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.
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.
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.
Nail-Laminated Timber (NLT) and box beam are efficient and economical engineered wood products. Although NLT has been used in North America for more than a century, only in recent years it has gained renewed interests as they have been seen as the most economical panel products used in mass timber buildings. Box beams, on the other hand, are lightweight and generally possess higher strength and stiffness than comparable-sized solid timber and are more efficient than solid timber large spans and loads.
In this report, existing design provisions and their limitations for the design and construction of NLT in box beam in Canadian standards are reviewed. For NLT, there is a general lack of information related to manufacturing, design and construction to ensure consistent manufacturing and installation practices. Therefore, it is difficult to research and document with confidence the full range of performance that can be achieved with NLT. It is therefore recommended that a North American product standard and design information on structural performance, floor vibration, fire resistance, acoustic performance, and construction risk mitigation measures (e.g. moisture and fire) be developed.
In CSA 086, design methods are limited to box beams with flanges and webs bonded with glue. As the flanges and webs of a box beam can be assembled by either glue or mechanical fasteners, it is recommended that design provisions for box beam with mechanical joints be also developed. With the information in Eurocode 5 and relevant supporting research papers, it is ready to be implemented.
The 2009 edition of CSA Standard O86, Engineering Design in Wood (CSA 2009), provides an equation for determining the deflection of shear walls. It is important to note that this equation only works for a single-storey shear wall with load applied at the top of the wall. While the equation captures the shear and flexural deformations of the shear wall, it does not account for moment at the top of the wall and the cumulative effect due to rotation at the bottom of the wall, which would be expected in a multi-storey structure.
In this fact sheet, a mechanics-based method for calculating deflection of a multi-storey wood-based shear wall is presented.
Wood-frame construction is the dominant building construction in low-rise buildings. The growth in the urban population and the need to meet sustainability objectives will mean having to allow taller buildings in areas that were traditionally low-rise construction. While the need for higher and environmentally sustainable building solutions increases, the Canadian codes responsible for the health and safety of buildings continued to limit wood building solutions to four storeys. Mid-rise (5- and 6-storey) wood-frame construction is a natural extension of low-rise wood-frame construction. In 2009, the BC Building Code (BCBC) was amended by the BC Building and Safety Standards Branch (formerly Policy Branch) to allow mid-rise wood-frame construction. The amendment brought the BC Building Code more closely in line with the U.S. states of California, Washington, and Oregon, where mid-rise wood construction is permitted. More than 100 mid-rise wood-frame construction projects in BC followed the BCBC amendment. Later, the provinces of Québec, Ontario, and Alberta took steps to permit mid-rise wood-frame construction, and finally the Canadian Commission on Building and Fire Codes (CCBFC) accepted code change proposals to allow 5- and 6-storey wood-frame construction in the 2015 edition of the National Building Code Canada (NBCC). NRC, CWC, and FPInnovations worked collaboratively on a project, funded by Natural Resources Canada and several provinces to provide additional technical information to support mid-rise wood-frame construction. This Handbook consists of ten multi-disciplinary chapters, which have been prepared to facilitate the design and construction of mid-rise wood-frame construction in Canada. Building on the information that formed the basis of Association of Professional Engineers and Geoscientists of British Columbia (APEGBC) Bulletin and the Régie du bâtiment du Québec (RBQ) guide, this Handbook covers broad design and construction topics and provides practical solutions by making use of the most recently developed technical and research information. The Handbook has been prepared to assist architects, engineers, code consultants, developers, building owners, and Authorities Having Jurisdiction (AHJ). It is designed to be used in conjunction with the upcoming 2015 edition of the NBCC and the 2014 Edition of the CSA Standard on Engineering Design in Wood. It also complements existing design aids such as the CWC Wood Design Manual.
Although energy dissipation is one of the key factors in resisting seismic force, current design codes only take into account the ductility of the backbone properties of hysteresis curves, and the energy dissipation is usually not accounted for. This paper focuses on understanding and assessing the influence of energy dissipation due to different pinching levels on the seismic performance of a light-frame wood shear wall system. Timber structures with identical backbone curves but different pinching levels were analyzed. Incremental dynamic analyses were run on a single-degreeof-freedom system with varying pinching stiffness and residual strength. The seismic evaluation is presented by the spectral accelerations causing failure of the structure and the hysteresis energy dissipation under a suite of 22 ground motions (2 components per motion) over a wide range of fundamental periods of typical timber structures. Results show that the effect of pinching on the seismic performance of timber structures is period-dependent. Short period structures are more sensitive to the pinching of hysteresis loops compared to long period structures. The residual strength of pinching loops has a greater influence on the seismic performance than the stiffness of the pinching loops. Hysteretic energy dissipation derived from standard reversed-cyclic tests can provide a better understanding on the seismic resistance of timber structures. However, the hysteretic energy under a seismic event at near-collapse stage neither agrees with quasistatic cyclic test’s energy dissipation nor is well correlated to the maximum seismic capacity of the structure.
