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
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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
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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].
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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.
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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. In segmental bridge construction process the construction is started from both the piers simultaneously with the help of Form Traveller (Gantry) hence time dependent effects are important to be considered in construction stage analysis. The weight of the form traveller applied is 500kN, which includes the formwork and its support devices is internally converted into a vertical force and a moment, which is then applied at the end of the cantilever segment.The loads applied on the bridge structure are generally on higher side than the vehicle loads for which it is designed, hence in construction stage highest deflection occurs during the construction, the two cantilevers undergo different creep, shrinkage and tendon losses, resulting in different stresses and deflections at the time of erecting the Key Segment, such differences need to be reflected in preparing the construction stages for analysis. Data Considered in the modelling and analysis: 1. Pier Section - 2 no’s RCC pier 2.5m x7.5m at 4.5 m c/c 2. Grade of Concrete used - M-40 for Pier and M-50 Superstructure 3. Pier Table - 15m wide at top and 10 m wide at bottom, height of 15m from the ground level 4. Superstructure - 12 segments of 3m length each, key segments of 2m length, 4no’s of end segments of 2.5 m length each 5.Form Traveller - Weight of gantry is taken as 500 kN. 6. Creep/Shrinkage - Start of loading at 10 days and end at 10000 days (As per IRC:112-2011
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Construction of the Substructure, Gantry Fabrication and Assembling
Erection of Pier Table formwork and Installation of Form Traveller (Gantry)
Erection of formwork, Fabrication of Reinforcement, Ducts etc.,(7 day Duration)
Casting of concrete segments, curing and post tensioning of ten dons (7 day duration)
Form Traveller is moved and installed to the next segment to be casted
On Completion of one span of casting, Form Traveller (gantry) i s moved to another pier
Erection of scaffoldings, formwork for end spans (Full S upport Zones)
Construction of key segments linking the adjacent cantilevers
Erection of scaffoldings, formwork for end spans (Full Support Zones)
Construction of key segments linking the adjacent cantilevers
Installation of bearings, post tensioning of bottom tendons
Bridge surface construction
Figure 4 Construction procedures involved
2.2. Time Dependent Material Properties In the construction stage analysis of highway bridges, time dependent material properties such as elasticity modulus, creep and shrinkage for concrete and relaxation for the pre stressed steel are considered. For example, strength of the concrete increase continuously at 7th, 28th and 1000th days of concreting. If these properties are not considered in the analysis, it may effect the analysis results. There are several material properties and phenomena that have some effect on the response of the structure, the evolution of material parameters such as elastic modulus, creep, shrinkage,, and relaxation has been described according to the methods in the design code IRC 112-2011. Creep phenomenon may be defined as the property of concrete by which it continues to deform with time under sustained loads at unit stresses within the acceptable elastic range. Shrinkage is the property of concrete to change in volume independent of the load it sustains. It is essentially due to evaporation of water from concrete and hydration of its components with time, which occurs without any external stress to the concrete. Shrinkage is usually expressed as a dimensionless strain under steady condition of relative humidity and temperature. The development of compressive strength of concrete depends on the type of cement, curing conditions and maturity of concrete.
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Figure 5 Definition of Creep and Shrinkage
2.3. Loads and Load Cases In the analyses of the bridge, the following load cases are considered; • Dead Load: Weight of all elements is considered, it is calculated directly from the software for the model input. • Additional Load: Weight of the pavement, crash barriers, footpath kerb is added as additional loads and is considered to be 30 kN/m distributed throughout. • Gantry: Load of the form traveller. This load is implemented to previous one before the construction of one segment and slide to next one after construction of the segment this load is considered as 500 kN. After the construction of the bridge, this load is removed wholly. • Pre stress: Post-tension cables are fixed according to the load charecteristics by the software itself. Post- tension loads are considered as strain. • Time dependent Loads: Creep, Shrinkage and Compressive strength gain charecteristics is evaluated at each stage of construction and age of concrete, which corresponds to the Modulus of Elasticity at the specified period of construction. 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.
3. ANALYSIS AND RESULTS
3.1. Maximum Bending Moments (Cantilever Moments) The Bending moments induced due to the Dead load, Prestress tendons, Erection loads, Creep, Shrinkage of concrete are tabulated as below considering a point at which maximum moment occurs (Pier Table) according to the stages of construction in the segmental bridge construction. The maximum bending moment at the final stage analysis without the construction stage analysis effect is found to be -141173.1 kN-m The graphical representation of variation of Bending moments according to the stages of construction at a point in the pier table is considered and shown below in Fig.6.
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Bending Moments in (kN-m) Construction Dead Erection Prestress Stage Creep Summation Load Load Tendon CS-1 -7343.63 -2500 8809.861 287.7247 -1033.772 CS-2 -15459 -4000 14570.25 -539.391 -4889.535 CS-3 -26745.2 -5500 21265.83 -2193.96 -10979.37 CS-4 -41075 -7000 29044.04 -4581.26 -19030.97 CS-5 -58353.6 -8500 37227.14 -7832.7 -29626.46
CS-6 -78490.7 -10000 45121.14 -12113.6 -43369.53 CS-7 -101443 -11500 60152.18 -15655 -52790.72 CS-8 -127345 -13000 74993.41 -19911.2 -65351.9 CS-9 -156149 -14500 89604.19 -24977.1 -81044.93 CS-10 -187918 -16000 104127.6 -30879.9 -99790.6 CS-11 -222748 -17500 119399.1 -37478.4 -120848.5 CS-12 -260730 -19000 129133.3 -45963.4 -150596.8 CS-13 -273530 -20500 139785.5 -54613.1 -154244.6 CS-14 -283598 -13267.8 150766.8 -56269.4 -144390.1
CS-15 -287114 -13267.8 150145.1 -61254.1 -146301.4 CS-16 -294760 -12381.8 144210.1 -123229 -150297.1
Figure 6 Variation of Bending Moment with Construction stages and load cases
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3.2. Behaviour of RCC Pier The variation of vertical reaction and the bending moments with the Construction Stages at the base of the Pier is as tabulated below:
Fz (kN) My (kN-m) Construction Stage Erection Erection Dead Load Summation Dead Load Summation Load Load CS-1 10926.8 500 11433.37 -800.515 -218.344 -919.55 CS-2 12087.63 500 12597.29 -1399.53 -298.377 -1508.8 CS-3 13216.71 500 13729.04 -2148.99 -376.782 -2189.5 CS-4 14321.49 500 14836.42 -3040.65 -453.966 -2947.4 CS-5 15405.41 500 15922.76 -4064.99 -529.928 -3813.8 CS-6 16475.67 500 16995.14 -5211.59 -604.82 -4817.6 CS-7 17531.21 500 18053.95 -6483.02 -678.944 -5685.3 CS-8 18590.68 500 19116.5 -7872.03 -751.613 -6675.1 CS-9 19645.52 500 20174.19 -9373.4 -823 -7781.8 CS-10 20699.16 500 21230.47 -10988.9 -893.249 -9001.5 CS-11 21755.04 500 22288.92 -12721.7 -962.477 -10298 CS-12 22826.34 500 23362 -14564.6 -1030.65 -11869 CS-13 25897.94 500 26454.74 -13424.5 -1098.33 -10515 CS-14 28544.68 2789.26 28332.25 -12638.7 874.3043 -8982.7 CS-15 26584.1 2789.26 25859.22 -14113.7 874.3043 -10618 CS-16 26909.44 3274.28 19003.57 -13901.6 613.6935 -29955
70000
60000
50000
40000 Fx(kN) Summation 30000 Fx(kN) Erection Load 20000 Fx(kN) Dead Load
10000
0 1 2 3 4 5 6 7 8 9 ------10 11 12 13 14 15 16 ------CS CS CS CS CS CS CS CS CS CS CS CS CS CS CS CS Figure 7 Variation of Moments (Reaction) with different Construction Stages
3.3. Displacement of the Box Girder The displacement of the top girder obtained for the last stage of construction which is maximum, the maximum displacement is found to be 30.04 mm ( http://iaeme.com/Home/journal/IJCIET 380 [email protected] Suhas S Vokunnaya, Ravindranatha and Tanaji.Thite Figure 8 Displacements at CS-16 (Maximum) 4. CONCLUSIONS • Large difference is observed between the results with and without the construction stages, a difference of 13071 kN-m (10 % deviation) . It can be stated that the analysis without construction stages cannot give the reliable solutions. • The bending moment increases as the new segment is added till a key segment is constructed (CS-13). • The variation in the Reaction parameters offered by the pier (column) has to be considered and incorporated in the design. • Maximum Bending moments occur on the Pier Table. REFERENCES [1] Richard Malm, Hakan Sundquist (2010) “Time-dependent analyses of segmentally constructed balanced cantilever bridges” Elsevier journal of Engineering Structures 32(2010). [2] Ahmet Can Altunisik, Alemdar Bayraktar, Bar Sevim, Süleyman Adanu and Arman Domaniç. (2010) “Construction stage analysis of Komura Highway Bridge using time dependent material properties”- Structural Engineering and Mechanics, Vol. 36, No. 2 ,Page no 207-223 [3] Parag D Patil, Dr.B.S Karkare, G.R.Chillal. (2010) “Construction Stage Analysis of Balanced Cantilever Bridge” International Journal of Advance Foundation and Research In Science & Engineering, Volume 2. [4] Adanur, S. and Gunayd M. (2010), “Construction stage analysis of bosporus suspension bridge”, Proceeding of 9th International Congress on Advances in Civil Engineering, Trabzon, September. [5] Altun k, A.C., Bayraktar, A., Sevim, B., Domaniç, A. and Adanur, S. (2009), “Construction stage analysis of bridges using time dependent material properties”, Proceeding of International Symposium on Earthquake, Sakarya, September. [6] JN Mahto and SC Roy, Experimental Analysis of Propagation of Crack In Brass Cantilever Beam Under Forced Vibration. International Journal of Mechanical Engineering and Technology, 7(5), 2016, pp. 316– 320 [7] IRC: 112 – 2011, ‘Code of Practice for Concrete Road Bridges’, India Road Congress, New Delhi. [8] IRC: 18 – 2010, ‘Design Criteria for Prestressed Concrete Road Bridges, India Road Congress, New Delhi. http://iaeme.com/Home/journal/IJCIET 381 [email protected] Construction Stage Analysis of Segmental Cantilever Bridge [9] ER. Hassan Mohamed Abdulnabi and Dr. V. C. Agarwal, Claims in Construction Projects "Design Errors and Change Orders". International Journal of Civil Engineering and Technology, 7(6), 2016, pp.123 – 130. [10] Sumesh Sudheer Babu and Dr. B. Sudhakar, Construction Project Management During Economic Crisis. International Journal of Management, 7(7), 2016, pp. 371–381 [11] IRC SP: 65 – 2005, ‘Guidelines for Design and Construction of Segmental Bridges’, India Road Congress, New Delhi. [12] IS: 456 – 2000, ‘Plain and Reinforced Concrete –Code of Practice’, Bureau of Indian Standards, New Delhi. [13] IS: 1343 – 2012, ‘Prestressed Concrete –Code of Practice’, Bureau of Indian Standards, New Delhi. [14] Raina.V.K, ‘Raina’s Concrete Bridge Practice-Analysis,Design and Economics, 2007. http://iaeme.com/Home/journal/IJCIET 382 [email protected]