Ductile detailing for earthquakes

Buildings are designed for earthquake forces well below the elastic demand, on the understanding that the structure will yield and dissipate energy without collapsing. That understanding holds only if the yielding is ductile and happens in the right places. Ductile detailing is the set of rules that makes it so, and in India it is given in IS 13920 for reinforced concrete.

The first principle is strong column, weak beam. Plastic hinges should form at beam ends, where many can form without endangering the building, and not in columns, where hinges in one storey create a sway mechanism that can collapse it. IS 13920:2016 requires the sum of column moment capacities at a joint to exceed the sum of beam capacities by a margin of 1.4. A related principle is that shear failure, which is brittle, must never precede flexural yielding, so shear is designed for the force that develops when the hinges reach their probable moment capacity.

Confinement is the second principle. Closely spaced closed hoops at column ends and in beam hinge zones hold the core concrete together after the cover spalls, prevent the main bars from buckling and keep shear capacity. The hoops must end in 135-degree hooks with an extension of six bar diameters, but not less than 65 mm, so they stay anchored in the core when the cover is lost. Laps are kept out of hinge zones and enclosed by hoops.

Beam-column joints, where beam bars pass through or anchor into columns, carry very high shear in an earthquake and need hoops through the joint. Shear walls need boundary elements at their edges when compressive stress is high. Each rule addresses a failure mode seen repeatedly in past earthquakes.