Bridge Infrastructure of Bijlmer Railway Station

Bridge Infrastructure of Bijlmer Railway Station Christophe BAUDUIN Project Manager Engineering Department, Besix Brussels, Belgium Christophe Baudui...
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Bridge Infrastructure of Bijlmer Railway Station Christophe BAUDUIN Project Manager Engineering Department, Besix Brussels, Belgium

Christophe Bauduin obtained his Master Degree in Civil Engineering at Ghent University in 1983. He worked at Delft Geotechnics (’83-’86) and at Construction Control Office Seco (8689). Currently he is Project Manager in the Engineering Department of BESIX. Since 1999 he has been associated professor in geotechnical design at the University of Brussels.

Summary The construction of new railway stations enclosing operational ones is a major but exciting challenge to designers and contractors. A typical example is the Amsterdam “Bijlmer” railway station project. Four new double track platforms, 300 m long, have to be built within the existing station, 8 m above (future) ground level. Value Engineering led to a concept consisting of adjacent precast prestressed box girders, 4 per span. Transverse post-tensioning assembles the girders into a monolithic deck. The present papers focuses on following aspects: -

Specific design aspects of substructure and precast decks Connection devices between individual girders ensuring a monolithic structure. Analysis of laboratory test results against the applied shear friction model defining the shear strength capacity of the post-tensioned connection between girders.

Keywords: Station, bridge, precast box girder, transverse post-tensioning, shear friction, test

1. Introduction The new railway and metrostation “Bijlmer” is a futuristic but functionally designed landmark for the people living and working in an expanding area of the city of Amsterdam. It will be built at the location as the existing station and is one of the important projects in the upgrading of the Amsterdam-Utrecht line. The station will become an intensively used junction for train, metro and bus traffic with an expected daily flow of 60000 passengers. The architectural concept is governed by 4 major goals: the trendsetting building fits and upgrades the architecture of the surroundings, is passenger friendly and ensures easy accessible transfer points. The new station has to link both parts of Bijlmer, separated by the rail tracks, in a clearly identified visual line fitting in the city plan. The existing station comprises two metro and two railway tracks, each with a separate platform, placed on an embankment six metres above ground level. The embankment is interrupted in the central part of the station by an approximately 20 m long underpass, by which access is provided to the platforms. Car traffic on ground level crosses the embankment through two underpasses. The new station will comprise eight tracks (two for metro and six for railway traffic) placed on four parallel railway bridges of 300 m length, about 8 m above ground level. The existing embankment and station will be entirely removed. The platforms are 7m wide and cantilever out from either side of the railway bridges. A trend-setting steel and glass shell will cover the bridges and the platforms. The space under the railway bridges will be used as bus station with roads and pedestrian traffic connecting the various districts of Bijlmer. Shopping malls and Station offices are located in this area. The new station has to be built in and around the existing one without interruption of the railway traffic on the embankment and car traffic on ground level. This requires a staged construction. The existing situation before start of construction works and the general phasing are illustrated on figure 1.

Fig. 1 General phasing of the construction works

In a first phase, the two outer bridges, parallel to the present tracks in service are constructed, followed by the transfer of the existing railway traffic to the new bridges. Passengers have access to the train via the outer platforms. Next, one inner bridge is built and the existing metro traffic is than transferred. Finally, the fourth remaining bridge is realised and leading to the final railway traffic situation is installed. The steel and glass shell (not shown on the figure) will be erected after completion of the bridges and platforms. Works require anchored temporary sheetpiles aside the tracks in service, staged demolition of the existing structures and excavation of the embankment. Cellars underneath the station, intermediate levels, stairs and elevators to the platforms are built in several stages allowing use of new and temporarily remaining existing facilities until the new station is fully completed. All these activities are performed in the existing station and along fully operational tracks. Stringent procedures for the safety of railway traffic, passengers and public as well as for the workmanship are of utmost importance.

2. The construction using precast elements Initially the bridges were conceived as cast-in-situ, multi-span, plain concrete, post-tensioned slabs with curved soffits. The precast platforms are fixed to the slabs and supported by inclined steel brackets. Columns (in pairs) directly support the slab without cross beams. The position of the columns result from the layout and organisation at ground level, leading to a large variety of span lengths. Tenders were submitted based on this basic design. However, the bidders and the Owner, NS RIB, did not reach an agreement, mainly because the bids significantly exceeded the budget. Besix (Brussels) was then invited to negotiate with NS RIB and with the Project manager and consultant Arcadis, with the aim to develop a value engineered solution within a target budget respecting the milestones of construction and track delivery planning. The designers of Arcadis and Besix developed an alternative design. The cast in situ deck is replaced by four adjacent precast prestressed box girders per span (see figure 2). Fig. 2 Cross section through the deck

Fig. 3 Deck construction: Placement of the girders on the crossheads

Fig. 4 Detail of connection between adjacent girders: left in situ nodes; right: surface of girder at connection with indents for transfer of shear force

Fig. 5 Mounting of the platforms after transverse tensioning

These factory-constructed box girders are placed on the cast in situ crossheads (see figure 3). The alternative deck design with precast girders had to be implemented in the existing architecture, without altering its concepts. To support the isostatic spans the continuous soffit of the bridge required crossheads. These are to be integrated into the deck construction height. Consequently, the crossheads have the same cross section as the deck The isostatic spans have an overall length of about 28,5 m (C/C) with a girder length of 25,5m between bearings. The pre-tensioning is achieved using 64 straight 15,7 mm diameter FeP 1860 tendons per girder. The reinforcement is prefabricated. Inside polystyrene blocks create the “hollow volumes”. The hollow volumes are separated by two unreinforced diaphragms. The girders are cast in one pour using self-compacting C65 concrete. The construction cycle, including hardening of the concrete and cutting of the pretension strands, takes 24 hours. The use of self-compacting concrete requires specific pouring techniques. One girder weights approximately 1200 kN. Each girder is supported by two neoprene bearings at both ends. High strength, cast-in-situ concrete nodes connect the upper and the lower flanges of adjacent girders (see figure 4). No mild steel reinforcement connects the girder to the in situ concrete node. The contact area between the girder and the concrete joint is smooth (due to the use of steel formwork), and indents transfer the shear forces. Transverse post-tensioning is applied through the upper and the lower flanges after hardening of the insitu concrete joints and assemble the four box girders into a monolithic deck. Once the transverse post-tensioning is applied, the precast platforms are mounted on the deck and on the crossheads (see figure 5). The transverse post tensioning typically consists of 6T157 FeP 1860 strands spaced at 0.865 m C/C in the upper flange and of 5 T157 strands spaced at 1.77m in the lower flange; the locations of the transverse post tensioning strands followed from the module of the precast platform (3.53 m wide, 10 mm joint) and supporting brackets.

Optimal use of precasting requires a high degree of repetition and this has lead to a rearrangement of the column lay-out to obtain the same span length, as closely as possible, and hence to minimise the number of different girders. Small deviations in span length, amongst other due to the curvature of the alignment in plan of the deck, were solved through the crossheads length. Very stringent requirements of construction tolerances were imposed for these cast in situ crossheads. Together with the rationalizing of the deck construction works, the foundation design was optimised for the temporary and final and design situations. The use of precast elements has simplified the temporary works, despite of constraints in terms of weight of the precast elements, phasing of erection activities and working area for the lifting. All construction activities have to be performed in the station, and have, therefore, been designed and planned to meet stringent safety requirements for train and passenger movement, as well as for construction workers. All the design changes arising from the decision to replace a cast-in situ deck with a deck formed by precast girders were evaluated and agreed in a short time through the close cooperation between the design teams of the consultant Arcadis, the contractor Besix and the girder and precast platforms supplier Betonson. One of the key factors to this cooperation was pro-active and open evaluation of the constraints for the different parties.

3. Some specific aspects of the structural design The structural design has been performed using the Dutch design codes: NEN 6720 “Concrete design” [1] and NEN 6723 “Design of concrete bridges” [2], complemented by the new, specific OVS “Requirements for railway structures” [3]. The design train speed is 200 km/h. 3.1 Piles, Columns and Crossheads The crosshead is supported by a single, central column with an oval section (2500 – 2800 mm, increasing in width over the upper half towards the crosshead; concrete grade C45) resting on a 1.5 m thick pile cap. A hollow section was chosen for the column to avoid excessive heat development during hardening of the concrete. Most of the pile caps are carried by vertical, precast pre-stressed driven piles. Were insufficient working space was available for heavy pile driving equipment, the foundations were executed using screwed steel pipes of two meters length welded together and filled by concrete after reaching the required depth. The crossheads have the same shape and construction height as the decks and support them via corbels. The girders end with an inverse corbel (figure 6). Due to the small dimensions of these corbels, especially at the ends of the crossheads, and the important deck loads (maximum design value of the reactions: 2800 kN and 2500 kN for the bearings under the outer girders and 1800 and 1700 kN for the bearing under the central girders) high concentrations of stresses and complex transfer of the deck reaction from the bearings to the column were expected. The stresses in the crossheads were analysed using an ANSYS 3D model with volume elements. The high compressive and shear stresses required the high strength (C65) concrete to be placed in one pour. The tensile forces in the corbels are resisted by 3 to 5 posttensioned Dywidag bars (diameter 32 mm, FeP 1030) under each deck bearing. The tensile forces in the other direction, Fig. 6 Longitudinal section of crosshead showing due to bending, torsion and shear are post-tensioned corbels of the crossheads. supported by heavy reinforcement. 3.2 Deck design The forces in the webs and flanges of the deck girders and in the connections between the girders were assessed using a three-dimensional FEM shell model. The calculation results of this model were used to determine the pretension forces and the reinforcement in the webs and flanges of the girders, according to the appropriate rules from the design codes.

Special attention was paid to the connections between the girders. The connection (height= 280 mm) is subject to transverse bending moments, normal forces and vertical and longitudinal shear forces. Rather important longitudinal shear forces qlongitudinal result from the differences of stiffness between the adjacent girders tied together to form the monolithic deck. The vertical shear is relatively small. As the contact area between the girder and the concrete joint is smooth (due to the use of steel formwork), indents transfer the shear forces. The transverse prestress force acts through the centres of the flanges and connections and is determined using the following criteria: minimum compressive stress under compressive forces (mainly from the cantilever platforms and the transverse post-tensioning), transverse bending moments in the connections is always larger than 0 N/mm². This requirement follows from considerations on durability and watertightness. transfer of the shear forces using a shear friction model according to NEN 6720 as below: Fig.8 Set up for the shear resistance of the connection qu,longitudinal / h = τu = kb fb + ks σn (1) between the flanges of adjacent girders where 7 qu,longitudinal: longitudinal shear force in ultimate state y = 1.1452x + 1.4784 R = 0.9326 6 τu: shear strength kb and ks: factors depending on the 5 roughness of the contact area 4 fb: tensile strength of the 3 concrete σn: design value of the 2 mean compressive stress on the contact area 1 resulting from the transverse prestress 0 force and the external 0 1 2 3 4 5 6 loads. COMPRESSIVE STRESS Values of ks = 0.4 and ka = 1.0 were NEN shear friction model assumed for calculating the transverse Test to failure (peak) prestress force using as per equation (1). Result: test not to failure These values are given by NEN 6720 for Shear friction model from test “rough made contact area”, and were Residual strength confirmed by tests in the laboratory. The Compressive and shear stresses from calculations principle of the test set-up simulating Linear (Test to failure (peak)) shearing of the connections subject to compressive forces is shown in figure 8. Fig. 9 Test results, deduced derived design shear During the test, the shear force is friction model at peak and residual resistance increased under constant compressive compared to NEN design model and calculated force until failure is reached. The results showed a peak resistance followed by a stresses “residual” strength corresponding to pure friction after rupture of the indents. The points of maximum shear resistance at failure are plotted in figure 9 against the applied normal stress. Two tests were interrupted just before failure. The corresponding compressive and shear SHEAR STRESS

2

stresses are also indicated in fig. 9. The figure shows design equation (1) according to NEN 6720 (kb = 0.4 and ks = 1.0; dashed line) as well as the best fit curves corresponding to the measured peak strength. The design curve derived from the test results by applying the partial factors on concrete strength and the reduction factor for long duration effects according to NEN 6720 is shown in full line. It appears that the equation (1) with the values of the factors ks and ka according to NEN 6720 leads to a safe estimate of the ultimate shear strength in the connection for mean compressive stresses σn above 1 N/mm². The figure also shows the design values of compressive and corresponding shear stresses obtained from the 3D model of the deck in various sections of the upper and lower connections between the outer and central girder and between the two central girders. The reduction of the mass of the deck led to a careful analysis of the response of the deck and the platforms to dynamic actions from passenger and freight trains. The dynamic response has been analysed using a 3D FE dynamic model and calculated accelerations were checked against strict criteria for the comfort of people on the platforms and noise in the station. This dynamic analysis led to determine the minimum length of plain cross section at the ends of the girders and the minimum connections between the cantilever platforms. The design had to deal not only with the final, permanent conditions, but also with temporary situations when only half of the bridge is in service (including the platforms), with platforms not yet placed at the other side. Due to the asymmetry of the loads, temporary ballasting of the “unloaded” half of the bridge was necessary to ensure the minimum required reaction on the outer bearings.

4. Conclusions Value Engineering has created a precast, pre-stressed box girder railway bridge deck as an alternative to the cast-in-situ post-tensioned deck. Transverse post-tensioning through upper and lower flanges assembles the girders into a monolithic deck. Good communication, including careful consideration of the specific architectural, design and construction constraints, was the key factor for a close cooperation between the design teams of the consultant, the contractor and the girder supplier. This led to the successful implementation of the several modifications and optimisations as the result of the alternative deck construction technique. The precast deck solution has proved to be effective in cost and construction time and allows to work in safe conditions for traffic and workmanship. During design special attention was paid to the dynamic loading of the lightweight structure and to the connection between adjacent girders.

5. References [1] [2] [3]

NEN 6720 “TGB 1990 Regulations for concrete. Structural requirements and calculation methods.” Nederlands Normalisatie-instituut, 1994 (in Dutch) NEN 6723 “Concrete Bridges. Structural requirements and calculation methods.” Nederlands Normalisatie-instituut, 1995 (in Dutch) Ontwerp Voorschriften voor Spoorwegbouw (OVS) “Ontwerpvoorschrift voor kunstwerken: Bijlage VI” Nederlandse Spoorwegen, december 2000 (in Dutch)

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