W. D. Liam Finn

Finite elements incorporating characteristics for one-dimensional diffusion-convection equation

Abstract A finite element method incorporating the method of characteristics for the solution of diffusion-convection equation with variable coefficients in one spatial dimension is developed. The method employs spatial-temporal elements with sides joining the nodes at subsequent time levels oriented in particular directions. This method is capable of solving diffusion-convection equation accurately for the whole spectrum of dispersion from pure diffusion, through mixed dispersion, to pure convection in particular, employing any mesh of reasonable size. The application of the method is demonstrated by solving three examples.

A Study of Piles during Earthquakes: Issues of Design and Analysis

The seismic response of pile foundations is a very complex process involving inertial interaction between structure and pile foundation, kinematic interaction between piles and soils, seismically induced pore-water pressures (PWP) and the non-linear response of soils to strong earthquake motions. In contrast, very simple pseudo-static methods are used in engineering practice to determine response parameters for design. These methods neglect several of the factors cited above that can strongly affect pile response. Also soil–pile interaction is modelled using either linear or non-linear springs in a Winkler computational model for pile response. The reliability of this constitutive model has been questioned. In the case of pile groups, the Winkler model for analysis of a single pile is adjusted in various ways by empirical factors to yield a computational model for group response. Can the results of such a simplified analysis be adequate for design in all situations?The lecture will present a critical evaluation of general engineering practice for estimating the response of pile foundations in liquefiable and non-liquefiable soils during earthquakes. The evaluation is part of a major research study on the seismic design of pile foundations sponsored by a Japanese construction company with interests in performance based design and the seismic response of piles in reclaimed land. The evaluation of practice is based on results from field tests, centrifuge tests on model piles and comprehensive non-linear dynamic analyses of pile foundations consisting of both single piles and pile groups. Studies of particular aspects of pile–soil interaction were made. Piles in layered liquefiable soils were analysed in detail as case histories show that these conditions increase the seismic demand on pile foundations. These studies demonstrate the importance of kinematic interaction, usually neglected in simple pseudo-static methods. Recent developments in designing piles to resist lateral spreading of the ground after liquefaction are presented. A comprehensive study of the evaluation of pile cap stiffness coefficients was undertaken and a reliable method of selecting the single value stiffnesses demanded by mainstream commercial structural software was developed. Some other important findings from the study are: the relative effects of inertial and kinematic interactions between foundation and soil on acceleration and displacement spectra of the super-structure; a method for estimating whether inertial interaction is likely to be important or not in a given situation and so when a structure may be treated as a fixed based structure for estimating inertial loads; the occurrence of large kinematic moments when a liquefied layer or naturally occurring soft layer is sandwiched between two hard layers; and the role of rotational stiffness in controlling pile head displacements, especially in liquefiable soils. The lecture concludes with some recommendations for practice that recognize that design, especially preliminary design, will always be based on simplified procedures.

Dynamics of gravity dam-reservoir systems☆

Abstract The response of a long concrete gravity dam-reservoir system for a harmonic rigid base acceleration normal to the dam axis is investigated. The hydrodynamic pressure acting on the vertical wet surface of the dam is first evaluated in closed form as a function of the unknown deflections of the dam-reservoir interface and the ground acceleration. Employing finite element techniques, the motion of the dam is investigated. The hydrodynamic pressures enter the equation of motion of the dam as loadings in excess of the inertia load. In this approach, the general flexibility of the dam cross-section and the compressibility of water are taken into account. The viscosity of water and the effect of the surface waves are neglected. The coupling between the dam and the reservoir results in changes in the mass and stiffness properties of the dam which depend on the excitation frequency. For an excitation frequency greater than the fundamental frequency of the reservoir, the damping properties of the dam are also modified.

Variable domain finite element analysis of free surface gravity flow

Abstract A variable domain finite element method is presented for the solution of problems in gravity flow with free surfaces. Novel features of the method are the use of a functional with a variable domain of integration and a finite element mesh that can adapt automatically to all compatible variations in the domain. The finite element solution gives the streamlines and the flow rate associated with any stagnation level and system geometry. The method is applied to flow over spillway and gives results that agree extremely well with experimental data.

Analysis of ground motions at Treasure Island site during the 1989 Loma Prieta earthquake

Abstract Site response analyses and coherence studies were conducted at the Treasure Island site where surface motions were recorded during the Loma Prieta earthquake. The analyses were conducted using a nonlinear dynamic effective stress method which took into account the effects of the liquefaction that occured at the site. The rock motions recorded at nearby Yerba Buena Island were used as input motions. Computed and recorded ground motions transverse to the direction of wave propagation and associated response spectra were in good agreement. Agreement was also good in the radial direction, except in certain frequency bands higher than 1·25 Hz. Coherence studies showed that some of these discrepancies may be due to low coherence between the Treasure Island and Yerba Buena motions in these same frequency bands.

Geotechnical Aspects of the Estimation and Mitigation of Earthquake Risk

This paper describes the procedures for characterizing the impact of geotechnical factors on the hazards and risks imposed by earthquakes. Definitions of the concept of risk and its constituent parts proposed by UNDRO (United Nations Disaster Relief Organization) (Varnes, 1984; van Essche, 1986), have been adopted here. A modified form of the definitions is given here specifically for earthquake risk.

The Evaluation of the Break-Out Force For a Submerged Ocean Platform

Describes a mechanism which offers an explanation of the breakout phenomenon. This mechanism leads to a simple analytical method for predicting the breakout force. One of the most important factors controlling ability to predict breakout force is the proper evaluation of the strength of ocean sediments. The method is applied to the analysis of data obtained by the Naval Civil Engineering Laboratory at Port Hueneme from breakout experiments in the Gulf of Mexico and San Francisco Bay. It is also applied to the problem of raising a pedestal for a well-head from the ocean floor.

Seismic Evaluation of Existing Basement Walls

Current state of practice for seismic design of basement walls is using the Mononobe-Okabe (M-O) method that is based on the Peak Ground Acceleration (PGA). The National Building Code of Canada (NBCC) has considerably changed the seismic hazard level from 10 % in 50 years in NBCC1995 to 2 % in 50 years in NBCC2010, which leads to doubling the PGA in Vancouver from 0.24g to 0.46g. The current design PGA leads to very large seismic forces that make the resulting basement walls very expensive. Because there is a little evidence of any significant damage to basement walls during major earthquakes, the Structural Engineers Association of British Columbia (SEABC) initiated a task force to review current seismic design procedures for deep basement walls. Presented in this paper are some preliminary results of the work conducted by this committee. A series of dynamic numerical analyses have been carried out on a typical basement wall designed using the M-O earth pressures with the NBCC1995 PGA for Vancouver. This wall is then subjected to three ground motions spectrally matched to the Uniform Hazard Spectrum prescribed by the NBCC2010 and the seismic performance of the wall under this level of demand has been presented and discussed. Particular attention has been given to the resulting drift ratio in the walls.

Offshore pile foundations in sand under earthquake loading

Abstract A new approach to the analysis of pile foundations, developed recently for the analysis of a pile supported offshore structure, is described. The method uses a coupled soil-pile analysis which takes into account the non-linear resistance of the pile to lateral deformation and the effect of progressively increasing pore water pressures on that resistance. The analysis allows also for radiation of energy away from the site. Typical of results given in the paper are: (1) the effect of pore water pressure increases on the API cyclic loading curves, (2) the degradation in lateral stiffness due to pore pressure increases for piles with fixed and free heads, (3) variations in deflections and moments with depth due to pore water pressure from those predicted using current API procedures.

Performance-Based Retrofit of School Buildings in British Columbia, Canada

In 2004, the Province of British Columbia, on the West Coast of Canada, announced a 10-15 year,