Uzay kafes sistemlerde alüminyum çubukların çekmedeki davranışı
Başlık çevirisi mevcut değil.
- Tez No: 55797
- Danışmanlar: PROF.DR. TEVFİK SENA ARDA
- Tez Türü: Yüksek Lisans
- Konular: İnşaat Mühendisliği, Civil Engineering
- Anahtar Kelimeler: Uzay kafes sistemleri, Çubuk elemanlar, Space truss systems, Bar elements
- Yıl: 1996
- Dil: Türkçe
- Üniversite: İstanbul Teknik Üniversitesi
- Enstitü: Fen Bilimleri Enstitüsü
- Ana Bilim Dalı: Belirtilmemiş.
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: Belirtilmemiş.
Özet
ÖZET Uzay kafes sistemler, 1950'li yıllardan beri özellikle hareketli yükleri az olan çaü ve benzeri bina bölümlerinde, büyük açıklıkları geçmek için yaygın olarak kullanılmaktadır. Bu sistemlerin çalışma şekli, yüklerin düğüm noktası tarafından alınarak bağlantı elemanlarıyla çubuklara aktarılması ve çubukların sadece eksene! kuvvet taşıması prensibine dayanmaktadır. Türkiye'de de, özellikle 1985'ten itibaren yaygın olarak kullanılmaya başlayan uzay kafes sistemlerin teşkilinde, büyük bir oranda Mero sistemi kökenli düğüm noktası detayı ve malzeme olarak da çelik tercih edilmektedir. Çeşitli endüstri dallarında karşılaşılması olağan bir malzeme olan alüminyumun, yapısal uygulamalarda adını duyurmaya başlamasından sonra uzay kafes sistem imalatında da kullanıldığı bilinmektedir. Bu yüksek lisans çalışmasında, uzay kafes sistemin alüminyumdan üretilmesi düşünülen elemanlarının çekme etkisi altındaki taşıma yükleri deneysel olarak incelenmiştir. Bu amaçla, USKON A.Ş. Fabrikasında yapılan deneylerle, çeliğe göre daha hafif ve korozyona dayanıldı bir malzeme olan alüminyumun Türkiye'de uzay kafes sistem üretiminde kullanılması yolunda bir adımatılması amaçlanmıştır. Bu çalışmada, çok kullanılan ve iyi bilinen bir malzeme olan çelikle karşılaştırmalı olarak incelenen alüminyum uzay kafes sistem elemanlarından alman olumlu sonuçlar, araştırmalara devam edilmesi gerektiğini ortaya çıkarmıştır. viii
Özet (Çeviri)
SUMMARY BEHAVIOUR OF ALUMINIUM TENSIAL MEMBERS IN SPACE FRAME STRUCTURES Light construction and economy in material are current requirements in the building field. The distribution of loads, spreading of forces, the reduction of bending moments and the utilization of material to its limits - these are the tendencies that become decisive for the civil engineer. Simultaneously, they imply a new concept in building construction, the space structure which is as useful technically as it is beautiful. It represents both superstructure and design element as well. Figure 1.1 An example of space frame structures ixSince 1950, space truss structures have been used widely, especially at roofs or similar parts of constructions on which there is no effective live loads. The cause of this choice, using space truss structures, can be defined as: -lightness of weight, -high degree of in determinancy, -almost no bending element so, using minimum quantity of materials, -freedom of drawing and forming, -great facility in erection, in disassemble and in changing the truss members. 'jy//w/s/s/////w///w/.'/te s/,'////;;/;/s/,v/j>s/,y//</,'7. Figure 2. The member of space frame structures To a certain degree, the space structure may be considered as a kind of framework extended into the third dimension. Forces which have been distributed up to now within a plane, are nowadays omni-directionally branching out into space. Most of the forces involved are balanced this way, forming a homogenous field without peak loads which is able to withstand almost any outside loading. Inner stresses are reduced, cross sections of compression and tension members are decreased - with the result mat less material with less weight is required. Prefabrication as well as saving of labour are further contributions to the outstanding economy the new construction principle will offer when widery used. An increasing interest in space structures has become very noticeable during recent years. Engineers realized many years ago that space structures require less material than ordinary linear systems and that, if properly designed, they can be highly economical in cost It cannot be considered or analysed as a plane system. It can resist loads applied at any point, at any inclination to the surface of the structure and acting in any direction. The use of aluminium alloys in civil engineering as a structural material represents a new trend. Engineers are more familiar with steel structures, which are better known and more widely used in metal fabrication. In this way a continuous comparison between the two different materials will be given, in order to emphasize the specific characteristics and the advantages and disdvantages of aluminium alloys. A preliminary presentation of this new building material - aluminium alloys-is made from the metallurgical and technological points of view.Considerable electric power is necessary to fabricate aluminium. On the other hand, owing to the low melting point of aluminium (660°C), the fabrication of aluminium alloys does not need a large amount of power. One of the biggest advantages of aluminium alloys is that they can be extruded by press. The extrusion process allows the manufacture of profiles of any shape that cannot be obtained by hot rolling. Aluminium alloys, which are usually called light alloys, have been developed in order to increase the strength of the base metal, aluminium, which is very ductile and corrosion resistant. It seems important to observe that aluminium alloys require very small quantities of additional elements. The main elements used in the alloys are magnesium, silicon, zinc, copper and manganese. Nickel, cobalt, choromium, iron, titanium and zirconium are used as additional elements. Table 1. Physical properties of aluminium, steel and stainless steel Casing alloys, which have to be remelted an then casted. Wrought alloys, which have to be hot worked or cold worked without being remelted. After cold working, strength is increased and ductility is decreased. Pure aluminium has low strength conversely, it is a very ductile materiaL If the material is cold worked the strength is increased up whereas ductility is drastically decreased. Pure aluminium is highly corrosion resistant. AlMgSil is the alloy that will be used during the experiments. Magnesium and silicon are the main alloying elements. They form the inter metallic compound MgaSi which readily passes into solution in the aluminium. These alloys are particularly suitable for extrusion. These alloys are used either in welded structures or in bolted and riveted connections. xiThe main physical properties of aluminium at room tmeperature compared with those of steel and stainless steel are given in Table 1. The density of aluminium is approximately one-third of the density of steel and Young's modulus is also approximately one-third that of steel. The thermal expansion coefficient of aluminium is twice that of steel The principal mechanical properties can be derived from a tension test. The load-displacement diagram is usually represented by a continuous curve without yielding. The diagram can be divided into a linear elastic portion up to the proportional stress a nonlinear portion up to a“knee”, and a strain-hardening portion ( the slope of which depends upon the type of alloy ). The stress corresponding to a residual strain of 0.2 percent is assumed to be the stress beyond which the behaviour of the material is not elastic. Ultimate strength /, corresponds to a strain value which can be defined as the limit of“uniform elongation”corresponding to necking of the specimen. At this point strees increases at the neck and large deformations of the cross-section can be observed. At the same time the testing machine records a load which is decreasing owing to the failure of the specimen, with a value of elongation equal to st. This value is usually used to evaluate the ductility of the material, that is, the capacity of the material to withstand large plastic deformations without experiencing feilure. It has already noted that the large family of duminium alloys covers a wide range of values of strength. All these alloys are sufficiently ductile to be used in structural applications. Furthermore, in aluminium alloys a transition temperature cannot be defined. The transition temperature is defined in steel as the temperature below which brittle fracture is most likely to occur. Usually aluminium needs no protection against atmospheric or chemical corrosive agent. In contrast to steel, in which the corrosion process in not naturally inhibited (apart from self-protecting steels), corrosion processes in aluminium tend to cease naturally. Corrosion can be divided into two classes: that which damages the aesthetics of the structure, and that which damages the safety of the structure. Corrosion in aluminium can be considered to belong to the first type. The more pure the aluminium, the higher is the corrosion resistance. When steel bolts are used to make a connection, the bolts should be galvanized or electroplated. On the other hand, Young's modulus is one-third that of steel, thus giving more frequent problems of deformation and of instability. Aluminium alloys have a strain-hardening portion without a horzontal line corresponding to yielding. Ultimate elongation is lower than that of steel. Howerer, it should be pointed out that it is not always possible to take advantage of this benefit offered by aluminium alloys, especially when the material is under compression. It should also be noted that the value of the coefficient of thermal expansion of aluminium is twice that of steel. In contrast to steel structures, aluminium alloys do not have a transition temperature. The physical and mechanical properties of aluminium alloys have been examined, and a comparison of these aloys with steel has been made. The following conclusions may be drawn : (a) Aluminium alloys represent a wide family of materials and cover the range of strength offered by the most commonly used mild steels. (b) Corrosion resistance normally makes it unnecesssary to protect aluminium structures. (c) It is possible to have bolted, riveted, or welded connections. d) The advantage of the lightness of the material is offset by the higher deformabiliy of aluminum, which gives a higher susceptibility to instability. xue) The material is not prone to brittle fracture, though particular attention should be given to those problems in which higher ductility is required. f) The extrusion fabrication process makes it possible to increase the geometrical propeties of the cross-section in designing a shape that simultaneously gives the minimum weight and and the highest structural efficiency. In particular, lightness makes it possible to have : Simpler erection Transport of fully prefabricated components Reduction of loads transmitted to foundations Economy of energy either during execution and / or sevice Reduction of physical labour. Corrosion resistance makes it possible to have : Reduction of maintenance expenses Better performance in corrosive environmets. xiii
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