Sisters Towers Project – Building Above the City

Pierre Leflour
Structural Engineer
Setec TPI, Paris

The Tour Sisters project, designed by the Pritzker Prize winning architect Christian de Portzamparc, is a mixed development project in La Défense, France. It consists of two high-rise buildings, 200 and 100 meters tall, respectively, located on both sides of a pedestrian bridge. The two towers are linked by a bridge-like structure 100 meters above ground.

The project is singular due to the lack of ground space to anchor the towers, given the density of the surrounding urban area. This context generates a lot of superstructure area overhanging without natural foundations, as well as predominance of the shorter towers. A significant number of structural systems have therefore been embedded in the project to lead and transfer the loads to the foundation system. As the buildings are tall towers, the design of these systems must be adapted to the magnitude of the loads involved. Additionally, the foundation system of the project must be carefully designed to avoid disruption to the neighboring structures.

The project includes the following transfer systems: a rigid grid structure in façade leading vertical loads, transfer walls bringing façade loads to the core, as well as arches. Lastly, the bridge structure rigidly links the two towers to stabilize the smaller one by anchoring it to its bigger sister. This presentation elaborates upon the design process of these systems (from the constraints of site geography, to the choice and design of the elements).