents the bending of the container and makes it easy to carry.

Similarly, in reinforcement steel, ribs are provided so that it can bond well with the concrete as its surface area increases.

Let us take another example. You all might have played game of building blocks. If you keep on placing blocks in a straight line, then there is tendency of after a certain height the blocks will fall off.

Similarly, slender members of a building have a higher chance of failure due to collapse. A firm base is required to prevent the collapse of slender members.

Structural Elements/ Structural Members

You have understood the meaning of structure earlier wherein structure is said to comprise different parts attached with each other at joints and transferring the loads. These parts are known as structural members or elements. Thus, different type of elements form a structure and these elements keep the structure functional as well.

Type of Structural Members

Different types of structural members are enlisted below:

Column
Beam
Plate
Slab
Arch
Truss
Frame

1. Column

It is a vertical member, which carries load from the beam or slab to the foundation. Column can transfer only axial force or bending force.

The design guidelines for RCC column can be found in IS: 456 – 2000 while the design of steel column can be found in IS: 800 – 2000.

2. Beam

Beam is a horizontal member and transfers the transverse load from slab to column.

It resists the lateral load and as a result, shear force and bending moment are produced in the beam.

3. Plate

A plate is a structure that spans in two directions. Slab is an example of plate structure.

4. Slab

Slab rests on the framework of beam in frame structure or on walls in case of load bearing structure.

It transmits the load to beam.

5. Arch

An arch is generally constructed from masonry. It can carry compression force only and that too in only one direction.

Care should be ascertained that the line of thrust remains within the depth of arch.

6. Truss

Truss is formed of members connecting axially as hinge joints.

The members can only transfer axial forces of compression or tension. As all the joints in truss are hinge joints, a truss cannot resist moment.

Usage of truss is favoured for large span structures where provision of solid beams may prove uneconomical.

7. Frame

Frame is a network formed of beams and columns. It makes the construction economical.

Loads that can Possibly Act on a Structure

Different types of loads act on a structure during its lifetime. If the structure does not possess enough strength to resist these loads, then the structure might fail.

Different classifications for the loads acting on a structure are given below:

Based on Origin/Source

Dead load
Self weight
Imposed load
Wind load
Earthquake load
Erection load
Temperature stress

Based on Direction of Action

Vertical Load
Horizontal Load
Longitudinal Load

Based on Mode of Action

Concentrated Load
Uniformly Distributed Load
Uniformly Varying Load

Based on Origin/Source

1. Dead Load

The weight of all the permanent constructions in the structure is taken as dead load. Self weight of the structure is also included in the dead load most of the times. Weight of floors, walls, beams, slabs and other elements of the structure is considered as dead load.

Following loads are included in dead load:

Floor
Column
Beam
Roof
Balcony
Wall
Lintel
Chhajja
Partition wall
Footing

Calculation of DL: Dead load can be calculated by calculating the quantity of the material and then multiplying it by its unit weight.

Dead load of a material can be calculated using the following formula:

D. L. = Quantity of Material * Unit Weight of Material

The unit weight of structural materials are provided in IS : 875 (Part I) – 1987.

2. Self-Weight

Self weight of the structure is often considered under dead load.

The self weight of the structural element is calculated first. For this, its volume is calculated with the help of its dimensions and is then multiplied by the unit weight of the material used for the construction of the respective structural member.

3. Imposed Load

Following types of loads are included in imposed loads-

Live load
Crane load
Snow load
Dust load
Wave load
Earth pressure load
Impact load

A. Live load

The loads changing their position with respect to time are termed as live loads.

Examples of Live Load:

Weight of People
Car
Furniture
Moving partitions

Live load is one of the crucial factors to be considered during the design of elements. The minimum value of live loads for different types of buildings is listed in IS : 875 (Part II) – 1987.

Live load depends on the type of building like residential building, assembly building, industrial building, etc.

Note that the imposed load acting at the same time is very rare in case multi-storeyed building. For this reason, the IL is reduced as the number of floors increases.

B. Crane load

The load of crane and other machineries acting on the structure are included in crane load.

The value of the crane load is taken as mention by the suppliers’ data.

Following crane loads are included in imposed load:

Vertical loads
Eccentricity effect happening because of vertical loads
Impact loads
Lateral and longitudinal braking factors

C. Snow load

Snow load is to be considered in the areas susceptible to snowfall. The snow load acts in vertically downward direction and is expressed as force per unit area.

The snow load is dependent on the following factors:

Shape of the roof
Capacity of roof to hold the snow

D. Dust load

Dust load is taken into consideration when there is a possibility of accumulation of dust on the roof for structures like cement plant, steel plant, etc.

The dust load is taken equivalent to the probable thickness of dust accumulated.

E. Wave load

Wave load is a random load taken into consideration for the offshore structures. A specialist is to be consulted to determine these loads.

For bridge piers, abutment, and waterfront structures, force exerted by the water current is also considered.

F. Earth pressure load

Earth pressure load is the pressure exerted by the soil or wter or both to the structures partly or fully below ground level.

Earth pressure load is considered for following structures:

Retaining wall
Basement walls
Bridge abutments
Underground water tank
Culvert

Note that the structures below ground level should be checked against overturning and sliding as well.

G. Impact load

The structures on which moving load is acting upon are subjected to impact load.

Empirical formula is used to convert the impact loads caused by vertical crane loads into static loads with the help of impact factor. Impact factor is normally a percentage of crane loads. Impact factors for different structures are given in IS : 875 (Part II) – 1987.

4. Wind Load

The horizontal component of the wind is taken into consideration for the design of tall structures.

The wind load is dependent on the velocity of the wind in the area where the structure is located and shape and size of structure.

If wind pressure is not considered while design of tall structure, then bending or tilting of structure may take place.

Design wind pressure is calculated for wind load with the help of the following formula:

Design wind pressure = 0.6 * square of basic wind speed

The procedure of complete calculation of the wind load is given in IS: 875 (Part III) – 1987.

5. Earthquake Load

Earthquake load is considered for the structure situated in earthquake prone areas. The earthquake causes movement of ground, which causes movement in the foundation of the structures.

The earthquake load causes movement in two horizontally perpendicular directions and vertical direction too. During earthquake, the load direction is reversed. Therefore, the structural members designed primarily to carry the vertically downward load are suddenly exposed upward loads too making them unstable.

The lateral forces of earthquake are calculated by the following methods as per IS: 1893 – 2002:

Seismic coefficient method
Response spectrum method

6. Erection Load

Erection loads are required to be carried by the structure or any part of structure during the time of erection of the structure. These may be due to storage of construction materials as well as erection equipment. The loads induced due to operation of the equipment are also included in erection loads.

Erection loads affect the design of some structures like cantilever bridges, cable structures, etc. in these structures, failures are observed during the construction rather than the post-construction failure. Therefore, the strength should be enhanced during construction of these structures by bracing on temporary basis.

Erection loads are not mentioned directly in any of the building codes. Along with erection loads, dead load, wind load, and imposed load also need to be considered at the time of erection.

7. Temperature stress

The ends of the members are restrained. Steel members undergo expansion or contraction under temperature and produce loading effects and thus temperature stresses are induced in the member.

Structures like bridges, microwave towers, transmission line towers, etc. are some of the structures where temperature stresses are considered. These need to be considered for the areas where large temperature differences are observed in a short period of time. The data for maximum and minimum air pressure across India is given in IS: 875 (Part IV) – 1987.

Based on Direction of Action

1. Vertical Load

The loads acting in the vertical direction are grouped under vertical loads. These loads are also called gravity loads as they act in the direction of gravity.

Following loads are included in vertical load:

Dead load
Imposed load
Impact load

2. Horizontal Load

The loads acting laterally on the structure are horizontal loads as they act in horizontal direction.

Wind load is example of horizontal load.

3. Longitudinal Load

Moving loads are included in longitudinal loads.

Based on Mode of Action

1. Concentrated Load

The load that acts at a particular point of a structural member is called concentrated load. It is also known as point load.

2. Uniformly Distributed Load

The load that acts on the complete span of structural member at a uniform rate is called UDL.

The load is distributed across the whole member and the load at each point throughout the member is uniform in case of UDL.

3. Uniformly Varying Load

The load that acts on the complete span of structural member at a uniformly varying rate is called UVL.

The load is distributed across the whole member. However, the load at each point throughout the member is not uniform in case of UVL.

Classification of Structures

Classification of structures on different bases is given below:

Based on Function

Building
Dam
Bridge
Cable ways
Chimneys
Industrial Sheds

Based on Load Transfer

Beam
Column
Floor Slab
Arch
Shell
Truss
Frame
Footing

Based on Analysis Perspective

One-Dimensional Structure: The structure transferring loads in one direction only are called 1-D structures. For instance, Beam, column, arch, rope, cable
Two-Dimensional Structure: The structure transferring loads in two directions are called 2-D structures. For instance, Slab, dome, plate
Three-Dimensional Structure: The structure transferring loads in three directions are called 3-D structures. For instance, Solid masses

Based on Static Determinacy of the structure

Statically determinate structure: The structure whose reactions at support and internal forces can be found out by the basic conditions of the static equilibrium.
Statically indeterminate structures: The structure whose reactions at support and internal forces cannot be found out by the basic conditions of the static equilibrium alone.

Based on Kinematic Determinacy of the structure

Kinematically determinate structure: The structure whose displacement components of joints can be found out by the compatibility equations alone.
Kinematically indeterminate structures: The structure whose displacement components of joints cannot be found out by the compatibility equations alone.

Structural Material

There are many materials used as a structural material in construction. The most common structural materials are as follows:

Structural steel
Concrete

1. Structural Steel

Structural steel is an alloy of iron and carbon. Sometimes other elements like nickel, silicon, copper, etc are added to alter its properties.

As concrete is weak in tension, steel is used to carry the tensile loads in a structure. Density of steel is around 490 lb/cu ft and coefficient of thermal expansion is 0.65 * 10^-5 /° F.

As the coefficient of thermal expansion of steel is almost similar to that of the concrete, it is the best-suited material that can be used as a reinforcement embedding it in concrete. If the coefficient of thermal expansion differs by huge amount, then there is unequal expansion or contraction when exposed to temperature variation and consecutively, thermal stresses are produced in the structure.

Reinforcement steel is available in the form of bars.

MS Bars– Mild Steel bars do not have ribbed surface.
HYSD bars
Torsion bars

2. Concrete

Concrete is generally used as construction material except for steel structures. To construct the structure with concrete, process of concreting should be known. You can know the steps involved in concreting process wherein they are described in detail.

The materials used in construction are all explained here- Building Materials used in Construction.

Equilibrium of the Structure

The concept of equilibrium is the most basic theory with respect to static structures.

A body is said to be in equilibrium if the resultant of all the forces acting on it is equal to zero. If a body is stationary, then it is under equilibrium and the conditions equations of equilibrium can be applied to compute an unknown force.

Equations for Equilibrium of Structure/ Basic Conditions of Static Equilibrium

The equations of equilibrium help in obtaining the internal and external forces acting on a body.

For 2-D Structures: There are three mathematical equations for a 2-D body to be in equilibrium as follows:

$\sum F_{x}=0$
$\sum F_{y}=0$
$\sum F_{z}=0$

For 3-D Structures: There are six mathematical equations for a 3-D body to be in equilibrium as follows:

$\sum F_{x}=0$
$\sum F_{y}=0$
$\sum F_{z}=0$
$\sum M_{x}=0$
$\sum M_{y}=0$
$\sum M_{z}=0$

Deflection of the Structure

Upon the action of applied loads, each member of the structure undergoes deformation. This results into the deflection of the axis from its original position. Besides applied load, deflection may also occur due to temperature variations.

Deflection may cause cracks in non-structural members sue to large deformations.

The compute the deflection in a structural element under particular loading condition, many methods are there.

The criteria for allowable deflection in structural members are given in IS: 456 – 2000.

Structural Design

Structural design includes the provision of the following details for a structure:

Type of material for the structural member
Dimensions of the structural member

The type of material is checked for its yield point and modulus of elasticity.

The dimensions of the structural member are provided after considering various requirements like

Deflection limit
Strength requirement
Check against shear

Structural Analysis

Structural analysis is the determination of strength, stiffness, and stability of the structure taken into consideration to ensure that the structure does not fail under loading conditions.

For analysis of structure, strength and stiffness of structure is calculated from the dimensions and the type of material of the structure.

Following checks should be observed during analysis:

Strength: Stresses in the member should not be more than critical values
Stiffness: Deflection in the structure should not exceed the critical value
Stability: Buckling and cracking in the structure should not occur

Key Takeaway

Structural engineering is a field of engineering which deals with the design and analysis of structures by computing probable loads on the structure throughout its lifetime.

A structure is formed of several structural members like beam, column, slab, arch, truss, frame, etc. These members transmit load coming onto the structure until it is dissipated in the surrounding soil.

The dimensions of the structure are computed during the design while analysis gives the amount of load a structure can withhold without failure.

There are different types of loads acting on a structure- dead load, live load, imposed load comprise gravity load acting in vertically downward direction while wind load acts in lateral direction. In addition, the load can act in the form of concentrated/point load, UDL or UVL.

The internal forces and support reactions can be found by equation of equilibrium only in case of statically determinate structures. The displacement component of joints can be found by compatibility equations only in case of kinematically determinate structures.

When the resultant of all forces acting on a body is equal to zero, the body is said to be in equilibrium. In such condition, the summation of forces acting on it in each direction and the summation of moment acting on it in each direction is zero.

FAQ

What is Structural Engineering?