This dissertation introduced a new hybrid building system in which the post-tensioned rocking CLT panels were coupled with traditional light-frame wood constructions. The initial study showed excellent self-centering and energy dissipation capacities of the hybrid walls. A finite element model was developed for rocking walls/columns and allowed to perform a global-analysis for structures using rocking elements. The model was validated by 3-D FEM models in ABAQUS and MATLAB, and an experimental test. A direct displacement-based design check procedure was proposed for CLT-LiFS buildings and illustrated by designing a six-story CLT-LiFS building. The FEM model for rocking elements was utilized to implement 2-D non-linear static analysis and non-linear time history analysis to check the design. After that, pseudo-dynamic hybrid simulation tests at three hazard levels were conducted for the six-story CLT-LiFS building, in which a two-story CLT-LiFS building was built and served as the physical substructure of the test. The tests showed minor damages and very small residual drifts to the building, even after MCE level. Finally, an optimization problem was developed for mid-rise to tall CLT-LiFS buildings using evolutionary algorithm. The variables including number of stories, hybrid wall length, CLT panel width, number of CLT panels and cable arrangement were considered so that the buildings were optimized in cost while still met their technical performance expectations. The normalized cost (for frame work) of optimum building configurations were in the range of 15.88 – 21.44 USD/sft. The study also archived several figures that will help select the building configuration in the design process of CLT-LiFS building.
A new type of interlocking timber-to-timber connection was designed to simplify the structural details and the mounting conditions between wall elements and ceilings or floor panels in timber frame constructions. An experimental test series on different connector geometries was performed due to the unclear component behaviour and failure mechanism in mode II. The connection types achieved sufficient capacity but do not reach the predicted loads according to EC5. Thus, a mixed mode failure of mode I and II obviously occurs. A design approach is provided recognising all influences on the load-bearing capacity.
This paper contains the research results of a study related to developing an approach to estimate the deflection of Cross-Laminated Timber (CLT) shear walls for platform-framed construction. In order to account for the total deflection at the top of the wall, the contributions of connections and the CLT panels are considered. The connection contributions are accounted for through wall sliding and rocking, whereas the contribution of the CLT panels is estimated from the bending and shear deformation under lateral loading. The influence of perpendicular walls and floors above on the in-plane deflection of CLT shear wall is also investigated. A step by step procedure to estimate the deflection of CLT shear walls without and with perpendicular walls and floors above is discussed with examples.
The research presented in this paper is related to estimating the in-plane stiffness and strength of CLT shearwalls with different connections for platform-framed construction. Finite element analyses (FEA) for CLT shear walls with various types of connectors for wall-to-floor, wall-to-foundation, and wall-to-wall joints were conducted. The CLT panels were modelled using plane-stress shell elements with elastic material properties and the connections were modelled using nonlinear springs. The joints, consisting of traditional steel brackets, hold-downs, and screws connections, were modelled using nonlinear zero-length spring elements with "pinching4" hysteresis properties calibrated from tests. A parametric study was performed on single and coupled CLT shear walls with the variation of the number and types of connectors. The results showed that strength and stiffness increased significantly with the increase in the number of connectors. Placing hold-downs on both sides of the coupled shear walls increased performance-i.e. 43% and 25% increase in strength and stiffness compared to coupled shear walls with hold-downs located at the outer edges only.
The research presented herein investigated the in-plane performance of cross-laminated timber (CLT) shear walls for platform-type buildings under lateral loading. Finite element models of CLT connections (i.e., brackets, hold-downs and self-tapping screws) were developed in OpenSees and calibrated against experimental tests to represent the connections’ hysteresis behaviour under cyclic tension and shear loading. The results were incorporated into models of CLT single and coupled shear walls. The results in terms of peak displacement, peak load and energy dissipation were in good agreement when compared to full-scale shear wall tests. Subsequently, a parametric study of 56 single and 40 coupled CLT shear walls was conducted with varying numbers and types of connectors (wall-to-floor and wall-to-wall) for evaluating their seismic performance. It was found that the strength, stiffness and energy dissipation of the single and coupled CLT shear walls increased with an increase in the number of connectors. Single shear walls with hold-downs and brackets performed better under seismic loading compared to walls with brackets only. Similarly, coupled shear walls with four hold-downs performed better compared to walls with two hold-downs. Finally, ductility of coupled shear walls was found to be 31% higher compared to that of single shear walls. The findings from this research are useful for engineers to efficiently design CLT shear walls in platform-type construction.
The latest developments in seismic design philosophy in modern urban centers have moved towards the development of new types of so called “resilient” or “low damage” structural systems. Such systems reduce the damage to the structure during an earthquake while offering the same or higher levels of safety to occupants. One such structural system in mass timber construction is the “Pres-Lam” system developed by Structural Timber Innovation Company (STIC) and Prestressed Timber Limited (PTL), both from New Zealand. FPInnovations has acquired the Intellectual Property rights for the Pres-Lam system for use in Canada and the United States.
The NMIT Arts building completed in 2011 in Nelson is the first three storey building using Expan type technology with energy dissipating rocking shear walls constructed from laminated veneer lumber (LVL). The long term wall post tensioning, wall shortening, and concrete composite floor deformations have been evaluated in more detail from April 2011 until October 2011. An overview is given of the building and overall instrumentation systems. Detail is given on the long term instrumentation on-site data logging systems, data transfer, Auckland monitoring equipment, data extraction and development of the web interface. The instrumentation is temperature sensitive but gross variations are predictable and corrected. The data will provide further opportunity to investigate hygro-thermal response. Preliminary results for the rate of compressive deformation were 0.4 to 1.4 mm in five months till mid October 2011 and the post tension loss is between 8 and 14.5 kN over the same five months. The post-tension losses since June 2010 vary from 13.9 to 20.6%.
Steel-timber hybrid structural systems offer a modern solution for building multi-story structures with more environmentally-friendly features. This paper presents a comprehensive seismic performance assessment for a kind of multi-story steel-timber hybrid structure. In such a hybrid structure, steel moment resisting frames are infilled with prefabricated light wood frame shear walls to serve as the lateral load resisting system (LLRS). In this paper, drift-based performance objectives under various seismic hazard levels were proposed based on experimental observations. Then, a numerical model of the hybrid structure considering damage accumulation and stiffness degradation was developed and verified by experimental results, and nonlinear time-history analyses were conducted to establish a database of seismic responses. The numerical results further serve as a technical basis for estimating the structure's fundamental period and evaluating post-yielding behavior and failure probabilities of the hybrid structure under various seismic hazard levels. A load sharing parameter was defined to describe the wall-frame lateral force distribution, and a formula was proposed and calibrated by the time-history analytical results to estimate the load sharing parameter. Moreover, earthquake-induced non-structural damage and residual deformation were also evaluated, showing that if designed properly, desirable seismic performance with acceptable repair effort can be obtained for the proposed steel-timber hybrid structural system.
In an effort to develop a high-ductility/low-damage lateral-load resisting system, an experimental shear wall consisting of rigid LVL panels is fabricated and tested with slip-friction connectors acting as hold-downs. The slip-friction connectors are fabricated in such a way that their force-displacement behaviour can be described by close to an ideal elasto-plastic shape. With the application of an increasing racking force at a top-corner of the wall, the slip-force in the connectors is eventually attained - at which point sliding takes place and the wall corner displaces upwards. The wall thus displays a form of rocking behaviour, and this places a cap on the activated base shear. In order to resist this activated base shear, a new type of shear key is included at the bottom centre of the wall. The shear key consists of vertical steel plates welded to the foundation, placed on both sides of the wall, and with shear pins inserted through the plane of the wall, bearing directly against the edge of the plates. In order to facilitate rocking and to reduce the influence of the shear key on the force-displacement behaviour, the plates are sloped at a slight angle to the vertical. An equation is derived that quantifies the effect on the wall strength due to the interaction of the slip-friction connectors and the shear key. It is found that the predicted wall strengths align closely with the experimentally measured values. Furthermore, as expected, the hysteretic behaviour of the wall is highly elasto-plastic, and the strength of the wall can be readily ‘tuned’ through the use of Belleville washers in achieving and maintaining bolt preload. The wall readily descends at one end, while uplifting at the other end - and this indicates that it would likely have good re-centring ability under an actual earthquake. The results show the possibility of inherently brittle structures being made ductile through the application of a simple and cheap steel friction device.
Cross-laminated timber (CLT) is gaining popularity in residential and non-residential applications in the North American construction market. CLT is very effective in resisting lateral forces resulting from wind and seismic loads. This research investigated the in-plane performance of CLT shear wall for platform-type buildings under lateral loading. Analytical models were proposed to estimate the in-plane stiffness of CLT wall panels with openings based on experimental and numerical investigations. The models estimate the in-plane stiffness under consideration of panel thickness, aspect ratios, and size and location of the openings. A sensitivity analysis was conducted to reduce the number of model parameters to those that have a significant impact on the stiffness reduction of CLT wall panels with openings. Finite element models of CLT wall connections were developed and calibrated against experimental tests. The results were incorporated into models of CLT single and coupled shear walls. Finite element analyses were conducted on CLT shear walls and the results in terms of peak displacements, peak loads and energy dissipation were in good agreement when compared against full-scale shear wall tests. A parametric study on single and coupled CLT shear walls was conducted with variation of number and type of connectors. The seismic performance of 56-single and 40-coupled CLT shear walls’ assembles for platform-type construction were evaluated. Deflection formulas were proposed for both single and coupled CLT shear walls loaded laterally in-plane that in addition to the contributions of CLT panels and connections, also account for the influence of adjacent perpendicular walls and floors above and illustrated with examples. Analytical equations were proposed to calculate the resistance of CLT shear walls accounting for the kinematic behaviour of the walls observed in experimental investigations (sliding, rocking and combined sliding-rocking) and illustrated with examples. Different configurations (number and location of hold-downs) of single and coupled CLT walls were considered. The findings presented in this thesis will contribute to the scientific body of knowledge and furthermore will be a useful tool for practitioners for the successful seismic design of CLT platform buildings in-line with the current CSA O86 provisions.
International Conference on Theoretical, Applied and Experimental Mechanics
At the beginning of the 20th century, a new wood manufacturing technology, i.e. cross-laminated timber (CLT), was started. In Taiwan, the manufacturing technology of CLT has just started recently. For the sake of safety, the information of stiffness and strength of the shear wall of the CLT are essential for structural designs. In this paper, by following the method B (ISO 16670 Protocol) of ASTM standard E2126-11, shear test of a real-scale CLT shear wall was performed. The measured shear modulus and cyclic test results of the CLT shear wall were reported in this paper. By using the three-dimensional digital image correlation technique, full-field deformation information of the CLT shear wall were obtained.
In the US, codified seismic design procedure requires the use of seismic performance factors which are currently not available for CLT shear wall systems. The study presented herein focuses on the determination of seismic design factors for CLT shear walls in platform type construction using the FEMA P-695 process. Results from the study will be proposed for implementation in the seismic design codes in the US. The project approach is outlined and selected results of full-scale shear wall testing are presented and discussed. Archetype development, which is required as part of the FEMA P-695 process, is briefly explained with an example. Quasi-static cyclic tests were conducted on CLT shear walls to systematically investigate the effects of various parameters. The key aspect of these tests is that they systematically investigate each potential modelling attribute that is judged within the FEMA P-695 uncertainty quantification process. Boundary constraints and gravity loading were both found to have a beneficial effect on the wall performance, i.e. higher strength and deformation capacity. Higher aspect ratio panels (4:1) demonstrated lower stiffness and substantially larger deformation capacity compared to moderate aspect ratio panels (2:1). However, based on the test results there is likely a lower bound for aspect ratio (at 2:1) where it ceases to benefit deformation capacity of the wall. This is due to the transition of the wall behaviour from rocking to sliding. Phenomenological models were used in modelling CLT shear walls. Archetype selection and analysis procedure was demonstrated and nonlinear time history analysis was conducted using different wall configurations.
This paper presents a numerical study of the influence of varying story strength on the seismic performance of multi-story wood-frame shear wall buildings. In the prior FEMA P695 studies of these buildings, the non-simulated collapse limit-state was exceeded primarily in the first story . This observation raised interest in quantifying the influence of varying strength from story to story on seismic response. In this study, four different distributions of strength are used as bounding cases. The Parabolic strength distribution (1) is based upon the ELF method in ASCE 7 and assigns lateral forces to each level based on weight and story height. The Triangular strength distribution (2) is based upon the simplified procedure in ASCE 7 and distributes lateral forces based on the seismic weight at each level. The Constant strength distribution (3) assumes the same shear wall design was used on all levels. The Baseline strength distribution (4) is from actual designs provided in the FEMA P695 wood-frame example and represents the practical implementation of the ELF method for designed shear walls. The FEMA P695 methodology, which quantifies seismic performance via adjusted collapse margin ratios, is employed in this study. The analytical models include P-Delta effects and utilize the 10-parameter CASHEW hysteresis model. Based on the analysis of a subset of index models from the FEMA P695 wood-frame example, it is observed that the Parabolic strength distribution, which facilitates dissipation of energy along the entire height of the building, has larger adjusted collapse margin ratios (lower collapse risk) than other strength distributions studied and reduces occurrence of concentrated inelastic deformations in a single story from the onset of an applied lateral force.
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 dissipators 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 of two main parts: material tests and system tests.
Cross-laminated timber shear wall systems are used as a lateral load resisting system in multistory timber buildings. Walls at each level typically bear directly on the floor panels below and are connected by nailed steel brackets. Design guidance for lateral load resistance of such systems is not well established and design approaches vary among practitioners. Two cross-laminated two-story timber shear wall systems are tested under vertical and lateral load, along with pull-out tests on individual steel connectors. Comprehensive kinematic behavior is obtained from a combination of discrete transducers and continuous field displacements along the base of the walls, obtained by digital image correlation, giving a measure of the length of wall in contact with the floor below. Existing design approaches are evaluated. A new offset-yield criterion based on acceptable permanent deformations is proposed. A lower bound plastic distribution of stresses, reflecting yielding of all connectors in tension and cross-grain crushing of the floor panel, is found to most accurately reflect the observed behavior.
Cross-laminated timber (CLT) is a relatively new type of massive timber system that has shown to possess excellent mechanical properties and structural behavior in building construction. When post-tensioned with high-strength tendons, CLT panels perform well under cyclic loadings because of two key characteristics: their rocking behavior and self-centering capacity. Although post-tensioned rocking CLT panels can carry heavy gravity loads, resist lateral loads, and self-center after a seismic event, they are heavy and form a pinched hysteresis, thereby limiting energy dissipation. Conversely, conventional light-frame wood shear walls (LiFS) provide a large amount of energy dissipation from fastener slip and, as their name implies, are lightweight, thereby reducing inertial forces during earthquakes. The combination of these different lateral behaviors can help improve the performance of buildings during strong ground shaking, but issues of deformation compatibility exist. This study presents the results of a numerical study to examine the behavior of post-tensioned CLT walls under cyclic loadings. A well-known 10-parameter model was applied to simulate the performance of a CLT-LiFS hybrid system. The posttensioned CLT wall model was designed on the basis of a modified monolithic beam analogy that was originally developed for precast concrete-jointed ductile connections. Several tests on post-tensioned CLT panels and hybrid walls were implemented at the Large Scale Structural Lab at the University of Alabama to validate the numerical model, and the results showed very good agreement with the numerical model. Finally, incremental dynamic analysis on system level models was compared with conventional light-frame wood system models.
This paper presents an experimental study into the lateral response of cross-laminated-timber (CLT) shear walls under cyclic loads with particular attention to the distribution of forces between the panel-to-floor connections. Six tests on full-scale specimens of different geometric characteristics and connection configurations are presented. The test set-up and wall configurations are described and a detailed account of the experimental results and observations is presented. The paper examines key response features including stiffness, strength, energy dissipation and ductility. Especial attention is given to an accurate measurement of the load sharing between different brackets and its evolution throughout the cyclic action. The results reported offer valuable information on the lateral forcedisplacement response of CLT walls and the applicability of widely employed design assumptions.
This paper presents selected results of connector testing and wall testing which were part of a Forest Products Lab-funded project undertaken at Colorado State University in an effort to determine seismic performance factors for cross laminated timber (CLT) shear walls in the United States. Archetype development, which is required as part of the process, is also discussed. Connector tests were performed on generic angle brackets which were tested under shear and uplift and performed as expected with consistent nail withdrawal observed. Quasi-static cyclic tests were conducted on CLT shear walls to systematically investigate the effects of various parameters. Boundary constraints and gravity loading were both found to have a beneficial effect on the wall performance, i.e. higher strength and deformation capacity. Specific gravity also had a significant effect on wall behaviour while CLT thickness was less influential. Higher aspect ratio panels (4:1) demonstrated lower stiffness and substantially larger deformation capacity compared to moderate aspect ratio panels (2:1). However, based on the test results there is likely a lower bound of 2:1 for aspect ratio where it ceases to have any beneficial effect on wall behaviour. This is likely due to the transition from the dominant rocking behaviour to sliding behaviour.
Joints and shear walls of buildings made from dowel-laminated timber were experimentally investigated and assessed. Based on cyclic tests on shear walls, a nonlinear dynamic building model was developed. The developed model served to evaluate the seismic behaviour of buildings made from dowel-laminated timber and to derive a preliminary behaviour factor q required for seismic design of this building typology.