Failure Modes of Masonry Structures
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.
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 per-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.