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  1. WOOD DESIGN FOCUS V. 26, N. 1 1 Wood Design A JOURNAL OF CONTEMPORARY WOOD ENGINEERING Editorial . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 CLT Research: Available and Accessible to North American Building Designers Lisa Podesto, Scott Breneman . . . . . . . . . . . . . . . . . . . . . . . . . . .3 Cross Laminated Timber in the U.S.: Potential Adopter Perceptions Maria Fernanda Laguarda Mallo, Omar Espinoza . . . . . . . . . . . . 8 Predicting Stiffness of CLT Beams Based on Constituent Lumber Properties Kristopher S. M. Beagley, Joseph R. Loferski, Daniel P. Hindman, John C. Bouldin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 Rolling Shear Strengths of Southern Pine Cross Laminated Timber Mengzhe Gu, Weichiang Pang . . . . . . . . . . . . . . . . . . . . . . . . . . 25 In This Issue: U.S. Research on Cross Laminated Timber (CLT) Volume 26, Number 1 Spring 2016 In The Next Issue: Inspection of Existing Decks
  2. WOOD DESIGN FOCUS V. 26, N. 1 2 Editorial WOOD DESIGN FOCUSPublished by the Forest Products SocietyEDITORIAL BOARD CHAIRDaniel P. Hindman, Ph.D.dhindman@vt.eduEDITORIAL COMMITTEELarry Beineke, P.E., Ph.D.Don Bender, P.E., Ph.D.Chris Brandt, P.E.Cheryl Cieko, AIA CSI Joseph R. Loferski, Ph.D.John “Buddy” Showalter, P.E.Thomas D. Skaggs, P.E., Ph.DFrank Woeste, P.E., Ph.D.Wood Design Focus (ISSN 1066-5757)is published quarterly by:Forest Products Society15 Technology Parkway SouthPeachtree Corners, GA 30092Phone: (855) 475-0291Fax: (301) 604-0158www.forestprod.orgThe annual subscription rate is free to members of the Forest Products Society and $125 USD for Institutions who are members. For nonmembers, the subscription rate is $155USD for individuals and $199USD for institutions and libraries. The Forest Products Society and its agents are not responsible for the views expressed by the authors. Individual readers of this journal, and nonprofit libraries acting for them, are permitted to make fair use of the material in it, such as copying an article for use in teaching or research. Permission is granted to quote from this journal with the customary acknowledgement of the source. © 2015 Forest Products SocietyCross laminated timber and the larger family of massive timber products have been receiving unprecedented attention for wood products. Interest from the architecture and engineering communities has been very strong. Several years ago, Wood Design Focus published an issue dedicated to CLT research. The issue was largely provided by researchers and collaborators with FPInnovations, who were one of the driving forces to bring CLT from Europe to Canada. Ever since that issue, interest in CLT has grown and several large-scale buildings have now been built in Canada and the United States. Many scientists and researchers have been involved with CLT research. Recently a Mass Timber Workshop was held at the Forest Products Laboratory in Madison, WI where over 120 engineers, architects, researchers and academics met to discuss current CLT efforts and develop a roadmap for future research and collaboration. Next March22-24, a Mass Timber Conference sponsored by the Forest Business Network will be held in Portland, Oregon (www.masstimberconference.com). This issue of Wood Design Focus is a continuation of the previous CLT issue. However this time, the issue focused on the research and development efforts of cross-laminated timber in the United States. Topics include a summary of research efforts on massive timber, a market analysis of CLT perceptions among architects, modeling the properties of yellow-poplar for use in hardwood CLTs, and a method for examining the rolling shear properties of CLTs. We hope you enjoy this issue. At the end of each article, author contact information is provided if you would like further information. Daniel Hindman, PE Editorial Board Chair, Wood Design Focus dhindman@vt.edu
  3. WOOD DESIGN FOCUS V. 26, N. 1 3 In 2010, cross laminated timber (CLT) took US designers by storm when architect Andrew Waugh toured the US sharing the amazing success of what was, at that time, the tallest modern timber structure in the world. A nine-story building constructed almost entirely of CLT seemed out of reach for designers in the US; yet here we are five years later with approved codes and standards inclusive of CLT construction up to six stories prescriptively and perhaps more using the performance path. Cross laminated timber and other mass timber technologies have captured the interest of designers, industry and governments alike. The result of all this interest is the need for lots of research and the need has not gone unanswered. An interesting aspect of CLT research to date is not only that there has been so much in such a short period of time but also that it has been uncharacteristically well organized and distributed; not always the norm in the world of research. The motivation for such a swift and organized effort stems from a perfect storm of factors surrounding this innovative structural system: Market need: Designers are looking for a low-carbon building material to address drastic reduction goals for atmospheric carbon emissions. Available resources: The North American public forest inventories are increasing in age and density, leading to increased potential for fire and disease. CLT and other mass timber products offer high value markets for low-value small diameter logs yielded from forest restoration projects. Social/economic motivation: Rural economies that rely on the forest products industry have been in steady decline. High-value products such as CLT provide opportunities to stimulate job growth in non-urban communities. In 2013, FPInnovations and the Binational Softwood Lumber Council in association with the USDA Forest Products Laboratory, American Wood Council, APA – The Engineered Wood Association and WoodWorks published the US Edition of the CLT Handbook (Karacabeyli and Douglas 2013), making relevant European and North American research accessible to US designers. By deciphering important results, offering conclusions, and identifying where further study was needed, that document has provided an important foundation for the research that has taken place since and is yet to come. Several sources of funding, including the Softwood Lumber Board, Binational Softwood Lumber Council, Natural Resources Canada, the USDA Wood Innovation Grant program and the Canadian NEWBuildS Network, are enabling research efforts to move forward for many new wood building systems, including CLT. These programs have the benefit of being well-coordinated and also having an outreach component. This prevents research institutions across North America from being islands of knowledge, allowing each research project to build upon the others and offering an opportunity for early input from the manufacturing and design industries. For example, the NEWBuildS program has a single steering committee reviewing all of the funding proposals from over 13 participating research institutions. Their website provides access to abstracts and papers on all CLT-themed research in a single location, making it easy to determine what’s being done without insider knowledge about which universities are working on what. Another site, the Wood Education and Resource Center hosted by the US Forest Service, contains similar information for USDA grant funded projects. Further improving access to research, the wood industry -- funded by the Binational Softwood Lumber Council and Forestry Innovations Investment -- is pursuing a single database to house information on completed and in-progress research on CLT and other mass timber Keywords: Cross Laminated Timber, North America, Research, Fire, Structural Properties, Seismic
  4. WOOD DESIGN FOCUS V. 26, N. 1 4 systems. The databased will be made available via the rethink Wood website (www.rethinkwood.com). In addition to widespread accessibility, CLT research has also been largely application based. Organizations like WoodWorks that are focused on assisting designers with non-residential and multi-residential projects are providing a mechanism for identifying the needs of architects and engineers and putting them at the forefront of research discussions. Structural and fire summits were held in November 2014 in association with WoodWorks’ Toward Taller Wood Buildings Symposium in Chicago, serving as a kind of research charrette for this purpose. Bridging the gap between data and design has been a strength of the organization, which has helped support the rapid growth of CLT manufacturing and the greater mass timber industry. The purpose of this article is to highlight research efforts recently completed and underway that address pressing questions related to the design and construction of CLT, thus allowing continued pursuit of this product as a mainstream building system. Since the CLT Handbook’s release, a code definition for CLT has been established through the code cycle process and now appears in the 2015 International Building Code (IBC). The definition recognizes the APA/ANSI PRG 320 Standard for Performance-Rated Cross-Laminated Timber (ANSI/APA 2012), and specifically calls out opportunities for its use in prescriptive Type IV construction. The American Wood Council also recently published the 2015 National Design Specification® (NDS®) for Wood Construction with specific design standards for CLT (AWC 2015). While research and the Handbook played a role in these achievements, the conclusions presented in the Handbook and subsequent inquiries from the design community highlight the need for further study, most notably in the areas of fire performance and seismic/structural analysis. Fire Performance Fire performance, and specifically exposed fire resistance, may be one of the most asked about areas in terms of additional research information—but, in reality, suffers more from misperception than lack of research data. The predictability of wood’s char rate has been well established for decades and has also been recognized for years in US building codes and standards. However, the use of existing code provisions has not been commonplace in modern commercial construction; therefore, jurisdictional comfortability with an expanded use of those provisions for the purpose of CLT design has presented a challenge. The 2015 NDS includes a char calculation procedure to provide calculated fire resistance of up to two hours. It expands on the design examples in the fire chapter of the US CLT Handbook by allowing for laminations of varying thicknesses. Further study and additional full-scale panel tests continue to be done, not necessarily to prove legitimacy of the CLT char methodology but to support expansion of its applicability. Areas of expansion include new assembly configurations (in pursuit at the Advanced Composite Lab and the University of Maine), exploring performance under non-standard fires and developing performance prediction tools (as was done at Carleton University under the NEWBuildS program). The Fire Research Team at the USDA Forest Products Laboratory is studying the fire resistance properties of CLT panels in order to improve their marketability for low to midrise construction. The goal is to find a panel layup that maximizes the hourly fire rating so structural panels can be used in a larger variety of situations. Recent studies on 25 CLT specimens have investigated how features such as the grade of wood, layout of individual boards, adhesives and protective membranes can be optimized to reduce the charring rate and increase the hourly rating of the panel. This research is being performed in conjunction with Virginia Tech, Clemson University and North Carolina State University. It is commonly asked why there are not Underwriters Laboratories (UL) or equivalent tested assemblies available for CLT and this area is often suggested for research. The truth is that the calculated method offers more flexibility to designers than a series of UL assemblies and provides more precision with regard to the panel thickness needed to accommodate fire-resistive requirements. When structural strength and fire resistance are so intertwined, a prescriptive method for determining fire resistance cannot offer material efficiency. A comparison of the ASTM E 119 fire tested CLT performance and the predicted performance using the calculated method demonstrates the reliability of char calculations for CLT. Such a comparison can be done by independent designers but is also shown in graphical form with tests done prior to 2013 in the fire chapter of the US Edition of the CLT Handbook (Karacabeyli and Douglas 2013). The impressive ability of CLT to meet two and three hours of fire resistance with and without gypsum protection seems to be overshadowed by concerns about its combustibility. The increase of wood volume raises necessary questions about the additional potential for structural contribution to combustion and what it means for fire safety. Full-scale fire tests completed by FPInnovations and funded by Natural Resources Canada
  5. WOOD DESIGN FOCUS V. 26, N. 1 5 and others are intended to help address this issue. In association with a 13-story mass timber demonstration project (12 stories of CLT over one story of concrete) in Quebec, the provincial government there funded full-scale CLT fire tests to prove CLT’s equivalence to 2-hour-rated non-combustible construction. One series of full-scale compartment tests compared the performance of light-gauge steel, light-frame wood and CLT. Tests included a three-story encapsulated CLT apartment simulation that ran for three hours. Details of this study are described in a previous WDF article (Dagenais 2015). Results of the apartment simulation show the effectiveness of encapsulation in significantly delaying CLT’s potential contribution to fire growth and proved that the structure can withstand complete burnout. The summary study went so far as to state, “Results show that, with encapsulation, the three test apartments constructed using wood structural elements provided the level of fire performance that meets the NBC intent statement assigned to the noncombustible construction requirement in limiting the involvement of the structural elements in fire and in limiting the contribution of the structural elements to the growth and spread of fire.” (Su and Lougheed 2014, p.105). Another test focused on a 25.5-ft CLT stair/elevator shaft (exposed on the inside face with two layers of gypsum protection on the fire side) and studied the smoke propagation and leakage as well as its structural stability as a fire exit. The test ran for two hours and showed no sign of smoke or heat penetration into the shaft. One area highlighted by the Handbook as needing more study was detailing for penetrations and concealed spaces. Research recently completed by FPInnovations and funded by Natural Resources Canada/The Canadian Forest Service evaluated the ability of selected fire stops and sealing joints in CLT assemblies, both for panel joints and around through-penetrations to prevent the passage of hot gasses and limit heat transfer. Results showed that products commercially available for use in light-frame and concrete construction are also feasible for CLT applications. (Dagenais 2014) Structural and Seismic Performance There has been a proliferation of industry and academic research initiatives to build out the body of knowledge on CLT structural performance in US applications. Some have pertained to standards and testing methods suitable to North America, such as the investigation of testing protocols for evaluation of in-plane shear strength of CLT panels (Gagnon et al. 2014). These and other efforts have led to the new Acceptance Criteria For Cross-Laminated Timber Panels For Use As Components In Floor and Roof Decks (AC455) from the ICC Evaluation Services. This product evaluation standard is generally compatible with the ANSI/APA PRG 320 qualification requirements with a notable addition of testing procedures for evaluating the in-plane strength of CLT panels. Having acceptance criteria for CLT panels allows manufacturers to pursue directed testing culminating in an Evaluation Service Report (ESR). ESR reports are helpful in gaining jurisdictional approval for new materials, further assisting designers. Current North American CLT manufacturers are promising ESRs in the near future. Research into connection technology for North American CLT has included static and cyclical testing of self-tapping screws for CLT-to-CLT and CLT-to-wood beams performed at the University of British Columbia in Vancouver, Canada (Hossain 2015 and Ashtari 2014). In addition to connection behavior, Ashtari et al. looked at the behavior of a horizontal CLT floor system as a diaphragm of a lateral force-resisting system. Using CLT components in lateral (wind or seismic) force-resisting systems is an area of considerable ongoing research. A much anticipated project is the Development of Seismic Performance Factors for Cross Laminated Timber with principal investigator John van de Lindt of Colorado State University. This project will follow the Federal Emergency Management Agency (FEMA) P-695 process, which is currently underway, to rigorously quantify seismic performance factors (R, Ωo and Cd) for a type of CLT shear wall system for use following seismic design procedures of ASCE 7. This comprehensive study was preceded by a site- and building-specific FEMA P-695-like study to estimate whether a seismic Response Modification Factor of R = 4.5 met the performance objectives of the candidate design (Pei 2013). To date, CLT shear wall systems for seismic resistance have been designed using conservative seismic performance factors or using advanced performance-based seismic design procedures. The completion of this research will be a significant step toward easier design of CLT shear wall systems for seismic resistance and eventual inclusion of CLT in the seismic structural design standards used throughout the US. Another research project evaluating CLT walls for seismic resistance is a Network for Earthquake Engineering Simulation (NEES)/National Science Foundation (NSF) project investigating seismic-resistant tall wood buildings for the Pacific Northwest (Pei 2014a). This multi-
  6. WOOD DESIGN FOCUS V. 26, N. 1 6 university project is executing an inclusive process to develop seismic performance goals, as well as a variety of potential high-performance/low-damage seismic force-resisting systems. Since the 2014 publication on this project, the research team has progressed to running a series of experimental tests of CLT rocking walls at Washington State University. Additional research is being performed on the design of CLT rocking walls at Clemson (Gu et al 2014) and the University of Alabama. For those wanting to know more about the history of CLT seismic research, the 2014 Journal of Structural Engineering forum article entitled Cross-Laminated Timber for Seismic Regions: Progress and Challenges for Research and Implementation is a very good resource (Pei 2014b). Additional Research In addition to expanding knowledge of CLT as it is currently manufactured and used, other research is exploring different ways to manufacture CLT by using different source material, for example, such as southern pine (Hindman and Bouldin, 2014) and hybrid poplar (Kramer et al., 2013), or the inclusion of voids within the panels (Montgomery et al. 2014). These new opportunities are intended to allow for greater utilization of lower-value small diameter timber stocks that are available across the country. The U.S. Tall Wood Building Prize Competition, sponsored by the Softwood Lumber Board, USDA and Binational Softwood Lumber Council, is also providing support for and helping drive research into practical applications of tall wood buildings across the U.S. WoodWorks is working to expand military, public, and private markets for wood construction by studying the blast performance of CLT wall systems with funding from a USDA Wood Innovation Grant. Karagozian & Case, an internationally recognized protective design consultant, with support from the Advanced Composites Lab at the University of Maine and several other research and government institutions, is collaborating with WoodWorks to test the dynamic performance of the material and propose a design methodology based on the results of this testing. Prioritizing the research needs for CLT was the focus of a recent Mass Timber Workshop hosted by the USDA Forest Products Laboratory. With a large diversified attendance of designers, researchers, and industry, outcomes are expected to influence upcoming funding allocations and help to ensure that the future of CLT research meets market needs. CONCLUSION As low carbon alternatives to other building materials, mass timber products are poised to revolutionize the landscape of the built environment. They’re also helping to bolster rural economies, because stronger markets for wood products provide an incentive for public and private landowners to invest in the long-term sustainability of North American forests. With tremendous interest in the potential of CLT in particular, prompt attention has been given to its inclusion in building codes and standards, with the awareness that a great deal of research is still underway. In addition to the research described in this paper, the depth and breadth of research on CLT is spreading to embrace other mass timber systems, including the development of mechanically-laminated products such as dowel- and nail-laminated timber, and the expanded use of glue-laminated timber. WEB RESOURCES http://www.na.fs.fed.us/werc/http://newbuildscanada.ca/http://www.bcfii.ca/tools-resources/market-research/REFERENCES ANSI/APA. 2012. ANSI/APA PRG 320: Standard for Performance-Rated Cross-Laminated Timber. APA-The Engineered Wood Association. Tacoma, WA, USA. Ashtari, Sepideh, T. Haukaas and F. Lam. 2014. “In-Plane Stiffness of Cross-Laminated Timber Floors”. Proceedings of the World Conference on Timber Engineering, Quebec City Canada, August 10-14 2014. AWC - American Wood Council. 2015. National Design Specification for Wood Construction. AWC. Leesburg, VA, USA. Dagenais, C. 2015. “Recent Advances in Fire Performance of Cross-Laminated Timber”. Wood Design Focus, 25(3): 22-26. Dagenais, C. “Fire Stops and Sealing Joints in Cross-laminated Timber Construction” FPInnovations. May 12, 2014. Gagnon, S., M. Mohammad; Williams Munoz Toro; and Marjan Popovski. 2014. “Evaluation of In-Plane Shear Strength of CLT”. Proceedings of the World Conference on Timber Engineering, Quebec City, Canada, August 10-14 2014.
  7. WOOD DESIGN FOCUS V. 26, N. 1 7 Gu, M., W. Pang and S. Schiff. 2015. “Displacement Design Procedure for Cross Laminated Timber (CLT) Rocking Walls with Sacrificial Dampers”. Proceedings of the ASCE/SEI 2015 Structures Congress, Portland, OR, April 23-25 2015. Hindman, D. and J. Bouldin. 2014. “Development of Southern Pine Cross-Laminated Timber for Building Code Acceptance”. Proceedings of the World Conference on Timber Engineering, Quebec City Canada, August 10-14 2014. Hossain, A., R. Lakshman and T. Tannert. 2015. “Shear Connections with Self-Tapping Screws for Cross-Laminated Timber Panels”.Proceedings of the ASCE/SEI 2015 Structures Congress, Portland, OR, April 23-25 2015. Karacabeyli, E. and B. Douglas. 2013. U.S. Edition of the CLT Handbook. FP Innovations and Binational Softwood Lumber. Pointe-Clair, Quebec. Kramer, A., A. R. Barbosa; and A. Sinha. 2013. “Viability of Hybrid Poplar in ANSI Approved Cross-Laminated Timber Applications”. Journal of Materials in Civil Engineering September 2013. Montgomery, W. G., S. Schiff and W. Pang. “Hollow Massive Timber Panels: A High Performance Long-Span Alternative to Cross Laminated Timber”.Proceedings of the World Conference on Timber Engineering, Quebec City Canada, August 10-14 2014. Pei, S., J. W. van de Lindt and M. Popovski. 2013. “Approximate R-Factor for Cross–Laminated Timber Walls in Multistory Buildings”. Journal of Architectural Engineering. v19(4) pp 245-255. Pei, S., J. Berman, J. D. Dolan, J. W. van de Lindt, J. Ricles, R. Sause, H. Blomgren, M. Popovski and D. Rammer. 2014a. “Progress on the Development of Seismic Resilient Tall CLT Buildings in the Pacific Northwest”. Proceedings of the World Conference on Timber Engineering, Quebec City Canada, August 10-14 2014. Pei, S.; J. W. van de Lindt; M. Popovski; J. W. Berman; J. D. Dolan; J. Ricles; R. Sause; H. Blomgren; D. R. Rammer. 2014b “Cross-Laminated Timber for Seismic Regions: Progress and Challenges for Research and Implementation”. Journal of Structural Engineering, 2014. Su, J.Z.; Lougheed, G.D; “Fire Safety Summary: Fire research conducted for the project on mid-rise wood construction”. NRC Publications Archive Report No. A1-004377.1. National Research Council Canada, Natural Resources Canada. December 31, 2014 Lisa Podesto PE is Senior Technical Director with WoodWorks. lisa@woodworks.orgScott Breneman PhD SE PE is Senior Technical Director with WoodWorks. scott.breneman@woodworks.org
  8. WOOD DESIGN FOCUS V. 26, N. 1 8 ABSTRACT Cross-laminated timber (CLT) is an innovative structural system based on the use of large-format, multi-layered panels made from solid wood boards glued together alternating the direction of their fibers. This cross-laminated configuration improves rigidity, dimensional stability, and mechanical properties of the panels, which can be used for many applications such as walls, floors, and roofs. Since its introduction, more than 20 years ago, CLT has been successful in Europe, and has made inroads in the Australian and Canadian markets. In the United States the adoption of the system is still in its early stages. In order to better evaluate the market potential for CLT in the U.S., this project aimed at assessing the level of awareness, perceptions and willingness to adopt the system by U.S. architecture professionals. To achieve these objectives, interviews with CLT experts were conducted, followed by a nation-wide survey of U.S. architecture firms. Results show that potential adopters identified the use of wood, a natural and renewable material, as the main advantage of CLT. Commonly cited perceived disadvantages of CLT were its acoustic and vibration performance. Notably, results show that the level of awareness about CLT is low among U.S. architects. Building Code compatibility, availability of the product in the domestic market, and cost were mentioned as the main barriers to the implementation of the system in the U.S. Architects are likely to adopt CLT for their near-future projects, especially for multi-family, commercial, and recreational buildings. Importantly, willingness to adopt CLT was positively correlated to the level of awareness with the system. Results show that diffusion of knowledge about CLT will be essential for the successful introduction of this new structural system into the U.S. market. Keywords: Cross-laminated timber, CLT, massive timber, engineered wood products, sustainable buildings, wood-based construction. INTRODUCTION One of the latest innovations in the area of wood construction has been the development of cross-laminated timber (CLT). CLT is a building system based on large-format solid timber panels. These panels are configured similarly to plywood, with boards that are glued side by side in a single layer and then glued to other similarly constructed layers placed at right angles. This configuration improves rigidity, stability, and mechanical properties. Because of their construction, CLT panels can take up forces in all directions, allowing them to be used as walls, roofs or slab elements (Lattke and Lehmann 2007; Lehmann 2012). Typically, a cross section of a CLT panel has between three and seven (odd numbers to achieve a balanced construction) glued layers. The final dimensions of the panels are typically between 2 and 9 feet wide, and up to 79 feet long (Crespell and Gagnon 2011). During the manufacturing of the panels, lumber is visually graded or machine stress-rated and kiln dried before boards are finger jointed and glued together using structural adhesives. After panels have been pressed and machine-surfaced, openings for windows, door and service channels, connections and ducts are cut using CNC (Computer Numerical Controlled) routers. Elements are then packed and sent to the construction site, ready to be put into place with cranes (Crespell and Gagnon 2011). CLT panels are connected to each other using metal connectors such as steel angles and metal splines. Screws are used to attach these connectors to the panels (Crespell and Gagnon 2011, Karacabeyli and Douglas 2013). CLT can be used in a wide range of applications, such as single- and multi-family residences, barns, power line towers, churches, bridges, and mid- and high-rise buildings. This versatility has added visibility and reputation to the system (Ceccotti et al. 2010, Sanders 2011).
  9. WOOD DESIGN FOCUS V. 26, N. 1 9 The use of CLT has become a popular and successful method of construction in Europe since its introduction, and has been recently introduced into the Canadian and Australian markets, with more than 50 buildings erected in the former using this building system (Crespell 2015). The U.S. market for CLT is still in its embryonic stage. So far, only a handful of small projects have been built with CLT, most of them with imported panels. The market development in the U.S. has been hindered in part due to the lack of manufacturing facilities in the country. As of December 2015, there were only three CLT manufacturers in the United States, and only one with APA/ANSI certification. Extensive research has been carried out to evaluate CLT performance as a structural system, such as mechanical properties, fire and thermal performance, and seismic behavior (Harris et al. 2013, Karacabeyli and Douglas 2013, Kuilen et al. 2011). However, market research has been scarce. To addresses this lack of information, this study aimed at assessing the market potential for CLT in the United States. METHODOLOGY To achieve the objective of this research, the project was carried out in two stages: (1) a set of interviews with CLT experts, and (2) a nationwide survey of U.S. architecture firms. A more detailed description of the research approach follows. Interviews With CLT Experts With the purpose of gaining understanding about the awareness, perceptions, and willingness to adopt CLT by the U.S. construction industry, a series of interviews with CLT experts were conducted during summer of 2013. Interviewees were chosen based on their experience and knowledge about CLT-related topics. Names and contact information were obtained from publications available online and recommendation by academic experts. The list of experts included professionals from the academic, manufacturing, architecture, and industry promotion communities. Participants were located in the U.S., Canada, and Austria. Interviewees were initially contacted via email to invite them to participate in the study and set up a convenient day and time for the interview. A list of ten questions was prepared. Questions covered the following categories: demographic information; benefits and barriers for the implementation of CLT in the U.S.; awareness of CLT in the architecture community; perceptions about CLT and willingness to adopt CLT by the U.S. construction industry. Interviews were conducted over the phone and were recorded (with the participant’s consent) for analysis and future reference. All interviews were fully transcribed, coded, and analyzed using established qualitative research methods. Survey of U.S. Architecture Firms The goal of this part of the study was to assess the market potential and barriers to adoption of Cross-Laminated Timber (CLT) in the United States through a survey of potential adopters in the architecture community. Results from the first part of the study (see previous section) provided the main input for this phase of the research. Specifically, a web-based survey was conducted to gather statistically representative information about the perceptions of U.S. architecture firms, very important players in the material selection for a construction project. The population of interest was further narrowed down to architecture firms working with commercial building construction, which was identified as one of the most likely market segments for CLT by the experts interviewed during the first part of the study. A distribution list was developed using the online database managed by the American Institute of Architects (AIA 2013). This association is the major professional association for licensed architects, according to personal communications with the Director of Component Communication & Resources of the AIA. The AIA’s member directory provides search tools to generate lists of firms using criteria such as geographic location, type of building projects, and zip code. A distribution list was compiled with names and addresses for over 1,600 firms. An 11-item questionnaire was developed using the results from preliminary interviews and the literature review as primary inputs. The questionnaire covered topics such as: company demographic information: location and size of company; awareness of CLT familiarity with CLT; perceptions towards CLT, and willingness to adopt the CLT building system in the future. An initial draft was sent to six professionals (including architects and academics) to assess its clarity. Next, a pre-test was conducted among 50 U.S. architecture firms, to identify clarity or inconsistency issues. Feedback from participants to the pre-test was used to improve the questionnaire. An initial email, inviting firms in the distribution listto participate in the study was sent on December 2013. Two reminder emails were sent to those participants that did not complete the questionnaire, with one week separation between communications. The survey was closed in January 2014. Responses were downloaded and analyzed using standard statistical techniques and statistical software SPSS (IBM 2013).
  10. WOOD DESIGN FOCUS V. 26, N. 1 10 RESULTS Interviews With CLT Experts In this section, the major results from the interviews with CLT experts are presented. Also provided are results from the research literature related to the experts’ assertions. CLT Characteristics as a Building Material In regards to its environmental performance, several studies have concluded that CLT could be used as a more environmentally friendly alternative to concrete and steel (Chen 2012, Darby et al. 2013, John et al. 2008). Almost all interviewees agreed that the carbon sequestration and the lower greenhouse gas emissions associated with the use CLT is one of its most important environmental benefits. Specifically, one respondent mentioned that Life Cycle Analysis (LCA) of existing CLT buildings showed that these buildings have a negative carbon footprint. CLT experts interviewed noted the favorable properties of wood in respect to heat transfer and storage, especially compared to traditional structural materials. Experts also mentioned that wood in CLT panels act as a thermal mass that stores heat during the day and releases it at night. According to one of the interviewees, the insulating capacity of CLT could also allow significant reductions in the amount of insulation needed to achieve a lower energy consumption (Reijnders et al. 1999). According to our interviewees, CLT allows the use of underutilized and low-quality timber, since CLT is made of small components assembled and glued together, and the quality of individual pieces is not as critical as with other timber-based building components. Two manufacturer representatives stated that the biggest environmental benefit of CLT came from the potential for using underutilized forest resources to manufacture the panels. The structural advantages of CLT mentioned by experts are in part related to the intrinsic physical and mechanical properties of wood as a construction material. The cross-laminated configuration of CLT panels act as reinforcement of the whole panel, adding to dimensional stability and allowing panels to span and carry load in both directions (Turner 2010, Van de Kuilen et al. 2010). One participant mentioned that “... [CLT is] as strong and [performs] as well as concrete but it weighs one-sixth of concrete.” In regards to the strength-to-weight ratio, one interviewee also stated that the reduced weight of the structure enabled substantial savings on the foundation construction. The majority of experts interviewed indicated that the structural characteristics of CLT make it a viable alternative to concrete structures, especially in high-rise constructions (over 6 stories). A report by the architecture and engineering firm Skidmore, Owings and Merrill proposed a 42-storey CLT-concrete hybrid building in Chicago, called Timber Tower Research Project (SOM 2013). More recently the architecture studio Rüdiger Lainer and Partners (RLP 2015) has designed the HoHo Wien, a project for a new 24 story CLT residential building in Vienna. One of the engineers interviewed for this study mentioned some skepticism about the fact that wood could be used for high-rise structures in the future, since so far the highest building modeled in the laboratory is 15 stories high. Regarding the seismic performance of CLT-base systems, interviewees with a background in engineering stated that CLT panels could perform satisfactory under lateral loading. It has been proposed by several authors that CLT-based constructions perform well under lateral forces and also possess ductility due to its multiple, small connections (Winter et al. 2010). Three CLT experts interviewed stated that one of the main benefits of using CLT came from its design flexibility. According to one respondent, CLT allows covering long spans without intermediate supports; something that would be too complex or impossible to attain using wood in traditional ways. For example a CLT panel with 7 layers (9 inch thickness) can be used to cover spans of up to 25 feet (Malczyk 2011). Some of the respondents also stressed the fire performance of CLT elements. Research conducted by several authors (FPInnovations 2013, Frangi et al. 2009, Karacabeyli and Douglas 2013) state that wooden structural elements of large sections such as CLT panels have desirable fire resistance properties, mainly because of wood’s particular charring properties. Respondents mentioned that this behavior allows the structural element to maintain its strength and dimensional stability without collapsing in an abrupt way, potentially providing time for the evacuation of occupants from the building. Moreover, one of the interviewees, working at a national research facility, mentioned that CLT panels, due to their thickness and airtightness connection between elements, provide an extremely advantageous barrier, limiting the spreading of smoke and fire. From the 10 expects interviewed, 9 recognized the speed of construction as one of the main cost advantages of building with CLT. More specifically, one interviewee mentioned that CLT panels “...can be
  11. WOOD DESIGN FOCUS V. 26, N. 1 11 installed in one-third of the time compared to other products.” Respondents also stated that faster construction means fewer chances for on-site accidents, all of which reduces total costs associated with the construction process. Interestingly, 2 out of 10 respondents mentioned that the speed of construction was particularly attractive to developers, who can recover their investments more quickly: “...[CLT] can go up so quickly that the people who are paying the bills for the projects can start making money from their investment much sooner” and “...so you save time, and get to move into that building sooner and the savings from that alone are quite substantial.” Interestingly, the large volume of wood required to manufacture CLT panels, which was mentioned by some of the respondents as an advantage (enhancing the environmental, structural, fire and thermal performance of the panels), was also cited as one of the system’s main drawbacks. One of the researchers interviewed, estimated that CLT panels need almost three times more wood to make than a wood-frame counterpart. Moreover, some interviewees added that the larger volume of material required could also affect the price-competitiveness of wood against traditional materials, and the public perceptions of the system, as one respondent pointed out: “...People will say that perhaps it is a lot of fiber engaged in CLT,” while another respondent added that the depletion of our forests “... Is a misconception because countries such as ours [Canada], with good forest management practices, do not have deforestation concerns.” The acoustic performance of CLT structures was also mentioned by some of the interviewees as a disadvantage. One researcher stated that acoustic problems arise due to lack of proper linings or mistakes during the installation of the system. Research conducted by Gagnon (2011) found that due to its massive and airtight nature, CLT assemblies could achieve good acoustic ratings and provide adequate noise control for both airborne and impact sound transmissions. Awareness of CLT in the architecture community Experts interviewed were also asked about their thoughts on the level of awareness of Cross-Laminated Timber among U.S. architects. Their responses can be grouped in two categories. Half of the experts interviewed indicated that awareness is about CLT is currently low or very low. Two researchers agreed that professionals not familiar with the wood industry are less likely to have heard about CLT, with one respondent stating that “... Because the marketing for this product is coming from the wood industry, people not connected with [the sector] are not aware of the product.” A second group of respondents indicated that the level of awareness of CLT in the architecture community was intermediate and steadily growing over the past five years as a consequence of the increased promotion of CLT by organizations that promote the use of wood in the U.S., such as WoodWorks (Woodworks 2015). Interestingly, one participant of the study stated that the level of awareness is likely to be “Higher in the architectural area [as] they tend to know about it, whereas from the engineering standpoint, it is lower, [since] they are less knowledgeable about wood materials.” The same participant justified the statement by explaining that the national engineering curriculum does not include in-depth information of wood as a construction material. For this reason, engineers tend to be less aware of wood-based construction alternatives, and thus more inclined to choose other materials that were extensively covered in their educational programs, such as concrete and steel. Barriers to the adoption of CLT in the U.S. The majority of respondents interviewed stated that, by the time this study was conducted, the main barrier for the adoption of CLT in the U.S. was its compatibility with the building code. At the time of this study, designers who wanted to use CLT for a project were required to request a special permission to local authorities. In 2013, the American Wood Council created task committees to begin the process of adding “heavy timber” (e.g., CLT) to its National Design Specifications (NDS) and ultimately to the International Building Code. The final decision on the proposed language was made in late 2014. The approved code, including CLT under the Code’s Heavy Timber classification, was made available for jurisdictions to adopt in early-mid 2015 (International Code Council 2015) Another major barrier mentioned by interviewees was the availability of information and education about CLT. The participants with a background in architecture stated, “...architects and engineers are aware [of CLT] but [between] being aware [and] being proficient, there is [a] lack of education.” The same respondent later added: “It is going to take a little time to understand the product and probably [there is a] need of training, seminars, or similar activities, to get people more comfortable [with CLT].” The limited availability of CLT in the domestic market was also seen as an important barrier to the successful adoption of the material in the U.S. market. One
  12. WOOD DESIGN FOCUS V. 26, N. 1 12 respondent declared, “...if you don’t have any capacity or any production in the United States then [...] no one knows about this product.” So far, only a handful of small projects have been built with CLT, with imported panels. As of December 2015, there were only three CLT producers in the United States; all of them in the Western half, and only one APA/ANSI-certified. Cost-competitiveness of CLT Experts were asked about the competitiveness of CLT compared to traditional building systems. The majority of interviewees agreed that CLT could be cost-competitive as an alternative to concrete structures and for building over 6 stories high. These results are in accordance with a preliminary research conducted by FPInnovations. A manufacturing representative from Europe mentioned that the cost-competitiveness of CLT should not only be seen as initial investment, but through the whole life cycle of the building, adding that “[CLT is] cost-competitive because it already has thermal insulation, [...] and for sure it might be a little bit more expensive in the beginning, but when you also include the maintenance costs it turns out to be absolutely cost-competitive.” Potential for adoption of CLT The last question asked to interviewees was intended to gain insight on the perceived potential for adoption of CLT in the U.S. Most experts interviewed agreed that the system had great potential to become an environmentally-friendly alternative to traditional materials like concrete or steel. Two researchers and one architect agreed that adoption was likely to depend on regional differences, with regions with a stronger tradition in wood construction more likely to adopt CLT. In regions were concrete and steel have a stronger presence, the adoption process could face more challenges. As one researcher stated, the adoption of CLT “...Would face challenges in the South because of [the concern for] termites, this might require [chemically] treating CLT. But in the Northern, Eastern, and Western regions of the United States, I think there is a big potential for buildings constructed with CLT.” Interestingly, one of the respondents stated that the potential for the wide adoption of CLT adoption was intrinsically related to the “green building” movement, indicating that this movement is the “wild card” to the successful adoption of wood-based construction materials such as CLT. Survey of U.S. Architecture Firms The online survey was sent to a total of 1,627 U.S. architecture firms, from which 351 responded. Accounting for incomplete responses, undeliverable emails and firms that declined to participate, an adjusted response rate of 22.7% was calculated. Table 1 shows de demographic information of respondents, specifically location and firm size. The responses to the survey are summarized in this section. Material’s attributes considered during the material selection process In order to learn what architecture firms evaluate when selecting a structural system, participants were asked to rate the importance of a number of attributes of construction materials. Table 2 shows the count of responses and percentages obtained for each characteristic. In accordance to the results obtained from the interviews to CLT experts, structural performance and durability, which are also related to the structural performance of a structure, were rated the highest. The same can be said for fire performance, which was also between the characteristics rated with the highest importance. On the other hand, earthquake performance, which is certainly related to the structural performance and safety of the structure was rated as one of the least important material characteristics; which may be explained by the uneven distribution of seismic activity across the U.S. When comparing how earthquake performance is perceived in the different regions, more Table 1. Survey Participants’ Firm Location and Size (by Number of Employees) . N=351 Respondent Characteristic Percent of Respondents (%) - - U.S. Region - - Northeast 14.5% South 30.2% Midwest 17.9% West 21.1% Alaska 0.0% Hawaii 0.6% Multi-Region 15.7% 1 - 4 Employment 52.4% 5 - 9 Employment 21.4% 10 - 19 Employment 10.5% 20 - 99 Employment 12.0% 100 or more Employment 3.4% - - Firm Size - -
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