by Sailesh Adhikari, email Sailesh@vt.edu
In North America, the structural use of the lumber is conventionally dominated by softwood species. In the past, hardwood species was studied for dimensional grading when grading techniques for structural timber has been implemented (Ding, 1987). However, due to low economic advantages, hardwoods have never been successfully manufactured as structural lumber (Grasser, 2015) and left behind with traditional sawing practice of random length and width. AHEC (2008) argued that because of low economic margin and sufficient availability of the softwood lumber and their mechanical properties to meet the design requirements, hardwood lumbers are never considered to standardize and continue to produce in random width and random length. Though there has been some effort to use hardwood lumbers in dimensional construction.
The first hardwood species which occasionally appeared on the structural market is yellow poplar (YP) (Green, 2005) because of some intrinsic properties of YP wood. Additionally, YP lumber is considered as easy to machine and plane which dry in faster pace compared to other hardwood species. Also, HMS (2013) reported that YP had shown stable connection and high quality finishing properties, that makes it more attractive for CLT application.
North American CLT standard and YP
CLT manufacturing with YP lumber is generally feasible as research and pilot projects have already proven (Slavid, 2013, Beagley et al., 2014; Espinoza et al., 2016). Given the intrinsic wood properties and existing ANSI/APA standard YP exhibits the properties to be considered as CLT raw material. The YP lumber properties and ANSI/APA standard are compared in Table 1.
Table 1: General requirements of North American CLT standards and Yellow- poplar lumber.
|Recognized by the ALSC under PS 20||No||(ALSC, 2010, ANSI/APA, 2012)|
|Specific Gravity, (0.42) above 0.35||Yes||(Bendtsen and Ethington, 1975, ANSI/APA, 2012)|
|Minimum lamination grade major strength direction: 1200f-1.2E MSR; Visual No. 2||Yes||(AWC, 2014, ANSI/APA, 2012)|
|Minimum lamination grade minor strength direction: Visual No.3||Yes||(AWC, 2014, ANSI/APA, 2012)|
|CLT Grade for YP||No||(ANSI/APA, 2012)|
Potential of YP lumber in CLT
Research conducted in Virginia Tech to utilize YP lumber in CLTs produce significantly positive results on the mechanical test. Six 5-layer CLT beams (101” x 6” x 3.13”) have been fabricated and were tested non-destructively to complete this study. The observed result concludes that the YP CLT is capable of matching strength requirements on effective bending stiffness (EIeff) and effective shear stiffness (GAeff) of the current North American CLT standard ANSI/APA PRG 320 (Mohammad et al., 2015). Research conducted at the University of Trento in Italy examined into strength properties for CLT concludes that YP CLT has three times more strength and stiffness under rolling shear than other softwood species that is commonly used in CLT fabrication (Slavid, 2013). The same research also concludes that yellow-poplar lumber is an ideal raw material source for CLT manufacturing given its properties.
The first YP CLT application in structure is Maggie’s Oldham from the UK, which is the world’s first building made from hardwood cross-laminated timber (CLT). Recently College of Architecture and Urban Studies and Department of Sustainable Biomaterial work together to construct a train observatory in Radford, VA registering as the first successful hardwood CLTs in structural application in the US.
Research and pilot studies conclude that technically and performance-wise YP lumber is a possible substitute or admix to softwood lumber, but there is some significant barrier to implement YP CLTs. Most notable of all the significant barriers to successful hardwood CLT implementation is the efficient manufacturing of hardwood lumber produced to the CLT standard. Currently, hardwood sawmills are designed for appearance grade lumber production and not for the required CLT standard. In the dimensional aspect of current hardwood lumber production Quesada (2018) and Espinoza (2016) note significant findings compare to the CLT standard ANSI/APA PRG-320 2017.
- The thickness of CLT layers should be between 5/ 8 inches and 2 inches, so lumber from hardwood logs should be sawn to a maximum of 2 inches thick.
- For lumber to be used in the parallel load direction, the width should be greater than 1.75 times the thickness of the lumber, which excludes 2×2,2×3 dimensions. Lumber in perpendicular layers must have a width to thickness ratio greater than 3.5, which excludes 1×2, 1×3, 2×2, and 2×4 dimensions. Thus, the minimum possible lumber dimension can be 2X6. Hardwood logs should be sawn to the dimension of 2X4 and higher for the parallel layer application and 2X6 and higher for the perpendicular direction.
- All lumber to be used in CLT has to be surfaced on all sides and trimmed with 2% and below dimensional tolerance(ANSI/APA,2017).
Second major limitation of hardwood CLT implementation is the raw material price (Grasser, 2015). The average price of the random length of YP lumber in March of 2018 was about $450 (AHEC, 2018), at the same time the average price of the southern yellow pine was about $360 (Madison Report, 2018) in North America. However, if we observe further, the price of low grade YP lumber, i.e., below 2com, the average price is less than $346 (AHEC, 2018). Thus there is an excellent opportunity to utilize lower grade YP lumber for CLT application.
The potential use of the YP in CLTs is an excellent opportunity to utilize the highly growing feedstock from the Eastern region of the US. At present, YP is one of the species that have a higher growth rate than harvesting rate, despite increased harvesting from the past. Additionally, it grows from the plain of south Texas to the northern part of Canada, as shown in Fig 1, so there is an abundant resource available for this particular market. As the production of the CLTs for structural application grow to industrial level in the US, CLT manufacturer has to depend upon the traditional dimensional lumber species to meet the demands that will trigger the increased competition and potentially increased lumber price. Thus, YP lumber can be marketed from now as the additional raw material to the CLTs market so that both CLT manufacturer, as well as lumber producer, will be benefited with additional market opportunity.
The first step to promote YP lumber in CLT application is to manufacture CLT mats because at present situation, it is the standard product of all CLT manufacturer in the US and manufacturing CLT mats does not require to meet PRG 320 standard. Such practice will help lumber manufacturer to find a new market and evaluate the cost factor to prepare the ready to use lumber for CLT application. On the other hand, CLT manufacturer with their manufacturing experience and capacity can work to establish the standard for the YP CLTs that will be crucial to recognize hardwood CLTs for the structural application.
- AHEC 2008. Guide to American Hardwood Products. Washington: American Hardwood Export Council.
- ANSI/APA. 2012. ANSI/APA PRG 320-2012 Standards for performance-rated cross-laminated timber. ANSI/APA, Tacoma, Washington. 23 pp.
- ANSI/APA. 2017. ANSI/APA PRG 320-2012 Standards for performance-rated cross-laminated timber. ANSI/APA, Tacoma, Washington.
- Beagley, K. S., Loferski, J. R., Hindman, D. P. & Bouldin, J. C. 2014. Investigation of hardwood Cross Laminated Timber design. World Conference on Timber Engineering. Quebec, Canada.
- Denig, J., Wengert, E. M., Brisbin, R., & Schroeder, J. (1984). Dimension Lumber Grade and Yield Estimates for Yellow-Poplar. Southern Journal of Applied Forestry, 123-126.
- Espinoza, O., Buehlmann, U., Laguarda, M., & Trujillo, V.R., 2016. Identification of research areas to advance the adoption of cross-laminated timber in North America. Bio-Products Business, 1-13.
- Grasser, K.K. 2015. Development of cross-laminated timber in the United States of America. Master’s Thesis, University of Tennessee, 2015. Retrieved from http://trace.tennessee.edu/ utk_gradthes/3479.
- Green, D. W., 2005. Grading and properties of hardwood structural lumber. Undervalued hardwood for engineered materials and components. Madison, WI: Forest Products Society.
- 2013. Species Guide – Poplar [Online]. Pittsburgh: Hardwood Manufacturers Association. Available: http://www.hardwoodinfo.com/articles/view/pro/24/315
- Mohamadzadeh, M., & Hindman, D. (2015). Mechanical performance of yellow-poplar cross-laminated timber.
- Quesada, H. (2018). Potential and limitations of using hardwood lumber as raw material for CLT. PowerPoint Presentation. West Lafayette, Indiana, USA. March 21, 2018.
- Slavid, R., 2013. Endless Stair – A Towering Escher-like structure made from American tulipwood CLT for the London Design Festival 2013.