General Design Requirements | Design of Steel Structures - Civil Engineering (CE) PDF Download

Introduction

The Structural engineering is a branch of engineering which deals with structural analysis and structural design. The structural engineering plays an important role in civil engineering, mechanical engineering, electrical engineering, naval engineering, aeronautical engineering and in all the specialized phases of engineering. The structural analysis deals with the development of suitable arrangement of structural elements for the structures to support the external loads or the various critical combinations of the loads which are likely to act on the structure. The analysis also deals with the determination of internal forces developed in the various members, nature of stresses or critical combination of the stresses at the various points and the external reactions due to the worst possible combination of the loads. The structural design deals with the selection of proper material, proper sizes, proportions and shape of each member and its connecting details. The selection is such that it is economical and safe. The structural design further deals with the preparation of final layout of the structure and the design drawings are necessary for fabrication and construction.

Definition

Construction or framework of structural elements (members) which gives form and stability, and resists stresses and strains. Structures have defined boundaries within which each element is physically or functionally connected to the other elements, and the elements themselves and their interrelationships are taken to be either fixed (permanent) or changing only occasionally or slowly.

Classification of Structures

The structures may be classified as statically determinate structures and statically indeterminate structures. When the equation of statics (ΣH = 0, ΣV = 0 and ΣM = 0) are enough to determine all the forces acting on the structure and in the structures are known as statically determinant structures. When the equation of equilibrium are not sufficient to determine all the forces acting on the structures and in the structure, then the structures are known as statically indeterminate structures. The equations of consistent deformations are added to the equations of equilibrium in order to analyze the statically indeterminate structures.
The structures are also classified as shell structures and framed structures. The shell roof covering of large buildings, air planes, rail road cars, ship wells, tanks etc are the examples of shell structures. The plates or sheets serve functional and structural purposes. The plates act as a load carrying elements. The plates are stiffened by frames which may or may not carry the principal loads. The framed structures are built by assemblies of elongated members. The truss frames, truss girders, rigid frames etc are the examples of framed structures. The main members are used for the transmission of loads.
The structures may be further classified depending on the materials used as plastic structures, aluminium structures, timber structures, R.C.C structures and steel structures.

Advantages of Steel Structures

1. Steel has a high strength and so steel components have smaller sections for the same strength compared to corresponding components of other material.The existing steel structures and structural component may be strengthened by connecting additional sections or plates.
2. Steel members are gas and watertight, because of high density of steel.
3. Steel structures can be fabricated at site easily.
4. Steel structures have great durability and serve for many years.
5. Steel members can be readily disassembled or replaced.
6. The existing steel structures and structural component may be strengthened by connecting additional sections or plates.

Disadvantages of Steel Structures

1. Steel structures are liable to corrosion and need painting frequently.
2. Steel structures have a low fire resistance and are liable to lose their strength and get deformed at high temperature. 

Structural Steel

The structural steel is the steel used for the manufacture of rolled structural steel sections, fastenings and other elements for use in structural steel works. Steel is an alloy of iron, carbon and other elements in varying percentages. The strength, hardness and brittleness of steel increases and ductility of steel decreases with the increase of percentage of carbon. Depending on the chemical composition, the different type of steel are classified as mild steel, medium carbon steel, high carbon steel, low alloy steel and high alloy steel. The mild steel, medium carbon steel and low alloy steel are generally used for steel structures. The copper bearing quality of steel contains small percentage of copper contents. The corrosive resistance of such steel is increased.
Mild steel is used for the manufacture of rolled structural steel sections, rivets and bolts. The following operations can be done easily on mild steel 1.Cutting, 2. Punching, 3.Drilling, 4. Machining, 5. Welding and 6. Forging when heated. All structural steels used in general construction, coming within the purview of IS:800-84 shall, before fabrication, comply with one of the following Indian Standard specifications

1. IS : 226-1975 structural steel (standard quality)
2. IS : 1977-1975 structural steel (ordinary quality)
3. IS : 2062-1984 weldable structural steel
4. IS : 961-1975 structural steel (high tensile)
5. IS : 8500-1977 weldable structural steel (medium and high strength qualities)

1. IS : 226-1975 structural steel (standard quality).
The mild steel is designated as St 44-S for use in structural work. This steel is also available in copper bearing quality in which case it designated as St 44-SC. The copper content is between 0.20 and 0.35 per cent. The physical properties of structural steel are given below:

• Unit weight of steel 78.430 to 79.000 kN/m³
• Young’s modulus of elasticity, E = 2.04 to 2.18 x 10⁵ N/mm²
• Modulus of rigidity, G = 0.84 to 0.98 x 10⁵ N/mm²
• Coefficient of thermal expansion (or contraction) α = 12 x 10^-6/˚C or 6.7 x 10^-6/˚F.

The tensile strength, yield stress and percentage elongation for IS : 226-1975 structural steel standard quality, determined in accordance with IS : 1608-1960. The steel confirming to IS : 226 is suitable for all types of steel structures subjected to static, dynamic and repeated cycles of loadings. It is also suitable for welding up to 20 mm thickness. When the thickness of element is more than 20 mm, it needs special precautions while welding.

2. IS : 1977-1975 structural steel (ordinary quality).
The steel which did not comply with IS : 226, was formerly called as steel of untested quality. The standards for such steel have been laid down in IS : 1977-75 (ordinary quality). There are two grades in this standard which are designated as St 44.0 and St 32.0. The steel St 44.0 is intended to be used for structures not subjected to dynamic loading other than wind loads e.g., platform roofs, office buildings, foot over bridge. The copper bearing quality is designated as St 44.0C.
The steel confirming to IS : 1977 is not suitable for welding and for the structures subjected to high seismic forces (earth quake forces). The steel structures using steel confirming to IS : 1977 must not be analyzed and designed by plastic theory.

3. IS : 2062-1984 weldable structural steel.
This structural steel intended to be used for members in structures subjected to dynamic loading where welding is employed for fabrication and where fatigue and great restraint are involved e.g., crane gantry girder, road and rail bridges etc,. it is designated as St 42-W and copper bearing quality is designated as St 42-WC. It is suitable for welding the elements of thickness between 28 mm and 50 mm. when the thickness of elements is less than 28 mm; it may be welded provided the limiting maximum carbon content is 0.22 per cent.

4. IS : 961-1975 structural steel (high tensile).
The high tensile steel forms a specific class of steel in which enhanced mechanical properties and in most of the cases increased resistance to atmospheric corrosion are obtained by the incorporation of low proportions of one or more alloying elements, besides carbon. These steels are generally intended for application where saving in weight can be effected by reason of their greater strength and atmospheric corrosion resistance. Standards of high tensile steel have been given in IS : 961-1975. It has been classified into two grades designated as St 58-HT and St 55-HTW. St 58-HT is intended for use in structures where fabrication is done by methods other than welding. St 55-HTW is intended for use in structures where welding is employed for fabrication. The high tensile steel is also available in copper bearing quality and two grades are designated as St 58-HTC and St 55-HTWC. The steel conforming to IS : 961 is suitable for bridges and general building construction.

5. IS : 8500-1977 weldable structural steel (medium and high strength qualities)
Various medium and high strength qualities of weldable structural steel are, Fe 440 (HT1 and HT2) Fe 540 (HT, HTA and HTB), Fe 570 HT, Fe 590 HT and Fe 640 HT.

Production of Steel

The steel is produced in the form of ingots and converted to different shapes. In our country, Tata Iron and Steel Company, Indian Iron and Steel Company, Mysore Iron and Steel Company and Hindustan Steel produce steel at their plants

Recent Developments in Material

A number of developments in material such as steel have been made recently. The weldable qualities of steel (IS : 2062) designated as St 42-W and IS : 961 designated as St-55-HTW are developed with the large scale use of welding. IS : 961 has been developed with high tensile strength and there is saving in weight due to enhanced mechanical properties. Its weldable quality is advantageous for composite construction.