Strength and stability of any engineered structure rely heavily on the connections that link its structural members together. Even though current Canadian code provisions for the design of timber bolted connections were essentially developed based on connections showing a ductile behaviour (based on the so-called European Yield Model), such provisions do not provide any reliable guidance towards predicting deformations attributed to a given loading state. Displacement-related properties, such as stiffness, ductility cannot be determined using the EYM approach. Such information is crucial for designers who are interested in designing for high wind and earthquakes, where stiffness and ductility, in addition to other serviceability issues, become a major issue.
This study provides critical information on the stiffness and ductility of bolted timber connections. Results from analysis made on available sets of bolted timber connections data that has been generated over the last 15 years at the Royal Military College of Canada (RMC) and at Forintek is presented in this report. Analysis results indicate that ductility ratio (m) is strongly dependent on the yield point estimate and the ultimate displacement used to calculate the ratio. The method developed by Yasumura & Kawai for the estimation of yield load and deformation was found to be more suitable for ductility calculations of bolted timber connections. Moreover, analysis have indicated that stiffness equation given in Eurocode 5 for bolted timber connections does not adequately predict the initial stiffness of bolted timber connections as the equation is expressed as a function of wood density and fastener diameter only. There is a need to incorporate other parameters such as the slenderness ratio and the connection geometric configuration which could improve predictions. Ductility ratio of bolted timber connections can be predicted reasonably well using the “Reserved Capacity” concept which is described as the ratio between the governing brittle failure resistance (i.e., RS, GT, T) and the ductile capacity of the connection as predicted by the new proposed design equations in CSA O86. Further verification of the proposed expression is needed to make sure that it is applicable to all bolted timber connection configurations and explore its applicability to other types of timber connections.
This work was launched in support of the new proposed Section on the “Lateral Load Resisting Systems Design” and the new design approach for fastenings in the Canadian Timber Design Code (CSA Standard O86). Efforts will be invested in the future in refining both the current Eurocode 5 formula for prediction of initial stiffness and the proposed expression for prediction of ductility ratio. Ultimately, there is a need to link the new proposed Lateral Load Resisting Systems Design Section to the Fastenings Section in terms of connections’ stiffness and ductility required to satisfy the specified system response.
This report summarizes the progress from Year 4 of the multi-year Lumber Properties project. All activities continue to conform to the guiding principles adopted by the Lumber Properties Steering Committee (LPSC) at the start of the program. This year support was provided to statisticians from the University of British Columbia’s Department of Statistics to meet and work with researchers and statisticians from the US Forest Products Laboratory (USFPL) in Madison, WI. All physical testing under the ongoing monitoring pilot study was also completed, allowing the UBC statisticians to continue work refining their global lumber properties simulator. Work is continuing on the collection of secondary properties for Norway spruce and on the analysis of the data collected to-date.
No activities requiring significant resources were carried out under the Resource Assessment and the Special Products Initiative. Instead, these resources were redirected to cover shortfalls in the provincial funding under the Strategic Framework Initiative, so that the statistical work with the USFPL could continue.
A round robin study using the CSA O112.9 standard was conducted with six participating test laboratories, three in Canada and three in the U.S., with the support of the CSA Wood Adhesive Sub-Committee. The tests included block shear (dry, vacuum-pressure, and boil-dry-freeze), delamination, and condition B1 creep. Test specimens were prepared using Douglas-fir as substrate and four adhesives (phenol-resorcinol formaldehyde, melamine-urea formaldehyde with 80% melamine, melamine-urea formaldehyde with 40% melamine, and catalyzed polyvinyl acetate).
Precision and bias statements (repeatability and reproducibility limits) were developed for the block shear test (strength and wood failure) and delamination test for O112.9. The statistical procedures of ASTM E 691 were found to be inadequate for the block shear test, so appropriate statistical procedures were identified and applied. However, a refinement of the E691 method was used to analyze the delamination test. The creep data was not analyzed because it showed huge variability across the laboratories. The rest of the data will be analyzed in the future.
In the block shear test, the repeatability estimates obtained from the round robin test tend to overestimate those of O112.9, that is, the outcomes from using the O112.9 test procedures would actually vary less than those indicated by the limits derived from the study.
This report of the research project "Improved prediction of seismic resistance of Part 9 Houses" under the CMHC External Research Program consists of a review and assessment of analysis methods; numerical evaluation of current seismic design requirements in Canada; and new formulations for seismic design of conventional wood-frame construction in Canada.
The relative performance of three mechanics-based methods is ascertained by comparing the test data of lateral capacities of partially restrained wall specimens having window openings with the predicted results from three calculation methods: Method 1 by Ni and Karacabeyli (2000, 2002) is the simplest to use and gave the most conservative results; Method 2 by Källsner et al., (2001, 2002) is less conservative but more complicated to apply to practical problems, and Method 3 by Källsner and Gurhammar (2005, 2006) gives non-conservative results. The suitability of other methods of analysis, (e.g. SAWS, Drain2D-X) was also examined. Method 1 was chosen as the principal analysis tool for this investigation.
The adequacy of the seismic provisions of the CWC 2004 Design Guide and of the proposals for Part 9 of the 2010 NBCC are assessed by the seismic design methods specified in Part 4 of the 2005 NBCC and utilizing the analysis Method 1. Two building types were used: a square building of 15.0 x 15.0 m plan and a rectangular one of 4.8 x 15.0 m, each of 1, 2 and 3 storeys height. The analysis indicates that neither the current CWC Guide nor the proposals for the 2010 NBCC Part 9 meet the seismic requirements of Part 4 of the 2005 NBCC for the higher seismic zones. The discrepancies are particularly pronounced for the shorter side of the rectangular buildings.
It must be noted that the buildings studied in this investigation represented worst case scenarios. In reality, wood-frame houses would generally contain more walls than the minimum wall lengths required by the CWC Guide and the proposed NBCC 2010, and thus would possess larger lateral resistance.
Following the numerical results of a parametric study of different wall constructions, two new approaches for the seismic provisions of conventional wood-frame construction in Canada are presented, an area-based method, and a method based on percentages of braced wall lengths. Both methods conform substantially to the seismic requirements of Part 4 of the 2005 NBCC.
For heavy construction the provisions for 1 and 2 storey buildings give reasonable agreement with those for 2 and 3 storeys of light construction.
Additional parameter studies should be carried out for irregular buildings, for heavy wall cladding such as stucco and masonry, and for minimum size of braced wall panels.
The objective of the project is to develop/improve practical, reliable and internationally recognized methods for assessing/pre-screening the long-term structural performance of engineered wood products used in residential and non-residential applications.
The main sources of lateral loads on buildings are either strong winds or earthquakes. These lateral forces are resisted by the buildings’ Lateral Load Resisting Systems (LLRSs). Adequate design of these systems is of paramount importance for the structural behaviour in general. Basic procedures for design of buildings subjected to lateral loads are provided in national and international model building codes. Additional lateral load design provisions can be found in national and international material design standards. The seismic and wind design provisions for engineered wood structures in Canada need to be enhanced to be compatible with those available for other materials such as steel and concrete. Such design provisions are of vital importance for ensuring a competitive position of timber structures relative to reinforced concrete and steel structures.
In this project a new design Section on Lateral Load Resisting Systems was drafted and prepared for future implementation in CSA O86, the Canadian Standard for Engineering Design in Wood. The new Section was prepared based on gathering existing research information on the behaviour of various structural systems used in engineered wood construction around the world as well as developing in-house research information by conducting experimental tests and analytical studies on structural systems subjected to lateral loads. This section for the first time tried to link the system behaviour to that of the connections in the system. Although the developed Section could not have been implemented in CSA O86 in its entirety during the latest code cycle that ended in 2008, the information it contains will form the foundation for future development of technical polls for implementation in the upcoming editions of CSA O86.
Some parts of the developed Section were implemented in the 2009 edition of CSA O86 as five separate technical polls. The most important technical poll was the one on Special Seismic Design Considerations for Shearwalls and Diaphragms. This technical poll for the first time in North America includes partial capacity design procedures for wood buildings, and represents a significant step forward towards implementing full capacity-based seismic design procedures for wood structures. Implementation of these design procedures also eliminated most of the confusion and hurdles related to the design of wood-based diaphragms according to 2005 National Building Code of Canada. In other polls, the limit for use of unblocked shearwalls in CSA O86 was raised to 4.8 m, and based on the test results conducted during the project, the NLGA SPS3 fingerjoined studs were allowed to be used as substitutes for regular dimension lumber studs in shearwall applications in engineered buildings in Canada.
With the US being the largest export market for the Canadian forest products industry, participation at code development committees in the field of structural and wood engineering in the US is of paramount importance. As a result of extensive activities during this project, for the first time one of the AF&PA Special Design Provisions for Wind and Seismic includes design values for unblocked shearwalls that were implemented based on FPInnovations’ research results. In addition, the project leader was involved in various aspects related to the NEESWood project in the US, in part of which a full scale six-storey wood-frame building will be tested at the E-Defense shake table in Miki, Japan in July 2009. Apart from being built from lumber and glued-laminated timber provided from Canada, the building will also feature the innovative Midply wood wall system that was also invented in Canada. The tests are expected to provide further technical evidence for increasing the height limits for platform frame construction in North America.