Page 29 - Advanced Design Examples of Seismic Retrofit of Structures

Example of a Two-Story Unreinforced Masonry Building Chapter 2 21

achieved. This type of damage usually depends on the acceleration level of the
seismic actions. The main parameters influencing out-of-plane behavior of
URM walls are as follows:
- Slenderness ratio (ratio of a wall’s height to its thickness): the larger the slen-
derness ratio is, the more vulnerable the masonry wall in out-of-plane direc-
tion becomes. For more information, see Chapter 6, “Results” section.
- Boundary conditions: connection of wall to roof and floor greatly enhances
the overturning capacity of URM walls. Also, existence of horizontal ties at
the base and top of walls and also vertical ties are key factors in determina-
tion of their out-of-plane performance. These ties can activate arching
actions in the walls; this greatly improves out-of-plane performance of such
walls. For more information, see Chapter 6, “Results” section.
- Load-bearing versus nonload-bearing walls: generally, load-bearing walls
can better withstand out-of-plane actions than nonload-bearing walls.
- Distance of intersecting walls: the perpendicular walls intersecting URM
walls can provide lateral support and improve out-of-plane resistance. Con-
nections to the perpendicular walls should satisfactorily withstand the
demand forces in order to maintain integrity of the walls system.

2.4.1.1 Collapse of Gable-End Walls
In many cases, URM gable-end walls are among the most vulnerable walls in
out-of-plane direction. Collapse of these walls in the past earthquakes is a fre-
quently observed damage type. The main reason of this vulnerability is the very
high slenderness ratio of these walls and poor attachment to the roof. Examples
of collapse of gable-end wall due to out-of-plane actions are shown in Fig. 2.7.

2.4.1.2 Out-of-Plane Flexural Cracks and Overturning of Walls
Out-of-plane flexural cracks is one of the most common types of damage in
URM walls. These cracks easily develop in URM walls, especially nonload-
bearing walls without suitable connections in their boundary conditions. The
out-of-plane flexural cracks in load-bearing walls can result in the total collapse
of URM buildings.
In addition, out-of-plane movement of walls can cause damage at intersec-
tions of perpendicular walls. These cracks occur due to the fact that out-of-plane
wall are low in stiffness, while in-plane walls act with considerable stiffness and
the inharmonic response of the perpendicular walls results in cracks at their
intersection. These through-wall vertical cracks usually cannot be prevented
and cause no major damage to URM buildings; however, if severe, they can
spoil the integrity of the walls and cause local collapse of URM buildings.
Examples of walls overturning due to lack of connection to the perpendicular
walls are shown in Fig. 2.8.