Page 39 - Advanced Design Examples of Seismic Retrofit of Structures

Example of a Two-Story Unreinforced Masonry Building Chapter 2 31

(A) (B)

(C) (D)
FIG. 2.15 Examples of diagonal cracks mechanism at walls corner. (A) Diagonal cracks at the top
of the walls. (B) Major diagonal cracks in walls and local collapse. (C) Major diagonal cracks in
walls and local collapse. (D) Mechanism of diagonal cracks at walls corners. (Permission from
DRES.)

2.4.9 Damage at Walls-Roof Connection
There are some cases in which the beams are directly placed on masonry walls
and no proper connection is considered between them. As a result, the roof
slides on the walls during seismic actions. Severity of this kind of damage is
usually slight due to the heavy weight of the roof and huge frictional resistance.
The lighter the roof, the more chances are of severe sliding, which can result in
the collapse of the roof. Examples of damage at walls-roof connection are pre-
sented in Fig. 2.19.

2.4.10 Damage Due to Soil and Foundation Condition

2.4.10.1 Damage Due to Damped Bases
Existence of moisture at the base of masonry walls can result in strength reduc-
tion and sliding or overturning of the upper portion of walls. Also, cyclic damp-
ening and drying of walls can decrease masonry strength considerably. It is
worth mentioning that for initiation and propagation of cracks due to moisture,
it is not necessary for walls to be damped during earthquakes; previously
damped walls that are now dried can be very vulnerable because of absorption
of salts and damage experienced prior to the earthquake. Fig. 2.20 shows two
examples of damage due to a damped wall’s base.