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Tekrarlı yükler etkisinde betonarme çerçevelerin düğüm noktalarının davranışının sonlu elemanlar metodu ile incelenmesi ve donatı düzenlenmesi

Başlık çevirisi mevcut değil.

  1. Tez No: 55886
  2. Yazar: EMAD ELTALWLY
  3. Danışmanlar: PROF.DR. MELİKE ALTAN
  4. Tez Türü: Yüksek Lisans
  5. Konular: İnşaat Mühendisliği, Civil Engineering
  6. Anahtar Kelimeler: Belirtilmemiş.
  7. Yıl: 1996
  8. Dil: Türkçe
  9. Üniversite: İstanbul Teknik Üniversitesi
  10. Enstitü: Fen Bilimleri Enstitüsü
  11. Ana Bilim Dalı: Belirtilmemiş.
  12. Bilim Dalı: Belirtilmemiş.
  13. Sayfa Sayısı: 130

Özet

ÖZET Son yıllarda birçok ülke araştırmacıları tarafından betonarme yapılarda düğüm noktalarının incelenmesine önem verilmektedir. Özellikle dünyanın çeşitli yerlerinde olmuş depremlerden sonra yapılan incelemelere göre birçok binanın göçmesi, düğüm noktaları detaylarına önem vermemekten kaynaklanmaktadır. Betonarme yapılarda düğüm noktaları tekrarlı yükler etkisinde kritik bir bölge olup davranışlarının incelenmesi ve bu noktaların itina ile teşkil edilmesi (donatılması) gerekmektedir. Bu tez çalışmasında betonarme çerçevelerinin orta, kenar ve üst köşe düğüm noktalarının tekrarlı yükler altında davranışları, düğüm noktasında birleşen kolon ve kiriş boyutlarının belirli oranlarına göre, sonlu elemanlar yöntemi kullanılarak incelenmiş ve bulunan gerilme dağılımından faydalanılarak donatı yerleştirilmesi esasları açıklanmıştır. Ayrıca konu ile ilgili yapılan araştırma ve yayınlar incelenmiştir. Altı bölümden oluşan bu çalışmanın birinci bölümünde problem tanıtılarak tezin amacı belirtilmiş ve konuyla ilgili çalışmaların özeti verilmiştir. İkinci, bölümde betonarme çerçevelerin tipik düğüm noktaları gösterilmiş ve çalışmada yapılan kabuller verilmiştir. Üçüncü, dördüncü ve beşinci bölümlerde sırasıyla betonarme çerçevelerinin orta, kenar ve üst köşe düğüm noktaları detaylı biçimde incelenmiş, gerilme dağılımları çizilmiş ve donatı düzenleme esasları verilmiştir. Altıncı bölümde ise konu ile ilgili sayısal örnekler çözülmüştür.

Özet (Çeviri)

SUMMARY SEISMIC BEHAVIOUR OF BEAM-COLUMN JOINTS IN REINFORCED CONCRETE FRAME USING THE FINITE ELEMENT METHOD AND REINFORCEMENT DETAILS It is surprising that until recently little attention has been given to the desgin of joints in reinforced concrete structures. It appear that after the evalution of working stresses in adjacent members, most designers normally assumed that conditions within the joint, which often had somewhat larger dimensions than the members it joined, were not critical. The gradual adoption of the philosophy of limit state design has exposed the weakness of this assumption. Joints are often the weakest links in a structural system. Much valuable work has been done in this area very recently. Usually causes of failure in reinforced concrete frames, which were seriously damaged or which collapsed during recet earthquakes, could be attributed to unsuitable energy dissipating mechanisms within the framing system and to poorly designed and detailed beams and columns. In this study, the behaviour of reinforced concrete beam-column joints under siesmic loads are modeled using the Finite Element Method (FEM). Two dimensional stress plane FEM with rectangular elements is used to find the stress distributions in typical beam-column joints and different reinforcement details are investigated. The study include six sections, in the first section; the purpose of study and a review of literature in the area of beam-column joints are summarized. In the second section; typical jonits of space frames and the assumptions which are used in the study are explained. In the third, fourth and fifth sections; the solutions of the Finite Element Method of the beam-column interior, exterior and corner joints model with different beam-column ratios are presented. Stress distributions, behavioural models and reinforcement details are discussed. In the last section; numerical examples with some applications are solved. DESIGN CRITERIA Mulistorey reinforced concrete frames in seismic areas are usually designed and detailed for ductility. The ductile design approach is associated with what is generally known as 'strong column-weak beam' behaviour whereby plastic hinges are designed to form in the beams rather than the columns. Beam-column joints in such a moment-resisting frame are subjected to large shear and bond forces. When such a frame is designed for sustained gravity loads and transient wind forces, the beam- column joints may need some attention in terms of adequate strength, with exterior joints being more vulnerable than interior joints. Induced joints shear forces in such case are relatively moderate when compared with those generated by a severe earthquake. The situation becomes more critical when cyclic reversals of earthquake actions need to be accounted for since beam-column joints are prone to extensive cracking, patterns of which can be seen in fig 1. During a severe eathquake, when inelastic lateral frame displacements take place mainly as a result of of plastic hinging XIin the beams, the beams usually develop their flexural strengths at the column faces. Columns above and below a beam-column joint should preferably remain elastic. Thus primary attention in design must be focused on the capability of each beam- column joint to transmit the necessary shear forces, both horizontally and vertically across the inevitably cracked joint core, without jeopardising the desired ductile response of the frame. Therefore, the joint should be considered as an integral part of the column. Design criteria adopted in Europe and New Zealand are intended to ensure that the strength of a beam-column joint core should not be less than that corresponding with the development of the selected plastic hinge mechanism in the frame and that capacity of a column should not be jeopardised by possible strength degradation of the joint. BEHAVIOURAL MODELS Under horizontal earthquake attack, the moments and shear forces generated in the beams and coluns of a building frame introduce internal stress resultants at the faces of joint cores, as illustrated in Fig 1. The stress resultants cause both horizontal and vertical shear forces to act on the joint cores. As a result, internal diagonal tensile and compressive stresses occur, which, if large enough, will lead to diagonal cracking of the core concrete. Unless adequate shear resistance is provided, eventually failure of the joint core may occur along a corner to corner diagonal plane. £ _s 1£M1 /\ / Ts !*E /c2 tS-j.' LFİİ: -) V-V (a) Exterior joint ci p0'“”^ \'clj ^ p" - S x ! - II) -. (b) Interior joint Fig 1 Forces acting on beam-colun joints under seismic actions xnFor the behavior of a typical interior beam-column joint of a seismic frame (Fig lb), from equilibrium conditions, the longitudinal shear force Vjh across the mid-depth of the joint core is Vjh = FsUFcb2+R2b-Vcl = Fâ+Fcb1 + R1b-Vc2 (la) whereas the vertical joint shear force VJV is given by Vjv = Fsc1+Fcc2+F^-Vbl = Fâ+Fcc1+F|f-Vb2 (2a) In the case of the common multilayered arrangement of column reinfocement, the derivation of vertical stress resultats is more cumbersome. By take into account the distances between the various stress resultants and the member dimensions, the following approximation for design purposes is considered to be acceptable: Vjv-Vj^ (2b) where hband hc are the beam and column depths, respectively. Designing for ductility implies that plastic hinges are expected to form in the beams, generally at the column faces. When this condition is reached, the tensile stresses in the longitudinal beam bars attaining forces Fsj and Fs2 can be significantly higher than that given by the characteristic yield strength of the steel, fyk. The stress in the tension steel can reach A,0fyk, where X0 is the over strength factor accounting for deviations from characteristic yield strength and also for strain-hardening due to cyclic inelastic strain reversals. In Nem Zealand and the United Stated, X0 is taken to be 1.25 in the design of ductile frames. For building frames with a regular layout, usually the earthquake-induced shear forces from the beams at the opposite sides of a joint core are similar. It may therefore be assumed that Vbi « Vb2 » Vb, as indicated Fig 2a. A similar acceptable approximation is Vcl = Vc2 = Vc. Noting that Fs2 = Fb2 +Fs'b, eqn. (la) can then be simplified to Vjh=FS+FS-Vc (lb) The magnitude of Vcusually ranges between 12 % and 20 % of (Fsb + Fs2). JOINT SHEAR STRENGTH Beam-column joint cores can be subjected to extremely high shear and bond stresses when subjected to severe seismic loading. If the beams and columns are detailed for adequate ductility, the joint cores could become the critical regions of the structure unless also carefully designed. XlllFig 2 illustrates an interior beam-column joint core which forms part of a moment-resisting frame subjected to earthquake seismic loading. Consideration of the concrete and steel forces acting at the boundaries of the joint core indicates that, to satisfy the equilibrium requirements of the joint core, there must be two mechanisms of joint core shear resistance, i.e. - a diagonal compression strut carrying the concrete compressive force and some bond forces from the longitudinal bars across the joint core. - a truss mechanism of joint core reinforcement carrying the remaining bond forces from the longitudinal bars across the joint core. The design horizontal and vertical shear forces, Vjh and Vjv, can be allocated to the above two mechanisms of joint core shear resistance, i.e. vjh = vch+vsh Vjv = vcv + vsv (3a) (3b) where Vch and Vcvare the vertical shear forces transferred by the diagonal compression strut mechanism (Fig 2b), respectively, and Vsh and Vsv are the horizontal and vertical shear forces transferred by the truss mechanism (Fig 2c). (a) Joint action in equilibrium (b) Concrete strut (c) Diagonal compression field Fig 2 Internal actions in equilibrium at an interior beam-column joint and joint shear resisting mechanisms The horizontal and vertical shear forces Vsh and V^, to carried across beam- column joint cores by the truss mechanism, require both horizontal and verical shear reinforcement which can be provided by column hoops and intermediate longitudinal column bars. It is also evident from Figs 1 and 2 that large steel forces need to be transferred by bond to joint cores over relatively short lengths of longitudinal beam and column bars. For interior joints, when plastic hinges form in the beam at the column faces, a beam bar will be yielding in tension on one side of the joint core and may be yielding in compression on the other side of the joint core. Hence twice the yielding force of the bar may need to be transferred by bond to the joint core. Hence extremely large bond stresses may develop in the joint core which could lead to bond degradation and slip of bars, resulting in yield penetration into the joint core. xivTherefore, the diameters of longitudinal reinforcing bars passing though the joint cores need to be small enough to avoid high bond stresses and excessive slip of bars through the joint core. This limitation can be expressed by requiring an anchorage length of a specified number of longitudinal bar diameters within the joint core. xv

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