Civil Engineering Reference
In-Depth Information
(iii)
Failure Mechanism:
shear failures can be generated by the presence of infi lls, especially in
multi-storey frames and where incomplete panel infi lling is used. In addition, 'pounding' and
brittle failure of the walls can undermine the seismic performance of the structure.
Extensive experimental and numerical simulations on steel frames infi lled with brick walls carried
out by Moghaddam
et al
. ( 1988 ) showed that infi lled frames have an increase in stiffness of 15 to 40
times over that of the bare steel frames. As a result of their contribution to the lateral stiffness of struc-
tures, infi lls undergo cracks and damage during earthquakes. Non-structural damage can be controlled
by imposing stringent drift limits. Consequently, the difference in the relative stiffness between frames
and infi lls is reduced. On the other hand, non-structural components in RC concrete structures often
crack prematurely when subjected to alternating seismic loads. Experimental and analytical studies have
demonstrated that infi lls continue to govern the overall response of the structure even after cracking
during earthquakes (Klinger and Bertero, 1976; Bertero and Brokken, 1983; Fardis and Calvi, 1995 ;
Fardis and Panagiotakos, 1997 ; Kappos
et al
., 1998). Cracking due to low tensile strength of the
masonry diminishes drastically the initial elastic stiffness of masonry panels; these are generally slender
and possess low out-of-plane stiffness (Abrams and Angel, 1993). The presence of masonry infi lls may
affect the response positively or negatively, depending on the bare frame period and its relationship
with the dominant period of input motion.
Problem 2.2
An eight-storey reinforced concrete building is to be constructed to replace an existing condominium
block that has collapsed during a major earthquake. Two options are available for the building lateral
resisting system. These are provided in Figure 2.23 along with the lateral capacity of the sample
structures obtained from inelastic pushover analysis. Calculate the elastic lateral stiffness and the
secant lateral stiffness at ultimate limit states for both multi-storey structures. If the property owner
decides to employ brittle partitions, which structural system is preferable and why? It may be
assumed that both structures behave linearly up to the yield limit state.
2.3.2 Strength
Strength defi nes the capacity of a member or an assembly of members to resist actions. This capacity
is related to a limit state expressed by the stakeholder. It is therefore not a single number and varies as
a function of the use of the structure. For example, if the interested party decides that the limit of use
of a structural member corresponds to a target sectional strain, then the strength of the member is defi ned
as its load resistance at the attainment of the target strain. This may be higher or lower than the peak
of the load-displacement curve, which is the conventional defi nition of strength (Figure 2.24 ). Target
strains may assume different values depending on the use of structural systems. For instance, strains
utilized in multi-storey frames for power plants may be lower than those employed in residential or
commercial buildings. Target strains can be correlated to the risk of failure, which in turn depends on
the use of the structure.
Strength is usually defi ned as a function of the type of applied action. Axial, bending and shear
resistances are utilized to quantify the capacity of structures and their components in earthquake
structural engineering. In the response curve shown in Figure 2.24, the shear capacity
V
of the system
is defi ned with respect to either the shear at yield
V
y
or at maximum strength
V
max
. Alternatively, the
shear capacity can be expressed at any intermediate point between
V
y
and
V
max
, e.g.
V
i
in Figure 2.24 .
Similarly, axial (
N
) and bending (
M
) resistances are evaluated through axial load-axial displacement
