Amaury Leroy
Head of Building Structural Engineering
Bureau d`etudes Greisch, Brussels

Luis Nosiglia
Project Director
Bureau d`etudes Greisch, Brussels

Solving Long-Term Creep Issues With Diamond-Like Precision

One of the challenges of using outrigger systems in tall buildings is accommodating the long-term differential vertical displacements between the columns and the core, resulting from creep and shrinkage. The adaptative outriggers at Silver Tower, Brussels, employ elongation devices integrated into the outriggers themselves. The outriggers consist of four welded composite steel sections, connected in pairs to form one upper member and one lower member of a narrow “diamond.” The upper and lower members intersect at their extremities along the longitudinal axis of the diamond, where they are attached to connection pieces embedded in the core and the columns. The geometry is tailored so that the frames fit perfectly within the connecting pieces like a stapler, thus allowing free rotation at the connections. In the center, an elongation adjustment device (similar to a hydraulic jack), ties the upper and lower members together.

This customized geometry gives the outriggers the ability to lengthen in the long term by shortening the elongation device, and thus admitting a differential core-column settlement (estimated at 30 millimeters over the building’s life) without clamping. In the short term, they provide the axial rigidity necessary to resist wind loads. The elongation devices automatically regulate the length of the outriggers, self-adjusting as required over the building’s life. The system designed for the Silver Tower can easily be adapted for other building geometries, presenting an effective and elegant solution to free the outrigger system of the lock-in forces due to differential shortening. The implementation of this solution has limited impact in the construction critical path, since the different elements can be pre-assembled, and are fully operational once installed.

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