Tall-Building trends

Tall buildings are a product of our modern world. They are enabled and driven by technological advancements, fuelled by economics and globalisation, and thus by definition sensitive to social, political and financial events, whether local crises or global emergencies. In this long read, AKT II’s design director Ricardo Baptista discusses the tall-building typology’s trends, whilst questioning its role within our evolving urban environment.?

A tale of three crises…

London has been accumulating the effects of several different crises, each adding a further layer of interrogation for the role, design and construction of tall buildings. NLA’s tall-buildings survey, which considers all current planning applications plus ongoing constructions, shows a clear relationship between these crises and the quanta of new planning applications and on-site starts.

The Brexit vote clearly had ramifications beyond the political and social, with the format of the UK’s future relationship with the EU remaining unknown for a lengthy period. The impact of this on the UK’s economy, tempered the development of tall buildings, particularly in the commercial sector, while the residential sector was still been feeling the effects of Stamp Duty changes.

A year later, we had the terrible fire at Grenfell Tower. The subsequent fallout is rightly impacting how tall buildings are designed, built and maintained, while inevitably adding a financial onus to these buildings’ viability.

And now the Covid-19 pandemic. More viral outbreaks and pandemics have occurred within the past 20 years than in all of the preceding century; it’s likely indeed that there will be a ‘new normal’. Whatever this looks like, it will force the construction industry to reflect on the flexibility of current and future projects, as well as that of existing buildings, and of their intended uses.

? New London Architecture.

…and one emergency.

But something much bigger looms over London and the rest of the world: the climate emergency, first declared in December 2016. The construction industry contributes disproportionately to carbon emissions, while the scale of the tall-buildings typology magnifies any client brief and every design and procurement decision – negatively or positively. Any obsolescence of buildings of this scale should be socially unacceptable, and this should be a founding principle for all future tall-building design. Clearly, more-sustainable materials must play a role, but other, more lasting typologies must moreover be designed and better constructed (or, most likely, assembled), to tackle this emergency holistically.

Why tall?

The rapid urbanisation experienced within the world’s larger cities has led to huge demand, which is not expected to abate, with 60% of the world’s population expected to be living in cities by 2030, and almost 70% by 2050.

? UN World Urbanization Prospects (2018)

This is further compounded by densification, with less urban land available, while consecutive layers of overdevelopment – particularly in historic cities such as London and New York – have sterilised the ground by making it extremely difficult, and therefore costly, to introduce new foundations. These factors combined have resulted in the rapid acceleration of tall-buildings construction, with more constructed within the past two decades than in the prior 120 years.

So what happens now? Against the backdrop of the climate emergency, and with globalisation in apparent retreat, is this the end of the mega-city? A renaissance for smaller towns of the suburbia and beyond? And is the pursuit of buildings that are dominated by single-use, particularly in city centres, the right approach? Or should uses be mixed into ‘vertical cities’, so that the transportation infrastructure is no longer a fuse point in future pandemics?

Reinvent existing towers.?

The most efficient and effective way to reinvent or refresh the existing built environment can be through the reuse of existing building stock – an approach with clear sustainable and economic benefits. This breathing of new life into existing structures will continue to be a frontier for every designer, and where possible should involve the extension of these structures through modern analysis and design techniques that explore the structure’s available redundancy.

A first of its kind, the redevelopment of the South Bank Tower proved this approach, by adding 11 storeys onto the existing 30-storey structure. This was achieved by coupling existing information with advanced engineering techniques, including CFD analysis to more accurately predict the building’s wind loading, as well as 3D geotechnical analysis of the soil-structure interactions to mitigate any foundation movement that would otherwise impair the adjacent below-ground assets. A similar approach has since been employed at HyLo (formerly Finsbury Tower) which adds 13 storeys to the existing 17, which is nearing completion.

South Bank Tower - 30 storeys extended by another 11. Hylo - 13 storeys added to the existing 17.

Both buildings are exemplars of the ongoing trend to convert old commercial buildings, deemed obsolete by current workplace standards, particularly in a post-Covid world, to new uses, often residential or mixed-use, which beyond lowering the projects’ carbon footprint, can contribute to making city centres more diverse.

These projects also highlight the growing importance of what is now the world’s most valuable commodity, data, and how the collection of design and construction information can yield significant benefits, particularly when revisiting existing buildings and their structures.

Universal tall buildings.

Buildings and their structures should provide an adaptable framework that allows multiple or hybrid uses throughout the project’s lifetime: they must become ‘transformers’, inherently able to adjust to future requirements, and designed for ‘long life, loose fit, low energy’. This adaptability will be proportional to the building’s ambition and budget.

A hierarchical structure could achieve this; split into primary ‘permanent’ and secondary ‘temporary’ elements, with the latter embodying an ability to easy adjust through time, to suit changing requirements and uses. This is the strategy adopted for the new Google headquarters in London’s King’s Cross – a 300-metre-long ‘groundscraper’ – where a permanent concrete backbone supports a hanging adaptable structure. This system allows the lighter, steel-and-timber structure to be added or removed to suit Google’s evolving demands, whether by creating double- or triple-height spaces or, at the other end, by maximising available floor area.

? BIG / Heatherwick Studio

This adaptable framework can translate between groundscraper and skyscraper, allowing the fabric of an entire block-section to be changed to a different material, such as timber, or internally reconfigured to suit differing use. This separation – villages within a vertical city – creates further opportunity: with plant levels spread more evenly throughout the volume, the core and risers require less floor area, while the services’ adaptability is enhanced. The technical areas can also contribute to the building’s environmental performance, by incorporating labyrinths at the base of each ‘block’ that allow air to flow through, to naturally cool the building while reducing wind loadings and aerodynamic effects – they can also accommodate transfer structures that allow changes in the column grid and can multitask as outrigger systems that link the core with the perimeter.

A hierarchical structure provides an adaptable framework

Tall buildings are no longer limited to simple extrusions of a single floor plate. An independent operation of vertical or horizontal sections will allow services and circulatory layouts to be optimised for each use. Residential floors can be served by centralised cores, while office floors can be served by cores at one edge to create large, uninterrupted functional areas. Amenities and other uses can be provided not just at ground level, but throughout the building’s height.

Technology will continue to inform the tall building’s function and appearance. Recent vertical-transportation advances, such as double-decker lift cars, require fewer lift shafts and thus allow more efficient core areas. Soon, ‘free movement’ lifts may surpass these, with cars operating both vertically and horizontally in a shared network of shafts. This will undoubtedly accelerate the rise of interconnected tall-building clusters, wherein buildings are no longer designed as stand-alone structures. Connective links may share more than just circulation, forming each cluster into a resilient, true vertical city.

With cores no longer expanding proportionally with building height, a paradigm shift in tall-building engineering could bring in more adaptable, permeable ‘soft cores’, opening up multiple avenues for building stability. This is something that we’ve explored on several projects over the years, where the building’s frame is engaged to simultaneously withstand vertical and horizontal loads, making the building inherently more permeable, and adaptable.

The Yellow Building, a 'soft core' approach. ?

Integrated design.

If more structure is moved to the building’s perimeter to ensure its stability (and adaptability), how will this then impact the envelope systems? New typologies that respond to the challenges of climate change – specifically solar gain, and overheating – can benefit from a more solid, less glazed envelope. This in turn will unravel opportunities for structural options that can perhaps become part of the architecture while also contributing to the environmental strategy.

This was the case for the Central Bank of Iraq, currently under construction in Baghdad, which features an exoskeleton that performs both structural and environmental functions. The building springs from its base as a solid tube that gradually opens up to allow daylight and views into the upper office floors, while also shading the critical solar directions - whereas the eastern and western elevations are virtually solid, to prevent solar gain from the shallow early and late sun, the southern elevation opens up, to allow sunlight in.

The Central Bank of Iraq: a structural exoskeleton driven by environmental strategy

The inverted tower typology may incidentally also be a solution for city-centre sites where available footprints are often constrained by existing substructure or infrastructure, and opens up possibilities for exploring ‘air space’. This integrated approach also feels particularly relevant for residential typologies. Here, the finely tuned exercise between minimising overheating and maximising daylighting ultimately requires a quantum of solid cladding, which can be engaged to maximise the perimeter structure and thus improve the overall structural efficiency.

Beyond acting as a passive system, which can provide shading, these structural frames can also contribute to further environmental strategies. The wider, ongoing drive towards fossil-free energy will lead to quieter urban environments with cleaner air, which in turn will allow thus-far-hermetically-sealed envelopes to become permeable with mixed ventilation modes that include natural ventilation and minimise MEP requirements, complete with thermal mass and cooling pipes. Other technologies, such as fa?ade photovoltaics and ground-source boreholes, are also becoming more widely adopted and are allowing MEP systems to benefit from free energy.

Old materials; new specifications.

More-sustainable materials will also have to play their part. Traditionally conservative timber codes are finally catching up with engineered-timber’s evolution, which is enabling built timber projects to approach 100 metres in height. Many further, taller timber buildings are being designed, and with super-tall timber buildings also being researched, the era of tall-timber-hybrid is upon us. In the UK, this typology has been impacted by the Grenfell fire, with regulations since updated to limit any use of timber cladding to a fixed height; this has inevitably delayed the progression of taller timber projects here.

Other materials however should not be dismissed, providing they are efficiently designed and appropriately specified. For the 45-storey Highpoint tower in south London, we developed a structural system that engages all elements that support the vertical loading to also withstand the horizontal loading, an exemplar of lean design. Here, the full extent of the solid facade area is again used to place the perimeter structure, which is then connected to the main core through diagonal party walls which stiffen the slabs and effectively act as mini-outriggers. This fully mobilises the full width of the tower as part of the stability system, while all vertical elements – a mix of twin walls and solid precast walls - were fabricated offsite.

This system has been tested in various project iterations over the years, and has proven inherently adaptable, with the potential to incorporate a multitude of layouts, within the structural framework.

Highpoint, multitasking structural solution

Build better; build faster.

Tall buildings must also address the challenges faced by contractors and the wider construction industry. Labour shortages will most likely remain a challenge, and with productivity levels continuing to disappoint, alternative approaches are paramount. Modern methods of construction (MMC) allow faster and safer on-site construction, requiring fewer personnel while minimising waste and transport, and can deliver a better-quality product if well-designed from the outset.

A growing number of our projects feature structural solutions, either partially or wholly manufactured offsite, based on an elemental ‘kit of parts’ – e.g. Highpoint – but a more holistic approach is needed to reap the full benefits of MMC. Together with Mace, we are currently exploring low-carbon, off-site high-rise solutions that fit structural modules with both services and envelope, allowing these to be lifted and assembled as one, thus drastically reducing construction times.

Given their potential benefits, these typologies raise questions about the timings of the contractor’s involvement within the design process. This applies particularly for tall buildings, the scale of which can maximise the benefits of a focused approach on buildability from the project’s inception. Perhaps, rather than shaping a form and then forcing the makings and workings of the building to follow suit, an ‘inside out’ approach could be pursued?

Off-site high-rise solution, developed with Mace.

Data-based design.

Another area where advances can be made is the use of sourced data to inform the design of tall buildings. Historically, the collection of structural data has been very limited, but the relentless technological and digital progress that is sweeping our industry is now changing this.

One Park Drive and Highpoint are two case studies where we have collected structural data, particularly on the building’s behaviour under strong winds, which can then be compared with our codified parameters and analysis. This iterative, post-processed approach allows a greater understanding, and a subsequent refinement, of the key parameters that drive the building’s structural performance, providing further opportunities to optimise our future tall-building structures.

Data sourcing at Highpoint

This data is also used continually to calibrate our in-house bioclimatic toolkit. This software toolkit is used to assess a development’s impact to the surrounding public realm and to the amenity spaces such as terraces, balconies and rooftops, and can also be used to assess how tall buildings withstand wind loadings. This includes computational fluid-dynamics analysis (CFD), at the early design stages, to quickly test design iterations within a ‘virtual wind tunnel’. Post-validation of the CFD results using a physical wind tunnel is still currently required, but the CFD accuracy can now closely match, and sometimes even surpass, the physical wind tunnel information. Altogether, this process allows clients and design teams more time and freedom within the design process.

Present technology also allows BIM models to expand into true digital twins, including the use of sensors that gather real-time conditions data which is then received, processed and monitored within cloud-based systems. These virtual models can capture the nature of the building itself, as well as its services and structural performance, with the latter information extending beyond the building’s operation and including its design and construction – this stream of data, which can include parameters such as the concrete strength gain over time, will be invaluable when it comes to the end of the building’s design life, allowing it to be extended.

Virtual wind Tunnel

The new, variable normal.

Whether through crisis or otherwise, change are coming. Trends continue to accelerate, including those of tall buildings such as timber-hybrid construction, computational design, offsite construction, and adaptive reuse. Universal tall buildings must entail more adaptable, resilient typologies that embody a greater integration between structure, envelope and services. These will be built through offsite manufacturing using more-sustainable materials, and will benefit from advances in automation, robotics and AI. Ultimately, these buildings must be designed around a loose-fit brief that aligns with an increasingly variable future.