Page 113 - Advanced Design Examples of Seismic Retrofit of Structures

Example of a Two-Story Unreinforced Masonry Building Chapter 2 105

This is a reasonable assumption for strength design since the flexural and shear
strength of the reinforced shotcrete overlay can be many times more than that of
the URM wall. This assumption may result in some cracking of the masonry as
the reinforcement in the shotcrete strains past yield. This may violate a perfor-
mance objective for immediate occupancy or continued operation [22]. The Ira-
nian Instruction for Seismic Rehabilitation of Schools [52] considers shear
failure for the shotcreted masonry walls only, and neglects the contribution
of URM to the strength of these walls. The expected capacity of the shotcreted
masonry walls according to this instruction is determined based on Eq. (2.44):

L
p ffiffiffiffi

0
Q CE ¼ 0:17 0 ; f inMPa (2.44)
f t c L + A s f y
c c
S
where:
0
f c ¼compressive strength of concrete which should be at least 20MPa dur-
ing construction; however, this parameter is conservatively assumed to be
15MPa in design calculations;
L ¼wall’s length;
t c ¼thickness of shotcrete layer;
A s ¼area of the each reinforcement bar in steel grid;
f y ¼yield strength of the steel grid; and
S ¼distance of the of each reinforcement bar in steel grid.
It is assumed that shear failure in shotcreted masonry wall is a deformation-
controlled action. According to Eq. (2.38), the m-factor and the knowledge fac-
tor are conservatively assumed to be 2.0 and 1.0, respectively. However, in this
chapter, the capacity of the shotcreted masonry walls is determined using the
relations proposed by Ghiassi et al. [50].

Failure Modes
Flexural According to Ghiassi et al. [50], the behavior of shotcreted masonry
walls can be similar to that of the corresponding reinforced masonry walls. Con-
sequently, the concepts for capacity determination of the latter for different fail-
ure modes can be adopted for the former. This is also the case for considering
acceptance criteria and m-factors in linear analysis. By assuming plane sections
remain plane after deformation, elastic-perfectly plastic behavior of steel bars,
neglecting tensile strength of concrete, and assuming the “a” and “α” factors as
0.85 and 0.85, respectively, as shown in Fig. 2.55, the moment capacity of the
shotcreted wall is determined based on Eq. (2.45) after some manipulations and
simplifications. It is worth noting that in determination of the flexural capacity
of the wall, it is assumed that compressive failure of masonry is unlikely. This
assumption is justified by the low reinforcement ratio commonly used for rein-
forcement of shotcrete. Moreover, the majority of masonry buildings are low-
rise, which means there are low-to-moderate vertical forces acting on the walls.
As a result, the flexural failure of these walls occurs when all the vertical
reinforcements yield in tension and compression.