The innovative design of the Lully viaduct proposes a light and transparent structure made of a triangular cross-section fabricated entirely from unstiffened circular tubes. The result is twin space trusses, with a typical span of 42.75 m. Each transversal triangular cross-section is 2.9 m high and 4.0 m wide, and is supported by a single slender pier. The largest diameters and thickness of the tubes are over 500 mm and nearly 70 mm respectively. One major difficulty during the design of the connections was to define the stress distribution along the complex intersecting perimeters of the tubes, and to calculate the hot spot stresses. Geometry calculations, precision cutting and edge preparation of the tubes were necessary for performing full penetration welds. The mobile formwork, which also ensures the stability of the tubular trusses during concrete pouring, also required special consideration. This paper describes the evolution of the project, from design, truss fabrication and welding, to construction on site.