Page 101 - Advanced Design Examples of Seismic Retrofit of Structures

Example of a Two-Story Unreinforced Masonry Building Chapter 2 93

(A) (B)
FIG. 2.46 Examples of using external reinforcement. (A) Using horizontal and vertical steel
bamboos [34]. (B) Using diagonal steel strips [35]. ((A) Permission from Taylor and Francis.
(B) Permission from ASCE.)

of the retrofitted wall and to fill the voids and cracks, which are present in the
masonry due to physical and chemical deterioration and/or mechanical actions.
For multiwythe masonry walls, injecting grout into empty collar joint enhances
composite action between adjacent wythe. The success of a retrofit by injection
depends on the injectability of the mix used, and on the injection technique
adopted [22].
The technique is effective at restoring the initial stiffness and strength of
masonry. A cement-based grout injection is capable of restoring up to about
0.8 of the unretrofitted masonry compressive strength [29], 0.8–1.1 of the unre-
trofitted wall in-plane stiffness, and 0.8–1.4 of the wall unretrofitted in-plane
lateral resistance [30–32]. In addition, a cement-based grout injection can
increase the interface shear bond of multiwythe stonewalls by a factor of
25–40 [33]. Walls retrofitted with epoxy injection tend to be stiffer than the
unretrofitted, but the increase in stiffness (10%–20%) is much less dramatic
than the increase in strength. The increment in lateral resistance ranged from
2 to 4 times the unretrofitted resistance. An example of grout injection of
masonry walls is shown in Fig. 2.46.

2.8.3.7 External Reinforcement
External reinforcement on masonry walls are usually performed by adding hor-
izontal, vertical, and/or diagonal elements in the form of steel strips and bamboo
canes. In designing the retrofitting elements, the relative rigidities of the unre-
trofitted structure and the new steel bracing are an important factor that should
be taken into consideration. In an earthquake, cracking in the original masonry
structure is expected and after sufficient cracking has occurred, the new steel
system will have comparable stiffness and be effective [36, 37].