Cross-laminated timber (CLT) is a construction technology developed in Europe in the early 1990s, where it has since then become a widely used building material. In Europe, CLT successfully competes with steel, brick and concrete in selected market segments, such as multi-family buildings.
The ANSI/APA PRG 320-2012 (2012) standard defines CLT as a “prefabricated engineered wood product made of at least three orthogonally bonded layers of solid-sawn lumber that are laminated by gluing of longitudinal and transverse layers with structural adhesives to form a solid rectangular-shaped element intended for roof, floor or wall applications.”
CTL panels are pre-fabricated, with openings for doors, windows and ducts precision-cut by CNC routers. The prefinished panels are transported to the construction site and put into place with cranes and a small construction crew.
Walls and floor systems are joined using metal connectors. Additional insulation layers can be applied to CLT walls and ceilings, or the surfaces can be left bare to take advantage of the warmth and aesthetics of wood. CLT’s attractiveness as a building system originates in part from the speed with which CLT buildings can be raises, resulting in considerable savings in labour and minimal disturbance to the site’s surroundings. In addition, part of the attentive given to CLT is due to its potential use in tall buildings.
Since its market introduction in the early 1990s, production of CLT has grown at a rapid rate. Most estimates put global annual production of CLT at over 600,000 cubic metres for 2014, a number that is expected to reach one million cubic metres by 2016 as operations in Finland, Latvia, Japan and the US come on line. CLT production will potentially reach three million cubic metres within the next 10 years, with most of the growth expected to occur outside Western Europe.
Currently, production is heavily concentrated in Central Europe, specifically the German-speaking countries, which hold just under 80 percent of the global installed production capacity as of 2015 (60 percent in Austria, 17 percent in Germany and three percent in Switzerland).
CLT is comparable and, in some aspects, superior to concrete or steel. As a building system, CLT allows long spans without intermediate supports. A CLT panel with seven layers and a total thickness of nine inches can be used to span approximately 25 feet. Variations of CLT such as ‘cassette’ or ‘folded’ floors, allow for even greater span.
Regarding structural performance, CTL panels can be used as load-bearing plates and shear walls, in contrast to other wood-based engineered composite panel products. CLT also has advantage regarding its fire performance because of the predictable burning properties of large-section wood structural elements. Furthermore, unlike a wood frame system, CLT constructions create limited concealed spaces, which reduces fire spread.
The seismic performance of CLT has been the subject of several studies. In one experiment, a seven-storey building specimen was subjected to severe earthquake-like motions, equivalent to 7.2 on the Richter scale. CLT showed excellent seismic behaviour, with maximum inter-story drifts of 1.5 inches and lateral deformation of less than 12 inches. CLT also performs well in respect to environmental performance when compared to steel, brick, glass, plastics or concrete.
The environmental performance attributes of CLT originate mainly from the basic characteristics of wood, which have been demonstrated in numerous life-cycle assessments as being extremely favourable. Given sustainably managed forests, replacing steel, concrete or plastics with wood as raw material reduces carbon emission.
A 2011 study by the US Forest Service concluded that wood has superior environmental performance over other materials such as concrete or steel, even when the wood stems from diseased trees. A number of independent studies compared the environmental performance of multi-storey buildings built with CLT and concrete.
These studies consistently concluded that CLKT buildings had lower embodied energy than concrete-based buildings and superior performance compared with concrete and steel with respect to ozone depletion, global warming potential and eutrophication. Furthermore, wood has the additional benefit of acting as a carbon sink. Lastly, tests in Canada showed that CLT’s volatile organic compounds and formaldehyde emissions can be below established standards.
Research Centres In Europe
Development of modern CLT was the result of a joint effort by the industry and academia, initiated in the early 1990s by the Swiss government as part of an effort to develop new markets for sawmilling by-products. What followed was remarkable interest in the resulting material, CLT. In 2012, there were reportedly over 100 CLT projects in Europe and considerable research activity in European institutions.
There is also interest in CLT internationally. In 2014, the joint World Conference on Timber Engineering and Forest Products in Quebec, Canada, listed eight sessions and over 56 presentations on CLT research, demonstrating global interest in CLT. As CLT today is being used globally, research into its use and advancement is also occurring on a global scale.
Given the large potential for expanded wood products used in the European building industry, a web-based target survey was conducted to collect the opinion of European CLT experts. Dillman’s tailored design methods for survey design and implementation were followed.
Web surveys are increasingly common and are a cost-effective method to collect information. Web surveys, when carefully done, can generate response rates and quality of responses comparable to the more traditional method of mailed surveys. The targeted sampling approach chosen for this survey was a non-probability strategy, where researchers do not have certainty of whether all potential respondents have the same change of selection.
Therefore, generalisation from the results of this survey cannot be made for the entire population of interest. However, for this survey, targeted sampling was chosen over random sampling due to time and resource limitations and because of the existence of a limited set of current and valid addresses.
For this survey, the population of interest comprised of European experts in timber engineering as well as civil engineers and researchers with a focus on wood construction. A non-probability sampling strategy was adopted. Therefore, results and conclusions are only valid for the sample. Numerous experts on the list assembled were part of the program on European Cooperation in Science and Technology (COST), a framework supporting trans-national cooperation among researchers, engineers and scholars across Europe.
Specifically, experts chosen belonged to the following COST groups: COST Action FP1402, a network on structural timber design, with the objective of bridging research results and the needs of designers, industry and regulators, and the management committee of COST Action FP1101, Assessment, reinforcement and monitoring or timber structures.
Experts included in the distribution list assembled worked in research and educational institutions, wood construction firms, manufacturing entities, state-funded laboratories, timber engineering consulting firms, private industry, industry associations, design and engineering firms and in structural software companies. The final distribution list contained 93 names and addresses.
Most of the respondents indicated ‘researcher’ as occupation (68.6 percent), followed by ‘engineer’ (51 percent) and ‘educator’ (49 percent). Numerous respondents reported more than one occupation, with the most common combinations being engineer and researcher (37.3 percent) and educator and researcher (31.4 percent). Lastly, 12 respondents (23.5 percent) indicated being an engineer, researcher and educator.
As for the geographic distribution of respondents, they were not concentrated geographically and were widely distributed over 25 European countries. However, most responses were received from Spain (19.6 percent), Sweden (9.8 percent) and Italy (7.8 percent).
Level Of Awareness
Respondents were asked about their perceptions of the level of awareness of CLT among building owners, contractors, construction managers, engineers and architects.
According to the participants, the level of awareness is low for construction managers, contractors and owners/imitators. In fact, 91.5 percent, 95.9 percent and 98 percent of the respondents rate the awareness as ‘low’ or ‘very low’ for these professionals. These results are quite surprising as CLT has been used in Europe for more than 20 years with extensive coverage in trade journals and in the news.
However, awareness was rated higher for engineers and architects with 32 percent and 28 percent of the respondents rating these professionals’ level of awareness ‘high’ or ‘very high’.
Respondents were asked to rate a list of potential barriers to the adoption of CLT in Europe. Barriers to CLT adoption in Europe perceived by respondents were ‘compatibility with building code’, which 51 percent of the respondents considered a ‘large barrier’, followed by ‘availability of technical information’, ‘misperceptions about wood or CLT’ and ‘cost’ (38.8 percent, 32.7 percent and 29.2 percent respectively). ‘Availability in the market’ and ‘volume of wood required for CLT’ were considered potential barriers (58.3 percent and 45.8 percent rated these factors as ‘may be a barrier’). Lastly, 60.9 percent of respondents indicated that CLT’s performance as a building material is ‘not at all a barrier’.
These results can be contrasted with those from recent research in the US, where a nationwide survey of architects was conducted. US architects coincided with European engineers in that building code compatibility is a large barrier (in the US study, just under 90 percent of respondents deemed building code compatibility as large or potential barrier).
Results were similar for cost and availability of technical information. Perceptions differed, however, for availability of CLT, with 94 percent and 67 percent of US architects and European engineers considering availability of CLT a large or potential barrier. This more pronounced concern for the availability of CLT in the US is not surprising as CLT is not yet readily available in the US. In fact, only one manufacturer produces CLT panels and only a handful of projects have been built with imported CLT.
Respondents were given the opportunity to indicate barriers not listed in the ‘other’ category. Seven participants entered suggestions, including: fear of drawbacks of wood as material (fire damage, decay, insect damage), thermal performance, lack of experience in design of CLT buildings, high cost (for residential buildings), fire performance (in tall buildings), calculation of connections, and lack of awareness by the public. All these barriers mentioned were considered as large barriers.
The fourth question in the survey asked about research needs related to CLT. Participants were presented with a list of six potential research topics and asked to rank these topics in order of importance.
To facilitate the interpretation, answers were grouped into three options: first and second ranked were grouped into a ‘high priority’ category, third and fourth into a ‘medium priority’ category, and items ranked fifth and sixth were grouped into a ‘low priority’ category.
Just over 90 percent of respondents indicated that ‘structural performance and connections’ were the most important research needs. Of all respondents, 76.5 percent, 64.7 percent and 45.1 percent regarded ‘moisture performance’, ‘market/consumer research’, and ‘acoustic performance’ as medium or high research priority. Interestingly, 96.1 percent of respondents indicated that ‘environmental performance’ is a ‘medium’ to ‘low’ research priority.
This was not expected, given that the environmental aspects of CLT are repeatedly mentioned as one of its biggest selling points. It can be speculated that for Europeans, where the environmental advantages of constructing with wood have been heavily promoted for the last 20 years or more, the respondents did not feel a need for much more effort in that area.
Another unexpected result is that structural performances was the subject identified as the area of greatest research need, whereas the same topic was rated as the least important barrier to wide adoption. One potential explanation for this apparent contradiction is the structural engineering research backgroundof most respondents.
CLT is an innovative wood-based building product, with significant economic and environmental advantages over traditional building materials. CLT buildings effectively store large amounts of carbon and have been shown to produce lower emissions than construction executed in concrete or steel. Furthermore, the prefabricated nature of CLT allows effective and efficient construction and minimal disruption to site surroundings. Structurally, research has shown that CLT can compete and even outperform more traditional materials.
These advantages have resulted in exponential growth in CLT production, primarily in Central Europe. Although production and construction is still highly concentrated in this area, interest is growing in other parts of the world including Australia, New Zealand, Japan and North America. In a similar way, research activity on CLT structural design, properties and testing has grown rapidly.
In this study, European experts in timber engineering, civil engineering and research voiced their perception about (a) the level of awareness about CLT in the construction community in Europe; (b) their view on the most pressing research needs for CLT, and also about (c) the perceived barriers to CLT adoption in Europe.
The major findings are summarised as below:
•Respondents deemed the perceived level of awareness of CLT in European construction industry as low, particularly among engineers and architects. These two groups were rated as having low or very low levels of awareness (72 and 68 percent respectively, were ranked in these two groups by the respondents). However, awareness was perceived to be lower for owners, contractors, and construction managers.
•The major barriers to CLT adoption, according to participants, were building code compatibility (51 percent considered it a larger barrier to adoption), availability of technical information (38.8 percent), public misconceptions about wood (32.7 percent) and cost (29.2 percent). CLT availability and volume of wood required for its construction were perceived as potential barriers by 58.3 percent and 45.8 percent of respondents respectively. Lastly, CLT’s performance as building material was not considered as a barrier by 60.9 percent of engineers.
•The most important research needs, according to survey participants, were CLT structural performance and connection (90.2 percent considered this as ‘high priority’), followed by moisture performance (37.3 percent) and market research (27.5 percent). CLT’s thermal (11.8 percent) and environmental performance (two percent) do not seem to be a high priority research need for European timber engineers.
Results from this study apply only to the participants because of the non-probability sampling strategy adopted. Generalisations to the entire European CLT professional community cannot be made.