2.2 Magnitude of the challenge
The viaduct resulting from this typological study will become , in terms of main span :
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The largest railway bridge ( and obviously HSR bridge ) in Spain .
� The 3 rd largest bridge in Spain for all traffic types . �
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The largest HSR arch bridge , surpassing the Dashegguan Bridge in China ( 336 m ).
The largest railway arch bridge made of concrete ( including non-HSR bridges ), surpassing by more than 100 m the bridge over the Froschgrund Lake , Nürnberg-Erfurt line , Germany ( 270 m ).
� The 3 rd largest arch bridge made of concrete without traffic-type distinction , only after the Wanxian Bridge in China ( 420 m ) and close to the greater of the two bridges between the Sveti Marko and Krk islands in Croatia ( 390 m ).
These facts give an impression of the magnitude of the challenge to conceive , design and construct such a major bridge project .
2.3 An appropriate design for the problem to be solved
2.3.1 . Suitable bridge type and material for the crossing problem
The open-spandrel deck-arch bridge as it was proposed , with 45m spans between piers and 42m spans between spandrel columns , is the most economical , in terms of construction cost , of those which have been analysed . Two of the reasons leading to this fact are the orographic features and geotechnical properties of the site . The existence of sound rock ( slate ) close to the soil surface and the shape of the valley make the deck-arch bridge design highly competitive . In addition , the deck was proposed as a continuous multi-supported prestressed boxgirder all along the viaduct and , by virtue of the pier and spandrel-column arrangement , it made it possible to be erected using a conventional overhead cast-insitu movable scaffolding system ( Figs . 5 and 11 ). This design is also the most appropriate from the durability point of view , so it will be also the most suitable of those studied when considering future maintenance costs and it is also the most sustainable .
Another favourable aspect of this alternative is its behaviour under dynamic effects . A better response to vibration phenomena than other solutions is achieved due to the mass and damping provided by the use of concrete for the whole bridge construction .
This bridge type is also more environmentally friendly than most of the analysed options , not only due to its integration in the landscape ( which is considered highly successful as well by the authors ) but also to the fact that the arrangement of its structural members , with a low number of them and quite massive , minimises accidental bird impact and death . This aspect is very important in a valley which is a corridor for migratory birds .
2.3.2 Specific features of the design which improve the basic bridge-type scheme
In addition to the previously-mentioned features , which make the open-spandrel deck-arch bridge made of concrete the best solution for the crossing problem , the designed bridge incorporates a series of specific features which significantly improve the basic performance characteristics . These features directly respond to the main span dimensions , location of the bridge and HSR traffic to be supported .
Joining together deck and arch at its crown is the first of these features . This joint forms the fixed point of the structure , taking advantage of the excellent characteristics of the arch as a horizontal-load transmitter , in addition to its main role as the verticalload supporting element . Consequently conventional expansion joints at both ends of an almost 1km long viaduct have been used ( Figs . 5 and 6 ).
The transformation of the single arch into two inclined legs is the second of these features . It improves the behaviour of the bridge under transversal actions ( both static and dynamic ) and its response to out-of-plane instability phenomena . These improvements are essential for a structure with both a reduced deck width ( due to its HSR nature ) and an exceptionally large main span . Splitting the arch transversely ( Figs . 5 and 6 ) and the variation of moment of inertia relative to the horizontal axis of its cross section , being a maximum at the springings and a minimum at the span centre , increases arch stiffness while maintaining a similar mass per unit length all along the arch . This leads to a better dynamic-effect response , to both vertical and horizontal effects caused either by wind or by trains , if compared with solutions with constant ( or less variable ) depth and width .
4 / 2016