With long heatwaves and skyrocketing average temperatures highlighting the need for sustainable construction techniques, architectural firms are looking at timber frame buildings and specialist suppliers as the future of construction.
However, whilst some designers are exploring the potential of hybrid and engineered mass timber as what ArchDaily describes as the “concrete of the future”, others are taking a less dogmatic approach and seek to combine it with existing, highly successful construction methods.
The hybrid skyscraper Atlassian Central, which will be the largest hybrid timber building in the world once it is completed in 2026, has led to questions about our typical assumptions surrounding building materials and how they are used.
The trade publication Architect Magazine went as far as to describe it as a “new species” of building, but is this truly the case? What has changed to make hybrid construction possible? And is it sustainable enough compared to redoubling efforts to make ever-taller ‘plyscrapers’?
What Is Hybrid Construction?
Architects, building managers and contractors typically discuss buildings in terms of the primary materials used.
Whilst the vast majority of buildings will feature elements of concrete, steel, glass, plastic, wood and countless other materials in various forms, there are typically one or two primary materials that form the structural core of the building.
In a timber frame building, this takes the form of mass timber, which includes engineered wood products such as cross-laminated timber (CLT), glued-laminated timber (glulam) and countless other coatings and products.
Whilst these differences help make wood stronger, more durable and improve fire resistance, none of this stops the building from being considered a “timber” building, any more than wooden window frames stop a concrete house from being considered as such.
Hybrid buildings are significantly more complicated than this, integrating steel, concrete and wood together in ways that accentuate the benefits of each material whilst minimising the flaws.
Atlassian Central does this with a steel exoskeleton and concrete foundational core, combined with columns and floor systems made from glulam and CLT, respectively.
It is difficult to truly describe it as one type of building or another, as it requires all three materials to be structurally sound, and maximises the benefits whilst minimising the flaws.
The concrete is rigid, secure and fireproof, the steel provides a degree of lateral stiffness through its I-beam superstructure, whilst the wood is a natural insulator, is lighter for its level of strength and allows for the sequestration of carbon during its construction.
The result is, according to WSP and the University of Warwick, a 60 per cent reduction of embodied carbon throughout the project, which is the carbon used throughout the entire production process.
Why Use Hybrid Timber Construction?
Every material used in construction has certain advantages and disadvantages, and the primary limitation at present that CLT has as a mass building material is its currently limited spans compared to steel.
Whilst steel frames can extend tremendous distances without the need for central columns or stabilisation materials, CLT alone has some limits when it comes to the span that it can take the strain.
An example of where hybrid timber became the optimal solution was Foundry South in Richmond, Virginia.
As a corporate campus complete with a gymnasium and auditorium, Foundry South requires large, uninterrupted floor plans that would typically be seen in steel production.
However, the designers also wanted the aesthetic appeal of mass timber and wanted to minimise their carbon footprint wherever possible, which made a hybrid approach essential.
What Has Changed To Make Hybrid Timber Buildings Viable?
Hybrid timber buildings have existed for a very long time, but they have often used concrete or steel elements out of necessity rather than entirely through choice.
The current tallest wooden skyscraper on Earth, MKE Ascent in Milwaukee, Wisconsin, uses post-stressed steel beams and a six-storey concrete car park underneath.
Such is the significance of its foundations that some parts of the architectural press describe the building as a hybrid structure, and its status as the world’s largest wooden skyscraper has been disputed as a result of this.
One of the biggest changes that has allowed hybrid timber buildings to become more viable is a much greater volume of applicable knowledge of how timber framing works at a much larger scale than in the past.
Mass timber’s advantages in terms of sustainability, lightness, astonishing strength and ease of construction once on-site have been known for decades, if not centuries.
However, the ability to use this in architectural design, surveying, manufacturing and assembly was somewhat more limited, which was part of the impetus to rely on established but expensive and carbon-intensive materials instead.
Will Hybrid Timber Replace Pure Timber Frame Buildings?
There are two schools of thought when it comes to hybrid timber.
Depending on who you ask, it is a question of whether the existing wave of plyscrapers will be replaced by a more hybrid approach in future, or conversely whether the regulatory necessities of existing hybrid buildings would disappear to allow for “pure” mass timber.
In other words, when does a mass timber building become a hybrid building?
As noted above, MKE Ascent has a concrete and steel core to hold the 25-floor wooden skyscraper, whilst the W350 Tower in Tokyo would use steel reinforcement to help the wooden structure endure the significant earthquakes characteristic of the nation.
In the latter’s case, ten per cent of the building’s mass by volume would consist of steel braces to help boost wood’s natural resistance to earthquakes and strong winds.
Accepting the premise that both are mass timber buildings, would a hybrid approach replace them in future? It appears somewhat unlikely.
The appeal of plyscrapers comes primarily from the sheer scale of what can be done with an often-misunderstood material.
Wooden skyscrapers and mass timber structures prove that it is not only possible to make strong, robust, long-lasting and highly practical buildings made primarily of engineered wood materials, but that there are significant benefits for doing so.
Whilst there is clearly a case for hybrid projects to fill the gaps that timber buildings cannot, it is more likely to reduce the carbon footprint of conventional steel and concrete skyscrapers rather than stopping the rise of mass timber in construction.