Page 27 - Advanced Design Examples of Seismic Retrofit of Structures

Example of a Two-Story Unreinforced Masonry Building Chapter 2 19

parts. As a result, these buildings show a seismic response inferior to that in
buildings that had not experienced any damage from preceding earthquakes.
Moreover, the strength of the damped walls reduces to the limit that the walls
cannot tolerate their own weight. Uniformity in seismic behavior of URM is the
key point in determination of earthquake damage in these buildings. Therefore,
attachment of perpendicular walls to each other and also to the roof, quality of
the mortar and blocks, and workmanship are of utmost importance.

2.3.5 Soil and Foundation Condition, Site Topography
Soil characteristics contribute in filtering and amplifying seismic signals; that
is, stiff soil filters low-frequency waves and amplifies the high-frequency ones,
impacting the seismic performance of many URM buildings. As a result, a typ-
ical URM building is expected to experience larger seismic forces when located
on stiff soil than on soft soil. Since URM buildings are mainly classified as low-
rise buildings with a lack of ability of masonry in tolerating tensile forces which
lead to foundation uplift, their foundation condition has not much to do with
overall structural performance. In this regard, some design codes give prescrip-
tive recommendations for foundations of URM buildings and do not necessitate
detailed design procedures for these structural parts. Note that this is not the
case for reinforced masonry (RM) buildings, as considerable tensile and shear
forces need to be transferred to the foundation.

2.3.6 Construction Quality

In some cases, it was observed that buildings with lateral load-bearing elements
which are considered engineered structures, experience severe damage and
sometimes collapse during moderate earthquakes. This may originate from their
poor construction quality, which itself may be because of lack of supervision
during construction. In some regions, engineers are not fully familiar with
the seismic code’s regulations. In the absence of proper supervision, some of
the design codes’ regulations may be neglected in order to reduce construction
costs. Excessive distance of shear reinforcement in confining ties, using
low-quality concrete which may be crushed by hand, inadequate connection
between horizontal and vertical ties in confined masonry buildings are common
examples of poor masonry workmanship in buildings. Examples of the effects
of construction quality on seismic performance of URM buildings are shown
in Fig. 2.6.

2.4 DAMAGE CLASSIFICATION

In the following, the main damage of URM billings due to past earthquakes are
reviewed with example photos and sketches.