Experimental and Numerical Investigation of the Flexural Behaviour and Effective Flange Width of Mass Timber Composite Panels

Abstract

The construction industry is increasingly adopting mass timber as a sustainable alternative for large-scale buildings, driven by the urgent need to limit global emissions. However, conventional mass timber floor solutions remain challenged in applications requiring long spans and open spaces (e.g., institutional, commercial, and industrial buildings). Compositely connecting cross-laminated timber (CLT) slabs and glulam beams to form ribbed or box type mass timber composite (MTC) panels has emerged as a promising solution for longer spans while maintaining the benefits of prefabrication and sustainability.

Despite their potential, MTC floor systems have seen limited adoption due to an incomplete understanding of their structural behaviour. Key challenges include limited experimental guidance on practical shear connection solutions, uncertainty in predicting serviceability-level performance, and a lack of validated methods for accounting for shear lag and effective flange width (EFW) in the CLT flanges. Existing research has primarily focused on mechanically fastened or glue-pressed connections requiring large hydraulic presses, and limiting the practical manufacturability. Screw-gluing has been proposed as an alternative construction method; however, its structural viability and applicability to large-scale components remain insufficiently understood. Furthermore, while EFW concepts are widely used for composite systems, the current Canadian Engineering Design in Wood standard lacks guidance on the EFW of CLT, and the proposed draft update to Eurocode 5 provisions have undergone limited experimental verification.

This thesis presents a comprehensive experimental and numerical investigation into the flexural behaviour of MTC panels, with emphasis on practical shear connections, serviceability (SLS) and ultimate-level (ULS) structural behaviour, and EFW at SLS. The research integrates connection level testing, full scale composite beam experiments, full strain field results using digital image correlation (DIC), and validated numerical modelling to improve understanding of MTC system performance.

A central contribution of this work is the experimental characterization of screw glued timber–timber shear connections using gap-filling and non-gap-filling adhesives, and screws at larger, more economical spacings. Experimental results demonstrate that screw gluing, when combined with appropriate surface preparation and detailing, achieved high strength and stiffness, providing performance comparable to glue-pressed connections and meaningfully exceeding that of conventional mechanical fasteners. These findings demonstrate that screw gluing is a viable, practical, and scalable alternative to glue-pressing for manufacturing large-scale MTC panels and realizing full composite action throughout the response.

Full-scale tests on CLT–glulam composite T beams further demonstrated that specimens incorporating screw glued connections exhibit highly consistent stiffness, linear elastic response virtually all the way to failure, and minimal connection slip, resulting in generally fully composite behaviour. In contrast, configurations with metal plate connectors exhibited increased slip, as well as SLS stiffness degradation and residual deformations when reloaded. Multiple governing failure modes were observed (particularly in shear), underscoring the need for comprehensive strength verifications in MTC system design.

This thesis also evaluates commonly used analytical methods for predicting the flexural stiffness of MTCs. Comparisons with experimental results showed that accurate SLS predictions are governed more by consistent treatment of shear deflections and EFW than by the choice of analytical method alone. The Gamma Method proved to have good estimations of the experimental bending stiffness, as well as possessing a direct way of addressing rolling shear in the CLT perpendicular layer. The Rigidly Bonded Method also provided good estimations, but showed inconclusive results using its shear correction factor to account for longitudinal shear deflections. A validated finite element modelling method, supported by full field DIC results, was used as an interpretive tool to assess shear lag in the CLT flanges. The model demonstrated good capabilities for predicting the experimental results, but slightly overestimated the DIC EFW results, with shrinkage-induced separation of the edge-gluing deemed a primary cause.

A major contribution of this work is the experimental and numerical demonstration that EFW in CLT is a governing, response-dependent design parameter. Results demonstrate that EFW is strongly influenced by load configuration, connection stiffness, and flange geometry (particularly the perpendicular CLT layer(s)), indicating that it is a response dependent parameter rather than a fixed geometric property. Existing design guidance was found to be generally overly conservative and, in some cases, non representative of observed flange participation, particularly for line load conditions. A new approach is proposed for combining layer level EFW from finite element modelling into a full depth CLT flange EFW, addressing a gap in current design guidance. The work also demonstrated that methods for capturing the full strain field are particularly beneficial over discrete strain gauges in order to mitigate the impact of local wood defects.

Overall, this thesis advances the knowledge surrounding the behaviour of MTC floor systems by providing experimentally grounded insight into practical shear connections, system level flexural response, serviceability level modelling, and the effective flange width in CLT, ultimately supporting the reliable design of long span mass timber floors.