Owing to their small scale and light load-bearing requirements, footbridges offer considerable freedom for engineering
innovation when compared with most other bridge types. This type of structure provides considerable scope
to exploit the wide range of concrete finishes, the mould ability of concrete either in in-situ or precast
form and of dense concrete, lightweight concrete or the use of ultra high strength concrete offering thin elegant
profiles. Pedestrian footbridges over busy roads or other obstacles give a safe passage for various types of user.
Practicability and aesthetics are important considerations and footbridges are less costly than other types of bridge,
so their size and form are not necessarily constrained by economics.
However, because of their slenderness, designers must be aware of issues such as wind, vibration and the effect of collision loads. In particular, long slender spans are often used, as they minimise the number of supports, which may be vulnerable to impact. 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).
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 improved
and the risk of column impact is reduced.
Details on clearances are given in Highways England CD 127(2). The minimum vertical clearance over highways is 5.7m, so the deck only needs to be designed to withstand nominal impact loads. CD 353(3) details minimum footway widths and ramp requirements.
Hamilton to Motherwell Rd, Lanarkshire
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.
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.
River Cherwell, Oxford
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), the UK NA to BS EN 1991-2(5) and PD 6688-2(6). Structure 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. Substructures should be designed for vehicle collision loads in accordance with BS EN 1991-1-7(7), but these loads may be able to be avoided by positioning supports outside the danger zone, normally 4.5m from the carriageway. Wind and pedestrian induced vibration must be considered in accordance with BS EN 1991-1-4(8), the UK NA to BS EN 1991-1-4(9), PD 6688-1-4(10), and the above footway loading standards.
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 requirements in Highways England CD 377 (11) which state that they should withstand a horizontal load of 1.4kN/m at the top. Attention should also be given to BS EN 1317(12) and BS 7818(13). 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.
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.
George Avenue, Stoke-on-Trent
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 with compressive strengths of 170 to 230 MPa being obtained using cement, sand, silica fume, silica flour, admixture, water and high-strength 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, impact and abrasion; 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 only one third to one half of its conventional volume; dramatically reduces the structural weight to be supported by a structure and can provide both direct and indirect cost saving.
Peace Bridge, Seoul
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. Also the CIRIA Bridge detailing guide, C543(14) gives guidance on details for bridges and retaining walls. It concentrates on the detailing issues that have proved to be reliable in everyday use, in terms of durability and ease of construction, inspection, maintenance and repair, and provides advice on the function and relative merits of various details.
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.
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.