法国阿赛洛钢铁集团专家桥梁学术报告Lectures

New trends in Steel Bridge Design and Construction in FranceDr Wasoodev HOORPAHCivil EngineerHead of Bridges Dept.OTUA, Paris, FRANCESummarySteel has gradually made a come-back in the deck structure of medium and long span bridges in France in the last twenty-five years. This trend started in the road bridges first and has now reached the maximum share in the bridges for the new high-speed railway lines. Along with economical structures like the twin girder decks, steel has kept on being used in innovative and technically challenging design. This paper shows the recent trends and the new markets in bridge design with steel taking place now in France.Keywords: Steel, composite, aesthetics, high-speed railways, twin girder, tied arch, double box section, construction, launching.1. IntroductionNowadays in France, above 80 % of bridges in the medium and long span range have a composite steel concrete deck. This trend started in the 1980’s in the road bridges. Before that in the 1970’s the development of prestressed concrete bridges in France had totally excluded steel from the bridge market.About ten years later, the same phenomenon was observed in the rail bridges for the high-speed lines. The first two lines in the 1990’s had only pre-stressed concrete viaducts, but the share of steel has gradually been increasing since that time.The comeback started with the twin girder composite decks and gradually gave birth to other types of structures. The new trends with steel can now be observed in all span ranges.2. The come back of steel in bridgesSteel for bridges is not an important market in France, the annual structural steel weight in this construction branch bridges varies between 30 and 40 thousand tonnes. Bridges however provide a highly mediatic image for steel and the steel industry is keen to promote this image. Although the total weight remains nearly the same, the percentage of deck surface in the medium and large bridges has been constantly rising since the 1970’s. In these lest years, major bridge projects are being built with steel both for motorways and high-speed railways.This comeback is due to the special research and development efforts of all the professionals involved in the steel bridges: owners, designers, builders, steel fabricators. The most important of these relate to the design of uncomplicated structures, the use of modern calculation rules based on the limit states concept and the availability of high quality thick steel plates.The French Steel Bridges Committee has also played an important role in the development of the steel bridge market. Since the early 1980s, for large and medium bridges a double tender has been made as often as possible, with two alternatives: prestressed concrete and steel-concrete composite deck. Professional training programmes have also been organised regularly to familiarise the design engineers with steel and composite bridges.Statistics of road bridges shows that a single type of structure accounts for the success of steel bridges: the twin girder composite bridge. This structural type accounts for over 80 % of new bridges. The economic advantage of the twin girder deck was first demonstrated in the motorway bridges. For this reason the French Railways with the help of the steel bridges constructors and the steel fabricators carried research programs and proved the competitiveness of these decks for the new railway bridges.102030405060708090100020406080100Fig. 1 : Market share of composite bridges with span length3. Road bridges3.1 Twin girder deckThe most common type of structure is the twin girder composite deck which consists of a reinforced concrete slab which is connected to two steel beams. The typical roadway widths correspond to two or three lanes. The main beams are cross braced by I -girders every 6 or 8 metres ( figure 1).These cross girders are welded to T -shaped vertical stiffeners. These structures are usually entirely welded by fillet welds, with the exception of the temporary wind bracing which is used during construction before the concrete slab is placed and which is assembled using bolts.The slab is generally connected by four rows of studs. Welded segments of angles have occasionally been used.Fig. 2 : Common twin girder composite deckThis basic form has undergone different variations : For very wide decks the slab is also supported by the composite cross girders whose upper flanges are welded to the upper surface of the main girders and these cross-girders sometimes extend under the cantilever parts of the deck as well.Though this structure appears simple, it nevertheless incorporates the latest developpments from the steel manufacturers :- very thick plates reaching 150 mm, thus without flanges consisting of plates welded ontop of each other;- longitudinally profiled plates, which alllows an optimal tailoring to the bending momentdiagram; ( Fig. 3)-thermomechanical steel with a better weldability, not requiring preheating on site.To enhance the aesthetic of the bridge, it is quite common to have girders of variable depth, especially for long spans reaching 100 meters. The launching of such decks is possible, though a bit more complicated than the constant depth girders.3.2 Box sectionsFor geometrically complicated bridges: curved or skew for example, it is usual to have a box section necessary to resist torsional effects. The most common shape is a trapezoidal form. In some cases the upper flange provides the closed section, but the open box with two upper narrow flanges is cheaper to fabricate and the slab concreting can be also easier.The aesthetics of these sections are highly appreciated by bridge owners; for this reason they have been chosen even for medium span crossings where the twin girders could have been economical.For large decks, up to now the cantilever on the outside was supported by crossbeams in the alignment of the diaphragms. The recent examples of Verrières and Valence viaducts have introduced diagonal tubular bracings which can also be inclined longitudinally. ( Fig.4)Fig. 4 : Box sections with tubular brackets3.3 Special composite bridgesConcrete bridge designers often find it interesting to use steel webs, for lightness and specially for construction ease. For the 240 m. span Bras de la Plaine bridge opened in 2002 in Réunion island, the deck consists of two prestressed concrete flanges connected to vertical tubes. These were essential for the cantilever construction of the deck. Additional external prestressing cables are also present .Fig. 5 : Viaduct of Bras de la Plaine in RéunionIn the Meaux viaduct near Paris, the box section has the upper and lower flanges in prestressed concrete and two ‘plano-tubular webs’ with additional tubular brackets. This 31 m wide bridge also has the signal span prestressed with underside cables. ( Fig. 6) This design benefited f r om a special innovation financing.Fig. 6 : Meaux Viaduct4. High speed railway bridgesThis comeback took place in the new TGV railway bridges only after 1990. The first high speed railways in the eighties: South East and Atlantic lines had only pre-stressed concrete viaducts while the North TGV line from Paris to Lille included some fifteen steel and composite bridges for a total weight of 20000 tons.This trend was confirmed in the new Mediterranean TGV with 44000 tons of steel in 23 bridges, counting for nearly two thirds of the total large crossings. This exceeds the current annual steel consumption in French bridges which reaches about 30 000 tons.4.1 The structural designSteel has played an essential role by enabling the high speed crossing of spans ranging from 25 m to over 100 m. For the longest spans above 63 meters, a warren truss girder was chosen in the TGV Nord line and tied-arch bridges for the TGV Méditérranée line; for the shorter spans plategirders decks were chosen: composite two or four beams, twin boxes or lateral girders. Table 2 shows the structural types with the total length and the steel quantities used.In the TGV Mediterranean line, the eight largest viaducts were designed with an architectural competition; among these five had a steel structure : tied arch bridges of La Garde Adhémar, Mornas, Mondragon, Bonpas and a special tubular truss at Viaduc de l’Arc.The conceptual design of steel bridges has evolved from the classical twin girder composite deck to more aesthetic tied arches. This was due of course to the increase in span length, but also to the aesthetic demands in the TGV Méditerranée crossings.1993 - 1996 1994 - 1996 1997 -1999Multi Girder 234 m 573 t 121 m 700 t 992 m 3554 tRAPL 581 m 3350 t 300 m 2000 t 245 m 2040 tWarren Truss 91 m 1000 t 0 0 0 0Twin box section 0 0 0 0 290 m 1022 tTied arch 0 0 0 0 730 m 12900 t0 0 0 0 300 m 1270 tSpecial tubulartrussTable 1 : Structural types of HSR steel bridges : total length and steel weightThe twin girder composite deck proved its highly competitive and economic value throughout all the lines.The largest span built up to now is 63 m. at Viaduc d’Orgon; span of 75 m. will be seen in the new TGV Est at Viaduc sur La Moselle. Its design has been optimised through easy to fabricate structural details especially for the lower bracing. In some bridges this bracing is replaced with concrete slabs, which are even more economical.The conceptual design was carried with special attention to the dynamic behaviour of the deck which had to be guaranteed under the high speed train. This is one of the main reason why the high speed railway steel bridges systematically incorporate a concrete slab which takes part in the global and local resistance as part of the composite deck structure. It also carries the ballast. The concrete also brings supplementary mass and damping, thus decreasing the noise emission under the TGV passing. High speed requires the steel deck to be very stiff and sufficiently heavy to limit dynamic phenomena, which have to be mastered in order to ensure the safety and comfort of the train passengers. The railway works regulations impose severe and precise criteria for these points. This also has some important consequences on the detailed design regarding the fatigue resistance.In all these bridges, particular attention was given to the integration of architectural and structural values in the engineer's work through a constant dialogue with the architect. This gave birth to the spectacular steel superstructures in the large tied-arch viaducts with complex fabrication details Mornas and Mondragon, La Garde Ahémar, South Avignon toll. And of course the Arc Viaduct with its particular tubular truss girders.In the new TGV Est line under construction, the lateral girder composite decks have been chosen for skewed crossings over the A4 motorway. For the large viaducts like the Jaulny viaduct, a composite deck on a double box sections with the outer web inclined has been chosen. This last kind of deck had been introduced in fact in the TGV Méditerranée line for short skewed crossings.5. Cable stayed bridges - The Millau Viaduct5.1 Preliminary designThe Millau viaduct under construction now will lead to the opening of an alternative to the North - South crossing of France by the A6 and A7 in the Rhone Valley. The by-pass of the town of Millau in progress now in Aveyron department has required the construction of the impressive world record high Millau Viaduct over the Tarn valley.The crossing of the Tarn valley represents a difficult obstacle for all crossings designed around Millau. Lengthy studies carried since fifteen years ago h ad first analysed the gross itineraries in order not to cut of the town from this new route, then the possible crossings of the Tarn Valley, with all the bridges and tunnels required by each route chosen. Geological conditions excluded some possibilities, also did the too steep alignments outside the motorway standards. ( Table 2)March 1988 First survey of possible routesJune 1989 Choice of « median » routeOctober 1991 Choice of the ‘’high’’ alternativeJuly 1993 Approval of preliminary design (APS)November 1993 Preliminary detailed project with 8 engineer and 8 architect teamsNovember 1994 Competition with 5 engineer-architect teams.January 1995 Official Public Interest Declaration (DUP) of Millau by-pass.July 1996 Choice of multi-span cable stayed bridge by team: FOSTER -SOGELERG -SERF - EEG .January 1998 Government decision for concession of the viaductOctober 1999 DUP of the concessionFebruary 2000 Call for bidsJuly 2000 The 4 candidates are accepted for the competitionDecember 2000 Return of the bid offer by 3 candidatesFebruary 2001 Choice of Eiffage group with the steel deck solutionApril 2001 End of preparatory works on siteSeptember 2001 Official government decree approving the concessionTable 2 : Preliminary design phases of the Millau ViaductThe valley is here about 2 500m large between two high points: Puech d'Ausset in the North and Puechass at the South. The deepest part, about 700m wide is over the river Tarn and is followed by the almost flat Plateau de France towards the North.As shown on Table 2, the French Government called for the competence of the French bridge engineering community to analyse all possible structural solutions for this crossing. The design teams - architects and engineers - studied various structural alternatives. For the final design phase there were six bridge types proposed : deck of constant or variable depth in steel and concrete, a large concrete arch, under-stressed truss girder and multi-span cable stayed viaduct. (Fig. 7)Fig. 7 : The different types of bridges in the initial design ( Source AIOA )For the final phase of these preliminary design steps, the following alternatives were proposed : •Pre-stressed concrete deck with spans of 175m•Steel box with orthotropic deck in 200m span•Concrete arch viaduct 500m wide.•Steel lattice deck with steel piers in 350m span•Multi-span cable stayed viaduct with pre-stressed concrete deck in current span of 350m From these proposals, an international jury led by the Road Director chose the cable-stayed bridge.5.2 Cable stayed viaductThis solution is a 2450 m long viaduct with 6 equal spans of 342m and end span of 204m. The particular mountainous configuration of the crossing imposed the pier implementations and the span distribution. A large signal span over the Tarn was not justified technically or architecturally, because this span would have been at one end of the viaduct without a global equilibrium and certainly unstable to wind loads. Besides this span would not seem logical from the different viewpoints as the Tarn crossing is never completely visibleIn this powerful landscape, the viaduct could not remain discreet; it had to mark its presence while at the same time remaining slim and elegant. The regular span distribution has a particular aesthetic aspect.Fig. 8: Photomontage of final design( Design team FOSTER-SOGELERG-EEG-SERF) ( Source AIOA)The Millau viaduct will the latest example of the new cable stayed bridge concept. Such multi span cable-stayed bridges provide an elegant solution for large valleys or strait crossing. The design of such bridges requires careful studies: an intermediate span loading causes uplift in the two adjacent spans. As there are no retaining cables from a pier on a back span or abutment, an internal stiffness must be provided within the bridge itself. This is achieved by the double pier tops and the pylon legs, which provide a rigid intermediate support. ( Fig. 9)Fig. 9 : Pier-deck-pylon junction ( Source N.FOSTER)5.3 The steel deckThe initial design was proposed with a pre-stressed concrete deck. The steel alternative had also been proposed by the client, but the unorthodox nature of the proposed construction method, led many professionals to give little chance to the steel against the concrete deck. As the in service behaviour of the two types of deck are nearly identical, the benefits of steel in the long term was put forward, for m aintenance and repair, the greater speed and reliability that industrial steel fabrication would offer, and the safety and ecological benefits of launching a fully constructed deck from the two abutments as opposed to concrete works at such a tall height finally gave an advantage to this solutionFig. 10 : Cross-section of orthotropic steel deck ( Source AIOA)The all steel deck consists of a 4m by 4.2m rectangular main box which forms the central spine of the 32m deck and whose primary function is to anchor the stay cables. The whole deck is a aerodynamic box section designed to resist the buffeting wind effects which are particularly strong at this height over the mountain valley. The upper flange of the deck is an orthotropic plate with the trapezoidal stiffeners designed to withstand also the axle loads of cars and heavy trucks on theasphalt. The corrosion protection of the inside of the box section will be obtained by the use of a drying system, which is more economical than a coating system. However the external surface of the steel deck will be protected by a classical multi layer painting system which will also give the final colour.The Millau Viaduct also demonstrates that steel pylons can be economical by reducing the work duration. The two pylons on the Tarn span will be used to retain the overhanging nose of the deck during launching, while the other pylons will be slid on the deck and upraised directly over the piers.The steel deck of Millau Viaduct will be fabricated with high performance weldable steel. Many of the steel elements such as the inner webs will have to resist complex stresses under the very high reactions during launching or the pylons and the stay cable anchorages in service. For these reasons most of the steel elements will be in S460 TM grade. On site welding of this grade can be done without pre-heating. This steel grade was first used in France in 1994 at the Remoulins Viaduct, and the Millau viaduct will be the first large scale utilisation : about 12000 tons on the total of 36000 tons. The other steel grade mainly used will be S 355.5.4 The construction methodThe Millau Viaduct has been for the specialist design teams, one of the bridges whose construction method proved to be really tough to imagine. This mountainous site rendered any construction method from the valley below quite impossible without a ballet of heavily loaded trucks hindering the local traffic. Besides the local authorities and the bridge owner were committed under the environment protection measures to limit the works nuisance. The construction method thus logically follows: the deck could only be constructed by launching the completely fabricated box section from either side of the large valley. Temporary steel lattice piers are being built between the definite concrete piers in order to cut the launching distance to half the final span, except over the deepest part of the crossing, just over the river Tarn, where the two parts will join together. ( Fig.10). The deck stability, against aerodynamic wind forces is naturally obtained with the profiled shape of the steel box.To control the end deformation of the deck during launching, this end is supported by the stay cables anchored on the end pylons. The deck will be pushed on special launching equipments called translators. It will be lifted on each support with vertical jacks, then pushed with horizontal jacks, thus translating the whole deck forward by 60cm steps. These operations carried out simultaneously on all the supports will be synchronised by computer control. In this system, contrary to a classical sliding method on neoprene pads, no longitudinal forces are injected on the pier tops. The launching will take place from one pier to the next one without intermediate stop, the deck end will thus be fixed during the stopping required to weld the deck sections at the back. As for the steel pylons reaching 90m over the deck, they will be completely assembled in the construction halls on the work platforms behind the abutments and slid horizontally on the launched deck by a special carrier on multiple wheels, then raised at their final position over the piers. For this raising, the steel contractor has designed a special rotating crane that will lift the upper end of the pylon with jacks and cables. ( Fig. 11).Fig. 12 : Some construction sequences in April 20035.5 FabricationThe foundation works have started at the beginning of 2002 on site. The steel fabrication has started at nearly the same period in the steel contractor, EIFFEL’s shops at Lauterbourg. The 36000 tons of steel which will be mounted for this viaduct represents a year’s consumption of steel in bridges in France. The inner box has web plates in thickness ranging from 40 to 90mm. The lateral parts consists of 12 and 14 mm plates with stiffeners in 6 and 8 mm cold formed trapezoidal section. The sections with an average length of 22m and breadth of 3.50m are fabricated with automatic welding machines and a welding robot. ( Fig. 12- Fig. 13)6. Conclusion7. References[ 1 ] G. LEBAILLY, B. PLU : Les ouvrages d’Art Métalliques sur LGV – JIS Dec. 2002[ 2 ] Ph. RAMONDENC : The design of the steel and composite bridges of the TGVMéditerranée. IABSE Conference , Madrid 12-14 June 2002[ 3 ] OTUA : Ponts du TGV Méditerranée - Bulletin Ponts Métalliques N° 19 - 1999 [ 4 ] OTUA : Ponts du TGV Nord - Bulletin Ponts Métalliques N° 16 – 1996[ 5 ]J. RAOUL : The design of common composite road bridges in France – ECCS Steel Bridges Symposium- Barcelona March 2003。