Enine Donatı Detaylarındaki Yetersizliklerin Betonarme Kolonların Eksenel Yükler Altındaki Performansına Etkisi

Abstract

Dünyadaki betonarme yapı stokunun çoğu eski yönetmeliklere göre tasarlandığı için yeteri kadar güvenli değildir. Bu güvensiz betonarme yapılar deprem anında hemen hasar görmekte ve telafisi mümkün olmayan sonuçların meydana gelmesine neden olmaktadır. Büyük bölümü deprem kuşağında olan ülkemizde ise meydana gelen son depremler, betonarme yapılardaki taşıyıcı sistem elemanlarında meydana gelen alışıla gelmiş hataları bir kez daha ortaya çıkarmıştır. Bu hataların başlıcaları, düşük beton dayanımı, yetersiz enine donatı, kötü işçilik olarak sıralanabilir. Belirtilen bu hatalar yapının deprem anında zarar görmesine neden olmakta ve çok fazla can ve mal kaybı meydana getirmektedir. Meydana gelen depremler sonrası betonarme yapılarda yapılan araştırmalar, yapının en fazla zarar görmesine neden olan yapısal elemanın kolonlar olduğunu ortaya koymuştur. Betonarme bir yapıda kolonlar eğer kendisinden beklenen performansı göstermiş ise yapı ayakta kalmış ve daha az hasarla depremi atlatmıştır. Betonarme yapıların deprem anında ayakta kalması büyük ölçüde kolonlar aracılığıyla olduğu için, yapılan çalışmalar bu yapı elemanı üzerinde yoğunlaşmakta ve yıllardan beri bu konu üzerinde sayısız bilimsel çalışma yapılmış ve yapılmaktadır. Özellikle de betonarme kolonları süneklik ve dayanım açısından belirli bir seviyeye getirmek için enine donatı(etriye) çok önemlidir. Çünkü betonarme kolonlarda enine donatı yerleşimi gerek dayanım gerekse süneklik açısından davranış üzerinde önemli etkilere sahiptir. Literatürdeki çalışmalarda genel olarak yönetmelik kurallarına uygun enine donatı yerleşimleri incelenmiş, sınırlı sayıda çalışmada ise yönetmelik kurallarına bütünü ile uymayan enine donatı yerleşimleri ve bu durumda davranışta ortaya çıkan zayıflıkların giderilmesine ağırlık verilmiştir. Bunlara karşılık yönetmelik şartlarını kısmen sağlayan, kısmen sağlamayan enine donatı yerleşimlerinin davranışa etkisi araştırılmamıştır. Bu konuya yönelik olarak İTÜ Yapı ve Deprem Mühendisliği Laboratuvarında çok kapsamlı bir çalışma sürmektedir. Bu tezde devam eden çalışmaların bir bölümüne ait sonuçlar sunulmaktadır. Farklı enine donatı yerleşimlerine sahip kare ve dikdörtgen enkesitli betonarme kolon numuneleri eksenel basınç etkileri altında deneye tabi tutulmuştur. Deneysel çalışmadaki parametreler kanca açısı, kanca boyu ve etriyeler arası mesafedir. Eksenel yükleme deneyleri için yerdeğiştirme kontrollü 5000 kN kapasiteli İnstron test cihazı kullanılmıştır. Deneyler ile ilgili yönetmeliklere kısmen uyan enine donatı yerleşimleri için yukarda belirtilen deney parametrelerinin değişimine bağlı olarak betonarme kolonların davranışı incelenmiş, deneysel olarak elde edilmiş olan davranış özellikleri, literatürde mevcut olan analitik modellerle, dayanım, süneklik ve enerji yutma kapasitesi açısından karşılaştırılmıştır.

It is well known that a high percentage of existing structures in earthquake zone of the world need to be strengthened for reducing the risk of heavy loss of life and economic losses. These structures were constructed according to older codes or inadequate construction practice. Particularly in developing countries, the seismic performance of the structures may be significantly poor due to insufficient ductility and low concrete strength. The structural members of this type of structure may experience severe damage due to low deformability and axial capacity of the structural members. Recent earthquakes also occurred in Turkey where the most of its parts in earthquake zones revealed usual errors that occur in structural elements of reinforced concrete structures once again. These errors are primarily low concrete strength, inadequate transverse reinforcement, poor workmanship can be listed as. Especially, low concrete quality and lack of adequate confinement reinforcement are among most common deficiencies for existing reinforced concrete structures and RC column elements. These deficiencies may cause structural damage leading to the collapse of structures during earthquakes due to insufficient compressive strength, deformability and ductility. To overcome these problems, RC column members that suffer from low quality of concrete or lack of adequate confinement can be wrapped by transverse reinforcement (stirrups). These stirrups can provide passive lateral pressure that results with enhancement in compressive strength, ductility and stress strain relation of the member. That errors as low concrete strength, inadequate transverse reinforcement, poor workmanship cause damaging to the structure during earthquake and a lot of loss of life and property. Researches in reinforced concrete structures after earthquake has showed that RC columns are critical structural members for earthquake behavior of the reinforced concrete structure systems. If the reinforced concrete (RC) columns show the expected performance in reinforced concrete structure during earthquake, generally building can stand and get over the earthquake. A building keeps up through the columns of reinforced concrete structures during earthquake, for that reason studies have focused on the structural element and countless scientific studies have been done and continued for many years. Some of the researchers who are working on this subject; [Kent and Park 1971; Sargin et al., 1911; Priestley et al., 1981; Park et al., 1982; Sheikh and Uzumeri, 1982; Mugurama et al., 1983; Dilger et al., 1984; Ahmad and Shah 1985; Mander et al., 1988a, 1988b; Mugurama and Watanabe, 1990; Saatcioglu and Razvi 1992; Cusson and Paultre 1995; Saatcioglu et al., 1995; Hoshikuma et al., 1997; Ilki et al., 1997] can be ordered. Especially, transverse reinforcements are very important to bring a certain level of strength and ductility for reinforced concrete columns. Because the configuration of transverse reinforcement has a major impact on the behavior of both strength and ductility of reinforced concrete columns. Although extensive experimental data is available on transverse reinforcement confined concrete members, most of these studies are small size experimental parameters without hook height, hook angle and distance of transverse reinforcement. An extensive analytical and experimental research program on the behavior of RC column members is under progress at the Structural and Earthquake Engineering Laboratory (STEEL) of Istanbul Technical University. In this study, relatively larger size specimens with different hook height, hook angle and distance between the stirrups were tested under axial compression. 7 square specimens and 12 rectangular specimens that are wrapped by transverse reinforcement of various hook height(80 mm,40 mm), hook angle(135o,112.5o,90o) and the distance between the transverse reinforcement(100 mm, 50 mm) are tested under axial load and cylinder concrete strength in all columns was 30 MPa. Dimension of square RC columns and rectangular RC columns are 250x250x500 and 250x375x500 respectively. Longitudinal reinforcement was not also used in this study to measure the effectiveness of transverse reinforcement, only the transverse reinforcement was used. Wooden slats instead of longitudinal reinforcement were used to connect the transverse reinforcements. The average unconfined concrete strength was 35.6 MPa for 7 square RC columns and 35 MPa for 12 rectangular RC columns. The parameters of the experimental work were hook height, hook angle and the distance between the transverse reinforcement. Ties were also used in addition to the transverse reinforcement in rectangular RC columns. After the tests of unconfined and confined specimens, it was observed that the contribution of the transverse reinforcement to deformability, ductility, stress strain relation and strength enhancement was remarkable for specimens with square and rectangular RC columns. RC column specimens were tested under monotonic or sustained compressive loads by using an Instron Satec 1000 RD universal testing machine with a capacity of 5000 kN. Testing machine is capable of making displacement and force controlled loading. And experimental results were compared with the analytical models. These models are Mander et al.(1988), Ilki et al.(2004) and Saatçioğlu and Razvi (1992). According to the test results, significant enhancement on stress strain relation, load carrying capacity, ductility ratio, energy dissipation and core concrete strength. After the axial compression tests on square reinforced concrete (RC) columns with adequate transverse reinforcement, the following conclusions were derived. Transverse reinforcement in square RC column specimens (h/b = 2) cross-sections significantly improves the compressive strength, deformability, ductility, stress-strain relationships of these specimen. As a result of these improvements, significant enhancement can be obtained for energy dissipation capacities and ductility of these members. Effect of confinement in specimens with a big hook angle and hook length became more effective than specimens with a small hook angle and hook length. The smaller the distance between the transverse reinforcements, the bigger ductility of square reinforced concrete column specimens. Depend on the angle and length of hooks descending parts of stress-strain relationships of specimens are very different each other. According to the comparisons of three analytical approaches proposed by Ilki et al(2004)., Mander et al(1988). and Saatçioğlu and Razvi(1992) with the test results, it has been observed that Ilki et al. model can accurately predict the stress-strain relation. But this comparison shows that the models do not predict the behavior of the RC columns with the non-code-complying transverse reinforcements accurately. Consequently, a model including the hook angle and the hook length is required for determining the performance of existing RC structures with the non-code-complying transverse reinforcement detailings.