The second crossing of the Strelasund

Together with the construction of the new Baltic autobahn A20 Lübeck – Stettin the Rügen feeder “B96n Stralsund/Rügen”, ap-prox 55 km long, between the A 20 in the region of Grimmen and the town of Bergen on Rügen has been realised. With the construc-tion of the new “B 96 n Stralsund/Rügen” Rügen feeder an effec-tive transport link between the largest German island, Rügen, and the German and European long-distance road network has been implemented. An essential component of the whole building pro-ject has been the second Strelasund crossing between the town of Stralsund and the island of Rügen. The main structure of the 2.8 km long bridge design is configured as a stayed cable bridge.

IntroductionThe connection of the island of Rügen to the German long-dis-tance road network improves considerably accessibility of the island for holiday traffic and transit traffic, running by ferry from Sassnitz to Scandinavia and Eastern Europe. A town bypass de-congests now trough-traffic in the centre of the city of Strelasund on the existing B96.Essential part of the Rügen feeder is the second Strelasund cross-ing running parallel to the existing Rügen causeway. The project followed the new construction of the town bypass of the city of Strelasund and runs over the Ziegelgraben to the island of Dän-holm and the sound of Strela to Rügen island and finishes at the

traffic hub Altefähr. The road comprises, in addition to the 2.8 km long bridge, an almost 1.3 km long route with embankments and cuttings. The old causeway with an integrated bascule bridge for road and rail traffic has been retained.

Design and constructionBridgeThe bridge with a total span width of 2,831 m consists of 6 individ-ual structures between both abutments; these units are connect-ed to each other on the separating piles by carriageway joints:

- BW 1.1: prestressed concrete T-beam over 10 spans with a total length of 327.5 m

- BW 1.2: composite box section over 6 spans with a total length of 317.0 m

- BW 2: cable-stayed bridge with a total length of 583.3 m- BW 3 to 5: prestressed concrete box sections over 10 spans with

a total length of 532.3 m, 532.2 m und 539.0 m

The bridge cross section is in total 15.0 m wide between the el-evated cornice head and is subdivided in an 11.5 m wide carriage-way and two cornice areas of 1.75 m width each. The large safety spaces of 1.0 m with safety barriers allow emergency path of 0.75 m width on each side of the carriageway.

above: Overall viewright: Seperating pile in axis 170

Structure 1.1

10 110 170 230 330 430 530

327.50 m 317.00 m 583.30 m 532.30 m 532.20 m 539.00 m

Structure 1.2 Structure 2 Structure 3 Structure 4 Structure 5

Structural analysis, construction and - mounting of the composite and cable-stayed bridge

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Structure 1.1 The structure 1.1 is the direct continuation of the bypass of Stre-lasund and is designed as prestressed concrete bridge with indi-vidual span widths of 29.0 + 30.5 + 7x33.5 + 32.5 m. The layout in the ground-plan changes from a clothoid to a radius and finishes in an asymptotic curve. The continuous 2-web T-beam cross sec-tion has a construction height of 1.80 m. Cast in-situ cross girders are only implemented at the superstructure ends.

In the bearing concept transverse fixations are planned on the abutments and the separating piles as well as on every second inner pile. In longitudinal direction the superstructure is fixed to a pile at the bridge centre. In addition to the concrete box abut-ment, the substructures are composed of two individual columns at each bearing axis, founded by a joint pile cap on driven piles.The cross section of the columns is drop-shaped, the tip point-ing to the outside. This design element is realised over the whole bridge ensemble. The bridge superstructure, longitudinally in-clined by 4 degrees from the abutment up to around 20 m above the ground, has been constructed span-wise with short cantilever

arms in 10 concreting sections on a scaffolding, whose interme-diate yokes are founded on driven piles like the substructures.

Structure 1.2 Structure 1.2 is a continuous girder with steel composite box sec-tion of individual span widths of 48.0 + 49.0 + 72.0 + 2x49.0 + 48.0 m. The bended layout in the ground-plan starts at the asymptotic curve, changes to a radius of R = 350 and then to a clothoid. The composite cross section, constantly 2.50 m high in the bridge axis, has a maximum carriageway inclination of 5.5 degrees in the arc. The 7.0 m wide bottom plate of the box section is horizontal in the cross section over the whole bridge length, resulting in different heights of the inclined webs according to the transverse inclina-tion To distribute torsion moments and to keep the cross section’s form, transverse frames are arranged at a distance of around 4.3 m. Every third transverse frame is supplementarily stiffened. Because of the flat character of the bridge’s cross section, instead of habitually planned diagonals, the lateral vertical web plates are widened disc-like from the chord connections downwards to

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1 m in front of the cross section axes. Additionally to the main opening, the superstructure is connected bending stiff to the re-inforced concrete pile by inclined steel bars arranged in V-shape. In the cross section, two individual bars are assembled, joint left and right to the bottom plate of the superstructure and connected to a shared pile.

The drop-shaped cross sections of the individual bars narrow from the bottom of the pile with b/dmax = 2.20/1.60 m to the super-structure to w/t = 1.20/1.00 m. These geometrically complicated components are to be implemented with plate thicknesses of 60 mm according to the structural analyses. The bar sections are produced from only one plate, welded with a longitudinal seam at the drop tip to form a box section. To distribute the moment at the framing corner, the inclined bar cross sections are contin-ued within the superstructure’s box section vertically up to the widened upper chords. Because of the penetration of these cross sections with the cross parts, necessary for distribution of trans-verse force and torsion moments, the drop bending within the

superstructure’s box section is shown with only small deviations like a polygon chain. The bending stiff connection of the steel bars to the concrete piles is ensured by linking the reinforcement to the steel bars at a depth of around 2 m. Force distribution from the steel to the concrete cross section is realised by shear studs. To absorb changes of force directions, the drop-shaped cross section is stiffened by a bulkhead plate at the connection’s area. In the other axes, the superstructure is, analogous to structure1.1, sup-ported on elastomer or deformation bearings on each time 2 indi-vidual supports, connected to a cross beam at the supports’ head. The supports cross sections are also drop-shaped. At each bearing axis, the structure is founded by a joint pile cap on large bored piles of diameter 1.50 m.

Structure 2Main load-bearing structureStructure 2 bridges the Ziegelgraben and is the predominant part of the bridge ensemble. Both main openings are spanned with stay cables. The individual span widths of the bridge running in a

1 View of structure 1.2 2 View of structure 2 3 Cross section and pier of structure 1.14 Cross section of structure 1.25 Cross section of the pylon

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170 180 190 200 210 220 230

55.00 72.00 126.00 198.00 72.00 60.30

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straight line are 54.0+72.0+126.0+198.0+72.0+59.3 m. By calotte bearings with a fixation on all sides on the pylon pile and by trans-versal fixation in axes 170, 190, 210 and 230, the superstructure with its pylon is sustained on individual supports with drop-shaped cross sections. Separating pile and pylon pile are walkable. The foundation is realised in the same way as for structure 1.2 on large bored piles with diameters of 1.50 m.

Bridge deckThe superstructure cross section is a three-cell steel box section with a constant construction height in the structure’s axis of 3.15 m. In the area of the lateral boxes the cross section narrows in direction of the external webs. The oblique arrangement of the external webs widens the cross section downwards.

Manholes in the interior webs at every fourth transverse frame make the complete box section accessible. Carriageway slab and footpaths are formed as orthotropic plates in consideration of recommendations of German Standard DIN-Fachbericht 103 for constructional formation of steel carriageway decks. Trapezoidal box stiffeners have been chosen as longitudinal rips just as for the webs and bottom plates. At a distance of 4.0 to 4.4 m, transverse frames are assembled assuring the bridge cross section’s form stability. Every second transverse frame is supplementarily stiff-ened between the main webs by diagonals made of round tubes. In the longitudinal axes, transverse bulkheads with manholes have been designed to assure distribution of transverse forces and torsion moments.

The minimum thickness of the carriageway slab is 14 mm in ac-cordance with DIN-Fachbericht. The webs and bottom plates are

determined at a minimum thickness of 12 mm in view of low weld-ing deformations. Larger plate thicknesses are only required in the areas of the supports and at the connecting point of the stay cables for static reasons. The trapezoidal box stiffeners at the car-riageway slab and the bottom plates are with 77 mm formed in the same way as those at the webs with 6 mm. The stiffness of the cable-stayed system is essentially a result of the cables pre-tension. The forces of the stay cables are absorbed in this present case by ballast concrete in the area of the cable anchoring of the shorter main span; tensile supports are thus avoided.

PylonThe pylon consists of two approximately 87 m high individual sup-ports connected rigidly to the bridge superstructure. To guarantee sufficient stability in transversal direction of the bridge, 3 cross beams connect both pylon stems to the framing system. The con-nection of the pylon stems to the supporting cross girders next to the carriageway necessitate at this point a widening of the bridge cross section. The emergency paths are led through 90 cm wide and 2.50 m high manholes in the pylon feet.

The drop-shaped cross sections of the pylon stems are constantly 3.03 m wide in the bridge elevation. In the cross section they nar-row from 4.01 m at the bridge deck to 3.54 m at the pylon top.

The pylon cross sections are stiffened with web plates in longi-tudinal and transversal direction of the bridge as well as by al-ready assembled platform plates at a distance of around 4 m. To make cable anchors and the pylon top accessible, the pylon is accessible by a vertical ladder next to cross section core. On the opposite inner side of the bridge is a transport shaft over the

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180 190 200 210 220 230

entire pylon height, which is completely free of stiffeners and slightly narrowed in the area of the connection points. Longitu-dinal bulkheads and bended external plates of the cross section core are implemented with plate thicknesses of up to 45 mm. The thicknesses of the adjoining plates are reduced by up to 12 mm according to structural requirements. Buckling stiffeners are not planned. The pylon is made of steel of quality S 355 J2G3 just as the bridge deck.

Stay cables and cable anchoringThe load-bearing structure is spanned at two levels in the axes of both pylon stems. In the bridge elevation, the individual ca-bles are arranged parallel to each other like a harp. The eleva-tion shows 8 cable anchors at a distance of 16.1 m in the main opening and of 12.9 m in the neighbouring spans, this results in a so-called multi-cable system which is distinguished by rela-tively small longitudinal bending moments in the stiffening gird-ers and the possibility of simple assembly by cantilever method.

With maximum service loads of the cables of 3,500 KN, the instal-lation of cable groups has not been necessary and the stresses during exchange of cables or in case of cable brake did not have to be taken into consideration in the calculations.

1 Static system of structure 22 Cross section of structure 2 3 Pylon

3 20.588

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A particularity of the bridge structure consists of the first-time use of strand bundles instead of closed spiral cables used commonly in Germany. System DYNA Grip of company SUSPA DSI with strands of 150 mm2 and quality St 1570/1770 is applied.The parallel strand bundles in the casing tubes consist of individual strands with 7 galvanised, cold-drawn smooth individual wires. The strands are each coated with corrosion protection as well as a tight PE cover. To obtain the necessary single case approval by the German Ministry of Transport, Building and Housing, a series of quality inspections of wires, strands, corrosion protection and the HDPE casing tubes had to be delivered. The Technische Universität München accomplished three combined fatigue/tensile test with σo = 0.45 σuts and Δσ = 200 MPa, which were all successful. Cable type C37, necessary for this structure, was tested with an anchor for at maximum 37 strands. Even if, from a structural point of view, 30 strands would have been sufficient for cables stressed at maximum, all cables of the bridge will be executed with 34 strands to comprise load-bearing reserves. This also meets requirements of the tender to create space in the anchor head for 3 supplementary strands in addition to the structurally required strands. Advantages of the use of strand cables are the simple assembly procedure of ‘strand-wise’ installation, the elimination of cable stretching and the possibility to exchange individual strands. Cable anchors are ar-ranged at the pylon between the two longitudinal webs of the cross section as well as next to the bridge deck at the corbel. The corbels at the bridge deck are connected to the bridge cross section at the lateral boxes. To avoid structurally and technically problematic details, the corbels with their webs inclined to the ca-ble axis have been detached from the carriageway slab and have only been welded to the external and internal webs of the bridge cross section. In the area of load insertion, the external webs of the

superstructure as well as the two transverse frames next to a cable anchor have been strengthened to meet structural requirements. Moreover, additional bulkheads between the corbels’ girders and the bottom plates are designed. Transverse tensile forces caused by the inclination of the external webs are hence distributed and for resulting plate bending of the external webs additional bulk-head plates are welded between the corbel girders and the bottom plates. The cables are tensioned only at the bridge deck. In the py-lon, fixed anchors are planned.

Structure 3 to 5Structures 3 to 5 are implemented according to the mandated alter-native offer as prestressed box sections with external reinforcement as mixed construction. The official design envisaged a composite box section just as for structure 1.2. Span widths of the structures that run in a straight line up to the connection areas to structure 2 are between 53 and 54 m. The substructures are composed in ad-dition to the abutments of 2 individual columns with drop shaped cross sections as it is implemented for the other structures. They are founded by a joint pile cap on bored piles with a diameter of 1.50 m.Construction of the substructures in Strelasund was executed with-in thick stiffened sheet pile walls and by applying an underwater concrete bottom.The superstructures are built span-wise by means of a launching truss.

Structural engineering of the cable-stayed bridgeStatic system and load-bearing behaviour of the structureCalculations were based on a structural model, which allowed, in addition to a safe and economic dimensioning of the load-bearing structure, a structured and comprehensible documentation of re-sults.

Cross section of structure 3 to 5

Stability verifications of the main load-bearing structure have been delivered for the final stage as well as all construction stages by using a spatial framework system representing the superstructure, both pylon stems, all cables of both levels and the supporting cross girders. In the framework system, the sub-structures have been modelled, too, in order to determine forces caused by deformation in longitudinal and transversal direction of the bridge from horizontal and eccentric loads at construction and final stage. The modelling of the superstructure deck as beam was justified for the stiffened 3-cell box section as effects from profile deformation and effects of the folded-plate structure can be ne-glected. Local load-bearing structures and nodes have been veri-fied on separated framework and folded plate models. By means of the finite element method the orthotropic slab, cable connection points and cross girders in the area of the pylon foot have been analysed. Furthermore, the penetration points of the cross beams and the individual pylons have been calculated with a finite ele-ment system as the pylon cross section could not be completely stiffened because of its separation in three parts by the stairwell, the cable anchors and the transport shaft.

Dead weight and cable pre-tensioningBy choosing the cable pre-tensioning, the state of dead weight of a cable-stayed bridge can be configured. With the general aim of balanced moments, the dead weight loads from the bridge super-structure are generally exceeded.

However, when choosing the cable pre-tensioning, particu-larities of each individual bridge have to be taken account of. For the Strelasund bridge, cable tensioning has been chosen in view of the following aspects:

- balanced moments in the bridge deck under dead weight with mostly maintained minimum plate thicknesses

- uniform use of strand cables type C37 from DSI because of the required individual case approval

- avoiding a deformation of the pylons in workshop form- avoiding of tensile stresses and cracking in state II in the com-

posite concrete and thus inaccuracies of the system description- full pre-tension of cables with the cantilever method; avoiding

post-tensioningIn the calculations of the superstructure, for verification of ulti-mate limit state, both portions of internal forces “dead weight su-perstructure” and “cable shortenings” have been analysed with the same safety factor due to their interdependence according to DIN-Fachbericht 103.

Stability verifications of the superstructureCross section valuesThe load-bearing behaviour of this cable-stayed bridge is essen-tially marked by the interaction of bending stiffness of the bridge deck and extensional stiffness of the cables.

The harp-shaped arrangement of the cables gives an increased importance for load distribution to the bridge deck, which is not notably slender for a cable-stayed bridge with its main opening of 1/63.Because of these characteristics of the load-bearing system, the stiffness of the bridge deck had to be described quite accurately to calculate structural stability and deformations. Basis for deter-mination of accurate participating widths have been the moment curves of the dimensioning load cases. In the description of the cross sections, the non-load-bearing chord surfaces have been

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taken into account to correctly analyse bending stresses by sepa-rating the chord plates according to the relation of participating and non-participating surface portions into coupled discs whilst maintaining the total cross section form and plate thicknesses, as well as by setting to zero the Young’s modulus of the non-participating plate portions. As normal forces are distributed by the whole cross section, the chord portions, together with the Young’s modulus at zero, have been inserted into the centre of gravity of the cross section with their real extensional stiffness EA.This procedure allows to take into consideration the bending stiffness EI of the superstructure’s cross section with the partici-pating width and, at the same time, assures by maintaining the cross section form with the existing plate thicknesses that the calculations with the extensional and shear stiffnesses EA, GA and GIT of the whole cross section result in correct spreading of normal and shear forces.

The concrete needed for ballasting of the stay-cables participates inevitably at the stiffened steel cross section. Correspondingly, for this area a composite cross section has been analysed, the concrete cross section reinforced and the shear connections between concrete and steel secured by shear studs in usual manner. The extensional stiffness of the 40 to 170 m long cables has been taken account of with the corresponding ideal Young’s modulus. Relevant sag of the cables was between 50 and 950 mm depending on the cable length. Changes of the ideal Young’s modulus depending on the cable sag and cable force of around 2 % could not be neglected and linear calculation of the load-bearing structures with superposition of all individual load cases had to be delivered.

StressesStresses caused by dead weight, traffic, temperature and wind were analysed as per DIN-Fachbericht and DIN V ENV 1991-2-4.Aspect ratios for wind loads have been determined in the wind channel during elaboration of the design. Especially for a structur-ally reasonable and economic dimensioning of the load-bearing structure at construction stages and bridge fitting it was crucial to know exact values. Constraints due to structure movements have been indicated in the geotechnical report. Actions from ship colli-sion and thermal ice load have been regulated in the construction description. For ship collision a frontal impact of 23.4 MN and a lateral impact of 6.8 MN had to be taken into consideration. Linear load of impress is 200 kN/m. Ship collision has been cal-culated dynamically in consideration of mass inertia. For the ca-bles, exchange and failure of individual cables has been analysed. Internal forces of the structural system under dead weight result from the addition of loads in all construction stages. Calcula-tion basis is thus the detailed assembly design of the executing company.

Stability verificationsVerifications of structural stability and serviceability have first been carried out at the undeformed static system. Normal com-pressive forces in the pylon and the bridge deck required supple-mentary considerations as per 2nd order theory. As expected, the influence of deformations on internal forces in the bridge deck was low with 5 % in case of the present load-bearing system with stay-cables connected to one fixed bearing and the comparably low slenderness of the superstructure, and thus not relevant for dimensioning. For the same reason and because of the harp-like cable geometry, a stress increase of only 10 % resulted for the

Finite element system of the pylon cross beam Stability computation

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pylon under unfavourable combination of stresses in longitudinal and transversal direction of the bridge. Local insertion of trans-verse forces and torsion moments into the 3-cell box section has been verified on the flat framing systems by observing the state of residual stresses of the transverse frames.For calculations of the cross girders of the carriageway slab, the influence of recesses in the web plates by trapezoidal stiffeners had to be taken into consideration. This effect has been relevant for the analyses because of the distance of the main webs of 8 m and with heights of the vaulted cross girders of 700 to 900 mm. Safety against buckling of the superstructure has been guaranteed by the arrangement of trapezoidal stiffeners. Based on profound verifications by means of a finite element model, supplementary buckling stiffeners could be eliminated in the drop-shaped pylon cross section next to the main webs and the platform plates. The result of the calculations of the framework structure of the main load-bearing system, the finite element analyses of load insertion and the keeping of minimum plate thicknesses set in the technical regulations has been a material distribution with minimum thicknesses in the bridge deck, with exception of the support and cable connection areas.A durable steel load-bearing structure necessitated in the detail conception, in addition to implementation of calculation results from the structural models, a comprehensible force flow and the structural consideration of all additional stresses based on the re-alistic deformation behaviour. In all detail, great importance was attached to the reduction of notch effects by smooth transitions of the constructional elements where uniform stiffness could not be completely ensured. Moreover, in the areas of load insertion the structure has been formed in such a way that all construction elements are accessible for inspections.

Fatigue analysis According to regulations of DIN-Fachbericht 103, fatigue verifica-tions of the main load-bearing elements are unnecessary when during detail formation values do not fall below detail category 71. A correct durable detail formation does in general not require fatigue verifications for the main load-bearing structure. The con-struction of stiffened transverse frames, in case of unfavourable detail categories, entails the consideration of double stress am-plitudes due to two-way traffic loads. In any case, fatigue stabil-ity of the cable anchors has to be subjected to analysis. As per fib recommendations and tender conditions, the fatigue stabil-ity at the completed bundles have been verified in a combined fatigue/tensile test with σo = 0.45 σuts and Δσ = 200 MPa. To take into account bending stresses due to installation tolerances, superstructure deformations and changes of the cable sag, the tests have been carried out with anchor plates rotated by +/- 0.6 degree to the cable axis by a wedge plate. In addition to maximum cable forces, the static analysis verified that stress amplitudes at the load-bearing structure under load of the fatigue load model are significantly below the admissible stress of Δσ = 200 MPa resulting from the test. Furthermore, it has been shown that the final rotation angle of the cables under the fatigue load model from translation and rotation of superstructure and pylon as well as changes of cable sag does not exceed the value of 0.3 degree. The reducing effect of the elastomeric retainer of the cable bun-dle in the anchor head has not been part of the analysis to be on the safe side. The supplementary restrictions of construction tolerances for fabrication of cable anchors to an angle deviation of 0.3 degree has been ensured so that the maximum final rotation angle under fatigue load model does not exceed the verified test value of 0.6 degree.

Workshop drawings:superstructure with anchorage Reforcement drawing for the nodes of structure 1.2

3 4

Moreover, verification has been delivered that the maximum final rotation angle of the cables does not reach in any load case the admissible value of 1.4 degree of the fib recommendations. To esti-mate existing securities, transversal bending stresses at the anchor plate, i. e. in the wedge area of the cable anchors, have been veri-fied under maximum arising loads and with the maximum admis-sible final rotation angle of the cables of 1.4 degree on a realistic model of the anchoring structure.

In addition to the strand bundles and the anchor head, the recess-es and bearing tubes have been shown as well as the elastomeric retainer of the cable bundle in the bundling area. Due to the elasto-meric retainer of the strand bundle in the bearing tube, a decrease of the rotation angle of 1.4 degree in front of the cable head to 0.27 degree in the wedge area was the result of the structural cal-culation. As this value is even below the value of 0.3 degrees for superstructure deformation on which the fatigue analysis is based,

the calculation resulted as expected in lower stresses of the indi-vidual strands in the wedge area than measured in the tests. A high level of fatigue strength is thus documented for the present load-bearing structure, as neither under the fatigue load model nor under maximum admissible final rotation angle and forces at the basis of the test, have load conditions been reached on the load-bearing structure.

DeformationsTo establish workshop drawings, the unstressed workshop form has to be known, which results from the addition of deflections of all construction stages. Moreover, because of compressive stresses of the bridge superstructure caused by pre-tensioning of the cables, the shortening of the superstructure has to be preset. Compression of the pylons in an area of around 10 mm could be neglected in the workshop form. Cable anchors at the bridge deck and the pylon are installed in consideration of cable sag under permanent loads.

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1 Inclined bars with shear studs 2 Mounting of the reinforcement cages for the nodes 3 Assembly of structure 1.2 4 Erection of the steel structure 5 Pier after assembly of the inclined bars Pi

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Analyses of bridge vibrationsDuring design, the vibration behaviour of the entire bridge as well as the cables has been analysed. Decisive for vibration security was the choice of an aerodynamically advantageous and torsion stiff cross section. Hence, for all excitation mechanisms harmless-ness has been proven. Critical for verification of vibration safety of the bridge is the construction stage with projecting cantilever structure, just before reaching the pile in axis 210. In this state, the eigenfrequency of the bridge is at 0.35 Hz for vertical vibra-tions and 1.7 Hz for torsional vibrations. Cable vibrations are already caused by small excitations because of the extremely low self-damping characteristic of the cables. Vi-brations are in general insignificant in view of the stability of the bridge due to the small forces that arise. However, clearly visible vibrations should be avoided. The excitation mechanisms ‘rain-wind induced vibrations’ and ‘footprint vibrations’ are analysed. Rain-wind induced vibrations should be avoided by a profiling of

the cladding tube of the cable so that a rivulet cannot form. A calculation of footprint vibrations resulted in insignificant excita-tions. As each calculation is only an approximation and the cables are extremely sensitive to vibration excitations, for each cable the possibility of a supplementary dampening has been planned in the design. After completion of the bridge, cable vibrations are measured over a longer period of time and individual cables are dampened where necessary.

AssemblyStructure 1.2Once the concrete superstructures were finished, the inclined bars and then the up to 52 m long and 210 t heavy sections of the super-structure have been mounted. The top points of the inclined bars were sustained on temporary supports, dismounted after welding of the steel structure. A special task for design and construction ex-ecution has been the connection of the inclined bars to the piles.

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The structurally needed 3-layer connecting reinforcement had to be adapted to the narrow distances of the shear studs in the steel bars, which form, in the ground plan as well as the elevation, an angle to each other with their drop-shaped cross section. To guar-antee installation, it was necessary to install the reinforcement bars on a vertical level despite the radial arrangement of the shear studs. The different interdependent penetration points of the in-dividual reinforcement layers with bar diameters of up to 32 mm led to different arrangements of the shear studs in the opposite lying steel bars. Reinforcement drawings have been elaborated 3-dimensionally. To produce steel gauges for the fabrication of the reinforcement cages, the spatial data of the reinforcement draw-ings were submitted digitally by the steel plant. After prefabrica-

tion of the reinforcement cages, the reinforcement of the nodes was mounted, aligned and the steel bars were assembled by push-ing them over the connecting reinforcement. The nodes were then grouted with self-compacting concrete. For this application a single case approval was necessary.The possibility to place even heavy steel elements precisely to the centimetre with the utilized crawler crane led to quick and flawless assembly on site, also because of the extensive and exact design and work preparation.

To avoid transversal bending of the inclined bars during concreting, straps with GEWI bars are arranged at the centre of the carriage-way slab at each transverse frame. Despite the structure’s curve in

Erection sequence of structure 1.2

superstructure

deck

110

110

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77.105

31.472 31.471 34.619 35.405 36.192 28.325 23.603 28.325 36.192 35.405 36.192

36.192 36.193 34.618 31.472

77.105

77.105

81.141

43.383

69.914

61.897

36.161

59.009

82.274

61.897

77.105

77.105

77.105

77.105

77.105

77.105

113.297

113.297

HS 2

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HS 3

HS 4

HS 4

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HS: flying shore

the ground plan, an assembly joint has not been implemented as the calculated deformation differences of the webs under concret-ing loads have been very low due to the compensating effect of the transversal inclination with the different web heights as well as the structurally analysed deformation increase in case of possible unplanned torsional stresses.After erection and welding of the steel structure, the construction of the carriageway slabs took place with two formwork travellers by pilgrim step method. To avoid considerable longitudinal deforma-tions of the bridge superstructure, which is curved in the ground plan, two concreting sections were grouted at the same time, symmetrically to the inclined bars. In addition to the reduction of extremely high absolute values of pre-deformation, which is in any case reasonable to avoid errors, the curved line of the structure in the ground plan required to absolutely avoid a predetermination of axial deformations, which would have been possible in other cases.

Structure 2The steel superstructure was delivered in six cross section parts with length of up to around 30 m. After welding of the 3-cell steel cross section, 50 to 55 m long and up to 470 t heavy superstructure sections of the edge spans were mounted with a crawler or swim-ming crane. For support and stabilisation of the first section on the pile in axis 190, a temporary inclined bar was placed on the pile foundation, connected to the pile head by a tie rod.

Assembly of the approximately 90 m long and 850 t heavy steel section in the smaller main span was accomplished at the same time by swimming crane and strand jacks. The used swimming crane TAKLIFT 7 of company SMIT has a maximum lifting force

of 1,200 t depending on its jib and radius, and maximum lifting height of 160 m. In the main opening the steel superstructure was erected by cantilever method in sections of 16.1 m length and a weight of 140 t each. Together with the cantilever part, the pylon segments and 4 cables for each cantilever section were assem-bled. The composite concrete was grouted during assembly in 3 concreting sections, thus avoiding concreting stages relevant for dimensioning and guaranteeing economical material distribution.The assembly and pretensioning of the cables was done strand-wise. The tensioning procedure with two hydraulic coupled mo-no-presses guarantees that after tensioning all strands of a cable have the same force. Each time the two cables of a bridge span were tensioned simultaneously and cables on land and on wa-ter were tensioned alternating in at least two tensioning steps, whereas the first tensioned cable was on land. Post-tensioning of the cables was not planned but is possible with a gradient press. Assembly was accompanied by a comprehensive measurement programme, documenting in table and graphic form for each con-struction stage, target height of bridge deck and pylon as well as uniform deformations to assess external influences from wind, temperature and position of the scaffoldings.

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Parts of cross section during the transportation structure 2

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Transport of the pylon cross beam Assembly of the part of the superstructure between axis 190 and 200

Assembly of structure 2

170

170

170

170

170

170

54.00

54.00

54.00

54.00

54.00

54.00

72.00

72.00

72.00

72.00

72.00

72.00

concrete ballast 52300 kN

concrete ballast 52300 kN

strandjack 6000 kN

strandjack 6000 kN

126.00

126.00

126.00

126.00

126.00

126.00

198.00

198.00

198.00

198.00

198.00

198.00

72.00

72.00

72.00

72.00

72.00

72.00

59.30

59.30

59.30

59.30

59.30

59.30

180

180

180

180

180

180

190

190

190

190

190

190

200

200

200

200

200

200

210

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210

220

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230

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External influences can however in general be excluded during deformation controls, which is achieved by carrying out measures before sunrise. There are possibilities to correct bending of the superstructure or the pylon in case of deviations of the real dead load and stiffnesses of the load-bearing structure from the calcu-lated assumptions. These possibilities are interactive pre-stress in several steps with intermediate controls and insertion of the cor-recting portion of the composite concrete only after application of secondary dead loads.

In addition to bending controls, axes and lengths were measured. Corrections of the bridge length are possible on three correction sections by cutting to length the concerned section.

Concluding remarkThe 2nd crossing of the Strelasund represents a striking and opti-cally attractive bridge within the European road network creating a performing traffic connection to Rügen island.

Construction started in 2004. The bridge has been completed in 2007.

Involved in the project

Client Federal Republic of Germany represented by the Land of Mecklenburg-Western Pomerania, repre-sented by DEGES, Deutsche Einheit Fernstraßen-planungs- und -bau GmbH

Executing company Max Bögl Bauunternehmung GmbH & Co.KG, NeumarktMax Bögl Stahl- und Anlagenbau GmbH & Co.KG, Neumarkt

Structural engineering Design joint ventureSSF Ingenieure AG, MunichBüchting + Streit, Munich

Authors of the article Dipl.-Ing. Stephan Otto, member of the extended management of Max Bögl Bauunternehmung GmbH & Co.KG, 92318 Neumarkt Dr. Ing. Klaus Thiele, manager of technical depart-ment at Max Bögl Stahl- und Anlagenbau GmbH & Co.KG, 92318 NeumarktDipl.-Ing. Hans-Joachim Casper, department manager, SSF Ingenieure AG, Leopoldstraße 208, 80804 MunichDipl.-Ing. Markus Karpa, project manager, SSF Ingenieure AG, Leopoldstraße 208, 80804 MunichDipl.-Ing. Frank Sachse, project manager, SSF Ingenieure AG, Leopoldstraße 208, 80804 Munich

bottom: Assembly of prefabricated segments by cantilever methodright: Pylon of the Crossing of the Strelasund

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