A VERY LONG TIBETAN BRIDGE (Val di Cembra, Italy)

The wind tunnel tests were carried out within the framework of a Research Agreement between the Comunità della Valle di Cembra and CRIACIV, and concerned a suspended pedestrian bridge connecting the villages of Gresta and Grumes (TN).

The research activity started with a detailed analysis of the structural geometry and dynamic characteristics, aimed at characterising the interaction between wind action and the structural response. This preliminary phase supported the definition of the experimental test programme and the design of a sectional model specifically conceived for wind tunnel investigations.

The experimental campaign included both static and dynamic tests, with the objective of characterising wind-induced loads and aeroelastic behaviour under representative operating conditions.

The investigated structure is a “Tibetan-type” suspension footbridge, conceived as a lightweight tensile structure for pedestrian use. The total span of the bridge is approximately 215 m, with a maximum height of about 109 m above the valley floor. The two bridge abutments are located at approximately the same elevation.

Unlike traditional Tibetan bridges, typically consisting of rope-based systems, the structure is equipped with a rigid walking deck to improve pedestrian comfort. The deck consists of braced steel members supporting a metallic grating with a width of approximately 1 m.

The structural system includes two upper load-bearing cables, acting as handrails and located at the top of the parapets, and two lower load-bearing cables aligned with the deck. The resulting cross-section has a trapezoidal geometry, fully open on the upper side. Two additional stabilising cables are also present and connected to the deck through a system of tie rods.

The geometric definition of the bridge and the finite element model, providing the modal properties of the structure, were supplied by the design office responsible for the structural design and used for the development of the wind tunnel model.

A single physical sectional model, built at a scale of 1:7.5, was used for both static and aeroelastic tests. The model had a longitudinal length of 1005 mm, a chord length corresponding to the deck width of 138.2 mm, and a section height of 202 mm.

To promote a quasi-two-dimensional flow around the model, it was equipped with vertical wooden end-plates. Thanks to the porous nature of the deck and side elements, the blockage ratio within the test section remained within acceptable limits. The total mass of the model, including the end-plates, was approximately 5.6 kg.

Static wind tunnel tests were performed to measure the aerodynamic forces acting on the bridge deck section. These measurements allowed the determination of mean aerodynamic coefficients as a function of the wind angle of attack, as well as the estimation of the Strouhal number associated with vortex shedding phenomena.

The experimental programme considered different geometric configurations, including variations in deck porosity and side mesh characteristics, in order to investigate their influence on the aerodynamic behaviour of the structure.

Dynamic (aeroelastic) tests were carried out to identify the flutter derivatives of the bridge deck section for several geometric configurations and wind angles of attack. Each configuration was tested at zero wind angle and at small positive and negative angles.

The model was connected to a dedicated aeroelastic setup based on a 4+4 spring system, allowing coupled vertical and torsional motion. Flutter derivatives were identified using a free-decay testing approach, both in still air and under wind excitation. Signal processing and parameter identification were performed using the MULS method, a time-domain technique based on complex modal analysis.

The wind tunnel tests provided a comprehensive experimental characterisation of the aerodynamic and aeroelastic properties of the bridge deck section. The measured static and dynamic coefficients were combined with the modal characteristics of the full-scale structure to evaluate, under consistent assumptions, the wind-induced static and dynamic response of the bridge.

The results of the study constitute a robust experimental basis for the assessment of wind effects on the structure and support the aerodynamic and structural evaluation of long-span lightweight pedestrian bridges in complex topographic environments.

Additional visual documentation of the bridge and its natural setting is available in a YouTube video by Mr. Massimo Zanier.