In timber structures, the connections are generally flexible in comparison to the members they connect, and so contribute significantly to the dynamic properties of the structure. It is shown here that a widely-used form of connection, the dowel-type connection, exhibits nonlinear stiffness and energy dissipation, even at pre-yield loads, and that this nonlinearity affects the modal properties of structures with such connections. This study investigates that behaviour by modal analysis of a portal frame and a cantilever beam constructed from timber with steel dowel connections. The observed nonlinearity is explained qualitatively by considering the measured force-displacement response of individual connectors under cyclic load, which show a reduction in stiffness and an increase in energy dissipation with increasing amplitude of vibration. The structures were tested by modal analysis under slow sine sweep and pseudo-random excitation. Under pseudo-random excitation, a linear single degree-of-freedom curve fit was applied to estimate the equivalent linear modal properties for a given amplitude of applied force. Under slow sine sweep excitation, the frequency response function for the structures was observed to show features similar to a system with a cubic component of stiffness, and the modal properties of the structures were extracted using the equation of motion of such a system. The consequences for structural design and testing are that two key design parameters, natural frequency and damping, vary depending on the magnitude of vibration, and that parameters measured during in-situ testing of structures may be inaccurate for substantially different loads.
This study investigated the vibration serviceability of timber structures with dowel-type connections. It addressed the use of such connections in cutting-edge timber structures such as multi-storey buildings and long-span bridges, in which the light weight and flexibility of the structure make it possible that vibration induced by dynamic forces such as wind or footfall may cause discomfort to occupants or users of the structure, or otherwise impair its intended use. The nature of the oscillating force imposed on connections by this form of vibration was defined based on literature review and the use of established mathematical models. This allowed the appropriate cyclic load to be applied in experimental work on the most basic component of a dowel-type connection: a steel dowel embedding into a block of timber. A model for the stiffness of the timber in embedment under this cyclic load was developed based on an elastic stress function, which could then be used as the basis of a model for a complete connector. Nonlinear and time-dependent behaviour was also observed in embedment, and a simple rheological model incorporating elastic, viscoelastic and plastic elements was fitted to the measured response to cyclic load. Observations of the embedment response of the timber were then used to explain features of the behaviour of complete single- and multiple-dowel connections under cyclic load representative of in-service vibration. Complete portal frames and cantilever beams were tested under cyclic load, and a design method was derived for predicting the stiffness of such structures, using analytical equations based on the model for embedment behaviour. In each cyclic load test the energy dissipation in the specimen, which contributes to the damping in a complete structure, was measured. The analytical model was used to predict frictional energy dissipation in embedment, which was shown to make a significant contribution to damping in single-dowel connections. Based on the experimental results and analysis, several defining aspects of the dynamic response of the complete structures, such as a reduction of natural frequency with increased amplitude of applied load, were related to the observed and modelled embedment behaviour of the connections.
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 article presents recent research on the seismic resistance of coupled post-tensioned timber walls for use in multi-story buildings. The walls are constructed from laminated veneer lumber (LVL), post-tensioned with unbonded vertical tendons, and coupled together with mild steel U-shaped flexural plates (UFPs) as energy dissipating elements. The timber wall design follows the same principles as used for post-tensioned precast concrete wall systems, using U-shaped plates to obtain a “hybrid” system, where energy is dissipated through yielding of the plates, while the vertical post-tensioning provides the restoring force. In this project, the same principles are applied to timber coupled walls. A series of quasi-static cyclic and pseudo-dynamic tests have been carried out to verify the applicability of the concept and the feasibility of the construction technology in timber buildings. The U-shaped plates showed stable energy dissipation characteristics and, in combination with the post-tensioning, desirable re-centering hysteretic behavior typically referred to as “flag-shape”. Because of the simplicity of these elements and the low cost of implementation, they have good prospects for practical application.
Seismically resilient, lateral systems for tall timber buildings can be created by combining cross laminated timber (CLT) panels with post-tensioned (PT) self-centering technology. The concept features a system of stacked CLT walls where particular stories are equipped to rock against the above and below floor diaphragms through PT connections and are supplemented with mild steel U-shaped flexural plate energy dissipation devices (UFPs). Experiments were conducted to better understand rocking CLT wall behavior and seismic performance. The testing program consisted of five single wall tests with varying PT areas, initial tensioning force, CLT panel composition, and rocking surface and one coupled wall test with UFPs as the coupling devices. The walls were tested with a quasi-static reverse-cyclic load protocol. The experimental results showed a ductile response and good energy dissipation qualities. To evaluate the feasibility and performance of the rocking CLT wall system, prototype designs were developed for 8 to 14 story buildings in Seattle using a performance-based seismic design procedure. Performance was assessed using numerical simulations performed in OpenSees for ground motions representing a range of seismic hazards. The results were used to validate the performance-based seismic design procedure for tall timber buildings with rocking CLT walls.
New Zealand Society for Earthquake Engineering Conference
April 26-28, 2013, Wellington, New Zealand
This paper describes options for seismic design of pre-fabricated timber core-wall
systems, used as stairwells and lift shafts for lateral load resistance in multi-storey timber
buildings. The use of Cross-Laminated Timber (CLT) panels for multi-storey timber buildings is
gaining popularity throughout the world, especially for residential construction. This
paper describes the possible use of CLT core-walls for seismic resistance in open-plan
commercial office buildings in New Zealand. Previous experimental testing at the
University of Canterbury has been done on the in-plane behaviour of single and coupled
Pres-Lam post-tensioned timber walls. However there has been very little research done
on the behaviour of timber walls that are orthogonal to each other and no research into
CLT walls in the post-tensioned Pres-Lam system. This paper describes the proposed test regime and design detailing of two half-scale twostorey CLT stairwells to be tested under a bi-directional quasi-static loading. The test specimens will include a half-flight stair case with landings within the stairwell. The “High seismic option” consists of post-tensioned CLT walls coupled with energy dissipating U-shaped Flexural Plates (UFP) attached between wall panels and square hollow section steel columns at the corner junctions. An alternative “Low seismic option” uses the same post-tensioned CLT panels, with no corner columns or UFPs. The panels will be connected by screws to provide a semi-rigid connection, allowing relative
movement between the panels producing some level of energy dissipation.
Currently the massive timber shear walls are mainly made from Cross Laminated Timber (CLT), which possesses a high in-plane shear strength and rigidity. But only part of its elements (mainly the vertically aligned laminae) are engaged in carrying the vertical load and that could be a limitation when designing taller timber structures or wherever higher vertical load is present. This project studied alternative solutions to massive timber shear wall system, based on Nailed Laminated Timber (NLT) and post laminated LVL (Laminated Veneer Lumber).
The test was conducted on three levels: shear test on glue/nail line, bending-shear test on a small element, and full size wall test under lateral loading. The former two tests investigated the properties of basic elements in NLT and post laminated LVL. The results were used to design and predict the performance of full size shear walls.
The NLT walls were tested under two conditions: without sheathing and with plywood sheathing. The wall without sheathing had the lowest load-carrying capacity and lowest stiffness. Adding plywood sheathing significantly increased its strength and stiffness. The failure in the wall with sheathing was at the sheathing connections, in the forms of nail withdrawal, nail head pull through, and nail breakage. The NLT wall with sheathing had a peak load up to 60% higher than the comparable light wood frame wall, also with a higher stiffness and better ductility. NLT shear walls have an internal energy dissipating capacity which CLT and post laminated LVL walls lack. The post laminated LVL walls behaved as a rigid plate under lateral loading, with little internal deformation. The failure occurred at the holdowns not within the wall. The size effect of its shear strength was studied and an equation was developed to calculate the shear strength of a large size wall plate.
Both products have efficient vertical load bearing mechanism by arranging all elements in the vertical direction. They may serve as alternative to light wood frame walls or CLT walls. Some guidelines for the application and design of NLT shear walls and post laminated LVL shear walls were proposed.
Laminated veneer lumber (LVL) structural members have recently been proposed for multi-storey timber buildings based on ongoing research at University of Canterbury, New Zealand. The members are designed with unbonded post-tensioning for recentering and energy dissipation through the ductile connections. This paper describes the experimental and numerical investigation of post-tensioned LVL walls coupled with plywood sheets, under quasistatic cyclic testing protocols. It is observed that energy is dissipated mostly through yielding of the nails, and the LVL walls return close to their initial position while remaining virtually undamaged. The same specimen has been tested under repeated cyclic loading to investigate the performance of the arrangement under more than one seismic event (a major earthquake followed by a significant aftershock). Different nail spacing and arrangements have been tested to compare their energy dissipation characteristics.
The results indicate good seismic performance, characterized by negligible damage of the structural members and very small residual deformations. The only component significantly damaged is the nailed connection between the plywood sheet and the LVL walls. Although the nails yield and there is a reduction in stiffness the system exhibits a stable performance without any major degradation throughout the loading regime. The plywood can be easily removed and replaced with new sheets after an earthquake, which are reasonably cheap and easy to install, allowing for major reduction in downtime. With these additional benefits the concept has potential for consideration as an alternative solution for multi-storey timber buildings.