Städel Museum Frankfurt

It looks like Frankfurt’s Städel Museum is expecting – a bigger collection at the very least. The museum and art academy, founded on a bequest in 1816 from Johann Friedrich Städel, is dedicated to both the creation and collection of art so expands continuously. The third and latest extension of the 1878 building was through an invited competition in 2007, and almost doubles the previous area of 4000m2. Local firm Schneider+Schumacher beat the likes of UN Studio, Diller+Scofidio and SANAA to win, with an intriguing subterranean €34m proposal that inserted all the new accommodation beneath the museum’s courtyard. Opening last year and accessed via a grand new stair from the museum’s original foyer, the result is a strange and beguiling courtyard of grass and circular rooflights that bulge at the courtyard’s centre – the only surreal evidence that something strange and interesting is going on beneath.

Inserting a new basement below the courtyard walls of a 19th century museum was not without its challenges. Existing foundations had to be stabilised and underpinned using high pressure injection techniques. Elaborate provisional structures to protect the upper floors were only removed once the foundations and new concrete 75m by 52m basement box had been constructed. The proximity of the River Main required the use of waterproof concrete throughout , a 500mm thick basement floor slab to resist upthrust and 13m-deep reinforced augur piles were used where there was no toploading in the form of walls or columns.

Measuring 55m by 48m in area, the concrete grid shell roof of the subterranean Garden Halls is crowned by a distinctive 26m diameter bulging concrete dome at its centre, raising the internal soffit from 6m to 8.2m. The roof’s form was designed using parametric software Rhino to resolve the circular dome and 12 internal perimeter columns that transfer the load to the concrete box. Engineer Bollinger+Grohmann used ANSYS for the structural optimisation. Cast in place, the flat roof structure and dome slab were pre-stressed with the tensioning rods assuming the curved line of the formwork as a result of their own dead weight. Complexity was increased by the fact that, with the architect demanding there be no suspended ceiling, all services had to be integrated into the concrete roof, to ensure a smooth soffit.

So it is the 195 roof lights, evenly spaced across the grassy hillock of the courtyard, that form the main visual element of the museum’s underground extension – as much a landscaping feature as structural signifier. Predominantly 1.5m in diameter, they increase to 2.5m towards the centre of the courtyard to accentuate the curvature of the dome. Lying flush with the grass surface, they are all safe to walk on. The roof lights are double glazed, formed of two layers of ­laminated safety glass – an outer pane cold-formed to a spherical curve and a flat inner pane. To be usable by pedestrians,  the outer curved layer has a non-slip surface and requisite strength ­of 5kN/m2, as well as sufficient bearing capacity in the case of breakage.

A steel frame around the perimeter of the glazed element secures it into the structure and houses the other automatic shading and lighting systems. Surrounding the rooflight are 44 luminaires, an equal mix of warm-white (2700K) and cool (5000K) LEDs. The whole system, which illuminates the galleries both naturally and artificially, is managed by a lighting BMS, achieving a constant level of illumination regardless of external conditions.

Being a garden itself, the roof forms part of its own SUDs strategy.  The Garden Halls’ roof is inverted, with the insulation sitting on top of the waterproofing membrane, and the 400mm deep intensive green roof skirting the concrete upstands of the individual roof light openings. The architects needed a long-term and homogeneous waterproofing system that would be flexible enough to deal with the roof’s constantly changing contours and root resistant, and robust enough to withstand footfall. In the end the contractor opted for a Kemper Systems cold ­liquid-applied waterproofing membrane. A drainage board is incorporated to divert excess water into the surrounding ground and land drainage systems. It’s also aided by the fact that the green roof is higher than the surrounding area, directing run-off away and preventing sumping. This is done via simple supporting gutters, cast into the upper face of the pre-stressed reinforced concrete roof form.  

Architecturally, the design of the Städel Museum extension has reconciled two goals – the creation of an ideal space for art beneath beautiful public gardens while at the same time preserving the gardens themselves. In a sense it fulfils the original brief perfectly, which asked for a balance ‘between respect for the historical building fabric on the one hand and the aplomb, individuality and distinctiveness one would expect of a new object. As such, Schneider+ Schumacher’s grassed hillock with its bold roof lights actually becomes an abstracted landscape – a piece of art in itself. A strange subversion of the lawn- a grass sculpture  that can be walked and laid  on, and simul­taneously a precision optical device creating the perfect viewing conditions for the artworks hidden beneath.