Dilimsel dengeli konsol yöntemi ile inşa edilen köprü üstyapısının yapım aşamalarına göre analizi
Construction stage analysis of segmental balanced cantilever bridge superstructure
Yazar: SHAMIL NURI
Danışman: DOÇ. DR. ABDULLAH NECMETTİN GÜNDÜZ
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / İnşaat Mühendisliği Ana Bilim Dalı / Yapı Mühendisliği Bilim Dalı
Derece: Yüksek Lisans, 2019
Konu: İnşaat Mühendisliği = Civil Engineering

Özet:
Dilimsel dengeli konsol yapım yöntemi dünyada uzun zamandır kullanılan ve birçok örneği bulunan köprü inşa yöntemidir. İç açıklıkları 300 metreyi bulan ve köprü toplam uzunluğu birkaç kilometreye ulaşan, ayrıca orta açıklıklar 80-200 metre arasında etkili olmakta olup kutu kesitli ardgermeli beton tabliyeler için öncelikli bir yapım yöntemi olarak kullanılmaktadır.
Açıklıklar belli uzunluklara sahip dilim şeklinde kolon üstü diliminden itibaren açıklık ortalarına doğru sağa ve sola ilerlemektedir. Bu halde sistem dengeli konsol şeklinde çalışmaktadır. Sistem açıklık ortasında kapatma dilimi döküldükten sonra sürekli hale gelmektedir. Böylece, dilimsel yapım yöntemi ile inşa edilen köprünün tasarım ve projelendirilmesinde iki ayrı aşama dikkate alınması gerekmektedir.
Bu çalışmada köprü üstyapısının yapım aşamaları anlatılmış, tasarımda dikkate alınacak yük ve yük kombinasyonları verilmiş, ve köprü üstyapısının dengeli konsol durumu analizi için gerekli yükler hesaplanmıştır.
Köprü üstyapısı farklı geometriye sahip dilimlerden oluştuğundan dolayı, ver herbir dilimin ardgerme tekniği yapım süresine bağlı olarak dilimlerin farklı beton yaşlarında uygulandığından zamana bağlı gelişen etkiler dikkate alınmalıdır. Dünyaca kabül edilen köprü tasarım şartnamesi AASHTO LRFD hükümleri doğrultusunda dilimsel dengeli konsol köprülerin analizi malzeme modelleri ele alınarak incelenmelidir.
Yürütülen tez kapsamında CEB FIP MC90-99, ACI209R-92 ve AASHTO LRFD tarafından sünme ve büzülme etkilerinin hesabı için bağıntılar ayrıntılı şekilde incelenmiştir. Bir çok parametreye bağlı sünme ve büzülme modelleri kullanarak sünme katsayıları ve büzülme birim kısalmaları elde edilmiş, ve çok daha az girdi parametresi gerektiren ve tasarım için yeterli mertebede sonuçlar veren AASHTO LRFD modeli tez çalışmasının ilerleyen aşamalarında ele alınmıştır.
İncelenen malzeme modellerinin dilimsel dengeli konsol köprüde kullanma amacındaki bir başka husus artımsal zaman adımlı hesap yöntemini kullanan özel köprülerin yapım aşamalarına göre analizini yapan bilgisayar yazılımları için kolay programlanabilmesi açısından uygun olmasıdır.
Zamanla gelişen sünme ve büzülme gibi etkilerden ve yüksek mukavemetli öngerme çeliğinin gevşemesinden dolayı kayıplar malzeme modelleri kullanarak incelenmiş ve hesapları yapılmıştır.
Bu çalışmada öncelikle dilimlerin yapım aşamaları artımsal zaman adımlı analiz yöntemine göre incelenmiştir ve her bir kesitte her bir dilimin yapımı tamamlandıktan sonra meydana gelen gerilmelerin tahkiki yapılmıştır. Yapım aşaması süresi boyunca hiçbir kesitte meydana gelen gerilmeler şartnamece belirlenen izin verilen gerilme sınırlarını aşmamıştır.

Abstract:
Segmental balanced cantilever construction method is a bridge construction method which has been used for a long time in the world and has many examples. The main spans can reach up to 300 meters and the overall length of the bridge can reach up to several kilometers. But as a priority in terms of economical construction method  and adequate design for box-section post-tensioned concrete decks, the effective main span length may vary between 80-200 meters. Furthermore, this method is easily adaptable to irregular bridge arrangement, congested project sites, rough and water terrain, and environmentally sensitive areas.
The bridge spans are divided into segments with variable length. For cast-in-situ method of construction segments range between 3 and 5 meters in length. The basic concept of segmental balanced cantilever construction is to attach the segments in an alternate manner at opposite ends of cantilevers supported by piers. The cantilever determinate system is subjected to changes during adding segments at each construction stage and after casting cast-in-place closure stitch segment the system become continuous and statically indeterminate system. As a result, the analysis and design of the structure must follow the construction stages. At construction stage negative moments occured at pier section will be resisted by temporary tendons, placed at top of sections, and for continuous system positive moments occured at span region are resisted by continuity tendons, placed at bottom of sections.
In general, this special type of bridges are designed and analyzed by softwares. The aim of this study to understand the main behaviour principles by performing hand calculations based on the theoretical background given in design specifications.
In this study the construction stages of bridge superstructure are explained by figures, load and load combinations to be considered in the design are given, and internal forces occured at each construction stage during determinate balanced cantilever system are calculated. Previously designed balanced cantilever bridge will be analysed in this study. For preliminary design check purposes section heights at top of piers and at midspans are checked by span over depth ratios. Also strands number is checked by preliminary method of analysis.
Since the bridge superstructure consists of segments having different geometries and unusual dimensions, level of posttensioning, construction aging, and concrete age at time of posttensioning for new erected segments, a refined method for time-dependent effects estimation shall be used. This method of estimation of prestressing losses due to each time-dependent  source, such as creep, shrinkage, or prestressing steel relaxation, can lead to a better estimate of total losses compared with the approximate estimatation method of time-dependent losses. AASHTO LRFD is used as the basis for the analysis of segmental balanced cantilever bridge analysis and reference material models indicated in this document shall be taken into a consideration during refined method of analysis.
Within the scope of the thesis, the expressions given in CEB FIP MC90-99, ACI209R-92 and AASHTO LRFD model codes were examined in detail. Creep coefficients and shrinkage strains were obtained by using creep and shrinkage material models which are dependent on many parameters. As a result of this examination AASHTO LRFD material model, which requires less input parameters and provides sufficient results for design, has been decided to be used in following stages of the thesis.
For segmental construction method and multi-stage posttensioning during the construction, calculations for loss of prestress should be made in accordance with an incremental time-step method. This method models the design life of a structure through discrete intervals of time. The deformations, stresses, and internal forces that develop in response to external loads and time-dependent material effects are computed for each time interval. The response of the structure at the end of any time interval is taken to be the summation of the responses of the preceeding intervals. For this purpose, computer softwares are used in practice in the construction stage analysis of balanced cantilever bridges. Creep coefficient and concrete shrinkage strain expressions given in reference material model codes can be easily used for computer software programming.
The time-dependent prestress losses given in specification are analyzed seperately by researching theorethical background. Remarkable growth of high strengh concrete applications especially in bridges has been seen the last thirty years. The aim of refined prestress loss investigations is to quantify the loss of prestress of high strength concrete bridge and to find justifications on increasing usage of high strength conrete for bridges.
Elastic modulus variation over time has been analyzed during the thesis. The first elastic modulus type taken into consideration is tangent elastic modulus. This parameted is given in specifications using specified compressive strength of concrete. Instead of using this value strength variation over time is used in elastic modulus calculations.
Time-dependent creep gain should be taken into account. For these purpose, various types of elastic modulus was improved by researchers. The main two are using in time-dependent calculation. The first one, effective elastic modulus is used in time-dependent deflection calculation. The second one, age adjusted elastic modulus was improved by adding aging coeeficient. The last one is used in time-dependent prestress loss calculations and given in the AASHTO LRFD Bridge Design Specifications.
Within the scope of the thesis, an analysis program has been developed by using Mathcad and Microsoft Excel Visual Basic for Applications (VBA) programming languages in order to ensure ease of calculations and to prevent the loss of time because of the continuous repetition of the mathematical operations. This program has been developed in accordance with the AASHTO LRFD material model code provisions and all the required parameters are taken into account.
Bending moments occured due to selfweight of segments, formwork traveler movement, and construction live load, are calculated seperately for each segment beginning and end sections. By using incremental time-step method stresses occured at top and bottom of each section at the end of each construction stage are calculated. Shear effects have not been taken into consideration. All the stress variations are given on the graphs and it is concluded that all the compressive and tensile stresses do not exceed allowable stress limits given in AASHTO LRFD provisions. As a result, uncracked section properties are used in deflection calculations. Also examining stress variation it can be seen that almost all the sections considered in the analysis are under compression during bridge construction before erecting closure stitch segment.
In this study only elastic analysis has been performed for high strength concrete posttensioned sections. Bending moment values have been calculated using service load combinations. These sections may be checked by ultimate limit state flexural analysis using load combinations given in AASHTO LRFD Bridge Design Specifications. Also, shear analysis may be performed for the segment mid and end sections.
Pier head segment has not been taken into consideration during this thesis. This segment has been assumed as a rigid member as it is connected to the pier and supported by diaphragm wall in the box section. An analysis for this segment may be performed by softwares using finite element method.  
For the construction of cast-in-situ and prefabricated segments, it is necessary to analyze each phase in the construction of the cantilever and to determine the deformation curve for each cantilever element, phase-by-phase. For both of construction method different segment placing techniques are used for estimation of deformation trajectory followed by the end of the cantilever at each phase of construction. By modifying the angular positions of the segments and making corrections during the erecting the modified profile is obtained which can effectively compensate for the future deformations occured during construction stages.
In this study an analysis has been performed only for balanced cantilever bridge superstructure during the construction stage. Time-dependent effects, multi-stage posttensioning, load variation and continuously increasing internal forces at this stage have been analyzed. As pier columns for this type of bridge are very high, performance analysis may be performed at eash construction stage for pier columns. Also, as cantilever lengths at construction stage and span lengths for continous system are very high, the vertical design response spectrum should be used in seismic calculations.