Study of the Aurora Bridge with a FRP-deck instead of

concrete.

Journal: 37th Madrid IABSE Symposium 2014

Manuscript ID: Madrid-0726-2014.R1

Theme: Innovative Design Concepts

Date Submitted by the Author: n/a

Complete List of Authors: van Ijselmuijden, Kees; Royal HaskoningDHV, Infrastructure Mikkonen, Atte; WSP Finland,

Material and Equipment: Composites, Concrete, New Materials

Type of Structure: Bridges, Lightweight Structures

Other Aspects: Innovative Structural Systems, Optimization

Kees Van IJSELMUIJDEN Senior Structural Engineer Royal HaskoningDHV Amsterdam, The Netherlands kees.van.ijselmuijden@rhdhv.com Kees van IJselmuijden, born 1972, received his civil engineering degree from the Hogeschool Amsterdam and betonverening TUDelft both in the Netherlands. He is a senior structural engineering at Royal HaskoningDHV. His speciality is the design and implementation of Fibre Reinforced Polymer in civil structures.

Atte MIKKONEN Project Manager MSc WSP Finland Ltd Helsinki, Finland Atte.Mikkonen@wspgroup.fi Atte Mikkonen, born 1972, received his Master’s degree for civil Engineering from the Helsinki University of Technology. He is a project manager in WSP working with bridge design.

Summary

Exceptional and innovative Aurora Bridge originates from a bridge design competition in Helsinki. In competition the area footpath crossed a busy city junction diagonally, where the land area for bridge structures was strictly limited. The winning entry was a curved slender concrete deck pedestrian fly-over, which is suspended by sculptural “half arch”. The design is built in 2012.

In this paper it is studied if FRP could provide a real alternative solution for the relative heavy concrete deck in this specific case with its strengths and weaknesses is studied for practical application. First the original design and execution is described followed by an alternative FRP-deck solution. Both designs results are compared together with the costs. Because of the lighter deck the dynamic behaviour is checked followed by our conclusion.

Keywords: Arch Bridge, Footbridge, Steel, Concrete, Fiber Reinforced Polymers (FRP)

1. Introduction

In 2010, the City of Helsinki arranged a design competition for a bridge to connect the last remaining parts of the Helsinki Central Park from South to North. The existing pedestrian network is equally important for refreshment and commutation, where the crossing over the Nordenskiöld Street was the last level crossing with traffic lights. The street to cross over, is a busy city cross link from East to West where also the city tram run on the street alongside the traffic.

The site is located in a built environment close to the Olympic Stadium, City Ice-hall, and to the Football stadium. The Bridge was to be a noticeable landmark frequently serving dense crowds of the nearby events. The built environment and the existing traffic network gave strict boundary conditions for the bridge location and geometry. The site has also several requirements for the traffic on the deck (accessibility, safety, ski-tracks on the bridge) as well as below the deck (collisions, free space and view requirements).

Fig. 1: Location map / plan view Fig. 2: Side view of the Bridge The city had prepared preliminary designs, which had not resulted in a satisfactory design. The

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solution was sought by a small invitational design competition. As an outcome of the competition, a unique and innovative half arch (arch-frame) bridge was built and inaugurated November 13th 2012.

2. Design Solutions in the original design

2.1 General

As the land area for the Bridge, especially at the bridge ends, was strictly limited it was governing the design. Cable stayed option would have been obvious and flexible for the site geometry, but prohibited in the competition program - the Client wanted something else. Plan geometry of the deck was designed to curve smoothly over the crossing, not influenced by below lane geometry providing open and pleasant continuity to the existing paths. A symmetrical arch was also studied in

the competition phase, but an aesthetically satisfactory solution was not found. There was not enough space for the arch structures to land down at the North side together with the chosen plan geometry. The designed deck geometry was made possible by bending down the classic arch to a supporting frame leg, immediately after the deck was curved away below. The special shape of the arch was found also aesthetically pleasant, giving a strong characteristic to the bridge. In the Aurora Bridge the slender deck is supported by hangers suspended to the

Figure 3: View for the slender suspended, cast in situ deck

slender steel box arch. This design made possible a low longitudinal slope on the deck and unobstructed views over the crossing under the bridge. The concrete deck is cast in situ, which is typical and a durable solution in Finland. Steel deck was considered less maintenance free and more expensive. The dynamic behaviour of the solid concrete deck was also found satisfactory in design. Various studies were made for different activities excitations, since the City marathon crosses the Bridge soon after the start. The geometry of the suspenders for the deck and the free height requirement for the traffic on the bridge resulted to some extra width to the structure in the inside curve. Any platform outside In the Aurora Bridge the slender deck is supported by hangers suspended to the slender steel box arch. This design made possible a low longitudinal slope on the deck and unobstructed views over the crossing under the bridge. The concrete deck is cast in situ, which is typical and a durable solution in Finland. Steel deck was considered less maintenance free and more expensive. The dynamic behaviour of the solid concrete deck was also found satisfactory in design. Various studies were made for different activities excitations, since the City marathon crosses the Bridge soon after the start. The geometry of the suspenders for the deck and the free height requirement for the traffic on the bridge resulted to some extra width to the structure in the inside curve. Any platform outside the parapets was not allowed, while the handrail was needed to define the free width with required free height. Solution was found by designing a parapet with varying inclination at the curved part. It has a frame and handrail made of steel, equipped with glass panels. When the railing has a curve in plan, in vertical and has varying inclination, the surface geometry of the glass panels does not form a regular shape. All the glass panels are different. The complicated geometry was solved by modelling all steel structures and glass panels for fabrication and by using clod bending for the glass at the site. That created a geometrical shape which is close but not exactly the theoretical one

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Fig. 5: The cross section of the deck

Fig. 6: Deformation of the structure under the permanent loading and the principle of horizontal tie forces.

Fig. 7: Temporary structures for the arch Erection

according the bridge geometries. The difference is hardly visible. Glass in the railings opened possibilities for supplementary additional lightning.

Fig. 4a and 4b: The shape of the parapet

2.2 Structural characteristics of the deck

The concrete slab is supported by tension rod hangers by approximately every six meters. The deck has roughly constant bending in transverse

direction similar as simply supported. In longitudinal direction the arch behaves similarly like a full arch with vertical hangers under non symmetric loading. Due the flexibility of the arch, the deck becomes bent also in longitudinal direction. The difference in behaviour compared to the full arch is that the vertical leg can’t provide horizontal support such like a full arch. The whole structure tends to deform in longitudinal direction. The inclined hangers provided a solution to tie the horizontal movement and reduce the bending in the vertical leg. Due the inclined hangers, the tension force is induced to the deck which by pre-stressing is able to transfer the tension forces to the abutment to which it is monolithically connected.

2.3 Construction of the bridge

The Construction works started in August 2011 and the Bridge was opened for the traffic in November 2012. The Arch erection, which required temporary supports, started in March. Temporary supports and scaffoldings were on place at the crossing area from April to September, approximately six months disturbing the traffic.

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3. The Alternative design

3.1 Introduction FRP

FRP is an upcoming material which can be used in civil structures, as well as for buildings. The benefits of the material are: light weight, low maintenance, sustainable, high specific strength, fatigue resistant, fast installation and freedom in shape. Although FRP has a lower stiffness than steel and FRP has relative higher material costs, FRP can be competitive in costs to steel, without taking into consideration the benefits of the maintenance. But can a FRP deck compare with the original concrete deck and reduce the amount of steel in the arch?

3.2 FRP deck design

In the original design, the weight of the concrete deck is the governing load for the steel structure. Replacing the concrete deck by a FRP deck will reduce the deck weight significantly. When the weight of the suspended deck is significantly smaller, the force effects to the arch are reduced. In this study, the comparison is made in two stages. First, with the original steel structure with FRP deck and then with an updated design in which the cross section of the arch is reduced.

Figure 8 Model for calculation with FRP-deck. Figure 9 Model for calculation of FRP-deck

The stresses in the FRP deck under self-weight and pedestrians are mostly less than 30 MPa. Only at the connection points it is higher. The structure is not optimized in this study, but FRP makes it possible to make this optimisations.

3.3 The FRP deck execution

In the FRP bridges the vacuum infusion technique is used. For infusion you need a mould to get the shape of the construction. To be cost effective, the mould is used several times. The bridge has different lengths between the supports; the mould is based on the biggest length and width of the bridge. Inserts can be placed at the end of the section to get the right dimensions of each section. The sections can be transported to the final destination and lifted to the (temporary) supports.

Figure 10: Picture from Netcomposites (UK) Figure 11: Picture from Boonboats (UK)

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Fig. 13: Detail connection process

Fig. 12: Installation of the deck general

The FRP deck section is able to span between the supports before they are connected together. At the cable support is a temporary support needed to connect the sections to each other. In the longitudinal direction the deck is glued together, see figure 13. In FRP adhesive connections are more used than concrete or steel. The use of adhesive between main construction parts is still not

accepted in the civil engineering world. Therefore at this time the adhesive connection must always be tested and there must always be a back-up scenario, so collapse will not occur. Of course it must be possible to repair the adhesive connection if it fails. Due to the large contact area, the stress in the adhesive connection is very low. The steel connection bracket functions as a second load path and backup. The single FRP deck section is able to bear its own- and pedestrians weight on a four pointed support at the cable connections. The FRP deck starts and ends with a vertical plate. The top and bottom plate will be rejuvenated at the end. First the 2 endplate adjacent panels are connected. Then the horizontal plate can be placed in the

rejuvenated part of the deck on the top and under the FRP deck. For the cable connection, there are several solutions. In this case we chose a steel C-profile. This profile encloses the deck at the cable connection. The steel C-profile and the FRP-deck are connected with adhesive and a bolted connection for back-up. The C-profile connection is used in the bridge “Nieuwe Houtenseweg” [4] in the Netherlands. Here the bridge was connected to the steel main construction, but then over the full length of the bridge. For the Aurora Bridge we only wanted to use this at the cable connection. We did not detail the connection in calculations in this study; we only suggested one possible solution, see figure 12 and 13. The C-profile connection can also be used at the connection with monolith connection at the abutment if this is necessary (for concrete it was). In figure 13, also a solution for the drainage of the bridge is shown. It is possible to integrate heating in the FRP pedestrian deck in the winter. In winter the top of the FRP-deck will remain 2º C, so the deck will not be covered with snow or frozen over. It has been tested for small surfaces and gave some good results.

4. Comparison of the results

The new design for the steel structure is done by maintaining the stress level at the same for the reference loads (dead load and live load) in the service limit state (eg. in the arch -120…70MPa and -240…250MPa in the leg). Minor loads are neglected in this design. Finally, the utility ratio for the axial load is checked to be equal or less than in the original design considering the increased

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slenderness of the compression members (buckling/2nd order). Based on this study approximately 40% of the structural steel (72 of180 tonnes) could be saved.

Table 1: Comparison of the main force effects for the steel structures

Original design FRP deck – Original Arch FRP deck – reduced Arch Load Normal My max Normal My max Normal My max [kN] [kNm] [kN] [kNm] [kN] [kNm] Steel structure 416 374 416 789 255 342 Superimposed dead load 481 136 450 184 451 184 Deck self weight 2264 800 409 140 411 197 Live load 657 213 654 205 658 265 SLS total 3818 1523 1929 1318 1755 988

Force effect of original design 51% 87% 46% 65% The concrete deck is much stiffer. Deflection under the live load is only 30mm compared to the deflection of 60 mm with FRP deck and the original arch design. With reduced steel cross sections the deflection increases up to 83mm. Anyway, compared to the span length 55m, the FRP deck design still fulfils the requirement of 110mm (L/500).

5. Costs

The original cost estimate was made based on the material quantities and a similar approach is used also in this study. The reduction in the weight also has an effect to the substructure design, especially to the arch foundations, and the cost influence is estimated only roughly. Table 2: Cost estimates of the original and the new design Cost estimate Original design FRP deck Substructures (piers and foundation) € 610.000,- € 457.500 (75% of original) Superstructure (pre-stressed concrete deck) Arch spans € 330.000,- (46% of

volume/52% of cost € 553.000,- (€1000,-/m2)

Approach spans € 300.000,- € 300.000,- Steel Arch € 950.000,- € 570.000,- Accessories € 290.000,- € 290.000,- General costs (for bridge structure 25%) €620.000,- € 542.625,- Related earthworks and landscaping € 580.000,- € 580.000,- Lightning € 220.000,- € 220.000,- Total € 3.900.000,- € 3.513125,- The rough cost estimate shows that the reduction in the steel compensates the higher cost of the FRP deck structure.

6. Dynamic behaviour

In the structural design of Aurora Bridge a detailed study for the pedestrian comfort was carried out. The Bridge was studied according to National guidelines for single groups of five joggers or walkers and for steady state of pedestrians according the NA to BS EN 1991-2:2003. Additionally, a study for different sizes of groups with varying speeds (run-event) were analysed by full transient method based on statistical load models and Monte-Carlo simulation. Maximum accelerations of 0.38m/s2 for groups of five and 0.61m/s2 for continuous pedestrian flow were estimated. For the run-event maximum acceleration of 0.92m/s2 was found. In this paper a steady state analysis, according the NA to BS [1] for crowds is presented to compare the different deck behaviour and the lateral lock in check is carried out according the JRC report for human induced vibrations [2]. The analyses are done for the original structure (to verify the simplified model), to the original steel arch with FRP deck and finally to the re-designed arch. The original arch structure with FRP deck is also analysed with 40mm asphalt surfacing and with a lightweight surface wear layer treatments (eg. Methyl Methacrylate (MMA) resin based screed).

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Fig.15: The first horizontal Eigen mode (0,65Hz)

Fig. 14: The studied vertical Eigen modes (1.11 Hz, 1.94Hz, 3.63Hz and 4.79Hz) of the new steel

design with FRP deck.

In the analysis the crowd load is 0.8 persons / m2, as recommended for urban routes subject to significant variation in daily use. In the dynamic analysis the FRP deck is calculated as orthotropic with different constant stiffness in perpendicular directions. The Eigen mode of the Bridge remains quite the same when the shapes are dominated by the arch. The Damping ratio 0.5% was originally used in the design (for suspended pre-stressed concrete deck). For the suspended FRP deck general design data is not available so the value 0.5% was also used. As reference for the Aberfeldy Bridge [3], vertical damping ratios 0.4…0.9 were measured with average value 0.7%. Table 3: Dynamic characteristics of the designs under a crowd loading Concrete deck FRP deck – original Arch FRP Deck – reduced Arch Damping 50% Damping Damping Eigen mode Freq.

[Hz] Acc. [m/22]

Eigen mode Freq. [Hz]

Acc. [m/22]

Eigen mode Freq. [Hz]

Acc. [m/22]

2 1.36 0.28 1 0.71 0.03 3 1.11 0.25 4 1.91 0.62 3 1.33 0.54 7 1.94 0.40 5 2.09 0.20 8 2.37 1.25 12 3.63 0.54 Lightweight surfacing 20 4.79 0.54 3 1.39 0.42 7 2.70 0.17 8 2.84 0.55

For the vertical accelerations the new design and the FRP deck was found to meet the requirements well. It must be noted that for the lightweight deck there are a few torsional Eigen mode under 5Hz which are not studied in this context. The lowest Eigen mode for the FRP deck is the first horizontal, with a frequency of 0.65Hz. This is quite close to the critical range 0.8-1.2Hz for lateral lock-in. In this bridge the main span is curved and the back spans are straight with guided bearings. The horizontal vibration mode of the main

span includes then the back spans vibrating along their axis. Thus the modal mass becomes big, and the triggering number of pedestrians (NL) is estimated for with equation (1) according the JRC Report (Eq 4-21).

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7. Remarks

People easily oppose the unknown and it is difficult to estimate viability of something, which is not generally available and without practical knowledge. In Finland there is no Bridges made of FRP yet and no experiences regarding durability in Nordic conditions. In this study the structures have been studied as they are in the original design. The approach spans are not studied in FRP, since it has been considered secondary. In real case, the design concept for the whole bridge would be considered for FRP and it would probably lead to different detailing and structural arrangements. Several countries are working on a National Guidance for FRP structures and there are some already in use, like the BÜV [5] in Germany and the CUR96 [6] in the Netherlands. In the Netherlands there is a new CUR96 for FRP structures coming this year that is in line with the Eurocode concrete and steel, but then for FRP. In Europe there is a Work Group 4 (WG4) working on a Eurocode FRP, where the draft version must be ready in June 2014. With this guidance’s the acceptance of the material will increase.

8. Conclusion

The concrete deck of the original design, cast in place, is a durable and cost effective solution compared to steel or wood. Dynamic behaviour of the concrete deck is good, but the weight increases the required amount of steel in arch. Also the cast in situ concreting required scaffoldings in the busy crossing area producing significant disturbance to the traffic. And of course, some maintenance will be later needed. In this study some design parameters are not verified (structural damping). However, based on this study the FRP is technically feasible material for a bridge deck to a bridge like Aurora Bridge. The alternative design fulfils the design criteria for deformations and has a good vibrational behaviour. Also the significant reduction in the deck weight results in savings in other parts of the design. Benefits for the construction due the light weight of the deck, can easily been seen. The whole span could be installed with one lift and the disturbance for the traffic becomes much lesser. Also the reduction in steel weight makes arch installation and lifts easier. With the experience in different countries in Europe and a Eurocode FRP coming up, new constructions will be built more and more and bigger and bigger in FRP, with the remark that the right material must be used for the right job to fulfil. [1] UK National Annex to Eurocode 1: Actions on structures – Part 2: Traffic loads on bridges,

British Standards Institution, NA to BS EN 1991-2, 2003, pp 24…31

[2] Design of lightweight footbridges for human induced vibration”, JRC Scientific and technical reports, 2009, pp .35…37

[3] CHAKRAPAN T, “Use of Fibre Reinforced Polymer Composite in Bridge Structures” Master’s Thesis for Civil Engineering in Massachusetts Institute of Technology, 2005, p 50.

[4] FiberCore “Factsheet hybride bridge”, http://www.fibercore-europe.com/images/downloads/factsheetHybride.LR.pdf, Rotterdam, The Netherlands.

[5] BÜV-Empfehlung “Tragende Kunststoffbauteile im Bauwesen [TKB], Kurfürstenstr. 129, 10785 Berlin, Germany.

[6] CUR-COMMISION C 124 “FRP IN CIVIL STRUCTURES, “Fibre Reinforced Polymers in civil structures”, CUR-recommendation 96, Gouda, 2003, Netherlands.

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Figure 2: Model for calculation of FRP-deck

Fig. 1: Side view of the Bridge

Kees Van IJSELMUIJDEN Senior Structural Engineer Royal HaskoningDHV Amsterdam, The Netherlands kees.van.ijselmuijden@rhdhv.com Kees van IJselmuijden, born 1972, received his civil engineering degree from the Hogeschool Amsterdam and betonverening TUDelft both in the Netherlands. He is a senior structural engineering at Royal HaskoningDHV. His speciality is the design and implementation of Fibre Reinforced Polymer in civil structures.

Atte MIKKONEN Project Manager MSc WSP Finland Ltd Helsinki, Finland Atte.Mikkonen@wspgroup.fi Atte Mikkonen, born 1972, received his Master’s degree for civil Engineering from the Helsinki University of Technology. He is a project manager in WSP working with bridge design.

Summary

Exceptional and innovative Aurora Bridge originates from a bridge design competition in Helsinki. In competition the area footpath crossed a busy city junction diagonally, where the land area for bridge structures was strictly limited. The winning entry was a curved slender concrete deck pedestrian fly-over, which is suspended by sculptural “half arch”. The design is built in 2012.

In this paper it is studied if FRP could provide a real alternative solution for the relative heavy concrete deck in this specific case with its strengths and weaknesses is studied for practical application. First the original design and execution is described followed by an alternative FRP-deck solution. Both designs results are compared together with the costs. Because of the lighter deck the dynamic behaviour is checked followed by our conclusion.

Keywords: Arch Bridge, Footbridge, Steel, Concrete, Fiber Reinforced Polymers (FRP)

1. Introduction

In 2010, the City of Helsinki arranged a design competition for a bridge to connect the last remaining parts of the Helsinki Central Park from South to North. The existing pedestrian network is equally important for refreshment and commutation, where the crossing over the Nordenskiöld Street was the last level crossing with traffic lights. The street to cross over, is a busy city cross link from East to West where also the city tram run on the street alongside the traffic. As an outcome of the competition, a unique and innovative half arch (arch-frame) bridge was built and inaugurated November 13th 2012.

2. The Alternative design

In the original design, the weight of the concrete deck is the governing load for the steel structure. Replacing the concrete deck by a FRP deck will reduce the deck weight significantly. When the weight of the suspended deck is significantly smaller, the force effects to the arch are reduced. In this study, the comparison is made in two stages. First, with the original steel structure with FRP deck and then with an updated design in which the cross section of the arch is reduced.

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Fig. 3: The first vertical Eigen mode (1.11

Hz) of the new steel design with FRP deck.

3. Comparison of the results

Based on this study approximately 40% of the structural steel (72 of180 tonnes) could be saved. The original cost estimate was made based on the material quantities and a similar approach is used also in this study. The reduction in the weight also has an effect to the substructure design, especially to the arch foundations, and the cost influence is estimated only roughly. Based on the rough cost estimate the reduction in the steel compensates the higher cost of the FRP deck structure.

In the structural design of Aurora Bridge a detailed study for the pedestrian comfort was carried out. Maximum accelerations of 0.38m/s2 for groups of five and 0.61m/s2 for continuous pedestrian flow were estimated. In this paper a steady state analysis, according the NA to BS [1] for crowds is presented to compare the different deck behaviour The Eigen modes of the Bridge remain quite the same, when the shapes are dominated by the arch. For the vertical accelerations the new design and the FRP deck was found to meet the requirements well.

4. Remarks

People easily oppose the unknown and it is difficult to estimate viability of something, which is not generally available and without practical knowledge. In Finland there is no Bridges made of FRP yet and no experiences regarding durability in Nordic conditions. In this study the structures have been studied as they are in the original design. The approach spans are not studied in FRP, since it has been considered secondary. Several countries are working on a National Guidance for FRP structures and there are some already in use, like the BÜV [1] in Germany and the CUR96 [2] in the Netherlands. In the Netherlands there is a new CUR96 for FRP structures coming this year that is in line with the Eurocode concrete and steel, but then for FRP. In Europe there is a Work Group 4 (WG4) working on a Eurocode FRP, where the draft version must be ready in June 2014. With this guidance’s the acceptance of the material will increase.

5. Conclusion

The concrete deck of the original design, cast in place, is a durable and cost effective solution compared to steel or wood. Dynamic behaviour of the concrete deck is good, but the weight increases the required amount of steel in arch. Based on this study, the FRP is technically feasible material for a bridge deck to a bridge like Aurora Bridge. The alternative design fulfils the design criteria for deformations and has a good vibrational behaviour. Also the significant reduction in the deck weight results in savings in other parts of the design. [1] BÜV-Empfehlung “Tragende Kunststoffbauteile im Bauwesen [TKB], Kurfürstenstr. 129,

10785 Berlin, Germany.

[2] CUR-COMMISION C 124 “FRP IN CIVIL STRUCTURES, “Fibre Reinforced Polymers in civil structures”, CUR-recommendation 96, Gouda, 2003, Netherlands.

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