75 When the Dutch Municipality of Utrecht was confronted with a railroad expansion that would take the place of one of the key access bridges to the city, their city planners had a vision. The new bridge would be double as wide as the one it was to replace and its new capacity would be used exclusively to give place to a dedicated bus lane travelling in and out of the city. Additionally, in the best of Dutch traditions, the pedestrians and cyclists were presented with a majestic, 7m wide sidewalk cantilevered on the south side of the bridge. The designers opted for a tied arch to span the 170m. The arches supporting the 27m wide deck would be 12° inclined to the centre of the bridge and would give it a very recognizable shape. Each arch was to be connected to the deck by means of 56 almost filigrane (d 140mm), diagonal ties. The deck as such– a mixed steelconcrete construction – would stretch 20m between the arches, providing 4 lanes of traffic and would extend on an additional 7m in cantilever providing for a spacious sidewalk. The total weight of the structure would amount to 6200t: 2200t structural steel, 3200t concrete, 680t road finishing and 120t street equipment. This bridge, the Combibrug, is to be placed adjacent to the existing bridge. Contrary to the erection of most buildings the erection of a bridge generally creates a whole intrinsic set of engineering problems. This is particularly the case for an arch bridge as the structure needs to be complete before it acquires its load bearing capacity. Since the bridge spans over the extremely busy Amsterdam-Rhine Canal temporary supports are not an option as this would severely reduce the capacity of the canal. This means that the bridge has to be prefabricated 500m down site and floated on a barge to be jacked in place during a 36h blocking of the canal. Although the principal calculation was finished even before the start of the bidding, a lot of engineering was yet to be done: precambering, pre-stressing the ties, transporting the 700t prefabricated pieces on-site and floating the assembled bridge into place. Since VBSC did not have access to this principal calculation - they were to be used as a mean of verification – and some of the tasks still required an accurate model of the bridge it was decided to build a full calculation model from the ground up. The floating of the bridge was the main source of technical challenges during the erection process: since the abutments are 80m further apart than the width of the canal, it was necessary to support the bridge far away from its intended bearings during the floating. This in turn meant that the concrete deck could only be applied after the bridge was put in place. The arches simply could not carry the weight of the concrete while only being supported at ca. 1/3 of their span. After the floating the bridge would span the 170m without the concrete deck and so without its main source of lateral stiffness. As the bridge has an asymmetrical cross-section – there is a 7m sidewalk on the south side but not on the north side – this causes a 3-dimensional, warped pre-cambering due to the lack of lateral stiffness. It is evident that these challenges could only be successfully tackled with a flexible and accurate calculation model of the bridge at hand. Engineering started with making a model of the bridge including its concrete deck to determine the pre-cambering. The model was set up in ESA-Prima Win using a line-model for most of the steel structure and using 2D-plates for the concrete deck and some steel plates. Later on came the transport of the pre-constructed pieces to the assembling site. Boundary conditions like the width of locks and the heights of bridges between the factory and the site, as well as the accessibility of the barge severely limit the operable positions of the hydraulic trailers supporting the pieces. Full use was therefore made of the model to verify the stability of the cross-beams during the ro-ro operations. The slenderness of these beams even necessitated to take the internal working of the trailers and their stiffness into account, all of which was easily accommodated by ESA Prima-Win. While the bridge features an aesthetically, as well as a structurally elegant design, the absence of stiffeners and the inclination of the arches provided some challenges in handling the pieces of the arches – their weight can go up to 200t. The special corrosion protection limited the possibilities for placing lifting hooks while they had to withstand forces from widely different angles with no stiffeners close. Yet, a FEM plate model allowed for a bolted connection to be used so the removal of these hooks would not damage the corrosion protection. The next step was to prestress the hangers. The tensioning sequence could be determined through the use of a single model that rendered the absence of ties and intermediate supports. Removing elements trough the use of absences turned out to be a very versatile tool. This brings us to the centrepiece of the engineering challenges involved in the erection of the Combibrug: its floating into place. The first one is the verification of the integrity of the bridge during its transport. As mentioned earlier on, the width of the canal and the placement of the abutments make it impossible to transport the bridge on its proper bearings. A suitable lifting configuration was found that kept stresses within limits in the arches and supporting girders, however, it caused compression in the ties. While the buckling stability of some of these extremely slender ties (l > 1000) is not covered by the governing norms, the use of analysis software provided an attractive solution. Rather than heavily reinforcing the ties as to deal with negative strains, a geometric second order analysis of the structure as a whole revealed something unexpected about their extreme slenderness. It turned out that under small negative strains they buckled out in an elastic manner. This is a prime example of how an integrated approach can yield unexpected solutions. The final engineering challenge concern the towers supporting the bridge during the floating operation. Only the arches can bear the full weight of the structure and the deck can not be vertically restrained due to large deflections involved. The towers supporting the bridge are essentially free-standing and go through a series of different configurations: on top of trailers on the assembly site, while transferring to the barge and finally while jacking to reach the level of the abutments (7m in total). To make this happen in an orderly and safe manner a virtual environment is required in which all aspects of the structure can be explored: from the non-linear behaviour of the bridge up to making provisions for the effects of settlements caused by the uneven terrain under the trailers. ESA provides just that and allows the engineer to concentrate on what is really making a difference. Combibrug Utrecht
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