Vibrations caused by earthquakes generate additional loading. Shear stresses develop which cause damage to structural elements. Since masonry, which can be stressed relatively high in compression, is weak in resisting bending and shear, collapse is often the result. The different failures modes of masonry are:
1. Sliding shear failure
It results in a building sliding off its foundation or on one of the horizontal mortar joints. It is caused by low vertical load and poor mortar. If the building is adequately anchored to the foundation, the next concern is for adequate resistance of the foundation itself, in the form of some combination of horizontal sliding friction and lateral earth pressure. The dislocation of a lightly attached roof is also an example of this type of failure. A wall with poor shear strength, loaded predominantly with horizontal forces can exhibit this failure mechanism.
Aspect ratio for such walls is usually 1:1 or less (1:1.5)
2. Diagonal cracks
Diagonal cracks in masonry walls when the tensile stresses, developed in the wall under a combination of vertical and horizontal loads, exceed the tensile strength of the masonry material.
3. Nonstructural failure
While structural elements of a building should be the prime concern for earthquake resistance, everything in the building construction should resist forces generated by earthquakes. Nonstructural walls, suspended ceilings, window frames and fixtures should be secure against movement during the shaking actions. Failure here may not lead to building collapse, but it still constitutes danger for occupants and requires costly replacements or repair.
Interior partitions, curtain walls, wall finishes, windows and similar building elements are often subjected during earthquakes to shear stresses, for which they do not have sufficient resistive strength. The most common damage resulting from this is breakage of window panes and cracks in internal plaster and external rendering. A possible remedy for the former is to isolate the window frames from the surrounding walls by the introduction of flexible joints; the latter can be avoided by reinforcing the plaster or to pre-crack it by introducing control joints (groves).
4. Failure due to overturning
The critical nature of the overturning effect has much to do with the form of the building's vertical profile. A wall that is too tall or too long in comparison to its thickness is particularly vulnerable to shaking in its weak direction. Thus the tendency of a wall to topple when pushed in the weak direction can be reduced by limiting its length-to-thickness and height-to-thickness ratios.