2.1.3. Failure Mechanisms Of earthquakes

ii. Seismic Load – Though the earthquake force acts in all the directions, for engineering calculations and seismic design it is considered as a resultant lateral force acting on the built structure and termed as a seismic load. The factors6 which affect the seismic load are – Earthquake zone factor, soil foundation factor, hazard factor, stiffness, and dampening of structure and weight of the superstructure.

2.1.3 Failure Mechanisms of Earthquakes

To study the seismic resistance of traditional structures it is important to understand the basic failure7 mechanisms of earthquakes. Figure 7 represents a basic masonry structure with openings and a flat roof on top. Wall A represents the longer wall whereas wall B represents the shorter wall connecting the two parallel long walls. When the resultant direction of force is along the x-axis, Fx, the longer wall acts as a shear8 wall, when the resultant is Fy, the shorter wall acts as a shear wall. During the former case, the shorter wall would develop cracks at the junction. In the latter condition the longer wall which has more mass will try to resist due to the inertia force. When the resultant force is Fx, the wall B will tend for out of plane failure, but the connections with wall A and the roof together prevents the complete failure, thereby acting as a box. Now, the roof plays an important role in the overall behavior of the structure. If the roof is rigid and connected well to the walls, it acts as a horizontal diaphragm9, its inertia will be distributed equally on the walls A and B. If the roof is flexible its inertia will go on the support wall.

6 The equation used for determining the seismic load is – F=S.Fs.I.C.W, where F is the seismic force, S is the earthquake zone factor, I is the hazard factor depending on the usage, C is the factor depending on stiffness and dampening and w is the total weight of the super structure. 7 Failure - when the material in a structure is stressed to its strength limit, thus causing fracture or excessive deformations. 8 Shear - When a structural member experiences failure by shear, two parts of it are pushed in different directions. 9 Diaphragm - diaphragm is a structural element that transmits lateral loads to the vertical resisting elements of a structure (such as shear walls or frames).