title: Robustness of Post-And-Beam Mass Timber Buildings Under Dynamic Actions
Creators: name: Cheng, Xinyi
Publisher: name: Griffith University
Year: 2022
Subjects: subject: Laminated veneer lumber (LVL)
subject: Mass timber building
subject: Dynamic increase factor (DIF)

description: Mass timber mid-rise to tall buildings are gaining international popularity owing to their lower global carbon emissions, fast erection and more aesthetically pleasing than traditional concrete and steel structures. Given the increasing height of mass timber buildings, up to 85.4 m currently, the consequences of a progressive collapse event of such building can be catastrophic. In Australia, mass timber buildings must be designed to resist progressive collapse according to National Construction Code (NCC). Moreover, a progressive collapse event is essentially dynamic and nonlinear. Dynamic effects therefore need to be taken into considerations. Since the actual dynamic analysis of a structure can be complicated, it can be also achieved by applying a dynamic increase factor (DIF) to a quasi-static response, which is a theoretical upper value of 2.0 obtained from elastic system, instead of the well-developed values in concrete and steel structures. Therefore, in this study, scaled-down dynamic experimental studies on 2-bay post-and-beam mass timber frames were carried out to fill the gap in this knowledge and gain a preliminary understanding of the dynamic behaviour of such structural system. It was found that experimental DIF values were dependent on the types of connections and their failure modes. Ductile connections tended to exhibit lower DIF values. Such DIF values are critical to progressive collapse prevention designs. Based on the previous experimental study on the structural behaviour of a mass timber frame in a global scale, it can be concluded that connections are the key, which therefore is the next focus in this study. Timber connections are often manufactured from a combination of multiple materials including timber, steel and aluminium, which experience different degree of sensitivities under the same level of strain rate. Such differences may further result in different failure modes and load carrying capacities in strain rates encountered during dynamic events when compared to quasi-static strain rates. While strain rate effects on the materials properties of steel and common aluminium alloys have been well researched, for timber, it was mainly focused on the different between quasi-static (low strain rates, e.g., 10-5) and blast loadings (high to extremely high strain rates, e.g., 101) which are not applicable to earthquake events (medium strain rates). Therefore, in this study, in total 400 material tests on softwood LVL were conducted on critical timber mechanical properties, including embedding, tension perpendicular-to-grain, fracture model I and fracture mode II, to gain a better understanding of these material properties under the strain rate range equivalent to quasi-static and earthquake events. It was found that significant increase in stiffness and strengths in timber embedding, while no clear tendency in tension perpendicular-to-grain and fracture energies under low to medium strain rate range. To further investigate the timber connections, finite element model was chosen as an economical option. This study selected a commonly used commercial timber beam-to-column connection that contains most of the features in timber connections including steel bolts and dowels, and aluminium slotted in concealed bracket. In this study, static FE models were established first to ensure the reliability of the models, which were calibrated step by step against timber half-hole embedding, full hole push-down, full-scale connection shear and bending static tests. Then the material properties under medium strain rates were incorporated into the static FE model to study the influence of those material properties alone to structural behaviours of such timber connection. Based on the findings of this study, further 3D mass timber frame with CLT panels is recommended to further study the DIF values in complete mass timber systems. Furthermore, while the FE models developed in this study represents a first step towards understanding the behaviour of timber connection under dynamic loading, it needs to be applied to other connection types and validated against corresponding experimental results.