An Office Tower with Public Spaces Open to the City

AKASAKA GREEN CROSS is a high-rise, multi-tenant office building positioned between the expansive scale of a government district and the finer grain of a human-scaled urban neighborhood in Akasaka, Minato City, Tokyo.

Combining a high-quality work environment with advanced environmental and disaster-resilience, the building promotes well-being through its strong connection to nature. The office entrance is positioned on the third floor, transforming the lower levels, from second basement through third floor, into public space open to the surrounding city. The ground floor functions as a welcoming and accessible “grand lobby” for the neighborhood, connecting seamlessly with surrounding streets. With entrances on all four sides, the building enables free pedestrian flow, while its integration with generous green open spaces enhances circulation and gently links the distinct identities of the two districts.

A multi-level public space with a dual character

Linking Tameike-Sanno Station at the second basement level directly to the third-floor office lobby, the design establishes a multi-layered public realm that extends to the station plaza. Around the central void spanning the first to third floors, aluminum pipes inspired by traditional sudare (bamboo blinds) create a permeable screen that unifies the space across levels. The use of sound-absorbent plaster throughout the office and grand lobby ceilings reinforces both visual continuity and acoustic comfort. While open and connected to the city, this lower zone also offers a more contemplative atmosphere where calm and activity intersect, offering office workers a subtle retreat from the intensity of everyday work.

A curtain wall integrated with the surrounding scenery

The exterior was designed as a minimalist glass curtain wall inspired by the shimmering surface of the former Tameike pond and the reflected landscape once seen in its waters. This allows the architecture to mirror the surrounding sky, clouds, and landscape, resulting in a structure that merges with its environment. The glass curtain wall was conceived as a unitized system using one seamless pane of glass without dividing the vision and spandrel zones, introducing a three-point support system that suppresses deflection and reduces the size of members. This approach helps minimize radiant heat transferred from the outside, considering environmental sustainability. By adopting mullions that are not only slender in profile but also shallow in depth, the design also maximizes openness and allows for unobstructed views. With a standard office floor of approximately 555 tsubo (around 1,835 m²), the layout provides well-proportioned, column-free spaces suited to diverse tenant requirements. Each floor is equipped with a green terrace, which not only adds an architectural accent to the facade but also provides access to fresh outdoor air and vegetation even on the upper levels. Portions of the exterior incorporate ultra-slender structural braces composed of interconnected triangles and expressed as a design feature. Gaps between structural components are utilized to integrate the building’s air supply and exhaust systems while also incorporating green wall components, introducing fresh air while creating an expressive exterior that engages with the city. This is a vertical green wall system for a high-rise building, extending continuously from ground level to the top of the tower. In an urban landscape crowded with inorganic skyscrapers, it presents a new architectural prototype in harmony with nature.

Ultra-slender perimeter braces in harmony with the exterior design

As a seismic measure, to suppress the sway of a building with a tower aspect ratio of 5 and a floor-plan aspect ratio of 3, multi-story braces in the shape of a lowercase “h” were placed at the extremities farthest from the center of rigidity, enhancing torsional resistance while maintaining bending stiffness in the short-span direction. In addition, on the short-span side of the building interior, column transfer occurs at the seventh-floor mechanical level. This floor was designed as a truss framework to serve as an intermediate outrigger, securing the building’s overall flexural rigidity. Thin braces generally have a high risk of buckling during earthquakes; however, the system releases stress by permitting part of the brace to slide under loads below the buckling limit, achieving non-buckling slender braces. The design also incorporates friction dampers at the intersections of the truss structure, allowing it to move during earthquakes and thereby dissipate seismic energy. The slender braces at the end walls and intermediate levels were specified as H-250mm × 250mm × 9mm × 14mm sections, with dampers concealed inside the horizontal members (H-400mm × 400mm × 13mm × 21mm). Moreover, the joints of the H-steel braces were detailed with bolted connections only at the web and interior flange, avoiding visible bolts or metal fittings on the building exterior.

Shaping urban spaces where plants can thrive

With the aim of restoring, even in part, the ecological network and climate-regulating role once supported by the water and vegetation of the former Tameike pond, the landscape design focuses on generating a substantial volume of diverse vegetation not only at ground level but also on the rooftop and facades. The publicly accessible plaza facing the intersection serves as a key pedestrian node connecting the two different neighborhoods to the north and south, and was therefore designated as “Forest Gate.” Forest Gate is composed of numerous large deciduous trees that help regulate seasonal sunlight, providing a pleasant environment where office workers and visitors can unwind and appreciate seasonal changes. Along the lively southern street lined with small-scale shops, a green promenade inspired by the former flow of the Akasaka River was created. The facades were also covered with vegetation extending to the upper levels to increase green coverage, while butterfly gardens were incorporated on inaccessible lower-level rooftops, creating spaces that contribute to the urban ecosystem.

Project name: AKASAKA GREEN CROSS
Building use: Offices, restaurants, retail, medical clinic, parking
Location: Minato City, Tokyo
Site area: 5,985.98 sqm
Total floor area: 73,453.14 sqm
Number of floors: 28 stories above ground, 3 basement levels, and 1 rooftop mechanical floor
Eave height / maximum height: 146.83 m / 147.98 m
Main structure: Steel, partial steel-reinforced concrete, reinforced concrete
Completion: May 2024

Client name: Sekisui House, Ltd./ Nippon Life Insurance Company
Lead architect: NIKKEN SEKKEI
Main scope of work: Basic Design, Design Development, Construction Supervision
Other design companies: OBAYASHI (detailed design of substructure), LIGHTDESIGN (lighting), Emotional Space Design (signage)
Construction contractor: OBAYASHI, Zenitaka & Iwata Chizaki Construction joint venture

Photo credits: Nacása & Partners Inc.