Page 59 - Advanced Design Examples of Seismic Retrofit of Structures

Example of a Two-Story Unreinforced Masonry Building Chapter 2 51

(A) (B)
FIG. 2.31 Significant mass of the roof an floors. (A) Excessive roof thickness. (B) The mass of the
removed unnecessary layers. (Permission from DRES.)

transferred to it. On the other hand, seismic forces have stiffness distributed pro-
portionally when the building has rigid diaphragms. In other words, the walls’
share from the total seismic demand is independent from the seismic mass of the
each wall. As shown by Yekrangnia [12], providing a roof with rigidity can
improve the seismic response of masonry buildings by 40% compared to that
of a flexible diaphragm. The effects of a diaphragm’s rigidity on walls’ defor-
mations are schematically shown in Fig. 2.31 for a typical regular masonry
building. In the case of a rigid diaphragm, the walls in each direction experience
the same amount of displacements; however, these walls, based on their stiff-
ness and lateral strength, experience different amounts of lateral displacements.

2.6.3.2 The Effects of Horizontal Ties
When horizontal ties are present in buildings with flexible diaphragms, they
greatly improve the seismic performance of masonry buildings by providing
“rigid lines” for each wall’s axis. As a result, the lateral force is distributed
between these rigid lines based on their share from the seismic mass of the entire
diaphragm. However, the walls in each axis carry the force based on their rel-
ative stiffness to other walls in that axis thanks to the rigidity from the horizontal
ties. In conclusion, the seismic performance of a typical masonry building with
a flexible diaphragm having horizontal ties is superior to that of a building lack-
ing horizontal ties, but is inferior to the response of the same building with a
rigid diaphragm.

2.7 ANALYSIS PROCEDURE

In this example, the linear static procedure (LSP) is utilized for determination of
the demands and capacity of each masonry wall.