Page 211 - Advanced Design Examples of Seismic Retrofit of Structures

204 Advanced Design Examples of Seismic Retrofit of Structures

4.3 DAMAGE CLASSIFICATION
The behavior of infill walls in infilled frames depends on several factors which
include but not limited to inclusion of openings in the wall, rigidity of the beam-
column connections, material properties of infill wall and frame, flexural stiff-
ness of the frame members compared to stiffness of the infill wall, ratio of
height to length of the frame, and connection between the infill and the frame.
Generally speaking, infill walls increase the stiffness and strength of frame
buildings (Fig. 4.3). The results of the tests by Moghaddam and Dowling [5]
indicate that the stiffness and strength of the infilled frame is 2.0 and 2.7 times
larger than those of the corresponding bare frame. Neglecting the added mass of
the infills, a higher stiffness usually leads to lesser drift ratios. However, infill
walls place considerable forces on the frame members and, if not designed for
these forces, the columns and beams may fail in shear or flexure. Moreover, the
beam-column connections may not be able to transfer the increased forces from
the infill walls. As an acceptable approximation, the effects of infill walls can be
replaced by a compression-only diagonal strut, schematically shown in Fig. 4.4.
Another negative effect from adding infill walls to the frame building is a
possible increase in the ductility demand. As infill walls tend to decrease the
natural vibration period of structures, they may also decrease the normalized
yield strength f of the structure, defined as Eq. (4.1) [6].
y
f y
f ¼ (4.1)
y
f 0
where f y ¼ yield strength of the elasto-plastic system and f 0 ¼peak value of the
earthquake-induced resisting force in the corresponding linear system.
As an example, the ductility demand for elasto-plastic system due to El Cen-
tro ground motion is shown in Fig. 4.5. In this figure, the possible effects of

(A) (B)
FIG. 4.3 Damage in frame members caused by infill walls. (A) Shear failure (Permission from
Springer) [3]. (B) Flexural failure due to shot column effect (Copyright holder: Elsevier) [4].