Structural Analysis of Historic Construction – D’Ayala & Fodde (eds)© 2008 Taylor & Francis Group, London, ISBN 978-0-415-46872-5

Strengthening historic pedestrian suspension bridges: Publicsafety goes first!?

I. Wouters, Y. Schoonjans, M. de Bouw & S. AdriaenssensVrije Universiteit Brussel, Brussels, Belgium

ABSTRACT: In the 1820’s an impressive amount of early iron suspension bridges was constructed. Since all ofthe large span suspension bridges of that period of the European continent have disappeared, the smaller pedestrianones gain interest. Only four iron suspension bridges built before 1825 remain on the continent (Vernosc-lesAnnonay, Bazel-Kruibeke, Saint Petersburg and Nuremberg). They are the last materialized witnesses of anexperimental phase in iron suspension bridge engineering. Given their antiquity and the modern demands, thosebridges must be renovated. Strengthening the bridge to comply the current safety standards is in direct contrastwith the wish to preserve the historic fabric. It is within this complex problem that different renovation strategiesfor the early 19th century pedestrian bridges, ranging from replica to contemporary interference, are analyzedand discussed.

1 EARLY IRON SUSPENSION BRIDGES

In September 1823, after returning from a study trip toGreat Britain, the Frenchmen C.L.M.H. Navier pub-lished his famous report ‘Rapport à Monsieur Bequeyet Mémoire sur les Ponts Suspendus’. Navier outlineda historical overview on the construction of suspen-sion bridges, starting with rope bridges in Peru, overrealizations of M. Poyet, M. Belu, J. Finley, M. Telford,S. Brown, M. Stevenson, M. Brunel, concluding withthe footbridges of M. Seguin in Annonay. Subse-quently, he established some general theories on thecalculation and the construction of suspension bridges.

Looking back at this impressive list of early ironsuspension bridges, one has to conclude that most ofthe mentioned bridges are lost. Some of them failedsoon after they were built, some were destroyed bywind or water, some were bombed, others were dis-mantled for safety reasons or replaced while upgradingthem to new design standards. The only suspensionbridges, listed by Navier, and still remaining, are theUnion Bridge over the RiverTweed (United Kingdom),built in 1820 by Samuel Brown, and the experimentalbridge in Vernosc-les Annonay, over the river Cance(France), built in 1822 by Marc Seguin. Moreover, theUnion Bridge is the oldest remaining vehicular ironsuspension bridge open for traffic.

Although Navier included many suspension bridgesin his Memoire, the list was not meant to be exhaus-tive. Most of the large span bridges were discussed, butthe smaller pedestrian bridges were not included, apart

from a few pedestrian bridges built in Scotland andFrance. Since the large span suspension bridges havedisappeared, these smaller pedestrian bridges gaininterest. In America, all the suspension bridges builtbefore 1825 are lost. In Britain several early sus-pension bridges remain. They are well documented(Paxton 1999, Ruddock 2003). So this paper will focuson the less known bridges on the vast continent.

When focusing on the remaining experimental ironsuspension bridges, built before 1824, on the vastcontinent, the following bridges can be listed:

– 1822, private bridge by Marc Seguin, Vernosc-lesAnnonay, France

– 1824, castle bridge by Jean-Baptiste Vifquain,Bazel-Kruibeke, Belgium

– 1824, Post office bridge by Wilhelm Von Traitteur,Saint Petersburg, Russia

– 1824, Kettensteg by Konrad Georg Küppler,Nuremberg, Germany

Remark: since information about small pedestrianand park bridges is rather scarce and the castle bridgesare often inaccessible, it is likely that more bridges willbe revealed in the near future.

These bridges are the only relic of this experimentalphase in iron suspension bridge engineering and sincetheir existence is a lucky coincidence rather than awell-considered policy of patrimony, we are curiousto see if these relicts represent the rich history theystand for.

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2 HISTORICAL CONTEXT

2.1 Suspension bridges

The beginning of the 19th century is a very fascinatingtime for engineering. Several iron builders and engi-neers experimented with different structures, amongwhich suspension bridges. Various construction sys-tems were in circulation originating from local habits.In America James Finley used chains to build up themain cables, in England they used flat rods or roundbars.The Frenchman Claude Navier illustrated the sys-tems in his widely spread Memoire. Due to the highmaterial costs compared to labour costs in France andSuisse, wire cables were less expensive than chaincables.As a consequence, the Frenchmen Marc Seguinstarted building suspension bridges using wire cables.Other engineers as H. Dufour, J. Chaley and L. Vicatwere inspired by the work of Seguin and Navier andadopted wire cables as well (Wagner 1989).

2.2 Engineering schools

Great Britain and the United States took the lead in theindustrialisation, instantly followed by France, some ofthe German States and Belgium. Although the largermodernisation of Russia will be realised only in the20th century, one cannot underestimate the local devel-opment in the beginning of the 19th century of theSaint Petersburg’s area. The relations of the tsar withFrance and the German state of Baden, where VonTraitteur had emigrated, have strongly contributed tothis development.

From the industrialisation on the one hand and thedemocratisation on the other, a need for new types ofconstructions (bridges, water ways, infrastructure. . .)and new types of buildings (factories, schools,stations. . .) was formulated. Those types were domi-nated by functional and technical requirements, whichthe traditional architect not always mastered.The poly-technic schools had to train a new group - the civil engi-neers – to fulfil this task with a controlled rationalityand a mature knowledge of the new materials and con-struction systems. The foundation in Paris of L’EcoleCentrale des Travaux Publics in 1794 (changed asl’Ecole Polytechnique in 1795) under auspices of theFrench Ministry of Defence, gave an adequate answerto the new construction requirements of the develop-ing modern society. In imitation of the Parisian EcolePolytechnique, similar schools were founded all overEurope. In 1806 in the German state of Baden, JohannGottfried Tullas laid the base for a local school ofengineering. The same was done by the FrenchmenAugustin de Bétancourt in Saint Petersburg (1809).This bilingual Russian/French polytechnic schoolembedded the French culture of engineering in Rus-sia (Federov 2000). It was in that context that the

young Von Traitteur travelled in 1814 with his Ger-man/French experiences, from Baden to Saint Peters-burg to cooperate in an intense way with de Bétancourt.At the end of the same year, the Belgian Jean-BaptisteVifquain obtained his engineering title at the FrenchEcole Polytechnique, in spite of his active participationin the military defence of Paris (de Bouw 2003).

2.3 Architectural detailing

Analysing their work reveals that Von Traitteur andVifquain do not focus exclusively on the design ofbridges and infrastructure, but at the same time onthe design of buildings. Their oeuvre is certainly notwithout architectonic ambition. With the designs ofan expedition building (1815–1818), the barracks atPella (1819), the transformation of a military school(1820), the school of building conductors (1820–26)etc., it is clear that the work of Von Traitteur mustbe written in a context of architectural classicism andearly historicism and eclecticism (Schoonjans 2007).In the same way, the work ofVifquain includes not onlybridges and infrastructure, but also several buildingsand urban plans as the Barricadesquare, the hospitalof the city of Tienen and the extension of the castleat Bazel-Kruibeke where he also designed the ironsuspension bridge in 1824. This broad practice ofthe engineer was not so exceptional at that time. Thewell-known J.J. Durand developed in 1805, with hisPrécis des leçons d’Architecture données à L’écolePolytechnique, in Paris a method to enable the engi-neer to design an architecture that was usable in allcontexts but still remained in contact with the ideaof a building as cultural artefact (Szambien 1984).It was the choice of the engineer to address ratio-nal constructions or to aspire within his oeuvre anarchitectural commitment. This was certainly the casewith Von Traitteur. His bridges were seldom consid-ered as purely rational constructions but had to beappropriate in larger urban ensembles for develop-ing the new cosmopolitan city. This is particularlyvisible in his design for the Panteleimon Bridge, theEgypt Bridge or the Lion Bridge over the Katharinacanal. Von Traitteur himself refers in his publicationsto the importance of using the correct proportionsand ornaments when sketching a bridge. The needto open a historical pallet and to consider forms ofthe past as a possibility to introduce elegance intoobjects of a pure utilitarian nature was present in thework of Von Traitteur. His bridges are a perfect matchbetween the technical and stability requirements andthe architectural demands within a typical 19th centuryembellishment of the city. But even the more straight-forward Post Office Bridge or Vifquain’s bridge atthe castle at Wissekerke, contain a particular architec-tural detailing. The portals and railings have a refinedornamentation beyond pure technicality. Whereas thePost Office Bridge is formulated within a classicistic

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ensemble, Vifquain (like at the Panteleimon bridge byVon Traitteur) uses a neo-Gothic language. This way,the design is clearly anchored in the culture of thesimultaneously developed English landscape gardenthat the Viscount Vilain XIIII commissioned aroundhis castle in Wissekerke. Such architectural ambitionis not found to the same extent at the Kettensteg ofKüppler or at the experimental bridge at Vernosc-lesAnnonay.

It is not clear whetherVonTraitteur was aware of thelatest work of James Finley, Samuel Brown, ThomasTelford, Marc Seguin, etc. (Peters 1987). However,it is without fail that the newly developed printingtechniques made it possible to produce rather cheapjournals and model books with very high-quality illus-trations. Within the network of firm contacts, thesepublications – and with them the knowledge andinsights – were rapidly spread throughout the west-ern world from Paris, over Brussels and Nuremberg toSaint Petersburg.

The period 1823–30 seems to be a pivoting momentwhere the innovative aspects of the iron suspensionbridges were elaborated.

2.4 Design issues

The questions that intrigued the engineers at the timewere aspects of the geometry, the calculation of thehorizontal and static forces and the creation of asufficiently rigid deck. In the United Kingdom, onedeveloped chains and eye-bars. French and Americanengineers experimented with (stronger) wire cables.The analysed footbridges in this paper clearly showthat the engineers Küppler, Vifquain and Von Trait-teur were informed on the French systems by meansof the Parisian Ecole Polytechnique as well as on theEnglish insights. It was rather common in Belgium thatengineers, like Vifquain, finalized their study with afieldtrip to Great Britain.

The design and the production of these iron suspen-sion bridges were however not generic.The outline wasnot only based on the more obvious aspects of the spe-cific static and dynamic forces, but also on the specificcondition of the local quality of iron and the possi-ble dilation, the urban or rural setting of the bridge,etc. The realisation of bridges was considered by VonTraitteur and Seguin, even more than by Vifquain orKüppler, as a laboratory of modern engineering andfor a modern society.

3 IRON WIRE SUSPENSION BRIDGE,M. SEGUIN, 1822

3.1 Historical value

After an experiment in 1821 Marc Seguin and hisbrothers built a small cable suspension bridge on

Figure 1. Iron wire suspension bridge built by Seguin in1822 in Vernosc-les Annonay, France (Photo: P. Stott, 2003,Structurae).

private land to span the river Cance in Vernosc-lesAnnonay. The private bridge was only 0.5 m wide,spanning 18 m. The cable consisted of iron wires. In1823–24 Seguin built a second experimental bridgeover the Galloire in St. Vallier with a span of 30 m.To stiffen the bridge he designed the railing as a truss.This intuitive solution appears to be valid and mustbe preferred to the theoretical solution of Navier, whoclaimed that the weight of the deck had to be increasedto stiffen the deck (Wagner 1989).

Together with Marc Dufour and Marc-AugustPictet, Marc Seguin used this experience to build theSaint-Antoine iron wire suspension bridge in Genève,with a span of two times 41 m. From then on, Seguinbuilt suspension bridges at a great pace. The Pontde Tournon in 1825 (ruined by flood in 1857), ThePont D’Andance in 1827 (partially destroyed in 1944,rebuilt in 1946), Passerelle d’Arcole in Paris in 1828(replaced in 1854), Pont de Sablons in 1828 (replacedin 1933), Pont du Doux (1828, ruined by flood in1857), etc. By 1841 more than 75 suspension bridgeswere built by Seguin.

At the time Navier wrote his Mémoire no one couldhave predicted the innovative role Seguin would play inbuilding wire suspension bridges. Nowadays, Seguinis considered as one of the few engineers who mas-tered the complete design and building process ofsuspension bridges.

3.2 Future

The bridge inVernosc-lesAnnonay was listed as histor-ical monument in 1981. Although this fact reflects thatFrance is aware of the historical value of the bridge,the local authority, who owns the bridge, is not in ahurry to take action. Figures 1 and 2 reveal the neglectof the historical monument and raise the question ifthis structure can be renovated or if it has to be labeledas lost already.

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Figure 2. Detail of iron wire suspension bridge in ruin inVernosc-les Annonay (Photo: P. Stott, 2003, Structurae).

Figure 3. Iron chain suspension castle bridge built byJ.B. Vifquain in 1824, Bazel-Kruibeke, Belgium (2003).

4 IRON CHAIN SUSPENSION BRIDGE, J.B.VIFQUAIN, 1824

4.1 Historical value

In 1824 viscount Philippe Vilain XIIII commissionedto alter the castle Wissekerke at Bazel-Kruibeke, toturn the park into an English garden and to build a pri-vate bridge over the pond. The viscount approachedthe Brussels engineer J.-B. Vifquain, who realizedimportant architectural projects and was involved inthe design of the canals Pommereuil-Antoing andCharleroi-Brussels (de Bouw & Wouters 2005). AsVifquain spent numerous time analyzing the work ofEnglish engineers it is not surprising that the bridge isdetailed according to the British examples.

He detailed the wrought iron cable as pinned eye-rods (Fig 4). The main cable spans from column tocolumn (distance 23 m) and consists of 23 chain links.The tension force in the main cable is guided downtrough compression forces in the cast iron columns and

Figure 4. Detail of the chains, hangers and railing. Ironsuspension bridge Bazel-Kruibeke (2006).

tension forces in the ties that go over in the abutments.The sag of the cable is low: 1 over 19. The woodendeck is 2 m wide.

The eye-rods and the hangers are connectedvia bolts. Although the wrought iron eye-rods(14 × 31 mm), the ties (15 × 36 mm) and the hangers(13 × 13 mm) can take up considerable forces, it arethe bolted connections that limit the permissible liveload to only 0.1 kN/m2.

To stiffen the suspension bridge and reduce themovements while crossing it, engineer J.-B. Vifquaindetailed the railing in a stabilizing way, forming stiffdiagonals. This solution is not only very efficient froma structural way of view, moreover it leads to a verylight and elegant bridge of extreme beauty. (Fig. 4)

4.2 Alterations

During the years, the wooden bridge deck and the con-nection between the bridge deck and the wrought ironhanger have been adapted. New U-shaped steel beamswere connected to the hangers to carry the woodenbridge deck.All other elements (cable, railing, hangersand columns) are still original.

The bridge was listed as a monument in 1981. From1998 onwards the viscount sold parts of his prop-erty to the local authority and private investors. Thelocal council of Bazel-Kruibeke bought the park sur-rounding the castle and opened it for public. Shortlyafter, in 1990, the bridge was closed because of its badcondition and unsatisfactory load-bearing capacity.

4.3 Future

In 2007 the mayor asked to strengthen the bridge suchthat it could serve as a public bridge. The imposedlive loads on public bridges are 50 times higher thanthe actual capacity of the former private castle bridge.

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Strengthening the suspension bridge would harm themonument in an irreversible way. As replacing origi-nal material by stronger alternatives was not an optionfor the council of Monuments and Sites two propos-als were presented: keeping the bridge as an historic‘object’, not making it accessible for people, or build-ing a present-day bridge within the existing one. Thelatter solution will be implemented: the inauthenticdeck will be replaced by a shallow box girder that lieswithin the height of the deck. This girder provides therequired bearing capacity for the deck and is connectedat its sides by a sliding connection with the historic sus-pension structure that will be restored. An additionalhandrail is designed to meet the stability and safetyrequirements. The handrail structure is fixed on thebox girder and consists of a steel frame filled witha fine stainless steel net. Its vertical members fol-low the rhythm of the vertical hangers of the initialbridge.

Looking at the final solution in terms of setting pri-orities, one can say that safety went first. It was thechallenge of the designing team to balance minimalvisual impact with total preservation of the historicparts. Furthermore, it’s remarkable that ‘keeping orig-inal material’ was set a higher priority than ‘theview/visual impact of the intervention’.

5 IRON CHAIN SUSPENSION BRIDGE,W. VON TRAITTEUR, 1824

5.1 Historical value

Colonel Wilhelm Von Traitteur, working for the engi-neer corps in Saint Petersburg, was charged in 1823with the construction of a traffic (Panteleimon Bridge)and a pedestrian bridge (Post Office Bridge) over theMojka. He convinced the duke Alexander of Würtem-berg that a suspension bridge was the best typology tomeet the need by pointing to the Report and Mémoireof Navier. But before using the theory and best prac-tices that were listed by Navier, Von Traitteur askedto start a research program to determine the strengthof the iron produced in Russia and to investigate howthe mechanical properties of iron changed when thetemperature dropped.

In 1824 the research project was completed andthe foundation was laid for the use of Russian ironin engineering constructions. Subsequently, the struc-tural elements of the suspension bridge could bedimensioned and the production of the constructionelements started. After proof testing the chains andthe assembled cable on a – for this purpose designed –apparatus, the structural elements of the Post OfficeBridge were assembled on site.

Each of the two main cables was built up by twochain cables, which consisted of nineteen identical

Figure 5. Iron chain suspension bridge built byW.VonTrait-teur in 1824, Saint Petersburg, Russia (Photo: O. Blinkova,2007).

Figure 6. Iron chain suspension bridge Saint Petersburg(Photo: O. Blinkova, 2007).

1.75 m long bars with a diameter of 35 mm. (Fig. 5)On every side of the bridge, 18 hangers, with a diam-eter of 16 mm, are attached to the cable, in a similarway as Samuel Brown did for his Union Bridge in1820. Because of the limited space available on thefootpath, Von Traitteur came up with a compact solu-tion: he added a quarter of a circle to the columnto lead the tension forces vertically into the founda-tions. In August 1824, the bridge was opened for thepublic.

The Post Office Bridge was the first iron suspen-sion bridge of Russia. Other suspension bridges wouldfollow at a great pace. The Panteleimon suspensionbridge, which is known for his architectural refinementand detailing, was opened that same year. In 1826, theEgypt Bridge over the Fontanka was opened for pub-lic, as well as two pedestrian bridges over the canalKatharina: the Lion Bridge and the Bank Bridge.

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The striking characteristics of the bridge designs ofWilhelm von Traitteur are the integration of the bridgeas an architectural object in the city landscape, thetransfer of the tension forces in the main cable to theabutments, and the possibilities he creates to adjustthe geometry of the bridge by detailing the connectioncable-tie and tie-deck.

5.2 Alterations

Almost immediately after the opening of the bridgeadditional actions were undertaken to stiffen the deckand to avoid the swinging movements. Wooden beamswere placed diagonally in the deck in order to increasethe rigidity.

At the same time the top of the cast iron columnsstarted to lean towards the canal. Wilhelm Von Trait-teur was optimistic and said the phenomenon wouldstabilize. To demonstrate the strength, he stated thathe already observed an assembled crowd of about 600to 700 people on the bridge.The weight of such a crowd(almost 6 persons/m2!) leads to a live load of 4 kN/m2.This number contrasts sharply with the load-bearingcapacityVonTraitteur communicates to C.F.v.Wiebek-ing, who was working on the publication ‘Mémoire surles ponts suspendus en chaines de fer’ published in1832:

Technical data of the Post Office Bridge:

– self- weight 0.7 kN/m2

– live load 1.5 kN/m2

– total load 2.2 kN/m2

In 1905, after the collapse of the Egyptian ironsuspension bridge, which was built in 1826, the confi-dence in the historic iron suspension bridges was lost.The Panteleimon Bridge was, under public protest, dis-mantled for reasons of security. The smaller pedestrianPost Office Bridge was strutted.

In 1935, the Post Office Bridge, as well as the LionBridge were recognized as historical relics by the cityof Saint Petersburg. This action could however notsafeguard the structure.

There were plans to restore the bridge to its orig-inal state while upgrading the load-bearing capacityto the current standards. Wrought iron was replacedby high tensile steel (limit of stretching strain 1200N/mm2). During the successive interventions in 1956and 1981–83, the cast iron columns, the wrought ironchains, ties and deck elements were replaced by steelspecimen.The used renovation strategy was to upgradethe structure to the actual safety standards and main-tain the appearance of the bridge by replacing theapplied materials by stronger materials. Although thiswas common practice in the 1980’s, nowadays thisstrategy is merely applied for renovations instead ofrestorations.

6 IRON CHAIN SUSPENSION BRIDGE,K. G. KUPPLER, 1824

6.1 Historical value

In 1824, the mechanic and later professor KonradGeorg Küppler, built an iron suspension bridge to spanthe river Pegnitz in Nuremberg, Germany. Küppler wasalso involved in the development of the first Germanrailroad between Nuremberg and Fürth in 1835.

The iron bridge consists of two suspension bridgesplaced successively with a span of approximately 33 mand 34 m. The cable starts in an elevated abutment,runs trough three portals and disappears again in theabutment on the opposite bank.

The main cable consists of iron rods (30 mm diam-eter) ending in a T, connected to the next bar by meansof two rings (Fig. 8). The hanger, also a bar (diameter20 mm), passes trough the chain rings, and is fixed byusing an iron plate. The other end of the hanger wrapsaround the timber beam that carries the bridge deck.Since no bolts are used in the chain connections, thedemountable details of the bridge look like the black-smith bridges that were built in Scotland from 1816onwards. The iron railing works structurally indepen-dent from the suspension chain and reduces the bridgedeck to 1.55 m.

Two cables run on each side of the deck. (Fig. 8)Since the 20 hangers are connected alternating to oneof both cables, the curvature of the two cables staggers.(Petri & Kreutz 2004).

6.2 Alterations

The columns that supported the main cable were orig-inally in oak. After the floods of 1909 the bridge wasserviced. The oak columns were replaced by steelframe columns. The timber elements of the bridgedeck were renewed, taking into account a live loadof 2 kN/m2.

Since the bridge was very lively, the connectionshad to be checked regularly. In 1927, the Nurembergpolice prohibited people to make the bridge swingwhen crossing it.

In 1931, while discussing the upgrading of thebridge to a live load of 5 kN/m2, the bridge was tem-porarily reinforced and stabilized. The detailing ofthe wooden bridge deck with extended timber crossbeams, lends itself to be carried by a steel I-section.

The I-sections, on their turn are supported bywooden posts (Petri & Kreutz 2004).

6.3 Future

Thanks to the supporting structure, the bridge couldbe preserved up till now. Nevertheless, the cables andhangers are reduced to ornaments. The university of

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Figure 7. Iron chain suspension bridge built by K.G. Kup-pler in 1824, Nuremberg, Germany (Photo: N. Janberg, 2001,Structurae).

Figure 8. Detail of the iron chains. Suspension bridgeNuremberg (Photo: E. Bernstorff, Bridgemeister).

Nuremberg and the local heritage foundation exertpressure to restore the suspension structure.

7 CONCLUSIONS

From the early period of iron suspension bridges (uptill 1824) there are still four bridges left on the vast

continent. In spite of the rare character of these bridgesthey are treated with lack of respect. The wire suspen-sion bridge in France is degenerating, the restoration ofthe iron chain bridge in Nuremberg is placed ‘on hold’.However, the best way to ensure the long-term survivalof historic buildings and structures is to guaranteetheir continuing usefulness. For early 19th centurypedestrian bridges, this comes to defining a restorationstrategy. Not only can we detect different problems interms of stability because of the aging of the bridge,at the same time the bridge should be upgraded tothe latest standards concerning comfort and safety. Asa consequence, taking structural measures becomesa necessity. In architecture it is possible to adapt thebuilding’s structure in an invisible when the structureis hidden by cladding. However, in structural workslike suspension bridges, this is far more difficult, sincethere is an interaction between the construction andthe structural behaviour. Moreover, there is a directvisual impact, since the construction itself is the form.Do we want to keep this form and create a replica insteel, like in the bridge at Saint Petersburg,? In thatcase the argument in favour lies in the aim to retainthe urban ensemble and its purely visual aspects. Butthen the discussion on the detailing and the imitation ofthat detailing starts. In the case of Bazel-Kruibeke, notthe form, but the preservation of the historic artefactitself was put forward. An almost invisible present-daybridge is built within the existing one.

Between these extreme options (replica versus totalconservation historic fabric) an interval of interestingconservation strategies is ready to be explored. Thesestrategies can initiate a more explicit dialogue betweenthe old bridge and a new solution. Arguments of sta-bility, public safety, authenticity of the artefact, thebridge as a landmark, and other will guide the chosenstrategy. It is the task of the design team to equilibratestrengthening versus conservation.

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