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Structural Steel and Classification of Steel Sections

Introduction to Steel Structure

Figure - 01

Steel structures are those structures which are made of steel components connected with each other to sustain and share applied loads with adequate safety and serviceability. As compared to other structures like concrete structure and timber structure in the place where other engineering materials are scarce and structure is supposed to work for the greater magnitude of loads, steel structures are the better alternative. The structural members may be in axial loading, torsion or bending or in their combined effect, depending upon their orientation, structural use, and type of loading. The various types of steel structures are the truss, tower structure, bridge, staircase, etc.

Structural Steel and Classification of Steel Sections
Structural steel can be defined as a steel construction material which is formed with a specific shape or cross section with certain standards of chemical composition and mechanical properties. The main governing factors of structural steel for their choice and use in the structure are shape, size, composition, strength, malleability, ductility, durability, storage, etc. The products of structural steel are available in various shapes and sizes so that the structural engineer could select the suitable sections to suit the requirements as per the design. Depending upon the manufacturing process, the steel sections are classified as hot -rolled section and cold -rolled or cold –formed section.

  • Hot - Rolled Sections
    Figure - 02
    The various products that are made by using this process are plates, strips, shapes and sections (angles, tees, beams, channels), flats and bars. They are classified according to the Bureau of Indian standards as follows;
    • Beams (ISJB, ISLB, ISMB, ISWB)
    • Columns/ Heavy -weight Beams (ISSC, ISHB)
    • Parallel Flange Beam and Column Sections (ISNPB, ISWPB)
    • Channels (ISJC, ISLC, ISMC, ISMCP)
    • Angles (ISA)
    • T -sections (ISNT, ISDT, ISLT, ISMT, ISHT)
    • Tubular sections
    • Rectangular/ Square Hollow Sections
  • Cold - Formed Light -Gauge Section
    Cold-formed light-gauge sections are those which are used where thicker hot -rolled sections become uneconomical, especially in small buildings subjected to lighter loads. These are produced from the steel strips generally not thicker than 8 mm. They are produced by cold rolling for mass production whereas smaller numbers of special shapes are produced on press brakes. As far as availability is concerned, they are available in the forms of equal angles, unequal angles, channels, hot sections and 'z‘ sections. They are designed by numbers denoting dimension in form of depth(mm) x width(mm) x thickness(mm).

Method of Analysis and Design
Structural analysis is necessary to determine the internal forces that are developed in members of the structures. For determining the design forces and moments in a member or connection complying with requirements of the limit stability, strength, and serviceability, section 4 of IS code 800:2007 permits the methods of analysis given below:

  • Elastic analysis
  • Plastic analysis
  • Advanced analysis
  • Dynamic analysis as per IS 1893 (Part 1): 2002 for seismic design.
    Similarly, the method of design consists in idealizing the structure, quantifying expected loads, carrying analysis to find the member forces and sizing the members that are based on possible failure criteria and economy of the material. Working Condition is kept as a fraction of the failure condition since there are limitations in precise modeling the structure.In the order of their evolution, the design philosophies used are listed below:
  • Working stress method (WSM)
  • Ultimate load design (ULD)
  • Limit state design (LSD)

Design Process and Basis for Design
The design process is such a process which decides shape, size and connection details of the members so that the structure that is being designed will perform satisfactorily during its intended life. With an appropriate degree of a safety, structures should have certain basis given below:

  • Sustain all the loads expected on it
  • Sustain deformations during and after the construction
  • Should have an adequate durability
  • Should have an adequate resistance to misuse and fire
  • Structure should be stable and have alternate load paths to prevent overall collapse under an accidental loading.

References
"Design of Steel Structures" K.S. Sai Ram, PEARSON Education
"Design of Steel Structures" Ram Chandra, Standard Book House
"Limit State Design of Steel Structures" S.K. Duggal Tata McGraw-Hill Publishing Com
"Design of Steel Structures" L.S. Negi, Tata McGraw-Hill Publishing Com.
Teachers note
Wikipedia


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