A study was conducted with the primary objective of examining the efficacy of a standard block shear test method to assess the bond quality of cross-laminated timber (CLT) products. The secondary objective was to examine the effect of pressure and adhesive type on the block shear properties of CLT panels. The wood material used for the CLT samples was Select grade nominal 25 x 152-mm (1 x 6-inch) Hem-Fir. Three adhesive types were evaluated under two test conditions: dry and vacuum-pressure-dry (VPD), the latter as described in CSA standard O112.10. Shear strength and wood failure were evaluated for each test condition.
Among the four properties evaluated (dry and VPD shear strength, and dry and VPD wood failure), only the VPD wood failure showed consistency in assessing the bond quality of the CLT panels in terms of the factors (pressure and adhesive type) evaluated. Adhesive type had a strong effect on VPD wood failure. The different performance levels of the three adhesives were useful in providing insights into how the VPD block shear wood failure test responds to significant changes in CLT manufacturing parameters. The pressure used in fabricating the CLT panels showed a strong effect on VPD wood failure as demonstrated for one of the adhesives. VPD wood failure decreased with decreasing pressure. Although dry shear wood failure was able to detect the effect of pressure, it failed to detect the effect of adhesive type on the bond quality of the CLT panels.
These results provide support as to the effectiveness of the VPD block shear wood failure test in assessing the bond quality of CLT panels. The VPD conditioning treatment was able to identify poor bondline manufacturing conditions by observed changes in the mode of failure, which is also considered an indication of wood-adhesive bond durability. These results corroborate those obtained from the delamination test conducted in a previous study (Casilla et al. 2011).
Along with the delamination test proposed in an earlier report, the VPD block shear wood failure can be used to assess the CLT bond quality. Although promising, more testing is needed to assess whether the VPD block shear wood failure can be used in lieu of the delamination test. The other properties studied (shear strength and dry wood failure), however, were not found to be useful in consistently assessing bond line manufacturing quality.
This chapter provides general information about the manufacturing of CLT that may be of interest to the design community. The information contained in this chapter may also provide guidance to CLT manufacturers in the development of their plant operating specification document. Typical steps of the manufacturing process of CLT are described, and key process variables affecting adhesive bond quality of CLT products are discussed. Proposed methods for evaluating panel quality are presented.
The North American product standard for performance-rated cross-laminated timber (CLT), ANSI/APA PRG 320, was published in 2012. The standard recognizes the use of all major Canadian and US softwood species groups for CLT manufacturing and provides design properties for specific CLT layups with visually graded and E-rated/MSR laminations. While design properties for CLT layups with Spruce-Pine-Fir and Douglas fir-Larch laminations are specified in the current standard, no design properties are indicated for CLT layups with Hem-Fir laminations.
Design properties for two proposed CLT grades manufactured with Hem-Fir lumber were developed. These include a CLT layup with visually graded laminations and another layup with E-rated/MSR laminations. Design properties for these two CLT layups were calculated separately for use in Canada and the US.
Supporting information for the addition of design properties for Hem-Fir grades to the CLT product standard was generated. Recommended amendments to the CLT product standard include durability and wood failure requirements of bondlines, and design properties for Hem-Fir layups.
Cross-laminated timber (CLT) products are used as load-carrying slab and wall elements in structural systems, thus load duration and creep behaviour are critical characteristics that should be taken into account in design. Given the nature of CLT with orthogonal arrangement of layers and either mechanically fastened with nails or wood dowels, or bonded with structural adhesive, CLT is more prone to time-dependent deformations under load (creep) than other engineered wood products such as glued-laminated timber.
Time-dependent behaviour of structural wood products is accounted for in design standards by providing load duration factors to adjust specified strengths. Since the Canadian Standard on Engineering Design in Wood (CSA O86-09) does not deal with CLT, it does not provide load duration and service condition factors. Until this can be rectified, two options are proposed for adopters of CLT systems in Canada. These include not only load duration and service factors, but also an approach to accounting for creep in CLT structural elements. The proposed recommendations are in line with the specifications in CSA O86-09 and Canadian National Building Code.
Le présent chapitre fournit de l’information générale sur la fabrication du CLT qui peut être intéressante pour les concepteurs. Les renseignements contenus dans ce chapitre peuvent également servir de guide aux fabricants de CLT quant au développement de leur cahier de spécifications d'exploitation d'usine.
Ce chapitre aborde également les étapes spécifiques du processus de fabrication de CLT et les variables de processus clés qui ont une incidence sur la qualité d’adhésion des lamelles des produits de CLT. On y retrouve également les méthodes proposées pour évaluer la qualité des panneaux.
Wood design standards in Canada and the United States provide design values for floor and roof diaphragms with sheathing thickness ranging from 9.5 mm (3/8 in) up to 18.5 mm (3/4 in), that are supported by joists spaced less than 610 mm (24 in) on centre. This range of sheathing thicknesses is adequate for housing and small buildings, but for large non-residential structures, diaphragms with thicker sheathing and wider joist spacing may be more appropriate.
This paper includes the findings of a study aimed at providing research information suitable for implementing design values for diaphragms with thick sheathing in the North American wood design standards. Results from quasi-static monotonic tests on fifteen full-scale 7.3 m (24 ft) long by 2.4 m (8 ft) wide diaphragms framed with 38x191 mm or 38x235 mm (nominal 2x8 and 2x10, respectively) solid sawn lumber or laminated strand lumber and sheathed with plywood or oriented strand board are discussed.
A numerical model was developed using the finite element method. The basic properties of the sheathing, framing members and nailed connections were implemented in the model to replicate the structural behaviour of the diaphragms with thick panels. The numerical model was successfully validated against the experimental data. The shear resistance values for the diaphragms with thick panels tested in this study were calculated. The model may be used to interpolate between various diaphragm configurations and calculate shear resistance values for other configurations of diaphragms with thick sheathing.
In the long run, it is hoped that the use of thicker sheathing will enable the use of structural systems that are cost effective for wider joist or beam spacing than systems made with dimension lumber and traditional sheathing thickness. The experimental data and the model developed in this project will be used to develop proposals for implementation of wood floor and roof diaphragms with thick panels in the Canadian and United States wood design standards.
The objectives of this project are to provide research information suitable for implementing design procedures for diaphragms with thick sheathing in the Canadian Standard for Engineering Design in Wood (CAN/CSA O86.1) and to make the information available to other markets by publishing the results and recommended procedures in a journal article.
This report describes work to provide research information suitable for implementing design procedures for diaphragms with thick sheathing in the Canadian Standard for Engineering Design in Wood (CAN/CSA O86.1) and to make the information available to other markets by publishing the results and recommended procedures in a journal article.
Cross laminated timber (CLT) panels were manufactured and tested to assess their time dependent behaviour. This study is intended to help guide the development of an appropriate test method and acceptance criteria to account for duration of load and creep effects in the design of structures using CLT.
Nine CLT panels of different qualities and using different wood species combinations were manufactured at a pre-commercial pilot plant out of local wood species. The CLT panels manufactured in this study were pressed at about 54% lower pressure than the minimum vertical pressure specified by the adhesive manufacturer due to a limitation of the press, so the CLT panels are viewed as a simulated defective sample, which may occur in a production environment due to material- or process-related issues.
Full-size CLT panels were initially tested non-destructively to assess their bending stiffness. Then, billets were ripped from the full-size CLT panels, and tested to failure in 1-minute and 10-hour ramp tests, or assessed in creep tests under sustained load. The constant loads imposed on the CLT billets tested in creep were calculated as to allow for a maximum deflection of L/180. Following two cycles of loading and relaxation, the CLT billets tested in creep were further tested to failure at the end. The principles of ASTM D6815-09 and those of an in-house FPInnovations protocol were applied to assess the time dependent behavior of the CLT billets.
The main test findings are summarized below:
In terms of residual stiffness, the percentage change in the initial bending stiffness for the CLT billets subjected to the 10-hour ramp test varied between 0-5%, showing a 3% drop in stiffness on average, while that for the CLT billets tested in creep ranged between 0-3%, showing a 1% stiffness drop on average. These are regarded as relatively small changes in bending stiffness.
In general, decreasing creep rates were observed on most of the CLT billets especially in the first cycle up to 90 days. The creep rates went up after 120 days of loading due to an increase in temperature and relative humidity conditions, which greatly affect the rate of deflection and recovery of wood products.
Fractional deflections were calculated for all the CLT billets after 30-day intervals and found to be less than or equal to 1.43.
Creep recovery was above 36% after 30-day, 60-day, and 90-day recovery periods in the first cycle. However, in the second cycle, creep recovery for some CLT billets dropped below 20% for certain time periods.
ASTM D6815-09 provides specifications for evaluation of duration of load and creep effects of wood and wood-based products. The standard was designed to accommodate wood products that can be easily sampled, handled, and tested under load for minimum 90 days and up to 120 days. The standard requires a minimum sample size of 28 specimens. Because of its large dimensions, CLT products are not feasible for experiments requiring such large sample sizes. However, the findings of this study revealed potential for some of the acceptance criteria in ASTM D6815-09 to be applied to CLT products. The CLT billets in this study were assessed in accordance to the creep rate, fractional deflection, and creep recovery criteria in ASTM D6815-09 standard. All CLT billets tested in this study showed (1) decreasing creep rates after 90/120 days of loading, (2) fractional deflections less than 2.0 after 90-day loading, and (3) higher creep recovery than 20% after 30 days of unloading, as required by ASTM D6815-09. A single replicate billet was used per CLT configuration instead of the minimum sample size required by the standard which may have an effect on the findings.
