The following topics in the field of seismic analysis and design of mid-rise (5- and 6-storey) wood-frame buildings are included in this paper: Determination of the building period, linear dynamic analysis of wood-frame structures, deflections of stacked multi-storey shearwalls, diaphragm classification, capacity-based design for woodframe...
FPInnovations carried out a survey with consultants and researchers on the use of analytical models and software packages related to the analysis and design of mass timber buildings. The responses confirmed that a lack of suitable models and related information for material properties of timber connections was creating an impediment to the design and construction of this type of buildings. Furthermore, there is currently a lack of computer models and expertise for carrying out performance-based design for wood buildings, in particular seismic and/or fire performance design.
In this study, a sophisticated constitutive model for wood-based composite material under stress and temperature was developed. This constitutive model was programmed into a user-subroutine which can be added to most general-purpose finite element software. The developed model was validated with test results of a laminated veneer lumber (LVL) beam and glulam bolted connection under force and/or fire.
Light-frame wood structures are the most common type of construction for residential and low-rise buildings in North America. The 2015 edition of the National Building Code of Canada has increased the height limit for light-frame wood construction from 4 to 6 stories. With the increase in building height, it was noticed that light-frame wood structures may be governed by inter-story drift under wind and seismic loads. To reduce the inter-story drift, a hybrid system, consisting of CLT cores and light-frame structures, is proposed. The efficiency of this hybrid system is dependent on the performance of the connections between the two sub-systems. In this project, self-tapping screws (STSs) were used to connect the CLT core and light-frame wood structures on the floor level. Monotonic and reversed-cyclic tests were carried out on CLT-wood frame connections connected with STSs inserted at 45°, 90°, and mixed angles (45° and 90°). The connection performance was evaluated in terms of strength, stiffness, ultimate displacement, ductility, and energy dissipation capacity. Results show that a joint with STSs inserted at 45° had high stiffness and ductility but low energy dissipation, while connections with STSs installed at 90° had high ductility and energy dissipation but low stiffness. Connections with STSs inserted at mixed angles (45° and 90°) achieved the advantages of both configurations when the STSs were inserted at 45° or 90° individually, i.e., high stiffness, ductility, and energy dissipation. The ductility and energy dissipation were significantly improved compared with connections with STSs only inserted at 45° or 90°. This mixed angle connection can be an ideal design for connecting light-frame wood structures to a CLT core to resist wind and seismic load.
Figure 1 shows a floor plan and elevation along with the preliminary shear wall locations for a sixstorey wood-frame building. It is assumed some preliminary calculations have been provided to determine the approximate length of wall required to resist the lateral seismic loads.
If the preliminary design could not meet the drift limit requirement using the base shear obtained based on the actual period, the shear walls should be re-designed until the drift limit requirement is satisfied.
This building is a typical one-storey commercial building located in Vancouver, BC. The plan dimensions are 30.5 m x 12.2 m (100’ x 40’), with a building height of 5 m. The walls are wood-based shear walls, with a wood diaphragm roof and a steel moment frame at the storefront. The roof plan is shown in Figure 1. The site is Seismic Class ‘C’. Wind, snow and seismic figures specific to the project location are taken from the current version of the British Columbia Building Code (2012). Roof dead load is assumed to be 1.0 kPa and the wall weight is 0.5 kPa. The weight of non-structural items including mechanical equipment and the storefront façade has not been included in this example for simplicity.
Wood-concrete composite slab floors provide a promising solution for achieving long spans and shallow wood-based floor systems for large and tall wood buildings. In comparison with conventional wood floor systems, such long span and heavy floors have a lower fundamental natural frequency, which challenges the floor vibration controlled design. A laboratory study, including subjective evaluation and measurement of the natural frequencies and one-kN static deflections, was conducted on wood-concrete composite floors. Method of calculation of the composite bending stiffness of the wood-concrete composite floor is proposed. The design criterion for human comfort was derived from the subjective evaluation results using the calculated fundamental natural frequency and 1 kN static deflection of one meter wide strip of the composite floor. The equation to directly determine the vibration controlled spans from the stiffness and mass was derived. Limited verification was performed. Further verification is needed when more field wood-concrete composite floors become available.
Mass timber (MT) building systems are gaining momentum around the world, especially in Canada where Federal and Provincial governments encourage the greater use of wood in construction projects through various promotion programs such as GCWood Program. In the meanwhile, seismic design provisions in the 2020 National Building Code of Canada have been revised, resulting in significantly higher seismic loads for structures in many locations. Consequently, there is a need to develop new lateral load resisting systems that allow mass timber structures to better compete against their counterparts in steel, concrete and masonry. Building on the success of midply shear walls for wood-frame construction, a multi-year research project was initiated at FPInnovaitons to develop MT version of midply shear wall systems that have greater structural capacities, fire, and acoustical performance.
In the first year of this project, literature reviews were conducted to identify the code requirements on MT components and to survey the available LLRSs used in the MTstructures. Conceptual MT midply wall systems meeting structural, fire, and acoustical performance requirements were proposed. An advisory group meeting was held to evaluate the practicability of the proposed MT midply systems. In the next fiscal year, the proposed MT Midply will be optimised further according to the comments and suggestions from the advisory group. Analytical evaluation of the proposed MT Midply wall systems along with necessary tests will be conducted. Based on the evaluation, a go / no-go decision will be made as to whether the study should be continued for the proposed MT Midply.
National Building Code of Canada (NBC) 2020 is the latest edition of the national model code that will be published towards the end of 2021. Based on the best available information from the Standing Committee on Earthquake Design (SCED) at the time of writing this report, the seismic design demand in the NBC 2020 has increased for all site classes for many locations across the country. Also, there are other changes in NBC 2020 that might impact the seismic analysis and design of timber buildings. The main objective of this report is to compare the NBC 2020 to the 2015 edition, with emphasis on the level of the seismic design loads (demands), and potential impacts on the analysis and design of timber buildings.
Midply shear wall, which was originally developed by researchers at Forintek Canada Corp. (predecessor of FPInnovations) and the University of British Columbia, is a high-capacity shear wall system that is suitable for high wind and seismic loadings. Its superior seismic performance was demonstrated in a full-scale earthquake simulation test of a 6-storey wood-frame building in Japan. In collaboration with APA–The Engineered Wood Association and the American Wood Council (AWC), a new framing arrangement was designed in this study to increase the vertical load resistance of midply shear walls and make it easier to accommodate electrical and plumbing services. In this study, a total of 12 midply shear wall specimens in four wall configurations with different sheathing thicknesses and nail spacing were tested under reversed cyclic loading. Test results showed that the modified midply shear walls have approximately twice the lateral load capacity of a comparable standard shear wall. The drift capacity and energy dissipation capability are also greater than comparable standard shear wall. Seismic equivalency to standard shear walls in accordance with ASTM D7989 was also conducted. Results show that an overstrength factor of 2.5 and can be used to assign allowable design strengths of midply shear walls with 7/16” and nail spacing at 4” or 3” on center. For midply shear walls with 19/32” OSB, a higher overstrength factor must be used to meet the ductility criteria. The information from this study will support code implementation of the midply shear walls in Canadian and US timber design standards, thereby providing more design options for light wood frame structures in North America.
Utilizing Linear Dynamic Analysis (LDA) for designing steel and concrete structures has been common practice over the last 25 years. Once preliminary member sizes have been determined for either steel or concrete, building a model for LDA is generally easy as the member sizes and appropriate stiffness can be easily input into any analysis program. However, performing an LDA for a conventional wood-frame structure has been, until recently, essentially non-existent in practice. The biggest challenge is that the stiffness properties required to perform an LDA for a wood-based system are not as easily determined as they are for concrete or steel structures. This is mostly due to the complexities associated with determining the initial parameters required to perform the analysis.
With the height limit for combustible construction limited to four stories under the National Building Code of Canada, it was uncommon for designers to perform detailed analysis to determine the stiffness of shear walls, distribution of forces, deflections, and inter-storey drifts. It was only in rare situations where one may have opted to check building deflections. With the recent change in allowable building heights for combustible buildings from four to six storeys under an amendment to the 2006 BC Building Code, it has become even more important that designers consider more sophisticated methods for the analysis and design of wood-based shear walls. As height limits increase, engineers should also be more concerned with the assumptions made in determining the relative stiffness of walls, distribution of forces, deflections, and inter-storey drifts to ensure that a building is properly detailed to meet the minimum Code objectives.
Although the use of LDA has not been common practice, the more rigorous analysis, as demonstrated in the APEGBC bulletin on 5- and 6-storey wood-frame residential building projects (APEGBC 2011), could be considered the next step which allows one to perform an LDA. This fact sheet provides a method to assist designers who may want to consider an LDA for analyzing wood-frame structures. It is important to note that while LDA may provide useful information as well as streamline the design of wood-frame structures, it most often will not be necessary.
