Page 58 - Advanced Design Examples of Seismic Retrofit of Structures

50 Advanced Design Examples of Seismic Retrofit of Structures

shear failure mode. This is mainly due the fact that: (1) these walls carry con-
siderable vertical loads from heavy jack-arch roofs which prevent the rocking
failure mode, although the vertical stress on these walls are not that large com-
pared to multistory masonry buildings to initiate toe-crushing failure mode; and
(2) URMs in Iran are categorized as strong bricks-weak mortar; therefore, the
cracks are initiated and propagated through bricks head and bed joints. Lack of
mortar in the head joints of URMs is another factor influencing the stair-stepped
diagonal cracks instead of the well-known horizontal bed-joint sliding cracks.
The importance of mortar’s shear-bond strength is highlighted more when the
possible ranges of other mechanical properties of material indicated in Code
360 [10] have negligible effects on the capacity of masonry walls. An example
of this is the compressive strength of masonry prism. The upper and lower
bounds of this parameter according to Code 360 do not have much influence
on the wall’s toe-crushing capacity, whereas the wide range of mortar’s shear
strength according to Code 360 significantly influences the shear capacity of
masonry walls.

2.6.2 Seismicity and Soil Conditions
Apart from the solutions regarding soil stabilization, foundation upgrading, and
special considerations during building construction on loose soils and on slopes
which are beyond the scope of this book, the effects of seismicity and soil con-
ditions can be summarized in the seismic demand force, that is, design base
shear. The latter shows itself in the form of fundamental acceleration (parameter
“A” according to the Iranian Code of Practice for Seismic Resistant Design of
Buildings (Standard 2800) [7]), and is accounted for in the reflection factor
(parameter “B” in Standard 2800, which also depends on the site’s seismicity).

2.6.3 Load Transfer Between the Walls
Once the total seismic demand of the building is determined, the distribution of
the seismic forces between the load-bearing walls can be greatly influenced by
the behavior of diaphragm in terms of rigidity and also existence of horizontal
ties. These factors are elaborated in this part.

2.6.3.1 The Effects of Diaphragms Flexibility
As previously mentioned, Iranian masonry buildings have one of two types of
diaphragms: jack-arch or filler-joist. Although the flexibility of the diaphragm
is determined based on the relative stiffness to that of the walls, both of these
diaphragm types are usually regarded as flexible. The rigidity of diaphragm has
a profound effect on the distribution of the seismic forces between the walls. In a
building which has a flexible diaphragm, the seismic demand forces are distrib-
uted between the walls based on their share of the seismic mass. The more each
wall carries the seismic mass from the roof, the more seismic forces are