RAYLWAY “ALGERIA” VIADUCT
Wind Tunnel Tests for aerodynamic and aeroelastic behaviour of a railway truss viaduct
Year: 2011
The wind tunnel experimental campaign was carried out to support the aerodynamic and aeroelastic assessment of a long-span viaduct located in Algeria, developed during the executive design phase. The infrastructure is part of the
Oued Tlélat – Tlemcen high-speed railway line. The primary objective of the study was to quantify the wind-induced actions acting on both the bridge deck and the pylons, and to investigate the potential susceptibility of the deck to vortex shedding–induced vibrations. Given the slenderness of the structural components and the limited availability of site-specific wind data, the use of wind tunnel testing was considered essential to complement code-based design approaches and to reduce uncertainties related to wind loading.
The experimental programme was conceived as a multi-level investigation, combining static aerodynamic tests, flow-induced vibration analyses and aeroelastic tests. Different physical models and experimental setups were adopted, each specifically designed to address a well-defined aspect of the problem. This strategy allowed separating the global aerodynamic behaviour of the structure from more localized phenomena, such as vortex shedding from individual components, while maintaining coherence among the various test phases.
Sectional model of the bridge deck
A key element of the experimental campaign was the definition of a representative sectional model of the bridge deck. The sectional approach was selected to accurately characterize the aerodynamic forces and moments acting on the deck cross-section, as well as to investigate possible flow-induced instabilities under controlled conditions. The model reproduces the main geometric and aerodynamic features of the real deck, including the overall depth, width and the presence of relevant appendages.
The same sectional model was used consistently across static and dynamic tests, ensuring full comparability of the results. Particular attention was devoted to the definition of the model scale, which was selected as a compromise between blockage constraints, achievable Reynolds number and manufacturability. Thanks to the bluff geometry and sharp edges of the deck, Reynolds number effects were found to be negligible within the investigated range.
Static wind tunnel tests were first performed to determine the nondimensional aerodynamic force and moment coefficients acting on the deck section as functions of wind speed and angle of attack. These measurements provided the baseline aerodynamic characterization of the deck and allowed a preliminary assessment of galloping susceptibility through quasi-steady criteria.
The results showed that the aerodynamic behaviour of the deck is strongly dependent on wind incidence and on the specific deck configuration. In some conditions, the slope of the lateral force coefficient suggested the potential for instability, justifying the need for further aeroelastic investigations. The static tests also allowed identifying the wind directions and angles of attack associated with the most critical aerodynamic loading scenarios.
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Vortex shedding analysis and flow-induced forces
The vortex shedding process from the deck was investigated by analysing the spectral content of the measured aerodynamic forces. Well-defined shedding frequencies were identified and expressed in terms of Strouhal number, allowing a direct link between wind tunnel results and full-scale behaviour. Due to the complex deck geometry, different characteristic dimensions were considered to interpret the shedding process consistently.
This analysis made it possible to identify wind speed ranges where synchronization between vortex shedding and structural vibration modes could occur at prototype scale, providing essential input for the subsequent aeroelastic tests.
Rigid model tests on deck–pylon interaction
To investigate the global aerodynamic behaviour of the viaduct, additional tests were performed on a rigid model representing the combined deck–pylon system. This model allowed measuring the aerodynamic forces acting on the pylons in the presence of the deck, thus capturing the mutual aerodynamic interaction between superstructure and substructure.
The results highlighted that the presence of the deck significantly modifies the flow field impinging on the pylons, affecting both the mean forces and the unsteady loading components. At the same time, the tests showed that the detailed structural connection between deck and piers plays a secondary role once the main geometric features are accounted for.
Isolated pylon tests and vortex shedding behaviour
A dedicated experimental phase focused on the aerodynamic behaviour of an isolated pylon. This simplified configuration allowed a clear identification of vortex shedding characteristics for different wind directions. A stable and well-organized shedding process was observed when the wind was normal to the widest face of the pylon, whereas more irregular flow patterns emerged for oblique incidences.
These results suggest that, under real conditions, vortex-induced effects on the pylons are likely to be mitigated by upstream interference from the deck and by the complex flow environment of the site.
Aeroelastic tests on the deck section
The susceptibility of the deck to vortex-induced vibration was finally investigated through aeroelastic wind tunnel tests. The sectional model was mounted on a dedicated aeroelastic rig designed to allow vertical motion only, reproducing the first vertical vibration mode of the deck. The structural properties of the system were carefully calibrated through free-decay tests, ensuring control of frequency, mass and damping.
The aeroelastic tests were carried out in smooth flow conditions, representing a conservative scenario. The results showed the possible occurrence of lock-in phenomena within specific wind speed ranges, together with the associated vibration amplitudes. These findings provided direct quantitative information on the expected dynamic response of the deck under vortex shedding excitation.
Discussion and engineering implications
The experimental campaign provided a comprehensive aerodynamic and aeroelastic characterization of the viaduct in its final design configuration. By combining static force measurements, vortex shedding analysis, deck–pylon interaction studies and aeroelastic testing, it was possible to identify the critical wind scenarios and to assess the robustness of the structure against wind-induced phenomena.
The results confirmed the necessity of wind tunnel testing for complex bridge structures, where simplified analytical approaches may fail to capture the interaction between geometry, flow and structural dynamics. The findings obtained in this study formed the basis for subsequent design decisions and for additional investigations addressing temporary construction stages, such as those involving the launching nose.
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