Concrete and Steel Design
Concrete and Steel Design covers limit-state design to IS 456 and IS 800: flexure, shear, bond, development length, columns, slabs, footings, tension and compression members, bolts, and welds. The exam tests code-based formulas and ductile detailing.
Core rules
- Limit-state design uses partial safety factors: 1.5 for dead and live loads, 1.5 for concrete, and 1.15 for steel.
- For singly reinforced sections, x_u,max/d is 0.53 for Fe250, 0.48 for Fe415, and 0.46 for Fe500.
- M_u,lim = 0.138 f_ck b d² for Fe415; 0.148 for Fe250; 0.133 for Fe500.
- Steel sections are classified as plastic, compact, semi-compact, or slender; plastic design requires compact or plastic sections.
- Bolt grade 4.6 means 400 MPa ultimate strength and a yield ratio of 0.6.
Worked example
An M20, Fe415 beam has b = 230 mm and effective depth d = 400 mm. The limiting moment is M_u,lim = 0.138 f_ck b d² = 0.138 × 20 × 230 × 400² = 101.57 × 10⁶ N·mm ≈ 101.6 kN·m. If the factored moment is below this, the section can be singly reinforced; if above, compression steel or a larger depth is needed. Note that d is the effective depth, not the overall depth.
Traps that cost marks
- Using overall depth instead of effective depth.
- Applying the Fe250 coefficient to Fe415 steel.
- Ignoring lateral-torsional buckling in steel beams.
- Confusing shear plane and bearing area in bolt design, or forgetting block shear.
Exam focus
Memorise the limiting-moment coefficients, the partial safety factors, and the section-classification limits; then check ductility and detailing requirements.