Mevcut öngermeli kirişli betonarme bir köprünün AASHTO LRFD'ye göre çelik I kirişlerle kompozit tasarımı ve karşılaştırmalı analizi
Design and comparative analysis of an existing prestressed concrete bridge with a composite steel I-girder bridge according to AASHTO LRFD specification
Yazar: GİZEM KOLGU YİĞİT
Danışman: PROF. DR. ELİŞAN FİLİZ PİROĞLU
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / İnşaat Mühendisliği Ana Bilim Dalı / Yapı Mühendisliği Bilim Dalı
Derece: Yüksek Lisans, 2019
Konu: Mühendislik Bilimleri = Engineering Sciences ; Ulaşım = Transportation ; İnşaat Mühendisliği = Civil Engineering
Dizin: Kara yolu köprüleri = Highway bridges ; Kompozit kirişler = Composite beams ; Çelik kirişler = Steel beams ; Öngerilmeli beton kiriş = Prestressed concrete beam ; Öngerilmeli kirişler = Prestressed beams

Özet:
Ülkemizde, karayolu köprülerinin inşaatında uygulamada sunduğu avantajlar, maliyet, basınca çalışan beton elemanların çekmede de etkin olabilmesinin sağlaması gibi birçok avantajı sebebiyle tipleştirilmiş öngermeli betonarme kirişli köprüler tercih edilmektedir. Bu metot kullanılarak köprü kirişlerinde sabit ve hareketli yüklerden dolayı oluşacak çekme gerilmelerinin, uygulanacak öngerme kuvveti ile azaltılması veya tamamen ortadan kaldırılması amaçlanmıştır. 
Kirişli köprüler kısa ve orta açıklıklarda en çok tercih edilen ve en basit olan köprü tipidir. Kesit tercihinin I kiriş olarak yapılması aynı zamanda kesitin kayma ve eğilme dayanımının yüksek olmasını sağlamaktadır. 
Çeliğin diğer yapı malzemelerine göre üstün olduğu hususlar ise; dayanımı ve sünekliğidir. Betonla kıyaslığında çelikte çekme dayanımı-maliyet oranı daha yüksek iken basınç dayanımı-maliyet oranı daha düşük olmaktadır. Bunun yanı sıra rijitlik-ağırlık oranları kıyaslandığında çelik betondan çok daha yüksek sonuçlar vermektedir. Bu nedenle çelik kirişler köprü inşaatlarında etkili ve ekonomik olup tasarımda tercih sebebidir. 
Diğer taşıyıcı yapı sistemlerine göre minimum ölü yükle daha büyük açıklıklarda daha ağır yükler taşıyabilmesi, bunun bir sonucu olarak da köprü temellerinin küçülmesine olan etkisi çelik köprülerin avantajlarındandır. Tüm prefabrik malzemeler gibi çelik köprü kirişlerinin de büyük bir kısmı atölyelerde daha önceden imal edildiğinden, inşa süresinde kazanç sağlamaktadır.
Bu tez çalışması kapsamında; köprüler ile köprü şartnamelerinin gelişimine kısaca değinilmiş ve mevcut öngermeli betonarme bir viyadük tasarımının en uzun hesap açıklığındaki üstyapısı ele alınmıştır. Birim fiyatının fazlalığı, hammadde temininde yaşanan zorluklar, tasarım ve uygulamada uzmanlık gerektirmesi nedeniyle kullanımı ülkemizde fazla yaygınlaşmamış ancak öngermeli elemanların yerini alarak daha optimize tasarımlar elde edilebileceği düşünülen çelik kompozit kirişler kullanılarak yapılan yeni bir tasarım ile mevcut öngermeli betonarme kirişli tasarım karşılaştırılacaktır.  
Beton tabliye ve çelik kirişten oluşan bu iki farklı yapısal eleman kayma bağlantıları kullanılarak birlikte çalıştırılmak suretiyle, dış yükler altında eğilme sonucu kesitte meydana gelen çekme gerilmelerinin çelik kiriş tarafından, basınç gerilmelerinin ise beton tabliye tarafından aktarılması sağlanmaktadır.  Birlikte çalışması veya yük aktarması sağlanmış malzemelerin daha yüksek eğilme dayanımları sayesinde daha uzun açıklıkların geçilmesi ayrıca mümkün olmaktadır.
Kompozit kirişlerin tasarımı Midas Civil 2019 bilgisayar programı kullanılarak yapılmış olup, tasarımda  AASHTO LRFD 2010 (American Association of State Highway And Transportation Officials-Load and Resistance Factor Design) şartnamesi esasları uygulanmıştır. Yük ve dayanım katsayılarının kullanıldığı bu felsefede yapının ekonomikliği, estetikliği ve güvenilirliği esas alınarak istatistikler sonucunda oluşturulan yük ve dayanım katsayıları kullanılmıştır.  
Kompozit tasarım incelenirken iki farklı malzemenin elastisiteleri oranında kesit dönüşümleri yapılarak; kompozit olmayan, kısa-dönem kompozit ve uzun-dönem kompozit etkilerine yer verilmiştir.  Sınır durumları baz alınarak yapılan analizlerde kesitlerin tüm koşulları sağladığı görülmüş ve Bölüm 3'te tasarım esasları detaylandırılmıştır.  Hareketli yükler altında en çok zorlanma dış kirişlerde meydana gelmiş olup, çalışma kapsamında boyuna kirişlere ek olarak enine kiriş, berkitme ve stud çivisi hesaplarına da yer verilmiştir.
Statik analiz sonrasında  mevcut köprü üstyapısı ile yeni üstyapı tasarımının maliyet analizi yapılmış ve tasarım özellikleri karşılaştırılmıştır. Öngermeli betonarme köprülerin söz konusu açıklıkta daha ekonomik olduğu, ancak çelik tasarımda kiriş sayıları ve enkesit alanlarının azaldığı, dolayısıyla üstyapı ağırlığında azalma meydana geldiği görülmüştür.

Abstract:
In Turkey, for construction of highway bridges, standard prestressed concrete I girder bridges are commonly preferred due to their advantages in application, cost and providing efficiency in tension for concrete compression members. By applying prestressing forces it is aimed to reduce or completely eliminate the tensile stresses of bridge longitudinal beams subjected to constant and moving traffic loads.
Girder bridges are the most preferred and the simplest type of bridges for short and medium span lengths. It is the simplest way to select an I-beam as cross-section which provides a high shear and flexural resistance for the beam.
Steel is superior regarding other building materials due its strength and ductility. Compared with concrete, the tensile strength-cost ratio is higher for the steel while the compressive strength-cost ratio is lower. Besides, when the stiffness-weight ratios are compared, the results obtained for steel are much higher than concrete. For this reason, steel beams are effective and economical in bridge construction, thus they are preferred in design.
Based on literature survey on development of steel beams; in a report by Paul Christophe in 1902, it is seen that slab and beam systems have been implemented in European construction market by using steel sections. 
Another example is Mathias Koenen's (1849–1924) flat-soffit, later (from about 1892) ribbed floor in which the underlying steel sections carry the tension, and the arching concrete infill sections are solely responsible for carrying the compression. Despite the lack of shear connectors, tests confirmed Koenen's assumptions regarding the structural behaviour. 
With the start of reinforced concrete applications, steel reinforcement systems are included. In the report written by R.A.Caughey in 1929, it was emphasized that there should be a mechanical connection between concrete and steel in order to provide horizontal shear strength. Following the first successful steel composite bridges having 6 ~ 24 m span lengths in America, the American Association of State Highway Officials (AASHO) standardized steel-concrete composite bridges in 1944. In 1960, it was decided to use standard stud nails as shear connectors.  As a result of the development on reinforcement technologies, connectors and cross-sectional properties in the late 1950s, the prestressed concrete structures were no longer able to compete with steel composite systems.
The advantages of steel bridges are the minimum dead load for other bearing systems and the ability to carry heavier loads at larger span lengths and so causing the reduction of bridge foundations. As all prefabricated materials, large part of steel bridge beams is already pre-fabricated in a workshop, they provide benefits during the construction period. 
Within the scope of this thesis; the development of bridges and bridge specifications are briefly discussed  by regarding the mid-span of an existing prestressed reinforced concrete multi-span viaduct design. The current concrete bridge design will be compared with the new design by using steel composite I girders which are not commonly used in Turkey because of the their high unit price and the need for expertise in design and application. However it is believed that composite girder designs will replace prestressed girder constructions for bridges.
Two different structural elements used for composite sections, i.e. concrete deck and steel girder, are operated by using the shear connectors where the tensile stresses in the section occurred as a result of bending under external loads are resisted by the steel beam while the pressure stresses are transferred by the concrete deck. Due to the higher bending resistance of materials as well as the cross section?, it is also possible to span longer lengths.
In this study, steel composite I girders were designed by using Midas Civil 2019 program according to the design principles of AASHTO LRFD 2010 (American Association of State Highway And Transportation Officials- Load and Resistance Factor Design). The load and resistance factors, which depend on economy, reliability and statistics for structures, were used regarding to this design philosophy. Another significant issue for steel composite I girder design is the used formulas which vary according to the compactness of the steel beam sections (web, flange etc.).
For the multi-span bridge the selected mid-span length was 41.5 m (from support to support), depth of built-up steel girder was 2.20 m and cross-section area of 8 main girders was approximately 0.114 m2. Six rows of cross-frames were used where their unbraced length was equal to the spacing of longitudinal beams, i.e. 2 meters. Studs with 19 mm diameter had 100 mm depth,  700 mm pitch and 100 mm transverse spacing. For each row, 5 stud type shear connectors were used.
The number of beams was reduced by ensuring the requirement given for the minimum beam height by AASHTO. While analyzing a composite section, its non-composite, short-term and long-term composite effects were considered on its cross-sectional deformations according to the ratio of the elasticity modulus of two different materials. In the analysis, based on the limit states, it was seen that the selected composite cross section resisted and satisfied all the required conditions and design principles given in Section 3. 
The maximum stress obtained under moving traffic loads occurred in outer beams. In addition to the longitudinal steel beams, cross frames, stiffeners and stud type shear connectors were also designed in this study.
Composite or non-composite I sections were examined under negative and positive flexure effects regarding the Strength Limit State. The longitudinal bridge beams designed within the scope of this thesis were assumed as a simple span beam and under positive flexure. 
For Service Limit State, flange stresses that will cause permanent deformations as well as flexural buckling expected to occur at the girder web were determined by using a computer software. Elastic deformation results were selected from computer program data, manually. 
For Fatigue Limit State, detail category C' was selected while all conditions for constructibility given in the specification were also provided.
By using special earthquake surveys, the spectrum function was updated in the new design by using 2475-year mean return period earthquake (an earthquake with a 2% probability of exceedance in 50-year.
After the design process, the cost analysis for the new superstructure design as well as for the existing structure was done and compared, finally both design features were discussed. It was  evaluated that prestressed reinforced concrete girder bridges were more economical in the aforementioned span lengths, however in steel composite design, the number of beams and cross-sectional areas could be decreased causing a drastic reduce for the bridge dead weight. As seen in Section 5, steel unit price reached a value corresponding to 10 times of the prestressed beam concrete cost in 2019, however it referred only twice of the total cost for the superstructure design. 
Beyond this, based on the composite design result obtained for the same span length it was revealed that the cross-sectional area of the reinforced-concrete beam was approximately 6 times greater than those for the steel beam section. It is believed that huge dimensions obtained in design will cause to remove from esthetic appearance for every structure as well as for a bridge construction.
In the case of prestressed concrete bridges, the cross-section forces due to the self-weight of the elements were higher than the effects of the moving traffic load. Based on the fact that steel material used for composite design was light, it was seen that instead of the dead weight loads the moving loads governed the design.
The main difference between the two design was caused by dead loads. Although the weight per unit volume of steel is 3 times greater than those of concrete, prestressed beams were obtained about 2 times heavier than steel beams. Thus, it was observed that this decrease in the dead weight for steel beams could provide a decrease for the pier as well as for the foundation sizes, finally a reduction in the amount of filler and excavation could also be achieved.
As a recommendation, while analyzing a steel composite bridge, shear connectors used between two materials to obtain higher strengths by working together, should be examined by means of shear effects between two different materials in particular as well as by fatigue effects under high number of moving axle loads.