Concrete and Steel Design
Concrete and steel design questions test limit-state reasoning under ECP 203 for reinforced concrete and ECP 306 for steel. You must size sections, place reinforcement or select steel sections, and verify flexure, shear, bond, deflection and stability without confusing characteristic strengths, design strengths and service loads.
Core rules and concepts
- Concrete materials: ECP 203 uses cube strength fcu and steel yield strength fy. Design strengths are obtained by dividing characteristic values by partial safety factors; the code's rectangular stress block is the normal basis for flexural design.
- Reinforcement limits: minimum and maximum steel ratios ensure ductility and crack control. A singly reinforced section is preferred; compression steel or a larger section is needed when the neutral axis becomes too deep.
- Shear: shear reinforcement is designed from the applied ultimate shear minus the concrete contribution, with spacing limits and minimum stirrups.
- Steel design: ECP 306 editions have used allowable-stress and limit-state formats, so identify the edition. Check section classification, lateral-torsional buckling, bolt and weld capacities, and connection eccentricity.
- Serviceability: deflection and crack width are limit states, not optional checks.
Worked example
A rectangular RC beam has b = 250 mm, d = 450 mm, fcu = 25 N/mm², fy = 360 N/mm² and Mu = 150 kN·m. Estimate the lever arm as z = 0.9d = 405 mm. Using As ≈ Mu/(0.87fy z): 0.87 × 360 = 313.2 N/mm², so As = 150 × 10⁶/(313.2 × 405) = 1182 mm². This is about 1.05% of bd, which is within normal beam limits, but the exact ECP 203 stress-block check, minimum steel and bar spacing must still be verified. If the required steel exceeded the singly reinforced limit, the section would be resized or compression steel added.
Traps that cost marks
- Using cylinder strength where ECP 203 requires cube strength, or vice versa.
- Designing for service moment instead of factored moment.
- Providing steel area without checking bar spacing, cover and anchorage.
- Treating shear as a nominal check and omitting minimum stirrups.
- In steel design, ignoring the unbraced length or assuming a compact section without classification.
- Mixing ASD and LRFD factors within one calculation.