SEISMIC PERFORMANCE

Earthquake performance of RC frame construction has been well documented.

Damage patterns in reinforced concrete frames during the 1971 San Fernando
(California) earthquake have been extensively studied. More recently, several destructive
earthquakes of the last decade, including the 1999 Athens (Greece) earthquake, the
1999 İzmit and Düzce earthquakes (Turkey), 1999 Chi Chi (Taiwan) earthquake, 2001
Bhuj (India) earthquake, and the 2003 Boumerdes (Algeria) earthquake, have caused
substantial damage to RC frame construction. These earthquakes have revealed the
following patterns of damages and failures in RC frame construction:

• Shear failure and concrete crushing failure in concrete columns. These are the
most undesirable nonductile modes of failure (Figure 5). This behavior can lead
to the loss of gravity load-bearing capacity in the columns and potentially a total
building collapse.

Figure 5: Shear failure of a reinforced concrete column in the 2001 Bhuj earthquake
(WHE Report 19, India)


Partial ductile design and detailing. Systems that exhibit some (limited) yielding
behavior can eventually form dangerous collapse mechanisms as a result of
stiffness or strength degradation at sections without ductile detailing.

• Conceptual design deficiencies. This includes such deficiencies as incomplete
load path and architectural planning deficiencies such as vertical and/or
horizontal irregularities. Architectural features play an important role in the
performance of RC frame buildings.

• Inappropriate column/beam relative strengths. This can lead to failure of
individual members and connections when the “weak column-strong beam”
mechanism develops.

• Inadequate detailing of reinforcement.

• Soft-story effects. In many applications, architectural considerations result in a
taller first story, which causes a soft-story formation due to drastic change in the
stiffness between adjacent stories (Figure 6). The presence of a soft story results
in a localized excessive drift that causes heavy damage or collapse of the story
during a severe earthquake (Figure 7). Another typical case of soft story arises
when the first floor is left open (that is, no infills) to serve a commercial function
(stores) or as a parking garage (very common in Turkey, India, and Cyprus), while
upper floors are infilled with unreinforced masonry walls. A relatively rare case
results when the strength of the two adjacent stories is significantly different (weak
story) leading to localized deformations similar to the soft-story mechanism.

• Short-column effects. The short- or captive-column failure occurs due to partial
restraining of the columns that are, in turn, subjected to high shear stresses and
fail in shear if unable to resist these stresses.

Figure 6: Soft-story mechanism
(WHE Report 61, Taiwan)

Figure 7: Building collapse due to soft-story
mechanism in the 2003 Boumerdes earthquake
(WHE Report 103, Algeria)










In several instances, seismic performance of RC frame buildings has been quite poor,
even when subjected to earthquakes below the design level prescribed by code.
One of the underlying reasons is the absence of an effective mechanism for code
enforcement in some countries. This deficiency in governmental oversight is linked to
several related factors, such as the lack of technical control and supervision, problems
with the legal framework, low engineering fees, and improper regional construction
practices. When one or more such factors are present during construction, the built
structure does not comply with many aspects of the design. As a result, its seismic
resistance becomes inadequate, with the consequence that unpredictable damage
or failure results when subjected to loads below the code-prescribed levels.

The key deficiencies identified in the RC frame construction practice include the following:

• Alteration of the member sizes during the construction phase from specifications
in the design drawings

• Noncompliance of the detailing work with the design drawings

• Inferior quality of building materials and improper concrete-mix design

• Modifications in the structural system performed by adding/removing
components without engineering input

• Reduction in the amount of steel reinforcement as compared to the design
specifications

• Poor construction practice