Yetersiz sargılanmış kare ve dikdörtgen kesitli betonarme kolonlarda enine donatı detaylarının sünekliğe etkisi
Effects of details of transverse reinforcement on ductility of lightly confined substandard square and rectangular columns
Yazar: EMİNE EVRİM GÜLEY
Danışman: PROF. DR. ALPER İLKİ
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / İnşaat Mühendisliği Ana Bilim Dalı / Yapı Mühendisliği Bilim Dalı
Derece: Yüksek Lisans, 2014
Konu: İnşaat Mühendisliği = Civil Engineering

Özet:
Depremin meydana getirdiği hasarları analiz etmek kadar, deprem sonrasında yapının kullanıcı durumu açısından kimliğini belirlemek de oldukça önemlidir. 2007 Deprem Bölgelerinde Yapılacak Binalar Hakkındaki Yönetmelik (DBYBHY-2007) mevcut bina performans değerlendirmesi ve güçlendirmesi konusunda mühendislere yol açmıştır. Bu yönetmelik kapsamında mevcut bina değerlendirmelerinde özellikle kolonların, yönetmelik dışı etriyelerle sargılandığında, gerçekten sargı etkisi gösterip göstermediği belirsiz bir problem olmuştur ve karar mekanizması bozulmuştur. 
Bu çalışmanın amacı, yönetmelikte belirtilen kuralların dışında üretilen etriyelerin ve çirozların eksenel yük altında kolonda nasıl çalıştığını araştırarak, elemanın sargılı- sargısız beton davranışından hangisine daha yakın olduğunu belirlemektir. Böylelikle, yönetmelik şartlarının tartışılabilirliği deneysel olarak sunulmuştur.
Çalışmada 8 adedi 25x25x50 cm³ boyutlarında kare prizma ve 14 adedi 25x37.5x50 cm³ boyutlarında dikdörtgen prizma olmak üzere toplam 22 kare ve dikdörtgen kesitli kolon numuneleri yapılmıştır. Bunlardan 2 adet kare ve 2 adet dikdörtgen numune referans numunesi olarak imal edilmiştir ve etriyesizdir. Her kolonun etriyeleri ve (6 dikdörtgen numunenin) çirozları 90 ̊ , 112.5 ̊ , 135 ̊ arasında değişen çeşitli kanca açılarına ve 40 mm ile 80 mm  kanca boylarına sahiptir. Mevcut binalara yönelik bir araştırma olduğu için, donatı olarak S220, Φ8 nervürsüz donatı ve 28 günlük beton basınç dayanımı yaklaşık 10 MPa olan düşük dayanımlı beton tercih edilmiştir. Ayrıca, sadece enine donatının çalışmasını görmek için boyuna donatı kullanılmamıştır. Etriyelerin iskelet halinde kalıba yerleştirilmesi için 0.5 cm x 0.5 cm kesit alanına sahip ahşap çıtalardan faydalanılmıştır.
Dayanımını almış numuneler eksenel basınç altında test edilmişlerdir. Test aşamasında belirli yükler altındaki şekildeğiştirme dataları alınmıştır. Bu datalar işlenerek her numuneye ait gerilme- şekildeğiştirme ilişkileri bulunmuştur. Elde edilen ilişkiler, aynı deneysel şartlar için hesaplanmış matematiksel modellerle ait  gerilme-şekildeğiştirme ilişkileriyle kıyaslanmıştır. Bununla birlikte dayanım, süneklik ve enerji yutma kapasitesi açısından değerlendirmeler yapılmıştır. Çalışmada kullanılan matematiksel modeller, Mander Modeli, Saatçioğlu & Razvi Modeli ve İlki ve diğ. Modelidir.       
Yapılan karşılaştırma sonuçlarına göre, beton dayanımı düşük olması sebebiyle, modeller yeterli tahmin oranını sağlayamamıştır. Bu da, düşük dayanımlı beton ve yetersiz sargılama durumu olması durumunda modellerin geliştirilmesi gerekliliğini göstermektedir.

Abstract:
It is well known that analyzing the earthquake damage is important, but determine the structure identity after the earthquake is also important. Due to existing structures were constructed according inadequate construction knowledge and practice, construction conditions were not sufficient and unpredictable. Mostly, using low strength concrete and making inaquate confinement on RC beams and columns effects to building performance level.  Performance levels, immediate occupancy, life safety, collapse preventation determine characterstics of failure and correspondingly the utility of building after the earthquake motion.

The performance calculation firstly used by SEAOC (Structural Engineers Association of California) in 1995. Also, ATC (Applied Technology Council) (1996) and FEMA (Federal Emergency Management Agency) suggested the calculation based on performance analysis. In Turkey, 2007- SSBDA (Specification for Structures to be Built in Disaster Areas) led to engineers about the performance evaluation and strengthening of existing buildings. These standards specify the strain boundaries and failure mode for structural members. So, the engineer must decide by taking into consideration of existing standards. But the main problem is, in the concept of regulations on assessment of existing buildings, what if building has got low strength RC columns confined with substandard lateral reinforcement? Which performance level should be provided then? Cause of acceptance of inadequate confinement reduces the performance level and maybe makes the engineer decition too much conservative. This question impairs the decision- mechanism. 

Basicly, in recent years theoretical and experimental studies proof that the lateral confinement increase the strength and ductility of concrete. Ductility defined as the ability of a material to withstand plastic deformation without rupture. Ductile materials show large deformation before fracture. Usually, if two materials have the same strength and hardness, the one that has the higher ductility is more desirable. At this point, adequate lateral confinement makes passive lateral pressure and resist expansion of concrete. This motion increases the capability of deformation of the concrete. 

The effect of lateral confinement on concrete's stress-strain relationship firstly considered by Consider and the first well-known study on the stress strain curve of concrete with and without confinement was conducted by Richart et al. . Since then there have been numerous analytical models presented in the literature which have been based either on tests on concrete specimens or columns with reinforcement. The earliest analytical models are attributed to Hognestad who used a parabolic expression for the stress strain relationship. Popovics, Sargin et al. proposed a mathematical fractional function for stress strain relationship. Further improvements were developed by Wang et al.. Kent and Park used a fractional equation for the ascending part of stress strain curve and a linear function for descending part of the curve. Sheikh and Uzumeri studied the effects of the distribution and amount of longitudinal and lateral reinforcement, as well as the spacing of lateral ties on the response of reinforced concrete columns. Saatcioglu and Razvi used a second order parabola for the ascending branch, a linear descending branch and a constant residual strength equal to 20% of peak strength.  Many authors looked at the behavior of reinforced concrete columns under compression and proposed confinement models which took account of various parameters such as tie spacing reinforcement, column shape and concrete strength. Some of the authors ave been mentioned above, while others include Hoshikuma et al., Hsu and Hsu, Razvi and Saatcioglu, Chung et al., Mander, Shah et al., Ilkı et al..   

The aim of this experimental study, to investigate the relations of lightly confined substandard stirrups and crossties under the axial compression and determine the behavior of confinement. This experimental work may be answer the questions about the inadequate confined RC column behavior.  
      
In this study, relatively larger size specimens with different hook length and hook angle were tested under axial compression. 6 square and 12 rectangular specimens that are confined by transverse reinforcement of various hook length ( 80 mm and 40 mm) and hook angle ( 90º - 112,5º - 135º ) and 4 specimen for reference totally 22 specimen  tested under axial load. To invesitage existing buildings in the research prefered as reinforcement S220 , Φ8 without rib reinforcement and a 28-day cylinder concrete compressive strength of about 10 MPa low - strength concrete. Dimension of square RC columns and rectangular RC columns are 250x250x500 mm and 250x375x500 respectively. Longitudinal reinforcement was not also used in this study to measure the effectiveness of transverse reinforcement, only wooden slats were used. 
Received samples were tested under monolotic compressive loads by using Instron Satec 1000 RD universal testing machine which capable of making displacement and force controlled loading with a capacity of 5000 kN. In the test setup, there were totally 8 strain gauge, 4 straingage replaced between the rods on each surface of sample and 4 strain gauge measured on the full length of the sample. During test, deformation datas were taken from these strain gauges under certain loads with datalogger. The displacement datas transformed to strain datas by dividing first length. The stress datas found by dividing the loads into the section area. For each sample, stress - strain relationship was found by process these datas. 

The resulting compressive strength, deformability, ductility and stress-strain relationships were compared with mathematical models results for the same experimental conditions. Ductility defined mathematically by the ratio of the deformation corresponding 85% strength on descending branch of stress- strain relationship to the strain corresponding to maksimum strength of confined concrete in this study. 



The mathematical models used in this study are Mander Model, Saatcioglu & Razvi Model and Ilki et al. Model. These three model are well accepted models for confined concrete. However, none of them designed for variable hook angle or hook length.  
	
According to the comparison results, due to low concrete strength , models could not provide adequate estimates the rate . This low strength concrete, and in the case of insufficient wrapping condition indicates need to develop models. However, It is observed that low quality of concrete or lack of adequate confinement still increases the expected strength and ductility surprisingly. Even 90 degree hook angle, confinement provided and the effective area of the column worked and kept going to deformation. Hook length was also a parameter on ductility by penetrated to concrete and secured the working effective area.  Depend on hook angle and length descending parts of stress- strain relationships of samples were variable and because of low strength concrete, the behavior of lateral transverse reinforcement could not observed exactly. During the test loading, firstly low strength concrete rose to the pick load and then slowly damaged by deformations. Thereby, the power dissipation on the concrete made separation and lateral confinement kept out of effective parameter.  

Due to analytical approaches proposed by Ilki et al., Saatcioglu and Razvi,  Mander et al. with the test results, Ilki et al. model predict the stress- strain relation and ductility with a high degree of probability. Then Saatcioglu and Razvi model give a good approximation on espacially maksimum strength, but the discending branch did not match with experimental datas. However, Mander Model was unsuccesful to determinate maksimum strength of concrete and ductility for rectangular and square columns. 

Consequently,  a new model must be proposed for existing RC structures by taking account of various parameters such as hook angle, hook length, tie spacing reinforcement and concrete strength.