an alternative proposal for the design of balanced cantilever bridges ...

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4 Proceedings IBSBI 2011 backfill inspection opening retaining wall lowered tendons counterbalance cantilever structural tie pile cap (Detail) straight tendons bearing 450mm Detail Figure 2. The abutment of the proposed construction method. 2,75 R=30m 2,75 R=30m 150mm The minimum height of the deck cross section is proposed to be not smaller than 0,80 m. After the curing of the casted cantilevers, the tendons are stressed. The design of the prestressing force is based on the objective of the method that is to provide a slight pre-cambering of the cantilevers that is a slight bending deflection upwards. Therefore, at this stage the cantilevers of the deck are set higher than the final design height of the bridge. After the application of prestressing, the steel formwork is removed and the construction procedure is repeated for the adjacent spans. The tendons of the subsequent spans are coupled with the ones of the casted cantilever and the adjacent cantilever is constructed. A detailed description of the prestressing application and the rebar of the deck is given in section 3 of the paper. After the completion of the deck construction and the application of the prestressing force, positive bending moments, which are caused due to the eccentricity of the straight tendons from the deck’s centre of gravity, are induced along the deck. These positive bending moments overbalance the negative ones that are imposed by the self-weight of the deck. Hence, the aforementioned pre-cambering of the cantilevers is achieved. The precambering was deemed necessary in order to compensate for the pre-determined long-term prestress losses due to the creep and shrinkage of the deck and due to the relaxation of prestressing steel. The rest of the vertical loads of the deck that are the additional permanent and the variable loading [7] are imposed after the completion of the total bridge system. Thus, the frame action of the total bridge structure, in which the meeting cantilevers are connected, receives the additional vertical loading. The final bridge system is then checked against the resulting bending moments, the shear actions and the torsion effects after considering the re-distribution of actions. In particular, the design of the deck against shear actions is facilitated due to the beneficial inclination to the horizontal of the compression zone of the deck in the critical section for shear, namely where the maximum shear stress is acting. Possible deficiency of the deck at the supports against the bending moments caused by either the ultimate or the serviceability limit states [6] [8] shall be covered by additional
I.A. Tegos and S.A. Mitoulis 5 reinforcement bars of ordinary strength steel. The additional reinforcements cover the safety criteria set by codes [6] [8] and the serviceability requirements by limiting the crack width according to the code provisions [5] [6]. 3 APPLICATION OF THE CONSTRUCTION METHOD TO A CAST-IN-SITU BRIDGE 3.1 Description of the benchmark bridge The bridge of Kleidi-Kouloura belongs to Egnatia Motorway that runs across Northern Greece. It is a cast-in-situ structure with a total of three spans and a total length equal to 135.8 m. More details on the bridge are given in an another paper of IBSBI 2011 conference. 3.2 Results The benchmark bridge was re-analysed and re-designed according to current code provisions concerning serviceability [6] [8] and earthquake resistance [9]. The re-design took into account the construction phases of the proposed method and the following predominant design parameters were revealed: (1) The required number of straight tendons was less than the one needed in case a classification category A or B was chosen, (table 4.118 in [6] [8]). However, the total number of tendons ensures that the bridge is classified in category C, when this requirement refers to the performance of the top fibre of the deck, while the use of ordinary strength reinforcements in the bottom fibre of the deck leads to the classification category D. It is noted that, the design of the prestressing force and the resulting number of tendons aims at providing the required pre-cambering of the cantilevers against the self weight of the bridge deck, whose length was half of the total span length that is 45,60/2 = 22,80 m. (2) The re-design of the prestressing showed that 15x19T15 (15 tendons of 19 wires with diameter 15mm each) of high strength steel St 1500/1770 are adequate to receive the bending moment of the deck above its support. Additionally, ordinary steel rebar 76Ø16 (76 bars with diameter 16mm each) above the support were utilized, which gradually reduced to 28Ø16 at the bridge mid-span. The tendons and the reinforcements needed in the top flange of the deck are illustrated in Figure 3. The ordinary strength steel bars, which are also required by the code [6], are the ones which allow the safe transition from the uncracked to the cracked deck section and the avoidance of non-ductile failure modes. The lengths of the steel bars were chosen to be sub-multiples, namely half, of the conventionally produced ones by the steel manufactures in order to avoid material waste. Figures 4 and 5 show in detail the reinforcement layout at the support and at the mid-span. Figure 6 shows the steel rebar of the bottom part of the deck. The bars are installed in couples that are 2x71Ø25 (71 couples of bars with diameters 25mm each) at the mid-span, while 2x41Ø25 were found to be required at the bottom flange of the deck at the supports. The
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