Introduction to Steel Structures

 A Steel Structure consists of members which either sustain axial, flexural, or shear stresses. The axial stresses can be compressive or tensile, and accordingly, the members are classified as Tension Members or Compression Members. The Flexural members bear Bending Stresses. The example of a Compression Member is a Strut or a Column / Post, and that of a Tension Member is a Tie. A Beam is a Flexural Member. A PEB can be either Framed Structure or a Truss. The Members of Framed Structures typically bear Flexure, Shear, and Axial Stresses, but in the case of Trusses, the members bear only Axial and Shear Stresses. This difference in the type of stress arises because of the type of Joint. The Trussed Structure has Pinned Joints, which result in only Axial and Shear Stresses Transmission, while Framed Structures have Rigid Joints which result in the transmission of all types of Stresses. The point to be noted here is that in reality, a Beam designated as a Flexural Member may have to resist the axial forces also, which may arise due to Temperature Stress. The members here are classified according to the principle load transmitted by them that is to say, the main aim of a beam for example is to sustain the dead loads and live loads which result in flexure, similarly to others.

Steel Structures are mainly Classified into two types:-

1) Sheet or Plate Structures, these include Chimneys, Bins or Silos, etc.

2) Framed Structures, these include Pre-Engineered buildings, Trusses, Girders, etc.

Advantages of Structural Steel:-

1) Steel has high Strength to Weight Ratio, making it a good fit for High Rise Buildings, Bridges, etc.

2) Steel is a Ductile Material, which does not allow it to fail suddenly.

3) Steel is Tough, i.e. it will not fracture easily.

4) Proper maintenance of Steel leads to long life.

5) Steel mostly does not have a loss of properties with time.

6) It is possible to add or alter steel structures easily.

7) It is possible to erect steel at a faster rate.

8) It has got highest scrap value and can be reused.

Disadvantages of Structural Steel:-

1) Frequent maintenance is required, otherwise, it may cause rusting.

2) It needs fireproof treatment, which increases its cost.

3) Steel catches fatigue easily, that is on reversal of stresses or cyclic loads there is a reduction of strength.

4) Steel may lose ductility under high-stress points.

Properties of Structural Steel:-

1) Modulus of Elasticity (E) = 2 x 10⁵ N/mm²

2) Shear Modulus (G) = 0.769 x 10⁵ N/mm²

3) Poisson’s Ratio (μ) :-

(i) elastic range = 0.3

(ii) plastic range = 0.5

4) Coefficient of Thermal Expansion = 12 x 10^-6 / ^oC

5) Unit Mass (ρ) = 7.85 x 10³ kg/m³

Rolled Steel Sections:-

Note:-

• All standard I – Beams and channels have a slope of 16 2/3 % on the inside face of the flange.

• Apart from Rolled Steel Sections, there are Compound Sections as well.

• During the manufacture of Structural Steel Members, residual stresses are developed. Though they don't affect the structure as a whole, but may develop some localized plastic areas under external stresses. The hotter part of the member develops tensile stresses and the cooler part develops compressive stresses.

Corrections and suggestions are most welcomed. Put your queries and questions in the comments.

References:-

1) Limit State Design of Steel Structures by S.K. Duggal

2) www.britannica.com/science/axial-stress