Recipient Organization
UNIVERSITY OF MAINE
(N/A)
ORONO,ME 04469
Performing Department
School of Forest Resources
Non Technical Summary
Cross Laminated Timber (a specific type of mass timber) is an engineered wood product, made of dimensional lumber or structural composite lumber glued together. These form multi-ply laminate panels with adjacent layers typically oriented perpendicular to one another. These massive panels (2 to 10 feet wide, with lengths up to 60 feet and thickness up to 20 inches) are used to form walls, floors and roofs, and are revolutionizing the building industry. CLT is an engineered product and its implementation requires an engineering/design component in their specification. This expands the overall economic impact as both skilled and unskilled labor is needed when this technology is implemented.Many experts believe the optimal market for CLT, in terms of competitiveness against steel and concrete, is mid-rise construction (8-14 stories), where cost savings are estimated at 15-50%. As CLT buildings are constructed from a renewable resource, there are also environmental benefits versus traditional materials in terms of significantly less embodied energy and a reduced carbon footprint. Typically wood buildings in the U.S. are limited to 5 stories, however, CLT is allowing for much taller structures, including the recently approved 12-story Framework building in Portland, Oregon, or the recently completed 18-story Brock Commons dormitory on the University of British Columbia campus. Some are predicting the coming of a "Timber Age", where CLT becomes a predominant building material being produced in, and thus benefiting, rural economies while being utilized in urban areas where housing needs are ever-increasing.Currently, there are two CLT plants in Canada (Quebec and British Columbia), and two in the U.S (Oregon and Montana). Within the last nine months, the author is aware of four additional facilities announced for construction. The intent to locate a mass timber (CLT and glulam) facility in Dothan, Alabama was announced in June 2017 by International Beams.Maine's softwood supply is more than capable of supplying all the lumber a new CLT manufacturer would require. A current CLT manufacturer projects a new mill would require 50 MMBF/year. Current numbers from the Northeastern Lumber Manufacturers Association (NELMA, located in Cumberland, ME) show Maine's annual output to be nearly 300 MMBF/year, with mills reporting capacity to increase should demand rise.This proposed project leverages recently obtained funding in combination with the McIntire-Stennis program to focus on innovation of mass timber in Maine through technological and development activities.
Animal Health Component
30%
Research Effort Categories
Basic
20%
Applied
30%
Developmental
50%
Goals / Objectives
Innovation is a key theme of national recommendations for forests and forest products research and development actions throughout the United States. The overall objective of this project is to conduct applied research to promote innovation in the use of lumber in mass timber and to be scanning for emerging technologies with industrial relevance in the realm of solid and composite wood markets. The outcomes of this research are anticipated to support forest products industry in Maine and other regions in the United States with a similar forest composition. Innovation for increasing markets for wood will promote rural economies and enable active management of our vital forest resource.The five specific objectives of the research program are:1] Determine the effect of gap size on the structural performance (static and viscoelastic) of Cross Laminated Timber panels manufactured from SPFs lumber.2] Development of modeling techniques applicable to a range of gap sizes to predict said effects, and the determination of whether significant reductions in CLT shear and creep performance, due to the existence of edge gaps of CLT manufactured with lumber can be mitigated with alternate materials, e.g., LSL.3] Develop information on CLT panels manufactured from E-rated (MSR) SPFs lumber which is needed for establishment of a new grade.4] Evaluate the performance and techno-economics of using 1-inch nominal thickness SPFs lumber for the production of CLT panels.5] Evaluate the role of emerging technologies for composite and solid wood industrial significance.
Project Methods
The full methods is too long for the 8000 character limit. For more detail, please see the full proposal attached as a pdf file or contact the project director.1. Effect of Gap SizeIt is inferred by the limitations put in place by the international performance standards, and the reported concerns by the ANSI committee, that the fundamental assumption is that gaps between boards is undesirable. Gaps may have a detrimental effect of both mechanical (e.g., rolling shear, creep) and physical properties (e.g. fire performance, air permeability). However, the presence of gaps between boards may serve as an attribute to a CLT billet. The first goal is to determine the influence of edge joint gaps in the inner layers of CLT on certain static mechanical properties. The second goal is to examine the effect of edge gaps on CLT creep.To assess the effect of edge joint gaps, four different CLT panel layups will be fabricated and tested both non-destructively (to determine shear modulus) and destructively (to determine shear strength). Panel manufacture will follow protocols developed in our laboratory and described in Willey (2016) with the exception that CLT materials will be manufactured with several different edge joint gap widths between boards in the inner layers of the finished panel. CLT panels will be produced with four gap widths: without any gaps (control) and gap widths of 0.25 inch, 3.25 inch, as well as an intermediate width. The determination of shear modulus will be conducted by subjecting the specimen to a bending moment and shear force by supporting it at two end reactions and applying a single transverse load at the midspan.Influence of Edge Gaps on Creep of CLTIt is known that due to its orthogonal arrangement of layers bonded with structural adhesive, CLT is more prone to time-dependent deformations under load (creep) than other engineered wood products such as glulam (Jöbstl and Schickhofer, 2007).Creep behavior of structural wood products is addressed in design through the use of load duration factors that adjust design properties. Current duration of load (DOL) factors for wood were determined years ago based on the work that led to the development of the Madison Curve, which shows the predicted relationship between bending strength and duration of load of solid wood. Some tests conducted in the development of this curve lasted as long as ten years; repeating this prior work to develop a similar curve for CLT has not been done. Without specific DOL factors for CLT, the current version of the National Design Specification (NDS) for Wood Construction (AWC, 2015) refers designers to load duration factors typically used for other structural wood products. However, when calculating the predicted total deflection of a bending member, the recommended time dependent deformation (creep) factor (Kcr) in the NDS is 2.0 for CLT (significantly higher than the Kcr = 1.5 for seasoned lumber, glulam, wood I-joists, and structural composite lumber used in dry service conditions), which is based on the factor used for wood structural panels such as oriented strand board (OSB) and plywood.2. Model development for gap effectsThe analytical calculations used in the derivation of design capacities of CLT grades listed in PRG 320 assume gaps are not present between boards within a lamina, and if gaps are present, they are small enough so that the effect on the CLT panel is negligible. This practice has shown to be acceptable, as evidenced by confirmation testing, in both bending and shear, required by the product standard.However the ANSI Committee, when originally composing the PRG 320 standard, was concerned about unbonded edge joints, which may reduce the effective shear strength and stiffness. Because rolling shear strength and stiffness in CLT has been identified as a key issue that can control the design and performance of CLT floor and wall systems, the investigation of this concern is warranted.The analytical calculation for shear strength capacities listed in PRG 320 is as follows:Where coefficient "c" is a reduction factor calculated as:For CLT strength in the minor direction, it is assumed the transverse layers do not contribute to bending (thereby reducing), and and do not include the transverse face layers. Therefore, the design shear strength capacity of 5-layer CLT in the minor strength direction is equal to that 3-layer CLT in the major strength direction. Using this assumption, the 3-layer L2 and H2 CLT layups (which will be less costly and easier to manufacture than 5-layer CLT) will provide valuable information on the behavior of 5-layer CLT in the minor strength direction.The determination of shear modulus will be conducted by subjecting the specimen to a bending moment and shear force by supporting it at two end reactions and applying a single transverse load at the midspan. Per D198, the specimen is deflected at a prescribed rate and a single observation of coordinate load and deflection is taken. This procedure is repeated on four different spans. The shear modulus is calculated by plotting l/Eapp (where Eapp is the apparent modulus of elasticity calculated under center point loading) versus (d/l)2 for each span tested. The shear modulus is proportional to the slope of the best-fit line between these points.A survey of the literature has shown that various methods of determining shear moduli, e.g. torsion, three-point bending, five-point bending, may result in significantly different results (Harrison 2006, Hindman 2003). However, these differences in the determination of the shear moduli are not a concern in this work due to the comparative nature of this project. The relative effect of gap size, and subsequent modeling of the data, is appropriate using this method since CLT generally will experience both bending and shear stresses in application, not solely shear as induced through the torsion test method.3. CLT Panels from MSR SPFs lumberThe proposed solution is to introduce two new "E" grades using MSR SPF-S lumber. The two grades being proposed would be "E5" utilizing 2,100 Fb/1.8E MSR lumber in the parallel layers, and "E6" 1,650 Fb/1.5E utilizing 2,100 Fb/1.8E MSR lumber in the parallel layers. Both of these grades are proposed to be manufactured with No. 3 SPF-S lumber in the perpendicular layers. Note that the new E5 grade would be the highest published grade of CLT, while E6 would be the same as Douglas-fir, a species known for its relatively high strength and stiffness. This would immediately allow for direct competition (mechanically) with any North American CLT producer.The procedure for qualifying a new CLT grade is laid out in ANSI PRG-320. Required testing includes face bond and end joint evaluation, flatwise bending, and flatwise shear. The E5 grade will be qualified using SPF-S 2"x6" lumber obtained by Smartlam from a local vendor in Montana. The E6 grade will utilize SPF-S 2"x6" lumber from Maibec's mill in Maine. All CLT panels will be manufactured by Smartlam at their facility in Columbia Falls, MT.4. Thin laminae in CLTThe use of nominal 1-inch thick lumber in CLT has not been confirmed for any U.S.-made CLT product. A CLT manufacturer located in Quebec (where average log diameter is 4-5") does utilize nominal 1-inch boards in transverse layers of several of their CLT products. This suggests that engineers are aware, and accepting of, CLT made with "thin" lumber (and surely also understand that when used strategically, thinner boards can provide desirable attributes).