Çelik kiriş-kolon birleşimlerinin düşük tekrarlı yorulma davranışına yükleme hızının etkisi

Abstract

İnşaat mühendisliği mesleğinin bir branşı olan yapı ve deprem mühendisliği, özellikle en yıkıcı doğal afet olan deprem etkilerine maruz kalan coğrafyalarda depreme dayanıklı yapıların tasarlanması ve inşasına yoğunlaşmış durumdadır. Bir binanın depreme dayanıklı şekilde projelendirilmesi aşamasında ilgili coğrafyada ekonomik inşaat yapılmasına imkan verecek yapı malzemelerinin seçilmesi doğaldır. Ülkemizde ekonomikliği nedeniyle sıkça tercih edilen betonarme yapıların yanında, özellikle endüstriyel tesislerde yapısal çelik kullanımı yaygın olup, mimari formları gerçekleştirebilmek adına gittikçe artan şekilde, özellikle çok katlı yapılarda da çelik kullanımı yaygınlaşmaktadır. Malzeme açısından fabrika ortamında kontrollü bir malzeme kalitesi elde edilebilen yapısal çeliğin ülke refah düzeyine bağlı olarak yoğun kullanım alanı bulduğu ülkelerde, özellikle son yirmi yılda gerçekleşen büyük ve yıkıcı depremler sonucunda, bilhassa bu çelik yapıların ana taşıyıcı elemanlarını oluşturan kiriş ve kolon elemanlarının birleşim bölgelerinde çeşitli tasarım ve imalat kusurları saptanmıştır. Depremlerin ardından gerçekleştirilen hasar tespit ve değerlendirme çalışmaları sırasında, o günkü şartnamelere uygun projelendirilmiş yapılarda bile deprem performansı açısından kusurlu ve hasara açık noktalar tespit edilmiştir. Bu sorunları irdelemek ve ilgili yönetmelik ve tasarım kurallarını yenileyebilmek için yıllar süren araştırma programları yürütülmüş, elde edilen sonuçlar ışığında ülkemiz dahil tüm dünyayı etkileyen revize tasarım kuralları ortaya konmuştur. Çelik malzemesi, hızlı yükleme sonucunda akma sınır gerilmesinde pozitif yönde değişim gösteren bir malzemedir. Bu değişim, yapının kiriş-kolon birleşimlerindeki enerji yutma kapasitesini etkilemekte, buna mukabil deprem esnasında yapının vereceği tümsel cevabı değiştirmektedir. Bahsi geçen araştırma faaliyetlerinde de vurgulandığı üzere, çelik yapıların deprem davranışını anlamanın en iyi yolu tam ölçekli deneysel çalışmaların yürütülmesidir. Konu hakkında araştırmalar, elastik ötesi çevrimsel davranış sırasında enerji yutulmasını sağlayan düşük tekrarlı yorulma davranışını, gerek artan, gerekse sabit genlikli deplasman kontrollü testler yardımıyla incelemektedir. Yapılan çalışmalar kapsamında, gerçek bir yapıdan seçilen bir kenar kiriş-kolon birleşimi geometrisiyle oluşturulmuş tam ölçekli deney numuneleri üzerinde sabit genlikli deplasman kontrollü yüklemeler yapılmış, farklı hızlarda gerçekleştirilen bu deneyler sonucunda düşük tekrarlı yorulma davranışını ifade etmekte kullanılan Coffin-Manson denkleminden hareketle oluşturulup literatürde yaygın şekilde kabul görmüş Krawinkler ifadesinde yer alan ve deneyle tespit edilen parametrelerdeki değişimler yükleme hızına bağlı olarak incelenmiştir. Deneyler sonucunda, birleşimlerdeki kümülatif hasarın, hasar oluşturan hareketin etkime hızına göre de değişeceği, ve bir kiriş-kolon birleşiminin deprem performansının, o yapıya etkiyen sismik etkinin yükleme karakteristiğine birebir bağımlı olarak ortaya çıkacağı sonucuna varılmıştır.

Structural engineering, which is a branch of civil engineering profession, has focused on designing and constructing resilient structures, especially in regions which are prone to the most devastating natural hazard : earthquakes. Before designing a structure, selection of building materials dominantly depends on local manufacturing, building and construction costs. In addition to wide usage of reinforced concrete due to relatively lower costs in our country, structural steel construction sector continuously has been expanding especially in industrial structures as well as building-type structures due to the need for complying with architectural design requests. Structural steel has a predefined and documented material quality owing to the controlled production environment in steel mills. However, after the reconnaissance efforts undertaken following the earthquakes experienced in the last two decades, various design flaws and construction problems have been observed in connections of structural framing members, even in countries where structural steel has found widespread use due to the countries' welfare level. So as to assess these observed design and construction flaws and problems, years-long research programs have been initiated under governmental financial support. After a detailed and fruitful research period, revised design rules as well as construction and fabrication recommendations have been devised, all of which have had profound effect on structural engineers' understanding of steel connection behavior and design rules and regulations throughout the world, including Turkey. Steel is a material of which yield strength increases as rate of loading increases. This change of mechanical behaviour affects seismic energy absoption capacity of beam-to-column connection regions, which in turn affects structure's global seismic performance. As is the case during aforementioned research efforts, the best way to understand seismic behavior of structural steel connections has been to conduct full-scale experimental research. As part of outcomes of seismic testing of connections, low cycle fatigue behavior, which describes seismic energy dissipation capability in inelastic regime, has attracted many scientists who employ either multiple-step or constant amplitude displacement loading histories during laboratory works. During this research, dimensions of an actual exterior beam-to-column connection has been used to produce full-scale specimens and these specimens have been subjected to constant amplitude displacement controlled tests. By conducting these tests with different rates of loading, changes in parametres used in Krawinkler equation which is devised based on Coffin-Manson characteristic equation have been quantitatively described. Experimental results yield that cumulative damage in beam-to-column connections differs with the change in loading rate, which is a direct reflection of seismic action characteristics of an earthquake. This dissertation starts with a brief outline of structural engineering and how societies employ building materials based on local geological and economical conditions. As part of this, advantages of using steel as the main structural building material is summarized. As the much respected characteristic of steel, ductility –the capability of undergoing large deformations prior to onset of failure– is remembered, and how engineering community takes advantage of this excellent behavioural property is explained. Failure prone regions of a steel structure such as welded connections as well as bolts are critically emphasized remembering the outcomes of recent strong ground motions that have shaken the world as well as structural and earthquake engineering community. Albeit no damage was observed initially, in depth reconnaissance efforts have shed a bright light on hidden failures in steel beam-to-column connections. Research efforts were undertaken by government authorities and various research groups at universities and legislative bodies, and after years of intensive research efforts, a brand new era of seismic structural steel design has begun that have been transforming design codes worldwide. Following this introduction, concepts of fatigue in structural steel material and components are explained, focusing on low cycle fatigue – that is, failure under inelastic excursions after relatively low repetitions of imposed load or displacement. This behaviour becomes extremely important during seismic events, and resistance to low cycle fatigue directly influences the overall structural resistance to seismic actions. However, low cycle fatigue behaviour is also dependant on rate of external actions, which transforms into strain rate within building components such as beams or columns, and especially at the most critical regions of a steel structure : the beam-to-column connection. In Chapter 2, literature survey has been provided that aims to bring together the current state of knowledge about this phenomena. The readers will easily realize that much research has been carried out in European research institutions and studies complement each other. Chapter 3 summarizes the procedure for selecting the dimensions of the full scale specimen to be tested as the key figure of this research. The following Chapter 4 delves into experimental efforts, starting with the performance at material level. Performance of ASTM-compliant steel coupon specimens are evaluated taking loading rate into consideration. Outcomes of these preliminary tests are employed for preliminary finite element analyses of full-scale specimens. Numerical results have provided valuable information which has been transferred to design of test-rig as well as auxiliary components such as lateral restraints close to cantilever tip of the full-scale specimen. Following are the full-scale tests with varying tip displacement amplitutes and loading ram rates. Test results reveal that failure mode differs slightly based on displacement rate, which has never been observed previously, probably due to capabilities of testing facilities and/or widely accepted quasi-static loading, a.k.a. loading the specimens so slowly that eye can detect and trace a crack propagation for a steel structure, if occurs. Chapter 5 deals with finite element analysis simulations of all the full-scale cyclic displacement test conducted and comparative assessment of results have been carried out. Hysteresis curves are obtained and compared with the test results accordingly. PEEQ index results are shown graphically. Finally, Chapter 6 summarizes the assessment of test results so as to obtain material constants and base equations that may be used to describe low cycle fatigue behaviour of welded-unreinforced flange moment connections employing a Coffin-Manson type equation. It is believed that the outcomes of this research will broaden our understanding of inelastic damage that the structures we design have to withstand during seismic events, from which humankind has suffered a lot and inherently has no chance of isolation due to economical aspects.