Sisters Towers Project – Building Above the City

Pierre Leflour
Structural Engineer
Setec TPI, Paris

The SISTERS towers project, designed by Pritzker prize architect Christian de Portzamparc ( is a mixed developement project in la Défense, FRANCE. It consists of two high-rise buildings respectively 200 and 100m high, located on both sides of a pedestrian bridge. The two towers are linked by a bridge-like structure 100m above ground.

The project is singular due to the lack of ground level space to anchor the towers, due to the very dense state of the urban network in the area. This context generates an lot of superstructure area overhanging no natural foundations, as well as a global imbalance of the smaller high-rise. A significant number of structural systems have therefore been embedded in the project to lead and transfert the loads to the foundation system, as well as balance the global project. As the buildings are high-rise, the design of these systems must be adapted to the magnitude of the loads involved. Lastly, the foundation system of the project must be carefully designed to cause no disruption to the neighbouring structures.

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