Water colour: as lead designer, Cecil Balmond completes his first bridge, in Coimbra.

Cecil Balmond and Arup's Advanced Geometry Unit (AGU) are well known for helping to make some of the world's most innovative architectural creations stand up. Working alongside leading designers, Balmond is no stranger to collaborations where the traditional roles of architect and engineer merge to enable structure, form, space, pattern and ornament to synthesise. Promoting an engineering philosophy that transcends measurable matters of efficiency and effectiveness, in pursuit of richer, more sensual realities, Balmond is an ambitious and prolific designer. In person he has an energy and optimism that supports his can-do attitude, and with his team's more measurable ability to rationalise and resolve, Balmond and the AGU have won the respect and loyalty of many so-called signature designers. On this project, however, Balmond was not working in collaboration with a Koolhaas or an Ito. Instead, the completion of this comparatively modest 274m footbridge, represents the culmination of a relatively low-profile relationship with Portuguese engineer and bridge designer Antonio Adao da Fonseca; a relationship that indirectly led to Balmond's more celebrated collaborations with Alvaro Siza and Eduardo Souto de Moura. As the conceptual author of this bridge, Balmond describes his relationship with Fonseca as 'relaxed and trusted', giving him some distance from the nitty-gritty of undertaking detailed engineering calculations.

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Remarkably, despite his 40 year career with Arups, this is the first bridge that Balmond has built. Like many of the buildings he has worked on, its rationale flits between conceptual clarity and complex layered metaphorical references. As he recalls decisions (or feelings) in sequence, the explanation begins with Balmond's so-called impossible sketch of a bridge of two halves that never quite meet. With two sweeping curves that seem to repel each other, Balmond wanted to create an elevated public space mid-stream above the river; a place that would become somewhere to pause and to appreciate the River Mondego and the beautiful setting of Coimbra's citadel. In his second diagram, the two halves of the bridge are joined by a double-width elliptical deck. Diagram three shows this arrangement rationalised into two straight trajectories, linked by an S-shaped platform.

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In section each of the two halves were designed to be mutually reliant on each other, with an asymmetrical and essentially unbalanced profile. The 'impossible' halves gain stability and balance by leaning against its partner. The centralised junction not only represents a point of transition for passers-by--reaching the highest point and subtly changing tack before beginning the graceful descent. The junction also acts as the principal point of structural transition, where eccentric forces are equalised and resolved, and where the bridge is seen at its most light.

When considering the view along the bridge, the effect of perspective inspired Balmond to create the apparently haphazard filigree of balustrades and zigzagged handrails that are set on the outside edges of the 4m wide decks. Concerned that an array of vertical balustrades would appear wayward and forced when turning to negotiate the kink, the triangulated frames were developed to soften the edge and to make the change of direction less abrupt. Each of these frames was then infilled with one of four coloured glass panels that cast subtle low-tone shadows, with a rose-tinted tone bringing out the warmth of the rose-tinted white steel paint. While these elements could be seen as whimsical and over-designed, particularly in relation to the elegance of the structure that skips gracefully over the river with an incredibly shallow arch, when seen together these finer elements allow the tempo of the three bay hop-skip-and-jump superstructure to be moderated. In what Balmond refers to as a three-speed shift in pace, the kink sets the major interval; 12m long hand-rail bays set an intermediate scale that gives passers-by reference points along the way and the minor bays of the glass panels set a rhythm that matches the pace of each walking stride.

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When describing the bridge, Balmond's broad range of references can seem contrived and overwhelming, as he recreates well-rehearsed series of sketches that describe structural, formal, compositional, perspectival, rhythmic, optical and tactile influences, all of which are described in a logical sequence that builds to the climax of the overall ensemble. In reality, however, the sophistication of the bridge is evident without such a detailed commentary, and when visiting the bridge in person, Balmond exhibits a visible pleasure and affection for the finished work that reveals far more intuitive reasons behind the decisions made. In reality, things just feel right, and Balmond shows himself to be a highly instinctive designer; strict theories become looser, and the work exerts its own unique identity.

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