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Construction Stage Analysis of Segmental Cantilever Bridge

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Construction Stage Analysis of Segmental Cantilever Bridge International Journal of Civil Engineering and Technology (IJCIET) Volume 8, Issue 2, February 2017, pp. 373–382 Article ID: IJCIET_08_02_040 Available online at http://iaeme.com/Home/issue/IJCIET?Volume=8&Issue=2 ISSN Print: 0976-6308 and ISSN Online: 0976-6316 © IAEME Publication Scopus Indexed CONSTRUCTION STAGE ANALYSIS OF SEGMENTAL CANTILEVER BRIDGE Suhas S Vokunnaya Department of Civil Engineering, Manipal Institute of Technology, Manipal, India Ravindranatha Department of Civil Engineering, Manipal Institute of Technology, Manipal, India Tanaji.Thite Design Manager-Structures, C V Kand Consultants Pvt Ltd, Pune, India ABSTRACT Bridge is a structure which is usually built over several obstructions or depressions like rail lines, water bodies, highways, pipelines, canals and also urban roads for decreasing the traffic congestion and directing the traffic to desired destination. Cantilever construction is a method of progressive construction of a cantilever in segments and stitching them to segments previously casted by prestressing. Failure analysis of the bridge during construction phase is very essential, in balanced cantilever construction of continuous bridge, Bending Moment in the bridge increases with addition of the new segment during construction. Once the cantilever segments are added in to both side of pier, the bending moment arise in the pier is negative and increases with the addition of each new segment. When the key blocks are added, the bridge is converted from cantilever form to a continuous form and the negative bending moments on the pier decreases and there arises a positive moment. If the design of the bridge is carried out using the final construction stage structural factors only, it may fail during the intermediate stage. For this operation a bridge model is created and analysed to observe the rate of change of bending moment, reactions and deflection at different stages of construction including the time dependent effects in the construction sequence. The final stage results of the Segmentally constructed bridge is compared with the results obtained considering the bridge as a single structure neglecting the stage wise increments and the difference is noted to prove the importance of Construction Stage Analysis in a Segmental Cantilever Bridge . Key words: Balanced Cantilever, Cast in-Situ, Construction Stage, Form Traveller, Segmental Construction. Cite this Article: Suhas S Vokunnaya, Ravindranatha and Tanaji.Thite, Construction Stage Analysis of Segmental Cantilever Bridge. International Journal of Civil Engineering and Technology, 8(2), 2017, pp. 373–382. http://iaeme.com/Home/issue/IJCIET?Volume=8&Issue=2 http://iaeme.com/Home/journal/IJCIET 373 [email protected] Construction Stage Analysis of Segmental Cantilever Bridge 1. INTRODUCTION Transportation facilities and related infrastructural developments play a key role in the overall progress of a country. In a developing country like India, land transport especially-Road and Rail network play a major role in this regard. Bridges are one of the most important engineering structures which are commonly used for interplant and intercity transportation. Bridge is a structure which is usually built over several obstructions or depressions like railways, rivers, highways, pipelines, canals and also urban roads for decreasing the traffic jam and directing the traffic to desired destination. Main goal of constructing the bridge refers to enhancing safety, ease of mobility, reducing time and cost, decreasing the traffic jam and congestion that has significant impact on the environment as well as socio-economic situation of the society. Strategic decision is to be made on the type of bridge to be constructed and is to be analysed for better decision making in this field such as importance of the bridge, length of the bridge, span of the bridge, the nature of the terrain, and the material used to make it, safety of drivers, pedestrians, traffic, quality of work, user costs and impacts on the business and society. 1.1. Segmental Cantilever Bridge A Segmental Bridge is built in short segments, that is one piece at a time, combined to span a bridge. The bridge is either cast-in-place or precast. Where site conditions at the bridge site prohibit the erection of scaffolding and centering on river bed and long spans are to be constructed to compensate for the high cost of tall piers and deep foundations, cantilever construction is elegantly convenient and competitive. The cantilevering segments are erected from pier outwards, one on either side, and stitched back simultaneously. The insitu construction is done by a pair of travelling gantries also called form traveller each weighing around 400-800KN .After constructing the pier head unit, a pair of gantry systems is erected on top, one on either side of pier. The gantries project beyond the pier head to support the hanging shuttering required for casting the next segment on either side. The external shuttering of the box section deck is supported directly from the gantry system. The internal shuttering is supported on a gantry girder running inside the box along the length of the bridge which in turn is supported at its forward end by previously completed decking. Each travelling gantry is supported when it is moving from completed section to forward section. The gantry systems proceed in a systematic manner from section to section on either side of the pier after the prestressing of segments last cast, they also support a suspended scaffolding for constructional convenience and labour safety. Segmental type of bridge has several advantages in comparison with conventional bridge construction methods such as faster construction, additionally it can be used for irregular and long span lengths with few repetitions, most advantageous part of using this method especially in urban areas refers to its construction technology that it does not need any temporary shoring without any disruption to traffic over water channels and in deep gorges which is very dangerous for construction workers. 1.2. Construction Stage Analysis The construction of bridges is the most complex and challenging operations in bridge design. Different methods and techniques are adopted for the construction of bridge superstructure. To achieve a safe and structurally sound economical strategy planning and implementation of the construction operations, the effects of the chosen erection methods needs to be considered [1]. http://iaeme.com/Home/journal/IJCIET 374 [email protected] Suhas S Vokunnaya, Ravindranatha and Tanaji.Thite Figure 1 Typical construction of a Cantilever Bridge These effects are seen at early age due to the time dependent effects for which construction stage (CS) analysis has to be performed on the structure. In balanced cantilever construction of continuous bridge, bending moment (BM) in the bridge increases with addition of the new segment during construction. Once the cantilever segments are added in to both side of pier, the bending moment arise in the pier is negative and increases with the addition of each new segment. When the key blocks are added the bridge converted from cantilever form to a continuous form and the negative bending moments on the pier decreases and arise a positive moment. So, if the design of the bridge is carried out using the service period structural factors only, it may fail during the construction stage. It is seen that displacements have an increasing trend towards to the middle of the bridge deck. But, bending moments have a decreasing trend. Because of the fact that the bridge system is statically indeterminate and the cantilever length is much long, the minimum and maximum bending moments are obtained in the middle of the bridge deck and on the bridge column, respectively [3]. Both displacements and bending moments are obtained symmetrically according to the middle point of the bridge deck. 2. PROJECT METHODOLOGY 2.1. Segmental Cantilever Bridge The bridge analysed is segmentally constructed prestressed continuous box Girder Bridge consisting of three spans with a total length of 200 m (55.5m+89m+55.5 m). The cross section height of the superstructure is 5 m at the pier support and 3 m at the midspan section which varies following a second order curve (Parabolically). The cantilever consist of 12 segment each 3 m long and where the two adjacent cantilever meet they are joined with the key segments of 2 m length to close the structure and the end segments are 2.5m long. Bridge width is 12m. The segments are constructed with the Form Traveller (Gantry) at an interval of 14 days each. Grade of concrete used in construction of segments is M-50. In a FCM bridge construction, the sections at the piers are deeper than those at the mid spans to resist high moments and shear forces for cantilevers. Prestressing cables are provided in the upper flange as they are necessary in the construction stage, the cables in the bottom flange are post-tensioned after the completion of the superstructure when the centre segment is cast. 15.2mm-7 ply strands are used throughout. Duct size of 0.15 m is provided. The RCC Pier chosen is a solid rectangular one of dimension 2.5m x 7.5 m ,the Pier is constructed using M-40 concrete. Each support has 2 rectangular piers at a spacing of 4.5m c/c of the pier. http://iaeme.com/Home/journal/IJCIET 375 [email protected] Construction Stage Analysis of Segmental Cantilever Bridge 12m 36m 15m 36m 2m 36m 15m 36m 12m 15m 200m Figure 3 Layout of the Bridge Model The segmental bridge is modelled using the MIDAS Civil-2016 where the structure is modelled using free cantilever method known as balanced cantilever method. During modelling, the bridge is divided into the segments from Construction Stage 1 to Construction Stage 16, with a construction period of 14 days for the 12 segments of length 3m each out of which first 7 days is assigned for installation of formwork, reinforcement bars, ducts etc and next 7 days for pouring of concrete and post tensioning of tendons, curing and 30 days is assigned for the construction of end segments and key segments each, At each construction stage the elements are activated.
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