Strong-Column Weak-Beam

Strong-column weak-beam (SCWB) is the capacity-design principle for ductile moment-resisting frames requiring that, at a beam-column joint, the columns be stronger in flexure than the beams so that, under severe earthquake shaking, plastic hinges form in the beams rather than the columns. Beam hinging is a ductile, distributed, stable energy-dissipating mechanism; column hinging leads to a brittle storey/sway collapse mechanism (especially soft-storey) with little warning — the cause of many earthquake building failures.

IS 13920 Cl. 7.2 enforces this by requiring the sum of column moment capacities at a joint to be at least 1.4 times the sum of beam moment capacities (in each direction), ensuring the intended weak-beam mechanism. Combined with closely-spaced confining ties in the plastic-hinge regions and joint shear design, it underpins the ductile detailing that lets a frame survive a design earthquake by yielding without collapsing. It is a defining check in seismic RCC frame design and a frequent finding (when violated) in the seismic evaluation of older buildings.