Collaborative Research: Multimodal Orthotropic Field Theory for Shear and Torsion Design of Mass Timber Structures

Mass timber buildings constructed from large, engineered wood panels, known as cross-laminated timber, are now permitted at heights up to eighteen stories. These buildings use domestic resources, support forestry economies, and can be constructed faster and with less waste than conventional alternatives. The engineering rules used to design them, however, have not kept pace. In the regions of a building where forces concentrate, such as around door and window openings, at notches and penetrations in beams, and at connections between floors and walls, timber panels can fail in splitting due to bending, shear, or in-plane torsion. Current modeling and design methods for these regions rely on simplified empirical rules rather than mechanistic behavior, which limits both the safety margins engineers can verify and the economy of the resulting designs. This award supports research that combines large-scale laboratory testing with new engineering theoretical models to explain and predict how engineered timber panels resist these complex force combinations. The results will support robust theoretical models for safer and more economical tall wood buildings, strengthen national design standards, expand mass timber construction, and train students and practicing engineers for a rapidly growing sector of the American construction industry. The research develops and validates a multimodal orthotropic shear field theory that provides a mechanistic basis for modeling and designing discontinuity regions in both lumber-based and veneer-based cross-laminated timber structures. The experimental program uses the Universal Panel Tester at the University of Houston, a facility capable of applying precisely controlled biaxial and torsional stress states, to isolate shear failure modes of cross-laminated timber, namely gross shear, net shear, and combined shear-torsion, for both lumber-based and veneer-based panels. These experiments will produce mode-isolated shear response curves, shear-torsion interaction laws, Poisson-effect constitutive parameters, and size-effect data that currently do not exist for these products. The theory extends fixed-angle shear field formulations to orthotropic multi-lamella composites by introducing individual crack angles for each lamella, interlayer torsion transfer, and biaxial Poisson coupling, with fracture mechanics criteria governing brittle failure initiation at re-entrant corners. Subassembly experiments at Oregon State University and the University of Utah, including beams with openings and notches, pier and joint elements, and multistory shear walls with openings, will validate the theory across length scales and support derivation of volume and stability adjustment factors compatible with national design specifications. Deliverables include validated theoretical and computational models, procedures for capacity-based design of discontinuity regions based on validated theory, publicly archived experimental datasets, and educational micro-credentials for practicing engineers. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.