22 The Structural Engineer 87 (22) 17 November 2009

Introduction

Stress ribbon bridges are an elegant form of construction found invarious countries around the world. They use the theory of acatenary transmitting loads via tension in the deck to abutmentswhich are anchored to the ground. The basic and most commonlyfound form consists of a precast concrete deck with steel tendons.This concept was first introduced by German engineer UlrichFinsterwalder (1897-1988).

Stress ribbon bridges are quick and convenient to constructgiven appropriate conditions. They can be built with minimalimpact to the surroundings and their slender form ensures avisually pleasing end result. A wide range of structures exist inEurope, USA and Japan, including multispan bridges, three wayspans and long spans up to 150m built in urban, rural andmountainous areas.

The Pai Lin Li Travel Award 2008 was awarded by theEducational Trust of the Institution to spend up to 6 weeks abroadresearching worldwide practices in the design and construction ofstress ribbon bridges. Meetings were arranged with designengineers and researchers involved in this field in Germany, CzechRepublic and Japan over a 4-week period. This paper summarisesthe results of this research and discusses reasons why this form isnot currently popular in the UK.

Superstructure

A typical stress ribbon bridge deck consists of precast concreteplanks with bearing tendons to support them during construction,and separate prestressing tendons which are tensioned to createthe final designed geometric form (Fig 1). The joints between theplanks are most often sealed with in situ concrete before stressingthe deck. Stressing the planks provides the deck with additionalstiffness and minimises cracking in the cast in place concrete.

The prestressing tendons transfer horizontal forces into theabutments and then to the ground most often using groundanchors. The tendons are encased in ducts which are generally

grouted after tensioning in order to lock in the stress and protectthem from corrosion. The largest bending in the deck and cablesoccurs at the abutments; accordingly the ends are detailedcarefully to deal with this. Since the bending in the deck is loweverywhere else, the thickness is often dictated by the minimumcover requirements for the cables. Minimising the depth of deckreduces the dead load and hence the horizontal forces at theabutments. Even the longest stress ribbon bridge with a span of147.5m has a depth of only 250mm (Yumetsuri–Bashi bridge,Japan, Fig 2).

Dynamic design is particularly important in these structures dueto the lightweight nature of the deck. It has been found in analysisand dynamic tests that modes often occur at low frequenciesbelow 2.5Hz and close together; 2Hz is best avoided as aresonant mode which occurs from walking. Although many ofthese bridges have low natural frequencies and slight vibrationscan be felt, the acceleration and displacement are carefullychecked to ensure they are below the limit for pedestriandiscomfort as specified in the relevant codes of practice, and therehave not been any reports of major post-construction remedialwork being carried out.

Substructure and ground conditions

The abutments are designed to transfer the horizontal force fromthe deck cables into the ground via ground anchors. Pedestrian,wind and temperature loads can cause large changes in thebending moments in the deck close to the abutments andaccordingly crack widths and fatigue in reinforcement must beconsidered.

The ideal ground condition for resisting large horizontal forcesfrom the ribbon is a rock base. This occurs rarely but suitablefoundations can be devised even if competent soils are only foundat some depth below the abutments.

The ground anchors are normally tensioned in two stages, thefirst set is tensioned before the deck is erected and the rest, afterthe deck is complete. If stressed in one stage only, there will be alarge out of balance force to be resisted by the abutments in thetemporary case. The soil pressure, overturning and sliding have tobe checked for the construction as well as permanent condition.

In some cases where soil conditions do not permit the use ofanchors, piles can be used. Horizontal deformations can besignificant and are considered in design. It is also possible to use acombination of anchors and drilled shafts. Battered micropiling isanother alternative which can resist the load from the ribbonbecause of its compression and tension capacity.

Paper: Pai Lin Li Travel Award 2008

Stress ribbon bridges

Roma Agrawal, MA (Oxon), MSc, DIC

WSP Cantor Seinuk

Keywords: Bridges, Stressed ribbons, United Kingdom, Czech Republic, Germany, Japan

© Roma Agrawal

1 2

1 Form of a stress ribbon bridge2 Yumetsuri-Bashi bridge, Japan (Image courtesy

of Sumitomo Mitsui Construction)

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The Structural Engineer 87 (22) 17 November 2009 23

Construction techniques

The abutments are built first with the first set of anchors tensionedto provide temporary stability. The bearing cables are then hungbetween the abutments, and precast planks can be slid along thecables or craned into their final position. The bearing tendonsgenerally support the structure during construction, and only rarelyis additional falsework used.

The prestressing cables are installed and the cables aretensioned in stages. Concrete is poured in the joints between theplanks and allowed to harden before the final tensioning is carriedout. Retarding admixtures may be used in the concrete mix toallow all the concrete to be placed before hardening occurs. Oncethe final tension has been jacked into the tendons and thedeflected shape is verified, the ducts containing the tendons aregrouted.

The tension in deck cables and abutment anchors are checkedagainst the design values, any adjustments required arecompleted. The bridge is then ready for dynamic testing and forpublic use.

In summary, some of the important aspects of stress ribbonbridges which must be considered include: – Design for all load cases both static and dynamic in construction

and final stages– Determination of a suitable sag for stiffness, vibration modes

and tension at abutments– Checking of deck displacement and acceleration for moving

loads– Calculation of tendon stresses and sequencing– Detailed construction planning, and analysis of all temporary

cases– Important details for efficient and long-term functioning of the

bridge: haunches at ends, protection of cables fromenvironment

– Shrinkage, creep and horizontal movement of abutments due totension in cables in determining sag, especially for long spanstructures

Codes and standards

There are currently no specific codes for designing stress ribbonbridges in the UK. When designed in the UK, the bridge code BS 5400 can be used to determine the static and dynamic loadcases. The design of the tendons and precast planks shouldconform to BS 5400-4 for post-tensioned structures (clauses 6.7and 7.2 respectively). Special care must be taken in ensuringdetails are designed for environmental degradation such ascorrosion from moisture.

Engineers designing stress ribbon bridges world-wide use theircountry’s bridge codes, including Eurocodes, where applicable.Research notes and papers published (see further reading, codes)

are also referred to in their design. Generally, analysis is done usingcomputer modelling with dynamic tests sometimes done on scaledprototypes or the built bridge.

Stress ribbon bridges in the UK

Historically, stress ribbon bridges have not been popular in the UK.Some possible reasons for this are: – Expensive foundations in the absence of ideal ground conditions– Problems experienced with post-tensioned structures in the

past- particularly the corrosion of tendons– Sag in the deck being restricted by DDA compliance standards

(maximum slope = 1:22) which leads to high tensile forces andexpensive substructure

– Lack of experience in the field– Public perception of bridges with a relatively large sag and

possible awareness of vibrationsIn fact, there is only one notable example of a stress ribbon

bridge built recently in the UK; the Kent Millennium MessengerBridge (Figs 3,4), which was designed by architect StudioBednarski and engineer Strasky Husty and Partners. It is the firstof its kind with a cranked alignment on plan, consisting of twospans with a total length of 101.5m. The out-of-balance forcesarising due to the kink in the deck are dealt with using a strut andtie system, the compression taken by concrete stairs and thetension by an inclined steel column.

The bridge was modelled as a non-linear structure in 3D withflexible supports. The tension from the ribbon is transferred intothe Weald clay found locally via a combination of vertical shaftsand raking tension micro-piles. It was constructed rapidly and withminimal impact on the surroundings resulting in a stunning bridgewhich complements its natural surroundings.

Stress ribbon bridges in the Czech Republic

The Czech Republic has a number of stress ribbon bridges ofdifferent forms around the country built over the last 40 years.Engineers are not only designing innovative structures, for examplea stress ribbon tunnel, but also research being carried out onanalysis, design techniques and software development is making asignificant contribution to the future applications of these bridges.

The DS-L Bridges are a group of seven structures built between1978 and 1985 which consist of precast segments post-tensionedin one, two and three spans. The Brno-Komin Bridge is anexample of a simple stress ribbon bridge which spans 78m over

3 4

3 Kent Millennium Messenger Bridge, UK4 Crank with strut tie system (Millennium

Messenger Bridge)

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24 The Structural Engineer 87 (22) 17 November 2009

the River Svartka (Fig 5) and is widely used by pedestrians andcyclists in an urban setting.

A development of the concept of the simple catenary is thearch-supported ribbon deck which is a self anchored structure.The tensioned deck, arch and compression struts form a closedforce triangle thus eliminating horizontal forces at the abutments,which in turn reduces the cost of the foundations. Two examplesof such bridges in the Czech Republic are that at Olomouc over amotorway (Fig 6) and another bridge over the river Svartka (Fig 7).The supporting arch over the river was brought to site as twoprecast concrete sections which were suspended from cables untilthe permanent joint was completed. This method was necessarydue to the steep banks and lack of space for construction plant(Fig 8). Once complete, the bridge was load tested by a number oftrucks and the design thus verified. This example demonstratesthe versatility of stress ribbon bridges for construction in tight sites.

An interesting application of the stress ribbon principle is the

‘ecoduct’ or tunnel (Figs 9,10) which was built as part of a largenetwork of motorways outside Brno. The theory is the same as aself-anchored arch but the geometry is much more complex. It is50m wide and spans 70m and a finite element programme wasused in its design. In this case, the arches were cast in situ usingformwork since the space was available. The geometry of thetunnel was dictated by the dead load of soil above it, which wasreinstated once the structure was complete.

Stress ribbon bridges in Germany

Stress ribbon structures date back to the 1950s in Germany. Theoriginal roof of the Berlin Congress Hall built in 1957 had asuspended roof formed from inclined arches. The stability of thesearches depended on tension members formed by stress ribbonsanchored to a central ring.

Since then, there have been a number of bridges and a tradefair roof constructed in Germany and research is being carried out

5 Brno-Komin Bridge, Czech Republic6 Arch supported bridge at Olomouc7 Load test on River Svartka Bridge, Czech

Republic (Courtesy of Brno Technical University)

8 Construction- installation and anchoring half preformed concrete arch (Courtesy of Brno

Technical University)

9 Ecoduct tunnel under construction, Czech Republic

10 Stress Ribbon Tunnel: plan elevation and section (Courtesy of Strasky Husty and Partners)

6

7 8

5

9

10

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The Structural Engineer 87 (22) 17 November 2009 25

on new materials and damping methods.A 13m span model stress ribbon bridge has been constructed

at the Technical University of Berlin using multi-layered carbon fibrestraps to support precast concrete planks (Figs 11,12). Theadvantages of these fibres are that their tensile capacity is 10times larger than steel, they are resistant to moisture and notaffected by fatigue. The practicality of constructing abutments toanchor the fibres is still being considered since fraying or notchingon the surface reduces their capacity. Furthermore, the model isbeing used to investigate the use of ‘intelligent’ damping systems.The balustrade posts are connected at the top with fibre tubes thatcontain air, which compresses and expands non-linearly as thebridge moves. One of the aims of the research is to take fulladvantage of the lightweight nature of carbon ribbons and producea deck with minimal weight and let the damping system control

dynamics.A multispan bridge has been built at Rostock that uses an

innovative method of controlling bending moments at the piers.The deck is supported on S690 steel ribbons with flexible pierheads in S355 steel which consist of layered steel platesconnected at the centre (Fig 13). The Rosenstein II bridge is apretensioned space truss (Fig 14): the deck is supported by twosagging cables which are stabilised by a cable of oppositecurvature and the dead load of the deck panels. This bridge doeshave perceptible vibrations but its users have come to accept thisand have nicknamed it ‘the swinging bridge’.

Another form of ribbon has been used on the Unterer GrundFootbridge (Fig 15) and the Pforzheim Bridge. The decks aresupported on wide steel plates which have studs in the case of theformer, and bolted connections in the latter. The Pforzheim Bridgehas rubber strips between the precast deck panels and betweenthe steel ribbon and concrete panels, and mesh handrails which allcontribute to the damping of the deck.

An interesting application of the stress ribbon principle is theStuttgart Trade Fair Hall roof (Figs 16,17). The suspended,asymmetric roof comprises a regular repetition of stressed trusseswith individual I-beam ribbons of S460 steel between them. Thetrusses function as strut and tie A-frames based on concrete stripfoundations and are tied back to the ground with anchors. Thestress in the ribbons and weight of its ‘green roof’ were used toresist wind uplift. During fabrication, each ribbon was formed toobtain the stress-free geometry according to its design. Theribbons were loaded with ballast on site to replicate their finalloading conditions, to limit shear between the ribbons and roofdeck and to eliminate uplift in the temporary case (Fig 18). A light

11 Model stress ribbon bridge in Berlin12 Carbon fibre ribbons strapped at ‘abutment’13 Pier of bridge at Rostock showing flexible

steel plates (Courtesy of Schlaich Bergermann und

Partners)

14 Rosenstein II Bridge, Germany15 Unterer Grund bridge with saddles at

abutments, Germany16 Stuttgart trade fair roof17 Diagram showing structural form of the trade

fair roof (Courtesy of Mayr Ludescher Partner)

11 12

13 14 15

16

17

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26 The Structural Engineer 87 (22) 17 November 2009

and iconic structure resulted which contributes to Stuttgart’seconomy.

Stress ribbon bridges in Japan

Japan has a mountainous terrain so its transport infrastructureincorporates a large number of bridges and tunnels. There are avariety of pioneering stress ribbon bridges in Japan, ranging fromthe longest single span bridge (147.5m clear span, Yumetsuri-Bashi Bridge, Fig 1) to the world’s first three-directional stressribbon bridge.

The Seishun and Seiun Bridges are described as self-anchoredcomposite truss bridges. A suspension system using the principlesof a stress ribbon was used to carry construction loading, since noformwork could be used, and later released into the decks in thefinal arrangement. There are only vertical reactions at theabutments in the final case. The Seiun bridge spans nearly 100mover a deep valley and consists of tensioned concrete upper andlower decks with steel diagonal members between them (Figs 19,20). The steel diagonals were stabilised with wire anchors duringconstruction and the structural conversion from a suspended toself-anchored bridge was done carefully in 12 stages. This createda stiff structure, the first of its kind in terms of scale and vehiclecarrying capacity.

A similar example is the Shiosai bridge (Fig 21) which has a four

span deck supported on columns based on a continuoussuspension chord. The suspension chord is a stress ribbonassembled from precast segments of varying depth tensioned withsteel tendons. The supporting columns are also precast. Thissystem is partially anchored and does transfer some horizontalforces into the ground.

The Kikkou Bridge built in 1991 is a three-directional bridge; anexceptional example of innovation in bridge engineering (Fig 22).There are three spans, with an angle of 120° between them, whichmeet in the centre over a lake without any vertical support. Thevibration modes were complex because of the numerous modeswith similar frequencies. Vibration tests were carried out to checkthat the amplitude and acceleration of the deck were belowpermissible values in the Japanese codes. An automaticmonitoring system was used throughout construction to ensurethat the central piece had the correct position and sag andtherefore correct balance at all times.

Future of stress ribbon bridges

One of the interesting concepts proposed by Prof. Strasky ofStrasky Husty and Partners is the design of a stress ribbon bridgecurved on plan. The deck would be suspended from bearingtendons situated above the deck and tensioned by a tendon atdeck level. The balance would be achieved by designing the forces

18 19

20 21

22

18 Construction of the roof showing ballast bags, Stuttgart (Courtesy of Mayr Ludescher

Partner)

19 Seiun Bridge, Japan (Image courtesy

of Sumitomo Mitsui Construction)

20 Construction sequence of Seiun Bridge(Image courtesy of Sumitomo Mitsui Construction)

21 Shiosai Bridge with abutment in foreground, Japan

22 Kikkou Bridge: three-span bridge, Japan (Image courtesy of Sumitomo Mitsui Construction)

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The Structural Engineer 87 (22) 17 November 2009 27

in the bearing tendons so that the vertical component of the forceis equal to the self weight of the ribbon and the horizontalcomponent balances the torsion moment.

Engineers are also keen to use this form for bridges carryingvehicles. The larger load creates increased sagging in the deckand solutions to this are being sought. The use of new materials,better damping methods, efficient ground anchoring systems andmore sophisticated analysis techniques will lead to longer spansand more slender decks. The stress ribbon theory will continue tobe used in novel ways, in temporary applications duringconstruction and in structures other than bridges. It is an excitingfield with a huge potential for technical advancement which canlead to a new generation of structures world-wide.

Conclusion

Stress ribbon structures have been built in numerous forms and avariety of sites around the world. Each country visited hasoutstanding examples of innovative bridges, roofs and tunnels andresearch is also being carried out to find more structurally efficientand cost effective forms.

Stress ribbon bridges are a versatile form of bridge: theadaptable form of structure is applicable to a variety ofrequirements. The slender decks are visually pleasing and have alow visual impact on surroundings giving a light aestheticimpression. Post-tensioned concrete minimises cracking andensures durability, and bearings and expansion joints are rarelyrequired, minimising maintenance and inspections.

There are also advantages in construction methods, sinceerection using precast segments does not depend on particularsite conditions and permits labour saving erection and a short timeto delivery. Using bearing tendons can eliminate the need for siteformwork and large plant, contributing to fast constructionprogrammes and preservation of the environment. They aresustainable structures since not much material is used in the deck,making them relatively easy to deconstruct and remove after theirdesign life.

There are some obstacles to overcome in order to popularisetheir use in the UK. Using such bridges at suitable sites anddesigning suitable forms will reduce the cost of the foundationsand hence the overall cost of these bridges. The slope at the endsof the bridge may need to be reduced compared to that found inEurope to comply with DDA requirements in the UK. The slightvibrations that can sometimes be felt and the visual sag may giveusers in the UK the impression of not being stable henceawareness of this form will need to be increased and the lack ofexperience in its design and construction addressed.

Stress ribbon bridges can make a significant contribution toengineering in the UK. A new form of bridge can add to thepassion and innovation of engineering in the country and helpfurther advance the infrastructure in a sustainable way. There is awide range of different topographies and soil conditions found anda number of areas which require aesthetic yet cost effectivepedestrian bridges to be built: Stress ribbon bridges could provideelegant solutions to these challenges in the UK.

Acknowledgments

Institution of Structural Engineers – Educational Trust: Pai Lin LiTravel Award 2008WSP Cantor Seinuk (UK): Stuart Alexander, John Parker, GrahamPocockCass Hayward (UK): Neil Sadler Strasky Husty & Partners (Czech Republic): Jiri Strasky, TomasDvorak, Libor Hrdina, Richard NovakTechnical University Brno (Czech Republic): Radim Necas, JanKolacek, Michala HrncirovMayr & Ludescher (Germany): Guido Ludescher, Frank Braun Berlin Technical University (Germany): Annette Boegle, AchimBleicherSchlaich Bergermann & Partners (Germany): Mike Schlaich, UweBurkhardtSumitomo Mitsui Construction Co Ltd (Japan): Naoki Nagamoto,Kenichi Saito

Bibliography

– Stress Ribbon and cable-supported pedestrian bridges, J. Strasky, ThomasTelford 2005

– Seventh report of the Committee for the two years ending July 1987,Standing committee on Structural Safety

– Bridges using high strength concrete, J. Strasky, I. Terzijski, R. Necas– Active vibration control with artificial pneumatic muscles for carbon fibre

stress ribbon bridge, M Schlaich, A. Bleicher– Carbon fibre stress ribbon bridge, M Schlaich, A. Bleicher– Stress-ribbon roof structures of the new Stuttgart Trade Fair Exhibition Halls,

G. Ludescher, F Braun, U Bachmann– Beneath sweeping canopies – The new Stuttgart Trade Fair Centre, F.

Jaegar, avedition 2007– Prestressed concrete, Sumitomo Mitsui Construction Co. Ltd.– Design and construction of composite truss bridge under suspension

structure, A. Kasuga, T. Noritsune, K. Yamazaki, M. Kuwano– Development of composite truss bridges using suspension structure, A.

Kasuga– Design and construction of double suspension structure, A. Kasuga, H.

Sakao, Y. Taira, M. Kuwano– Design and construction of a three directional Stress Ribbon Bridge, H.

Nishiki, T. Kumaoka, E. Itai– Construction of the world’s longest pedestrian stress ribbon bridge, Y.

Shiogata, S. Ito, K. Hata, Y. Izumi– Stress ribbon bridges: Worldwide practices and advantages they can bring

to the UK, R. Agrawal

Design codes:Czech Republic

CSN 73 0035: Actions on structures (1986) CSN 73 6203: Actions on bridges (1986) CSN 73 6201: Standard Specifications for Bridges (2008) CSN 73 6205: Design of Steel Bridges (1999) CSN 73 6206: Design of Concrete and Reinforced ConcreteBridges (1972) CSN 73 6207: Design of Prestressed Concrete Bridges (1993)

Germany

DIN 18000: Structural SteelDIN 1045: Reinforced ConcreteDIN-Fachbericht 101: Einwirkungen auf Brücken (Actions onBridges)DIN-Fachbericht 102: Betonbrücken (Concrete Bridges)DIN-Fachbericht 103: Stahlbrücken (Steel Bridges)DIN-Fachbericht 104: Verbundbrücken (Steel/Concrete CompositeBridges)

Japan

Japan Prestressed Concrete Engineering Association documentsSpecifications for Highway Bridge Part I to V (Japanese andEnglish)Specifications for Small Suspension Bridges (Japanese)Design Manual for Aerodynamics of Highway Bridge (Japanese)Design & Construction Standard for prestressed concrete StressRibbon Bridges (Japanese)

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