Page 96 - Advanced Design Examples of Seismic Retrofit of Structures

88 Advanced Design Examples of Seismic Retrofit of Structures

example, PGA. The path ABC indicates a typical unretrofitted structure in
which point “B” is the threshold of the irreparable damage. As can be seen,
a small increase in earthquake intensity from point “B” leads to a large increase
in damage index up to the point “C.” If retrofitted according to the strength-
based approach, the performance of the building follows path DEF.
A significantly higher earthquake intensity is required to cause low to interme-
diate levels of damage index. However, the performance improvement in larger
earthquake intensities are small compared to the corresponding unretrofitted
building. In other words, in severe earthquakes, the vulnerability of the building
retrofitted according to the strength-based approach decreases marginally.
Nonetheless, considerable reduction in the vulnerability of the building is
achieved thanks to the retrofitting method based on the stability-based
approach. However, marginal improvement compared to the corresponding
unretrofitted building results in the lower earthquake intensities.
During earthquakes, tensile, shear and even compressive stresses in masonry
parts exceeds their strength and these results in cracking and crushing of URM
buildings. However, the main weakness of many URM buildings is not lack of
strength, but lack of ductility. The extent of damage in ductile structures
depends on the earthquake’s magnitude. In other words, in an earthquake with
the magnitude of 7.0, severe damage can be expected in the central parts of the
affected areas and by receding from the epicenter, the severity of damage
decreases. However, this does not follow for URM buildings which collapse
in central parts, and suddenly we encounter some areas with almost unaffected
URM buildings. The reason behind this is the behavioral parameters of URM
buildings and also records’ characteristics. Seismic performance of a URM
building can be summarized as follows [3]:
(A) Earthquake is not strong enough to demolish URM building and URM
building maintains its strength capacity.
(B) Earthquake can cause some damage in URM building in the last seconds of
excitation. However, since these changes occur in the last seconds of the
earthquake, they cannot cause major damage in the building.
(C) Earthquake intensity is high and, in the initial seconds, causes structural
damage. Severe strength and stiffness degradation occurs, which results
in the collapse of the building.
These three stages depend on earthquake magnitude and usually stage
C happens in earthquakes with large magnitude for the masonry buildings
located in areas close to the epicenter. Since the dissipation of the seismic waves
with distance, URM buildings can act according to stages A or B according to
Fig. 2.43. As a result, borders in which performance of URM buildings changes
from stage C to stage B can be considered. The area inside this border is named
the “cracking threshold area.” For instance, in the Manjil earthquake, the
cracking threshold area was found to be 48km from Manjil, with the local
PGA of 0.36g.