Cristiano Loss

Floor diaphragms are structural elements mainly responsible for transmitting lateral loads to the adjoining vertical members of the Seismic-Force-Resisting System (SFRS). Under significant seismic events, the actual in-plane stiffness of floor diaphragms affects patterns of load distribution in-between horizontal and vertical elements and, as such, contributes to the local nonlinear response and the global dynamic behavior. However, for SFRS with mass timber and hybrid timber-based floor diaphragms, neither comprehensive code design provisions nor accurate procedures exist to account for its in-plane flexibility. This knowledge deficit becomes more apparent if compared to the standard reinforced concrete flooring system. To gauge influences of the actual in-plane stiffness of diaphragms on a timber-steel hybrid building system, this research analyzes its seismic performance via a series of nonlinear dynamic analyses. Three finite-element building models were developed by adopting the OpenSees framework and were characterized by (i) rigid diaphragms, (ii) actual stiffness of the CLT diaphragm subassemblies, including the panel-to-panel slab connections, and (iii) stiffness of the CLT diaphragm subassemblies without accounting the panel-to-panel slab connections, respectively. In addition, the SFRS of the building entails a Special Concentrically Braced Frame whose nonlinear behaviors were explicitly simulated, including global buckling, tensile yielding, and post-buckling behaviors. Numerical models of steel struts and hybrid CLT diaphragms were calibrated and validated upon experimental datasets. Specifically, this paper outlines the global collapse capacity under varying ground motion intensities among the three building models. The collapse fragility analysis reveals that the conditional probability of exceeding all three considered limit states given any level of ground motion intensity for the SFRS equipped with the CLT-steel floor diaphragm is close to that of the SFRS with rigid diaphragm behavior. Removal of CLT panel-to-panel connections attracts disproportionate and excessive lateral deformation demands towards the unbraced frames due to the increased in-plane diaphragm flexibility.

View record

The use of sustainable design methodology in construction has seen major traction due to numerous regional and worldwide net zero emissions targets. These methods have advanced the technological development of systems in innovative ways, resulting in systems that reduce the carbon footprint of structures through the materials used, construction sequences, and end-of-life processes. Furthermore, particularly in seismically prone areas, the advent of resilient systems has seen a surge in ingenious solutions. In this study, a novel high-performance hybrid steel-timber rocking shear wall system is introduced for the next generation of resilient, sustainable structures. The wall system combines the favourable properties of steel plates and CLT panels, resulting in a composite CLT-steel wall section. This wall is pinned at the base and incorporates Resilient Slip Friction Joint dampers to effectively dissipate energy during rocking behavior. The cross-section of the walls is designed using readily available plates and panels, ensuring commercial viability. Through optimization of the composite modules and construction methodology, a modular system that delivers exceptional resilient structural performance, streamlines the prefabrication process, and provides a rapid construction process has been developed. The shear wall system is applied to a benchmark structure located in Vancouver, BC, and the composite section is modelled analytically using ANSYS, where modeling parameters, material thicknesses, strength and other design parameters are varied to determine the most favourable combination for the load scenario. The behaviour of the system is then implemented and calibrated in OpenSees, where a series of nonlinear seismic analyses were executed using selected crustal, subcrustal, and subduction ground motions present in the region according to NBCC 2020 guidelines. Results confirm that system performance exceeds drift requirements as per the current NBCC 2020 building code and yields an adjusted collapse margin ratio of 7.07, 5.61, and 3.90 for crustal, subcrustal, and subduction hazards respectively, resulting in a weighted average of 4.63, exceeding FEMA P-695 guidelines.

View record

All over the world, the mass timber construction industry is experiencing unprecedented growth. However, as mass timber buildings reach new heights, designers are faced with new challenges regarding constructability, sustainability, and compliance with performance-based design requirements. In particular, there is a need for novel connection solutions that are conducive to off-site prefabrication, quick on-site assembly, and that can provide required seismic resistance without suffering damage, creating the potential for deconstruction and reuse. This research investigated the structural performance of a novel multi-material shear connector for mass timber and hybrid timber-based buildings, consisting of a threaded steel rod embedded into Cross-Laminated Timber (CLT), reinforced with a ring-layer of epoxy-based grout. The protruding rods may be bolted to steel beams or hold-down plates to form hybrid timber-based floor and shear wall structural assemblies, respectively. The shear connector is to be capacity-protected, resulting in a damage-free connection, allowing for disassembly and potential reuse of the structural timber components. The response of shear connectors with varying rod diameter and steel strength-class, grout thickness, and CLT grade was analyzed. An insight into the behaviour under quasi-static monotonic incremental loads is given based on a comprehensive experimental campaign, with a total of 240 push-out tests performed on full-scale squared CLT specimens, including baseline samples without grout reinforcement. Test results revealed significant improvement in shear capacity and stiffness when a grout layer is included, without negatively impacting ductility and failure modes. Strong relationships between rod and grout diameter and yield and maximum shear resistance were established. Reliability analyses established a resistance factor in order to achieve similar levels of reliability across connector types and with dowel-type connectors already in the Canadian wood design standard CSA-O86. The results are encouraging and serve as a foundation for further research on this novel connector, including testing CLT assemblies and developing reliable mechanics-based models. From a design perspective, the studied multi-material shear connector has great potential for tall and large-scale timber building applications, giving designers a high-capacity alternative to traditional timber connectors.

View record