The Art of Designing a Pedestrian Bridge
Footbridges are enjoying a design renaissance, with increasing design sophistication and use of challenging materials creating sophisticated, beautiful structures for us all to enjoy. Davood Liaghat of Buro Happold argues that good footbridge design requires not only sound engineering principles but also the flair and aesthetics of architecture.
“To build a bridge! Is it not that one of the noblest of man's endeavours? To link terra firma with terra firma; to throw a path across a void. The ignorant say it defies nature, yet it rests upon her co-operation. … Almighty has given to the humblest bird the gift of wings with which to perform the same feat, but only to man has He given the power of Design, which is the first principle of His universe.”
Introduction
The creative process of bridge design demands images and ideas. Traditionally, the design of bridges has been the domain of the engineer. However, nowadays, at least within Europe, architects are becoming more and more interested in the visual and technical issues surrounding the designing of bridges as well. This is especially the case in the design of pedestrian bridges because of their enormous impact on the local urban and social environment. It is also true that clients are increasingly in search of landmark and iconic bridge structures. But even with the contribution of architectural ideas and vision, footbridges are still predominantly engineered structures.
Pedestrian Bridges
A footbridge allows the walking public to overcome obstacles such as a road or a river. The loading on footbridges is light compared with highway bridges and one can therefore design slender and elegant structures. Since footbridges are used by slow moving pedestrians compared with high-speed vehicles on highway bridges the interaction between the structure and the user is quite intimate. In terms of detail, the components of a footbridge should be of finer quality. The main span and ramps of a footbridge should be aligned along a route for the convenience of the public. It is important for pedestrians to have a clear view of where they are going and should not be intimidated by having to negotiate hidden corners on a footbridge. Good use of glazing on footbridges helps avoid vandalism.
Ramps are unsightly and should be avoided if at all possible. If ramps and steps are necessary they should be pathways built into the ground providing direct routes spanned by the structure. Manipulating the topography is probably the best way of minimising the impact of ramps. Ramps should ideally be at a slope of 1:20. If the location means that greater gradients are unavoidable landings should be provided at intervals suitable for negotiation by the disabled and the elderly. Structural ramps should have a simple geometry, ideally along the desire line. It is preferable to maintain the same structural section for the ramp as of the main span.
Some pedestrian bridges may also be used by cyclists and equestrians. In these cases, the barrier required to contain them, stopping these users from falling over the side of the bridge, should be higher than when the bridge is dedicated for pedestrians only. In some cases light vehicular traffic or public transport is allowed to mix with the pedestrians crossing the bridge, provided a speed restriction is maintained. These types of mixed use bridges are most common-place in mainland Europe.
Special Requirements
Within the past twenty years it has become common practice to hold design competitions as a means of promoting and procuring pedestrian bridges. The competitions are usually organised by municipalities, county councils, local authorities or organisations such as the RIBA. The purpose of these competitions is primarily to inject imagination into the design of bridges.
The footbridge should be iconic and easily identifiable. It should be able to manage many different uses. It should be simple and elegant in its design, be economical, functional, buildable and easy to maintain. The bridge must sit comfortably in its location and be subtle in form. A combination of these factors will contribute to the development of a pedestrian bridge that will make it an enduring contribution to its surrounding environment.
Bridge Types
Beam and Slab
One of the simplest forms of bridge construction is the constant depth beam. Larger spans can be achieved when steel or pre-stressed concrete is used. If the deck is made continuous over the supports shallower depths can be realised. This leads to fewer expansion joints which are a source of maintenance problems like water penetration and staining of the piers and abutments. In the design of the superstructure of a footbridge, the slenderness ratio (span/depth of beam) is generally between 20 and 30. Naturally the numbers of beams and spacing between them will affect this ratio. The beams should not be so closely spaced that access for inspection and maintenance is compromised. The way the edge of the deck beams are detailed and the how the parapet cantilever and the fascia beam are designed can affect the appearance of the bridge. By carefully controlling the fabrication process web distortions can be avoided and by making judicious use of stiffeners on webs of beams the appearance of the bridge will not be compromised.
Arch
Arched bridges can be constructed from masonry, steel, concrete or even timber. An arched bridge is made up of the arch, the deck and quite commonly supports from the arch to the deck. Arches work well because they are an efficient way of transmitting compressive forces, caused by self-weight and live loads, to the abutments, which must resist the lateral thrust of the arch. Arched bridges tend not to work well with point loads and high bending moments, so a good bridge design will use the compressive carrying capacity of an arch to its best extent. Some arches can be tied in order to avoid the need for constructing a large abutment to cope with large thrusts.
Truss
A truss is a very versatile form of structure, which can be made from timber, steel and sometimes concrete. It is made from smaller structural members than for beam bridges. It can be in three dimensions or as flat planes with bracing. Trusses should be as light and transparent as possible. Nodes should visually function as points of load transfer. Diagonal bars and ties should geometrically relate to each other. Generally structural members should intersect on their centrelines. If at all possible, secondary members should be avoided, as they tend to add complexity to the structure. The cross sections of members should be kept consistent and excessive mixing of sections should be avoided to avoid cumbersome connection details.
Suspension
These include cable stayed and suspension bridges. In pedestrian bridges, cable stayed structures are much more common because suspension structures are really more suited to long spans. Bridge cables have a definitive visual impact, which generates drama and can lead to very pleasing structures. The arrangement of the cables must be unambiguous if they are to be fully effective. In general it is preferable to have a single plane of cables. If two or more planes are used, the harp configuration provides a stronger visual impression. If a fan arrangement is chosen, then the two planes of cables should be tilted inwards so that meet at a point.
The deck and the tower of the cable stayed bridge should have the same impact as the cable configuration. The deck should appear slender and be in proportion to the tower. Cable anchorages should not be overwhelming, and should express their function of transferring the load to the stiffened parts of structure or to the ground. The tower should appear to rise from the ground or water to the top in a monolithic fashion. The advantage of the cable stayed structure is that it breaks the span of the bridge in an unobtrusive fashion which allows the deck to become slender and thus visually more light.
Finishes and Materials
The finish of the surface of the bridge and its components will affect the user in a visual and physical manner. The finish may vary from being a smooth bland surface of a concrete beam and slab to a painted steel circular hollow member of a truss. The detailing of connections should express how the joint works and the nature of the materials it is connecting. It is generally good form to use similar material for the components of the bridge for example if one were to design a truss bridge the deck and the truss members should be made from steel. Likewise piers, abutments and foundations could all be of made of concrete.
Materials which are used in the construction of footbridges are either the traditional materials such as normal steel or weathering steel, cast iron, in-situ concrete and reinforced concrete, timber, masonry or the newer materials which have found their way into bridge constrcution. These are plastics, glass, carbon fibre and aluminium. The materials of any suspended ramps or stairs should generally be similar those of the main span.
Supports and Structure
Footbridges are supported on columns and/or abutments. They suffer particular problems because they are light and are vulnerable to damage from impact by vehicles. It is a general requirement that footbridge columns within 4.5m of a highway kerb should be able to resist vehicle impact loads. Within this zone bridge columns tend to become very large in relation to the deck which is generally quite slender.
For appearance purposes it is better not have columns wider or thicker than the deck. Oval or circular columns are not only more efficient structurally than square sections but they also give a better appearance from all angles. It is therefore better to avoid the problem by keeping all support structures clear of the vehicle impact zone.
In cable stayed and suspension bridges the pier requirements can actually be used to provide a stronger visual effect. The back spans for these bridges can be used for ramps and the supports made to look less conspicuous.
Conclusions
A good design of a footbridge requires not only sound engineering principles but also the flair of engineering and architecture. The judgement of a good design is a subjective one but can be roughly summarised as follows:
Attention to form and function
Avoiding contrivance for the sake of it
Following good engineering principles
Respecting the urban environment
Paying attention to detail
Providing a pleasant and safe experience for the pedestrians using the bridge
Davood Liaghat BSc PhD MICE MIHT CEng DMS
Bridge Group Leader at Buro Happold
020 9279 9791
References
(1) “Ever After”, a novel by Graham Swift (Picador) 1992
Acknowledgements
The author wishes to thank Powell-Williams Architects for its contribution to the designs of Ponte della Musica Bridge and Kelvin Footbridge and also to Norman Foster Architects for their collaboration on the design of the Light Weight Bridge Design concept in Coalbrookdale.
Note: All bridge designs and concepts shown in this article are the property of Buro Happold Limited except where acknowledgement is given to other contributors.
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