It is the aim of the development to achieve a safe structure that can be manufactured cost effectively by means of a resin infusion process. The design is driven by the weight target (9,500 kg) and costs. Especially the labour for manufacturing and assembly is determining for the cost. In figure 3 a few examples of structural concepts are shown.

	Fig.3 examples of structural concepts

All structural concepts can be single layer or double layer, i.e with a separate road deck. The separate load deck can be transverse, for better spreading of the load, but also longitudinal, in which the deck beams contribute to the stiffness required for the spanning function of the bridge.
A first selection was made based on the number of elements (labour cost for manufacturing and assembly), structural efficiency (material cost) and fail safety of the structure. The second concept shown in figure 3 was rejected for that reason: when the coupling layer on top of the boxes fails, there is a high risk on desintegration of the structure. The first concept in figure 3 was not efficient enough: the transverse webs do not contribute to the spanwise stiffness.
The most important decision to be taken in this stage was whether or not to apply a separate road deck. If this concept is feasible, see Concept IV, it means a large reduction in manufacturing and assembly costs. The web distance of the longitudinal beams is much smaller than for the other concepts, because it is limited by the width of the wheel print. Without a separate deck, damage on the road surface leads to damage on the principal structure, whereas when a road deck is applied, this functions as a safety buffer. Only the road deck needs to be repaired in that case, meaning replacement of elements, instead of on site repair of the main structure.
However, costs are important as well. For a concept without separate deck safety criteria must be formulated. It is necessary to take measures to protect the structure against fire and checked by a fire test. Impact tests must be performed to verify the residual structural performance of the structure. For fire the criterion is that structural integrity must be maintained, sufficient for firemen and ambulance cars to access the bridge. The impact requirement is that invisible damage caused by impact may not grow up to a level of failure, during the lifetime of the bridge.
Concept IV is analysed using Finite Element Methods to check the transverse load spreading of the concept. It is estimated that 80% of the beams contributes to the load carrying capacity, which was higher than expected, and sufficient. Based on the evaluation, it was decided to continue with a single layer (all-in-one) concept, without separate road deck, see concept IV in figure 3.

In this section, in short the most important aspects of the bridge design are discussed. The resulting structure consists of a single deck, integrating the road deck and span function into a single row of 16 beams, coupled by means of adhesion and a top and a bottom plate of 14 to 24 mm thickness, see figure 4.

The load of the wheel print causes interlaminar shear stresses in the top layer. The most critical loadcase is the combination of a wheel print load and impact damage, for example caused by a truck losing a load. To check the residual strength of the top layer, impact tests were performed on a reference specimen consisting of the toplayer, and approx. 50 mm of webs. A cylindrical weight of 50 kg was dropped on the top surface from a height of 1.3 m, see figure 5.

A damaged region of approximately 60 mm with delaminations (white area) could be seen on the lower side of the top surface for the glass polyester specimen (fig. 6). For the glass epoxy specimen, no whitening of the laminate could be seen. It is clear from the test that it cannot be expected that this kind of damage is detected from the outside. Remarkable conclusion of the test was, that the granulate top surface of the bridge deck plays an important role in the dissipation of impact energy. As it crushes at the contact with the impactor, it protects the underlying composite structure.
The impact criteria says that the damage may not grow during the lifetime of the bridge. In a three point bending set up the specimen were loaded with a cyclic load varying between 0 - 80 kN, 106 cycles. The specimen with glassfibre - polyester did not sustain this test: the damage zone (fig. 6) extended quickly under the load cycles of the traffic. The specimen with glassfibre-epoxy showed no damage growth.
This is an important aspect during manufacturing. When a top layer is laminated onto the structure in a two steps, secondary bonding imposes the risk of low ILS-strength. Out of cost consideration, initially glass polyester was preferred. However, based on this test it was decided to use an epoxy resin in the final design.

The many opening and closing cycles of the bridge impose fatigue loads on the joints of the hydraulic cylinder and hanging supports, but the main fatigue load is of course caused by the traffic. Fatigue is most critical in the dimensioning of the joints. An example of a joint is shown in figure 7 and 8. Figure shows the interface of the steel parts of the rotation point and the composite structure. This type of connection using close tolerance bolts was used at the rotation point and the locking mechanism. At the joint of the cilinder, and the hanging rods, a different type of load introduction was applied, see figure 8.

Design rules and safety factors were applied to dimension and determine the strength of the joints. The joints are considered critical and a single lap test on ultimate strength and fatigue with large bolts in a thick laminate was performed as a proof of strength.The test set up is shown in figure 9. Loads were applied including all safety factors. The beam to beam bonded joint is also considered a primary load path and this joint was also tested on ultimate strength, see figure 10.

The ultimate strength test of the bolted joint showed a margin in strength of 76%, at 397 kN. The joint failed in the steel plate. The composite had some initial failure, see figure 11, but had not failed. On fatigue however, the steel bolts failed due to the bending introduced by the eccentricity of the single lap. The composite part of the joint sustained. It showed some delaminations and ovalisation of the hole, but did not fail. The double lap joint on the lower side of the specimen did not show any damage. It is concluded that this type of joint can be applied when rotation of the bolts is prevented, i.e when the single lap is supported. This is the case in the joints of the bridge, as can be seen in figure 7. The bonded joint was not tested on fatigue, but on ultimate strength. The joint failed in the upper corner of the triangulair adhesive area and from there grows along the edge downwards, see figure 12. The beam to beam adhesive joint showed a safety margin of 20%. The specimen failed at 144 kN, whereas 120 kN was required.

A single layer concept as shown in figure 4 can be used for this type of bridge. Critical aspects are the transverse stiffness, fatigue of the load introductions and the damage tolerance of the structure after impact. These items have been succesfully demonstrated with tests. The weight of the composite structural parts is 7,780 kg.