Timber has been linked to architecture ever since humanity decided to leave the cave. As architecture moved forward so has done the knowledge and technics surrounding this material. Even in the ancient Rome, well-known for its brick, stone and marble architecture, architects found in timber a solution for its overcrowded capital: the Insulae. This building typology was composed by a commercial base made of brickwork and a main body of apartments with a timber structure. The lightness of wood allowed these buildings to rise higher, up to 6 floors. The building codes enacted during the time of Julius Cesar to limit construction heights up to 21 m prove that such heights were possible (2).

Asian architecture also used timber for its highest religious temples: Pagodas. This illustrates the durability of timber when it is well treated and taken care of. The Horyu-Ji religious complex host a 32.55-meter-high pagoda (Figure 1) that dates to the year 711 AC. It is still standing even though it is located in Nara, Japan, known for being a seismic region. Even the Kobe earthquake of 1995, that scored 6.9 in the Richter scale, wasn’t able to produce any structural damage. The structure of pagodas is composed by 2 parts, a mass timber central column (Shin-Bashira) and several storeys of timber frameworks (3). While the Shin-Bashira is isolated from the rest of the structure by 1 cm gaps, the surrounding timber structure is made of individual storeys that rest on top of each other. The combination of both strategies allows the structure to dissipate the oscillatory movement produced by earthquakes.

This tipology spread across Japan reaching heights over 50 meters (Figure 1). However, the highest historical timber pagoda that has survived to this day is Sakyamuni with a height of 67.31m. Built in 1056 AC, in the county of Yingxian, China, another seismic region, but with a different strategy. In this case, the stony base of 4m moves with the soil, and the oscillating part is reduced to 63m, remains a very important height.

Although the predominant material for structures in conventional architecture has been timber, it reached the XX century as a discredited material (4). Wood is easily associated with tragedies, such as the Great Fire of London in 1666, which prompted the first regulations against timber use in construction, or more recently the Great Chicago Fire in 1871 and the fire following the San Francisco Earthquake in 1906. All of them important cities linked to industrial development, that decided to trust in brick and stone, and later in concrete and steel, instead of timber. Therefore, timber was left out of the cities and relegated to rural architecture (5).

However, its actual downfall came by the hand of the industrial revolution. The timber industry wasn’t strong enough to take advantage of it. Besides, the development of materials as concrete and steel soon took over the construction industry thanks to its uniform and predictable behaviour. Something that wood couldn’t assure due to its natural background.

Iron and steel were materials that existed before the industrial revolution. However, this movement led to a democratization of these materials thanks to the new production methods that made it a cheaper and faster process. Both materials had a rapid development and acceptance in the industry, while timber barely evolved. At the same time, in Chicago the race to conquer the sky had already begun. A race boosted by the improvements made on elevator’s security and the reinforcement of these new materials.

The main features required to any structural material are a great strength, σ, a low specific weight, ρ, and a great stiffness, E. The following is a detailed analysis of relationship between them. The “structural scope” of the material, A, is the height at which an unloaded column will fail under its own weight, also called maximum height or insurmountable size (16), is the determining parameter for a material to be appropriate for its use in high-rise structures. It is the maximum height that a one-material element could reach maintaining its section before collapsing by its own weight [1].

The main reason that has biased our reaction to timber for structural purposes is its behaviour when exposed to a fire. However, these mass-timber products are extremely hard to set on fire and once they do the behave predictably. In a fire, the outer layers of the wood burn and chars. The charred layers lose their structural capacity but isolates the rest of the wood from the fire due to a low heat transfer coefficient (Figure 3). This coefficient in regular timber is λ = 0.14 W/m·°K, and when it chars it is reduced to λ = 0.03 W/m·°K. (19).

Besides conductivity, the main factors that affect timber combustion are: its specie (Softwood burns more easily due to resins and flammable oils contained inside it), its density (lighter woods are more porous and therefore burn faster), its sizing (larger timber pieces have more volume to heat which delays their flash point), its shape (slanted surfaces favour ignition), and its moisture content (the higher it is the slower it burns).

The charring speed of the fire-exposed surfaces is between 0.55 and 0.80 mm/min, although it might variate between wood species. This speed is used to calculate how much extra material is needed to prevent the structure from collapsing during the fire.

The main strategy to improve the fire resistance of the timber structural members is to increase the section dimensions in accordance with the extra time required. Another option is to cover the structure with gypsum or other fire-retardant materials. The last option would be intumescent paint, but it isn’t very developed for timber (19).

The accidental fire hypothesis (20, 21) establishes security and simultaneity coefficients that imply a reduction coefficient around 0.65 in conventional buildings. This means it isn’t always necessary to oversize the structure.

The Eurocode 5 considers different methods. The simplified, the charring method, which is also reflected in the Spanish code (20) is the most common. Whereas the reduced strength and the advanced method are used lees often.

Another strategy against fire is the encapsulation method. This method hides and protects the timber structure with fire-retardant materials like gypsum. Furthermore, timber isn’t a conductive material as steel is (Figure 4). This avoids the spread of the high temperatures to more vulnerable parts of the global structure far away from the fire that might be bearing greater loads.

Joints are the weakest spots during a fire. Charring depths are deeper due to the joint design or because of the use of steel connectors that carry the heat to the core of the junction. Therefore, steel connector must be hidden inside the timber and protected by it.

In recent decades there has been a rediscovery of timber, especially in industrialized countries, where contemporary awareness advocates the need to protect natural resources. The large amounts of energy and high greenhouse gas emissions needed to produce high-tech building materials, such as steel or concrete, are incompatible with this growing concept of environmental sustainability.

In this sense, wood constitutes the supporting structure of the tree and has the advantage of being a structural product at source, without the need for an industrial transformation process associated with the high energy cost that this entails. Once a tree dies, the CO₂ absorbed during its life starts returning into the atmosphere as the tree decays. However, the wood elements placed on site fix the carbon captured in their cell walls throughout the useful life of the structure, contributing to a fully sustainable development. In addition, the use of wood certified with FSC, PEFC, etc. seals, which guarantee that wood comes from sustainably managed forests, contributes to increasing the rate of atmospheric CO₂ fixation. Sustainable management ensures that volume of cut wood is below the production capacity of forest mass and the regeneration of all felled trees. During their growth, new trees are able to fix a much higher amount of CO₂ than previous adult trees, because their photosynthetic activity is more intense.

After the wood elements have been used once, they can be reused in other buildings, recycled for other uses, and even burned or left to decompose in landfills, in which case the stored CO₂ is returned to the atmosphere. Therefore, the longer the life of wood elements, the greater the benefit to the environment.

It is estimated that production of one cubic meter of timber is able to absorbe between 1 and 1.6 tons of CO₂ (24). According to Green (23) a 20-storey building with a timber structure could store 3150 tons of CO₂. If the same building had a concrete structure it would generate 1215 tons. Therefore, the difference of material implies 4365 tons of CO₂ in our atmosphere, the equivalent of 900 cars off the road for a year. Steel and concrete, in comparison to timber, require 26% and 57% more energy, release 34% and 81% more greenhouse gases, generate 24% and 47% more pollutants into the air, unload 400% and 3000% more pollutants into the water, produce 8% and 23% more solid waste and use 11% and 81% more resources (percentages obtained regarding the weight of material used).

The ecological advantages of timber aren’t limited to the production and construction phase. Timber also helps to the overall energy efficiency of the building as insulation. Timber has a heat transfer coefficient (0.14 W/m·K) 14 times better than concrete (2 W/m·K) and 350 times better than steel (50 W/m·K) As a matter of fact, it’s only 4.5 times less insulating than medium density rock wool (0.04 W/m·K), one of the most common insulating materials (25).

As any prefabricated product, engineered timber involves a reduction of time in the construction process, which reduces costs. After timber layers are glued together, the material is cut precisely with computer numerical controlled (CNC) machines. This way the pieces can be placed and joined easily during construction. These ready-to-place products allow a just-in-time delivery method, which avoids the need of on-site storage, as products are delivered the same day they are needed. It also reduces the waste produced on site and the waste produced on the workshop can be recycled into other products.

To set a couple examples, the Stadthaus had a construction pace of 1 floor per week (26). While the Bridport House only took 12 weeks, 4 workers and 1 supervisor to build an 8-storey apartment building (15). None of these 4 workers were specialised in CLT nor timber construction as these products use very simple construction methods. In short, these prefabrication methods reduce cost and time during construction, nevertheless, they require extra time in the design process.

Regarding stability, the maximum slenderness is obtained from checking for horizontal displacement due to the gravitational forces, P, whose effect is stabilizing, and the horizontal forces, W, whose effect is destabilizing (Figure 5) [2].

The horizontal displacement at the top its linked to its slenderness. Horizontal actions cause a horizontal movement on the top of the building. The Spanish building code (CTE) (27) limits the displacement, δ, to 1/500 of its total height, H. Therefore, the maximum slenderness depends on the unitary deformation, ε. In the vertical elements, or supports, part of this deformation is due to the shortening under compression.

The rest will be an increment by compression or tensile stress produced by the wind. Oscillations and aerodynamic effects are not considered.

To obtain the unitary deformation due to the wind from the total deformation a section of the bracing is analysed (Figure 6). Both ends are compressed due to the gravitational forces. However, while one end is compressed the other end is decompressed due to the wind forces.

In the decompressed end (left side on the Figure 6), the axil value of decompression due to the wind (unfavourable effect), N_wdlim, can’t be higher than the compression axial generated by the favourable effect, N_Gf. Al most, they can be equal, so that the support does’nt reach a tensile stress, and don’t pass the tensile stress on to the foundations [5].

In the past 10 years the number of architects interested on the timber properties to create light, resistant and environmentally friendly buildings have increased significantly. In order to be able to build one, these architects often need a reformulation of their country’s construction code, usually too conservative towards timber. For example, in Germany, until 2002 timber structure buildings could only reach a height of 3-floors. And even 6 years after Germany’s code was modified, the 7-storey E3 building from Berlin), whose structure is made of mass timber, was required to build a concrete emergency staircase detached from the building (28).

These buildings, pioneers, have paved the way for new timber constructions. The technology has been available for years; however, the problem is the scepticism towards this material. In 2008, there was only one timber building that surpassed the 8-storey limit. In June 2017 there were already 40 buildings constructed, in construction or planned (9).

The Stadthaus from London (Figure 8), was built in 2009 reaching a height of 30,3 m and 9 storeys. It stands out for showing the feasibility of timber structures made of CLT and Glulam as well as its technological and economic competitiveness compared to steel and concrete. It creates a pattern, highly repeated afterwards, the use of CLT panels as vertical structure (walls) as well as horizontal structure (slabs) on top of a concrete 1-storey podium that accommodates the ground floor (29).

In 2015 with the construction of The Treet in Norway (Figure 8) the symbolic height of 50 meters is finally surpassed. Constructed by BOB BBL, it is conceived as a pilot project to show the feasibility of creating apartment buildings following the Passivhaus standards using a prefabrication method (30). Even though the original costs are higher than a conventional concrete structure building, the prefabrication of both, the Glulam main structure and the apartment CLT modules, lead to a reduction in construction times that lead to a reduction of the overall cost of the building. Once again, the timber structure lies on top of a concrete base, although this time instead of the ground floor is the basement. The building incorporates glulam bracing beams. This glulam structure can be seen from the inside and from the outside although it is protected by a curtain wall. The main load bearing structure is designed to receive the apartment modules as if they were «drawers» (31). The building includes concrete floors to increase its weight and reduce the sway produced by heavy winds.

In 2017 the Brock Commons Tallwood House Student Residence (Figure 9) set a new height record: 57.9 meters. Its construction took advantage of the control of the British Columbia University over the building code that affects the buildings within its campus (32). The bearing structure is made of CLT and Glulam pillars and 2 concrete vertical cores. A wooden structure that like the Stadthaus stands on a 1 story concrete base.

Even though the main reasons for choosing timber over concrete or steel might be the carbon sequestration and its innovative character, Acton Ostry suggests that by increasing the time during the design phase using BIM technology to define every detail and enhancing its prefabrication led to a reduction in the overall cost of the building. They designed a steel connector (Figure 9 right) to pass the loads on between pillars in consecutive storeys. It also helps the assembly of the building which cut construction times significantly by 2 to 3 months (33) In order to manage fire resistance required (120 minutes) the structure was covered by gypsum panel, which sadly, hide away the true materiality of the structure. The two concrete cores give the building better stability against horizontal actions (33).

While only a few buildings dare to surpass the heights of their predecessors, many others are being built to already stablished heights (Table 4).

Timber structures are still looking for new limits. Nowadays, there are several buildings under construction, or ready to start it, that will surpass the height limits already stablished (Figure 10), such as the Haut in the Netherlands or the Terrace House in Canada. However, two of them stand among these buildings: the Mjostarnet in Norway and the Hoho in Austria. Both will reach heights over 85.4 meters and are set to be completed along 2019, which will clear the way for future buildings.

Mjostarnet (Figure 10) follows the typology set by the Treet: a structure completely made of timber with glulam diagonals to brace the building. The ecological aspect of the building is enhanced by the use of local resources. Again, the connectors required special attention as they are the most delicate point of the structure. After several tests from the SWECO company, a steel connector embedded in the glulam beams was decided to be the best solution. The Steel connector is also covered by an intumescent paint layer so the R120 fire resistance could be granted (34).

The HoHo building (Figure 10) has a closer relation with the Brock Commons structure. However, the large size of its concrete core sparked a debate about when a structure stops being considered a timber structure and starts being considered a composite structure. Eventually, the CTBUH decided that at least 85% of the structure needed to be timber, which isn’t the case for the HoHo with only a 75% of timber (35).

In the past years several architecture firms have stepped in and tried to make their own contributions to this topic. These contributions have been either intensive research that help the evolution of the technique as well as the building codes or speculative projects that show us new typologies and what a building of this characteristics could look like. As many of these projects aren’t intended to be built, they are freed from following the most restrictive code issues and from waiting for a developer willing to innovate to invest his money.

One of the most complete research among the industry is Michael Green’s work (23), where he approaches technical, economic and social issues. In this paper he also proposes a feasible timber structure for 12 to 20-storey buildings and a construction process called FFTT (Figure 11). The FFTT solution is a tilt-up system of large CLT panels that compose the main load-bearing components of the building, an inner core, the external walls and the floors between both. The large size panels allow to assemble up to 6 floors at a time, which reduces erection costs. The FFTT solution proves that timber buildings can also be cost competitive.

SOM, a practice specialised in skyscrapers (Willis tower and Burj Khalifa among others), is also researching this topic (36, 37, 38, 39). They state that composite structures (timber-concrete or timber-steel-concrete) (Figure 12) where timber is the main material could reduce the building’s footprint around 60% to 75%. Even though an only-timber structure might reduce even more the building’s footprint, they consider that it’s not worth the high costs and technical complexities. Therefore, for the moment, they opt for composite structures and continue testing different technics.

Perkins & Will and the consultant Thorton Tomassetti proposed the River Beech project (Figure 13) as part of the urban planning of Chicago’s riverfront. The building is composed by 2 slender volumes, built by modules with a structural diagrid, connected by glulam diagonals creating community spaces between them. However, the project is on hold until Chicago’s building code is redeveloped (7).

In order to attract the general public many architecture studios, real estate developers and timber companies are producing model projects to show timber benefits. Therefore, the aim of these projects is to eradicate the bias that might be against timber, grow its acceptance and, consequently, speed the evolution of building codes. From an architectural point of view, these projects also help to develop new typologies inherent to wood instead of copying the steel and concrete typologies. The Oakwood tower or the Baobab are two fine examples of these proposals.

However, there are also proposals that hope to be built but are on hold until the researches, techniques and codes evolve enough. The HSB – Västerbroplan, a 223 meters-high wooden skyscraper scheduled for 2023, or the W350 tower, an ambitious project lead by Sumitomo Forestry that seeks to reach a height of 350 meters before 2041, are two of these projects.