One of the current challenges in physics-based ground-motion simulations is to refine the modeling of local site effects. These effects require a finer spatial resolution in the material modeling than that generally considered in regional-scale simulations. Because of this, empirical amplification factors are typically applied to capture these unmodeled phenomena. The ergodic nature of this approach suggests that there is room for improvement. In this study, the predictive capability of simulations is evaluated using alternative methods for capturing local site effects. In addition to the conventional empirical approach, two methods are examined that allow for more site-specific information to be incorporated: the square-root impedance method and the 1D time domain site-response analysis. The three approaches are tested using 1000+ observed ground motions from 150+ small-magnitude events (3.5 ≤ Mw ≤ 5.0), recorded at 20 strong-motion stationsin the Canterbury, New Zealand, region. These 20 well-characterized sites represent a wide range of soil conditions, including stiff gravels with Vs30 values greater than 500 m/s, and sand and silt deposits with Vs30 valuesless than 200 m/s. Multiple intensity measures are computed and prediction residuals are partitioned using mixed-effects regression to rigorously assess the relative performance of the different approaches considered. The results indicate that the benefit of using more sophisticated methods is highly dependent on the characteristics of the site. Key site parameters and trends are identified and discussed in light of the assumptions and limitations of each approach.
The lateral capacity of a conventional CLT shear wall is often governed by the strength and stiffness of its connections, which do not significantly utilize the in-plane strength of the CLT. Therefore, CLT shear walls are not yet being used efficiently in the construction of mass timber buildings due to a lack of research on high-capacity connections and alternative wall configurations. In this study, cyclic experiments were completed on six full-scale, 5-ply cantilever CLT shear walls with high-capacity hold-downs using mixed angle screws and bolts. All specimens exhibited significantly higher strength and stiffness than previously tested conventional CLT shear walls in the literature. The base connections demonstrated ductile failure modes through yielding of the hold-down connections. Based on the experimental results, numerical models were calibrated to investigate the seismic behaviour of CLT shear walls for prototype buildings of 3 and 6-storeys in Christchurch, NZ. As an alternative to cantilever (single) shear walls, a type of coupled wall with steel link beams between adjacent CLT wall piers was investigated. Effective coupling requires the link beam-to-wall connections to have adequate strength to ensure ductile link beam responses and adequate stiffness to yield the link beams at a relatively low inter-storey drift level. To this end, three beam-to-wall connection types were developed and cyclically tested to investigate their behaviour and feasibility. Based on the test results of the critical connection, a 3-storey, 2/3-scale coupled CLT wall specimen with three steel link beams and mixed angle screwed hold-downs was cyclically tested to evaluate its performance and experimentally validate the system concept. The test results showed a relatively high lateral strength compared to conventional CLT shear walls, as well as a high system ductility ratio of 7.6. Failure of the system was characterised by combined bending and withdrawal of the screws in the mixed angle screw hold-downs, yielding and eventual inelastic buckling of the steel link beams, CLT toe crushing, and local CLT delamination. Following the initial test, the steel link beams, mixed angle screw hold-downs, and damaged CLT regions were repaired, then the wall specimen was re-tested. The repaired wall behaved similarly to the original test and exhibited slightly higher energy dissipation and peak strength, but marginally more rapid strength deterioration under cyclic loading. Several hybrid coupled CLT shear walls were numerically modelled and calibrated based on the results of the coupled wall experiments. Pushover analyses were conducted on a series of configurations to validate a capacity design method for the system and to investigate reasonable parameter values for use in the preliminary design of the system. Additionally, an iterative seismic design method was proposed and used to design sample buildings of 6, 8, and 10-storeys using both nonlinear pushover and nonlinear time history analyses to verify the prototype designs. Results of the sample building analyses demonstrated adequate seismic behaviour and the proposed design parameters were found to be appropriate. In summary, high-capacity CLT shear walls can be used for the resistance of earthquakes by using stronger base connections and coupled wall configurations. The large-scale experimental testing in this study has demonstrated that both cantilever and coupled CLT shear walls are feasible LLRSs which can provide significantly greater lateral strength, stiffness, and energy dissipation than conventional CLT shear wall configurations.
