Page 103 - Advanced Design Examples of Seismic Retrofit of Structures

Example of a Two-Story Unreinforced Masonry Building Chapter 2 95

2.8.3.9 Posttensioning
There has been little application of this technique; posttensioning is mainly used
to retrofit structures characterized as monuments. Tendons are placed inside
steel tubes (ducts) either within holes drilled along the mid-plane of the wall
or along groves symmetrically cut on both surfaces of the wall. Holes are
cement grouted and external grooves are filled with shotcrete [40, 41]. In this
case, the tendons are fully restrained (i.e., not free to move in the holes). This is
true even if the tendon is unbonded, that is, no grout is injected between the duct
and the tendons [42].
One of the major drawbacks for using steel bars is corrosion. However,
fiber-reinforced plastic presents a promising solution for this problem [43];
in addition, this method as a retrofit solution may result in a negative response
of masonry walls with significant vertical loads, such as walls in the lower
stories of multistory building and also walls with low-quality material.
Vertical posttensioning results in substantial improvement in wall ultimate
behavior for both in-plane and out-of-plane; in addition, it improves both crack-
ing load and redistribution of internal forces. Rosenboom and Kowalsky [40]
show that for cavity walls, the posttension grouted specimen has lateral resis-
tance much higher (40%) than the ungrouted one. For grouted specimens,
although to bonding the bars have insignificant effect on lateral resistance;
the specimen which has unbonded bars has higher lateral drift (70%) over
the bonded specimen. The unbonded grouted specimen has a drift up to
6.5%. However, unbonded posttension tendons may show low energy dissipa-
tion due to the lack of yielding of reinforcement [44]. An example of grout
injection of masonry walls is shown in Fig. 2.48.

2.8.3.10 Safe Room
The main aim of a “safe room” is to mitigate human casualties during earth-
quakes. In other words, the performance improvement of the building retrofit-
ting with a “safe room” cannot be considered more than “life safety.” The “safe
room” is a 3D steel, concrete-filled frame which is installed in some rooms
which are considered vulnerable against earthquakes. This room will prevent

(A) (B)
FIG. 2.48 Schematic view of the safe room [45]. (A) Safe rooms extracted from the building.
(B) Safe room elements.