Foundation, Concrete and Earthquake Engineering

Concentration of earthquake Stresses in Structural Joints

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))

Failure  at construction joint
Figure 1 (a) Failure  at construction joint
This leads to shear or bending failure at or very close to the joint. The onset of high bending moments may cause yielding or buckling of the steel reinforcements.

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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.

Spalling of concrete
Figure 1 (c) Spalling of concrete

 Diagonal shear crack
Figure 1 (d) Diagonal shear crack
Bond failure is also observed, in case, lap splices are too close to the joints. Indian codes suggest methods that attempt to delay all these failures through a sound reinforcement detailing (IS 13920:1993). However, in many structures these details have not been followed due to perceived difficulties at site. In most of the structures in Gujarat lack of confinement and shear cracks have been found to be most common causes of failure. A rehabilitation and retrofitting strategy must alleviate these deficiencies from the structures.

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