Building higher on the shores of the Mediterranean: requirements for stability and comfort

La Marseillaise Tower. Client: Constructa. Architect: Ateliers Jean Nouvel Photo: Michèle Clavel

The weather conditions on the Mediterranean coast, characterized by winds from the sea and land, must be considered during construction. The CSTB assisted in designing the 100-meter high towers for housing units, offices and services on a coveted shoreline where land is scarce. It used its expertise in aerodynamics to ensure stability and wind comfort in buildings constructed along the Côte d'Azur, which are crucial for durability and safety. It also makes it possible to offer comfortable balconies and terraces and avoid wind corridors at the foot of towers.

Local climatic conditions

The coastal areas on the Mediterranean are known for their unobstructed opening onto the sea and their varied landforms, from beaches to 1000-meter high hills. In some places, they plunge directly into the sea, as in Monaco. These hills have an effect on the wind, whether coming from the sea or the north (mistral, tramontana), and a building is either protected or not by the environment, depending on its location. Within just tens of meters, the steep terrain of the Côte d'Azur can provide good shelter or leave you exposed to the wind.

The CSTB performs a precise analysis of the double impact of wind and topography on buildings, especially high-rises, which are particularly susceptible to weather conditions. Using its expertise in aerodynamics, it guides engineering firms and builders in optimizing the sizing of such structures and their facades. The challenge lies in the accuracy of input data and its analysis to produce a customized design.

The first high-rise to be built on the Arenc waterfront in Marseille, La Marseillaise, will welcome its first occupants at the end of summer 2018. The CSTB helped design this new office tower as well as residential projects in Monaco, like Testimonio II, slated to open in 2020.

Stability and wind comfort

Wind stability: this is the primary purpose of CSTB aerodynamics studies, ensuring the safety and durability of high-rise buildings. For La Marseillaise, for example, the structural optimization work focused on the core of the tower, made of concrete, to prevent occupant discomfort in windy conditions. It is possible to feel “seasick” on higher floors because of the movement of the top of the tower in strong winds.

For Testimonio II, balconies facing the sea on all 25 floors made it necessary to check the wind comfort of occupants throughout the year. Similarly, when the CSTB worked on the Tour Odéon, in Monaco, it was involved in optimizing the sizing of the sail-shaped glass parapet wall* bordering the terrace on the top floor (170 m) to ensure the comfort of users.

Finally, concerning the foot of the building: The higher the tower is, the more it is going to “suck” wind from the top and channel it to its foot. Pedestrians are then subjected to winds as strong as those at the top of the building. This effect can be accentuated by the crowding of other high-rises nearby. Planting trees can help protect pedestrians from these wind corridors.

CSTB expertise in Aerodynamics

To conduct its aerodynamics studies, the CSTB uses a specific approach, starting with the creation of a wind rose for each site of a future building. From the data collected at the nearest weather station and a topographic survey of the area, the CSTB develops a precise climate map. It considers the terrain, densely built-up areas and forests to estimate the level of wind that will have an impact on the building facade and structure.

Then, a combined digital and experimental simulation allows the CSTB teams to finely characterize the pressures exerted on the planned structure. Tests are conducted in a wind tunnel to measure the peak forces on a structure during a storm. To do this, a scale model of the future structure is tested in its immediate environment (radius of 400 to 500 meters). Numerical simulation is used before the test (to set the potential aerodynamic stress conditions and metrology, etc.) and afterwards, because it provides a very instructive representation of wind flow and facilitates analysis of the data collected in the wind tunnel.

The CSTB tailors this method to each project. Starting in the design phase up to the laying of the last stone, the CSTB helps stakeholders optimize their projects.

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* Here, a parapet wall refers to a structure concealing a flat roof or terrace.