Throughout the past 12 months, the physical boundaries created by our built environment have become ever more noticeable and increasingly important to us. Safety concerns throughout lockdowns due to the global COVID pandemic have raised awareness as to the barrier between the conditioned and unconditioned environment. The political unrest and protests in various parts of the world have further highlighted the physical safety net that our buildings provide, and just how vulnerable they can be to unexpected events of attack.
The size of our individual worlds has been somewhat reduced and focused towards our more immediate surroundings – particularly the buildings which we inhabit. For many, the boundaries of home and work have been blurred and the functionality of our homes has been stretched and put to the test as we attempt to live, work and play within the confines of our homes. The safety and security that we feel within our buildings has become increasingly important whether it be our own home or a building we are visiting. The perceived threats, from the external environment, onto our wellbeing are all the more apparent and as building designers we need to respond to this.
This article considers the role of the façade in responding to the increased requirements we are placing on our buildings and the protective, yet flexible, safety barrier we want it to provide.
Facades as a window to the world
Transparency has long been a coveted feature of our buildings. The use of glass in buildings, once was as much about the iconography of a building as it was for the benefit of the building user. Fully glazed buildings were designed as status symbols often having poor energy efficiency, difficult temperature control and glare issues. Now, on the whole, we are much more balanced about the use of glass applying it in a measured and purposeful way, appreciating the key benefits it can provide for the building users. There is clear guidance and research demonstrating that consistent exposure to biophilia, views of the sky, fresh air and natural daylight supporting our circadian rhythms is immensely beneficial to our physical and mental wellbeing. The tightrope we walk, as building designers, is maximising these benefits whilst ensuring that the fabric of the building does not excessively contribute to the embodied and operational carbon produced by the building.
During the COVID pandemic we have been spending an increased amount of time at home, and ongoing research suggests that a blended agile working pattern is likely to continue even as our office buildings begin to open up again. This places additional importance on how we design residential buildings to be flexible and comfortable in providing home office space for a greater proportion of the population. The awareness that people have about the quality of the spaces their home can provide for them has become amplified as the flexibility to use different buildings for all the normal parts of our daily lives has been restricted. Comfort is key – the ability to open a window and get fresh air, the need for noise control and quiet spaces, natural daylight and views of the sky are all features that our facades must provide. New residential developments without balcony provisions have become harder to sell as people appreciate the need to be able to open a door and step outside whilst also being protected and safe within the boundaries of their home.
Case Study 1 – Principal Tower, London (by Claudio Marini, Graduate Façade Engineer)
Principal Tower is a residential building part of the Principal Place mixed-use scheme in central London, which includes commercial, residential and office spaces. With circa 300 apartments and a height of 161m, the 50-storey tower ranks as one of the tallest residential buildings in Europe and represents a bold landmark in London’s cityscape. The residents can benefit from new public spaces and local amenities, as well as indoor leisure facilities available at mezzanine level, such as gym, sauna, swimming pool, cinema room and lounge.
Together with architects from Foster and Partners and contractor Lindner, Buro Happold engineered the façade of Principal Tower, where glass plays a central role with floor-to-ceiling glazing, curved balustrades at corner balconies and curved double-storey glazing at ground floor and penthouse levels. The elevations are characterised by horizontal lines defined by aluminium solar shading, smoothed at corners by the curved balconies, and alternate visual glazing and opaque metal panels.
The full-height glazing allows indoor spaces to be naturally lit and create a direct connection between the enclosed artificial environment and the external landscape, with views over the City and East London. In-out visual and physical connection is enhanced by the generous size of the glazing and the balconies, which extend the internal space toward the open air. Manual operable windows and blinds allow the user to control natural ventilation and daylighting, according to the outdoor conditions and activity taking place indoor, so that, for instance, glare effect can be mitigated while working from home on a laptop during a bright sunny day – a situation which is now a common requirement. Acoustic PVB interlayers were utilised at locations where enhanced performances were required, so that the façade is able to guarantee acoustic comfort to the inhabitants.
The typical façade system is a bespoke structural silicone glazed unitised system with double glazed units directly bonded to the aluminium thermally broken profiles, for a frameless appearance. The whole system depth is less than 200mm, from the outermost face of the glass to the back of mullion. Glazing is generally comprised of double glazed units, with laminated inner and outer panes, and dimensions up to circa 4200 mm wide and 1540 mm tall. At corners of the building, laminated curved safety glass form the balustrades at balconies.
Ensuring people feel safe and secure within a building is of utmost importance. Inherent safety such as dealing with structural loads including wind, barrier and snow is the foundation of façade design. Feeling safe against fire and additional threats such as vandalism and attack is also critical to ensure the wellbeing and safety of the building users is maintained. The type of threats to our buildings has become more varied and our response as designers must be to manage the risks posed by these threats ensuring minimised damage and appropriate protection to the building users and members of the public. Threats can include various types from blast events with explosives, vandalism – both opportunistic or planned, direct impacts from vehicles or ballistic events from gunshot. These threats are typically related to the specific use of the building or location. The status or value of the contents of a building will raise its risk profile. But the risk of these types of attack occurring can be assessed and the façade can be designed to mitigate and reduce injury and damage.
Typically, heavier solid facades are associated with providing higher levels of security. Balancing the security needs of a building with the desire to provide transparency and operability of our facades requires a careful engineering approach assessing the various performances by using a multitude of strategies and assessment methods to simulate and design against particular threats. This may include a test plan to evaluate the response against threats (e.g. soft body impact, hard body impact, axe and sledgehammer attack, blast) or a structural strategy aimed at minimising wandering debris, in case of explosion, or avoiding disproportionate collapse of a glass structure, in case of a local failure. Identifying, designing and incorporating key security requirements at an early stage in the development of the building can maximise the opportunity for more transparent facades ensuring that a more rounded and holistic response to people’s expectations for the building envelope is provided, giving them a protective, flexible, safety barrier that is additive to their physical and mental wellbeing.
Case Study 2 – Odeon Leicester Square, London (by Ben Sochacki, Senior Façade Engineer)
The Odeon cinema in Leicester Square is the flagship venue of Odeon Cinemas Group in central London. As part of the recent refurbishment works to the original Art Deco building, a structural glass enclosure extending out over Leicester Square and the red-carpet walk has been constructed. The space within hosts a chic cocktail bar with an exclusive view over the vibrant Leicester Square.
Due to the high-profile events that will take place in this well-known public location the threat of a ground-based attack occurring during the use of the building could not be discounted. The design team had to ensure that the design of the transparent glass assembly will safely deal with the extreme loads that arise during such an event, whilst not further endangering the public in the aftermath. At the same time, special attention was given to the high visual and performance characteristics of the glass as well as the detailing of load bearing glass-to-glass connections to achieve the premium aesthetics.
The glass enclosure is supported one storey above the ground level of Leicester Square on top of a cantilevering balcony structure. The 4-sided glass enclosure is 16.5m long with a height of 4.1m and is composed of 30 double glazed units each with laminated inner and outer panes. A total of 26 structural glass vertical and horizontal elements, each composed of four plies of heat-toughened glass laminated with structural interlayers, support the glazing panels.
The design strategy adopted is to mitigate the risk of hazardous debris in the event of an explosion. To achieve this the principles of a balanced elastic design methodology were applied to all constructions. In practical terms this means that the primary structure connections are stronger than the beams, the beams are stronger than the glass retention system and the glass retention is stronger than the glass panels. The glass is then allowed to surpass its elastic limit in such an event but its retention to the rest of the structure is ensured via glass lamination, engineered structural silicone and bolted angles to the glass fins.
The vertical and horizontal fins are connected together in pairs using lapped glass laminate layers and large diameter stainless steel bolted connections to form 12 L-shaped structural beams. The bolted connections allow small rotations which is necessary to accommodate the movements of the slender cantilevered support and builds in further resilience to the multiple scenarios of an attack. Global stability of the glass structure is provided by the in-plane stiffness of the glass panels which avoids the need for additional structural bracing and introduces redundancy to disproportionate collapse in an attack event.
Case Study 3 – The Tottenham Experience, London (by Panos Papanastasis, Associate Façade Engineer)
The Tottenham Experience Building sits in front of the Tottenham Hotspur stadium and houses a museum depicting the history of the club, the largest club retail store in Europe, and ticketing sales. The building incorporates the refurbished Grade II listed Warmington House which is nestled at the centre of the building.
The new building incorporates one of the largest cast iron facades in the UK. The building also incorporates two incredibly impressive glass boxes and slender 12m tall glazing strips interfacing with the listed Warmington house, linking to the idea of transparency used within the Stadium to provide a glimpse of the hive of activity within whilst respecting the existing heritage assets.
The glass boxes consist of 12x2m glass panels, silicone-bonded to continuous multiple laminated glass beams on site, that give an overwhelming transparency to the façade. Glass panes are supported along four edges and bear on the supporting framing members through a minimum rebate which was informed through project specific assessments and FEM analyses backed up by existing test data. The glazing units include laminated panes with structural interlayers to enhance their post-breakage behaviour and increase their flexural capacity.
The members supporting the glazing were designed to develop their full capacity under the predicted loads. The connections were, in turn, designed to resist the forces arising from the capacity of the member to prevent premature failure to the supporting substructure. Appropriate safety factors were used throughout to increase robustness and add redundancy to the design.
Stadia become crowded places during different day or night time activities or events, this can simply be a team training session with supporting staff and media coverage, a sporting event or a music concert. Thereby stadia and supporting infrastructure can also become an attractive location for organised crime which could include terrorism. This has been demonstrated on many occasions globally, where criminal activists will target locations and events which are easily accessible, and regularly available for example premier sporting events. Being part of the entry walk to the stadium, strategies to enhance robustness in order to reduce debris and facilitate evacuation as well as to prevent collapse were implemented. Additionally, the glass boxes were also designed to resist large crowd loading.
Part of a larger multidisciplinary team, the architect Populous, facade consultant Buro Happold and contractor Octatube had to work together to ensure the technical challenges were tackled successfully against the tight programme. Buro Happold held the role of developing the design intent, advising, monitoring and coordinating the envelope performances and testing requirements. This ensured design developments would be compliant with the client aspirations but also statutory requirements.
This article was originally published in IGS Magazines Spring 2021 Issue: Read the full Magazine here for more thought-leadership from those spearheading the industry
Anna Wendt is a Partner at Buro Happold and a Fellow of the Chartered Institute of Building Services Engineers. Her primary focus is on interdisciplinary project delivery; she has extended her deep technical facade expertise to bring together a range of highly specialist engineering disciplines. This approach allows the unlocking of design opportunities and the delivery of seamless solutions to specific engineering challenges within the built environment. Through her strategic specialist advice to clients, architects, contractors, and drawing on the huge wealth of expertise within Buro Happold, Anna is able to deliver the highest quality design, engineering and consultancy projects for a diverse portfolio of projects across a variety of sectors and global regions.