Located in Islington, a diverse and trendy London borough, the HYLO Tower is an innovative office and mixed-use space within walking distance to shops and restaurants. The neighborhood is where old and new collide, and the structure itself is the renovation and extension of the former Finsbury Tower. After the project team thoroughly examined the original structure, they decided to extend it by 13 stories rather than replace it.
Two unusual features distinguish the new high-rise structure. First, whereas many office towers have all-glass facades, the HYLO features brick and precast concrete cladding, which helps integrate the structure with adjacent brick residential buildings. Second, by retaining the 16-story existing structure and extending the building upward by 70% and sideways another 24%, the net rentable space was doubled while the embodied carbon was reduced by 35% as compared with new construction.
The original Finsbury Tower sat clumsily on a low podium stretching further to the north and south. The tower has now been brought to the ground, and the two podium elements were extruded upward from four to seven stories. Because the existing lift (elevator) shafts were insufficient to service the extra space, two new cores were built on the northern corners of the tower to house high-speed double-deck lifts. The project team evaluated the existing structure using time-dependent, finite element analysis to understand existing and proposed construction through past and future life cycles. The analysis showed that the substructure and superstructure were capable of supporting a steel frame and composite concrete floors with some localized column strengthening.
“The remodeling and extending of the existing form has enabled a larger building to sit more comfortably in its context,” says Stephen Cherry, director, Horden Cherry Lee Architects. “The sculptured form and the play between flush and highly recessed glazing subtly express the memory of the former building. Cutbacks create landscaped terraces for the office tenants. The existing and new concrete structures have been left exposed to reflect the building’s origins and add character to the new interiors.”
Lightweight Building Envelope
Masonry cladding on traditional precast concrete panels was originally considered but deemed too heavy. The project team preferred a unitized curtainwall system for speed and simplicity. They therefore selected a unitized aluminum-cladding system with brick-faced ultra-high-performance concrete (UHPC) panels attached. Bespoke brick slips were used to minimize the weight and were cast into precast UHPC panels that are just 75 mm (3 in.) thick (half the thickness of a standard panel). The double-height panels create solidity, and the textured bricks with flush lime pointing resemble the look of neighboring 19th-century brick buildings.
The glass and aluminum panels were made by a Turkish cladding specialist, and the precast concrete brick panels were made by another Turkish company, Fibrobeton. The two elements were brought to the site from their respective locations, which simplified logistics and avoided the damage to the panels that might have occurred if they were transported as one unit. The two elements were assembled on site and installed as unitized cladding.
This unusual approach for the cladding system lessened the weight loaded on the old structure and existing foundations. The special Fibrobeton panels of glass-fiber-reinforced concrete (GFRC) were cast with stiffening steel frames and custom dimensions. Panels range in size from 1008 × 512 mm (3.3 × 1.7 ft) to 8728 × 500 mm (28.6 × 1.6 ft). The total area of the precast concrete elements for the different geometries, including panels with numerous angled corners wrapping the outer edge of the building, totaled 6200 m2 (66,700 ft²).
Precast concrete production was adapted to accommodate the vast quantity and complexity of shapes and sizes, with the brick slips carefully inlaid on each panel face and set to rigorous tolerances. UHPC was poured over brick slips in special molds to ensure that the panels were thin and incorporated fixing bolts in defined locations.
Panel installation was challenging. The panels were launched from the slab edge adjacent to the cores and moved around on a monorail system. The installers worked from a cradle in front of the panels, attaching the pieces to brackets on the core. Between panels, custom movement gaskets were installed to accommodate settlement and movement over the life of the structure.
“The result is a building that is uplifting and honest and that, on close inspection, reveals the memory of the original building,” says Cherry. Internally, the tower is a modern, attractive workplace with desirable amenities such as high-speed lifts, landscaped terraces, and services. In addition to smart building features, HYLO is environmentally friendly and is certified WELL Gold and BREEAM Excellent. At street level, 17,000 ft2 of retail space spans across two buildings fronting Bunhill Row.