Cross-Laminated Timber (CLT) is a relatively new construction material that has not gained popularity in Hungary yet. Producing such building elements using Hungarian raw materials may help to establish this technique. The purpose of our research was to examine the possibility of producing CLT using Hungarian I-214 hybrid poplar. One three-layer panel was produced using Hungarian hybrid polar and polyurethane resin, and tested in bending. The MOR of the poplar CLT was found to be comparable to low-grade softwood CLT, but the MOE was lower than the requirement. Poplar raw material may be suitable for CLT production by selecting higher grade raw material using nondestructive testing, or as a secondary raw material mixed in with softwood.
One of the recent additions to the panoply of engineered wood products is cross-laminated timber (CLT). CLT is a prefabricated, large-scale, solid wood panel that consists of multiple layers of lumbers stacked together, with each layer arranged perpendicular to the next layer, glued with structural grade adhesives, and pressed. The use of massive CLT panels in wood construction provides several advantages over the traditional wood frame systems, making it particularly attractive for tall wood building construction. These main advantages are satisfactory distribution of defects, adequate seismic performance, ability to carry large loads, improved strength and stiffness, adequate acceptable fire performance, acceptable acoustic performance, and improved pre-fabrication.It is expected that as the CLT market will continue to mature, more diversified grades and special CLT products will be introduced into the markets. One special CLT product developed in at Oregon State University has been designated as hybrid CLT. Hybrid CLT refers to CLT panels manufactured with layers of high- and low-grade and low-density species, which aims at improving the economic efficiency and sustainability of the CLT industry with focus on the North America market.
One of the potential issues with hybrid CLT panel application is related to the unknown performance of the connection systems which are highly dependent on the density of the wood in which the fasteners embed. Most of the existing models that have been developed for estimation of the fasteners capacities in withdrawal and lateral loading scenarios are developed based on the assumption of uniform density profile across the layers to which fasteners penetrate. In a hybrid CLT panel, there is a possibility of a variation in density profile along the panel thickness so that the fasteners can be driven into wood of different densities and driven in directions parallel and perpendicular to grain. Because of the potential variation in density profile in the hybrid CLT, the connection system performance cannot be predicted using design models used for uniform density profile applications similar to the models in National Design Specification (NDS). Therefore, there is a need for evaluation of connections performance in hybrid layup.
The main objective of this work is to characterize the performance of connection systems for hybrid CLT. This is achieved through testing and modeling of single fastener connections and then testing and modeling of the typical connection systems. So, the specific objectives are: (1) evaluate the single fasteners performance to account for density variation and compare the results to a proposed modified model, (2) perform an experimental program to test different connection systems with different hybrid CLT panel layups, (3) develop a numerical algorithm based on the use of meta-heuristics tools to fit the optimal parameters for constitutive models to match the experimental data for the connection systems, (4) obtain the optimal parameters for constitutive models of the connection systems tested.
Cross-laminated timber (CLT) is a massive engineered wood product made of orthogonally bonded layers of solid-sawn lumber, and is intended for roof, floor, or wall applications. Although it was developed in Europe in the early 90s, CLT is relatively new to North America. CLT products must be certified for structural use. First North American product standard stipulating test methods and qualification criteria for benchmark structural properties and adhesive bond integrity in structural CLT is ANSI/APA PRG320-2012. These methods and criteria have been adapted from existing laminated timber products (glulam), sometimes disregarding substantial differences between parallel laminates and CLT, in which layers are perpendicular to each other.
From the point of view of long term sustainability of the CLT industry in North America, the critical questions are: 1. Is it possible to use low-grade timber harvested in the Pacific Northwest region in CLT products without compromising critical engineering parameters? Utilization of low- grade lumber, which is typically under-valued, in value-added engineered products should reduce the pressure on the high end structural lumber supply and may also provide a substantial outlet for lower-grade lumber timber species, including beetle-killed pine (BKP) harvested in the affected areas. 2. Can alternative adhesive systems, currently used in related engineered wood products and manufactured by domestic industry, be successfully used in CLT production? This is an important question, and is related to the fact that polyurethane (PUR) is the primary adhesive currently used by CLT manufacturing industry, and is supplied worldwide by a single Europe-based company. This adhesive is optimized for the species commonly used in CLT products to-date. ANSI/APA PRG320-2012 standard allows alternative adhesive types (PRF and EPI are specifically named), but to-date, only one alternative (MUF) has been used in commercial products.
The objective of this project is to determine effective adhesive systems and bonding pressures for the hybrid cross-laminated timber (CLT) combinations. A secondary objective is to evaluate the testing methods prescribed in PRG 320-2012 for cross-laminated bond integrity. Integrity of hybrid CLT layups was evaluated on small specimens derived from CLT billets fabricated in-house using test procedures and qualification criteria specified in ANSI/APA PRG 320-2012 section 8.2.3. Test results were compared to prescribed qualification criteria. The Hybrid CLT combinations for this study include both structural grade lumber and low-grade lumber. For a reference species, lodgepole pine was selected, since it is a member of the US-SPF group closely related to the European species commonly used for CLT construction. The structural-grade, local species will be represented by Douglas-fir, while the low-grade species will be represented by low-grade lodgepole Pine, Douglas-fir, and Western Hemlock.
The two adhesive systems investigated were 1) polyurethane-based PUR (currently the most common adhesive used by the CLT industry), which will serve as a reference system, and 2) phenol-resorcinol formaldehyde (PRF), which will represent a potential domestic alternative. PRF was chosen because it is a cold setting adhesive commonly used by the engineered wood products industry in North America; however, no CLT manufacturers utilize this adhesive system. The variables included species combinations (6), adhesive types (2), and clamping pressures (3), with repetition of 9 specimens per combination coming from at least three different CLT billets. The specimen’s bond integrity was assessed by the qualification panel requirements in PRG 320-2012 section 8.2. The qualification tests are block shear and cyclic delamination. A combination must pass both of the test requirements to qualify. The results of the study show that, of the 36 combinations, six failed the block shear test requirements and twenty-five failed the delamination test requirements. The 10 variable combinations that passed both requirements were DDL10F, DDL40F, DPL40F, PPH10F, PPH69F, PPH10U, PPH40U, PPL10U, PPL69U, and PHL69U. Initial inspection of test results show that no single variable that seems to make a significant impact on the bond integrity. It did reveal that no combinations with the use of Douglas-fir as a face material and PUR as an adhesive met the requirement, and only one combination with western hemlock as a core material met the requirements.
It is evident that the delamination test was the major restriction on whether or not a combination passes the bond qualification. We believe that the adaption of a delamination test standard designed for layers with parallel grains makes the passing requirement too strict for an orthogonally bonded product. In conclusion, there were 10 combinations that passed both bond integrity test requirements. It was unclear whether the species and/or grade combination, adhesive system, or clamping pressure made the biggest impact on the bond integrity. Relative to the reference adhesive (PUR), and species combination (lodgepole pine), the hybrid panels performed similarly and showed that certain species and/or grade combinations could pass the qualification requirements for specific requirements. The knowledge gained by this screening study will allow further qualification testing of the passing combinations per PRG320-2012. This also has the potential to supply the CLT manufacturing community with greater flexibility of manufacturing techniques and materials, as well as offer value to underutilized lumber.
Project contact is Stacey Fritz at Cold Climate Housing Research Center – National Renewable Energy Lab (NREL)
Summary
This project will design, produce, test, and integrate engineered timber products for a modular building system with potential for national applications. The Cold Climate Housing Research Center (CCHRC) in Fairbanks, Alaska, is combining advanced building technologies into a high performance and interoperable kit-of-parts building system called “New Iglu” to meet the increasing demand for affordable, flexible housing solutions. CCHRC is prototyping its innovative New Iglu project, which utilizes vacuum insulated panels, with support from the Department of Energy’s Advanced Building Construction Initiative. With this Wood Innovation Grant, CCHRC will partner with Oregon State University (OSU) and University of Oregon (UO)’s TallWood Design Institute (TDI) to leverage TDI’s specialized research laboratory facilities and expertise in engineered timber, prototyping, and structural engineering. The goals are to prototype modular engineered timber structural frame components for the New Iglu system, demonstrate the commercial viability of low-value timber, and disseminate results to stakeholders. TDI will develop frame components, including reusable structural connections, that integrate with New Iglu and meet current U.S. buildings codes and standards.
Sustainable solutions to building construction can help improve material utilization efficiency while providing economic development. This paper focuses on the development of low-grade hardwood CLT made with Yellow-Poplar (Liriodendron tulipifera) as an exemplar species. Analysis programs developed at Virginia Tech (CLT-VT) investigate whether design methods developed for softwood species are suitable for use with the mechanical properties of hardwoods to predict structural behaviour of CLT panels. The CLT-VT programs will include the analytical design methods defined in the CLT Handbook for floor/roof and wall systems, and beams/lintels. The study will assist in further development of a sustainable building product while adding value to under-utilized low-grade hardwood lumber and creating a road map for the production of CLT materials from most every domestic wood species available in the United States.
This study examines if Cross-Laminated Timber (CLT) design methods approved for softwood species can be used with hardwood species, specifically low-grade hardwoods. Analytical predictions from researcher-generated computer programs will be compared to data from experimental evaluations of hardwood CLT. Successful completion will allow for an under-utilized timber resource to be incorporated into CLT production.
Fibre-reinforced polymers (FRPs) are effective in the flexural stiffening and strengthening of structural members. Such systems can be optimised if accurate numerical models are developed. At present, limited information is available in the literature on numerical models that can predict with good accuracy the nonlinear behaviour of FRP reinforced low-grade glued laminated timber beams. This paper discusses the development of a finite element model, which incorporates nonlinear material modelling and nonlinear geometry to predict the load–deflection behaviour, stiffness, ultimate moment capacity and strain distribution of FRP plate reinforced glued laminated timber beams manufactured from mechanically stress graded spruce. Beams with and without sacrificial laminations are modelled and their performance is compared to unreinforced glued laminated timber beams. The model employed anisotropic plasticity theory for the timber in compression. The failure model used was the maximum stress criterion. Strong agreement was obtained between the predicted behaviour and the associated experimental findings. It was deduced from comparing the results from the numerical model with experimental findings that the FRP plate succeeds in increasing the performance of the adjacent timber significantly. The model is a useful tool for examination of the effect of reinforcement percentage and will be used for optimisation of the hybrid beam.
In Australia CLT has a big potential but has to be imported from overseas to date for quite high prices. Milling of Pinus Radiata using optimised sawing patterns for yield and consecutive mechanical grading lead to a substantial amount of boards, which cannot be used for structural purposes directly. Therefore it should be economically interesting to produce CLT using this resource. The authors performed a considerable amount of mechanical tests using various setups and optimised layups in order to investigate the mechanical properties of Pinus Radiata CLT using non-structural boards. The results showed that depending on the layup of the CLT the used resource leads to a product that performs similarly to the ones on the market in Europe.
The proposed paper presents two alternative strategies for using fast-growing, low-grade softwood for modern engineered wood products. A chemical based strategy is explored first with the testing of polymer-impregnated small clear wood samples. A second mechanical based strategy based on the tectonics of stress-laminated bridge decks is examined in further detail with 1:10 scaled structural models, followed later on by full-scale testing. The relative benefits and disadvantages of each strategy are compared to each other, and benchmarked against regular sawn timber and conventional engineered wood products like glulam and cross-laminated timber.
International Scientific Conference on Hardwood Processing
Research Status
Complete
Notes
September 25-28, 2017, Lahti, Finland
Summary
Low-grade hardwood logs are the by-product of logging operations and, more frequently today, urban tree removals. The market prices for these logs is low, as is the value recovered from their logs when producing traditional forest products such as pallet parts, railroad ties, landscaping mulch, or chips for pulp. However, the emergence of cross-laminated timber (CLT) for building construction in North America may provide an additional and possibly a more valuable product market for low-grade, low-value hardwood logs. Using the RaySaw sawing and ROMI rough mill simulators and a digital databank of laser-scanned low-grade yellow-poplar (Liriodendron tulipifera) logs, we examine the yield-recovery potential for lumber used in the production of CLT.
Utilization of Low-Value Lumber from Small-Diameter Logs Harvested in Pacific Northwest Forest Restoration Programs in Hybrid Cross Laminated Timber (CLT) Core Layers: A Market Response
This thesis is part of a larger project where lumber from small logs harvested in restoration programs was examined for use in CLT; panels were manufactured utilizing lumber from small logs and the mechanical properties were assessed. While another team focused on mechanical testing to examine the technical viability of this concept according to current manufacturing standards, the objective of this thesis was to assess the practical feasibility of this conception within the supply chain. This research focuses on the Blue Mountains–a region in eastern Oregon.
The approach was based on analysis of previous literature, Forest Service Cut & Sold reports, and semi-structured in-person and telephone interviews of federal timberland foresters, sawmill personnel, and management of current and potential cross-laminated timber manufacturers.
This research suggests that if the regional supply chain in the Blue Mountains were to process more small diameter logs into lumber, this will increase the available supply of 2x4 and 2x6 dimensions of lumber for use in a variety of different markets rather than for exclusive use in the center layers of cross-laminated timber panels.
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