The second crossing of the Strelasund
Structural analysis, construction and - mounting of the composite and cable-stayed bridge
Introduction The connection of the island of Rügen to the German long-distance 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 decongests 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 crossing 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 Dänholm and the sound of Strela to Rügen island and finishes at the
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Structure 1.1
Structure 1.2
Structure 2
327.50 m
317.00 m
583.30 m
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above: Overall view right: Seperating pile in axis 170
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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 construction Bridge The bridge with a total span width of 2,831 m consists of 6 individual structures between both abutments; these units are connected 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 elevated cornice head and is subdivided in an 11.5 m wide carriageway 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.
Structure 3
Structure 4
532.30 m
532.20 m 330
Structure 5
539.00 m 430
530
Picture credit: Florian Schreiber Fotografie for SSF Ingenieure AG
Together with the construction of the new Baltic autobahn A20 Lübeck – Stettin the Rügen feeder “B96n Stralsund/Rügen”, approx 55 km long, between the A 20 in the region of Grimmen and the town of Bergen on Rügen has been realised. With the construction of the new “B 96 n Stralsund/Rügen” Rügen feeder an effective 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 project 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.
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Structure 1.1 The structure 1.1 is the direct continuation of the bypass of Strelasund and is designed as prestressed concrete bridge with individual 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 section 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 abutment, 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 pointing to the outside. This design element is realised over the whole bridge ensemble. The bridge superstructure, longitudinally inclined by 4 degrees from the abutment up to around 20 m above the ground, has been constructed span-wise with short cantilever
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arms in 10 concreting sections on a scaffolding, whose intermediate yokes are founded on driven piles like the substructures. Structure 1.2 Structure 1.2 is a continuous girder with steel composite box section 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 inclination 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 reinforced 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 superstructure 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 continued 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 transverse force and torsion moments, the drop bending within the
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Structure 2 Main load-bearing structure Structure 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
transport – opening Picture credits: SSF Ingenieure AG
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s uperstructure’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, supported on elastomer or deformation bearings on each time 2 individual 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.
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stairwell
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anchorage 1 View of structure 1.2 2 View of structure 2 3 Cross section and pier of structure 1.1 4 Cross section of structure 1.2 5 Cross section of the pylon
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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 transversal 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 deck The 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 stiffened 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 accordance with DIN-Fachbericht. The webs and bottom plates are
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determined at a minimum thickness of 12 mm in view of low welding 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 carriageway 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 pretension. 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. Pylon The pylon consists of two approximately 87 m high individual supports 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 connection 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 narrow 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 longitudinal and transversal direction of the bridge as well as by already 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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entire pylon height, which is completely free of stiffeners and slightly narrowed in the area of the connection points. Longitudinal 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 anchoring The load-bearing structure is spanned at two levels in the axes of both pylon stems. In the bridge elevation, the individual cables are arranged parallel to each other like a harp. The elevation 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 relatively small longitudinal bending moments in the stiffening girders and the possibility of simple assembly by cantilever method.
Picture credits: SSF Ingenieure AG
With maximum service loads of the cables of 3,500 KN, the installation 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 2 2 Cross section of structure 2 3 Pylon
MW–0.09
approx.–37.20
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 arranged 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 cable 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 bulkhead plates are welded between the corbel girders and the bottom plates. The cables are tensioned only at the bridge deck. In the pylon, fixed anchors are planned. Structure 3 to 5 Structures 3 to 5 are implemented according to the mandated alternative 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 addition 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 within 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 bridge Static system and load-bearing behaviour of the structure Calculations 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 results.
Cross section of structure 3 to 5
15.00 5.75
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Picture credit: Florian Schreiber Fotografie for SSF Ingenieure AG
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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 substructures 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 neglected. Local load-bearing structures and nodes have been verified 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 element 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.
- 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 composite concrete and thus inaccuracies of the system description - full pre-tension of cables with the cantilever method; avoiding post-tensioning In the calculations of the superstructure, for verification of ultimate limit state, both portions of internal forces “dead weight superstructure” and “cable shortenings” have been analysed with the same safety factor due to their interdependence according to DIN-Fachbericht 103.
Dead weight and cable pre-tensioning By 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 superstructure are generally exceeded.
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 determination 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
However, when choosing the cable pre-tensioning, particularities 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:
Stability verifications of the superstructure Cross section values The load-bearing behaviour of this cable-stayed bridge is essentially marked by the interaction of bending stiffness of the bridge deck and extensional stiffness of the cables.
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Finite element system of the pylon cross beam
Stability computation
taken into account to correctly analyse bending stresses by separating 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 nonparticipating 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 participating 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.
Stresses Stresses 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 structurally 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 collision 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 calculated dynamically in consideration of mass inertia. For the cables, 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. Calculation basis is thus the detailed assembly design of the executing company.
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 loadbearing structures with superposition of all individual load cases had to be delivered.
Stability verifications Verifications of structural stability and serviceability have first been carried out at the undeformed static system. Normal compressive forces in the pylon and the bridge deck required supplementary 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
Picture credits: 1 + 2 + 4 SSF Ingenieure AG / 3 Max Bögl Stahl - und Anlagenbau GmbH & Co.KG, Spezialbau Engineering GmbH
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Workshop drawings:superstructure with anchorage
Reforcement drawing for the nodes of structure 1.2
pylon under unfavourable combination of stresses in longitudinal and transversal direction of the bridge. Local insertion of transverse 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 realistic 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 verifications 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 construction of stiffened transverse frames, in case of unfavourable detail categories, entails the consideration of double stress amplitudes due to two-way traffic loads. In any case, fatigue stability of the cable anchors has to be subjected to analysis. As per fib recommendations and tender conditions, the fatigue stability 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 bundle 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.
In addition to the strand bundles and the anchor head, the recesses and bearing tubes have been shown as well as the elastomeric retainer of the cable bundle in the bundling area. Due to the elastomeric 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 calculation. As this value is even below the value of 0.3 degrees for superstructure deformation on which the fatigue analysis is based,
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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
the calculation resulted as expected in lower stresses of the individual 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 loadbearing structure. Deformations To 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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Picture credits: Max Bรถgl Bauunternehmung GmbH & Co.KG
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 estimate existing securities, transversal bending stresses at the anchor plate, i. e. in the wedge area of the cable anchors, have been verified under maximum arising loads and with the maximum admissible final rotation angle of the cables of 1.4 degree on a realistic model of the anchoring structure.
Analyses of bridge vibrations During 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 harmlessness 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 vibrations 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. Vibrations 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 ‘rainwind induced vibrations’ and ‘footprint vibrations’ are analysed. Rain-wind induced vibrations should be avoided by a profiling of
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the cladding tube of the cable so that a rivulet cannot form. A calculation of footprint vibrations resulted in insignificant excitations. 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. Assembly Structure 1.2 Once the concrete superstructures were finished, the inclined bars and then the up to 52 m long and 210 t heavy sections of the superstructure 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 execution 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 guarantee 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 individual 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 drawings 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 pushing 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 carriageway slab at each transverse frame. Despite the structure’s curve in
Erection sequence of structure 1.2 superstructure 110
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HS 4 36.161 59.009 82.274 61.897
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deck
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Picture credit: Florian Schreiber Fotografie for SSF Ingenieure AG
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Picture credits: Max Bรถgl Bauunternehmung GmbH & Co.KG
Transport of the pylon cross beam
Assembly of the part of the superstructure between axis 190 and 200
the ground plan, an assembly joint has not been implemented as the calculated deformation differences of the webs under concreting 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 deformations 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.
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 assembled. 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 strandwise. The tensioning procedure with two hydraulic coupled mono-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 water 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 construction 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.
Structure 2 The 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 swimming 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.
Parts of cross section during the transportation structure 2
2
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
1
6
5
3
4
Picture credits: SSF Ingenieure AG
Assembly of structure 2
54.00 170
72.00
126.00
72.00
59.30 230
220
210
200
190
180
198.00
concrete ballast 52300 kN strandjack 6000 kN
54.00 170
72.00
126.00
72.00
198.00
59.30 230
220
210
200
190
180
concrete ballast 52300 kN strandjack 6000 kN
54.00 170
54.00 170
72.00
54.00
72.00
54.00 170
126.00
72.00 180
198.00
126.00
72.00
198.00
126.00 190
59.30
72.00
230
59.30 230
220
72.00 210
230
220
210
198.00 200
59.30 220
210
200
190
72.00 210
200
190
180
198.00 200
190
180
170
126.00
72.00 180
59.30 220
230
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 remark The 2nd crossing of the Strelasund represents a striking and optically 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.
bottom: Assembly of prefabricated segments by cantilever method right: Pylon of the Crossing of the Strelasund
Involved in the project Client
Federal Republic of Germany represented by the Land of Mecklenburg-Western Pomerania, represented by DEGES, Deutsche Einheit Fernstraßenplanungs- und -bau GmbH
Executing company
Max Bögl Bauunternehmung GmbH & Co.KG, Neumarkt Max Bögl Stahl- und Anlagenbau GmbH & Co.KG, Neumarkt
Structural engineering
Design joint venture SSF Ingenieure AG, Munich Büchting + 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 department at Max Bögl Stahl- und Anlagenbau GmbH & Co.KG, 92318 Neumarkt Dipl.-Ing. Hans-Joachim Casper, department manager, SSF Ingenieure AG, Leopoldstraße 208, 80804 Munich Dipl.-Ing. Markus Karpa, project manager, SSF Ingenieure AG, Leopoldstraße 208, 80804 Munich Dipl.-Ing. Frank Sachse, project manager, SSF Ingenieure AG, Leopoldstraße 208, 80804 Munich
Picture credits: right Florian Schreiber Fotografie for SSF Ingenieure AG / left Max Bögl Bauunternehmung GmbH & Co.KG
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 calculated assumptions. These possibilities are interactive pre-stress in several steps with intermediate controls and insertion of the correcting portion of the composite concrete only after application of secondary dead loads.
Title: Florian Schreiber Fotografie for SSF Ingenieure AG