Clause 10.1 codifies the strong-column-weak-beam (SCWB) principle, which is the cornerstone of earthquake-resistant design. At every beam-column joint, the sum of the moment capacities of the columns framing into the joint must be at least 1.1 times the sum of the moment capacities of the beams framing into the joint. This ensures that plastic hinges form in beams rather than columns, maintaining the gravity-load-carrying capacity of the frame during an earthquake.
Key Requirements
•At every beam-column joint, ΣMc ≥ 1.1 × ΣMb where ΣMc is the sum of moment capacities of columns and ΣMb is the sum of moment capacities of beams framing into the joint
•Column moment capacity shall be calculated for the factored axial force that results in the minimum moment capacity (typically the lowest axial load case)
•Beam moment capacity shall be calculated using the actual reinforcement provided, considering both sagging and hogging capacities as appropriate for the direction of seismic loading
•This check shall be satisfied for both directions of seismic loading (both sway directions) at every joint
•If the SCWB condition is not satisfied, the column shall be redesigned with increased section or reinforcement
Formulas
ΣMc ≥ 1.1 × ΣMb
Strong column – weak beam check at every beam-column joint
ΣMc = Sum of moment capacities of columns above and below the joint, calculated at the face of the joint, for the factored axial force giving minimum moment capacityΣMb = Sum of moment capacities of beams on either side of the joint at the face of the joint (hogging on one side + sagging on the other, as applicable for the direction of seismic loading)
Practical Notes
✓The 1.1 factor is an overstrength factor accounting for strain hardening and material variability. It ensures a real margin exists between column and beam capacities, not just a theoretical equality.
✓The check is at the JOINT face, not at midspan or at the column centre. Use the moment capacity at the face, considering the actual reinforcement and the axial load on the column at that joint.
✓In practice, this clause often governs the column size — if beams are heavily reinforced, columns must be enlarged to satisfy SCWB, which can cascade through the design.
Common Mistakes
⚠Using the maximum axial load (gravity-dominated) for the column capacity check — you should use the axial load that gives the MINIMUM column moment capacity, which is usually the lowest axial load case (earthquake uplift case).
⚠Checking SCWB in only one direction of seismic loading — it must be satisfied for both principal directions at every joint.
⚠Ignoring the SCWB check entirely for interior joints where beams frame in from both sides — the check is required at every joint without exception.