N’Albero, A Temporary Spatial Structure Made of Steel Scaffolding System: Loading Tests as Tool to Check Structural Design

The paper deals with design and construction stages of N’Albero, a temporary spatial steel structure built to be the main 2016 Christmas touristic attraction on Naples seafront. Opened to the public for three months, it was visited by more than 250.000 people, estimating the simultaneous presence of 750 visitors. N’Albero main structure consisted of 20.000 prefab steel pipe elements, adopting Allround System by LAYHER, a fast-assembly, safe and economic solution. The whole spatial system was about 45 m-high, with a squared base (30 m × 30 m), tapering gradually to the top through three sky terraces. Using light multidirectional scaffoldings, the main design aspects to consider were: stability, uplift or sliding of the structure; strength of components sufficient to withstand loads without failing; serviceability of a light structure under large deflections. Estimating a dead load of about 200 tons, a ballasting load of 130 tons was used to ensure global stability against wind force. For this deformable system, loading tests were performed during construction to check design, applying inclined (45°) concentrate forces at each terrace level: the effect of these forces, assigned at +13,00 m (F = 5t), +19,00 m (F = 5t) and +31,00 m (F = 2t), was obtained using steel tendons, connected to the structure through an additional steel truss, while the acting hoist was anchored to a crane as counterweight. During tests, all the applied forces were registered by a dynamometric cell. No visual damages or elements instabilities occurred in situ, validating design calculations. Furthermore, these tests revealed a high deformability of the structure, not readable from FEM model, requiring precautionary changes to the original solution. This makes loading tests a reliable way to check and improve structural design in the case of framework with high deformability.