Balastsız çelik kafes demiryolu köprülerinin balastlıya dönüşüm etkilerinin incelenmesi
Analysis of the transformation effects of ballasted steel truss railway bridges
Yazar: SÜMEYRA DEMİRALAY
Danışman: DOÇ. DR. KADİR ÖZAKGÜL
İstanbul Teknik Üniversitesi / Lisansüstü Eğitim Enstitüsü / İnşaat Mühendisliği Ana Bilim Dalı / Yapı Mühendisliği Bilim Dalı
Derece: Yüksek Lisans, 2021
Konu: İnşaat Mühendisliği = Civil Engineering

Özet:
Tez kapsamında, hizmet veren 50 ve 100 m açıklıklı farklı tip iki adet çelik demiryolu köprüsü orijinal paftadaki kesit ve ölçülere göre sonlu elemanlar yöntemi ile SAP2000 programında modellenmiştir.
Modellenen köprüler, dinamik arazi ölçümleri sonucunda elde edilen mod şekli ve frekans verilerine göre kalibre edilmiştir. Kalibrasyon öncesi ve sonrası olacak şekilde köprü frekans ve mod şekillerin karşılaştırması yapılmıştır. 50 m açıklıklı köprü frekans kalibrasyonu sonucunda maksimum %4, 100 m açıklıklı köprü frekans kalibrasyonu maksimum %2.2 yakınsalık elde edilmiştir. Ek olarak ilgili frekans değerlerindeki model mod şekilleri, arazi mod şekillerine uygunluğu kontrol edilmiştir.
Dinamik arazi frekans ve mod şekillerine göre kalibrasyonu tamamlanmış modeller üzerine hesaplanan uygun balast yükü enleme ve boylama elemanları üzerine etki edilmiştir. Balast yükünün köprüler üzerindeki frekans ve mod şekil değişimleri incelenmiştir. Balast yüklemesi ile ilgili mod şekillerine ait artan yük sonucunda frekans değerlerinde azalma gözlemlenmiştir.
Köprülerimizin katar yükü altındaki davranışlarını da inceleyebilmek için köprülere adım adım olacak şekilde standart katar yükü modelde etkitilmiştir. Her iki köprü için ölü yükteki, balast yüklü durumdaki ve katar yüklü durumdaki eleman gruplarının en yüksek değerdeki analiz sonuçlarına göre yük taşıma oranları ve güvenlik indisi hesaplanıp karşılaştırılmıştır. Hem balast yüklemesi hem de katar yükü etkitilmesi sonucunda her iki köprü eleman gruplarının yük taşıma oranı ve güvenlik indisi değerlerinde azalma gözlemlenmiştir. 50 m'lik köprü yük taşıma oranı ve güvenlik indis değerleri güvenli tarafta iken 100 m'lik köprü boylama elemanında belirlenen sınırları aştığı gözlemlenmiştir fakat takviye levha ile belirlenen sınırlar içerisinde kalması sağlanmıştır.
Her iki köprü sistemine ait yük taşıma oranı, köprü eleman grubu içerisindeki en küçük değerdeki yük taşıma oranı ile belirlenmiştir. Her iki köprü sistemine ait güvenlik indis değeri ise köprüleri seri sistemli olduğu kabulü ile hesaplanarak belirlenmiştir.
Balastsız durumda köprülerin ilgili yükler dahilinde güvenli olduğu saptanmış olup balastlı durumda çok az elemanda sınır durumun altında kaldığı görülmüştür.
Bu çalışma ile hizmet veren köprülerin balast yükü altındaki davranışı incelenmiş olup ileriki çalışmalara ışık tutuması hedeflenmiştir.

Abstract:
Throughout the history, structures have been renovated or rebuilted because of the variety of reasons. For last decades, many developments about railways have happened and so, auxiliary structures like bridges have also developed. One of the most researched and developed topic about railway bridges is ballast layer which is used under the traverses for more uniform train load distribution to the ground and its' behaviour on the bridges are investigated many times. Effects of ballast layer to the bridge is not well known although, there are many research. According to the literature, after ballast layer is implemented to the bridges, most common results are increased damping ratio and rigidity furthermore, the others are increased displacements and decreased frequecy of the bridges. Moreover, several variants like train speed, elastisite modulus of ballast, rezonans situation and presence of the shear force between ballast layer and bridges are also researched. In the literature about evaluation of the existing bridge, it has been observed that the rating factor of the structure obtained as a result of the tests performed on the structure is higher than the analytical rating factor. Load distribution factor, LDF and girder distribution factor, GDF were used in the literature. According to the results obtained, it can overestimate the bridge capacity as it includes all the beneficial effects with the GDF approach. However, it has been observed that the safety index values calculated for the design situations using the current and recommended load and strength factors in AASHTO and the safety index values obtained as a result of the calculations are quite compatible.
Within the scope of the thesis, different two steel railway bridges with 50 and 100 m span length were modeled in the SAP2000 program using the finite element method according to the original sections and dimensions. In order to monitor the changes in the results according to the bridge type and span length, 2 bridges were selected. Both bridge sections are formed to assemble the corners and plates with rivets. Rail and traverse weights are loaded to stringer elements as dead load.
Modeled bridges are calibrated according to the mode shape and frequency data obtained as a result of dynamic test results. To give briefly information about the dynamic test, after the train passes over the bridge, the remaining unforced vibrations that occur in the bridge are recorded by accelerometers and this recorded datas are used by using fast forrier transformation to obtain the bridge frequency and mode shapes. But in this thesis, already obtained frequency and mode shapes values are used. The calibration process is provided to the model with the springs assigned to the supports from three direction and springs` values are changed until bridge frequency and mode shapes converged to test results. While comparing mode shapes, the locations of accelerometers used in field test measurements on the bridge were used. Bridge frequency and mode shapes were compared before and after calibration. As a result of the 50 m bridge frequency calibration, a maximum convergence of 4%, 100 m bridge frequency calibration, a maximum 2.2% convergence was obtained.In addition, the model mode shapes in the relevant frequency values were checked for compliance with the test mode shapes.
The 30 cm thickness ballast layer load calculated on the calibrated models has been implemented on the cross beam and stringer elements as rail and traverse loads. To carry ballast layer, trapezoidal steel profile decking sheet is used and IPE80 profile is also used under the sheet 1 m apart. The frequency and mode shapes of the ballast load on the bridges were obtained. As a result of the increasing load of the ballast loading, a decrease in the frequency values related to the mode shapes was observed. Due to the ballast dead load, the frequency values of the 1st bending mode decreased by 40% in both bridges. While the frequency value of the 1st torsion mode decreased by 40% in the 50 m bridge, it decreased by 10% in the 100 m bridge. It was thought that this difference was caused by the truss systems of the bridges.
In order to examine the behavior of our bridges under the train load, the standard UIC train load has been applied to the bridges step by step. After implemented train load, section moment and forces are taking into account as living load.
Rating factor and safety index calculation methods are given at AASHTO. For both evaluation methods, LRFR method equations are used. The coefficients of variables in the equation have been taken to be compatible with our bridges and bridge elements. Many variables are taken differently considering to element groups.
Rating factor are greater than or equal to 1 indicates that the bridges can safely carry the desired live load profile. Similarly, if the safety index obtained are the target safety index used by TCDD, 3 or more, the bridges are safe. To mention differences of both evaluation methods is rating factor calculates the bridge strength and the load on it with a more deterministic approach. The rating factor cannot be calculated with many variables in more complex situations. Since the safety index calculation is based on the probability approach, it is possible to calculate with more variable inputs, but there is increased complexity in the calculations.
For both bridges, rating factor and safety index were calculated and compared according to the highest value analysis results of element groups in dead load, ballast loaded condition and train loaded condition. As a result of the effect of both ballast loading and train load, a decrease was observed in the rating factor and safety index values of both bridge element groups. While the 50 m bridge rating factor and safety index values were on the safe side, it was observed that the 100 m bridge stringer element exceeded the limits determined, but it was ensured that it remained within the limits determined by the reinforcement plate.
The rating factor of both bridge systems was determined by the smallest value rating factor in the bridge element group.
The safety index value of both bridge systems was determined by calculating the bridges assuming that they are serial systems. After calculating the safety index of the bridge elements, the structural system safety index values for both bridges were calculated with specified equation, taking into account the failure probabilities corresponding to the safety index values of each bridge system element type.
To more detailed look to the results, in the ballasted condition, rating factor decreased by 15% on average for 50 m bridge elements, and decreased by approximately 23% for 100 m bridge elements. While the rating factor of the 50 m bridge was 1.05 in the ballasted condition, the rating factor of the 100 m bridge increased to 0.9 as a result of the addition of 166x25 mm upper head reinforcement plate for the stringer element.
It has been observed that the safety index calculation results show parallelism with the rating factors. After ballast loading, the safety index of 50 m bridge elements decreased by 29% and the safety index of 100 m bridge elements decreased by 43%. Under the influence of ballast load, the structural system safety index value of the 50 m bridge decreased by 29% and was 3.5, while the structural system safety index value of the 100 m bridge became 4.16 with an increase of 38% as a result of the top reinforcement of the stringer elements.
It is seen that the graph obtained as a result of the comparison of the safety index and rating factor values of the bridge elements is an increasing graph for both ballasted and unballasted condition. In other words, the safety index value of the element with a high rating factor is also high.
In the un-ballasted condition, the bridges were found to be safe within the relevant loads, and in the ballasted condition, it was observed that very few elements were below the limit state.
With this study, the behavior of the bridges serving under ballast load has been examined and it is aimed to lighten on future studies.