Thu Thiem 2 Bridge in Vietnam has an unusual design – both above and below the water surface –  which needed to be aerodynamically verified before construction.

Ho Chi Minh City (formerly known as Saigon) is the largest city in Vietnam and growing rapidly. To connect the city with the new urban area, Thu Thiem, it is planned to build a large cable-stayed bridge over the Saigon River.

The design of the Thu Thiem 2 Bridge is exceptional. It is a single pylon cable-stayed bridge with a 200 m long main span and two cable planes carrying the 33 m wide steel-concrete composite deck – with the curved pylon bending towards the back span.

Investigating the wind effects of Thu Thiem 2 Bridge

WSP Finland Ltd. has made the basic design of the bridge. As part of the preparations, WSP contracted FORCE Technology in order to investigate the wind effects on the Thu Thiem 2 Bridge through various model tests covering tests of a section model as well as tests of the full-bridge model in a 1:100 scale.

Aerodynamic behaviour

In October 2016, wind tunnel tests of a section model of the bridge deck (at a 1:55 scale) were performed in our wind tunnel to assess the effects of wind on the bridge deck for the in-service condition. The test programme consisted of static and dynamic section model tests which were carried out in order to characterise its aerodynamic behaviour. The static wind load coefficients, which are essential for the designer’s structural analysis, were determined. Furthermore, it was investigated if vortex-induced vibrations are likely to occur since significant vortex-induced vibrations may reduce the user comfort and cause fatigue problems and damaging of structural parts, resulting in increased maintenance costs and reduced design life.

Following these tests, aeroelastic full-bridge model tests were conducted in February 2017 in our wind tunnel to verify the overall dynamic behaviour of the full bridge. The model tests were carried out on a 1:100 scale model of the full-scale structure, and the model was exposed to wind speeds corresponding to full-scale values up to approximately 80 m/s (i.e. very strong typhoon).

Two versions of the aeroelastic full-bridge model were tested, representing the full-scale structure in its in-service condition (completed condition) and in a construction stage (longest cantilever), i.e. the most critical phase of the construction process.

The aeroelastic full-bridge model tests were used to predict the total wind-induced response for the bridge, including effects like aerodynamic damping, turbulence coherence and aerodynamic admittance.

Long piles affect the bridge movements

During the project, our wind experts were faced with the added challenge that the bridge will be situated in a riverbed with low bearing capacity, and thus it has been designed with long pile foundations. These long pile foundations will have a significant impact on the movements of the bridge compared to a bridge with short pile foundations. Therefore, the effect of the pile foundations was incorporated into the model design.

“We elevated the entire model and supported the aeroelastic full-bridge model by rods to include the effect of the pile foundations. In this way, it was possible to obtain the correct scaled mode shapes and eigen-frequencies, though it required an elevated floor all around the model to simulate the water level at the correct height,” explains Mads Beedholm Eriksen, Project Manager at FORCE Technology’s Maritime Division.

Based on the test findings, WSP has now obtained verification of the aerodynamic aspects of the bridge in the engineering design and the project investor, Dai Quang Minh Real Estate Corporation, can continue with the construction process.