Manfred and I have some common interests,
among which the history of concrete structures
and for this occasion I have chosen a topic in this
field. On 18 February 2016, Manfred received the
Sarton Medal at Ghent University as a recognition
of his unique and pioneering research work on the
historie aspects of concrete structures and the link
with modern strengthening and renovation techniques.
At that occasion he presented a lecture on
"What European History, Legendary Bridges and
the Design of the Euro Have in Common", dealing
with the different bridges appearing on the Euro
bank notes. This paper deals with a challenging
bridge type, which is not very well known: self-anchored
suspension bridges with prestressed concrete
deck. Some of these bridges were built in the
1950's over a canal around the city of Ghent after
a design by Prof. Daniël Vandepitte (1922-2016).
This paper is also a modest personai tribute to Prof.
Vandepitte, who recently passed away at the age
of 94 years and who was a brilliant teacher in structural
analysis. He was a successor of Prof. Gustave
Magnel (1889-1955) in the field of structural analysis
and he designed several remarkable bridges in
the early 1950's before he was appointed at Ghent
University

Taerwe, Luc

English

Ghent University Academic Bibliography

Prof. dr. ir. Habil. Luc fl. TaerweHead Department of Structu- ra! Engineering, Grient Univer- sity Grient, Belgium Director Magnel Laboratory for Concrete ResearchLuc TaerweSelf-anchored suspension bridgeswith prestressed concrete deck: historie examplesIntroductionManfred and I have some common interests, among which the history of concrete structures and for this occasion I have chosen a topic in this field. On 18 February 2016, Manfred received the Sarton Medal at Ghent University as a recognition of his unique and pioneering research work on the historie aspects of concrete structures and the link with modern strengthening and renovation tech- niques. At that occasion he presented a lecture on "What European History, Legendary Bridges and the Design of the Euro Have in Common", dealing with the different bridges appearing on the Euro bank notes. This paper deals with a challenging bridge type, which is not very well known: self-an­chored suspension bridges with prestressed con­crete deck. Some of these bridges were built in the 1950's over a canal around the city of Ghent aftera design by Prof. Daniël Vandepitte (1922-2016). This paper is also a modest personai tribute to Prof. Vandepitte, who recently passed away at the age of 94 years and who was a brilliant teacher in struc- tural analysis. He was a successor of Prof. Gustave Magnel (1889-1955) in the field of structural analy­sis and he designed several remarkable bridges in the early 1950's before he was appointed at Ghent University.General conceptand survey of existing bridgesThe principle of self-anchoring eliminates massive anchorage structures, which have to withstand large horizontal forces, and which are necessary for classical suspension bridges. Instead, the cables are secured to each end of the bridge deck, which resists the horizontal component of the cable ten-280Fig. 7Self-anchored prestres- sed suspension bridge with a central span of lOOm at Merelbeke near Ghent(Photo: collection Department of Structural Engineering)sion. Therefore, the end supports resist only the vertical component of the cable tension, an advan- tage where the site cannot easily accommodate external anchorages [1].Because the stiffening girders support the cable tension. these girders must be placed before the main cable can be erected. This construction se- quence, which is opposite of that of a conventional suspension bridge, limits the self-anchored form to moderate spans and suitable site conditions [1].Vandepitte [21, [3] points out that when the concept of self-anchoring is applied to a steel bridge, a considerable amount of additional steel is required in the superstructure as compared to that of a true suspension bridge in order to ena- ble the stiffening girders or trusses to resist thethrust as well as the bending moments without being endangered by instability. The large thrust produced in the suspended bridge deck is, on the contrary, highly beneficial in the case of a concrete deck, for it acts as a prestressing force in the stiffening beams and helps them to with- stand the bending due to live load. In the con­crete case, instability is normally not a problem of any consequence, owing to thé cross-section being naturally more sturdy than that of a steel suspended structure, For the same reason, a prestressed concrete suspension bridge is much stiffer than its steel counterpart and aerodynamic instability is also much more unlikely. However, most of the self-anchored suspension bridges have a steel deck, as the advantage of the ab­sence of massive anchorage blocks apparently predominates the mentioned disadvantages.281Fig.2Cross-section of the Although the concept of was mentioned for thebridge deck first time by Langer in 1859 and independently by(taken from 121) Bender in 1867, it became common in Germany in the beginning of the 20th century only [1], The first large scaie self-anchored suspension bridge, built over the Rhine river in Köln-Deutz with a central span of 185 m, was finished in 1915. The most no- table of the German self-anchored bridges was the Mülheimer Brücke in Cologne (1929) with a cen­tral span of 315 m, which was destroyed in 1945. In the United States, three nearly identical bridg­es were constructed over the Allegheny River in Pittsburgh from 1925 to 1928. In 1955, the bridge in Duisburg with a span of 230 m was completed. Completed in 1990, with a main span of 300 m, the Konohana bridge in Osaka is the first large-scale, self-anchored suspension bridge built for vehicular traffic since 1955 and points to a renewed interest in this bridge type. In addition to its self-anchor- ing, this bridge is the first large-scale mönocable suspension bridge, with the main cable and in-clined hangers aligned in a single vertical plane in the centre of the road way. Very similar is the Yong Jong bridge near Seoul (2001), having the same main span length and A-shaped towers. It is the first combined road and rail bridge of its type and has a sag to span ratio of 1:5 compared to 1:6 for the Konohana bridge [4], These ratios are consider- ably greater than those of externally anchored sus­pension bridges, which typically are around 1:10. These more recent bridges show that for spans on the order of 250-400 m, three span-self-anchored suspension bridges can offer a competitive design solution, white maintaining a traditional suspension appearance [11]. The San Francisco Oakland Bay Bridge, opened for traffic in 2013, is the largest sin­gle tower self-anchored suspension bridge in the world, with a main span of 385 m.Fig. 3 Front view of a tower and cross-section of bridge deck (taken from [21)282All the previous examples are bridges with a steel deck. How- ever, the self-anchored suspen­sion bridge can also be obtained from a conventionally post-ten- sioned concrete bridge deck where, instead of keeping the tendons inside the concrete sec- tion, the tendons leave the gird- ers [2], [3], This allows to obtain significantly larger eccentricities which leads to a more economi- cal solution in case of significant dead weight. The hangers pro- vide the connection between the suspension cables and the bridge deck and transmit the upward forces created by the curved ca­bles to the bridge deck.The first self-anchored suspen sion bridge with a concrete deck was built in 1950 at Saint-Germain-au-Mont-d'Or (France), with a main span of 57.9 m and side spans of 21.8 m, very similarto the bridge W13 which is discussed in the next section. As far as we know, Vandepitte was not aware of the existence of this bridge.F.ig. 4Latera! view on one of the towers and V-sha- ped hearing walls (Photo: Luc Taerwe)Fig. 5Freyssinet hinge at lower part of V-shaped. hearing walt (Photo: Luc Taerwe)Original projects in BelgiumVandepitte designed three self-anchored pre- stressed suspension bridges with a concrete deck of various spans over the ring canal around the city of Ghent between 1954 and 1964. This section mainly deals with one of these bridges.Jörg Schlaich and his partners designed sever-al remarkable self-anchored pedestrian bridges The bridge in Merelbeke (designed as W12), throughout Germany, [5], shown in fig. 1, was finished in 1964 and has a283Fig. 8 Scaffolding of the bridge deck and the cables(Photo: collection Daniël Vandepitte)Fig. 7 Cable geometry (taken from [8])central span of 100 m, a total length of 192 m and the suspended structure is 21.6 m wide [2], [3), [8]. A cross-section of the bridge deck is shown in fig. 2.Each of the main cables consists of 910 paral­lel galvanized steel wires 7 mm in diameter. The sag of the cables in the central span equals 9 m which corresponds to a sag to span ratio of 1/11.1, which is smaller than the ratios mentioned befo- re for the steel bridges. The two stiffening girders are continuous box girders with a constant depth of 1.93 m which corresponds to 1/52 of the cen­tral span length. These girders are prestressed by the action exerted by the suspension cables only. There are no prestressing tendons in the suspen­ded structure itself, which is independent of the towers. The tensioning of the cables and conse- quently the prestressing of the superstructure was achieved by jacking up both towers with respect to the piers, which was a quite audacious and specta- cular operation.284The two cables are supported above each pier by a tower consisting of two legs, a flat arch connec- ting their tops, and two coupling beams connec- ting them underneath the roadway (fig. 3). On top of each leg, a cast iron saddle is positioned. The towers are wholly independent from the roadway structure and from the V-shaped hearings connec- ting the deck with the pier (fig. 4). These V-sha­ped hearings consist of concrete walls which have Freyssinet hinges at both ends (fig. 5). They are located in between the two coupling beams with sufficiënt spacing.Fig. 9First self-anchored suspension bridge <W13)(Photo: Luc Taerwe)The plane of the hangers and the cables almost coincides with the plane of the outer webs of the box girders. The distance between the hangers equals 5 m. At each of these locations, a transverse beam is positioned below the bridge deck (fig. 2). These transverse beams are partially prestressed, which was not a common technique at that time.Each tower was cast 0.67 m below its final de­sign position, before the main cables were built up, wire by wire, without any tension and were connected to the concrete structure' at their ends by means of the cable bands and of the hangers. Prestressing of the superstructure was achieved by jacking up both towers (not the roadway structure) with respect to the piers. Hydraulic jacks placed under the tower legs were used for that purpose (fig. 6). The jacking forced the cables to elongate and hence tensioned them, and it simultaneouslyFig. 10Deviation of the conti- nuous cable at one of the ends of the bridge deck: lateral and plan view (bridge W13)(taken from 171)285Fig. 11Bridge W16 at Mariaker- ke (Photo: Luc Taerwe)produced a total prestressing force of 43.9 MN in the longitudinal girders, for which a lifting force per tower leg of 17.5 MN was needed.In fig. 6, the positioning of the final supporting block is also shown. The top surface of this block is slightlyrounded and serves as the lower part of the Freyssinet hinge located at the bottom of the tower leg (slightly visible in fig. 4). The mortar layer between the top part of the concrete block and the bottom part of the leg measures 135 cm by 38 cm. The locally wider part at the bottom of the tower legs, which was necessary286to position the jacks, was removed after the jacking operation.At the abutments, the horizontal component of the cable force is transmitted to the longitudinal girders as prestressing force, but its vertical com­ponent also needs to be resisted. This is achieved by fixing a concrete box filled with sand, below the transverse end beams.The sags f and f, of the parabolic cables in the central span and lateral spans respectively were chosen such that f/L2 = f/l2 with L and I the corre- sponding span lengths (fig. 7). This means that the upward force per unit length exerted by the cable on the bridge deck is constant over the full length of the bridge. As this load was chosen to be initially 19 % higher than the dead weight of the bridge deck, upward reaction forces occur at the bridge piers under certain loading arrangements. Hence, the V-shaped bearings, mentioned before, were post-tensioned vertically to compensate the tensile force created by the negative support reaction.The effect of the increase in tendon force AP in a regular prestressed concrete beam due to the deflection generated by live load is generally ne- glected. However, in the case of a self-anchored suspension bridge, where the cable has a large eccentricity, this beneficial effect is not negligible. Denoting by f the cable sag, the additional moment generated by the cable force increase AP equals-f. AP which reduces the positive beam moment due to live load. Por the bridge W12 under consid- eration, the reduction of the bending moment at mid-span due to the full live load is 9.6 %. For other cable and bridge geometries, this reduction can be substantially higher.The concrete bridge deck was cast on scaffold- ing over its full length (fig. 8), which was obvious giving the particular situation that the canal to be bridged was not yet dug at the time of construc- tion. As this situation is not common, this bridge type has not been widely used. Moreover, in the 1960's cable stayed bridges came into use, which turned out to be more efficiënt in construction.Fig. 72Lower part of one of the piers of bridge W16 (Photo: Luc Taerwe)i287Fig. 13 Xiaolongwan Bridge in Nanjing (P.FI. China)(Photo provided by Zhao Liu)The bridge W12, discussed so far was the third one in a series of three. The first bridge of this type (designated as W13) that was built over the ring ca- nal in 1954-1955, had smaller spans: a central span of 56 m only and two lateral spans of 18 m. In the lat- eral spans no hangers are present and the cables are straight (fig. 9). This and the following bridge have in fact one continuous cable, which loops around the bridge deck at its ends (fig. 10). For this purpose, the cable is locally splayed out in three parts and deviated in the vertical plane by means of a con­crete deviation saddle. As the friction between the curved cable parts and the bridge deck was releasedshock wise during the tensioning operation, causing unexpected loud bangs, two separate cables were applied in the third bridge W12.The second bridge in the series (designated as W16), which was finished in 1958, is 'located in Mariakerke and has a central span of 100 m and lateral spans of 40 m (instead of 46 m for the bridge W12). In fig. 11 it can be noticed that the hangers are anchored in the ends of the transverse beams, which protrude from the bridge deck. This is not the case for bridge W12 (fig. 1) where the lateral view shows a continuous box girder, which is aes- thetically more pleasing. Fig. 12 shows the lower part of one of the bridge piers where the lower flange of the l-shaped stiffening girder can be no­ticed. Below the legs of the towers, steel hinges are provided and the steel rods which are visible besides the vertical wall supports have to resist the upward reaction force, while the walls resist the downward reaction force. As mentioned before, in bridge W12 the post-tensioned wall supports can resist both positive and negative reaction forces.Applications in ChinaAccording to personal contacts, many self-an- chored suspension bridges with prestressed con­crete deck have been built in China. The first one is Jinwan Bridge in Dalian (2002), with a total length of 198 m and the length of the main bridge being 24 + 60 + 24 m.288Recently, the attention of the author was drawn [3] tothe Xiaolongwan Bridge (2013) in Nanjing, shown in fig. 13, which is of the same type and has with 44 + 96 + 44 m similar span lengths as the bridge W12 in Merelbeke. However, in this case the cables were stressed by tensioning the hangers, as was performed at San Francisco's Oakland Bay Bridge, in combination with a stepwise pushing up of the saddles [5],References[1] Ochsendorf, J.; Billington, D.: Self-anchored suspension bridges. ASCE Journal of Bridge Engineering 4 (1999) 3, 151-156[2] Vandepitte, D.: Self-anchored prestressed concrete suspension bridges with parabolic cables. In: Laboratório Nacional de Engen- haria. Proc. of Symposium on suspension bridges, Nov. 1966 in Lisbon (Portugal), paper no. 38, 8 p.Vandepitte, D.: Le pont suspendu a poutres de rigidité en béton précontraintes par les cables porteurs. Annales des Travaux Publiés de Belgique (1955) 5, 3-28[4] Gil, H.; Cho, C.: Yong Jong Grand Suspension Bridge in Korea. Structural Engineering Inter­national (1998) 2, 97-98[5] Holgate, A.: The Art of Structural Engineering - The Work of Jörg Schlaich and His Team. Edition Axel Menges, 1997[6] Zhuo, W.; Liu, Z.: Key techniques of main cable installation of Nanjing Xiaolongwan self-anchored suspension bridge. Structural Engineers 30 (2014) 5[7] Vandepitte, D.: Hangbruggen van voorge­spannen beton. De Ingenieur (1959) 13, 29-39[8] Vandepitte, D.: De hangbrug van voorge­spannen beton W12. Cement XVII (1965) 7, 436-441289

Self-anchored suspension bridges with prestressed concrete deck : historic examples

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Self-anchored suspension bridges with prestressed concrete deck : historic examples

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Ghent University Academic Bibliography