Discrete element method for bearing capacity analysis

Analysis based on Discrete Element Method (DEM) is presented for estimating bearing capacity of foundations. Soil mass in the present model is treated as comprising of blocks which are connected by elasto-plastic Winkler-springs. By considering the conditions of compatibility, the boundary stresses on the failure surface and the ultimate bearing capacity of shallow foundations can be obtained. The computed boundary stresses from the present method satisfy equilibrium conditions and do not exceed the material strength. Formulation of the method is presented. Examples are shown to demonstrate the applicability of the method to the analysis of bearing capacity of shallow foundations. Effects of stiffness of Winkler-springs and mesh pattern on the computed results are investigated. The present method is shown to be a useful tool for analyzing bearing capacity of foundations with unusual geometry and loading conditions.     

Dual compensation strategy for real‐time hybrid testing

Real‐time hybrid testing is an experimental technique for evaluating the dynamic responses of structural systems under seismic loading. Servo‐hydraulic actuators, by nature, induce inevitable time delay between the command and the achieved displacements. This delay would lead to incorrect test results and even cause instability of the system; therefore, delay compensation is critical for stability and accuracy of hybrid simulations of structural dynamic response. In this paper, a dual delay compensation strategy is proposed by a combination of a phase lead compensator and a restoring force compensator. An outer‐loop feed‐forward phase lead compensator is derived by introducing the inverse model in the z domain. The adaptive law based on the gradient algorithm is used to estimate the system delay in the format of parametric model during the test. It is shown mathematically that the parameter in the delay estimator is guaranteed to converge. The restoring force compensator is adopted to improve the accuracy of experimental results especially when the structure is subjected to high frequency excitations. Finally, analytical simulations of an inelastic SDOF structure are conducted to investigate the feasibility of the proposed strategy. The accuracy of the dual compensation strategy is demonstrated through several shaking table tests. Copyright © 2012 John Wiley & Sons, Ltd.     

Soil–gas monitoring: A tool for fault delineation studies along Hsinhua Fault (Tainan), Southern Taiwan

Many studies have shown the soil gas method to be one of the most reliable investigation tools in the research of earthquake precursory signals and fault delineation. The present research is aimed finding the relationship between soil gas distribution and tectonic systems in the vicinity of the Hsinhua Fault zone in the Tainan area of Southern Taiwan. More than 110 samples were collected along 13 traverses to find the spatial distribution of Rn, He, CO₂ and N₂. The spatial congruence of all the gases shows that N₂ is the most probable carrier gas of He, whereas CO₂ seems to be a good carrier gas of Rn in this area. From the spatial distribution of Rn, He, CO₂ and N₂ the trace of Hsinhua Fault and neotectonic features can be identified. The spatial distribution of studied gases shows a clear anomalous trend ENE–SWS along the Hsinhua Fault.