WIND TUNNEL TESTS ON A TIED-ARCH BRIDGE (Kukes, Albania)

The Drini Bridge is a major infrastructure crossing the Black Drin, an artificial lake created by the construction of the Fierza hydroelectric dam in 1978. Set within a landscape characterized by deep canyons and small islands, the bridge plays a key role in improving mobility and connectivity across the region. The structure consists of three spans, with a central span of 270 metres that places it among the longest tied-arch bridges in Europe. The steel arch rises approximately 55 metres above the deck level and splits into two branches near the abutments, where the loads are transferred to reinforced-concrete piers, some of which are founded directly in water. The deck is 26 metres wide and the entire structure is built in weathering steel. Construction was carried out through extensive pre-assembly operations onshore, supported by temporary towers, followed by the launching of the structure into its final position using a specially built barge tailored to the exceptional dimensions of the bridge.

Within this context, a comprehensive wind-engineering study was undertaken to investigate the aerodynamic and aeroelastic behaviour of the bridge, with particular attention to the deck and to the interaction between deck and arch. The objective of the campaign was to provide design-oriented information on wind-induced actions and structural response, supporting the overall verification of the bridge under wind loading and contributing to a robust and reliable wind-resistant design.

The experimental activity focused on wind tunnel testing of a high-quality sectional model of the bridge deck, specifically conceived to allow both static and dynamic investigations. Static measurements were carried out to characterize the aerodynamic load coefficients over a range of wind incidence conditions representative of the site exposure and of possible operational scenarios. These tests provided a clear picture of the mean aerodynamic forces acting on the deck and of their sensitivity to wind direction, forming the basis for subsequent dynamic assessments. Particular care was devoted to the treatment of non-structural aerodynamic details, whose final definition was still evolving during the design phase. Alternative configurations were therefore considered in order to explore their influence on the overall aerodynamic response without binding the analysis to a single detailed solution.

The same sectional model was then employed in aeroelastic configuration to investigate wind-induced vibrations of the deck. The experimental programme addressed both vertical and torsional response, allowing the identification of vortex-induced vibration phenomena and their dependence on wind speed, angle of attack and key dynamic parameters. Tests were conducted in smooth-flow conditions, enabling a clear interpretation of the underlying excitation mechanisms and a reliable comparison between different configurations. Given the intrinsic uncertainty associated with full-scale structural damping at the design stage, the interpretation of the results adopted a conservative and parametric approach, exploring a meaningful range of normalized damping levels rather than relying on a single nominal value. This strategy made it possible to assess trends, sensitivities and relative criticality of different response conditions in a manner directly useful for engineering decision-making.

In parallel with the deck investigations, the aeroelastic behaviour of the arch system was assessed through a dedicated analytical framework based on consolidated literature for rectangular cross-sections representative of the bridge geometry. This step aimed at providing a rational screening of potential wind-induced instability mechanisms affecting the arch, while explicitly accounting for uncertainties in damping and in the dynamic interaction between arch and deck. By considering limiting interaction scenarios, the study framed the possible range of structural response and clarified the parameters governing the onset of wind-induced vibrations, thus supporting a conservative and informed evaluation of structural performance.

Overall, the wind tunnel campaign and the associated analyses delivered an integrated understanding of the wind-induced behaviour of the Drini Bridge, combining experimental evidence with established aeroelastic knowledge. The results provided valuable support to the design process, highlighting the governing aerodynamic mechanisms, the influence of key geometric and dynamic features, and the importance of reliable dynamic characterization in long-span tied-arch bridges. Within this framework, the study also illustrated engineering strategies commonly adopted for similar structures to ensure adequate performance under wind loading.

Additional visual documentation of the bridge and its natural setting is available in a YouTube video by MEAG C.