Citius, altius, fortius
A good part of multistoried residential construction was executed, from the start, in wood. This was how it was until the time between the early 19th century and the early 20th, when things changed. Its availability, its versatility, and the favorable relationship between its mechanical features and its weight make timber an efficient element in managing the lateral stability and vertical load transfers of residential buildings in urban encalves (although stone remains known as the noble material for social representation). Nevertheless, neither in Madrid of the Habsburgs nor in the Paris of Haussmann do residents notice the structural timber; unless, of course, they are socioeconomically disadvantaged, and excepting the wooden frames of certain rooms in stately homes.
Comparison with ‘modern’ materials suggests that the neglect of wood coincided with the development and heyday of reinforced concrete and steel. Reinforced concrete had a relatively brief crossing through the desert: from the time Le Corbusier declared it to be the quintessence of the future of construction, in the 1920s and 1930s, to the 1950s, when it became structural material par excellence, only two or three decades passed (and only four or five since the ‘foundational’ period of the Moniers, Perrets, or Hennebiques). For its part, steel settled in a comfortable position as the material that got most attention from 19th-century engineering (the ‘century of engineers’). Reflections having to do with the mechanical performance of a beam of Galileo involved wood; but already in the 19th century, in works by Navier, Culmann, or Catigliano, wood remained anecdotal (other than being often labeled as comparatively suspicious).
When exactly did wood lose hold? It was probably a simple question of economy. At the end of the 18th century, the industry of structural timber was rather miserly and it never managed to engage in the great movements of research and capital that led to the Industrial Revolution, despite some exceptions of little operational importance. The Industrial Revolution was accompanied, at the close of the 19th century, by the blossoming of the structural typologies that separated enclosure from structure, and gave rise to a race of tall buildings in the United States.
So by the mid-20th century, almost all medium-height residential and office buildings in the world were done in reinforced concrete, and with hybrid systems made of reinforced concrete and metal frames. These materials penetrated and filled the structural materials market without needing flashy advertising campaigns: the advantages were appreciated directly and intuitively by architects, engineers, contractors, and developers. Terms like reliability, speed, control, industrialization, or modernity became key words, but they could not be translated to the language of the industry of timber structures, given the state of these during those times. Key words led easiily to ‘idols’: being against, or being on the margins, was ‘taboo.’
We are in the second decade of the 21st century: we had to wait until the end of the first to see a ‘high-rise’ timber building, the Murray Grove, get built. It went up in London, and rises just eight floors. Why were no tall buildings built with wood before? Was there some technological impediment? It does not seem so; take the slenderness of 150-meter-tall pagodas that have stood for centuries in environments of much seismic activity, or the diaphanousness of the transversal section of medieval buildings of Scandinavian or British tradition. As for the problems associated with the risk of fire and the durability of materials, it has been some time now since serious analysis of statistics made us see that it was not particularly a problem of wood. So what is now happening with wood? The same thing that happened with reinforced concrete and steel: the agents of construction have started to associate timber structures with virtues like reliability, speed, control, industrialization, or modernity. Since the fading of wood and its abrupt withdrawal from the structural market, three changes have taken place.
Technology and Environment
The first has to do with the actual evolution of society, or rather its models of economic growth. No sector of industrial activity can avoid declaring the environmental impact of its activity (term to be taken with the degree of generality desired). In this sense, timber is practically the only material with the capacity to altogether eradicate the environmental costs of construction (at least in the area of structures). The difference, here, between wood and any of the alternatives is not one of nuance; it is huge.
The second change is the progress made in scientific-technical knowledge on the material. Since the mid-1980s, the amount of available knowledge about timber structures has been comparable to that about structures made of steel or reinforced concrete, to the point that current codes are calibrated directly so that the constructions resulting from applying them have the same degree of structural feasibility, regardless of the material used.
The third change is the emergence of different ways of using the raw material. From the invention of laminated timber in the early 1900s to that of cross laminated timber in the final years of that century, passing through all varieties of components (plywood boards, particles, or microlaminates), the 20th century left a ‘toolbox’ ready for addressing the building demands, whatever they are, of the 21st. Finally, cross laminated timber (CLT or X-lam) has allowed a substantial qualitative leap like few previous ones, in such a way that the rise in acceptance of this material – since the first factories in Central Europe some twenty years ago – has no equal in the recent history of the wood industry. It is highly probable that a significant part of high-rise construction in a not very distant future will be based on the use of cross laminated timber, in combination with other components of wood (solid, laminated, or particles), or in hybrids with concrete or steel.
On the other hand, there is nothing new. The shields of the Roman army, by the end of the republican period, had three layers of solid wood laminates set in alternating directions and glued and nailed; that is, cross laminated timber. These shields, in turn, were versions of the different ways in which the armies of those times made shields. And it is not rare to find cross lamination (nailed or bolted in wood) as a central component in siege tower systems, one of the most complex structures since antiquity because of the way they deal with lateral pressures. The same structural concept is ubiquitous in the tradition of naval carpentry. What’s the cardinal advantage now? The favorable resistance/ weight relationship inherent in wood, combined with the industrialized production of flat formats in an almost unlimited choice of sizes. Right now, cross laminated panels are manufactured in a maximum size in the order of 20 x 4 x 0.5 meters. The result is a superficial structural component with a base that is only a fifth part of concrete in weight, an eighth in thermal transmittance, and half in rigidity and resistance.
It is important to consider that these new materials facilitate structural design in an extraordinary way, while reducing to a minimum actual work on the construction site, which, moreover, does not require elevated qualifications. On the other hand, adaptation to work in BIM is immediate and prefabrication is not optional, but the natural path. Any structure of this kind takes up many more work hours in a technical office and in the industrial process than on the actual building site, and it is immensely simple to build with workers accustomed to assembling structures of concrete or steel. These touches give an idea of how building in wood today responds to the theoretical desires of the Modern Movement in terms of how construction ought to be.
The 2008 construction of the Murray Grove building in London – designed by Andrew Waugh – was a major step forward in the development of highrises in timber, and was economically feasible thanks to the systematic use of cross laminated timber. Nevertheless, to promote high-rise building with wood we must mention precedents in various Scandinavian programs (which gave rise to several experiments with heavy and light frames) or in similar programs developed in Central Europe. And of course we should highlight the efforts made, in relation to these programs, to gradually eliminate the limitations imposed on raising tall buildings with timber; limitations which are not based on evidence. In this way, some of the first tall buildings to go up in Sweden and Finland in the 1980s tended to require special permits to skirt the limitations. Today, standards that groundlessly limit the heights of timber-structured buildings are rare in economically developed countries, and everything seems to point to their imminent disappearance. In contexts where such limitations do not exist (as in much of North America), and in those where this favorable situation coincided with a strong and consolidated industry of building with timber, there seemed to be an unwritten law against exceeding four floor levels. Various European experiments with light timber frames (as in Scotland and Bavaria), undertaken from the late 1970s on, also maintained a maximum of six stories. And within this bracket of 3 to 6 floors, controlling the lateral stability of the frames becomes expensive and difficult. A construction system cannot have two greater enemies.
As for frames of the heavy kind, because of the very nature of their joining systems and in spite of (or precisely on account of) the large number of solutions available, they have over the decades been circumscribed in rather limited segments of the market, and with few exceptions they have stayed on the margins of efforts to promote high-rise residential construction in timber. Nevertheless, it is true that the use of heavy frames in increasingly demanding structures (towers 150 meters tall, roofs with openings of 80 to 200 meters) has brought about a comfortably grounded feeling of control over the material comfortably grounded feeling of control over the material.
The appearance of cross laminated wood solves the problem of managing lateral stability without committing the ‘mortal sins’ of increased costs and complexity of control. Moreover, this appearance happens when knowledge about the behavior of light and heavy frames in traditional tall structures had already generated an exhaustive body of research, knowledge, and innovation. Incorporating all this information into the well-established tradition of high-rise construction in steel and reinforced concrete is much less complex that it might seem at first.
Now that we are able to raise ourselves on the shoulders of these giants, highrise timber building – whatever the limits – is now an additional option we have; an architectural design option or a decision informed by the desired degree of environmental impact. In no case is it a decision conditioned by economic motives, nor by regulatory or technological limitations of any sort.
Shortly after the London building mentioned above, other economically developed countries (such as France, Germany, Australia, Italy, and Austria) saw the rise of constructions falling within the bracket of 8 to 15 floors. In fact today it is hard to say how many buildings of ten floors or so, and with structures of timber, are going up in the world. On the other hand, we can already find technically and economically consistent studies for buildings of 100 meters, a height which took on undeserved symbolic value ten years ago. Vancouver will in 2017 be seeing the completion of the first timber tower 50 or so meters tall, and Amsterdam the start of construction of one approaching 70 meters. For the time being, proposals exceeding 100 meters are at the exploratory stage (with studies for towers in Stockholm, Paris, or London), but they are realistic. Even in zones of strong seismic activity it is increasingly clear that wood, with the knowledge so far obtained, is already an option to consider when plans to build a tower are made.
From 1879 – when Le Baron Jenney erected the first ‘tall’ building with a metal structure, in Chicago of the 1930s, when 50-floor structures began to be frequent in New York and Chicago – nearly a century passed; a period sustained by a powerful industry and a university environment that irrigated this industry with trained engineers. On the contrary, in this same period structural timber continued to be a Cinderella in university programs, and figures in the wood industry were ridiculous compared to those in the concrete and steel industries. But now, with the development of structural timber technology, even in countries where wooden constructions have been absent since the 1940s, as in Spain, it could well take on the sheen of ‘normalcy.’ Between 2013 and 2015 I had the opportunity to design timber structures for buildings of 4-6 floors in Lleida, Madrid, Granada, and so on. This year, 2016, we begin the construction of a 7-floor cooperative housing development in Barcelona. If there is something to note about all these experiences with timber, it is the sense of normalcy that has quite speedily taken hold of the various agents involved. Here and there, colleagues have with increasing frequency been raising buildings of 3 or 4 stories in wood. We have even come to a point where some government organisms have started to include among its conditions the requirement that wood be used as the principal structural material in buildings of several floors.
We don’t know what the characteristic appearance of future highrise timber constructions will be, as we still need experience and maturity in this typology. The earliest structures of reinforced concrete imitated vaulted constructions or traditional wooden slabs. In some way, we are going through a parallel process: for designing wooden towers we lean on traditions of structural design in steel and concrete. Without a doubt we are in for a swift evolution towards specific types and details. There is an essential change in acoustic control and management of lateral stability when you go from the usual heavy structure to the extremely light one. In fact, at present a main challenge is to solve one of these problems without avoiding the other. We can also expect intense debate on striking a balance between hybrid solutions (incorporating steel and concrete at one level or another) and the desired reduction of environmental impact, which always seeks to maximize use of timber. But up to what point? There will also be debates on balancing between the advantages of the thermal stability of wood and the disadvantages of its hygroscopic instability. Traditional carpentry still provides us with simple but surprisingly efficient solutions that we ought to experiment with on a large scale. Which are better? The tools of this tradition or those of high-rise construction with conventional materials? The answers will come through deeds.