IRELAND | RETROFIT wide vertical joint between units. The doubleskin faade has a U-value of 1.1 Wm-2K-1. A series of glass fins, positioned in line with the joints on the outer leaf, separate the outer leaf from the inner unit. The fins are hung from the buildings roof. The weight of the outer glazed skin is supported on a bespoke stainless steel frame fixed back to the bottom floor slab. We worked diligently with the contractor to develop bespoke toggled connections that allowed accommodation of the differential vertical movements with the fins providing horizontal restraint to both skins, explains Lee Corcoran, senior faade engineer at Arup. An additional movement challenge was that the cantilevered structure onto which the faade was attached was predicted to deflect under imposed loads. Corcoran says: We worked closely with Octatube and structural engineers Torque Consulting Engineers to fully understand the movements associated with the structure at faade connection points so that we could design the system to accommodate the anticipated racking movements and minimise the joint sizes with minimal visual impact. The glazed corners of the outer skin were carefully considered through extensive structural calculations and detailing. The final solution to lock the corner units incorporates a closed loop structural solution; the large glass panels transfer the lateral load into bespoke stainless steel connections and Tom Johnson House is a five-storey 1970s office that will become home to Irelands Department of the Environment, Climate and Communications 38 September 2023 www.cibsejournal.com concealed stainless steel tension rods hidden within the silicone joints at the head and base of the units. It is a fantastic, discrete solution that no-one will ever see, which is aligned with the essence of the project, says Corcoran. Arup used a low-iron glass for enhanced clarity in both the inner and outer units. In the drive to maintain transparency, the architect was keen that the office floors were kept free of window blinds and that the double-skinned faade remained uncluttered by interstitial maintenance walkways. A high-performance solar control coating on the outer skin of the double-glazed unit combined with a solar control PVB interlayer on the laminated single-layer outer skin and ventilation to the interstitial cavity removes excess heat. To finalise the glazing and PVB selection, Octatube built a series of small-scale mockups at its Delft HQ. Once the selection was confirmed, a full-scale mock-up was built and subjected to the CWCT sequence B weather performance test. Access for cleaning within the cavity is provided by a single walkway concealed at the base of the faade. To enable personnel to move between the two skins, Arup worked with Octatube to shorten the glazed fins so that they stop 1.6m above this walkway. A bespoke abseiling solution was developed to allow for cleaning of individual bays in between the glass fins. The location of the glass box above the existing warehouses meant that when it came to constructing it, sequencing was key. As part of the restoration of the warehouses, all of their roof trusses were removed and taken off-site for restoration. This allowed construction of the core and structure to support the glazed box. The box was preassembled as far as possible to minimise work at height, ensure build quality and to complete the installation before the warehouse roof trusses were reinstalled. What made the project so successful was our involvement at such an early stage, which meant the design was well considered quite early in the process and that allowed us to engage with a specialist contractor very quickly, Corcoran says. The 85,000ft2 office is now entirely leased to TikTok. The location of the glass box above the existing warehouses meant that when it came to constructing it, sequencing was key