Figure 1. Photograph of the Villabona-Zizurkil footbridge. The bridge connects the two urban spreads that stretch along both sides on the Oria river. After the bridge was built, seating areas, a bike path and large boardwalk were created along the riverbank. Photo: Tracy Huynh.
Design engineer: Mario Guisasola (Anta Ingeniería Civil)
Owner: Villabona and Zizurkil City Councils
Contractor: Construcciones Mariezcurrena
Built: February 2009 - July 2009
Main span: 50.4 meters (165 feet)
Total length: 53 meters (176 feet)
Structural system: Two-span continuous beam
The Villabona-Zizurkil footbridge (see Figure 1) is situated in the small town of Villabona, 13 miles from San Sebastian in the north of the Basque Country. Finished in 2009, the bridge spans across the Oria river forming a vital connection between the two urban spreads on either side . The bridge designer is Mario Guisasola, an engineer who has designed several footbridges with a straightforward design approach that produces humble, yet compelling and efficient structures.
The 53 meter long Villabona footbridge is remarkable for its simplicity, elegance and slenderness. They are achieved through an inventive form-finding methodology and an asymmetric form. Built out of weathering steel with slender stainless steel railings, the bridge is an element of beauty that blends in seamlessly in the urban landscape.
The bridge’s unusual form is the result of a structural optimization process that emanates from the bridge support conditions, the bridge loads and the resulting bridge internal forces (bending moments) (see Figure 2). The form thus provides the bridge with an elegant, almost fluent outlook, as well as ensuring an excellent structural behavior. Thanks to his innovative approach, Guisasola succeeded in creating an elegant landmark structure at a remarkably low cost..
Figure 2. The form of the bridge (top left) is found by flipping over the bending moment diagram (middle left) that is obtained based on the imposed support conditions. Each cross-section (right) is then optimized so it has sufficient capacity to resist the internal forces. Sketches by Mario Guisasola.
Mario Guisasola (Figure 3) was born in 1967 in Bilbao, Spain. He graduated from the Universidad Politécnica de Madrid as a civil engineer in 1992, and obtained his masters at École Nationale des Ponts et Chaussées, Paris, in 1993. He began to work as a civil engineer in 1993 at Tonello Ingenieurs Conseils in Savoy, France, and served as the director of the structural engineering department at Injelan Engineering between 1994 and 2003. In 2003, Mario Guisasola founded ANTA, an independent civil engineering design firm based in San Sebastián that provides design, drafting, and construction management of civil engineering projects with a focus on bridge design. He has served as visiting lecturer of mechanics and structures at the Escuela Politécnica de Donostia in San Sebastián from 2005 to 2008, and continues to serve as an occasional lecturer. He has made several appearances as a speaker at international conferences on the topic of pedestrian bridges. He served as the director of the structural department of the civil engineering firm Fulcrum between 2011 and 2013, and is currently the bridge department director in Grontmij, France.
Figure 3. Mario Guisasola, design engineer of Villabona Bridge. Photo. Tracy Huynh.
Mario Guisasola’s first bridges (around the 2000s) exhibited generally the same design approach of defining the shape of the bridge according to the bending moment diagram and the support conditions provided by the site . Since then, Mario Guisasola has developed new ways to refine this design methodology to achieve a more efficient global process. The ultimate goal is to make his true vision a reality for each bridge in terms of its smooth integration into its environment. Guisasola also expanded his design perspective from two-dimensional (longitudinal experience) to three-dimensional by incorporating variability in the bridge sections. His techniques for creating a slender appearance evolved from altering the railing of the bridge to changing the main bridge girder itself. For example, the technique called corne de vache (literally bull’s horn in English) developed by the French engineer J.R. Perronet during the 18th century inspired ANTA for the changing cross section of the bridge. Figure 4 shows an example of a bridge using the corne de vache.
Figure 4: This 1887 engraving by George J. Stodart of the Telford’s Bridge at Gloucester, England, shows the classical use of the corne de vache technique to allow for more lightness in the bridge aspect. Public domain image.
Mario Guisasola and the engineers at ANTA, Esther Azcona and Haritz Iriondo, developed a new design concept in 2012 called “Urban Bridges Predesigned”, (see Figure 5) which introduces a systematic application of innovative designs of bridges . His comprehensive design approach begins with first assessing the site that will receive the future bridge. The existing configuration of the topography determines the support conditions. Subsequently, the resulting bending moment diagram of the structure inspires the final shape of the bridge. The result is not only an elegant shape, but also a structure that efficiently provides more strength in regions that are expected to experience larger stresses. Mario Guisasola is continuously refining this design approach with every generation of bridges to improve their appearance, efficiency, constructability, and integration with the environment.
Figure 5. Models of a series of footbridges by Mario Guisasola, all employ the same initial concepts of a shape inspired by the bending moment diagram, which are affected by the boundary conditions imposed by the construction site. The use of elastically fixed connections is a recurrent theme. Source: M. Guisasola .
The structural design of the Villabona footbridge.
Despite the bridge’s minimalistic appearance, the journey to obtaining the final form was filled with structural complexity. The bridge is simply supported on one end and elastically fixed on the other (see Figure 6). These supports were determined by the boundary conditions imposed by the rivers embankments: a vertical wall on one side, and a slope on the other. This led to decision of a pinned support on the vertical wall and a fixed support on the sloped wall. The fixed support was executed through a system of two pinned supports close to one another, resisting opposite vertical reactions . This special case is often referred to as an elastically fixed support. The use of this elastically fixed support system reduces the bending moment in the unsupported span of the system, allowing for a more slender appearance that the one that can be achieved by a simply supported beam.
Figure 6. Bending moment diagrams under a uniform load due to pedestrian corresponding to the simply-supported beam (orange) and the elastically-fixed beam (blue). Source: Charpentier et al. .
To add to the bridge’s structural complexity, the cross sections vary throughout the length of the bridge (see Figure 7) with a total of 28 different cross-sections. The geometry of each section takes into account not only the optimization the structure’s efficiency, but also considers the overall aesthetic of the bridge. As mentioned above (see Figure 4), this technique is a design aspect inspired by an 18th century method called the corne de vache (cow’s horn) approach of French engineer Perronet. This technique skews the curved length of the bridge for a more slender appearance, further dissolving its visual impact on the overall landscape. The Villabona Bridge is composed of weathering (Corten) steel and finished using slender stainless steel railings and a wooden deck. The natural and unimposing materials allow the structure to blend in well with the surroundings (see Figure 8).
Figure 7. The cross-sections of the bridge change along its length creating a structurally efficient and very elegant structure. Sketches by Mario Guisasola.
Figure 8. The bridge looks very slender and matches the landscape harmoniously.
The desire to interfere minimally with the environment is particularly important in the case of Villabona, as just 150 m north lies a historic stone bridge from nearly 400 years ago (see Figure 9 and Figure 10). As Mario Guisasola comments himself: “This other bridge has been there for 400 years, who are we to hide it?”.
Figure 9. The bridge blends in perfectly into the surroundings. Remark the elegant curvature and the simplicity in the details, such as the stainless steel railings that appear to be almost transparent.
The bridge does not distract from the calm pace of life in Villabona. It curves gracefully and follows the slope of a hill in the distance. Its form allows it to subtly coexist with the historic stone bridge. Moreover, it functions as an elegant viewing platform to better appreciate the historic structure.
The Villabona Bridge is both elegant in its simplicity, intriguing with its structural complexity, and powerful in its respect to its surroundings. The result is minimalistic and modest, but the design process was a rigorous dialogue between the demands of the environmental constraints and available structural capacities. This bridge illustrates perfectly the design philosophy of ANTA and Mario Guisasola.
 Amasa-Villabona. "Amasa-Villabona: Our People". www.villabona.eu. Retrieved on November 12th, 2014.
 M. Guisasola. “Predesigned Bridges”. Proceedings of the 37th Symposium of the International Association for Bridge and Structural Engineering (IABSE), 2014.
 Anta Ingeniería Civil. www.anta-ic.com. Retrieved on November 5th, 2014.
 M. Guisasola. “Proyecto de pasarela sobre el río Oria entre Villabona y Zizurkil”, 2005. In Spanish.
 M. Guisasola. Interview conducted by V. Charpentier, T. Huynh and T. Michiels in the basque Country, November 2nd, 2014.
 V. Charpentier, T. Huynh and T. Michiels. “Villabona Footbridge”. Report submitted for the fulfillment of the requirements of the course “CEE 463. A Social and Multi-Dimensional Exploration of Structures”, Princeton University, 2015.