Page 57 - Advanced Design Examples of Seismic Retrofit of Structures

Example of a Two-Story Unreinforced Masonry Building Chapter 2 49

In Stiffness (Soft Story)
The stiffness of the seismic-force-resisting system in any story shall not be
<70% of the seismic-force-resisting system stiffness in an adjacent story above
or <80% of the average seismic-force-resisting system stiffness of the three
stories above [2, 10]. According to Section 5.4.2.2 of ASCE 41-13, for checking
the soft story irregularity, an analysis shall be performed in accordance with
Section 5.2.4 using the linear dynamic procedure. The adequacy of all elements
of the seismic-force-resisting system shall be evaluated in the noncompliant
stories in accordance with Section 5.2.5 of that document. Based on stories’
total stiffness presented in Table 2.5, the building under study is regular.

2.6 DEMAND-TO-CAPACITY PARAMETERS

In this part, the important parameters that affect the seismic demands and the
building capacity are studied in detail. In design code methodologies, capacity
refers to the permissible strength or deformation for a component action, and
demand is the amount of force or deformation imposed on an element or com-
ponent [2]. Although the example building is an existing school building with
known behavioral parameters and site conditions, it is intended to study the
effects of each of these parameters and conditions of the vulnerability of
masonry buildings. In response to this need, the more important behavioral
parameters and site conditions are considered as variables which lead to several
cases. The results of these cases are then compared to each other, which even-
tually leads to a better understanding of the sensitivity of seismic vulnerability
of the considered building to each of these parameters. As previously men-
tioned, the more important parameters related to vulnerability assessment of
the example building are considered below. There are several parameters which
play significant roles in determination of a building’s seismic vulnerability.
These parameters, however, can hardly be determined in practice; as a result,
they are assumed to be typical fixed quantities. For example, the compressive
strength of masonry prism is important because this parameter is directly related
to the toe-crushing capacity and also the modulus of elasticity of masonry.
However, this parameter is not very feasible to determine for the existing build-
ings because it necessitates extracting several standard specimens from the wall
by cutting through them and then testing in the laboratory. Another solution is to
perform double flat jack testing, which is usually practiced for historical build-
ings but is rarely practiced for ordinary masonry buildings, especially on a large
scale as in the case of tens of thousands of Iranian masonry school buildings.

2.6.1 Mortar Shear Strength
Shear-bond strength of mortar is a very important parameter for masonry build-
ings, especially those experiencing shear-sliding failure. As indicated by
Yekrangnia et al. [11], the majority of Iranian masonry buildings experience