Structural Design of Steel Structures
Domain overview
This section tests limit-state design of steel members and connections: section classification, bending, shear, compression buckling, bolts and welds. The exam expects you to state the limit state and show the controlling resistance, not merely pick a section.
Core rules
- IS 800 design strength in bending is Md = βbZp fy/γm0 for plastic or compact sections; γm0 is commonly 1.1.
- Compression resistance uses buckling class, effective length, slenderness and imperfection factors; local buckling depends on section class.
- Bolt shear capacity includes threads and shear planes; bearing, block shear and prying action may govern.
- Deflection, fatigue and fire are serviceability or special checks, not substitutes for strength checks.
Worked example
A laterally supported beam must carry a factored moment of 150 kN·m. Using fy=250 MPa and γm0=1.1, the required plastic section modulus is Zp,req = Mdγm0/fy = 150×10⁶×1.1/250 = 660,000 mm³ = 660 cm³. Select a section with Zp at least 660 cm³, then verify shear, web bearing, lateral-torsional buckling if restraint is incomplete, and deflection under service loads. If the section is semi-compact, βb is less than 1 and the required Zp increases.
Traps
Using elastic section modulus for a plastic design, forgetting the 1.1 partial factor, assuming full lateral restraint when the compression flange is unrestrained, and checking only bolt shear while ignoring bearing or block shear.