Engineering and architecture firm Nikken Sekkei has used an innovative structural system to meet the challenges that Japan’s frequent earthquakes pose, building a seismically isolated skyscraper of unprecedented height in the bustling city of Osaka.
The 200-metre Festival Tower in the city's Nakanoshima district has 39 floors and three basement levels. The tower, which offers a concert hall and shopping facilities as well as office space, is the new local headquarters for the Asahi Shimbun newspaper.
The challenge for the design team was that the 2,700 seat concert hall had to be made with reinforced concrete walls to form a rigid frame that supports sound isolation and acoustic performance. The offices above also ideally needed to be column-free for maximum flexibility.
The solution was a mega-truss and an open lobby level. The Sky Lobby, including the central core, is glass-clad, clearly revealing the mega-truss and its transfer of force from the tower to the podium.
For the office tower, the firm adopted a round-cornered square with a central core surrounded by 15-metre deep office space. These rounded corners and the deep eaves of the lower levels were designed to help counteract wind excitation in the vicinity of the building.
Columns are spaced at 1.8-metre intervals around the periphery, creating a double skin and a completely uniform office space. A total of 220,000 large beige tiles, chosen to match the brickwork incorporated in the podium, are dry-fixed to the aluminium curtain wall and provide the detailing for the exterior facade.
The lower-level floors are made of steel reinforced concrete, while those from the ninth floor up are steel framed.
A mid-storey seismic isolation system was installed immediately above the new Festival Hall between the lower-level and mid-level floors. Rigidity and strength are achieved in the upper floors via the earthquake resistant brace assembly of the centre core frame and the perimeter frame surrounding the building with 128 H-shaped steel columns spaced 1.8 metres apart.
The core frame is equipped with oil dampers to reduce motion caused by either seismic force or wind force. The top of the building has a “hat-truss” which reduces warping of the building as a whole.
Between the 13th and 15th floors, there are two major trusses; a mega-truss and a belt-truss. The mega-truss is a huge three-dimensional structure standing 20 metres in height which supports the column axial force of the upper level core and transmits the load of the upper floors to 16 "big" columns, each three-by-1.5 metres, directly below the outer perimeter of the upper-level floors.
The belt-truss is a two-dimensional truss around the outside of the 14th floor. It serves to distribute the force of the 128 perimeter steel columns around the upper-level floors to the big columns. The big columns support the entire load of the building from the 13th floor up, and the two huge trusses make possible the vast space obtained for the hall in the lower part of the building.
The solution received recognition from the Council for Tall Buildings and the Urban Habitat at the organisation's recent conference and awards ceremony.
The team is now working on a twin skyscraper in an adjacent space. The buildings will subsequently also serve as the disaster prevention base for the region.