Clause 6.1 sets the dimensional and reinforcement limits for beams in ductile RC frames. Beams must have a minimum width of 200 mm and a width-to-depth ratio not less than 0.3. The minimum longitudinal steel ratio is 0.24√fck/fy on the tension face, ensuring ductile flexural behavior before shear failure.
Key Requirements
•Minimum beam width shall be 200 mm
•Width-to-depth ratio of the beam shall not be less than 0.3
•Minimum longitudinal reinforcement on any face at any section shall be not less than 0.24√(fck)/fy
•Maximum longitudinal reinforcement on any face at any section shall not exceed 2.5%
•Beams shall preferably have a width equal to or less than the width of the column it frames into plus 0.75 times the depth of the column on each side
•The factored axial stress on the member shall not exceed 0.1 fck
Formulas
ρ_min = 0.24 √fck / fy
Minimum tensile reinforcement ratio for beams in ductile frames
ρ_min = Minimum reinforcement ratio (As/bd)fck = Characteristic compressive strength of concrete in MPafy = Yield strength of reinforcing steel in MPa
Practical Notes
✓For M25 concrete with Fe500 steel, ρ_min = 0.24×√25/500 = 0.24%, which is higher than the IS 456 minimum of 0.2% (0.85/fy) — always use the IS 13920 value for seismic beams.
✓The 200 mm minimum width effectively rules out very slender beams — most practical seismic beams are 230 mm or 300 mm wide.
✓The 0.3 width-to-depth ratio means a 200 mm wide beam cannot exceed ~667 mm depth — for deeper beams, increase width proportionally.
Common Mistakes
⚠Using the IS 456 minimum steel ratio (0.85/fy = 0.17% for Fe500) instead of the higher IS 13920 requirement of 0.24√fck/fy.
⚠Designing beams wider than the column they frame into, which creates eccentricity and joint detailing problems.
⚠Ignoring the 0.1 fck axial stress limit — members with higher axial loads must be designed as columns, not beams.