Chong Tang

Axisymmetric Lower-Bound Limit Analysis Using Finite Elements and Second-Order Cone Programming

AbstractIn this paper, the formulation of a lower-bound limit analysis for axisymmetric problems by means of finite elements leads to an optimization problem with a large number of variables and constraints. For the Mohr-Coulomb criterion, it is shown that these axisymmetric problems can be solved by second-order cone programming (SOCP). First, a brief introduction to SOCP is given and how axisymmetric lower-bound limit analysis can be formulated in this way is described. Through the use of an efficient toolbox (MOSEK or SDPT3), large-scale SOCP problems can be solved in minutes on a desktop computer. The method is then applied to estimate the collapse load of circular footings and uplift capacity of single or multiplate circular anchors. By comparing the present analysis with the results reported in the literature, it is shown that the results obtained from the proposed method are accurate and computationally more efficient than the numerical lower-bound limit analysis incorporated with linear programming.

Lower-Bound Limit Analysis of Seismic Passive Earth Pressure on Rigid Walls

AbstractStatic and seismic passive earth pressure coefficients for the general case of an inclined wall and a sloping cohesionless backfill were computed by using finite-element lower-bound limit analysis based on second-order cone programming (SOCP). The comparison with the best existing upper-bound solution and the widely accepted solution obtained by the method of stress characteristics has shown that the present lower-bound solutions can provide a safe estimate of the passive earth pressures. Therefore, combining present results and upper-bound solutions can bracket the true value of passive earth pressures. In addition, design tables are presented that allow geotechnical engineers to easily use the present results in practice.

Model Uncertainty for Predicting the Bearing Capacity of Sand Overlying Clay

In this paper, 62 centrifuge tests are collected from the literature and used to perform a statistical evaluation of the model factor for conventional methods to calculate the bearing capacity of dense sand overlying clay. The model factor is defined as a ratio of measured capacity to calculated capacity. The variations of the model factor with input parameters are established as regression equations based on the results of finite-element limit analysis (FELA). Conventional methods that multiply by regression equations are shown to be more accurate on average for all data sets. Further verification exercise from 27 additional centrifuge tests indicates the modified method could also be applicable for medium dense sand overlying clay. The mean and coefficient of variation of the model factor for the modified methods are finally characterized as a lognormal random variable with a mean of 1.06 and coefficient of variation of 0.17. DOI: 10.1061/(ASCE)GM.1943-5622.0000898.© 2017 American Society of Civil Engineers.

Model Uncertainty of Eurocode 7 Approach for Bearing Capacity of Circular Footings on Dense Sand

AbstractThis paper presents a critical evaluation of the model factor M = qu,m/qu,c for Eurocode 7 calculating the bearing capacity of circular footings on dense sand, where qu,m = measured capacity and qu,c = Eurocode 7 calculated capacity. Regression analysis is required to remove the dependency of M on the input parameters. Because the input parameters cannot be varied systematically in load tests, previous studies showed that finite-element limit analysis (FELA) can be used as an alternative to load tests for regression. This is further verified from the model factor MFELA = qu,m/qu,FELA with a mean of 1 and a coefficient of variation (cov) of 0.1, where qu,FELA = FELA predicted capacity. A correction factor (Ms = qu,FELA/qu,c) is next defined, which can be decomposed as a product of a systematic part f and a residual part η (i.e., Ms = fη), which is modeled as a lognormal random variable with mean = 1 and cov = 0.11. Finally, a new model factor (M′ = qu,m/q′u,c = qu,m/fqu,c) is defined. The model sta...