Why ductile detailing is needed in joints

A beam-column joint transfers the moments and shears of the beams into the columns. During an earthquake it is subjected to very high horizontal and vertical shear and bond demands, and its failure can bring down the whole frame. Ductile detailing is therefore needed because:

• High shear in the joint core: the opposite forces from the beam top and bottom bars produce a large diagonal tension that can crack the core.
• Bond and anchorage stress: a bar passing through the joint may be yielding in tension on one face and in compression on the other, so it may slip. Hence, at an exterior joint, the beam bars must be anchored by their full development length from the inside face of the column, with a $90^\circ$ bend.
• Loss of confinement: the joint concrete spalls under cyclic reversals. Closed hoops with $135^\circ$ hooks hold the core and the bars together.
• The joint should not fail first: a joint failure is brittle and very difficult to repair, so the joint must be stronger than the members framing into it.
• Cyclic loading: stiffness and strength degrade quickly in unconfined joints.

As per IS 13920, the special confining hoops of the column ends are continued through the joint at 75-100 mm spacing (up to 150 mm when beams frame in on all four sides). Lap splices are never made in the joint.