2014 Structural Engineering Traveling Fellowship
Learning from Footbridges: Improving Urban Structural Connections

In 2008, the US National Academy of Engineering published Grand Challenges for Engineering, a document that identifies fourteen challenges currently facing our society yet deemed to have attainable, sustainable solutions. Of these fourteen challenges, the one most pertinent to the field of structural engineering is a call to “restore and improve urban infrastructure.” This challenge was chosen as a direct response to the 2005 American Society of Civil Engineers (ASCE) Report Card for America’s Infrastructure, which designated an average grade of D for the condition of US infrastructure. The most recent 2017 ASCE Report Card only elevated this grade to a D+, demonstrating that sustainable structural solutions for improving infrastructure still require research and implementation. As the 2017 ASCE Report Card specifically assigned the Bridges category a grade of a C+, classified as mediocre, there is significant room for improvement. To change this rating and improve urban infrastructure, it is necessary to first define what is considered to be good bridge design.

According to the ASCE grade classifications, a designation of good earns a grade of a B, while an A is denoted as being exceptional. David P. Billington states that good bridge design exemplifies the three criteria of structural art: minimum use of materials, minimum cost, and maximum aesthetic expression. These tenets of efficiency, economy, and elegance when paired with the concepts of accessibility and usability define inspiring and exceptional bridge designs. A bridge designed to be efficient, economic, and elegant serves no improved function apart from decoration in the context of urban infrastructure if it incurs minimal use or has unsuitable access. If US bridges are classified as mediocre, then the search not only for good but also exceptional bridges propels this research pursuit to evaluate the design of footbridges in the urban environment.

Although not included in the ASCE Report Card, buildings are an integral part of urban restoration efforts. Current assessments of good building design are based on the number of sustainable solutions that can be integrated into planning and construction processes. An exceptional building design cannot simply be attained or dictated by achieving a maximum rating system designation, as these point-based ratings systems cannot inform creativity and honesty of structural expression. At the skyscraper scale, an exceptional building can contribute toward the very identity of a city by reinvigorating its skyline as a recognizable icon while still expressing its “natural strength,” as stated by Fazlur Khan. The innovative bundled tube structural system that Khan developed for the design of the Willis Tower, not only supported a standout marker of the Chicago skyline as the tallest building in the city but also created an identifiable image that even in silhouette is defined by its structurally efficient, stepped profile.

In dense urban environments, the sheer verticality of a skyscraper hides its structural expression from the street level pedestrian and only reveals itself to the viewer from afar, e.g., on the transit approach to the city or from a vantage point provided by another skyscraper. These urban settings are also defined by low-rise buildings, such as museums and libraries, which can seemingly grow out of their surroundings and yet remain unnoticed, nestled within the context of the surrounding structures. While the tendency in designing these structures is markedly playful in nature, play is successful in structural engineering only when paired with discipline. This discipline for building design is described by Italian master builder Pier Luigi Nervi as “structural correctness, which is identical with functional, technical, and economic truthfulness” and “is an indispensable condition of good aesthetic results.” This mix of discipline and play can lead to exceptional small-scale, urban buildings.

An often-overlooked intersection between building and bridge design is in the design of skybridges. Criticism of these structures refers to them as eyesores and obstacles to viewing the urban landscape, with some city inhabitants calling for their dismantlement; these enclosed structures provide an essential means of circulation, forming an extensive pedestrian network connecting miles of urban real estate. With 50% of the world’s population living in urban areas, increasing to 60% by 2030 and 70% by 2050, skybridges have received renewed attention as fundamental components of urban infrastructure. Contradictory schools of thought exist in the urban planning field regarding the aesthetics of these structures. One advocates for uniformity, intending these structures to blend in and complement the connecting buildings, while the other promotes individualism, deeming these structures be distinct in relation to both the attached building aesthetic and the appearance of other walkways in the city. As little to no discussion exists outside the field of urban planning regarding the aspects of good design and integration of skybridges within the surrounding urban fabric, these structures require investigation from a structural engineering perspective.

Open-air footbridges designed for an urban environment can be categorized according to the type of obstacle spanned, e.g., railway, motorway, or waterway; these man-made and natural obstacles can define as well as divide a city, which can impact the aesthetic and structural form selected to foster a communal desire to use a footbridge. For a motorway or railway overpass, noise mitigation may be a primary concern of the user, whereas additional railing and clearance specifications are core concerns for the designer. While the user of an overpass may seek minimal interaction with the surrounding environment, the user of a waterway crossing typically seeks the unique perspective afforded by the new vantage point over the waterway. A firsthand survey of these diverse design elements present in the spectrum of footbridges constructed in urban environments are documented in this report.

Aesthetic expression is an invaluable part of sustainable, restorative infrastructure design. While a structural engineer should strive for maximum aesthetic expression, one cannot use design equations or optimization methods to maximize aesthetic expression in the same way as one would minimize materials for efficiency and cost for economy. Aesthetic expression and successful user interaction require more than just mathematics and theory, one needs to visit footbridges to fully experience how pedestrians interact with the structural form and how these structures in turn relate to their environment.

Conclusion

As footbridges constructed in urban environments typically experience the greatest pedestrian and cyclist traffic due to the density of the surrounding population, these structures serve as accessible benchmarks of current solutions to spanning obstacles that might otherwise disrupt the connectivity of the city pedestrian and cyclist network. Accordingly, a collection of footbridges in populous cities in Italy, Spain, France, England, Scotland, and Canada were chosen as case studies for evaluating the elements of footbridges constructed in city environments that could inform exceptional bridge designs that restore and improve urban infrastructure. A firsthand survey of these footbridges was conducted by accessing these footbridges as a pedestrian and, when viable, as a cyclist to assess these metrics from the user’s perspective. An assessment of the structural and aesthetic components of these footbridges as well as their state at the time of visiting were presented in this report to provide a personal perspective on the properties of footbridges that contribute toward successful and unsuccessful integration within the surrounding urban environment. A discussion of the lessons learned through these on-site visits and evaluations is provided in this section.

Establishing the term accessible as a metric of exceptional bridge design relies on two definitions: global versus local. In the global sense, accessibility pertains to the development of a traversable design for persons with disabilities as well as the practical placement of a footbridge within an urban pedestrian and cyclist transportation network. In the local sense, accessibility pertains to the ability to directly interact with the structural and nonstructural components of a footbridge. While global accessibility is fundamental to the successful integration of a footbridge within an urban fabric, the local accessibility of individual footbridge components to the user can be perceived as the provision of a blank canvas. Assessing the damage present during the on-site visits helps define aspects of footbridges detrimental to the desired aesthetic.

In serving as tangible support for the user when traversing a bridge, railings are typically the most accessible footbridge components and, as such, were found to be the most susceptible to damage. Graffiti, stickers and sticker residue, scratches, cracked or shattered glass, and locks were found to be the most common culprits of damage to footbridge railings. While graffiti can be viewed as an aspect of local expression, the presence of graffiti on a footbridge can deter use of the crossing if it is perceived as an unsafe environment while also tainting the intended bridge aesthetic. Counteractive measures can be taken by applying anti-graffiti coatings to susceptible surfaces or installing passive measures such as security cameras. Although painting can be argued as an expensive maintenance requirement, the preservation of an aesthetic can encourage use of the bridge and prevent deterrence due to a damaged aesthetic.

While the caveat to the tangible accessibility of footbridge components is the provision of a blank canvas, the components containing extensive amounts of graffiti were typically ones in accessible, yet out-of-sight locations; e.g., abutments, piers. As a result, there is a default to less conspicuous methods of defacement, such as stickers and scratches, on well-monitored footbridge components; even options such as the placement of locks is part of a socially acceptable norm of romantic gestures and not viewed as detrimental to the structure. While locks may appear to be harmless, an accumulation can add an unexpected load to the railing and damage the components to which they are attached; in addition, locks can obscure the desired transparency of a bridge railing.

Given the likelihood of damage to footbridge railings, panelization provides easy to install and readily replaceable railing systems; although repetition can be viewed as monotonous, variability can be achieved via the material choice for the protective panel elements. Steel mesh, perforated plate, cable, and traditional horizontal rod or vertical plate systems can vary from the transparent to the opaque; while the type of obstacle spanned may dictate the height of the railing, an element of play can be achieved via patterning, orientation, and form. While glass provides the greatest transparency and gives the illusion of slenderness by isolating the railing component from the bridge deck, it can yield the greatest maintenance burden. In terms of efficiency, integrating the railing with the superstructure not only yields a cohesive aesthetic but also can lead to a reduction in superstructure depth to achieve a slender footbridge form.

As a pedestrian, typically one can only comprehend the global structural form of a footbridge when viewed from an adjacent bridge or sidewalk. When the structural system disappears into the urban fabric, the tangible aesthetic is the railing, which can add character to an otherwise unseen urban infrastructure. As skybridges provide an aboveground pedestrian network, their impact on the urban aesthetic is unavoidable. The logistical necessity of sheltering the pedestrian from the environment should not adversely affect the user experience within the skybridge by imposing a feeling of enclosure. As glass is the default material for defining the environmental barriers, the choice of color, flooring, and structural system can contribute to an open internal atmosphere. The integration of barrier and structure can reduce the opacity caused by the juxtaposition of mullions and structural elements. Indoor pedestrian comfort prescribes the use of heating, ventilation, and air conditioning systems to create a space usable year-round; these systems can either remain exposed or contained within a dropped ceiling, which may augment the sense of enclosure. Whether skybridges should integrate into the surrounding urban aesthetic imposed by the buildings to which they connect or conspicuously stand out, a sense of play can be established in the two environments they define, external and internal, via structural form, materiality, and color.