This article explores into the distinctive aspects of footbridge design, including the reduced live loads, the freedom to explore different structural forms, the importance of user experience, and the materials typically used
Footbridges, though smaller in scale than their vehicular counterparts, offer a unique set of challenges and opportunities in design. This article provides an in-depth exploration of the key elements that differentiate footbridge design from that of road and rail bridges. While the fundamental principles of structural engineering remain consistent across all bridge types, footbridges require special consideration due to the lower design loads, the slower pace at which pedestrians traverse them, and the need for a higher degree of aesthetic and functional detail.
This article explores into the distinctive aspects of footbridge design, including the reduced live loads, the freedom to explore different structural forms, the importance of user experience, and the materials typically used. It also touches on essential considerations such as access, maintenance, water management, lighting, and containment.
The Distinctive Characteristics of Footbridge Design
Footbridges stand apart from road and rail bridges primarily due to the significantly lower live loads they are required to bear. According to Eurocode 1, footbridges are designed to accommodate a characteristic crowd loading of 5kN/m². However, this load can be adjusted downward in scenarios where high-density crowds are not anticipated. In contrast, Load Model 1 for vehicle bridges includes a 9kN/m² uniformly distributed load, coupled with a 300kN tandem load, which far exceeds the loads typically encountered on a footbridge.
When there is a possibility of a vehicle accidentally accessing the bridge, an accidental vehicle load of 180kN spread over two axles must be included in the design. In the United States, design codes require consideration of a 90kN accidental vehicle load across two axles. This accidental load often governs the design of orthotropic steel deck plates, leading designers to opt for 8mm longitudinal stiffeners spaced between 700mm and 800mm to meet deflection and dynamic criteria.
The design life of bridges in the UK is standardized at 120 years, applicable to both road bridges and footbridges. While there may be opportunities to reduce this design life, any such decisions must be agreed upon with the technical approval authority (TAA) and evaluated on a project-by-project basis.
Aesthetics and Appearance in Footbridge Design
One of the defining aspects of footbridge design is the greater freedom to explore various structural forms due to the lower design loads. Footbridges are typically narrower than road bridges, and this slenderness often becomes a key aesthetic feature. However, it is crucial that the pursuit of aesthetic appeal does not come at the expense of the bridge’s functional integrity.
Slenderness is a primary aesthetic criterion for footbridges, often necessitating the use of tuned mass dampers (TMDs) to satisfy acceleration limits caused by pedestrian-induced vibrations. Some designers have achieved slenderness ratios of 1/67 by incorporating TMDs into their designs.
The user experience is another critical consideration in footbridge design. Pedestrians, unlike vehicles, move slowly across the bridge, allowing them to engage with the details more intimately. Elements such as the height of the handrail, the inclusion of a kick plate, the gradient of the surfacing (both longitudinally and transversely), and how the bridge integrates with the surrounding infrastructure need to be carefully considered early in the design process.
Essential Considerations in Footbridge Design
Footbridges offer a unique experience for pedestrians, and there are numerous ways to enhance this experience through thoughtful design. A well-designed footbridge should not only be aesthetically pleasing but also functionally sound, integrating seamlessly with its environment. The design process should begin with an evaluation of the factors that will influence the aesthetic quality of the completed structure, including its position in the landscape and the social, cultural, and heritage impacts on local communities.
At the macro scale, various ancillary details can complicate a footbridge design if not adequately addressed from the outset. Late additions such as drainage systems, anti-slip surfacing, electrical cabling, and anti-vandalism measures can disrupt an otherwise well-conceived design. This underscores the importance of a holistic, multidisciplinary approach that brings together diverse elements into a cohesive whole. With careful attention to detail, a footbridge can become a masterpiece, embodying both aesthetic appeal and functional clarity.
The geometry of the footbridge must provide an accessible walkway in accordance with relevant design standards and guidelines. In the UK, the Equality Act (2010) mandates equal access to pedestrian environments for all, including footbridges. This is further reinforced by technical design standards that specify requirements for walkway gradients (not exceeding 1 in 20), clear width (not less than 2000mm), and guidelines for shared access areas where cyclists are expected.
Inclusivity should be a central consideration in footbridge design, extending beyond the basic dimensions of the walkway. An increasing number of users present with balance disorders, which can be exacerbated by poorly designed deck geometry and detailing. While intricate walkway layouts may seem like a sensible solution to challenging site constraints, they often have the unintended consequence of making the structure difficult or intimidating to navigate.
Containment and Safety Measures
The design of the parapet is a crucial aspect of footbridge design, significantly influencing how the bridge is experienced by users. The parapet must provide adequate containment, with the minimum height measured from the lowest point adjacent to the parapet. In the UK, the minimum containment height varies depending on the type of user (1100mm for pedestrians, 1400mm for cyclists, and 1800mm for equestrians) and the environment (1800mm over railways, regardless of the user).
Redundancy is an important consideration in the design of the parapet. The connection between the superstructure and the parapet must be designed to withstand a characteristic load that is 1.25 times the capacity of the parapet, as specified in EN 1992-1. In the event of an accidental load, such as a vehicle collision, the connection should be designed to yield, allowing for the parapet to be replaced without damaging the superstructure.
Material Selection and Application in Footbridge Design
Footbridges can be constructed from a variety of materials, with steel and concrete being the most common. However, alternative materials such as fiber-reinforced polymers (FRP) and timber are gaining popularity, particularly as the focus on reducing the embodied carbon of bridge structures intensifies.
Steel
Steel is frequently used in the superstructure of footbridges due to its high yield stresses in both tension and compression. It is an ideal choice for sites where clearance envelopes are a constraint and where the designer wants to minimize structural depth. Steel is particularly suited for locations that are conducive to prefabrication and where installation time is limited, such as over railways.
Carbon steel requires painting or galvanizing to ensure a suitable lifecycle, so access and maintenance requirements must be considered during the design phase. Orthotropic steel decks are popular for lightweight footbridges, but designers must evaluate the impact on the structure’s embodied carbon.
Weathering steel offers a low-maintenance option in difficult-to-access sites but can be less effective in aggressive environments, particularly in marine settings or areas with high levels of pollutants. The protective patina that forms on weathering steel takes time to develop, and this sacrificial layer must be accounted for in the design. Proper detailing around the supports is essential to prevent staining, and all surfaces must be designed to prevent water ponding to ensure the durability of the steel.
Bimetallic corrosion is another factor that must be addressed in the design, particularly when different metals are used together. Neoprene isolation pads can be employed between dissimilar metals to prevent corrosion. This is especially important for attachments such as handrails, lighting fixtures, and cameras.
Stainless steel is a viable option for footbridges, though it is often cost-prohibitive despite its lower maintenance requirements compared to carbon steel. Stainless steel is typically used for the parapet or other ancillary components.
Concrete
Concrete is another common material used in footbridge construction. Reinforced concrete is suitable for shorter spans, though in situ construction is generally not preferred due to the extensive temporary works required, particularly when bridging over hazards such as railways. Precast concrete offers an alternative that minimizes the need for temporary works.
Reinforced concrete can also be used for the deck in combination with permanent formwork, adding mass and stiffness to the structure and improving its dynamic response. The substructure of a footbridge typically uses reinforced concrete, though steel-cased piles are also common. Given the lower loads associated with footbridges, pad foundations are often favored over pile foundations, resulting in a significant reduction in embodied carbon.
FRP and Timber
While steel and concrete remain the dominant materials in footbridge construction, the use of alternative materials such as timber and FRP should not be overlooked. Timber, available in various forms, can be used as the sole material for the superstructure or in combination with steel to achieve greater spans.
FRP is a lightweight alternative to traditional materials, offering lower embodied carbon and reducing the broader impact of the project. For example, FRP can shorten the duration of railway possessions and road closures during construction. It is most commonly used as permanent formwork for bridge decks, eliminating the need for striking formwork at height over hazards such as waterways or railways.
Dynamic Considerations in Footbridge Design
Footbridges, by their nature, are less stiff than road bridges, making them more susceptible to dynamic responses from human-induced vibrations and aerodynamic excitation. Both of these factors must be thoroughly investigated during the design process.
Pedestrian-induced vibrations can occur in both vertical and horizontal directions, with the critical frequency range differing for each. While general guidelines exist for simpler bridge typologies, deviations from these norms require further analysis. This analysis can range from a single-degree-of-freedom model to the implementation of a response spectra method, the latter requiring a finite-element model and forcing functions
Also See: Construction of Concrete Bridges – Selecting a Bridge Layout
Sources & Citations
- Bourne S. (2019) ‘An introduction to bridges for structural engineers (part 1)’, The Structural Engineer, 97 (1), pp. 13–19; https://doi. org/10.56330/UZHI9445
- Bourne S. (2019) ‘An introduction to bridges for structural engineers (part 2)’, The Structural Engineer, 97 (3), pp. 16–23; https://doi. org/10.56330/PGKZ9349
- Nugent P. (2024) ‘Designing footbridge: an introduction’, The Structural Engineer, 106 (5), pp. 14–28
