MS13.7 - Hybrid analyses, experimental tests and numerical modeling in earthquake engineering

The time history of the design ground motion used in seismic design is suggested to be used either by using observed records or artificially synthesized to match the spectrum. In this case, the design response spectrum is a simplified representation of a statistical combination of observed earthquake events that have occurred in the past, and does not represent the ground motion with specific earthquake magnitude or distance. However, since the design response spectrum represent the seismic load according to the site characteristics and the importance of the structure for each natural period, If the similarity with the design ground motion is low, there is a possibility of underestimating or overestimating the ground response analysis. Therefore, input ground motions for response history analysis need to be selected reasonably. In recent design standards, when creating acceleration time histories corresponding to design ground motions, there is a trend to use seismic records observed at the site in order to consider the characteristics of the tectonic environment at a site. Among the seismic events observed in Korea, significant earthquakes as design ground motions include the Gyeong-ju earthquake in 2016 (Mw5.8) and the Po-hang earthquake in 2017 (Mw5.4). However, these earthquakes are not compatible with the design response spectrum. Therefore, it is necessary to convert the recorded ground motion in Korea to a model similar to the design response spectrum. In this study, the Gyeong-ju earthquake event from the MKL station was used and a virtual fault was modeled. And several approaches to adjust the spectral acceleration level at each period range were tested. These are the intrinsic and scattering attenuation considering the earthquake environment, magnitude, distance change by the green’s function method, and a rupture propagation direction’s directivity effect. In the case of the Gyeong-ju earthquake, since seismic wave energy was concentrated in the short-period, the long-period component of 0.1 second or more was insufficient compared to the design response spectrum. Therefore, it was attempted to amplify the long-period component compared to the short-period. As a result of conversion using each parameter, the amplification of the long-period component was greater when the epicenter was more than 30km, when the scale was large, and when the forward direction effect was applied. In addition, based on these results, a regression analysis was performed on the “short-period to long-period amplification ratio” according to the rupture propagation direction’s directivity effect and the magnitude and distance to suggest the optimal conditions to match the design response spectrum and validation was performed by converting the recorded ground motion.

Testing soils for earthquake and dynamic loads requires advanced equipment able to control the effects of hydromechanical coupling on the soil response. The majority of laboratory element tests are either “slow tests”, which intend to approach drained conditions throughout the soil sample in order to obtain reliable pore water pressure measurements, or “fast undrained tests”, where flow is prevented by closing the drainage lines. However, many natural loads, including earthquakes, impose a wide range of high loading frequencies, typically triggering a partially drained response in the field.. Although the rate effect does play an important role in soil behaviour, its investigation is hindered by the limitations of existing apparatus or sensors. In addition, the ability to apply multidirectional loading to soil elements in the laboratory is important to fully understand the soil response under earthquakes. Currently, multidirectional simple shear devices are used to study soil behaviour to multidirectional loadings. Nevertheless, many shear devices suffer from stresses and strains non-uniformities, which could potentially mislead interpretation of the data and constitutive model development. This paper presents an innovative multidirectional shear device developed in the section of Geoengineering at TU Delft, which can apply higher loading frequencies compared to previous equipment and a wider variety of multidirectional cyclic loading patterns. The apparatus is equipped with advanced sensors, also developed at TU Delft, to capture the local response of specimens. The sensors are installed to reduce a priori assumptions on the soil response, better interpret the element experimental results and further investigate the rate effect of applied loading. Preliminary test results on Dutch organic soft soils are provided to illustrate the complex load conditions which can be achieved.