Undersøgelse af lange hængebroer: Storebæltsbroen

Investigation on Long-Span Suspension Bridges: The Great Belt East Bridge

Simonsen, Kenneth ; Stevens, Robert

4. term

Structural and Civil Engineering, Master

2008

Denne afhandling undersøger aeroelastisk flutter på Storebælts Østbro i to byggefaser: med kun 15 % af brodækket monteret og når broen er fuldt opført. Begge faser analyseres numerisk med Computational Fluid Dynamics (CFD), som simulerer luftstrømmen omkring broen. Med udgangspunkt i Scanlan-teorien beskrives de aerodynamiske belastninger gennem såkaldte aerodynamiske derivativer (parametre, der kobler bevægelse til kræfter), bestemt ved en todimensional analyse af broens tværsnit via en tvungen oscillationstest. Disse derivativer bruges til at beregne den kritiske flutter-vindhastighed for den fuldt opførte bro, hvor CFD giver 71,9 m/s—nært de værdier (70–74 m/s), der er målt i vindtunnel. Under opførelsen udføres to tredimensionelle analyser. Her gennemføres en tvungen vibrationstest analogt med 2D-tilfældet, men med luft, der kan strømme rundt om broens frie ende, hvilket introducerer endeeffekter. Den strukturelle respons under byggeri viser, at den laveste vertikale egenfrekvens ligger meget tæt på den laveste torsionsfrekvens. Det antyder en lavere kritisk flutter-vindhastighed i denne fase; den beregnes til 47,6 m/s, mens vindtunnelforsøg viser 43,3 m/s. Rapporten undersøger også en fuldt koblet fluid-struktur-interaktion (FSI) til bestemmelse af flutterhastigheder. Bevægelsen af det antaget stive brodæk modelleres som et system med to frihedsgrader, og en førsteordens backward Euler-integrationsmetode anvendes i Ansys 11. For at reducere beregningstiden er masken i fluiddomænet grov. Simulationerne er gennemført, men med udfordringer omkring dæmpning; testen giver en kritisk hastighed på omtrent 20–25 m/s, markant lavere end vindtunnelresultaterne. Metoden beskrives dog som værdifuld for visuel og fysisk forståelse af aeroelasticitet.

This thesis examines aeroelastic flutter on the Great Belt East Bridge at two construction stages: with only 15% of the deck installed and when the bridge is fully completed. Both stages are studied numerically using Computational Fluid Dynamics (CFD), which simulates airflow around the bridge. Following Scanlan theory, aerodynamic loads are expressed through aerodynamic derivatives—parameters that link motion to aerodynamic forces—obtained from a two-dimensional cross-section analysis via a forced oscillation test. These derivatives are then used to compute the critical flutter wind speed for the fully built bridge; CFD predicts 71.9 m/s, close to wind-tunnel results of 70–74 m/s. For the erection stage, two three-dimensional analyses are performed. A forced vibration test, analogous to the 2D case, allows the fluid to flow around the free end of the deck, introducing end effects. The structural behavior during construction shows the lowest vertical natural frequency is very near the lowest torsional natural frequency, suggesting a lower critical flutter speed in this phase. It is found to be 47.6 m/s, while wind-tunnel tests give 43.3 m/s. The report also explores a fully coupled fluid–structure interaction (FSI) approach to determine flutter speeds. The deck, assumed rigid, is modeled as a two-degree-of-freedom system, and a first-order backward Euler integration scheme is used in Ansys 11. To reduce computation time, the fluid mesh is very coarse. Simulations were run but encountered damping issues; the test indicates a critical speed of about 20–25 m/s, far below wind-tunnel findings. Nonetheless, the method provides strong visual and physical insight into the aeroelastic phenomenon.

[This abstract was generated with the help of AI]

Aeroelasticity ; Flutter ; Long-Span Bridges ; Aerodynamic derivatives ; Fluid Structure Interaction ; Critical Flutter Wind Velocity ; Ansys CFX ; The Great Belt Bridge ; Erection ; Aeroelasticitet ; Flutter ; Lange hængebroer ; Aerodynamiske afledede ; Fluid Struktur Interaktion ; Kritisk flutter vindhastighed ; Ansys CFX ; Storebæltsbroen ; Opførelse

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