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Decisive for Ibe award decision was hence not only Ibe offered price but even to a higher degree Ibe form and functionality of Ibe structure. Often conflicting aspects of architectural demands and effective construction costs had to be combined to one optimum overall concept to be successful in Ibe bidding process. The particular wish of Ibe population not to sacrifice design and environment to infra-structural needs of Ibe bridge was taken into account. Awarding of design and execution of a construction project in Ibe framework of a de-sign and build process to one and Ibe same contractor allows an optimum solution refiectiog inpidual public interests, and incorporates Ibe construction companies' know-how about construction execution early on in Ibe project. The bidding companies, designers and archi-tects are given Ibe opportunity to concentrate not on Ibe price but on an idea competition in view of highest functiona1ity, economic efficiency and sustainability. Design and built means that the client is not the interface between designer and contrac-tor of the construction services with all the well-known risks in tenDs of time and finances. The awarding procedure gives the client the opportunity to hand over the overall respon-sibility for design. cost calculation and construction execution to the contractor and provides him, alongside highest possible cost and schedule reliability, with a structure tailored to his wishes.
3. Design of Load-Bearing Structure In close dialogue with the design and build team, architect Paul Wmtermans created the design, which plans a slender, :flowing load-bearing structure :intended to integrate itself mod-estly into the flat Dutch landscape. 'Monuments' that highlight themselves and relegate their surroundings to the background were quickly dismissed. --qs 2 52$? $ r:= , Figure 1: View of the structure ' I'~ ... ,- I ] J I ~ 0...-. -' . ~ , , , ~.J~~ _____ 0 __________________________ __ _ _ Figure 2: 2 Ground plan of the structure I~ = -:-- .. .:~ -- -- ---l _"'o .. A truss-arch above the river followed almost conclusively. The arch's milrimum. span width was set at 150m in the project requ:irements. To reduce: the construction height the arch's form was designed as continuous system. over the 75m wide neighboming spans. The result was a rise of the arch of 14.5m, only amounting to I: 10.3 in relation to the width of the main span. For design reasons, the outside surfaces and the bottom. views of the main girders are in-clined by 10 degrees and 6.5 degrees respectively. Architectural aspects led to a 10 degree in-clination of the truss-arches to the inside as even continuation of the outer webs' inclination. Keeping the regular clearances for railway traffic compelled to a continuous widening of the truss's lower chords as the arch height rises because the inside edge of these hollow box girders, just as the ballast borders and cable ducts, have to run with a constant distance to the track. axis. For architectural reasons there are no connections or cross beams between the arches. Maximum t:ranspa.rency in the bridge's front view required a minimisation of the mun-her of diagonals in the truss arch. Structural considerations and the fact that further reducing the diagonals would have been uneconomic due to the significantly increased need of struc-tural steel, limited the node distances to 30m. The foreland areas had to be spanned by a main load-bearing structure as slender as possible with a constant construction height. The structural height of the bridge's main girder was very limited anyway by the alignment of the railway line's gradient and the clearance between the Ijssel and the road at Gelderse Dijk. As lower longitudinal girders arranged below the track: installations would have entailed ridiculously short distances between bearings, the main girders were arranged on the sides of the track. In consideration of design prescriptions, 2.6Om construction height was determined for the main girder,lowered at some points to 1.95m above the road at Gelderse Dijk. The decision. influenced by architecture and construction operation" for uniform materi-als, and the implementation ofthc main load-bearing structure in steel, led to the choice of regular span widths of 40m and shorter edge spans for the foreland areas. The slenderness of the longitwlinal girders is with 1: 15.4 a technically and economically advantageous value for a railway bridge, the span length of 40m is beneficial for fabrication and assembly. Another design featme was not to integrate the foot and cycle path into the mam cross section of the bridge which would have made it more massive. Attached on the side of the railway superstructure it is intended to be carried by the later as optically independent cross-ing structure. The flat 1atcral views of the longitudinal girders at railing height as well as the lower edge of the foot and cycle path were inclined according to the design of the mam cross sec-tion. In the design. the substructures were V-shapcd piles optically inconspicuous underneath the load-bearing structure and adapting to the surrounding with their concrete colour, high-lighting the red superstructure as the main load-bearing structure crossing the river. In the design of the load-bearing structure and the bearings system, aspects of the load-bearing structure itself and the track superstructure had to be weight up against each other. To minimise maintenance works on the load-bearing structure and the superstructure, the decision's premise was to keep the number of bearings and joint as small as possible. Accord:ingly a load-bearing structure continuous over the whole length of927m without joints was designed Expansion joints were restricted to the superstructure's end. Decisive criteria were also that around 100m from the bridge's end in Hattem. switches had to be in-stalled on the superstructure and that the bridge was in a bend in some areas. With the present design prescriptions and regarding the length of the bridge, a complete elimination ofrail expansion joints was not feasible. However, it was possible to limit them to one expansion joint in the radial 1rack at the abutment Zwolle. On the side of Hattcm the superstructure together with the cycle path construction was connected to the abutment because of high horizontal bearing forces, and as well as the expansion joint, a difficult and necessarily replaceable bridge bearing was eliminated. High-quality rail expansion joints with large elongation at the end of long bridges are not to be assessed disadvantageously in view of serviceability, maintenance and repair com-pared to a high number of small transitions. On the contrary standard transitions of simpler design show weaknesses of unguided areas of the rail entailing excessive wear, higher main-tenance works and more noise. The shear-resistant connection of the cycle path on the side, acting in conjunction with the whole cross section, meant that high yet manageable stresses on the bridge consoles were to be expected. A regular distribution of joints was however dismissed because of the struc-tural di:fficulty, susceptibility to damage and maintenance efforts as well as the architcctura11y desired even and undisturbed visible smfaccs.