79 Introduction to the project The incessant endeavour to build more graceful, slender and more attractive bridges leads their authors to propose new construction systems and more resourceful designs including the higher value added to basic materials. Also the bridges using “extradosed” system shall be included in this group. To increase the effectiveness of external tendons of box girders leads to raising them above a deck. Increased uplift effectiveness of external tendons passing through a deviator above the bridge pier enables to reduce substantially the section depth. This type of prestressing and tendons is called “extradosed”. General description of the project The advantages of this system were utilized also when preparing the designs of 2 long-span bridges within 9.7 km long section of the D1 motorway, passing Považská Bystrica (Slovakia). One of them is the motorway Bridge over the River Váh in km 9.068, which carries the D1 motorway over the river and its flood area at the end of motorway section “Sverepec – Vrtizer”. Bridge superstructure The overall bridge length of 317 m will be constructed as a continuous girder with 5 spans (40 + 62 + 110 + 62 + 40 m). The depth of deck is 4,0 m; the height of the pylon is 11 m. The double-cell box girder, 29.85 m wide, has the 6.2 m long cantilevers supported by reinforced-concrete struts. The longitudinal prestressing of the deck including inclined ties in the section is similar to cable stayed systems. The bonded tendons were used during the cantilevered construction method in the first stage and extradosed type of external tendons was continually added. The continuity external tendons are assumed to be tensioned again after concrete pour of the last section. The first and last spans are built conventionally using falsework supported directly from the ground. The construction method of the rest of the bridge is similar to “free cantilever”. The double-cell box girder built by the “free cantilever” method will be gradually widened by struts and cantilevered deck slab to full section width. After concrete pouring and hardening, the deck slab is prestressed transversally by monostrands. Bridge substructure The circular bridge piers are situated only in a flood area. Because of the geologically heterogeneous soil they are supported by concrete piles (dia.1200mm). Basic materials used and indicators of their quantities Because of the complexity of the design, it was necessary to use C45/50 concrete. The steel reinforcement 10 505 (R), the 15 and 19-strand prestressing tendons (Ls 15,5 - 1800 MPa) and tie prestressing bars of CPS Ø 32 will be used. The indicators of quantities of basic materials (bridge over the Váh River): • Concrete: 0.95 m3/m2 • Steel reinforcement: 156 kg/m2 • Prestressed reinforcement - Longitudinal: 37.2 kg/m2 • Transverse 3.6 kg/m2. These indicators are approximately in accordance with indicators of other types of superstructures with similar span. However, the main advantage of this type of structure is the possibility to lower substantially the vertical alignment of the road and narrow substructure width, which is significant particularly in case of bridges situated high above urban areas. Used software solutions Due to staged construction, was it neither possible nor convenient to analyse the whole structure by one model. The global effects were assessed by the beam model taking into account construction stages and material rheology. To assess local effects, we had to create more than 20 models using shell and beam elements Crosssection stages - Three stages in transverse direction are required to build a section. At first, a double-cell core section without overhangs and concrete struts is poured, and then reinforced concrete struts with portions of overhangs and the rest of overhangs. In the beam model, longitudinal flexural stiffness of concrete struts was ignored and they were taken into account as a load. Prestressing – Two types of tendons (bonded and external) were applied in the beam model. Presstressed ties ensuring transfer of vertical forces of extradosed cables into vertical walls of a box girder section were neglected in the beam model but their effect was analysed by 3D shell element model. Construction stages – 80 construction stages were analysed by the beam model. The falsework supported on ground was modelled by “point-line” support and ageing of concrete of free cantilever by a negative relative time of a section concreting. Results - We used the TDA (time-dependant analysis) module to analyse the bridge superstructure. The design was carried out in accordance with the Slovak national standards (allowable stress and degree of safety design method). The core sections are fully presstresed (no tension in concrete) and the overhangs at the walls are designed for tension less than concrete strength in tension (limited prestressing). We studied the following results: • Concrete stresses • Stress in bonded tendons • Stress in external tendons • Principal stresses in concrete • Degree of safety The longest span of this bridge is 110 m and the width of the bridge is 29.85 m. Area of this span can be represented by half a football field. The bending moment at pylon was 810 MNm and 270 MNm in the midspan. These huge forces made the design extremely complicated and time consuming with more than 30 alternatives. Motorway Bridge over River Vah in km 9.068
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