Boyutlandırma kriterlerinin çok katlı çelik yapıların lineer olmayan davranışına etkisi

Abstract

Yüksek Lisans Tezi olarak sunulan ve " Boyutlandırma Kriterlerinin Çok Katlı Çelik Yapıların Lineer Olmayan Davranışına Etkisi " ni inceleyen bu çalışma altı bölümden oluşmaktadır. Birinci bölümde, yapı sistemlerinin düşey ve yatay yükler altındaki lineer olmayan davranışı incelenerek yapıların göçme güvenliklerinin belirlenmesini amaçlayan genel yöntemler özetlenmiş ve çalışmanın amacı ve kapsamı açıklanmıştır. İkinci bölüm çok katlı çelik yapıların boyutlandırılmasına ayrılmıştır. Bu bölümde, çalışmada esas alman ortak boyutlandırma kriterleri, burkulma boylarının bulunması, güvenlik gerilmeleri ve taşıma gücü ilkelerine dayanan boyutlandırma yöntemleri ve yönetmelikler, boyutlandırmada yararlanılan bilgisayar programları hakkında bilgi verilmektedir. Yapı sistemlerinin lineer olmayan teoriye göre hesabı amacıyla geliştirilen ve bu çalışmada yararlanılan bir yük artımı yönteminin dayandığı kabuller, yöntemin esasları ve matematik formülasyonu, hesapta izlenen yol ve yöntemin pratik uygulamaları üçüncü bölümde açıklanmıştır. Dördüncü bölümde çok katlı çelik yapıların olası pratik uygulamalarını temsil etmek üzere seçilen yirmi katlı, üç açıklıklı bir taşıyıcı sistem modeli üzerindeki sayısal incelemeler ve bu incelemelerin sonuçları geniş şekilde yer almaktadır. Bu bölümde, taşıyıcı sistem modeli önce TS 648, AISC-ASD, DİN 1050, 4114 ve AISC-LRPD yönetmeliklerine göre boyutlandırılarak enkesitleri ve sistem özellikleri belirlenmekte, sonra birinci ve ikinci mertebe elastoplastik teoriye göre hesaplanarak göçme güvenlikleri, yük parametresi-yerdeğiştirme bağıntıları, plastik kesit oluşumları ve ikinci mertebe etkileri elde edilmektedir. Beşinci bölümde, taşıyıcı sistem modelinin dört farklı yönetmelik esaslarına göre boyutlandırılması ve boyutlandırılan sistemin elastoplastik analizi ile elde edilen sayısal sonuçlar karşılaştırılmıştır. Altıncı bölüm bu çalışmada elde edilen sonuçların değerlendirilmesine ayrılmıştır.

In. this study, the effect of design criteria and codes on the elastic-plastic behavior and collapse safety of multistory steel structures subjected to gravity and lateral loads is investigated. The numerical results obtained in the course of the study are presented and discussed in detail. This study which is submitted as Master Thesis consists of six chapters. In the first chapter, the subject is introduced and the scope and aim of the investigation is presented. In recent years, the design and construction of tall buildings became more popular in our country, especially in major cities. Although most of the tall buildings are designed as reinforced concrete structures, it is believed that, in the near future the use of structural steel in tall buildings will gain importance due to several practical and economical reasons. The recent developments in the non-linear analysis methods of plane and space structures enable engineers to reach more realistic and economical solutions. Further, by the use of these methods, the non-linear behavior and collapse safety of building structures designed by the current codes can be studied in detail. By considering the facts above, a research project on the " Determination of Collapse Safety of Multistory Steel Structures Under Seismic Loads and Earthquake Resistant Structural Design " has been conducted under the sponsorship of Turkish Scientific and Technical Research Council. This study which is a part of the above mentioned research project aims : a- to investigate the collapse safety and the elastic-plastic, second-order behavior of a sample frame designed according to the provisions of the current steel design codes based on both allowable stress and ultimate strength principles, b- based on the numerical results obtained through the non-linear analyses of the sample frame, to discuss the current steel design VI codes with special emphasis on seismic safety and economical design of multistory steel frames. The procedure followed in this study has following steps : a- Selection of a sample structure which represents the tall office buildings commonly encountered in practice. This is a twenty-story, three-bay steel frame. b- Design of the sample frame according to the provisions of the Turkish code TS 648 and the major steel design codes based on the allowable stress method and the ultimate design principles. c- Analyses of the designed structural systems according to the second-order, elastic-plastic theory under constant gravity and increasing lateral loads as well as proportionally increasing gravity and lateral loads by using effective computer programs developed for the practical applications of the non-linear theory. d- Discussion and comparison of the numerical results obtained in these analyses. In this chapter, the second-order, elastic-plastic behavior of structural systems subjected to gravity and lateral loads is also discussed. The non-linear behavior of structures is caused by two different reasons, such as geometrical and material non-linearities. Material non-linearity represents the load carrying capacity of material beyond the proportional limit. As the gravity and lateral loads are increased starting from the initial state, plastic deformations develop at sections where the internal forces reach the limiting values which correspond to the proportional limit. In the case of structures made of ductile material such as steel, the plastic deformations are assumed to be accumulated at certain sections which are defined as plastic sections. This assumption is called as " plastic hinge hypo-thesis ". Geometrical non-linearity represents the effect of geometrical changes on the equilibrium equations. As it known, the theory which considers the geometrical non-linearity is called as " second-order theory ". When both non-linearities are considered in the analysis of a structural system, the collapse of a structure occurs at a load parameter of 3s L2 through the loss of stability. This load parameter is referred to as the VII After the formation of each plastic section, the plastic rotation at this section is introduced as a new unknown. Besides, an equation is added to the system of equations to express the incremental yield conditions. Since the system of equations corresponding to the previous load increment have already heen solved, the solution for the current load increment is simply obtained by the elimination of the new unknown. As it is clearly seen from the above discussion that, the determination of the second-order limit load of a structure is reduced to the determination of an extended system of linear equations and the solution of this system and its subsystems. The fourth chapter is devoted to the presentation of numerical results obtained through the detailed investigation. A twenty- story, three-bay steel frame representing the tall office buildings is selected for numerical study. The selected frame is first designed in accordance with different design codes explained in the second chapter. Then the structural systems designed by these codes are analyzed according to the non-linear theory. Six different analyses are performed for each design. These are first-, and second-order, elastic-plastic analyses under both proportional and non- proportional loadings. The following results are presented for each structural system designed by the codes investigated : a- member sizes of columns and beams and the governing loading conditions, b- characteristics of the structural system, such as weight of structural system, drift ratio, natural period, buckling load parameter, c- first-, and second-order limit loads determined by the non-linear analyses, d- load- drift curves, e- plastic section patterns, IX f- " P-A " effects at the levels of service load and collapse loads, and the variation of these effect along the height of the building. In the fifth chapter, the characteristics and the non-linear behavior of structural systems designed by the application of different design codes are compared. The results obtained in the course of this study are discussed in the six chapter. The main results are given in the following : a- The application of AISC-LRFD code which is based on the ultimate strength design principles results in more economical solution as compared with other codes. b- The weight of the structural system obtained by the application of American AISC-ASD design code is approximately 4 % less than obtained through the Turkish TS 648 Standard. The difference is caused by two reasons. i) The allowable bending stress adopted in AISC-ASD is 10 % above than that adopted in TS 648.. ii) According to AISC-ASD code, the allowable stresses for combined gravity and wind loading are increased by 33 % instead of 15 %. c- The collapse safety of structural systems designed by Turkish standard and DIN norms is above 1.50 for both proportional and non-proportional loading, while collapse safety of 1.44 - 1.45 is reached by American codes under proportional loadings. d- The collapse safety of structures against gravity and wind (HZ) loads is generally 50 % above the safety against gravity and earthquake (HD) loads. e- The governing loading condition for the design of structural system is generally gravity+earthquake (HD) loading. However, in the application of Turkish standard and DIN norms, the design of some of the lower story columns is governed by the gravity+wind (HZ) loading. f- When plastic hinge patterns obtained for wind and seismic loadings are compared, it is seen that the plastic hinges corresponding to wind loading generally accumulate in lower stories. g- The second-order ( P - A ) effects are 5-6 % for service load level and 15-16 % for collapse load levels.