Öngerilme kayıplarının çeşitli şartnamelere göre incelenmesi
An Investigation of the prestress losses according to standarts
- Tez No: 66698
- Danışmanlar: PROF. DR. MELİKE ALTAN
- Tez Türü: Yüksek Lisans
- Konular: İnşaat Mühendisliği, Civil Engineering
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 1997
- Dil: Türkçe
- Üniversite: İstanbul Teknik Üniversitesi
- Enstitü: Fen Bilimleri Enstitüsü
- Ana Bilim Dalı: İnşaat Ana Bilim Dalı
- Bilim Dalı: Yapı Bilim Dalı
- Sayfa Sayısı: 158
Özet
ÖZET Betonun, basınç mukavemeti yüksek olmasına rağmen çekme mukavemeti çok yüksek değildir. Betonun bu dezavantajını kapatmak için çekme bölgelerine çelik donatılar konularak betonarme bir cisim oluşturulmuş ve beton ile çelik birlikte kullanılmıştır. Normal açıklıklar, betonarme taşıyıcı sistemle projelendirilebilmekte fakat büyük açıklıklarda, taşıyıcı sistemde büyük çekme gerilmeleri oluşmaktadır. Çekme dayanımı yüksek çelik donanlar kullanılarak bu problem giderilmesine rağmen, betonun çekme dayanımının yüksek olmaması nedeniyle taşıyıcı sistemde büyük çatlaklar oluşmaktadır. Büyük açıldıkların betonarme ile projelendirilmesi güvenlik ve ekonomik açıdan mümkün olmamaktadır. Betonun her bölümde, basmç gerilmeleri oluşturacak, suni dış yük uygulanarak, öngerilmeli beton meydana getirilmiş ve büyük açıklıklı taşıyıcı sistemlerde, çekme gerilmelerinin oluşmaması ya da kısmen oluşması sağlanmıştır. Dış yük olarak, betonun içinden geçen, çelik kablolar kullamlmaktadır. Öngerilmeli beton izostatik taşıyıcı sistemlerde kullanılabileceği gibi hiperstatik taşıyıcı sistemlerde de kuUanılmaktadır. Bu çalışma, genel olarak üç aşamadan oluşmaktadır. Birinci aşamada, öngerilmeli betona genel bir bakış alfanda inceleme yapılmıştır. İkinci aşamada, ongenime kayıpları hesap esasları çeşitli ülke şartname ve yönetmeliklerine göre incelenmiştir. Üçüncü aşamada tek açıklıklı bir karayolu köprüsü öngerilme kayıpları, şartnamelere göre hesaplanıp karşılaştırılmaları yapılmıştır. xrv
Özet (Çeviri)
AN INVESTIGATION OF THE PRESTRESS LOSSES ACCORDING TO STANDARTSUMMARY Concrete is the most important structual material in costruction. The biggest defect of concrete is that its resistance to tensile stress is not too high. For this reason, steel reinforcements are placed in concrete sections which are subject to tensile stress. The resistance of steel reinforced against tensile stress is high. Concrete and steel are used together to form a reinforced concrete structure. Many structures are designed on reinforced concrete basis. In structures with large spans, like bridges, huge amounts of tensile stresses are formed. Steel reinforcements with high strength are used to take up these tensile stresses. However, due to the poor tensile strength of concrete, large and important tension cracks are formed in concrete. This is a situation which is not desired in term of structural safety. In order to prevent the formation of tensile stresses, prestressed concrete is developed. Prestressing is the deliberate creation of permanent internal stresses in a structure or system in order to improve its performance. Such stresses are designed to counteract those induced by external loadings. Prestressing generally involves at least two materials, the stressor and the stresses which, when acting together, perform better than either one taken separately. The principle of prestressing is indeed very convenient and has been widely applied. Its application to steel and concrete is relatively recent but has taken by far the widest proportions. The application of prestressing to concrete is in a way a natural result. Concrete is strong in compression and weak in tension. For design xvpurposes its tensile resistance is discounted. Prestressing the concrete would produce compressive stresses, either uniform or nonuniform, which will counteract tensile stresses induced by external loadings. The original concept attempted to counteract tensile stresses entirely, thus producing a crack-free material during service. However, it has since evolved to counteract only in part externally induced tensile stresses, thus allowing tension and controlled cracking in a way similar to reinforced concrete. This has led to what is called partial prestressing. Partial prestressedconcrete occupies the whole spectrum of the reinforcing range between fully reinforced and fully prestressed concrete. In a way it is a combination of both. Today it has become difficult to talk about either material separately, without talking about their combination. As essentially they use the same basic compenents, steel and concrete, their historical development will eventually be adressed simultaneously. When statically determinate systems are designed with prestressing, a prestressing moment is formed, which is expressed with the formula; Mp = P x e where P is the prestressing force and e is the eccentricity. The situation is different in statically mdeterminate systems, where the value of prestressing moment is not as large as P x e due to bearing reactions. XVIFigure 1 «- ?? N/A M/W Mp/W « - " Figure 2 N/A M/W Mp/W In this thesis study, firstly prestressed concrete is investigated such as prestressed steel, prestressed concrete and their mechanical properties. In the second stage of the thesis study, an investigation of the prestress losses according to standarts and regulations are compared and their foundations are considered. xvnWhat are the prestress losses and their sources ? The total loss of prestress is generally attributed to cumulative contribution of some or all of the following sources ; 1. Stress loss due to the elastic shortening of concrete: Because the concrete shortens when the prestressing force is applied to it, the tendons already bonded to the concrete shorten, simultaneously losing part of their stress. 2. Stress loss resulting from the relaxation of steel: The loss of force in the tendons allowed for in the design should be the maximum relaxation after 1000 hour duration. Tendons at high temperatures or subjected to large lateral loads, greater relaxation losses will occur. 3. Stress loss due to the shrinkage of concrete: The graduall loss, with time, of free water from the concrete, called shrinkage, mduces a shortening in the concrete which leads to a loss of stress in the attached tendons. 4. Stress loss due to the creep of concrete: Creep is caused by the compressive stresses in the concrete. It induces a shortening strain in the concrete in excess of the elastic strain which increases with time and leads to a loss of stress in the attached tendons. 5. Stress loss resulting from friction between the cable and its cover : Friction loss occurs during tensioning of post-tensioned tendons. It represents the difference in stress between the jacking end of the tendon and a section along the member. 6. Stress loss due to the sliding of anchorage wedge : Many post-tensioning anchorages of the wedge type require that the wedge a certain distance in order to lock the tendon at end of jacking. This set or slip leads to a loss of stress in the tendon. xvmEach of the above sources leads to a separate prestress losses in the tendons. Prestress losses occur either instantaneously or with time. Instantaneous losses in pre-tensioned members are generally reduced to the effect of elastic shortening of the members at transfer of prestress. In post-tensioned members they mclude, in addition to the partial effect of elastic shortening, the effect of anchorage set and frictional losses between steel and concrete. Time dependent losses mclude the effect of relaxation in the steel, as well as the effects of shrinkage and creep in the concrete. They effect all prestressed concrete elements. In the third stage of the thesis work, step by step, stress losses of the bridge which has a I precast cross-section and one span ( span length being 25.5 m ) are examined according to regulations. The bridge has two traffic lanes and is in the class H30 - S24 standart truck load is used as live load. To begin with, dead loads and hve loads are calculated. Along the bridge cross-section the unsuitable loading which is formed of Hve loads is determinated. The hve load moment and shear force are found at the span. After the determination of dead load moment and Hve load moment, the prestressing moment is calculated. After that, stress losses of the TS 3233, ACI 318-89, AASHTO and IS-1343 are examined and their advantages and disadvantages in relation with each other are determined. XIX20 120 25 m Figure 3 A View Of Bridge A 850 75 - I k 750 75 i 22Lh. K !s AAA \ / v vs Figure 4 Bridge cross - section A - A xx
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