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Transactions on the Built Environment vol 57, © 2001 WIT Press, www.witpress.com, ISSN 1743-3509
Effect of structural irregularities and short
columns on the seismic response of buildings
during the last Turkey earthquakes
H. Sesigiir, O.C. Celik, F. Cili, K. Ozgen
Faculty of Architecture, Department of Tlieory of Structures,
Istanbul Technical University, Ttrrkej.
Abstract
During the last eight years, Turkey hasexperienced many severe earthquakes:
Erzincan earthquake (Ms=6.8) o f March 13, 1992, Dinar earthquake (Ms=6.0)
o f October 1. 1995, Adana-Ceyhan earthquake (Ms=6.2) of June 27. 1998.
Kocaeli earthquake (M,=7.4) of August 17, 1999 and Duzce earthquake (Ms=7.2)
of November 12. 1999. Most of them affected heavily populated city centers
leading to huge loss of human life and economical crisis in the country. The last
two earthquakes were rather catastrophic and affected a wide area in the
Marmara region. This study mainly aims to evaluate the effect of structural
irregularities and short columns on the seismic response of buildings during the
last Turkey earthquakes, based on the site investigations of the authors. In this
respect, buildings having regular and irregular structural systems are discussed as
reference to the old and new Turkish Earthquake Code. Common types of
irregular buildings in the big cities of Turkey are typically summarized. Many
outstanding photographs of damaged and undamaged buildings taken from the
earthquake areas are given. In addition to their irregular structural systems, many
buildings were heavily damaged or collapsed due to short column failures during
the earthquakes. Since this kind of damage to reinforced concrete (RC) framed
buildings is frequently encountered. short column incidents are seperately
investigated. Further. architectural based damage to buildings are discussed and
possible solutions are proposed to minimize collapses d u e to structural
irregularity and short columns.
Transactions on the Built Environment vol 57, © 2001 WIT Press, www.witpress.com, ISSN 1743-3509
84 Earthquake Resistant Engineering Structures III
1 Introduction
In Turkey, many of the heavily populated and industrialized cities are on or near
the North Anatolian Fault Line (NAF). The region is one of the most important
earthquake prone area in Turkey. Records of past earthquakes reveal that
numerous earthquakes of M,=7.4 and above occurred on this fault line. In the
20th century, Adapazarl, a town very close to Kocaeli (~zmit),experienced an
earthquake of M,= 7.1 in 1967. In terms of the devastation it caused, the Kocaeli
earthquake of August 17, 1999 was the most serious one to hit a Turkish city
centre, and is the second largest earthquake of the century after the 1939 Great
Erzincan earthquake. For the same reasons, it is one of the most serious
earthquakes to have occurred worldwide [ l ,2,3].
There are many lessons to be learnt from these latest earthquakes, as from
others in the past. The reasons for the damage caused are not as simple as just the
use of substandard, poor materials by contractors as many assume. The authors
believe that responsibility must be shared by an entire group of people ranging
from technical personnel involved in the construction process, the owners, and
the authorities, as well as the contractors. Commonly encountered causes of
structural and nonstructural damages are typical such as poor quality of materials
used for the load bearing system (reinforced concrete, reinforcing bars, rolled
steel profiles, wood, stones and brick), irregularity in the load bearing system,
arrangement of the reinforcement in reinforced concrete buildings (especially
beam-to-column joints), inadequate element dimensions, short columns, short
beams, insufficient lateral rigidity (P-A effect), sowweak stories, pounding
between adjacent buildings, local soil conditions and the earthquake source
parameters. Liquifaction phenomenon was widely observed during the Kocaeli
earthquake of August 17, 1999 and many buildings were damaged in Adapazarl
resulting from unsuitable, poor soil conditions.
RC framed buildings have been widely used in Turkey (-95% of the
existing building stock). Most of the low-rise office and residential buildings
with 2-6 stories are generally designed and constructed with moment resisting
RC frames with masonry infill walls. Infill walls made of hollow-brick are
mostly used as nonstructural partition walls. These nonstructural components are
neither well separated nor totally integrated with the building frame. Therefore,
these components of buildings experienced moderate or heavy damage in case of
insufficient lateral stifhess. It is well-known that the stiffer the structure, the less
sensitive the interacting nonstructural components to damage. In this respect,
both positive and negative effects of infill walls were observed during the recent
earthquakes. Steel framed buildings are generally used as one storey, single or
multiple bay industrial buildings. Except for a railway wagon factory in
Adapazarl, no important damage was observed on steel structures during the last
two earthquakes. On the contrary, many of reinforced or prestressed concrete
prefabricated industrial type buildings of 1-2 stories had severe damage or
totally collapsed. This weak performance was generally due to poor joint
detailing and inadequate lateral stifhess. The majority of collapses during the
earthquakes were attributed to poor performance of nonengineered RC frames.
Transactions on the Built Environment vol 57, © 2001 WIT Press, www.witpress.com, ISSN 1743-3509
85
Eartliyzde Reslstani Eng~neerlngStructures 111
Most of the RC buildings with 4-7 stories suffered severe damage during
the last earthquakes. On the other hand, one or two storey nonengineered
masonry or wooden buildings (Figure: l ) and many of engineered RC h e and
frame+shear wall dual systems (Figure:2) survived the strong motion. Slight or
no damage was observed on these kind of buildings. There are several reasons
for the catastrophic losses.
I
1
Figure 1: Undamaged traditional house.
I
Figure 2: Undamaged RC
building with fiame+shear
wall dual system.
This study focuses on the reasons of damages due to irregularity and short
columns in structural system.
2 Effect of structural irregularities
As in many earthquake codes, the new Turkish Earthquake Code "Speczfication
for Structures to be Built in Disaster Areas, 1998 [4] encourages design and
construction of structures having regular structural systems in earthquake prone
areas. It is defined in the code that an irregular building is that "its design and
construction should be avoided because of its unfavourable seismic behaviour".
Structural irregularities are generally classified with respect to their significance.
Structural irregularities in the Turkish Code are classified into two main groups:
Irregularities in Plan and Irregularities in Elevation. Plan irregularities are
frequently encountered as torsional irregularity (Al), floor discontinuities ( M ) ,
projections in plan (A3) and nonparallel axes of structural elements (A4).
Elevation irregularities are described as interstorey strength irregularity-weak
storey (B l), interstorey stiffness irregularity-soft storey (B2) and discontinuity of
vertical structural elements (B3). Detailed site investigations after the last
earthquakes, showed that great majority of fiamed buildings with several
structural irregularities responded in the inelastic range developing heavy
structural and nonstructural damage, while many of regular buildings responded
in the elastic range with minor damage. During the last earthquakes in Turkey,
Transactions on the Built Environment vol 57, © 2001 WIT Press, www.witpress.com, ISSN 1743-3509
86
Earthquake Resistant Engineerzng Strzlcrwes III
most of the building collapses could be attributed to irregular structural
configuration in pladelevation and poor beam-to-column joint behaviour. Site
investigations for seismic performance evaluation of existing buildings show that
the simpler the building the better the seismic behaviour. In other words,
complexity of the structural system leads to unavoidable torsional effects which
are the weakest points of buildings. Figure: 3 and 4 show examples of RC
Figure 3: Torsional irregularity.
Figure 4: Damaged building with
several irregularities, shear failure
of RC shear wall.
A combination of damage types was observed on damaged or collapsed
buildings having several irregularities. Many buildings having large openings in
plan behaved poorly due to lack of diaphragm effect during the earthquake and
experienced heavy damage or collapsed. In such buildings having no RC shear
walls, the nonstructural elements had also severe damage. Most of the RC
buildings with lack of symmetry in the structural layout and elevation had visible
structural damage due to excessive torsional effects. In many of the collapsed RC
buildings, columns which are not connected by the girders in two principle
directions and common examples of cantilevers without continuity have been
encountered, Figure:5,6. These kinds of architectural irregularities in buildings
caused extensive damage or generally collapses due to excessive relative storey
drifts and overloaded structural elements. Irregularities due to architectural
design exist in most of the buildings in the earthquake region. The authors
believe that one of the main reasons in catastrophic damage and loss of human
lives during the earthquakes is the architectural based irregularities in the
building load bearing systems. The collapsed RC buildings having more than
four stories and of low ductility level had no shear walls in the structural layout,
[5,6,7,81.
Extensions above ground floors of buildings, as permitted by the building
regulations, are accomplished by cantilevers of the structural system. One of the
possibie reasons for structural and nonstructural damages in buildings is due to
these cantilevers [7].
Transactions on the Built Environment vol 57, © 2001 WIT Press, www.witpress.com, ISSN 1743-3509
Earthquake Resista~tEngineering S r ~ w ~ t z ~1r11
es
Figure 5: Column failure.
87
Figure 6 : Building with several irregularities.
The vertical deflection at the end of the frequently encountered 1S0m span
cantilevers with nonstructural external walls gradually restricts the serviceability
limits. In Turkish construction practice, in case of the cantilevers not being
continuations of internal beams, in other words, when they protrude from
columns, the adverse effects mentioned above are more pronounced and cause
cracks on the adjacent floors, Figure :7,8. The new Turkish Earthquake Code [4]
includes that, "in all seismic zones, columns at any storey of the building shall in
no case be permitted to rest on the cantilevers or on top of or at the tip of gussets
provided in the columns underneath." This is named as B3 type irregularity in
the Code. In these buildings, the structural and nonstructural problems are also
encountered under the vertical loads. Cantilevered buildings in the Marmara
region where the last earthquakes hit, experienced most significant and
unrepairable damages.
Figure 7 : B3 type building
(before earthquake).
Figure 8 : Compression +shear
cracks in RC column
In Turkey, many of the RC karned buildings have also somweak ground
stories due to the inexistence of partition walls for the commercial use. The
Transactions on the Built Environment vol 57, © 2001 WIT Press, www.witpress.com, ISSN 1743-3509
relative storey displacements were too much (P-A effect) and this naturally
resulted in collapses in these buildings, Figure: 9,lO. Soft storey failures were
more catastrophic in case of buildings having a higher first ground storey,
Figure: 6,10. In addition, lack of structural RC shear walls accelerated the
structural and nonstructural damages with the formation of plastic hinges at the
end points of columns.
Figure 9: Soft storey, timber building.
Figure 10: Soft storey, RC building.
3 Effect of short columns
As well experienced from the past earthquakes, short columns in building frames
are subjected to high cyclic shear forces during the dynamic excitation. The
existence or formation of short columns in earthquake prone areas mostly results
in inevitable shear cracks leading a considerable reduction in the storey yield
strength. Although some precautions can be taken during the design and
construction steps of the building, the occurrence of short columns is not
recommended by the contemporary earthquake codes. It was widely observed in
the earthquake area that many of the collapsed or heavily damaged RC buildings
had short columns to satisfy the architectural needs, Figure: 11,12,13,14.
Figure 1 1: Short column failure.
(Typical band window)
Figure 12: Short column failure.
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Earthquake Res~srantEngineering Srr-uctzrr-es111
89
It was also observed that the compressive strength of concrete in the
structural elements was about 8-12 ~ l m m These
~ . values are rather below the
minimum values recommended by the old and new Turkish Earthquake Codes.
Where compressive strength of concrete was low, the failure mechanism of
buildings with short columns was more catastrophic. In addition, many detailing
deficiencies such as use of inadequate amount of stirmps, lack of appropriate
anchorage of reinforcements resulted in spalling of concrete and buckling of
column longitudinal reinforcement were observed.
Figure 13: Short column failure.
Figure 14: Short column failure.
The Turkish Earthquake Code also gives some strict design and construction
rules related to short columns [4].
4 Conclusions
The last Turkey earthquakes resulted in loss of many human lives and yielded
economic crises within the country. The effect of the soil conditions on the
dynamic response of low-rise RC framed buildings was clear. Buildings having
architectural based irregular structural systems were heavily damaged or
collapsed during the earthquake. Cantilevers of irregular buildings have again
proven to be the primary source/cause of seismic damage. Many buildings
having regular structural system but roughly designed performed well with
minor damage. It should be recognized that the problem lies in frequently
repeated errors, and that the solution is simple.
Beam-to-column joint behaviour mainly affected the performance of RC
framed buildings. The earthquake performance of prefabricated buildings was
poor due to inadequate joint detailing and weak lateral stifhess. Excessive use of
soft stories, short columns, lack of column confinement and the use of framed
systems having strong beam-weak column joints are the reasons of the
catastrophic damage.
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90
Earthquake Resistant Engineering Structzuw III
Use of adequate amount of RC shear walls in RC buildings and use of
ductile steel bracing systems in steel framed multi-storey buildings are strongly
recommended to control storey drifts.
Many slightly or moderately damaged buildings have being repaired and
strengthened after the earthquakes. During this process, a detailed site
investigation based on damage assessment and material testing should be done.
A proper repair and strengthening method has to be selected depending on the
damage level of the building. It is essential that repair and strengthening in the
wake of the earthquakes should be carried out according to design projects,
which are carefully controlled, in strict compliance with the relevant principles,
and take account of the individual characteristics of each building and the extent
to which it is damaged. Buildings having several irregularities but have no
damage may also be appropriately strengthened to minimize irregularity effects.
Buildings, strengthenedlupgraded in this way prior to the recent earthquakes
performed well.
References
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earthquake of 17 August 1999. Yapl, 2 18, pp.65-76,2000.
[2] Cili, F., Celik, O.C. & Sesigiir, H. Architectural based damages to buildings
during 17 August 1999 Kocaeli Izmit) Earthquake. Proceedings of the Third
Japan-Turkey Workshop on Eart quake Engineering, Istanbul, vol. l , pp.45 1458,21-25 February, 2000.
[3] Celik, O.C. Observed bearn-column joint failures during 17 August 1999
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Paramount Role of Joints into the Reliable Response of Structures-From the
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[4] Ministry of Public Works and Settlement. Specification for %r~cturesto be
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[8] Cih,F., Ozgen,K., & Celik,O.C. Evaluation of seismic dama~es to RC
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