Braced timber frames (BTFs) are one of the most efficient structural systems to resist lateral loads induced by earthquakes or high winds. Although BTFs are implemented as a system in the National Building Code of Canada (NBCC), no design guidelines currently exist in CSA O86. That not only leaves these efficient systems out of reach of designers, but also puts them in danger of being eliminated from NBCC. The main objective of this project is to generate the technical information needed for development of design guidelines for BTFs as a lateral load resisting system in CSA O86. The seismic performance of 30 BTFs with riveted connections was studied last year by conducting nonlinear dynamic analysis; and also 15 glulam brace specimens using bolted connections were tested under cyclic loading.
In the second year of the project, a relationship between the connection and system ductility of BTFs was derived based on engineering principles. The proposed relationship was verified against the nonlinear pushover analysis results of single- and multi-storey BTFs with various building heights. The influence of the connection ductility, the stiffness ratio, and the number of tiers and storeys on the system ductility of BTFs was investigated using the verified relationship. The minimum connection ductility for different categories (moderately ductile and limited ductility) of BTFs was estimated.
Midply shear wall (hereafter Midply), 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 and make it easier to accommodate electrical and plumbing services. In this study, a total of 14 Midply specimens in six wall configurations with different sheathing thicknesses and nail spacing were tested under reversed cyclic loading. Test results showed that Midply has 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 walls. For Midply to use the same seismic force modification factors as standard shear walls, seismic equivalency to standard shear walls in accordance with ASTM D7989 was also conducted. Although Midply has superior lateral load and drift capacities, it does not seem to be as ductile as the standard shear walls at the same over-strength level. Additional testing and dynamic analysis are recommended to address this issue.
Le bois lamellé-croisé (CLT), un produit novateur mis au point en Autriche et en Allemagne il y a environ 20 ans, s’est acquis depuis une popularité grandissante dans plusieurs pays européens grâce à ses applications résidentielles et non résidentielles. L’expérience européenne prouve que la construction en CLT peut être concurrentielle, notamment pour les immeubles de moyenne et grande hauteurs.
Le présent chapitre comprend une revue de la documentation des travaux ayant été menés autour du monde sur le rendement sismique des panneaux de murs et des structures en CLT. Cela fait suite aux résultats obtenus grâce à une série de tests quasi statiques réalisés sur des panneaux de mur en CLT dans les laboratoires de FPInnovations à Vancouver, où divers assemblages et configurations de panneaux en CLT ont été évalués. Ces configurations comprenaient des panneaux de mur simples présentant trois différents rapports de forme, des panneaux de mur multiples avec des joints à demi-bois et différents types de vis pour les assembler de même que des assemblages de mur à deux étages. Les fixations permettant d’arrimer les murs à la fondation incluaient des ancrages d’acier standard avec des clous à filet annelé, des clous torsadés et des vis, la combinaison d’ancrages d’acier et de mécanismes de fixation, des vis longues disposées diagonalement, et des ancrages faits sur mesure utilisés avec des vis pour bois d’oeuvre. Les résultats ont démontré que les murs de CLT offrent une performance sismique adéquate lorsqu’ils sont fixés aux ancrages d’acier avec des clous ou des vis. De plus, l’utilisation de clous et de fixations à chaque extrémité du mur améliore la performance sismique. L’emploi de vis longues disposées diagonalement pour fixer le mur de CLT au sol n’est pas recommandé dans les zones à haut risque sismique en raison du comportement ductile réduit du mur et du mécanisme subi d’arrachement des vis. Par contre, l’installation de joints à demi-bois sur les plus grands murs peut non seulement s’avérer une solution efficace pour en réduire la rigidité, ce qui atténue la charge sismique entrante, mais aussi pour améliorer ses capacités de déformation. Enfin, l’utilisation de rivets pour bois d’oeuvre regroupés en petite quantité et de joints à ancrages faits sur mesure a aussi fait ses preuves quant à l’assemblage des panneaux de mur en CLT.
De surcroît, ce chapitre procède à un relevé des méthodes potentielles permettant la mise en oeuvre et l’évaluation des valeurs de modification des forces (valeur « R ») pour la conception de différentes structures. On y soulève aussi les résultats de recherches poursuivies en Europe sur la détermination de la valeur « q » (l’équivalent européen de la valeur « R ») quant au comportement des structures de CLT. Enfin, d’après les données recueillies, on effectue les estimations des valeurs « R » des structures de CLT selon les normes du Code national du bâtiment du Canada et on réalise l’ébauche de procédures de conception de structures de CLT d’après leur capacité.
This report presents the seismic design of a 10-storey Cross Laminated Timber (CLT) building in Vancouver, BC, conducted according to the National Building Code of Canada. The multi-storey condominium consists of 20 apartments for a total floor area of about 2000 m2. First, a preliminary simplified model is formulated assuming the same stiffness per meter for each wall of the building. The Equivalent Seismic Force Procedure is applied and the results serve for a preliminary design of all the major connections that play a significant role on the lateral stiffness of the building, assuming rigid in plane floor diaphragms and well-anchored CLT walls. Based on the results of the preliminary design, a 3 dimensional finite element model is created, describing analytically the modelling approach adopted, and both the Equivalent Seismic Force Procedure (referred as static analysis) and the Modal Response Spectrum Method (referred as dynamic analysis) are applied to obtain the design forces for each wall of the building. Based on the results from the dynamic analysis, the final seismic design of the building is performed and the results are presented for connections dedicated to transfer (i) shear forces from floor diaphragms to walls below and from walls to diaphragms below, (ii) uplift forces for each wall, (iii) boundary forces between CLT panels within the same walls, (iv) boundary forces between perpendicular walls, and (v) boundary forces between CLT floor panels. All connections prescribed to provide ductility and energy dissipation are designed to fail in ductile failure mode according to the CSA 086-09 while connections that should remain within the elastic range to allow the ductile connections to yield are designed with overstrength factor.
Cross-laminated timber (CLT) is an innovative wood product that was first developed some 20 years ago in Austria and Germany and ever since has been gaining popularity in residential and non-residential applicationsin Europe. European experience shows that this system can be competitive, particularly in mid-rise and high-risebuildings.
In this chapter, a literature review on the research work conducted around the world related to the seismic performance of cross-laminated timber (CLT) wall panels and structures is included. This is followed by the results from a series of quasi-static tests on CLT wall panels that were conducted at FPInnovations’ Wood Products laboratory in Vancouver. CLT wall panels with various configurations and connection details were tested. These configurations included single panel walls with three different aspect ratios, multi-panel walls with step joints and different types of screws to connect them, as well as two-storey wall assemblies. Connections for securing the walls to the foundation included off-the-shelf steel brackets with annular ring nails, spiral nails, and screws; combination of steel brackets and hold-downs; diagonally placed long screws; and custom made brackets with timber rivets. Results showed that CLT walls can have adequate seismic performance when nails or screws are used with the steel brackets. Use of hold-downs with nails on each end of the wall improves their seismic performance. Use of diagonally placed long screws to connect the CLT walls to the floor below is not recommended in high seismic zones due to less ductile wall behaviour and to the sudden screw pull-out failure mechanism. Use of step joints in longer walls can be an effective solution not only to reduce the wall stiffness and thus reduce the seismic input load, but also to improve the wall deformation capabilities. Timber rivets in small groups with custom made brackets were found to be effective connectors for CLT wall panels.
In addition, this chapter includes a survey of potentially available methods for development and assessment of R-factors for different structural systems. Studies conducted in Europe on the assessment of the behaviour q-factor (European R-factor equivalent) for CLT structures and their findings are also discussed. Finally, based on all available information, estimates were made on the values of R-factors for CLT structures according to the National Building Code of Canada, and capacity-based design procedures for CLT structures were drafted.
An analytical study to examine the seismic performance of wood-frame podium buildings up to 8 storeys is presented in this report. Simple archetype podium buildings of 5 to 8 storeys in total height were designed in accordance with the two-step analysis procedure given in 2015 NBCC or ASCE 7-10. Nonlinear time-history dynamic analyses were conducted using earthquake ground motions selected and scaled based on the guidelines proposed by Tremblay et al. to match the reference design spectra in NBCC. Using the performance-based seismic design criteria established in the NEESWood project, it was found that:
Podium buildings with a building period ratio of 1.1 (ASCE 7-10) did not meet the performance criteria, thus the period ratio requirement of 1.1 was not appropriate.
A stiffness ratio of not less than 10 times (ASCE 7-10) was more appropriate as a requirement of using two-step analysis procedure for wood-frame podium buildings up to 8 storeys, compared to that of not less than 3 times (NBCC Commentary). With a higher stiffness ratio, the seismic response of the upper wood-frame structure of podium building was closer to that of the pure wood-frame structure.
The results of this study will be used to guide the assessment of the feasibility of constructing wood-frame podium buildings of 8 storeys in height and the development of design guidelines. This would also guide the longer-term goal of proposing changes to the building codes.
Braced mass timber (MT) frames are one of the most efficient structural systems to resist lateral loads induced by earthquakes or high winds. Although braced frames are presented as a system in the National Building Code of Canada (NBCC), no design guidelines currently exist in CSA O86. That not only leaves these efficient systems out of reach of designers, but also puts them in danger of being eliminated from NBCC. The main objective of this project was to develop the technical information needed for development of design guidelines for braced MT frames as a lateral load resisting system in CSA O86.
In the first year of the project, the seismic performance of thirty (30) braced MT frames with riveted connections with various numbers of storeys, storey heights, and bay aspect ratios were studied by conducting non-linear pushover and dynamic time-history analyses. Also, fifteen (15) glulam brace specimens using bolted connections with different slenderness ratios were tested under monotonic and cyclic loading. Results from this multi-year project will form the basis for developing comprehensive design guidelines for braced frames in CSA O86.
Seismic isolation is a method to decouple the structure from the damaging effects of horizontal ground motion in the event of an earthquake. This is achieved through the lengthening of the natural period of the structure, and adding damping, thus reducing the demand as determined from the typical 5% damped design response spectra. As seismically isolated buildings have very small inter-storey drift, and much lower floor accelerations in comparison to fixed base structures, seismic isolation is often used in buildings that are expected to be “operational” or “fully operational” after a major earthquake.
Since the 1970’s the use of seismic isolation has continued to increase worldwide, especially in countries after they experienced a damaging earthquake; for example this was evident in Japan following the 1995 Kobe earthquake, in Italy following the 2009 L’Aquila earthquake, and in Chile following their 2010 earthquake, where in all cases the use of seismic isolation increased significantly after such events. In Canada, implementation of seismic isolation in buildings has been limited so far. This is due to the lack of provisions in the National Building Code of Canada (NBCC), provincial building codes or municipal by-laws. The 2015 edition of NBCC is the first one to include provisions for seismic isolation in the body of the code. It is expected that more base isolated buildings will be used once provinces and municipalities adopt the NBCC 2015 into their codes in the very near future, including the provisions for seismic isolation.
Although there has been a steady growth in the application of seismic isolation in concrete and steel structures (outside of Canada) that are designed to have low or no damage under major earthquake events, the application in wood structures is essentially stagnant in North America. There are a number of impediments to the implementation of base isolation in wood structures. It is perceived that a base isolated building may cost much more than a conventional wood building. As wood-frame construction has generally performed well during major earthquakes by just meeting the building code design criteria, it could be challenge to convince the developer and/or the building owner of the value of the seismic isolation system relative to its cost. In addition, the installation of a seismic isolation system requires the introduction of a stiff base slab at the first floor of the wood-frame superstructure, so that loads from the wood-frame building can be transferred to the isolation system and structural integrity of the wood-frame superstructure is maintained. While this can be an additional requirement for wood buildings, it can be accommodated in podium buildings where a wood super-structures is supported by a concrete or steel sub-structure. Also, isolators are usually not suitable for resisting large tensile loads, which usually occurs at the edges of shear walls or the columns in braced or moment-resisting frames due to the light seismic mass of wood structures. The effects of uplift must be carefully examined and evaluated. The added mass that is necessitated to improve the acoustical performance does increase the mass of the wood structures, hemce partiallt alleviate the issues of large tensile and overturning forces.
Market study and cost analysis show that the use of base isolation on average will increase initial construction costs up to 6% and possibly vary according to economies of scale. However, when construction costs are compared between wood-based buildings with base isolation (adding the maximum premium of 6%) versus conventional concrete-based buildings, wood buildings mostly maintain a competitive price advantage per square feet of floor area.
Therefore, there seems to be a potential for base isolated timber structures to be economically viable options for building construction. Further studies and research, however, are needed to address the identified impediments in order to allow wide-spread adoption of seismic isolation on wood buildings. Carrying out the studies and research now would help to prepare for the future if and when the seismic design philosophy in the building code moves towards the stricter criterion of low and no damage for all buildings and not just the essential “post-disaster” buildings. Given the generally light weights of wood structures and the need for a rigid base for wood-frame superstructures, one main focus of future research would be on development of new isolation systems that could accommodate these properties.