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1. Introduction

In the past decade Lightweight Structures B.V. designed three different composite bridge structures for three different types of bridges. Underneath the resulting designs are shown and it is explained how these solutions relate to the selected manufacturing methods. Also some examples of pultruded bridge structures are shown. The characteristics of both infused and pultruded structures are compared and evaluated.

2. Composite bridge structures

Bridge structures made by hand lay-up or vacuum infusion
The three designs that are presented underneath have a main structure manufactured by vacuum infusion or hand lay-up. The projects aimed at the manufacturing of a one-off bridge structure and the main argument for the selection of these processes is the low mould costs. The designs are all vehicular bridges, either VOSB1995 Class 600 [1], i.e. designed for three axle loads of 200 kN, or VOSB1995 Class 450, i.e. three axle loads of 150 kN. The three designs (named after their location Andel, Den Dungen, Friesland) show an evolution in structural design, in which a tendency is shown towards simpler elements.

Andel
In 1995 Lightweight Structures B.V. designed its first composite bridge structure, in assignment of the Dutch Ministry of Transport, Public Works and Water Management. It is a structure for a lift bridge. Figure 1 shows the cross section of the bridge structure.

Fig. 1 bridge structure of Andel	Fig. 2 prototype section of infused deck

This bridge structure consists of two layers, the deck and main structure. The main structure is built up out of large Z-beam members, that are placed with lapped flanges back to back and face-to-face, to create closed triangular sections. These large members of 1380 mm height and a width of 505 mm are dimensioned for manufacturing by hand lay-up.
This process was selected to make the structure feasible for a large number of manufacturers in the Netherlands. At that time the number of companies that mastered vacuum infusion was very limited.
The deck consists of small rectangular box-elements with dimensions of 50 mm x 169 mm. The road deck cannot be made by hand-lay up. By using a very light precut foam core as a plug and applying saw cuts in the foam acting as resin flow channels, this structure can be made by vacuum infusion in a well controlled manner. Deck elements of one meter in width with twenty box beams were manufactured in one step.
A prototype structure was manufactured and subjected to full scale wheel load test. Figure 2 shows a sample of the infused deck. A static and dynamic test representative for the traffic loads proved the durability of the structural concept.

Den Dungen
The second bridge design, a draw bridge has also been executed in assignment of the Ministry of Transport [ref 2], [ref3]. The cross section at the rotation points is shown in figure 3.
Because of the small width to length ratio of this bridge (3.5 m / 10 m), an integrated deck and main structure was designed (single layer structure). The width of the beams is determined by the size of the wheel loads, and the height of the beams is determined by the stiffness requirement of the bridge.

Fig. 3.1 Integrated deck and main structure bridge of Den Dungen-cross section at the rotation points

Fig. 3.2 sample of a box beam

Because the number of potential manufacturers had increased since the first project, this structure is designed for vacuum infusion rather than hand lay-up.
Unlike the road deck of the Andel bridge, the manufacturer decided to manufacture the beams separately. Main reason was to reduce the risk of failure and to be able to inspect the webs after the infusion. In a second process step the beams are adhesively bonded together. To achieve a well defined outer surface the beams are made in a female mould with a flexible inner mould.

Friesland
In 2002 a third project on the design of a composite bridge was started in assignment of the province of Friesland, funded by the Ministry of Economic Affairs. This design is a fixed structure to be constructed with modular elements.

Fig. 4.1 Modular bridge	Fig. 4.2 Infusion of the omega shaped main beam

Fig. 4.3 Protoype section

From the design of the Andel bridge deck it became clear that the manufacturing of a closed cell deck structure in vacuum infusion is possible, but quite elaborate. In the past years, more and more pultruded road deck profiles have become commercially available. This design therefore combines a pultruded deck with an infused main structure.
Furthermore, in the design of the bridge for Den Dungen it was seen that closed box shaped elements take a lot of effort in the positioning of the dry fibers. Inspection of the product after injection is difficult. Therefore the main structure consists of two elements: an omega-shaped beam and a flat top plate. Both elements can be made in simple moulds and can be fully inspected before assembly (see figure 4).

Examples of pultruded structures
The number of composite or hybrid steel-composite bridges is increasing worldwide. Many of these bridges make use of pultruded elements. Standard composite deck elements are now available on the market. Pultrusion is a relatively low-cost automated process for the manufacturing of prismatic profiles. The flexibility in geometry and material built-up is limited. Low cost however can be achieved, at a minimum purchase of length of profiles. The resulting structure is built up out of a large number of structural profiles. Figure 5 shows a typical example of a pultruded pedestrian bridge. The bridges shown in figure 5.2 and 5.3 are examples of vehicular bridges.

Fig. 5.1 Kolding bridge (Fiberline)	Fig. 5.2 Bond's Mill bridge (Maunsell)

Fig. 5.3 Westmill Bridge Oxfordshire (ASSET)

3. Vacuum infused versus pultruded bridge structures

Pultrusion is the most cost-effective process for the fabrication of prismatic structural profiles. It is an automated process with a good quality control.
Vacuum infusion will result in a higher price per unit weight of material than pultrusion, approximately a factor 2.5. However, the flexibility in geometry (curvatures) and size is almost unlimited. There are no restrictions in types of reinforcement. Though for reasons of structural efficiency, resin infused bridges make use of conventional structural shapes such as C-profiles or omega profiles, non-conventional and even organic shapes can be realized as part of the load bearing structure, making way for new architecture in bridges.
For vacuum infusion, in order to be competitive with pultrusion, use must be made of these advantages. In the following this is evaluated for the road deck and main structure and the composite structures are compared to conventional materials in terms of weight and cost.

Deck structure
Only for small width to length ratios the road deck and mainstructure can be integrated. For wider structures a separate road deck is necessary. To carry the load of the wheel print and spread it out over the main members, the deck consists typically of profiles with vertical webs at a spacing of max. 200mm.
The dimensioning of the deck is determined by fatigue, ILSS and buckling of the webs. The stiffness of the deck depends on the size and spacing of the supporting structure. The deck structure can be standardized for pedestrian and vehicular bridge classes and can be the same for bascule bridges as well as fixed bridges.
Considering this aspect of standardization, the main advantages of vacuum infusion being flexibility in material built up and freedom in shape, can not be exploited in the deck. The remaining advantage of vacuum infusion over pultrusion is that a larger structure can be made in one piece, meaning less joints and a faster assembly.

Comparison of composite with conventional materials
Fatigue, moisture and corrosion resistance of the composite material in combination with low weight make it a competitive material with conventional deck materials. Increased life-time and reduction of maintenance cost are the most important qualities in competition with steel as well as wood. Low weight is especially important for rehabilitation of bridge structures. The deck can be as much as 10 times lighter than a concrete deck, with a typical arial weight between 90 kg/m² and 120 kg/m².

Main bridge structure
The design of the composite main structure is stiffness dominated. The main stiffness requirements are the eigenfrequency of the bridge and the comfort of the users of the bridge, expressed in a limitation of the vertical acceleration of the drivers, cyclists or pedestrians. Because composite materials have a much lower stiffness than steel, smaller spans are preferred and higher cross sections for an larger moment of inertia are required. Preferably the railings are included in the structural cross section, as is seen in some cable stayed pedestrian bridges (fig. 5.1).
Size is the limit in pultrusions. A beam of a pultruded mainstructure has to be created by assembling a number of profiles (fig 5.2 and 5.3) whereas in vacuum infusion this can be produced in one step. Each joint substantially increases the cost/kg of the structure, especially when it has to be assembled on site in a less or even non-conditioned environment. Each joint introduces a risk of failure or delay in the installation, both very costly.
In the main structure more use can be made of the flexibility in shape and material built up of vacuum infusion. As the number of joints per cross section of pultruded bridge structure increases, vacuum infusion becomes more and more competitive with pultrusion. It would be interesting to investigate the break even point for smaller and larger bridge structures.
As composite structures are still under development, modularity and standardization are necessary to reduce the number of tests to proof the strength of the design. Modular mould systems with smart integrated reusable injection tooling must be developed to make production more efficient and cost- effective.

Comparison of composite with conventional, materials
A full composite structure is probably more feasible for a pedestrian bridge than for vehicular bridges, as they are usually made of wood and steel instead of concrete.
A concrete bridge is approximately 1.7 to 2 times cheaper (first cost) than a composite bridge. Low weight (10 times lower) and quick installation are the main advantages of the composite bridge structure. Low weight means easier transport and handling, but the reduction in time and cost generally does not outweigh the lower material cost for concrete.
The cost for stripping and recoating a steel structure impose additional cost for protection of the environment, closure of the road, or cost for a temporary bridge. The comparison showed that if these costs are included, composites can be an interesting alternative for steel, for example in an ecologically protected environment or in areas where road closure highly affects the economic activity.

Hybrid Glass-Carbon reinforced Composite Bridges
In the design of the bridge for Friesland the effectivity of a hybrid carbon-glass material built up was investigated for the main structure. For a balanced laminate, a maximum of 55% UD-fibers is assumed, with 15% in each of the other directions [ref. 4]. In a hybrid carbon-glass built-up, the UD-glass fibers can be replaced by one-third of the thickness in carbon fibers. The resulting typical mechanical parameters with epoxy resin and a fiber volume fraction of 50% are shown in table 1.

Table 1 Typical mechanical properties of a glass and hybrid reinforced lay-up

Laminate (Vf = 50%)	Ex [MPa]	Ey [MPa]	Gxy [MPa]	r [kg/m3]	Ex/r [m/s2]
Glass: 55% 0º,15% 45º, 15% -45º,15% 90º	25789	15943	5552	1825	14.1
Hybrid: Carbon 28% 0º, Glass 24% 45º, 24% -45º,24% 90º	41191	18502	7005	1720	23.9

For the same structural height the weight determining parameter is proportional to E/r and the hybrid structure can be approximately 1.7 times lighter. Assuming that the cost/kg of carbon fibers are 2.5 times higher than the cost/kg of glass fibers, the cost/kg of the hybrid material is approximately 1.4 times higher thann the glass built up. Additionally in relation to a full glass fiber reinforced built-up the hybrid built up will reduce the number of layers with 36%, saving labour hours in production accordingly. This evaluation shows that hybrid material can result in a cheaper structural solution.

4 Conclusions

Pultrusion is more cost-effective for deck structures than vacuum infusion. Because of the durability composite deck structures can compete with conventional deck materials such as wood and steel. Because of the low weight especially for rehabilitation of bridges composite decks are a good and economic solution.
For main bridge structures, both pultrusion and vacuum infusion are interesting. Depending on the type of bridge it must be evaluated which is best. Under certain conditions, full composite bridge structures can be cost competitive with conventional materials other than concrete, such as wood and steel. For the main structure, hybrid material can result in a cheaper structural solution

Acknowledgement