Kolon Süreksizliğinin Taşıyıcı Sistem Deprem Davranışına Etkisi

Abstract

Düzenli taşıyıcı sistemler çok fazla varsayıma gerek olmadan analiz edilebilir ve bu sistemlerin deprem davranışları kolayca tahmin edilebilir. Bu tip sistemlerde yükler en üst kattan temele kadar en kısa yoldan, birtakım ek zorlamalara sebep olmadan aktarılır. Genellikle düzenli sistemler sünek bir davranış sergilerler ve bu nedenle deprem bölgelerinde tercih edilirler. Diğer taraftan taşıyıcı sistemler mimari veya estetik kaygılar gibi birtakım zorlayıcı sebeplerle çoğu zaman düzensizdir. Deprem Bölgelerinde Yapılacak Binalar Hakkında Yönetmelik (2007)’ de bu düzensizlikler planda ve düşey doğrultuda düzensizlikler olmak üzere iki kısımda incelenmiş ve düzensiz sistemlere yapıların sismik kapasitesinin arttırılmasına yönelik birtakım ek yaptırımlar getirilmiştir. Deprem Bölgelerinde Yapılacak Binalar Hakkında Yönetmelik (2007)’ nin izin verdiği düşeyde düzensizlik durumu sadece kolonların iki ucundan mesnetli bir kirişe oturması halidir ve bu durumda mesnet kiriş ve bu kirişin birleştiği düğüm noktalarına birleşen diğer kiriş ve kolonlarda, deprem ve düşey yüklerin ortak etkisinden bulunacak tüm iç kuvvetlerin %50 oranında arttırılması şart koşulmaktadır. Bu çalışmada Deprem Bölgelerinde Yapılacak Binalar Hakkında Yönetmelik (2007)’ de tanımlanan düzensizlik durumlarından birisi olan kolonların iki ucundan mesnetli bir kirişe oturması hali ve bu düzensizliğin taşıyıcı sistem deprem davranışına olan etkisi detaylı bir biçimde incelenmiştir. İnceleme için üç tip çerçeve tasarımı yapılmıştır. İlk çerçeve hiçbir düzensizlik içermeyecek şekilde tasarlanmış ve referans çerçevesi olarak kabul edilmiştir. İkinci çerçevede birinci katta kolon süreksizliği bulunmakta olup ikinci kat kolonu birinci kat kirişine açıklık ortasında oturmaktadır. Üçüncü çerçeve ikinci çerçeve ile aynı özellikleri taşımakta olup mesnet kiriş ve bu kirişin birleştiği düğüm noktalarına birleşen diğer kiriş ve kolonlarda deprem ve düşey yüklerin ortak etkisinden bulunan tüm iç kuvvet değerleri %50 oranında arttırılmıştır. Bütün çerçeveler 4 açıklıklı ve 7 katlı olup aynı yük ve açıklık değerlerine sahiptir. Analizde eşdeğer deprem yükü yöntemi ve zaman tanım alanında doğrusal olmayan analiz yöntemi kullanılmıştır. Zaman tanım alanında analizde 3 adet deprem kaydı kullanılmış, kayıtlar Deprem Bölgelerinde Yapılacak Binalar Hakkında Yönetmelik (2007)’ de verilen tasarım spektrumuna benzeştirilmiştir. Sonuçlar taban kesme kuvvetleri, deprem talep deplasmanları ve eleman şekil değiştirme talepleri üzerinden çeşitli grafik ve tablolar yardımı ile karşılaştırmalı olarak verilmiştir.

Earthquakes are undoubtly one of the most important natural disasters in the world. During past years, many earthquakes have occured around the world and Turkey in particular. Turkey is located at one of the most active zones in the world and has faced lots of earthquakes. These earthquakes have caused thousands of life loss and significant economic loss. To prevent these losses structures must be designed regarding the earthquake resistance. Regular structural systems can be analyzed straightforwardly without requiring much assumption and their behavior can be predicted easily. In these systems loads are transferred in the shortest to the foundation without causing additional stresses. Generally the lateral load capacity of these structural systems is high and they display ductile behavior, when they are subjected to the ultimate capacity. That is why, these kinds of structural system is preferred in earthquake prone regions. On the other hand structural systems are often designed having various irregularities due to architectural requirements and esthetic necessities. In seismic codes, structural irregularities are commonly classified as planar and vertical irregularities as it is the case in the Turkish Seismic Code and irregular structural systems are penalized. This is done by increasing seismic demand and by requiring additional reinforcement detailing. The Turkish Seismic Code only allows the structural configuration where a column rests on a beam which is supported at both ends. However, the code requires an increase of 50% in all internal forces and moments at all the cross sections of the beam which supports the column and those of the beams and the columns connecting to the supporting beam. In this study, one of the vertical irregularities defined by the Turkish Seismic Code, i.e., column discontinuity is considered and its effect on the seismic behavior of the structural system is investigated in detail. To investigate this type of irregularity and the approach of the Turkish Seismic Code, three types of planar frames having this specific irregularity are designed. First type of the frame has no irregularity and is considered as the reference frame. It assumed that the second frame has vertical irregularity due to column discontinuity at the first story but internal forces and moments are not increased to investigate the effect of irregularity on the earthquake performance of the frame. Finally, the third frame is designed the same as the second frame but all the internal forces and moments are increased of %50 at the supporting beam and at the beams and the columns which connect to the supporting beam as the Turkish Seismic Code requires. All the frames have four bays, seven stories and same gravity loads. Investigation is carried out by using the static pushover and the time history analysis considering three different earthquake records scaled to the spectrum given by the Turkish Seismic Code. Effects of the column discontinuity on the earthquake performance of the structural system are obtained by adopting the nonlinear static pushover analysis and the nonlinear time history analysis. The results are displayed in various figures and discussed comparatively. Emphasize is put on nonlinear strain demands at the structural elements and the approach of the Turkish Seismic Code are investigated and discussed. The thesis consists of five chapters. First chapter includes introduction, basic concepts about earthquake motion, some of the important earthquakes happened in Turkey and review of previous studies. Second chapter covers brief information about irregularities defined by Turkish Seismic Code 2007. In the third chapter, concrete models such as Mander, Modified Kent and Park, Sheikh and Uzumeri, Saatcioglu and Razvi Models, the concept of ductility, basic principles of plastic hinge, performance based design concept, fundamentals of nonlinear static pushover and nonlinear dynamic time history analysis, structural performance levels and ranges given in Turkish Sesimic Code (2007), Eurocode 8 (2005) and FEMA 356 (2000) are investigated. Fourth chapter contains a numerical study about vertical irregularity which is produced by column discontinuity. To investigate this type of irregularity three types of planar frames are designed as mentioned above. This chaper also includes defining the cross sections using Section Designer option in SAP 2000. Evaluation of structural strain demands and determination of performance levels for structural elements using nonlinear static pushover and nonlinear dynamic time history analysis are explained in detail. Detailed and comparative results in terms of base shear, top displacement and strain demands obtained by using both methods are given in this chapter as well. Fifth chapter covers general results and suggestions. The results obtained by using nonlinear static pushover and nonlinear time history analysis can be summarized as following: i) Column discontinuity changes the load path in irregular systems. Axial force at columns which are placed on the left and right axes where the discontinutiy exists, increases significantly when compared to regular system. On the other hand axial force at the middle columns decrases because the support beam does not support the columns properly due to column discontinuity. Column which rests on a beam can be assumed to have an elastic support. ii) Although there are small differences for some elements, nonlinear strain demands obtained with nonlinear static pushover and nonlinear time history analysis can be regarded as consistent for the regular and irregular system where all internal forces are increased of 50% as Turkish Seismic Code requires. These differences are more noticable for the beams which are joining to the columns at the middle axes where irregularity takes place in irregular system where internal forces are not increased. iii) When the evaluation is carried out in terms of top displacements, it is seemed that both results obtained by using nonlinear static pushover analysis and nonlinear time history analysis are in good consistency. iv) Increasing of 50% in all internal forces and moments at all the cross sections of the beam which supports the column and those of the beams and the columns connecting to the supporting beam is found to have a positive effect on the seismic behavior of the irregular frames. v) In pushover analysis where the lateral load pattern is assumed to be proportional to first mode amplitude, base shear values are found to be smaller than the base shear values obtained by using time history analysis. Adaptive pushover analysis can be used for better comparison. vi) When all internal forces are not increased in irregular system, earthquke damage accumulates on the beams which are joining to the columns on the irregular axes. For that reason the frame does not ensure the life safety performance level which is the target performance level recommend by Turkish Seismic Code for residential buildings. With increasing all the internal forces, irregular frame ensures the life safety performance level.