300m Span FRP Footbridges 2014

Footbridge 2014 5th International Conference Footbridges: Past, present & future

FIBRE REINFORCED POLYMER FOOTBRIDGES SPANNING 300m

David KENDALL Structural Engineer Optima Projects Ltd, Lymington, UK [email protected]

Summary

Fibre Reinforced Polymer (FRP) Composites have been used in the design of 3 long-span footbridge concepts with single clear spans of 300m. These long spans have been achieved without the need for masts and cable supports, made possible by the use of ultra-lightweight carbon fibre composites dramatically cutting the dead load on the structure and improving dynamic performance.

The feasibility of the proposed structures is demonstrated through static and dynamic finite element analysis. Three different geometric designs are presented and it is shown that through geometric development and optimisation the structural efficiency of the bridge can be improved, increasing stiffness and dynamic properties with the same material content and overall weight.

It is envisaged that such concepts will bring significant advantages and be cost-effective in certain locations where it is impractical or expensive to provide intermediate supports. This could include river crossings, where piers in the river are very expensive and time-consuming to construct or over infrastructure, for example over major railways or roads, where finding space on the ground for intermediate supports is impractical or disruptive. Modular, off-site construction will also bring advantages in time and construction costs, minimising the amount of work carried out on site.

The use of FRP composites opens up new and exciting aesthetic possibilities, which have yet to be fully explored.

Keywords: aesthetics; carbon fibre; dynamics; structural concepts; new materials; fibre reinforced polymer composites; FRP; lightweight footbridges; long-span footbridges.

1. Introduction

Footbridges have generally been built from the same small selection of materials for the last 100 years most often steel or concrete, with occasional uses of aluminium or timber. Fibre Reinforced Polymer (FRP) Composites have been used in aerospace and marine applications for over 50 years and are now beginning to be used in bridge structures around the world. Initial developments have been in relatively small footbridges and have demonstrated the feasibility of using such materials, which are now being applied to larger footbridges.

Long-span footbridges, in excess of 200m clear span have most often been constructed using tall masts and steel cable supports, which can create significant maintenance liabilities through the life of the bridge. Using significantly lighter construction materials such as carbon fibre composites it is feasible to create much larger spans without the need for masts or cables.

Bridge owners are beginning to appreciate the advantages that FRP composites provide, especially reduced maintenance and through-life cost. Designers, Architects and Engineers are finding considerable technical benefits from these materials, enabling new and original designs to be developed. Contractors will see advantages in build times with off-site construction and reduced foundation and installation costs due to the dramatic weight reduction.

FRP composites have not yet been applied to long span bridge structures, but it is proposed that the advantages seen in shorter spans will be even more significant in long spans as weight reduction becomes more beneficial.

Footbridge 2014 5th International Conference - Footbridges: Past, present & future

2. FRP Materials

The primary material options for FRP laminates for such a structure are glass or carbon fibres in a polyester, vinylester or epoxy matrix. Most small FRP bridges built to date have been based on glass fibre reinforcements, sometimes with some local use of carbon fibre in high-load areas. As the span increases it becomes increasingly difficult to meet stiffness and deflection criteria using glass fibres and the use of carbon fibre is expected to increase in long-span bridges. For the initial stages of this study the entire bridge has been designed in carbon fibre, based on industrial grade high-strength fibres with a fibre modulus of 230 GPa, resulting in lamina properties shown in Table 1.

As stiffness is one of the driving design requirements for such structures the use of industrial grade high-modulus carbon fibres is also of interest as these can provide a 3-4 times increase in modulus and also increased structural damping. These will lead to a further weight saving, but produce lower strength laminates, especially in compression, so will need to be used with care and may not be suitable for the entire structure. Such options may be the subject of future studies to optimise these structures further.

Comparative material properties for different materials are available in [1].

3. Manufacturing options & viability

Manufacturing options are fundamentally split between pultrusion and moulding, with a wide selection of different moulding technologies possible such as contact moulding (hand layup), infusion, wet-preg and pre-preg moulding. The intention in this study was to design an aesthetically striking bridge with interesting geometric forms and it has therefore been based on a moulded solution, based on resin infusion inside temporary female moulds.

The wind turbine industry has demonstrated the ability to manufacture very large FRP structures in a single piece and is currently producing blades over 85m long. The marine industry has also built FRP boat hulls around 75m long in a single moulding, see Fig.1.

The use of pultrusion, with the benefits of automated construction and reduced labour costs is also attractive and will be suitable for some long-span bridges, but these will result in very different structural solutions and potentially many more joints due to the limited width of a single pultrusion. There is also less opportunity for laminate optimisation as the laminate will need to be uniform along the length of the pultrusion. However, it is envisaged that pultrusion could be utilised for some components of the proposed structures, for example deck panels, to reduce manufacturing costs.

Fig. 1 Mirabella V - Inside the hull mould and on sea trials

Table 1 Typical Unidirectional Carbon Fibre Lamina Properties

(High-strength fibres)

Fibre Volume Fraction Vf 55%

Tensile Strength (MPa) 1500

Compression Strength (MPa) 1000

Longitudinal Modulus (GPa) 130

Transverse Modulus (GPa) 7.5

Density (kg/m3) 1500


4. Design Criteria

Loading criteria for an FRP bridge will be the same as any other footbridge and may be calculated from existing codes.

Live loading from pedestrians has been calculated in accordance with the UK National Annex to Eurocode 1 [2 & 3] based on a loaded length of 300m, as follows;

qfk = 2.0 + 120 / (300 + 10) = 2.39 kN/m2, but qfk should not be less than 2.5 kN/m2.

Therefore, design live load qfk = 2.5 kN/m2.

Allowable deflection under live loading assumed to be span/300 [4], which for 300m span = 300000/300 = 1000mm.

5. FRP long-span footbridge designs

5.1 Shallow arch bridge

The author started development work on long-span FRP bridges in 2004, initially designing a very simple single-span shallow arch structure spanning 330m as shown below, similar in overall length to the Millennium Bridge in London over the River Thames. This design is detailed in a previous paper [1] so details will not be repeated here but some previous results will be compared to later designs detailed below.

Fig. 2 Previous shallow single arch design

Previous studies showed the original single shallow arch to be feasible, but it was considered that there was considerable potential for geometric optimisation of the structure and improvement in the aesthetic design of the bridge.

5.2 Flared arch bridge (FAB)

This is a development of the shallow single arch design, with the width of the structure increased and split into two sections towards the ends of the bridge to increase lateral stiffness. The depth of the structure is also increased to improve vertical stiffness.

Fig. 3 Flared Arch Bridge visualisation


Fig. 4 Flared Arch Bridge plan, elevation and sections

The structure flares out in plan to become very wide at the abutment to create a dramatic entrance to the bridge from the embankment and also to significantly improve the lateral stiffness to resist lateral wind loads and to minimise the risk of horizontal lateral vibration. To reduce the total deck width and amount of structure and also to add further aesthetic interest, the outer sections of the span are penetrated with three large voids passing all the way through the structure to provide views of the river below. These are connected with two transverse sections of deck to maintain a structural connection and also allow pedestrians to pass from one side to the other in these outer sections of the span.

5.3 High arch bridge

The Flared Arch design has been extended into a more three-dimensional geometry as shown below, with a much higher arched structure supporting a high level central walkway. To comply with maximum gradient requirements for disabled access the central walkway cannot follow the main structural arches and may require long approach ramps, stairs &/or lifts for access. However, in locations where the abutment topography requires a high-level deck this could be very suitable and highly efficient. This has been considered initially as a structurally efficient solution, but could also be developed further into a dramatic, sculptural and aesthetic architectural design, providing a more dramatic user experience with high-level views from the bridge and greater clearance below, which may be required in some locations.


Structural analysis this has demonstrated the potential efficiency of this concept producing a much stiffer structure with a similar material content to the previous designs. This will also result in less demand on the foundations as the bending moments to be resisted by the foundations will be considerably lower and could even be designed as pinned connections, although there will be some reasonable horizontal thrust forces to be resisted due to the arch action.

Fig. 5 High Arch Bridge with high-level walkway

5.4 Structural arrangements

All three designs shown above consist of stressed skin monocoque structures of complex moulded forms. The bridges are designed from carbon fibre laminate sandwich structures with structural foam cores to provide panel stiffness and buckling resistance. An arrangement of internal FRP frames will provide additional support to the skin panels to resist local loads on deck panels from live loading or on other panels from local wind pressures.

In these preliminary studies the laminate sandwich skin thicknesses vary from as little as 1mm on internal frames and range from 3 to 10mm on most of the external shell panels. In local highly loaded areas around supports then thicknesses will be significantly increased with extra reinforcement.

Sandwich core thicknesses also vary considerably in different parts of the structures from 30mm in relatively small panels and internal frames up to 150mm thick in large deck panels. Such thick sandwich structures avoid the need for additional local stiffeners to provide panel support and stability. However, a combination of additional extra frames and stiffeners allowing much thinner cores could also be considered in a future structural optimisation to further improve the efficiency of the structures.

6. Flared Arch Bridge Structural Analysis

All three designs have been analysed using finite element analysis (FEA) and results are presented below for the Flared Arch Bridge design.

The total live load applied in the analysis is 7,752 kN (775 tonnes) on the whole bridge.

The FEA model includes the following masses (whole bridge);

FRP structure = 253 tonnes Additional mass for parapets, finishes etc = 33 tonnes Total model mass = 286 tonnes


6.1 Deflection Results

Deflection results are shown below for Dead + Superimposed Dead Loads (parapets, finishes, lighting etc) and also for Live Loads.

Fig.6 Deflection under Dead and Superimposed Dead Loads = 325mm

Fig.7 - Deflection under Live Load = 919mm as shown above (1/4 model of bridge).

6.2 Stress Results

Typical stresses even under full Dead plus Live Load are very low compared to the laminate strengths and will result in high margins of safety and exceptionally good fatigue life. This is typical for an FRP bridge, where the design is stiffness driven and could be capable of withstanding far greater loads. This high strength capacity also results in exceptional damage tolerance, for example if damage were to occur from impacts or vandalism for example.


6.3 Modal Results

Eigen-value results are shown below showing modes and frequencies of vibration. The vertical modes at 1.0 and 1.6 Hz could potentially be excited by pedestrians walking over the bridge and it is proposed that these would be controlled within acceptable limits by fitting tuned mass dampers (TMDs) inside the structure at the locations of maximum amplitude of these modes, requiring three TMDs to be fitted.

The first horizontal mode (mode 2) at 1.13 Hz could also be excited by pedestrians and if possible should be increased to 1.5 Hz [3] clause NA.2.44.7. This could probably be achieved by the selective use of some high modulus carbon fibre to increase stiffness and/or with some geometric modifications. Alternatively, the TMD fitted at mid-span to control vertical motion in Mode 1 could also be designed to control horizontal vibration. The higher horizontal mode at 2.1 Hz should not be excited by pedestrian movement as this is above the 1.5 Hz limit.

Table 2 shows the primary global modes of vibration for all three designs. This demonstrates that for a similar overall material content and mass for each bridge the stiffness and structural efficiency can be dramatically improved through geometric optimisation.

Table 2 Frequencies of vibration of different bridge designs

Mode Original design [1] Flared Arch Bridge High Arch Bridge

1st Vertical 0.7 Hz 1.03 Hz 1.03 Hz

1st Horizontal 0.66 Hz 1.13 Hz 1.6 Hz

2nd Vertical 1.1 Hz 1.6 Hz 1.6 Hz

2nd Horizontal 2.1 Hz

Torsion 1.6 Hz 2.0 Hz

Mode 1 = 1.03 Hz Vertical

Mode 2 = 1.13 Hz Horizontal

Mode 3 = 1.6 Hz Vertical higher order

Mode 4 = 2.1 Hz Horizontal higher order

Fig.8 Flared Arch Bridge Modal Results


7. Aesthetic options & visualisation

The low-level bridge geometric forms will be striking in their simplicity and flowing geometry and could compliment other adjacent iconic structures or surrounding buildings, old or new, rather than trying to fight against them for attention.

Fig.9 View over the Flared Arch Bridge

The choice of finishes and colours will dramatically change the aesthetic appeal of such a bridge and these would need to be selected in accordance with the site location and surrounding architecture.

Although this would initially appear to be a very futuristic design, with the selection of traditional colours and finishes the bridge could be designed to have minimal visual impact on its surroundings, which may be desirable in some locations. In other settings more vibrant colours and finishes could be utilised to create a modern and very unique design.

The High-Arch Bridge design could impose a very dramatic statement and exciting user experience with dramatic vistas over the surrounding area below.

The flared Arch Bridge has been subject to further development, some of which is presented below. This shows a slightly deeper structure to improve structural efficiency further and shows some concepts on how a slightly higher walkway could be arranged to allow an embankment path to pass through the structure and allow a variety of entry and egress routes to and from the deck. This design has also been developed with flatter faced and sharp-edged geometry to demonstrate how the concept can be tweaked to change the aesthetic to suit different site contexts.

Fig.10 Further developments of the Flared Arch Bridge

8. Cost analysis

Preliminary cost estimates have been produced. It is predicted that the bridge superstructure will be more expensive than a more conventional three-span superstructure, but considerable savings will be made from not requiring piers in the river due to achieving a single clear span. More efficient offsite manufacture and quicker installation minimising the amount of work onsite will provide additional savings. Overall it is predicted that such a bridge could be constructed and installed within the cost of similar landmark bridges such as the Millennium or Hungerford Bridges in London. Once through-life costs are considered there are expected to be significant savings in adopting an FRP composite solution.


9. Conclusions

FRP composite materials can be a cost effective solution for manufacturing very large footbridges and will provide a significant weight saving over conventional materials such as steel or concrete, better durability and architectural freedom to produce some unique designs. It has also been demonstrated that very long clear spans can be achieved, removing the need for intermediate supporting piers, minimising the impact on land or infrastructure below the bridge and reducing construction costs and time on site.

The footbridges outlined in this study are sufficient to clear the River Thames in London in a single span, but could also be used over many other major rivers or to clear major road or railway interchanges or other areas of infrastructure, where it is either impractical or very expensive to provide intermediate supports. Most existing river crossings of this span will include at least two intermediate piers in the river, sometimes many more, but these will form a significant part of the overall cost, are difficult to construct and result in an additional hazard to river traffic and generally will have to be designed to withstand significant impact loading in areas where vessels such as ferries or barges may pass below the bridge.

It is therefore considered very desirable to avoid any structure in the river and to produce a clear span if possible. Most footbridges of such significant span have been based on suspension or cable-stayed designs, generally with steel masts and cables. Such structures can be very aesthetically pleasing and dramatic in their design, but will also be difficult and expensive to maintain over a typical bridge design life of 120 years. In busy city locations where several bridges are required within sight of each other, several different cable-stayed bridges can produce a rather busy and cluttered landscape, especially where there are also significant buildings adjacent to the site. In such situations a sleeker, more elegant and low-profile design may be desirable, made possible by the use of lightweight FRP composites.

It is intended that the structure will be moulded in very large sections by resin infusion to minimise the number of parts and joints. The outer shell, internal frames and deck will be assembled offsite into large sections, complete with finishes and parapets etc. These sections can be floated to the site and placed on temporary jack-up platforms at the side of the river, where they will be assembled using laminated and bonded joints. Alternatively, if there were sufficient space ashore adjacent to the site, then the bridge could be manufactured and assembled ashore in a temporary factory, avoiding any transportation requirements. The whole bridge will then be lifted into position using two mobile cranes, minimising work on site, reducing installation cost and risk. As most cost escalations and accidents occur during site work this will have both financial and health & safety benefits.

10. Acknowledgements

The Author wishes to thank the following for their contributions to this work;

Keith Piggott, Ramboll for finite element analysis

Jarrod Watson, Realucs for visualisations

Grimshaw Architects for visualisations in Fig.10

11. References

[1] KENDALL D, Large Span FRP Composite Bridges Bridge Engineering Conference, Rotterdam, June 2006.

[2] BS EN 1991-2:2003, Eurocode 1 Traffic Loads on Bridges.

[3] NA to BS EN 1991-2:2003, UK National Annex to Eurocode 1 Traffic Loads on Bridges.

[4] HIGHWAYS AGENCY DESIGN MANUAL FOR RAODS AND BRIDGES BD90/05, Design of FRP Bridges and Highway Structures.

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300m Span FRP Footbridges 2014