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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. Transactions on the Built Environment vol 57, © 2001 WIT Press, www.witpress.com, ISSN 1743-3509 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. Transactions on the Built Environment vol 57, © 2001 WIT Press, www.witpress.com, ISSN 1743-3509 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 [ l ] Celik,O.C., Ci11,F. & O z g e n , ~ . Observations on the Kocaeli (Izmit) 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 Kocaeli (Izmit) and 12 November 1999 Diizce, Turkey earthquakes. The Paramount Role of Joints into the Reliable Response of Structures-From the Classic Pinned and Rigid Joints to the Notion of Semi-rigidity, NATO ARW(AdvancedResearch W o r h h o ~ )Ouranoupolis, , Halkidiki, Greece, Vol. 4, pp. 197-206, 21-23 May, 2000. [4] Ministry of Public Works and Settlement. Specification for %r~cturesto be Built in Disaster Areas, 1998. [5] Youd, T.L., Bardet, J.P. & Bray, J.D. (eds). Kocaek Turkey, Earthquake of August 17, 1999 Reconnaissance Report, Earthquake Spectra, E E ~vol. , 16A, 2000. [6] Cili,F., Celik,O.C., & Sesigiir,H. Seismic damage evaluation in reinforced concrete buildings after 17 August 1999 Kocaeli (Ianit) earthquake. Proceedings of ITU-IAHS International Conference on the Kocaeli Earthquake, Istanbul, 02-05 December, pp.3 13, 1999. [7] Celik,O.C., Cih,F., & Ozgen,K. Cantilevers in reinforced concrete structures. Proceedings of l lrhEuropean Conference on Earthquake Engineering, Paris, 06-1 1 September, CD-ROM, 1998. [8] Cih,F., Ozgen,K., & Celik,O.C. Evaluation of seismic dama~es to RC buildings during 1992 Erzincan earthquake. Proceedings of 10' European Conference on Earthquake Engineering, Vienna, 28 ~ ~ ~ September, ~ ~ t Vol. l , pp.773-778, 1994. I -