A study was conducted with the primary objective of gathering information for the development of a protocol for evaluating the surface quality of cross-laminated timber (CLT) products. The secondary objectives were to examine the effect of moisture content (MC) reduction on the development of surface checks and gaps, and find ways of minimizing the checking problems in CLT panels. The wood materials used for the CLT samples were rough-sawn Select grade Hem-Fir boards 25 x 152 mm (1 x 6 inches). Polyurethane was the adhesive used. The development of checks and gaps were evaluated after drying at two temperature levels at ambient relative humidity (RH).
The checks and gaps, as a result of drying to 6% to 10% MC from an initial MC of 13%, occurred randomly depending upon the characteristics of the wood and the manner in which the outer laminas were laid up in the panel. Suggestions are made for minimizing checking and gap problems in CLT panels. The checks and gaps close when the panels are exposed to higher humidity.
Guidelines were proposed for the development of a protocol for classifying CLT panels into appearance grades in terms of the severity of checks and gaps. The grades can be based on the estimated dimensions of the checks and gaps, their frequency, and the number of laminas in which they appear.
The work presented in this report is a continuation of the FPInnovations' research project on determining the performance of the CLT as a structural system under lateral loads. As currently there are no standardized methods for determining the resistance of CLT shearwalls under lateral loads, the design approaches are left at discretion of the designers. The most common approach that is currently used in Europe and North America assumes that the resistance of CLT walls is a simple summary of the shear resistance of all connectors at the bottom of the wall. In this report some new analytical models for predicting of the design (factored) resistance of CLT walls under lateral loads were developed based on connection properties. These new models were than evaluated for their consistency along with the models that are currently used in North America and in Europe.
In total five different design models (approaches) were used in the study, the two existing models and three newly developed ones. All models were used to predict the factored lateral load resistances of various CLT wall configurations tested in 2010 at FPInnovations. The analyzed walls had different aspect ratios and segmentation, different vertical load levels, different connection layouts and different fasteners in the connections (ring nails, spiral nails and screws). The design values obtained using the various analytical models were compared with the maximum forces and yielding forces obtained from the experimental tests. Ratios between the ultimate loads obtained from experimental tests and design values obtained by the five analytical design models were used as a measure for the consistency of the models. Newly developed models that account for sliding-uplift interaction in the brackets (models D3-D5) showed higher level of consistency compared to existing ones. The analytical model D4 that accounts for sliding-uplift interaction according to a circular domain, is probably the best candidate for future development of design procedures for determining resistance of CLT walls under lateral loads. In case of coupled CLT walls, contribution of vertical load to the wall lateral resistance was found to be two times lower than in case of single wall element with the same geometry and vertical load. Special attention in the coupled walls design should be given to step joints between the adjacent wall panels. Over-design of the step joint can result in completely different wall behavior in terms of mechanical properties (strength, ductility, deformation capacity, etc.) that those predicted.
It should be noted that conclusions made in this report are made based on the comparison to the tested configurations only. Additional experimental data or results from numerical parametric analyses are needed to cover additional variations in wall parameters such as wall geometry and aspect ratio, layout of connectors (hold-downs, brackets), type and number of fasteners used in the connectors, and the amount of vertical load. The findings in this report, however, give a solid base for the development of seismic design procedure for CLT structures. Such procedure should also include capacity based design principles, which take into account statistical distributions of connections resistances.
At the request of the Council of Forest Industries, a simplified (hand-calculation) design method has been proposed for estimating the ultimate shear capacity and load-deflection response of wood-framed and panel-sheathed diaphragms, by Forintek's Wood Engineering Department. In its current form, the simplified code design approach can predict ultimate shear capacity for a wide range of (blocked) diaphragm constructions, sheathed with panels ranging from 7.5 mm (3/8") to 18.5 mm (3/4") in thickness, with a 22% coefficient of variation. This is comparable to the 21% variability exhibited by the current Canadian diaphragm design method, and not alarmingly larger than the 16% shown by the more detailed APA design method. Indeed, for the three high-shear diaphragms sheathed by 18.5 mm (3/8") Douglas-fir plywood panels, the proposed simplified design method yielded shear prediction errors of only 0%, 1% and 6%. Furthermore, simplified functions were also able to provide good estimates of diaphragms failure mechanisms, and their load-deflection patterns, as measured in earlier verification tests. Predictions from the simplified model need to be experimentally verified for high shear capacity diaphragms sheathed in thick panels fastened to glulam frames by large diameter nails; before its possible introduction to the Canadian wood design code.
A literature survey on experimental data and analytical studies of the structural behaviour of wood framed shear walls and diaphragms has been carried out. The utility of various analytical methods for the study of internal forces in these structural elements due to external static or dynamic forces has been noted. It also has been concluded that the complexity of these analytical methods precludes their use as a tool for standard designs on a daily basis. For these standard designs currently established design methods will likely be continued to be used for some time. Establishment of design data by means of testing for panel thicknesses currently not included in the Canadian design code is recommended.
In order to maintain the competitive advantage in existing and new markets situated in seismic and high wind zones such as the Pacific Rim and the southeastern U.S., it is proposed to study deflections in walls, floor and roof assemblies. The proposed project will also be very useful in: a) setting deflection criteria as will be demanded by performance-based codes, and b) responding to the inevitable transition to displacement-based seismic design.
In the new 2005 edition of the National Building Code of Canada, the permissible deflections under earthquake conditions will be much more restrictive and could potentially become the governing factor for the design of wood frame construction.
To proactively respond to the code changes, this three-year project was to develop design procedures for determining the stiffness or deflection of shearwalls and diaphragms under these extreme seismic and wind load events. While investigating the formulae for predicting deflections, issues related to the overall strength or load carrying capacity of shearwalls were also addressed.
Using a “mechanics-based” approach, deflection formulae were developed for unblocked shearwalls, two-sided shearwalls (gypsum wallboard on one side and wood-based panels on the other side), and shearwalls without hold-downs. In collaboration with staff from the Canadian Wood Council, these deflection formulae will be submitted for implementation in the next edition of the Canadian Standard for Engineering Design in Wood (CSA O86), which in turn forms the basis for acceptance under the National Building Code.
The work has also helped to address the following issues in the CSA O86:
· Height limitations for unblocked shearwalls (currently capped at 2.44m)
· The use of diagonal lumber as sheathing for walls and diaphragms. This information, which is particularly important in upgrading wood buildings to new code requirements, addresses concerns raised by designers.
Technical information generated from this project was disseminated in the wood engineering community and CSA O86 committee meetings. Two papers entitled “Lateral Resistance of Tall Unblocked Shearwalls” and “Deflections of Nailed Shearwalls and Diaphragms” were presented at the 8th World Conference on Timber Engineering in Lahti, Finland in June 2004. An article entitled “Racking Performance of Tall Unblocked Shearwalls” has been submitted to the ASCE Journal of Structural Engineering for publication. Another article entitled “Performance of Shearwalls with Diagonally Sheathed Lumber” is being prepared and will be soon submitted to the ASCE Journal of Structural Engineering.
By proactively responding to future design code changes, this project will allow the construction industry to take advantage of various shearwall options in their designs of wood frame buildings, and will assist the wood products industry to maintain its competitive advantage in existing and new markets situated in seismic and high wind zones such as those around the Pacific Rim and in the South Eastern United States.
The objective is to broaden the market for sheathing panels by establishing shear strength values for the design of floor and roof diaphragms using 18.5mm and 20.5mm thick plywood panels. As a result of an emergency no work was done on this project.
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.