Extension beneath the Garden – the Städel Museum in Frankfurt am Main
Visitors who use the main entrance of the museum have access to the former foyer via the central lobby and two new flights of stairs, which are integrated quite naturally into the existing layout. The route then leads via a spacious single flight of stairs down to the garden halls. In order to construct the new staircases and install a combined goods and passenger lift that links all floors of the building, it was necessary to insert a new basement under the 19th- and 20th-century structures. Up to that time, the existing basement levels housed only service runs and provided protection against groundwater and rising damp.
This process of inserting a new storey beneath an existing one called for ingenious structural planning as well as precise coordination: when and where a section of the existing structure could be removed, and whether it had to be supported (temporarily or permanently) or could be re-erected. High-pressure injection techniques were use to seal the excavation pit and bear the loads of the adjoining foundations. To support the upper floors, elaborate provisional structures had to be erected that could be removed again only after the foundations and the new basement had been constructed.