Eight underground lines within walking distance, five international airports, 20 million people within an hour’s commute: the location of the “22 Bishopsgate” skyscraper, completed in the midst of the Covid-19 pandemic, in the centre of London’s financial district could not be more ideal. The story of the project’s history, its special architecture and the building’s exemplary sustainability, designed by PLP Architecture, are equally impressive. The “closed cavity façade” installed by specialist façade contractor Josef Gartner with around 70,000 square metres of Cradle-to-Cradle certified glazing from AGC Interpane/AGC Glass Europe sets new technical and aesthetic standards.
Every day a little bit more height, a new part of the façade glazed – architecture rising before your eyes. Historically, 22 Bishopsgate was the polar opposite. The 278-metre-high skyscraper, which was completed in 2020, passed through numerous design incarnations: In 2005, an earlier design for the same site by the architects Kohn Pedersen Fox was to have an externally vented double-skin façade and sufficient solar panels that it could generate up to 200 kilowatts of electricity. To achieve this, the building was to have a spiralling roof and curved tapering plan form. The first test drilling of the subsoil and demolition of the existing buildings on the site began in 2006, alongside archaeological investigations. In February 2007, a Middle Eastern investment consortium acquired the site and the building was renamed “The Pinnacle”. Construction work began in 2008 and in March 2009 the largest foundation piles the United Kingdom had ever seen were sunk: 48.5 metres below sea level and 65.5 metres below the building site. A year later, excavation began for the construction of the basement levels, followed by the arrival of the first cranes in late 2009. In 2012, the construction was put on hold in the wake of the financial crisis, the subsequent recession and lack of preletting agreements. By then, only the concrete core of the first seven floors had been completed. Years of review and modification of the original designs followed.
In 2013, it became clear that all previously constructed parts should be demolished and rebuilt from scratch based on a more cost-effective plan. This demolition was never carried out and core and basement were sold to a new owner. The building’s site was sold to a consortium led by AXA Real Estate in 2015. A new design was worked up by PLP Architecture and submitted for planning permission in 2015. The central concrete core of the original design was completely removed in December 2015 – after which the design of the new building began utilizing as much of the existing basements and piling as was feasible. In 2016, it was announced that real estate company Lipton Rogers and its joint venture partner AXA IM – Real Assets were expected to complete the project in 2019. At 278 metres, it was to become the tallest building in the City of London to date. The City of London granted permission in June 2016.
Finally, the successful design
In 2020, the design by PLP Architecture was, at last, completed: The overall twenty-three-sided, faceted building form of this “vertical city” expresses an impressive aesthetic in which the wellbeing of the users was placed at the centre. The architects combined influences from art and craft, creating impressive spaces with maximised daylight control and fresh air to support the wellbeing of people working within the office spaces. A total of 128,500 square metres of net internal space is available, with more than 10% of the area devoted to facilities to optimise everyday life for its 12,000 regular users, such as a bicycle park with around 1,700 parking spaces, showers, lockers, a fresh food market, open kitchens, event areas and outdoor terraces, and even discounted rents for qualifying start-ups, co-working spaces and many facilities for networking and events. In “The Gym”, athletes climb a glass climbing wall and find special training facilities such as a high-altitude room. Users can find relaxation, Pilates, yoga and health services in “The Retreat”. In “The Club”, tenants and clients can socialise in an informal, bookable, flexible space.
PLP Architecture aimed to create a positive relationship between the building, its occupants and the public. At the base of the building, pedestrians have free access to a landscaped public open space. The architects promote a sense of wellbeing by maximising occupant views out across London and public views into the building. At street level large canopies were incorporated to minimise adverse wind effects and deliveries were reduced by bundling at an offsite facility. The multi-storey foyer is designed as a gallery for temporary art exhibitions, while at the same time permanent installations are intended to inspire visitors, such as the glass canopies incorporating artwork by Alexander Beleschenko. In addition to the two large canopies along Bishopsgate and above the main entrance with a total of 115 panes, the artist provided artwork for another 50 or so insulating glass panes for the lobby façade as well as several wind-migration screens. All the panes were printed by Sedak (Germany, Gersthofen) in a ceramic digital printing process according to the artist’s file templates. The wind-mitigation screens are also technically unique: PLP Architecture worked with wind tunnel and CFD engineers to model the shapes of the building and its effects on wind circulation. Multiple designs of canopies and screens were explored to minimise possible negative effects on pedestrian comfort at ground level. Hundreds of designs were tested before the optimum solution was chosen.
Other artistic applications in the building include the leather work in the reception library by Bill Amberg, sculptural wooden furniture by Pierre Renart and the colourful works of Bruce McLean, who designed a unique piece for each car in London’s fastest lifts. Smart and sustainable technologies can be found in every corner of the building. Queues to enter and leave the building or use services are a thing of the past thanks to the use of opt-in facial recognition technology.
State of the art Closed Cavity Façade
For the special “Closed Cavity Façade” manufactured by Josef Gartner (part of the Permasteelisa group), highly transparent glazing from AGC Glass Europe / AGC Interpane was selected to meet the design intent. The glazing units were manufactured to the highest quality in Germany at AGC Interpane plants in Belgern and Plattling in collaboration with Josef Gartner and the Design Team. The fully glazed façade utilises an intelligent blind system, which is the major contributor in mitigating solar gain and maximising energy efficiency. The glass façade sets new standard in terms of energy efficiency and day-light, contributing to the BREEAM rating of “Excellent”. The basic philosophy behind the Closed Cavity Façade (CCF) is “simplicity and efficiency”: the energy-saving, closed and double-skin technology was developed by façade supplier Josef Gartner to maximise the economic / environmental balance and transparency of buildings. In the case of 22 Bishopsgate, up to 6,000 Millimeter storey-height units of varying widths were incorporated as the inner skin of the CCF units with an iplus Low-E coating. The outside skin of the CCF unit is formed by laminated safety glass panes with an “ipasol bright white” solar control coating. The external glass is bonded directly to the PPC thermally separated aluminium profiles with an anodised carrier frame.
The maintenance-free interior of the Closed Cavity Façade is completely sealed and lightly pressurised with dry air, so it cannot become contaminated with dirt and moisture and consequently does not need to be cleaned. The need to clean the cavity of a conventional double skin façade would also have meant that the inner skin would need to be opened regularly, causing disruption to occupants and which would have required a large increase in framings, seals, fittings, etc. – the sustainability disadvantage of this large number of additional components is obvious. Since the Closed Cavity Façade is permanently conditioned with clean and dry air, which is generated by a centralised dry air plant, no condensation can form on the outer pane even when the temperature changes. The external skin of the CCF is made of daylight-optimised low iron float glass, which transmits more daylight into the building than standard float glass. The inner double-glazed unit is formed from mid iron glass. The Closed Cavity Façade matches the performance of conventional double skin façades in terms of solar control and improves on the performance in terms of thermal insulation and soundproofing, which ultimately significantly increases comfort for the building’s occupants.
The path to perfect glazing
Conceptually, PLP Architecture was concerned early on with the aesthetic effect of the large area glass façade, especially in the choice of any coatings. To the outside, the façade was to have a slight, discreet reflection to give it “presence”. Depending on the incidence of light, the viewing angle and the surroundings, the look should change in order to make the faceted character of the building more “legible”, to break up the visual mass and to emphasise the verticality of the individual angular planes. The design team ultimately made the choice of exterior glazing together with the client and preferred façade builder Josef Gartner. This involved virtual prototyping by PLP to assess the look of the glass and coatings, both at a city-wide and local scale, using sophisticated CGI rendering software from OCEAN (now part of AGC Glass Europe). Samples and studies showed that an external reflectance of 22 per cent most precisely achieved the desired aesthetics. In the end, the choice fell to the “ipasol bright white” solar control coating on a laminated Clearvision low iron float glass. The individual panes were assembled into laminated glass in the non-tempered state to avoid the risk of visual problems such as wave distortions and anisotropies. Ultimately, the desired aesthetics were achieved: The look of the façade changes over the course of the day as a fluid reaction to the surrounding environment, alternating between opaque, translucent and completely transparent.
The choice for an active solar shading system was made by PLP in order to maintain exactly this variable aesthetic. Conventional façades are covered with multi-layer coatings to minimise heat loss on cold days and reduce the solar gain in summer. Due to the laws of physics, these coatings reduce daylight in the building – to a greater or lesser extent depending on the layer composition. The higher the reflection, the less daylight transmission. Daylight and transparency, essential to the concept of 22 Bishopsgate, could consequently only be achieved through an active system that adjusts the performance of the façade to ensure a high level of daylight when little solar shading is required and vice versa when it is, as well as fluid adjustment options in between. The traditional way to achieve this would be a ventilated cavity façade. The natural buoyancy of the warming air mass in the cavity automatically drives the ventilation. The main disadvantage of this system is that moist, contaminated air enters the cavity, which has to be cleaned regularly, for example by opening the inner skin. The consequence is that the space on the room side becomes compromised due to the need to open the inner skin.
A Closed Cavity Façade (CCF) is a generic term for a double skin façade that has a shading device in the cavity, similar to a ventilated façade, where the cavity is tightly sealed and under slight pressure to prevent possible ingress of outside air. The façade regulates solar radiation into the interior through a series of steps: First, some of the radiation is reflected by the outer surface of the exterior glazing. A reduced amount enters the cavity, where it is reflected outwards or absorbed by the blinds and heats the air in the cavity, which then radiates the heat back outside. The inner and low-E coated insulating glazing reflects some of cavity energy back to the outside. Only a small part of the warming solar energy finally enters the building as little as 10 percent when the blinds are fully closed.
The façade of 22 Bishopsgate is a top-hung system, spanning from floor to floor on a nominal module 1.5 metres wide and 4.0 metres high. On some upper floors these panels are up to 2.2 metres wide and 6.0 metres high. The frames of the glass façade are powdercoated in a matt, dark metallic grey and thermally broken. The outer glazing consists of laminated glass with the slightly reflective coating ipasol bright white, the inner double glazing has a thermally toughened outer pane, a 16 millimetre wide argon-filled cavity and a laminated glass pane with an iplus Low-E coating. The motors of the internal blinds are not located in the cavity, but are mounted above the ceiling on the room side in the ceiling void. This keeps them cooler, which extends their life and facilitates access for routine maintenance. Dry, clean air is supplied via pipes in the ceiling from a separate technical room.
“PLP had a very clear idea as to what the glass should look like on the completed building and the way we wanted it to change across the day due to varying light conditions and cloud cover. The building externally was to be all about the glass so ensuring it matched these expectations was critical. We found conventional rendering in house or by third party visualisation companies struggled to replicate this anticipated look, so we invested considerable resources into developing a method ourselves that would be accurate and technically defendable. This involved exploring various rendering platforms, finally settling on OCEAN’s software as part of our process, backed up with detailed waveband photometric data from AGC Interpane for the various glass products we were considering.
We also undertook detailed laboratory scanning of glass samples ourselves where necessary to fill in the data we needed. This meant our representations were very accurate in terms of colour, optical transmission and reflection. As we undertook a lot of full-size benchmarking we were also able to cross-check our rendering exercises to ensure they were producing reliable representations. This gave confidence well in advance of panel manufacture. It became clear very quickly as the facades were being installed that the glazing did indeed look and behave as expected. Now the building is completed and thankfully contributing to the city views as we had hoped, we can say that the time and effort put in by the various parties involved in this process was invaluable and wholly worthwhile”
– ROB PEEBLES PLP / ARCHITECTURE
A conventional good quality single-skin façade with a high-performance triple-silver coating has a g-value around 28 per-cent and a daylight transmission of about 56 percent, depending on the thickness of the glass and coating type. The U-value of the entire façade is usually around 1.4 W/(m 2 K). The closed cavity façade of 22 Bishopsgate has a g-value of 41 percent and a daylight transmission of 63 percent with the blinds fully open. When the blinds are lowered with the slats horizontal, the g-value is 18 per cent, and when they are at an angle of 45 degrees, it is 14 per cent. When completely closed, the g-value is 11 per cent.
The building received BREEAM Excellent rating: compared to the requirements of the current building regulations, CO2 emissions were reduced by 35%. PLP aimed for a U-value (Uw) of 1.2 W/(m2K) for the entire façade system and were ultimately able to achieve 1.1 W/(m2K). The blinds can also be fully closed at night to further reduce heat loss in the colder months. The blind motors are fully addressable and are controlled individually via the building management system. The motors provide feedback on their position, enabling the recording of movement data and error logging. The software that controls the blinds has a detailed building model that takes into account the orientation of each of the 22 façade zones including shading provided by adjacent buildings and responds to information on air temperature and solar radiation collected by sensors. The blinds also provide a degree of glare protection if required. They consist of 60 mm wide powder-coated aluminium slats that are light grey on top to reflect heat outwards and dark grey on the underside to reduce inter-reflection. Building users can also control the louvres manually by switch, hand control or via electronic devices (computer, tablet, smartphone). The slats are 4% perforated to allow some visibility to the outside even when closed.
22 Bishopsgate form is complex: the building consists of 23 corners and 23 facets. The overlay of the 1.5 m office planning grid with the façade form determines the position of the mullions and thus the façade panel module width. The angle between the individual façade facets in conjunction with the mullion position determines the geometry of each individual panel. The panels can be divided into the following types: typical straight-line panels and one-piece 90-degree corner panels, typical two-piece inner and outer corner panels, and the double-height room façade consisting of two vertically stacked CCF panels. Externally, these panels are indistinguishable from the single storey panels above and below. In addition, there is a double-height plant room façade consisting of two vertically stacked louvre panels. From the outside, these panels are partially glazed to ensure continuity of the glass surface across an otherwise abrupt interruption.
As the quality of the float glass was of paramount importance to 22 Bishopsgate, all the manufacturing processes were meticulously monitored from float production and processing, through heat treatment and coating, to lamination and assembly of the insulating glass units. Quality benchmarks were carried out at all stages of production, from the individual components and materials to the panels in production on the assembly lines and the finished panels before shipment from the Josef Gartner factory in southern Germany to the UK. High-temperature endurance tests were carried out on the blinds and motors over a three-month period to test the resistance of the motors and all moving parts to temperatures in excess of 90°C. During these tests, the blinds went through their full movement cycles more than 20,000 times. The materials and surfaces of the blinds were tested for wear, fading and failure. Full-size production units were tested for air and water tightness and structural properties in accordance with CWCT standards. These tests culminated in impact tests of the glass and frame. The loads were among the highest that façade manufacturer Gartner had ever tested. In the test, the glass was allowed to break, but not to detach from the frame. To the surprise of many, neither breakage nor permanent deformation occurred.
This article was originally published in IGS Magazine’s Autumn 2021 Issue – Glass Retrospective: Read the full Magazine here for more thought-leadership from those spearheading the industry
Marc studied humanities at the Technical University of Braunschweig (Germany) with a focus on the psychological and sociological contexts of internet-based communication. After a total of 14 years in PR agencies, he was Head of Marketing Communications in the glass industry for 6 years before starting his own business in spring 2021. With a clear focus on sustainability topics, he now supports companies from the construction industry and architects in matters of strategy, communication, networking and events with his agency “Marc Everling Sustainable Communication”.