PROCEEDINGS PAPER

Long Span PrestressedConcrete Bridges in Europe

by F. Leonhardt*

INTRODUCTION

Long span prestressed concretebridges were built very early inEurope. Freyssinet built the first ofhis famous series of Marne bridges,the Luzancy bridge, in 1946, with aspan of 245 ft. In 1949 the authorbegan to build the first prestressedbridges with continuity over severalspans including the bridge acrossthe Neckar-Kanal in Heilbronnwith a main span of 320 ft. In 1950Finsterwalder built his first bridgeby the free cantilevering methodand in 1952 constructed the spec-tacular bridge over the Rhine atWorms with three spans of 330,371 and 340 ft. Between 1950 andnow, more than 300 bridges withspans over 250 ft. have been builtin Europe and many different con-struction methods have been de-veloped. The longest span of pre-stressed concrete is now 780 ft. andit was designed and constructed byEuropean engineers for crossingLake Maracaibo in Venezuela(Fig 1).

Thus, prestressed concrete hasproved its feasability for long spanbridges, mainly by virtue of itsgreat advantages: economy, dura-bility, low maintenance costs, high

*Consulting EngineerWest Germany

62

fatigue strength and the possibilityof achieving extreme slenderness.

For long spans, only the post-tensioning method is used. Differ-ent systems of prestressing tendonsare utilized including high tensilebars with a prestressing force of 30to 50 tons, wire cables in circularsheaths with forces between 50 and180 tons and finally the so-calledconcentrated cables, mainly built upwith seven-wire-strands, with pre-stressing forces between 1000 and3000 tons per cable.

METHODS OF CASTING IN PLACE

From the very beginning, therehave been different methods of con-struction. Freyssinet assembled hisMarne bridges with precast ele-ments having ducts through whichthe prestressing cables were pulledin and tensioned. The Heilbronnbridge was cast in place on a center-ing, and Finsterwalder's bridgeswere cast in place in short portionson cantilevering steel falsework.These three methods are still in useand the oldest method, casting theconcrete in place, is still very popu-lar in Europe, especially in Germany.Making forms on falsework orcentering formerly required a largeamount of skilled labor and expen-sive timber. However, this work hasbeen greatly simplified and stand-

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Fig. 1—Bridge Across Lake Maracaibo, Venezuela

Fig. 2—Light Steel Centering for Casting in Place

and parts have been developedwhich enable the forms to be as-sembled largely with unskilled laborand with less than half the man-hours formerly needed. The mainsavings have been obtained by theuse of special equipment for thecentering (Fig. 2). This consists oflightweight steel-shoring, tripodshoring towers or lattice type hori-zontal shoring which can be alteredin length by telescoping (Fig. 3)to adapt them to the necessaryspan and can also be modified intheir load-carrying capacity by theinsertion of additional chord-mem-

bers. Usually, several shoring towersare combined by light lattice-workto increase the resistance to windforces. The lengthwise spacing ofthese towers varies from 30 to 90ft. depending upon the height andthe load to be carried. The girders,bridging the gap, can be camberedto compensate for their deflection,thereby avoiding the troublesometimber-fillers formerly used on rolledsteel-beams.

Very large bridges have recentlybeen built using such centering andforms and they have proved to beeconomical in keen competition with

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Fig. 3—Lattice Type Horizontal Shoring

other methods such as pre-fabrica-tion.

MERITS OF CONTINUOUS BRIDGES

Casting in place also helps makelong bridges continuous over manyspans, thereby eliminating expan-sion joints and saving steel. We donot fear differential settlement ofpiers because the slender prestressedconcrete beams are not muchaffected by such settlement. Thebending moments caused by the:settlement of a pier will be elimi-nated to a large extent by the creepof the concrete as was proved inReference 1. Recently, we built along beam-bridge, continuous over12 spans, in a mining area in whichground settlements of about 15 ft.are expected during the next 25

years (Duisburg on Rhine, Fig. 4) .The column footings of this bridgecan be hydraulically adjusted in thevertical direction and rest on rollerbearings to permit horizontalground-movement. Even for suchextreme cases, continuity of pre-stressed concrete beams proved ad-vantageous over a steel bridge, andespecially over statically determi-nate single beams with which itwould have been almost impossibleto keep the bridge under trafficduring periods of settlement. Thecapacity for resilience following in-elastic deformation is much greaterwith prestressed concrete beamsthan with steel beams.

COUPLING OF TENDONS

These continuous bridges are nowoften constructed span by span oncentering with the tendons or cablesbeing coupled at the 1/s-span sec-tion (Fig. 5). With this construction

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Fig. 5—Coupling of Tendon at 1/5-span Section

method, the centering and formscan quickly be reused. In somecases, the centering is taken down

Fig. 4—Bridge Across Mining Area in Duisburg

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Fig. 6-4ypical Elevated Highway (Dusseldorf)

and reerected, in other cases thewhole centering is moved on rails.In this way, long elevated highwayshave been built (Fig. 6). The com-pleted bridges are often curvedand have total lengths of up to2800 ft. with continuity over 34spans (Hochstra j3e Dusseldorf).

Some of our contractors have de-veloped systems which are almostmachines providing forms attachedto steel-trusses which span frompier , to pier, and which can bemoved span to span by rolling onother steel-girders (Fig. 7). The

trusses carrying the forms are firstplaced in position for placing theconcrete in the right span. As soonas the concrete has been partiallyprestressed, the hauling girders areconnected to the cantilevering endof the concrete bridge and thenthe trusses can be moved to thenext span. When the trusses be-tween the main concrete girders arebeing moved to the next span, theouter trusses with the forms of thecantilevering portions of the road-way slab are still in the formerspan. The hauling girders and the

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Fig. 7—Movable Steel Formwork (Strabag -Ko1n)

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trusses are carried by the final piersso that no extra foundations areneeded. For one bridge with 120 ft.spans, it took two weeks to con-struct a span. Prestressing com-menced four days after the concretehad been placed.

A simpler but similar method isshown in Fig. 8. Hollow box steelgirders placed below the concretebridge to be built are supported bycross-beams resting on the bridge'spier columns. These box girderscarry the forms. After the prestress-ing of the bridge, girders and forms

are lowered hydraulically andmoved to the next span usinglattice-girder exensions to reachthe next supports. This equipmentproved very successful in difficultterritory. For example, Fig. 9 showsit being used for a bridge alongsteep hill slopes in the Rhine valley.

Casting in place into forms car-ried by steel trusses supported byauxiliary piers has been used alsofor the construction of the mainspans of the Maracaibo Bridge(F'ig.10).

Fig. 8—Simpler Movable Forms (Polensky u. Zollner, Koln)

Fig. 9—Use of Equipment of Fig. 8

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Fig. 10—Steel Trusses Carrying Forms—Maracaibo Bridge

Fig. 11—Bendorf Bridge (Finsterwalder)—Dywidag System

FREE CANTILEVERING METHOD

The free cantilevering method is,of course, known in the UnitedStates. Some of the latest achieve-ments using this method to cast theconcrete in place include the Med-way Bridge near London with a500 ft. span, and a bridge across theRhine in Bendorf (Fig. 11) with aspan of almost 680 ft. In the latterthe equipment for cantilevering wasmuch simplified by replacing thecounterweight with vertical anchorsinto the finished part of the bridge.

The deflections of the slendercantilevering beams are influenced

greatly by the early creep of theconcrete and, therefore, by themoisture and temperature of the airduring the hardening period, bothof which change with the weather.Therefore, much engineering skillis needed to make the method fullysuccessful.

In several cases, the free canti-levering method has been used tobuild parallel girders continuousover several spans. Of course, suchbridges cannot be freely canti-levered, but must be supportedtemporarily by diagonal steel tiecables (Fig. 12) which run over thetower above the pier of the bridge.

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The auxiliary support can also beprovided by steel-columns (Fig. 13)within the span of the bridge, ascan be seen in bridges near Duis-burg which have been built crossingover factories.

With the Dywidag system, pre-stressing rods are used for this freecantilevering method and they arebuilt in directly and coupled everytwenty feet. These many couplersare costly and, since the prestressingforce of such a bar is limited, a largenumber of such bars are needed forlong spans. Large prestressing ele-

ments provided by wire cables are,of course, preferable. In fact, oneof our firms has built a 470 ft. spanacross the River Main near Frank-furt with the cantilevering methodusing 100 ton prestressing tendonswhich are threaded through theducts as they are needed for re-sisting the cantilevering moment. Atthis bridge, the joint in the middleof the main span between the endsof the cantilevering beams has beenclosed and full continuity has beenachieved by careful arrangement ofthe tendons.

Fig. 12—Free Cantilevering Method Using Tie Cables

Fig. 13—Movable Cantilevering Equipment (Dywidag)

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There is no doubt that the freecantilevering method has great ad-vantages, especially if large tendonscan be used. The work cycle is re-peated every three days step bystep and the team of workmen canattain a very high standard ofefficiency and workmanship by theserepetitions.

ADVANTAGES OF CASTING IN PLACE

As a final reason for continuingto cast concrete for bridges in place,I would put forward the fact thatthe site-mixing equipment for con-crete is today almost fully mech-anized and can be erected any-where on short notice. Usually twomen are sufficient to operate it.Therefore, factory-made or ready-mixed concrete no longer has anyadvantage over site-made concreteif large quantities are needed. Somefurther advantages of casting con-crete in place must not be over-looked:

1. There are no difficulties withjoints and a fully homogenousstructure is obtained.

2. The designing engineer hasmuch more freedom to choosethe most economical cross-sec-tion for the girder system andto consider all the irregularfeatures which are so frequent-ly encountered in crowdedareas such as skew-crossings,curves, progressively wideningor flaring portions of thebridges, etc.

PREFABRICATION

Total Span Length Girders

The subject of long span bridgesfor which prefabrication has beenused will now be discussed. Thereis no doubt that in many cases pre-casting has great advantages over"casting in place, especially if many

equal spans are constructed.The simplest type of bridge for

prefabrication is that comprisingsimply supported beams with openjoints over the piers. Especially inthe United States, this type ofbridge has been highly developed.However, the spans are normallylimited to about 120 ft. becauselonger girders become too heavy fortransporting and handling. For longbridges it pays to erect a plant forprefabrication close to the bridge.It also pays to use special equip-ment for hauling and placing heavygirders; for example, the bridgeacross the St. Lawrence River inCanada where 176 ft. long girders,each weighing 180 tons, were in-stalled with a launching bridge madeof steel trusses.

Simpler means for moving suchgirders into the final position havebeen developed lately in Luxem-bourg and Germany.

Nenning puts a simple steelbridge over two spans (Fig. 14) in

Fig. 14—Launching Precast Girders (Nanning.Luxemburg)

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gaps on the piers at an elevationjust below the girders. He movesthe girder on this steel-bridge withlong hydraulic jacks with an engineof only 3.5 h.p. The girders are thenmoved transversely into the finalposition.

The author has developed meth-ods of establishing continuity insuch girders after their erection andcan thereby effect a saving of about20% in the amount of prestressingsteel in addition to a savings inweight. For spans of 190 ft. and aspacing of the main girders of 12 ft.the prefabricated part of the maingirders (Fig. 15) weighed only 140tons (normal weight concrete), and10 prestressing cables of 72 tonseach were sufficient. These 10 cables

are prestressed in three differentstages and coupled at the jointabove the pier (Fig. 16) in such amanner that longitudinal compres-sion is also obtained in the deckslab above the piers thereby provid-ing resistance to negative momentswithout tension in this slab.

This method might, however, belimited to spans of 200 ft. becauselonger girders for the full spanlength may become too heavy tobe handled in one piece.

If the bridge is to be builtacross water with sufficient depthfor the use of floating cranes, someof which have carrying capacitiesup to 200 tons, such cranes mightbe used for placing even longerand heavier prefabricated girders.

Fig. 15—Cross Section of Bridge with Precast Girders

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Fig. 16—Tendons of Girders in Fig. 15 at Pier

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For the Maracaibo bridge, girdershaving a 154 ft. span and weighing180 tons were placed in large num-bers.

The French engineer, NicolasEsquillan, and his firm Boussironhave prefabricated the full spanlength and the total cross-sectionof a double deck bridge with a hol-low box-section for a highway ontop and a railway running throughthe box at Abidjan on the IvoryCoast in West Africa. The 140 ft.long superstructure weighing 800tons was prefabricated on the shoreand then floated to its final positionon two barges. There is no doubtthat even larger bridges can beprefabricated in this way andfloated into their position. Good ex-amples for these possibilities havebeen given in this country by theprefabrication and floating of largetunnel sections or sections of float-ing bridges.

These examples relate to the pre-fabrication of girders for the fulllength of the span, and there areonly longitudianl joints if separateunits are used. It is, of course, alsopossible to prefabricate short lengthsof the total cross-section or of partsmaking up the cross-section. Theearliest example of this constructionmethod was provided by Freyssinet

for his five bridges over the Marne,which I have mentioned at the be-ginning. Another well known andmore recent example is the con-struction of the Hammersmith fly-over in London for which a center-ing was necessary which wouldcarry the total weight of the bridge.

Prefabrication with TransverselyCut Units

A new method has been de-veloped by the author with a viewto using longer and heavier unitsfor the prefabrication of large-spanbridges (Fig. 17). This method isillustrated by the Ager Bridge inAustria, which has four spans, thelargest being 314 ft. long. The crosssection of the bridge shows twohollow box-girders, each carryingthree lanes of the Autobahn, about40 ft. wide. The depth of 14 ft. isconstant. For prefabrication, thesebox-main-girders have been dividedtransversely in 30 ft. long portionsand these portions with the fullwidth, weighing about 180 tonseach, were prefabricated using sta-tionary forms. The reinforcementwas pre-assembled and placed bya tower crane. Eleven men wereable to make each section withinfour days. Since these pieces weretoo heavy to be moved by normal

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Fig. 17—Ager Bridge, AustriaFebruary 1965 71

Fig. 18-30 ft Sections of Ager Bridge Sliding on Timber Rails

cranes, they were moved by slidingon two timber rails with lubricatedsurfaces (Fig. 18). These rails weresupported between piers and tem-porary towers by a narrow centeringalong the valley so that all thesections could be placed one be-hind the other over the four spansto form the bridge.

The concentrated tendons werethen placed along the sides of thewebs within the hollow box tunnel.A jeep, running on the bottom slabof the hollow boxes, was used forplacing the strands. After the cableswere installed, the gaps betweenthe sections were concreted, in-cluding transverse frames with thecable-bearings for resisting thevertical forces due to the changeof direction of the cables. Thecables were then prestressed bymeans of movable anchor-blocks atthe two ends of the bridge accord-ing to the Baur-Leonhardt method.Very low friction for the long andmulticurved cables was secured byusing teflon between the slidingplates at the cable bearings.

A good bond (Fig. 19) betweenthe cables and the webs is achievedby stirrups anchored in the websand by the concrete cover whichalso protects the cables against cor-rosion. So far, this construction

method has been found to requirethe least amount of labor and a verysmall quantity of material consider-ing the large spans.

These large prefabricated unitscan also be used to build long span.bridges across waterways by meansof the free cantilevering method ifthe bridge portions can be floatedto the site (Fig. 20).

Such a bridge has been fullydesigned for spans of 240 ft. andthe prefabricated units shall belifted with the equipment whichhas been developed in this countryfor the lift-slab method, hydraulicjacks resting on cantilevering steel-girders. After the portion of thehollow box-girder has been lifted toits final elevation, the prestressingcables will be pulled in throughthe ducts in the top-slab and ten-sioned. Then the next piece can belifted. It is obvious that long andmultispan bridges can be built rathereconomically and quickly in thisway if there are a sufficient numberof spans so that the forms for thelarge units can be reused oftenenough.

Another variant of this method isbeing used to build a bridge acrossthe River Caroni in Venezuela withsix spans, four of 320 ft. and two of160 ft. span length. Large rivers in

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Fig. 20—Precast Box Sections Set with Lift Slab Jacks-233 ft. Spans

tropical countries are often liable to center of gravity and produce axialheavy flooding so that centering compression. In this way, the struc-cannot be used. For this reason the ture can resist variable positive orentire bridge with a total length of negative moments along its whole1660 ft. is being prefabricated on length during the subsequent han-the approach in the same way as dling of the bridge (Fig. 21) . Thethe Ager Bridge. The large pre- prestressing is done from the farstressing cables along the inner end by moving one large anchorsides of the webs are, however, in- block. The elongation of the cablestalled straight in the first stage of is about 8 ft.construction at the elevation of the The bridge as a whole, weighing

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Fig. 21—Caroni River Bridge, Venezuela

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Fig. 22—Design for Indus Bridge by Author

8,400 tons, will be moved hydraulic-ally across the river. A new type ofsliding bearing will be used atspaces of 160 ft. along the approachand on top of the five permanentand four temporary auxiliary piers.Two hydraulic jacks of 250 tonscapacity each will do this job. Theyare located at and act against theabutment. In order to decrease thecantilever bending moment of thehead of the moving bridge, a lightsteel truss is fixed to the head. Afterthe final position is reached, thecables will be moved vertically up-wards at the supports and down-wards in the spans, and fixed intheir final curvature so that the

bridge will be supported over thelarge spans by the forces due to thecurvature of these cables. Theauxiliary piers may then be re-moved.

The full advantages of prefabrica-tion are obtained by reusing theforms, placing fully pre-assembledreinforcement and concreting in onespot immediately beside the auto-matic concrete mixing plant. Thenumber of joints and amount of con-crete to be cast in place are thusreduced to a minimum.

In these last-mentioned examples,the prestressing tendons have beeninstalled after the assembly of theprefabricated parts by threading

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wire cables into ducts or by spinningstrands so as to form cables overlong lengths. Both these methodsreduce to a minimum the amountof labor involved in making cablesso that post-tensioning has becomevery economical, especially if largediameter wire such as 10 to 12 mm,and large cables developing over100 tons of prestressing force areavailable.

Long Spans with Tie Cables

In tropical countries, it is oftennecessary to build bridges acrosslarge rivers where poor soil condi-tions make very deep foundationsmandatory. In such cases, spans of400 to 600 ft. are economical and,formerly, steel trusses were almostthe only solution. For such cases,the author has developed a systemusing prestressed concrete in a veryeconomical way (Fig. 22) .

On top of the well-foundations,an A-shaped tower is erected withthe horizontal leg at the elevation ofthe roadway. This tower carries along and slender beam, cantilever-ing to both sides, supported byinclined tie-cables every 60 ft. to 80ft. The tie-cables are anchoreddirectly to the beams, which areedge beams and which simultane-ously form the railing. With thismain-girder arrangement, the edgebeams can be prefabricated in sec-tions about 60 ft. to 80 ft. long,which can easily be erected by acrane or by a special transportationwagon which runs under the road-

way and between the towers on acheap trestle bridge. As soon as theedge beams are hung to the tie-cables, the prefabricated elementsof the road-slab are placed betweenthe edge beams as can be seen fromthe cross-section in Fig. 22. Theends of the cantilevering main-girders between two towers areclosed by a suspended span. Thequantities of concrete and prestress-ing steel are remarkably low for thistype of long span prestressed con-crete bridge and only three differ-ent precast elements are neededfor the superstructure. The towerscan be built with slip forms, castin place or with precast elements.

The edge beams are prestressedlongitudinally with two or threetendons which will be pulled intotheir ducts after the erection of thecantilevers is finished. The bendingmoments in these main-girders arerelatively small if parallel wire-cables are used for the inclined ties.

CONCLUSION

The author hopes that he hasshown that the field of long spanprestressed concrete bridges givesgood opportunities for the civil engi-neer for using and proving intuitionand creativeness and he is sure thatfurther progress in this field is stillahead of us.

REFERENCE

1. Leonhardt, F., Prestressed Concrete De-sign and Construction, English Edition,Wilhelm Ernst u. Sohn, Berlin, 1964.

Presented at the Ninth Annual Convention of thePrestressed Concrete Institute, San Francisco, October, 1963.

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