Since 2012, the glass roof of the Rennes’ University Library protects and welcomes new readers. The renovation of the town of Rennes’ old university library gave a perfect opportuntiy to ELIOTH to show the full range of its expertise. The project signed by the architect, Bruno Gaudin, covered issues which dealt with structural engineering, façades and environment. The challenge that represents the modernizing of an existing building to optimize its performance all the while preserving its pervading spirit « genius loci », finally led us to designing a courtyard covered by a glass arch. Like a blown transparent membrane, the glass roof symbolizes the osmotic relationship with the town. The geometry of the arch strives to interrelate the work of architecture with structural and energy efficiency. Thus, its form is the result of a research process to obtain high structural efficiency.
Its typology, the relationship between solid structures and voids, are part of a study to seek a balance between thermal, visual and acoustic problematics.
A joint work on geometry, structural refinement, innovation in glazing and the definition of climate and light control has developed a unique spatiality, assuring a perfect reading comfort. Elioth ensured the full structural design of this rooflight, isostatically inserted into the existing buidling. The definition of the geometric area was the object of a dedicated mathematical analysis which resulted in a significant outcome for the glazed panels: all resulting glass rhombuses are flat while the overall roof glazed surface is twisted !
To cover the span between the four sides of the courtyard (21 x 28 m), Elioth designed a homogeneous grid shell whose form ressembles a double-curved cushion. The structural elements of the covering shoot out diagonally to form rhomboidal meshes. The shape of the roofing is sought out among translation surfaces so as to obtain a detailed layout with the quadrangular windows. Moreover, for the roofing over the patio bioclimatic aspects were introduced. The placing of a « lid » over the building renders it more compact and gains in density, and consequently reduces energy consumption. By adding sunbreakers combined with coated double glazing, the wall surface meets the comfort requirements in winter and summer. Comfortable sound levels are obtained by alternating opaque panels with glass modules. The design of the wall surface and the manner of filtering light have not been thought out for the sole purpose of controlling natural lighting but indeed to emphasize the visual aspect of the building in its setting, the exchange of light and dark shades, the glazed structure and the casting of shadows.