Earthquake generates ground motion both in horizontal and vertical directions. Due to the inertia of the structure the ground motion generates shear forces and bending moments in the structural framework. In earthquake resistant design it is important ensure ductility in the structure, ie. The structure should be able to deform without causing failure. The bending moments and shear forces are maximum at the joints. Therefore, the joints need to be ductile to efficiently dissipate the earthquake forces.
Most failures in earthquake-affected structures are observed at the joints. Moreover, due to the existing construction practice, a construction joint is placed in the column very close to the beam-column joint (fig. 1(a))
|Figure 1 (a) Failure at construction joint|
Figure 1(b) Crushing of concrete
The high compressive stress in concrete may also cause crushing of concrete. If the concrete lacks confinement the joint may disintegrate and the concrete may spall (fig. 1(b,c)). All these create a hinge at the joint and if the number of hinges is more than the maximum allowed to maintain the stability of the structure the entire structure may collapse. If the shear reinforcement in the beam is insufficient there may be diagonal cracks near the joints (fig. 1(d)). This may also lead to failure of the joint.
Figure 1 (c) Spalling of concrete
Figure 1 (d) Diagonal shear crack