Why steel structures
Structural steel is strong, stiff and ductile, and it is made in factories to tight tolerances. That makes it the natural choice for long-span roofs, industrial sheds, tall towers, railway and road bridges, and buildings that must go up quickly.
Steel's strength-to-weight ratio is high, so foundations are smaller. Members are fabricated off site and bolted or welded together on site, which shortens the construction programme and reduces wet work. Steel frames are easy to modify, extend or dismantle, and the material is recyclable at the end of its life.
Its weaknesses are just as clear. Thin steel parts can buckle locally or overall, so slenderness governs much of steel design. Unprotected steel loses strength in fire, so members need fire protection in buildings. Steel corrodes in damp or coastal air and needs paint systems, galvanising or weathering grades, plus maintenance.
Design in India follows IS 800 using limit states. The same ideas appear in AISC 360, EN 1993, AS 4100 and CSA S16, each with its own resistance factors.
Design tensile strength by yielding of gross section: T_dg = A_g f_y / γ_m0, with γ_m0 = 1.10 in IS 800
Problem. An angle tie has gross area 1 000 mm², f_y = 250 MPa. Find the design strength governed by gross-section yielding.
- T_dg = 1000 × 250 / 1.10 = 227 273 N.
Answer. About 227 kN (net-section rupture and block shear must also be checked)
Quick quiz
1Name two advantages of steel structures.
High strength-to-weight ratio and fast, prefabricated erection.
2Why does steel need fire protection?
It loses strength at high temperature.
3What failure mode makes slenderness important?
Buckling.
- IS 800 (India): General construction in steel: limit state design of steel members and connections.
- AISC 360 (USA): Specification for structural steel buildings.
- EN 1993 (Eurocode 3) (Europe): Design of steel structures.
Clauses and documents to read
- IS 800:2007, cl. 5.4.1 (Table 5): partial safety factors for materials
- IS 800:2007, cl. 6.2: design strength due to yielding of gross section
Cited by number and clause only. We do not reproduce clause text, tables or figures (BIS Act 2016, s.10(5) and s.11). Check the current edition and amendments before use.
References and further reading
Standard texts, listed in no order of merit. We do not copy text, figures, tables or solved problems from any book; our explanations and examples are original.
- N. Subramanian, Design of Steel Structures, Oxford University Press, latest edition.
- C. G. Salmon, J. E. Johnson, F. A. Malhas, Steel Structures: Design and Behavior, Pearson, 5th edition.
- W. T. Segui, Steel Design, Cengage, latest edition.
Free and official sources
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