Page 35 - Advanced Design Examples of Seismic Retrofit of Structures

Example of a Two-Story Unreinforced Masonry Building Chapter 2 27

(A) (B)

(C)
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FIG. 2.12 Examples of collapse of facade. (A) Collapse of facade from two-story CM building
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due to in-plane deformations in the first story. (B) Collapse of facade from two-story CM building
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due to amplified out-of-plane acceleration on the second story. (C) Collapse of facade after 2012
Ahar earthquake. (Permission from DRES.)
supports can greatly improve seismic performance of URM walls in the out-of-
plane direction, which is one of the main failure modes in URM buildings.
It has been observed that these supports can themselves experience some
damage in the form in-plane diagonal cracks or toe crushing. Failure mecha-
nisms and the severity of damage in these elements depend on the extent of ver-
tical loads, material quality, and the load-bearing span. Examples of damage to
walls’ support in past earthquakes are presented in Fig. 2.14.

2.4.7 Damage at Walls Corners
During seismic actions, stress concentration at the intersection of perpendicular
walls especially in unreinforced unconfined masonry buildings, results in
detachment of parts of the walls’ corners.

2.4.7.1 Diagonal Cracks at the Top and Bottom Corners
If diagonal cracks initiate from the top corner, they can expand further, to the
base of the wall. On the other hand, cracks at the bottom corner usually occur at
the walls’ intersection and can be accompanied by wall toe crushing and local
collapse. Examples of diagonal cracks in masonry walls are shown in Fig. 2.15.