Page 94 - Advanced Design Examples of Seismic Retrofit of Structures

86 Advanced Design Examples of Seismic Retrofit of Structures

f UF
DCR t ¼
V tc
6944
DCR t ¼ ¼ 1:40
4961
Therefore, rocking is the failure mode of this wall.
From these figures, several conclusions can be drawn, as follows:

l The inclusion of horizontal ties can significantly reduce the vulnerability of
the building; especially for the internal longer walls.
l The building with the rigid roof has the least vulnerability.
l Generally speaking, the longer walls are more vulnerable in the building
with a rigid roof. This is because the shear force demand is distributed
between the walls stiffness-proportionally and longer walls contribute more
to load-bearing.
l Because in the transverse direction, all the walls have the characteristics in
terms of the length and gravitational load-bearing contribution, the inclusion
ofhorizontaltiesand/orarigidroofisineffectiveinreducingtheirvulnerability.

2.8 RETROFIT MEASURES
2.8.1 Strength Versus Stability-Based Design Approach

According to the concept introduced by Tolles et al. [16], two fundamental
design approaches can be taken to improve the earthquake performance of
masonry buildings: strength-based design and stability-based design. The for-
mer, which is the general traditional design approach, assumes the elastic
behavior of the building. The latter is concerned with the overall performance
of the building and with assuring structural stability during the postelastic, post-
yielding phase. Stability-based design features can reduce the potential for
severe structural damage and collapse after yielding has occurred.
Implementation of strength-based design usually indicates that masonry
buildings would not perform well during even moderate seismic ground motions,
during which the masonry material will fail. Because masonry buildings have
massive walls and the masonry itself is a low-strength material, the dynamic
or equivalent static forces are large and the tensile properties of the material
are easily exceeded. While a strength-based analysis can accurately predict when
cracks will occur, it cannot provide insight into the postelastic performance of
masonry buildings. On the other hand, a stability-based design analysis can take
advantage of the unique characteristics of the postelastic performance of adobe
and the effects of a proposed retrofit system. The extent of retrofit intervention
required to stabilize different parts of a typical masonry building is often rela-
tively small and relies on many of the inherent properties of these buildings.
Nevertheless, the current trend in engineering design, that is, performance-
based design (PBD) is to design for multiple, specifically defined levels of
performance at different earthquake levels. The fundamental goal of