Modern construction in natural stone: an elegant saddle-shaped roofed entrance
Bari Politecnico’s Giuseppe Fallacara used tensioned steel bars to fasten the hovering hypar structure
Steel reinforced concrete experienced its heyday in the mid-20th century when architects took to constructing wide spanned roofs in dynamically curved shapes. Going with the times, the constructions seemed almost floating and weightless. Today such buildings can also be realized using the ancient material: stone. Bari Politecnico’s Giuseppe Fallacara recently completed such an object: It goes by the name of „Hypargate Vela“ (Hypargate sail) and stands in Sainte-Savine, France adorning the SNBR headquarters.
Let us take a closer look at technical terms. Fallacaras’ construction is a so-called „Hypar“, in geometric nomenclature a hyperbolic paraboloid. For details see the Researchgate portal (link below).
The hypar ensues when points A and B of a grid are extended upward while at the same time pulling points C and D down. The resulting shape is a sort of steep saddle.
Steel-reinforced concrete roofs were typically constructed in this shape in the 1960s to adorn posh coffeehouses.
Giuseppe Fallacara constructed his hypar in natural stone: specifically 100 single bocks each weighing 120 kg arranged in a 10×10 grid.
The construction spans 22 m² without extra support, resting its entire 12 t weight on two points ground wards.
Each of the 100 blocks is perforated, allowing sunlight to flood the underside.
But, plain to see, the high-reaching overhang cannot be self-supporting. They are pulled tight with the aid of steel bars.
The steel bars run through the seams between the blocks from one side to the other. To avoid interference, the holes were bored in two levels.
The boreholes are filled with mortar to ensure that tension is transferred to the stone evenly. The steel bars are protected against rust.
Complex static calculations are part of the run up work. Every stone has its own individual surface slant and was cut to exact precision using CNC technology.
The tension of the steel bars also had to be calculated precisely. 4 t of tension hold the construction in place.
The construction team chose steel bars instead of ropes because ropes are difficult to place precisely while holding tension. Also tightening bars after a time is easier than picking up slack on a steel rope.
The density of the stone is another factor which must be taken into consideration. If the stone is too soft or porous the tension applied might make it crack. Semond Limestone was used for the job at hand provided by Burgundy-based Lippiello Frères.
Our photo shows two other spectacular constructions by Fallacara next to the „Sail“: a triangular roof in the background rests on an arch of stone with a steel rope tension construction on the interior, and right: a self-supporting spiral staircase.
The stone for „Hypargate Vela“ was cut using CNC-Machine by T&D Robotics, Carrara.
Concept and design: Prof. Dr. Giuseppe Fallacara
3D drafting and optimization: Giuseppe Fallacara, Nicola Martielli, Maurizio Barberio, Bertrand Laucournet
Structural analysis (prestressed reinforced stone):
Prof. Ing. Nicola Rizzi, Valerio Varano, Daniele Malomo
Fabrication and construction: S.N.B.R. Société Nouvelle le Batiment Régional
Photos: Atelier Fallacara
(14.11.2016, USA: 11.14.2016)