The Normandie Bridge, France:A New Record for Cable-Stayed BridgesMichel VirlogeuxProf., Ecole Nationale des Ponts et Chausées, Paris. France

Landmark Cable-StayedBridges

On August 8, 1994. the last steel platewas welded to close the main spanof the Normandie Bridge. which, at856 m. is the longest cable-stayed spanin the world today (Figs. 1. 2). TheNormandie Bridge will begin its ser-vice life in January 1995. This is an ap-propriate occasion to analyse its de-sign and review the experience gainedduring the construction thus far.

In the 1970s and '80s, it was generallyconsidered that 500 rn was a limit forcable-stayed bridges, and almost allprojects were conceived with such alimit in mind. Consequently, therecord span progressed slowly: 404 min 1975 (Saint-Nazaire. France), 430 min 1983 (Barrios de Luna Bridge,Spain). 465 m in 1986 (John FrazerBridge to Anacis Island, Canada) and490 m in 1991 (Ikuchi Bridge, Japan).

But engineers already had some indi-cations that cable-stayed bridges werevery far from their limits: three majorbridges had been built in Germanywith a single pylon: the Köln SeverinBridge (302 m in 1959). the DüsseldorfKniebrucke (320 m in 1969) and theDüsseldorf Flehe Bridge (368 m in1979). For those who could foresee it.these three bridges proved that spans

of 600—700 m could be built from twopylons without major problems.

Some projects had been studied withlong spans, but the bridges had notbeen erected at the time: a first designwas done for the Normandie Bridgebetween 1976 and 1979, with a mainspan 510 m long: and a cable-stayedsolution was proposed in 1978 for theEastern Bridge of the Storeblt, Den-mark, with a span of 780 m.

Fritz Leonhardt proposed a cable-stayed solution in 1968—1 970 for cross-ing the Messina Straits with two pylonsin the sea and a main span 1300 mlong. He was followed by RenéWalther. who proposed that concretecable-stayed bridges can be economi-cally built up to 600 m, and compositeones up to 800 m.

Recent Progress

The preliminary design of the Nor-mandie Bridge — called the HonfleurBridge at the time — was developed be-tween September, 1986 and Spring,1987. The project was presented in thefirst conference devoted to cable-stayed bridges, in Bangkok, in Novem-ber 1987.

Since that time, the world record pro-gressed with two bridges designed and

built very quickly. probably helped bythe Normandie Bridge project. whichpsychologically opened the way forvery long spans: the Skarnsund Bridgein Norway (530 m in November, 1991),and the Yangpu Bridge in Shanghai,China (602 m in October. 1993). Twoother projects were clearly inspired bythe Normandie Bridge: the HonshuShikoku Bridge Authority decided, af-ter the Bangkok Conference, that theTatara Bridge would not be a suspen-sion bridge, but a cable-stayed one. Itserection began in 1993. and it will be-come. in 1998 or 1999, the new worldrecord with its main span 890 m long.Danish engineers designed a new ca-ble-stayed solution for the East Bridgeof the Storeblt. extending the mainspan to 1200 m. All problems foundappropriate solutions, illustrating thefantastic possibilities of cable-stayedbridges, but navigation requirementsfinally called for a 1624 m long mainspan. longer that the longest suspend-ed span in the world, and the cable-stayed solution was abandoned.

It is now interesting to compare thecable-stayed bridges which held thesuccessive world records:

— Saint-Nazaire Bridge. 1975:steel orthotropic box-girder

— Barrios de Luna Bridge. 1983:prestressed concrete bridge

— Anacis Bridge, 1986:composite deck with two I-shapedbeams and a concrete slab

— Ikuchi Bridge, 1991:steel main span (and concrete ac-cess spans. like the NormandieBridge) made of two parallel box-girders

— Skarnsund Bridge. 1991:prestressed concrete

— Yangpu Bridge, 1993:composite construction

— Normandie Bridge. 1994:steel orthotropic box-girder for itsmain span.

The cycle is closed, and a concrete ca-ble-stayed bridge with a main span ofabout 1000 m cannot be expected: nor

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Fig. 1: The Norinandie Bridge

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Fig. 2: Elevation

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Fig. 3: Cross section of the prestressed con-crete access span deck

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Fig. 4: Cross section of the steel box girderdeck of the main span

probably a composite one due to high-er weight and increasing costs of ca-bles. But the competition which exist-ed during twenty years between con-crete, composite and steel decks is an-other indication that the limits havenot been reached.

Main Aspects of the Design

While the Normandie Bridge is likelybe surpassed by even longer bridges inthe years to come — the Tatara Bridgeand still longer ones very soon there-after — it is of major importance inthe technical evolution of long-spanbridges. It is the first cable-stayedbridge entering the domain of verylong spans, which was reserved up tonow for suspension bridges. For thisreason. it is worth pointing at the mostimportant aspects of its design.

Wind-Governed Design

The design of long span bridges is gov-erned by wind and wind effects. TheNormandie Bridge helped or inspiredthe design of other bridges, and it isalso true that the Normandie Bridge it-self was very much inspired from thesuspension bridges designed by Free-man Fox and Partners: UK's SevernBridge and Humber Bridge, andTurkey's first Bosphorus Bridge. AsKlaus Ostenfeld once remarked whendiscussing the new cable-stayed solu-tion for the Storeblt in Denmark."Engineers are climbing over eachother's shoulders.

The main aspects of the wind design ofthe Normandie Bridge are:

— The streamlined cross section of thedeck, to reduce wind forces and toincrease the aerodynamic stabilityof the bridge. The streamlining isclearly inspired from the Englishbridges mentioned above, but the fi-nal shape was selected for specificreason: it had to be adapted to bothconcrete and steel structures, sincethe deck is in prestressed concretein the access spans (Fig. 3) and insteel in the central part of the mainspan (Fig. 4).

— A high torsional rigidity, to clearlyseparate the vibration periods intorsion and vertical bending. Forthis reason, the deck is a box girdersuspended on both sides. In addi-tion, the pylons have the shape of aninverted Y to concentrate the high-er anchorages of cables on thebridge longitudinal axis.

— The shape of the pylon — an invertedY — is also extremely efficient atresisting transverse wind forces(Fig. 5).

— The concrete and steel compositedeck, with concrete access spans onclose supports extended at a dis-tance of 116 m from each pylon in

the central span. as well as the rigidconnection between deck and py-lons. increases the structure's rigidi-tv. Wind-induced deflections aredrastically reduced. Alan Daven-port compared the deformabilitv ofthe Normandie Bridge with the Lit-tlebelt suspension bridge — also effi-ciently built with a streamlined deckbased on the English experienceand a main span of only 600 m — andconcluded that the NormandieBridge behaves like a cable-stayedbridge with a much shorter mainspan and is much more rigid than asuspension bridge with a span of500—600 m.

Composite Construction

The second major point in the designof the Normandie Bridge is the combi-nation of prestressed concrete and

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547.75Concrete access span

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737.50Concrete access span

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steel. Composite designs. where con-crete and steel are used to their great-est efficiency, are strongly endorsed bythe designers of the NormandieBridge.

The Normandie Bridge combines con-crete and steel for the design of the

deck, prestressed concrete in the ac-cess spans. on close supports. with anextension in the main span on bothsides. Only the central part of the mainspan is an orthotropic steel box girder.much lighter (9 t/m, instead of the usu-al 45 t/m) to limit the cable size. Theuse of concrete in the access spans re-duces total costs and increases thebridge's rigidity, as well as the back-staying efficiency of all rear cables.

This efficient combination of concreteand steel in cable-stayed decks hadbeen used before for the design of theTampico Bridge in Mexico (360 m,1988) and of the Ikuchi Bridge inJapan (490 m. 1991). And prior to that,much valuable experience had beengained about using various materials.such as traditional and lightweightconcrete, in Dutch bridges built by thecantilever method (Nijmegen bridges.around 1970). The experience gainedin using different weights for a specificstructural purpose proved to be ex-tremely useful (the bridges at Ott-marsheim and Tricastin and the cable-stayed bridge over the Elorn River).

The Normandie Bridge also uses acomposite design for the upper part ofthe pylons. where cables are anchored(Fig. 6). It is far more efficient to de-sign a steel anchorage box to anchorthe cables, since steel plates easily car-ry tensile stresses from back-stays tocables suspending the main span. Inaddition. it is much easier to fabricatethese steel anchorage boxes — or the el-

ements which will constitute them — ina factory than on site in concrete 100or 200 m above ground. To achieve theproper geometry. it is necessary to pre-cisely adjust the position of steel ele-ments which are later completed byconcrete walls.

Probably the first application of thistechnique was in Belgium. for the con-struction of the Ben Ahin and \VandreBridges, designed by René Greischand Jean-Marie Cremer. The idea wasused again for the Evripos Bridge inGreece and for the Chalon-sur-SaOneBridge in France. The problem wasmore complex in the NormandieBridge. with the transverse inclinationof cables. A design was developed withJean-Claude Foucriat. introducinghorizontal prestressing tendons topress the concrete walls against thesteel anchorage boxes to help thetransfer of vertical forces from steel toconcrete.

The steel anchorage tower (Fig. 7) wasdivided into 21 elements (the lowerone being divided into two half-ele-ments) to be lifted by the site crane(capacity: 20 t). and welded on site.The typical element was designed toanchor a pair of cables on each side(Fig. 8). The main plates were dividedin ties for the transfer of forces fromthe main span to back-stays in order tolighten the elements, reduce in situwelds and facilitate access from the lat-eral cells of the pylon — with a lift — tothe anchorages.

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Fig. 8: Installation of an anchorage box

Fig. 6: Cross section of the composite pylonwith steel anchorage element encased be-twee,z two concrete walls

Fig. 7: Installation of the first series of an-chorage boxes, south pc/on

High Performance Concrete

The main advantage of high perfor-mance concrete for standard andmedium span bridges is substantiallyenhanced durability. But for heavilyloaded elements, such as the pylons ofcable-stayed bridges with long spans,or the concrete deck of the NormandieBridge. which has to balance highstresses from wind effects. high perfor-mance concrete has great structuraladvantages.

All concrete on the Normandie Bridgecontains silica fume for a characteristicstrength of 60 MPa. This allowed for areduction in the cross section of theconcrete in the pylons and deck andthus a reduction in weight and founda-tions.

Erection of the Access Spans

The erection of access spans on bothbanks required the contractors to de-velop a new technology. Classical erec-tion techniques, with Teflon pads.would have produced very significanthorizontal forces due to friction (up to5 %) and to the slope of the accessramp (6%). For this reason the initialdesign did not use the incrementallaunching method, although it was ofgreat interest due to the complex crosssection shape and to the high rein-forcement ratio necessary to resistwind forces.

To be able to use it despite the slope.the contractors invented a so-called"staircas&' method for horizontal spanlaunching (Fig. 9). The deck is support-ed on each pier h' two trapezoidalblocks — one on each side — which canslide horizontally on the pier. Thismovement is permitted by specialbearings, made of a series of smallrollers, on top of the pier. After theforward movement, the deck is liftedby jacks commanded from a centralcomputer and the trapezoidal blocksare pushed backwards, ready for a newlaunching step. The launching opera-tion proceeds by successive launchingsteps: 15 cm horizontally and then 9mm vertically to correspond to theslope of6%.

Such a procedure was only made possi-ble h the use of a series of sensors. tocontrol horizontal and vertical move-ments on all supports. and of a centralmicrocomputer which could commandhorizontal and vertical movements. Itwas of special importance. of course.that vertical movements be the sameon all supports at any time.

In addition, this new technique re-duces the necessary manpower duringlaunching, since control is only neces-sary at supports, which can be done atthe central command from measure-ments obtained by sensors or videocameras.

Erection of the Main Span

The 116 m long concrete cantilever,which extends the side spans in themain one on each bank, and the 96 mlong last side span have been built bythe balanced cantilever method fromthe pylon with the help of temporarystays (Fig. 10). In the last side span. theclosure was made 6 m before reachingthe pier with the incrementallylaunched typical spans.

The steel part of the main span, 624 mlong, has been erected by the can-tilever method from the completed ac-cess spans with the help of a mobilederrick to lift the successive segments.19.6 m long, on each bank (Figs.11—13).

A Vew Generation of Cables

The preliminary design called forlocked coil cables, which were consid-ered very well adapted to such longspans. but which are unfortunatelyvery heavy. Their erection cost thusproved prohibitive.

For this reason, the contractors pro-posed alternative cables made of indi-vidually protected strands of hot-dipgalvanised wires which were re-drawnto keep all their structural characteris-tics. After coiling and after the corre-sponding thermal treatment. the voidsbetween wires were filled with oil waxto repel any water. The strand wasthen protected by extruded high densi-tv polyethylene at least 1.5 mm thick.

These strands were placed and ten-sioned one-by-one. Individual place-ment was extremely economical. buttensioning required a new technique.already developed by the cable suppli-

er for the erection of three bridges:the Arrade and Guadiana Bridges inPortugal, and the Chalon-sur-SaôneBridge in France. The first strand ofeach cable is tensioned to a computedvalue and equipped with a pressurecell which gives the tension at anytime. Each new strand, when installed.is tensioned to have exactly the sametension as the pilot strand at that pre-cise moment, which is given by the cell.All strands thus receive the same ten-sion. which is the desired one if the ini-tial tension of the pilot strand hadbeen appropriately computed. If not.an adjustment is made the same way.This process is not susceptible to influ-ences from teniporary operations, suchas the movement of constructionequipment.

Finally, cables received an externalduct made of a series of two half-ele-ments which are forced into each oth-er. These ducts are not for corrosionprotection: they are air and water per-meable. They aim at reducing drag

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Fig. 9: Concrete access spans launched from the north bank

Fig. 10: Balanced cantilver construction ofthe deck at the south pylon

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forces and avoiding rain-induced vi-brations of the cables. In addition. theytotally eliminate the vibrations ofstrands in the bunch which makes eachcable, which are produced by wind in-teraction between strands by a kind of"wake" effect.

Interconnecting Ropes

In his design for the Nlessina Straits,Fritz Leonhardt envisioned connectingall cables in each plane of the cables bytying ropes. which aimed at increasingthe apparent modulus of elasticity ofthe suspension. lowered by sag effectsin long cable-stayed spans. In someother bridges, such as the Faro Bridge

in Denmark or the two cable-stayedbridges of the Kojima-Sakaide route ofthe Honshu-Shikoku link, ropes wereinstalled to limit cable vibration whichwas rain-induced in the Faro Bridgeand coming from the wake effect in theJapanese bridges.

The purpose is totally different in theNormandie Bridge: due to the verylong span of the bridge, the main vi-bration period for vertical bendingwould have been of the same magni-tude as the vibration period of thelonger cables. 4.5 s, compared to about4.0 s. In this situation, it was fearedthat cable vibrations would be inducedby the deck movements. Interconnect-ing ropes were designed to totallychange the vibration periods of cables,at least transversally, reducing them to1.25 s and less.

Four ropes connect all cables in eachplane of stays. Their tension was se-lected to avoid dc-tensioning from vi-brations produced by wind turbulence.Their constitution is composite. withsteel and plastic to increase fatigue re-sistance because it is obviously diffi-cult to simultaneously achieve a highdamping coefficient and a high fatigueresistance.

Concluding Remarks

The design and construction of verylarge bridges which go beyond existinglimits require the strongest determina-lion from the Owner, who must investenormous confidence in. and support

of. the engineers in charge. The mostdangerous tempests that audaciousprojects face are not produced by windon site. but by antagonistic opinionsthat find a willing audience.

The success of the Normandie Bridgeis due in large part to the confidenceand support that the project receivedfrom the Owner. the Road Directorand the local authorities. Some organi-sational aspects and some episodesduring construction indicate the deci-sive importance of human factors.

The Owner gave the design engineerstotal responsibility for the design andgranted them complete freedom to as-semble the design team. Under thesecircumstances, improvements could beintroduced at each step of the project.with no consideration other than effi-ciencv. This is far superior to designcompetitions, now preferred by someadministrations, where projects can beselected based not always on structuralaspects. and where designers can be-come prisoners of their initial sketchesand of premature options and deci-sions.

Although there was no public moneyin the Normandie Bridge. the Frenchgovernment had to approve the pro-ject. The Road Director at the time,Jean Berthier, decided to invite an as-sessment of the design by an interna-tional group of experts: Marcel Huet(Project Manager of the TancarvilleBridge), Henri NI a thieu. CharlesBrignon, Roger Lacroix. RenéWalther and Jorg Schlaich. This groupproposed various amendments. someof which were included in the final de-sign.

Nevertheless, some engineers fromone of the erection contractors consid-ered the wind forces to have been un-derestimated and, thus, the safetyquestionable. The debate became pub-lic, even international.

The Owner and the Road Director de-cided to consult Alan Davenport toevaluate the wind tunnel tests and theestimated wind forces. He approvedthe performed analyses and recom-mended some additional wind tunneltests. the results of which were evenmore favourable than the first evalua-tions. This confirmation of the designhelped the project er much, andfrom the summer of 1991. all contrac-tors worked with enthusiasm and ener-gy to complete the bridge on schedule.within budget and up to the prescribedstandards of quality.

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Fig. 11: A steel segment of the main span be-ing lifted from a barge

Fig. 12: Derrick lifts a segment of the main span near the north pylon

Any decision can be questioned, anyaction criticised. The clear conclusionis that a complex and ambitious pro-ject like the Normandie Bridge cannothe successfully realised without astrong Project Manager — as BertrandDeroubaix has been for this project —to guide it over the years from concep-tion to completion. even when ques-tioned from many sides. Going furtherthan ever before in any given field callsfor courage. The Owner and the localauthorities remained totally confidentin the design and in the engineers incharge. even in difficult times. This wasdecisive for success; complex struc-tures cannot be built with hesitationsand doubts!

References

[1] VIRLOGEUX. NI.: FOUCRIAT.J.-C.:DEROUBAIX. B. Design of the .\o,-mandie Cable-Stayed Bridge near Honfleur.Proc. of the mt. Conf. on Cable-StayedBridges. Bangkok. pp. 1111—1 122. Novem-ber. 1987.

[2] VIRLOGEUX. NI. Projet dii Pont deNor,nandie, Conception générale de l'ou-vrage. Proc. of the 13th IABSE Congress,Helsinki. IABSE. June, 1988.

[3] DEROUBAIX. B.: VIRLOGEUX. NI.Design and Construction of the NormandieBridge. Proc. of the IABSE Symposium.St Petersburg, Russia. September. 1991.

L4] VIRLOGEUX. M. Wind Designand Analysis for the Normandie Bridge.In: Aerodynamics of Large Bridges.A. Larsen. ed. Balkema. Rotterdam 1992.pp. 183—216.

[5] VIRLOGEUX. M. Normandie Bridge:Design and Construction. Proc. of the Inst.of Civil Engs.. Structures and Buildings.August, 1993. pp. 281—302.

[6] DEROUBAIX, B. Presentation duprojet et développement de la construction.In: Le point sur le Projet du Pont de Nor-mandie. Annales de l'ITBTP. Paris. Sep-tember—October 1993.

[7] LEGER, P. Finance,nent dii Pont deNormandie. ibid.

[8] DAVENPORT, A. Analyse des etudesdes effets dii vent sur le Pont de Normandie.ibid.

Owner:Chambre de Commerce et d'Industriedu Havre

Project Managetnent:Mission du Pont de Normandie

Design:SETRA, Sofresid. Quadric. SEEE,Sogelerg, Setec and Europe-EtudesGecti. Architect: Charles Lavigne

Wind Laboratories:CSTB and ONERA

Contractors (concrete):OlE du Pont du Normandie(Bouygues, Campenon Bernard.Dumez, GTM. Quillery. Sogea.Spie Batignolles

Contractors (steel):Monberg and Thorsen

Sub-contractors:Bilfinger + Berger. Freyssinet. Munch,Lozal, VSL. SDEM

Service date:January 1995

Fig. 13: Segment being lifted into position

[9] VIRLOGEUX. M. Le projet dii Pontde Normandie. ibid. Photos: G. Fourquet, SETRA-CTOA

Fig. 14: North bank side of the Norinandie Bridge, September 1994

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