Feature

Piano's lid

Renzo Piano and Expedition Engineering top Athen's new cultural centre with a record-breaking ferrocement canopy

The architect’s Athens cultural centre incorporates the world’s largest-span ferrocement roof. By Pamela Buxton

Canopy seems rather an inadequate word to describe the spectacular structure over the Stavros Niarchos Foundation Cultural Center (SNFCC), designed by Renzo Piano Building Workshop in Athens.

Believed to be the largest ferrocement span in the world, the solar collector canopy stretches 100m x 100m to cover a vast terrace with views over both city and sea. In doing so, it forms the landmark flourish of the 22,000m2 building, which will house the Greek National Opera and Greek National Library within a new public park. The site previously contained a car park left over from the 2004 Olympic Games and is located in Kallithea, once one of Athens’ main seaports.

The main building structure has a conventional reinforced concrete frame, incorporating large expanses of fair-faced concrete and a number of long-span transfer structures over atriums and auditoriums. For the canopy, the initial plan was to use a lighter, steel-framed structure but it became clear that this couldn’t deliver Piano’s desired aesthetic.

“It was very important that it should be visually very light and fine and with a completely un-jointed bottom surface,” says Bruce Martin of UK structural engineer Expedition, which worked on the project with local partner OMETE. After exploring other materials and consulting with experts at Manchester University, Expedition settled on ferrocement, a material familiar to Piano’s team, which had previously built a ferrocement boat.

“We saw it had the potential to deliver the finish, and form curved profiles, to a very high quality,” says Martin, adding that ferrocement, unlike clad steel, can combine structure, waterproofing and finish in one element.

Given ferrocement’s rare mainstream use in construction, Expedition needed to demonstrate that a canopy in this material could not only meet performance standards but could also be tendered, with the monocoque ferrocement produced and installed successfully using local labour with no prior experience of the material. After extensive research and development including the production of more than 100 large ferrocement panels by a local contractor, the team created a 15m x 6m prototype and stacked it with sandbags to test its strength to destruction. It passed with flying colours – despite their best efforts, the team failed to destroy it before the stack became too high to continue with the test.

Production was further tested with the construction of two small ferrocement pavilions for the SNFCC park, before manufacture of the canopy itself began. The canopy tapers in depth from 4.4m to just 23cm at its edge. It is formed from two load-bearing “skins” linked by tubular steel bracing that carries shear forces. Each ferrocement skin was created using up to six layers of fine diameter steel mesh to limit cracking (the outer mesh layers are just 0.8mm and 1.6mm in diameter), encased in a cement mortar with a top and bottom outer “cover” layer.

The skins range in depth from 20mm to 55mm, with thicker areas designed where there are connections to the bracing, and in the areas immediately adjacent to the supporting columns. Almost all of the 3m x 7m panels were unique, to take into account variations in curvature. All have bi-directional ribs to prevent buckling.

In total, 717 prefabricated panels were produced using a temporary local factory with three parallel, automated, ferrocement production lines. These were then spliced together to form the final canopy. “Every one that was manufactured was used. I find that amazing,” says Martin.

The canopy is supported by 30 tapered cigar steel, braced columns with integral rainwater pipes stretching 9m high on the seaward side and 18m high on the landside. The canopy cantilevers 12.5m and has clear internal spans of up to 75m.

Particular attention was paid to the way the canopy connects with the columns. Column heads are sprung to limit stresses in the (inherently very stiff) canopy caused by wind and seismic loads. They also had to cope with differential settlement of the main building structure and differential thermal expansion of the canopy, whereby higher temperatures on the top of the canopy could lead to a “banana-ing” effect on the structure. The solution was a damper system of fluid-polymer springs adapted from technology used in French rail buffers. This limits stresses in the canopy by creating a soft connection with the columns, thus distributing loads more evenly onto the structure below.

Mindful of the possibility of variations in the colour of the white mortar mix over the expanse of the canopy underside, the design team opted to paint the finished surface, which has the added advantage of helping with durability. The canopy shelters a vast terrace and also a 30m x 30m, glass-walled library reading room which sits on top of the fly tower of the 1,400-seat auditorium below.

Although the privately-funded SNFCC was launched last year it is yet to open fully. It does however have a LEED Platinum rating – a first for Greece – with the canopy’s photovoltaics able to provide the base power supply for the whole cultural centre.

PROJECT TEAM

Architect Renzo Piano Building Workshop
Local architect Betaplan
Structural engineer Expedition Engineering/OMETE
Contractor Salini Impregilo-TERNA

Photos: Michel DenancĂ©; Yiorgis Yerolymbos