Moment-resisting timber frames combined with cross laminated timber walls for multistorey timber buildings

Abstract

The increase in the world population results in higher urbanization, which poses a significant challenge due to the substantial contribution of construction industry to greenhouse gas emissions. The increased urbanization results in further increase in greenhouse gas emissions, leading to adverse environmental impacts. Consequently, the incorporation of sustainable building materials becomes imperative. Timber, as a renewable material, offers a viable alternative to concrete and steel in reducing the adverse environmental impact of construction activities. Timber has a very good strength-to-weight ratio, attributed to its lightweight nature. However, the lightweight and moderate stiffness characteristics of timber buildings result in serviceability issues, particularly excessive lateral displacements and accelerations under service-level wind loading. Excessive lateral displacements can impact the functionality of buildings and cause damage to non-structural elements, while large accelerations may result in discomfort for occupants. Therefore, it is crucial to ensure that displacements and accelerations are within acceptable limits. Lateral displacements can be reduced by increasing the lateral stiffness of the building, such as using stiffer connections. Wind-induced accelerations can be reduced by increasing the lateral stiffness of the building, the mass of the building, the damping, or a combination of these. Lateral bracing with diagonal elements is widely used to provide the lateral stiffness for multistorey timber buildings. However, this requires substantial bracing elements that extend throughout the height of the building, potentially limiting architectural flexibility. A common alternative is the use of Cross Laminated Timber (CLT) walls as lateral load-resisting elements. Nevertheless, buildings with CLT walls tend to be material-intensive and characterized by cellular configurations. Moment-Resisting Timber Frames (MRTFs) offer a viable option as a lateral loadresisting system for multistorey timber buildings. In MRTFs, the beam-to-column Moment-Resisting Connections (MRCs) provide the necessary lateral stiffness. Compared to buildings with diagonal bracing or CLT walls, MRTFs offer increased open space and architectural flexibility. Additionally, incorporating stiff connections in timber floors can enhance their performance with respect to human-induced vibrations. Nevertheless, the use of MRTFs is less common in timber construction, mainly due to their flexibility. The stiffness of MRTFs can be enhanced by use of stiffer beam-to-column connections, thus making them more practical and feasible. A feasibility study (Vilguts et al. 2021) of MRTFs using Glued Laminated Timber (glulam) has shown that they can hardly be used for more than 8 storeys with a small out-of-plane spacing (2.40 m) between adjacent frames due to increased windinduced accelerations and lateral displacements. The use of MRTFs in combination with CLT walls (i.e. dual frame-wall) can allow for greater number of storeys and larger out-of-plane spacing between the frames. Moreover, stairs and elevators always exist in multistorey buildings, and are often surrounded by walls, hence employing CLT walls in combination with MRTFs presents a practical consideration. In this thesis, a parametric study was performed to investigate the feasibility of a dual frame-wall structural system. The feasibility of employing outriggers in combination with the dual frame-wall system was also explored. An extensive database was created for MRTFs with and without CLT walls, by use of linear elastic Finite Element Analysis (FEA). The FEA database was used to derive simplified analytical expressions for the fundamental frequency, mode shape, topfloor displacement, inter-storey drift, and top-storey wind-induced acceleration by use of nonlinear regression. The FEA database was also used to train Artificial Neural Networks (ANNs) that can be used to predict the aforementioned response parameters with an improved accuracy. The derived expressions and the ANNs can be used for preliminary assessment of MRTFs (with and without CLT walls) considering serviceability requirements. Experimental studies on pullout behaviour of threaded rods screwed into glulam have demonstrated their high strength and stiffness. As a result, these rods can be utilized in timber connections that demand high capacity and stiffness. Research conducted on pullout behaviour of threaded rods screwed into CLT elements is limited. In this thesis, a series of experiments was conducted on threaded rods screwed into the narrow face of CLT elements to evaluate their withdrawal properties. Penetration length, loading type (tension, compression, and fully reversed), loading configuration (pull-pull and pull-push), and angle to the grain (parallel and perpendicular) were varied during the testing. The use of self-tapping screws as reinforcement was also explored. Rods inserted perpendicular to the grain have demonstrated high withdrawal capacity and stiffness. Rods inserted parallel to the grain have shown high withdrawal stiffness, but the withdrawal capacity exhibited significant variability. Timber MRCs can be realized by either laterally-loaded or axially-loaded fasteners. Although MRCs with laterally-loaded fasteners typically exhibit high ductility, testing of such connections shows low stiffness and considerable pinching under cyclic loading. In contrast, MRCs with axially-loaded fasteners, such as screwed-in threaded rods, exhibit higher stiffness and lower pinching. MRCs using threaded rods screwed into glulam have demonstrated the possibility of stiff connections. However, the failure mode was brittle splitting. CLT comprises timber boards in two orthogonal directions, which may prevent the splitting reported in the MRCs made with glulam and achieve a higher capacity. In this thesis, an innovative slip-friction connection that can be used in combination with screwed-in threaded rods, featuring ease of assembly and disassembly, high stiffness under service-level loading, and damage-free ductility via friction sliding under ultimate-level loading is presented. Four full-scale, beam-to-column, MRCs with CLT and screwed-in threaded rods have been tested. All specimens have been subjected to service-level cyclic loading, followed by a destructive cyclic loading. The connection demonstrated high stiffness in the range of 10000-20000 kNm/rad (under service-level loading) and high moment capacity in the range of 164-180 kNm (under destructive loading). In view of the results of the performed parametric studies (no seismic design) and the full-scale tests of the developed MRCs, the dual frame-wall system can be used for multistorey timber buildings up to 10-12 storeys with 5.0 m out-of-plane spacing between adjacent frames. Employing outriggers can allow for up to 16 storeys.