mb concrete footbridges july2012

An overview of design, engineering and case studies

Concrete Footbridges

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IntroductionFootbridges offer design freedom and opportunity for innovation for architects and engineers as they typically have light load-bearing requirements and are small in scale.

A footbridge can be designed with in-situ or precast concrete and can utilise lightweight concrete and ultra high strength concrete. Concrete has a wide range of visual finishes and as a material is uniquely mouldable, being able to deliver curved and elegant profiles. In addition concrete can produce structures that are robust, cost-effective, easily maintained and that contribute to a sustainable built environment.


About this publicationThis publication is intended to be used by all members of the design and construction team in the initial stages of a footbridge project. It includes details on the main stages of footbridge design and uses case study examples to illustrate these points. This publication is intended to be read in conjunction with the dedicated website www.cbdg.org.uk which features many more case study examples to aid specification and design.

Front cover: The Juliana Bridge is situated in a wide bend of the river at Zaan in the Netherlands. No set of piers is equal, with their width increasing incrementally towards the deck pivot. The pier underneath the deck pivot is the highest deepest and widest of them all and the entire moving mechanism is hidden inside this pier. Photo courtesy of Royal Haskoning

This page: This 700m long multi-span stress ribbon footbridge was intended to provide an attractive and practical connection between two seaside towns. The deck was designed with 4m long precast deck units post-tensioned together, increasing the stiffness of the structure and intended to provide durability in the harsh marine environment. Image and design courtesy of Flint & Neill.

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DesignAesthetics and practicability are important considerations when designing concrete footbridges. However, because of their potential slenderness, designers must be aware of issues such as wind, vibration and the effect of collision loads. In particular, if long slender spans are used a balance between the number of supports and protection against impact needs to be achieved.

Kent Messenger Bridge, River Medway, Maidstone. Winner of a 2002 RIBA Award, the bridge is a ‘cranked’ stressed ribbon bridge. Architect: Studio Bednarski. Engineer: Strasky Husty

& Partners with Flint & Neill. Photo courtesy of Flint & Neill.

Pedestrian footbridges over busy roads or other obstacles give a safe passage for various types of user. Consideration must also be given to the needs of disabled users, pushchairs or cyclists, and parapet requirements have a large influence on safety, function and appearance. Footbridges which also form part of bridleways have special requirements, specified by the British Horse Society [1].

These considerations are explored in this section as well as the engineering and case study sections that follow.

Appearance

Compared with most types of bridge, footbridges offer greater flexibility for layout and form. Since footbridges are used at a slow pace by pedestrians, quality of detail and surface texture are important. In some cases, footbridges are a visual improvement on motorways or other locations where they contrast with adjacent low-key structures or surroundings.

Layout and headroom

Footbridges and approach ramps should be on the desired line so that

detours and short-cuts are discouraged. To reduce bridge length, square

spans are generally preferred: these also offer the possibility of limiting

intermediate supports near to running traffic. Visibility for drivers passing

under the bridge is then improved and the risk of column impact is reduced.

Details on clearances are given in Highways Agency Standard

TD 27/05[2]. The minimum vertical clearance over highways is 5.7m, so

the deck only needs to be designed to withstand nominal impact loads.

BD 29/04[3] details minimum footway widths and ramp requirements.

Width and gradient

Bridge width will depend on frequency of use and user type: the absolute minimum is 1.2m, but 2m is the desirable minimum to allow users to pass easily in opposite directions. If a bridge is to be used by pedestrians and cyclists, they should be segregated with a minimum width of 3.5m and a clear dividing line, warning pedestrians not to wander into the path of faster-moving cyclists.

To give access to all types of user, ramps are normally needed. The preferred maximum ramp gradient is 1 in 20, but space limitations may require steeper ramps, 1 in 12 being the absolute limit. Horizontal landings, 2m long, should be provided for every 3.5m increase in elevation. Stairs may provide alternative access. Riser heights are typically from 125mm to 150mm maximum, with a maximum of 20 steps between landings, which should be at least 2m wide, or 12 steps if there is no change in direction at the lower landing.

Lighting

Lighting is needed only in urban areas or where lighting is already present. Existing road lighting is often sufficient, except for covered bridges. It should be carefully integrated into the structure using recessed units, if possible.

Construction

The location of the structure and potential disruption to traffic often determine the method of construction. Supports should be built as far from the carriageway as possible, and precast deck units that can be lifted into place during a short traffic closure are frequently the preferred method of construction.

Kingsgate Bridge, Durham. This Grade I listed bridge was designed by Sir Ove Arup

himself. It was constructed over the river banks, then pivoted horizontally and

connected by a combined shear pin and expansion coupler at mid-span.

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Peace Footbridge, South Korea. Ultra high strength concrete (Ductal) is used for this footbridge. It is made up of six precast elements, each of 20m in length and 1.3m thick. This

supports a deck which is only 30mm thick. Photo courtesy of Lafarge.

EngineeringConcrete can be used to deliver the design ambitions of the project and meet the engineering constraints of loading, vibration and durability.

The use of new materials such as ultra high-strength fibre-reinforced concrete and innovative design such as ribbon bridges can be used to meet the demand for cost-effective, sustainable and aesthetic designs. Conventional in-situ and precast concrete with appropriate formwork can also achieve a flexibility of shape and finish.

Design loading and vibration

The pedestrian live loading applied to footbridges is typically 5kN/m2. For longer spans, a lower intensity may be appropriate, as described in BS EN 1991-2[4], National Annex to Eurocode 1[5] and BS PD 6688-2[6]. Structural deflection under live load should generally be limited to less than 1 in 250 of the span. Precamber under dead load should be provided to compensate some or all of this. Attention should also be given to BS EN 1991-1-7[7]. Substructures should be designed for vehicle collision loads in accordance with BS EN 1991, but these may be able to be avoided by positioning supports outside the danger zone, normally 4.5m from the carriageway. Vibration must be considered, which is covered by BS EN 1991-4.

Form and materials

Concrete bridges will use either in-situ construction or precast units. Conventional bar or prestressing strand may be used as reinforcement. The best examples of bridges are usually cast in situ, and specially created shapes can be used to improve the appearance. Soffits and ramps may be curved to give geometrically flowing solutions, and in-situ construction normally has advantages over precast construction when structurally continuous decks are needed, as site joints are not required.

Arched bridges are elegant and keep concrete in compression. Several manufacturers offer precast deck units, usually pre-tensioned beams. These beams frequently take the form of a box, tee or double-tee section, generally in rectangular and straight layouts.

Recent developments in the use of non-ferrous reinforcement have resulted in a few bridges using carbon fibre tendons.

There is also the development of ultra high strength concrete (UHSC) with compressive strengths of 170 to 230 MPa. UHSC consists of cement, sand, silica fume, admixture, water and steel fibre.

The durability properties of UHSC are those of an impermeable material with a resistance to permeability 50 times better than normal high strength concrete. Its other advantages are: no need for conventional reinforcement; resistance to aggressive environments and loading from blasts; permits the use of much thinner sections; provides complete freedom on the shape of the section; reduces the concrete volume of a structural member to one third to one half of its conventional volume; dramatically reduces the structural weight to be supported by a structure and provides both direct and indirect cost saving.

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Parapets

Parapets at least 1150mm high must be provided, with no foothold or gap more than 100mm wide. On cycle bridges, they should be 1400mm high, and if used by horses and riders should be 1800mm. They should conform to the P4 requirements in TD 19/06[8] which states that they should withstand a horizontal load of 1.4kN/m at the top. Attention should also be given to BS EN 1317[9] and BS 7818[10]. A 1500mm solid elevation parapet is required above railways. At some locations, it may be necessary to consider a full enclosure to prevent objects being dropped from the bridge onto traffic below.

Detailing

Durability of the structure is a primary objective. Bridges shorter than 60m should be designed without movement joints and bearings where possible. Deck waterproofing is compulsory and surface drainage may also be needed. The CIRIA Bridge Detailing Guide, C543[11] gives guidance for engineers and technicians engaged in the preparation and development of details for highway and accommodation bridges, subways, culverts and retaining walls. It concentrates on the detailing issues that follow conceptual and analytical design and explores basic principles, that have proved to be reliable in everyday use, in terms of durability and ease of construction, inspection, maintenance and repair. Intended for use by consultants, contractors, bridge owners and their maintaining agents, it provides advice on the function and relative merits of various details.

Case Study: Kruiswegbruggen, Hoofddorp, Netherlands This giant ‘surfboard’ crosses the Kruisweg near Schiphol Airport and spans six lanes of traffic without a central support. The span was made possible by using “finback” construction; the centre line of the deck comprises an arched concrete beam that protrudes from the surface like a fin. The soffit of the deck is shaped like a smooth hull without dilatations and the rounded edges provide the slender appearance. For more information visit www.cbdg.org.uk

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Photo and drawing reproduction courtesy of Royal Haskoning


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Case studies The case study examples contained in this publication are explored in more detail on the CBDG website, www.cbdg.org.uk. The website contains more projects than we have been able to feature here and will be updated with case studies from CBDG members as well as those submitted by visitors to the website.

Case study: Pedro Gómez Bosque, Valladolid, SpainThe Pedro Gómez Bosque spans the river Pisuerga in the city of Valladolid and set a new record for the longest hanging stressed ribbon footbridge. Designed by the architects Carlos Fernández Casado the bridge type was selected because of the two metre difference in height between the two bridge ends. The length of the bridge between the two ends of the stirrups is 100m, with a main span of 85 m. The four metre wide platform is made of prefabricated reinforced lightweight concrete.

The bridge is a very slender structure with a distinctly non-linear design but also uses rigidity and resistance to secure the deck and their heavy loads through traction. During construction the bridge was monitored to ensure that the vibrations caused by pedestrians or the wind would not affect its performance. Once completed, load tests were carried out as well as tests with pedestrians to make sure using the bridge was comfortable.

Case Study: George Avenue, Stoke on TrentA multi-span reinforced concrete trapezoidal deck supported on reinforced concrete intermediate piers and mass concrete abutments. The three main spans are 24.7m/24.7m/23.0m and there are four spans forming a spiral approach ramp with spans of 15.0 metres. This bridge also has hooped lighting columns.

Brick facing

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Photo courtesy of CITOP (Spain)

Drawing reproduction courtesy of URS

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See: www.cbdg.org.uk for further details of the case study examples shown.

Bridge at junction A74/A732 on the road from Hamilton

to Motherwell. Photo: Courtesy of Transport Scotland

Bridge over the A34 at the A4185 junction, Chilton.

Photo: Courtesy of CBDG

SummaryThis publication, the range of examples shown and the online resources at www.cbdg.co.uk are intended to provide confidence to the engineer or architect and demonstrate the ability of concrete to create footbridges that perform on the basis of appearance, contribution to the built environment and cost.

References and further reading 1. Standards and dimensions on Bridleways and Byways. British Horse Society, Kenilworth, 2010

2. TD 27/05: Cross-sections and headrooms, Design manual for roads and bridge, Volume 6, Section1, Part 2 . HMSO, 2005

3. BD 29/04: Design criteria for footbridges. Design manual for roads and bridge, Volume 2, Section 2, Part 8. HMSO, 2004

4. BS EN 1991-2:2003. Actions on structures. Traffic loads on bridges. BSI, 2003

5. National Annex to Eurocode 1. BS EN 1991-2:2003 Actions on structures. Traffic loads on bridges. BSI, 2008

6. BS PD 6688-1-1:2011. Recommendations for the design of structures to BS EN 1991-1-1. BSI, 2011

7. BS EN 1991-1-7:2006. Eurocode 1. Actions on structures. General actions. Accidental actions. BSI, 2006

8. TD 19/06: The design of highway bridge parapets. HMSO, 2006

9. BS EN 1317-1:2010. Road restraint systems. Terminology and general criteria for test methods. BSI, 2010

10. BS 7818:1995. Specification for pedestrian restraint systems in metal. BSI, 1995

11. Bridge Detailing Guide. CIRIA, 2001

� BA 41/98: The design and appearance of bridges, Design manual for roads and bridges. Volume 1 Section 3 Part 11. HMSO, 1998

� TD 9/93: Highway link design. Design manual for roads and bridges -Volume 6 Section 1. Part 1 TD 9/93 inc. Amendment No 1. February 2002. HMSO, 1993

� The appearance of bridges and other highway structures. HMSO, 1996

� TILLER, R. Concrete footbridges. Cement & Concrete Association, 1973

Bridge over the M40 at March Lane, Mollington.

Photo: Courtesy of AECOM

Bridge over River Cherwell. Photo courtesy of CBDG.

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© MPA - The Concrete Centre 2012

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mb concrete footbridges july2012

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