Page 107 - Advanced Design Examples of Seismic Retrofit of Structures

Example of a Two-Story Unreinforced Masonry Building Chapter 2 99

retrofitting the vulnerable walls, all walls of half of the classrooms on each story
are retrofitted in the three considered cases and the results are compared. The
schematic representation of the retrofitted walls is shown in Fig. 2.50. The shot-
crete layer is assumed to be 6cm in thickness and the compressive strength of
the concrete is 18MPa. The reinforcement of the shotcrete layer is assumed to
be a grid made of φ8@200mm with the yield strength of 400MPa for the bars.
Some important steps for shotcreting of URM walls are shown in Fig. 2.51 in
the form of example photos of some retrofit projects. Moreover, there are some
important points to consider for construction of shotcreting of URM walls
which are shown in Fig. 2.51.
There are, however, some cases in which implementation of shotcrete faced
with some problems. The common construction problems of shotcreting of
URM walls are listed in Fig. 2.52. These problems can be easily avoided by
proper supervision on different phases of the retrofit projects. For more infor-
mation about the requirements for implementation of shotcrete, the reader is
referred to the Practical Instruction for Shotcrete in Seismic Rehabilitation of
Schools [51].
Besides shotcreting of walls, many retrofitting projects consist of several
steps on other structural parts of URM buildings. The most important steps
of these projects related to structural parts other than walls are shown in
Fig. 2.53.
Progression of the retrofit project on the example building is shown in
Fig. 2.54. In this figure, the constructed retrofit details consisted of shotcreting
of all the peripheral walls which is different from the assumed retrofitting
scheme in this example.

2.8.4.2 Determination of Shotcreted Walls Stiffness
Based on the studies by Ghiassi et al. [50], the relation for stiffness determina-
tion of shotcreted masonry walls is similar to that of the unretrofitted masonry
wall, except for the equivalent modulus of elasticity and shear modulus, which
are replaced by their corresponding modulus for masonry material. As an exam-
ple, consider ASCE 41-13 [2] relation for determination of masonry wall’s stiff-
ness as Eq. (2.44):
1
K e ¼ (2.40)
H 3 H
+
λE m I m A v G m
where:
H ¼wall’s height;
E m ¼equivalent modulus of elasticity of masonry material;
G m ¼equivalent shear modulus of masonry material;
A v ¼wall’s cross-sectional area; and
λ ¼3 and 12 for cantilever and fixed-fixed walls, respectively.