Encircled by a tangle of highways, acoustics at Auditorio di Roma, a complex of three concert halls now under construction in Rome's Stadio district, are not just an internal issue. Outside, the halls must deflect or absorb 70dB of noise from flyovers almost overhead.
Designed by Italian superarchitect Renzo Piano Building Workshop and structural engineers Amideo Vitone and Piano Structural Engineering, the halls are 'beetle-like' with compound-curved roof shells. They are ranged atop a 5ha, two storey 'basement' structure.
Cost manager for German project management firm Drees & Sommer, Fabio Sarti, explains that the basement structure will house foyers, office space, rehearsal halls, shops, a library and multi-media centre and recording studios. Structurally the basement is 'entirely conventional - it is really not very exciting', says Sarti.
Climbing scaffolding to the roof of the Auditorio's 1,200 seater medium size concert hall where work is most advanced, Sarti's enthusiasm mounts. Contractors Impregilo and Colombo Costruzione have almost finished placing the main structural elements and the roof's final form is becoming clear. It is preparing to start on the roofs of the larger 2,700 seat hall and a small 750 seat structure.
Roofs for all three halls are composite sandwich structures supported by massive laminated wooden frames.
Looking at the sandwich from the inside out, steel-fibre reinforced concrete is poured and hand skimmed onto profiled steel decking to create a primary, 100mm deep shell. Waterproofing and insulation is laid on top of this and a veneer of wood applied. An air space is provided and more insulation and a second waterproof layer are added, followed by a second skin of steel-fibre reinforced concrete.
Another sheath of wood is applied before mountings are fixed for the final titanium skin.
'The roof has to be this incredibly elaborate to absorb and deaden all of the different sound frequencies, ' Sarti says.
Excluding structural timbers, weight of the roof is 500kg/m 2.The roof of the largest hall, covering 6,000m 2, will total 3,000t.
Teflon-coated rubber mountings are used to fix the steel deck, the first layer of the shell, to longitudinal timber purlins. This allows some movement - roofs have two longitudinal and three transverse expansion joints.
But just 60mm of deflection in the roof, overall, is allowed.
Main structural members, pairs of curved beams, form pinjointed arches, tied and braced with steel rods. Positioned transversely at 5m centres, the laminated timber beams are 1.5m deep and 300mm thick. On the medium-size hall the largest beam, spanning 60m, weighs 30t.
On the 2,700 seat hall, beams will weigh up to 50t.
One end of each arch is mounted on a pin bearing, with a roller bearing at the other end accommodating any movement caused by deflection under dead and wind loads. Movement is expected to be a maximum of 60mm, says Sarti. Computercontrolled hydraulic rams will adjust tensions in the roof to prevent all but the slightest changes in geometry.