Lack of research and design information for the seismic performance of balloon-type CLT shear walls prevents CLT from being used as an acceptable solution to resist seismic loads in balloon-type mass-timber buildings. To quantify the performance of balloon-type CLT structures subjected to lateral loads and create the research background for future code implementation of balloon-type CLT systems in CSA O86 and NBCC, FPInnovations initiated a project to determine the behaviour of balloon-type CLT construction. A series of tests on balloon-type CLT walls and connections used in these walls were conducted. Analytical models were developed based on engineering principles and basic mechanics to predict the deflection and resistance of the balloon-type CLT shear walls. This report covers the work related to development of the analytical models and the tests on balloon-type CLT walls that the models were verified against.
The latest developments in seismic design philosophy have been geared towards developing of so called "resilient" or "low damage" innovative structural systems that can reduce damage to the structure while offering the same or higher levels of safety to occupants. One such innovative structural system is the Pres-Lam system that is a wood-hybrid system that utilizes post-tensioned (PT) mass timber components in both rigid-frame and wall-based buildings along with various types of energy disspators. To help implement the Pres-Lam system in Canada and the US, information about the system performance made with North American engineered wood products is needed. That information can later be used to develop design guidelines for the designers for wider acceptance of the system by the design community. Several components influence the performance of the Pres-Lam systems: the load-deformation properties of the engineered wood products under compression, load-deformation and energy dissipation properties of the dissipators used, placement of the dissiaptors in the system, and the level of post-tensioning force. The influence of all these components on the performance of Pres-Lam wall systems under gravity and lateral loads was investigated in this research project. The research project consisted on two main parts: material tests and system tests.
In the material tests part of the program, a total of 110 compression tests were conducted to determine the load-deformation properties of four different engineered wood products (LVL, LSL, Glulam and CLT) in various directions. The LVL, LSL and Glulam specimens tested under compression parallel to grain had similar linear elastic behaviour with limited ductility. The CLT specimens tested under compression in the major-axis direction had linear elastic behaviour with moderate plasticity. Depending on the type of engineered wood product, typical failure modes included crushing, shear, wedge split and splitting. The compressive strength of the products tested ranged from 42.1 to 53.5 MPa, the global MOE (of the entire specimen under compression) varied between 6390 and 9554 MPa, the local (near the crushing surface) MOE parallel to grain was in the range of 2211 to 5090 MPa, while the local to global MOE ratio ranged from 29.2 to 58.0%, and was higher with the increase in the oven-dry density.
The specimens of the four different engineered wood products tested under compression perpendicular to grain or in the minor-axis direction had elastic-plastic behaviour with a clearly defined plastic plateau. Crushing (densification) of the fibres perpendicular to grain was the main failure mode for all specimens, and was in some cases followed by in-plane shear failure or cracking perpendicular to grain. Compression parallel to grain in the middle layer that was followed by its delamination and buckling was a unique failure mode for CLT specimens tested under compression in the minor strength direction. The compressive strength of the engineered wood products tested were in the range of 4.8 to 27.8 MPa, while the global and local MOE perpendicular to grain were in the range of 244 to 2555 MPa, and 320 to 1726 MPa, respectively. The compressive strength and global MOE perpendicular to grain increased with an increase in the oven-dry density. The results show no well-defined trend for the local MOE perpendicular to grain. The specimens loaded in the centre perpendicular to grain had higher strength, global and local MOE than those loaded at the end.
A convenient and timesaving design for the axial energy dissipators (fuses) was developed by replacing the epoxy in the original design with two half-tubes. Compared to the original design of fuses with epoxy, the new design with two half-tubes had similar necking failure mode and a longer failure displacment, thus providing user-friendly fuses that performed similar or even better than the original design.
In the system tests part of the program, a total of 17 different PT and Pres-Lam CLT walls with six different configurations were tested under monotonic and reversed cyclic loading. The studied parameters included the level of PT force, the position of the fuses, and the number of UFPs. CLT shear walls subjected only to post-tensioning, had non-linear elastic behaviour. The behaviour of the PT walls with and without energy dissipators was relatively similar under monotonic and cyclic loading. The strength degradation observed during the cyclic tests was low in all wall configurations suggesting that very little damage was inflicted upon the structure during the first cycles at any deformation level. Four major failure modes, including yielding and buckling of fuse, crushing and splitting of wood at the end of wall, and buckling of lumber in the exterior-layer of CLT wall, were observed in the tests. The yielding in fuses occurred at the early stage of loading as designed and the other failure modes happened when the lateral drift reached or beyond 2.5%.
The initial stiffness of the single-panel PT CLT walls tested ranged from 1.80 to 2.31 kN/mm, the load at the decompression point and 2.5% drift were in the range of 4.2 to 14.9 kN and 32.7 to 45.9 kN, respectively. The initial stiffness of the single-panel Pres-Lam CLT walls tested ranged from 1.69 to 2.44 kN/mm, the load at the decompression point and 2.5% drift were in the range of 21.0 to 30.2 kN and 59.6 to 69.8 kN, respectively. All the mechanical properties increased with an increase in the PT force. The average initial stiffness and the load at 2.5% drift of the coupled-panel Pres-Lam CLT walls tested were 4.59 kN/mm and 151.3 kN, respectively, while the load at the decompression point increased from 58.4 to 69.7 kN by increasing the number of UFP. The test results show that the behaviour of the Pre-Lam CLT shear walls can be de-coupled and a “superposition rule” can be applied to obtain the stiffness and resistance of such system.
The test results gave a valuable insight into the structural behaviour of the PT and Pres-Lam CLT shear wall under in-plane lateral loads. The data from the testing will be used in the future for development of numerical computer models. They will also be used for development of design guidelines for this system. All tests conducted in this study and the analyses in the future modelling research will form the basis for developing future design guidelines for PT and Pres-Lam mass timber systems.
Lors de la conception des structures en bois d’oeuvre, il importe que les éléments de la charpente, telles les poutres, les colonnes et les fermes, soient conçus de manière à résister aux charges prévues, puisqu’une chaîne n’est jamais plus forte que son maillon le plus faible. Il est tout aussi important que les assemblages de ces éléments soient conçus avec soin. Un assemblage doit pouvoir transmettre la charge et ses contraintes, d’un élément à un autre, en respectant des limites acceptables de déformation. Le rendement adéquat de l’assemblage importe particulièrement dans le cas des structures construites dans les régions sismiques, où la défaillance d’un assemblage peut entraîner l’effondrement de la structure lors d’un séisme important. L’introduction de divers produits du bois sur le marché a accru les occasions d’utiliser le bois dans diverses applications structurales, ajoutant ainsi à l’importance du rôle des assemblages dans les éléments structuraux.
A new design Section on Lateral Load Resisting Systems (LLRSs) was introduced in the 2009 edition of Canadian Standard for engineering Design in Wood (CSA O86). The activities presented in this report (development of technical papers, development of technical polls and attending various code committees) have a goal to continue the work in this field by further improving the new Section on LLRSs by implementing additional design information for other wood-based structural systems and assemblies. During the last two years, several technical polls and papers were developed and presented to various code committees for future code implementation. These activities will help design engineers to use timber in structural systems in residential and non-residential buildings in Canada and the US.
A new design Section on Lateral Load Resisting Systems (LLRSs) was introduced in the 2009 edition of Canadian Standard for engineering Design in Wood (CSAO86). The activities presented in this report (development of technical papers, development of technical polls and attending various code committees) have a goal to continue the work in this field by further improving the new Section on LLRSs by implementing additional design information for other wood-based structural systems and assemblies. During the last two years several technical polls and papers were developed and presented to various code committees for future code implementation. These activities will help design engineers to use timber in structural systems in residential and non-residential buildings in Canada and the US.
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.
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.
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.
European experience shows that Cross-Laminated Timber (CLT) can be competitive in mid-rise and high-rise buildings. Although this system has not been used to the same extent so far in North America, it can be viable wood structural solution for the shift towards sustainable densification of urban and suburban centers. For these reasons FPInnovations has undertaken a multi-disciplinary project on determining the performance of a typical CLT construction, including quantifying the seismic resistance and force modification factors for CLT buildings in Canada and the US.
In this report, a performance-based seismic design (PBSD) of a CLT building was conducted and the seismic response of the CLT building was compared to that of a wood-frame structure tested during the NEESWood project. A suitable force modification factors (R-factors) for CLT mid-rise buildings with different fasteners were recommended for seismic design in Canada and the US. The six-storey NEESWood Capstone building was redesigned as a CLT building using the PBSD procedure developed during the NEESWood project. The results from the quasi-static tests on CLT walls performed at FPInnovations were used as input information for modeling of the main load resisting elements of the structure, the CLT walls. Once the satisfactory design of the CLT mid-rise structure was established through PBSD, a force-based design was developed with varying R-factors and that design was compared to the PBSD result. In this way, suitable R-factors were calibrated so that they can yield equivalent seismic performance of the CLT building when designed using the traditional force-based design methods.
Based on the results of this study it is recommended that a value of Rd=2.5 and Ro=1.5 can be assigned for structures with symmetrical floor plans according to NBCC. In the US an R=4.5 can be used for symmetrical CLT structures designed according to ASCE7. These values can be assigned provided that the design values for CLT walls considered (and implemented in the material design standards) are similar to the values determined in this study using the kinematics model developed that includes the influence of the hold-downs in the CLT wall resistance. Design of the CLT building with those R-factors using the equivalent static procedures in the US and Canada will result in the CLT building having similar seismic performance to that of the tested wood-frame NEESWood building, which had only minor non-structural damage during a rare earthquake event.
The seismic and wind design provisions for engineered wood structures in Canada have to be enhanced to be compatible with those already available for structures built according to other material standards. Such design provisions are of vital importance for ensuring the competitive position of timber structures relative to reinforced concrete and steel structures. This report provides information on the framework needed for development of such enhanced provisions for design of lateral load resisting systems in engineered wood buildings. Although the framework presented here is suggested for implementation in the Canadian wood design standard (CSAO86) in correlation with the National Building Code of Canada (NBCC - the Canadian model design code), the information provided can be used as a template for other national and international codes and standards.
In the beginning, a comprehensive review on the development of the seismic design procedures in Canada and the US is presented. This is followed by the information on seismic design principles and methods of seismic analysis throughout the world. In addition, a state-of-the-art survey is included on the development and current activities related to the performance based design procedures throughout the world. This is followed by the details that should form the new design section on lateral load resisting systems (LLRS) to be developed in CSAO86 during the next code cycle. It is suggested that in response to the changes already in place in the 2005 edition of the NBCC, besides systems such as shearwalls and diaphragms, the section should include subsections for braced frames and moment resisting frames. It is also suggested that a strong link should be made between the design section on connections and the new one on LLRS. Future editions of the CSAO86 should also be encouraged to use the reliability and performance based design provisions, as the codes in the area of seismic design worldwide are moving in that direction.
There are several ongoing research activities in the field of wood-based lateral load resisting systems and connections throughout Canada and the US. Cooperation between researchers, engineering community and regulatory representatives is vital for successful delivery of the design guidelines mentioned above.