No. R781
Poulos, HG
A Method for Analyzing Piled Embankments
Abstract
This paper discusses the various issues involved in the design of piled embankments, and then describes the application of computer analyses of piled strip and raft behaviour to the piled embankment problem. These analyses are implemented via a program called GASP (Geotechnical Analysis of Strip with Piles). Using this program, the effects of other key parameters are also studied, and the design of transition areas is discussed. It is shown that optimum pile design will generally require the use of piles of different length and/or stiffness in order to even out the settlements. The ground movement arising from the piling platform construction may also have a major effect on this design.
No. R782
Xu, KJ and Poulos, HG
Principles of Program GEPAN for General Piles Elastic Analysis
Abstract
This paper presents the principles of general pile elastic analysis in a program called GEPAN (General Pile Analysis). It describes the development of a boundary element analysis for the behaviour of single piles and pile groups subjected to general three-dimensional loading and to vertical and lateral ground movements. Each pile is discretised into a series of cylindrical elements, each of which is divided into several sub-elements. Compatibility of vertical, lateral and rotational movements is imposed in order to obtain the necessary equations for the pile response. Via hierarchical structures, twelve non-zero sub-matrices in a global matrix are derived for the basic influence factors.
Solutions are presented for a series of cases involving single piles and pile groups. In each case, the solutions are compared with those from more simplified existing pile analyses such as those developed by Randolph and by Poulos. It is shown that for direct loading effects (e.g. the settlement of piles due to vertical loading), the simplified analyses work well. However, for “off-line” response (such as the lateral movement due to vertical loading) the differences are greater, and it is believed that the present analysis gives more reliable estimates.
Keywords
Pile groups; 3-D coupled problems; elastic analysis; boundary element method; influence factors.
No. R783, June, 1999
Wilkinson, T and Hancock, GJ
Tests of Cold-Formed Rectangular Hollow Section Portal Frames
Full Report in PDF
Abstract
This report describes tests on three large scale portal frames manufactured from cold-formed RHS in both Grade C350 and Grade C450 steel. The loading simulated gravity and wind loads. The connections, lateral restraint mechanism, loading method and test procedure are explained.
The results indicate that a plastic collapse mechanism was formed in each frame. There was no failure associated with any of the joints. Simple plastic analysis provided a reasonable estimate of the ultimate load, even though the analysis did not include second-order effects or non-linear material properties. An advanced analysis which included second order effects, material non-linearity, and member imperfections slightly over-predicted the strength of the frame, since the analysis underestimated the deflections and hence the second-order effects, as the analysis did not consider the small loss of connection rigidity. A second-order inelastic analysis without member imperfections provided the best estimates of the strengths of the frames. The distribution of curvature throughout the portal frame was analysed. The results indicate that for the portal frames tested a rotation capacity of R = 4 is adequate to form the plastic collapse mechanism.
Although the cold-formed RHS do not satisfy the material ductility requirements for plastic design, there was no failure associated with lack of material ductility in the three frames tested, demonstrating that the restriction on cold-formed RHS from plastic design is unwarranted provided that the connections are sufficiently ductile for any required plastic rotations.
Keywords
Cold-formed steel, hollow sections, RHS, frames, plastic design, connections, joints, bending, rotation capacity, local buckling.
No. R784
Poulos, HG
Design of Slope Stabilizing Piles
Abstract
This paper discusses a procedure for the design of slope stabilizing piles in which the resistance provided by the piles is assessed via an analysis of their response to lateral ground movements. The mechanics of such pile-soil interaction are discussed and it is shown that there are a number of modes of failure, involving yield of the soil and/or the piles themselves. For the ultimate case, a series of design charts are given to assist in the assessment of pile resistance. A conceptual approach for designing piles to limit slope movements is also presented. Finally, the application of the approach to two documented case histories is described.
No. R785
Trahair, NS
Column Bracing Forces
Abstract
This paper is concerned with the forces for which steel column braces should be designed. It reviews a previous study of column brace forces, and analyses the elastic and first yield behaviour of simply supported columns with central braces.
It is shown that the brace force depends principally on the initial crookedness of and the design load on the column, but also on the brace stiffness and the column slenderness.
It is concluded that the simple AS4100 (SA, 1998) requirement that the brace be designed for 2.5% of the column design load is over conservative, and a reduction to 1.5% is recommended. A brace which is designed for this force will be stiff enough to brace a straight column into the anti-symmetric buckling mode of a rigidly braced column.
Because the brace force reductions corresponding to reductions in the brace stiffness and the column slenderness are generally small, except for stocky columns with slender braces, there is no compelling need to make any further reduction in the design brace force.
No. R786
Poulos, HG
Some Aspects of Pile Skin Friction in Calcareous Sediments
Abstract
This paper reviews the development of skin friction of piles when installed in calcareous sediments. Data from extensive series of laboratory tests are summarised, with the following aspects being considered:
- Skin friction along piles while being driven
- Skin friction under static loading conditions, for both driven pile sand bored piles, including the effects of such factors as cementation, over consolidation, and relative density of the soil
- The effects of cyclic loading, with respect to both skin friction degradation and accumulation of axial movements
- The relationship between volumetric strains developed during cyclic loading and cyclic degradation
- Group effects.
Procedures for numerical modelling and design are then considered, and a comparison is made of the predicted response of a pile as computed from alternative computer approaches.
No. R787
Liu, MD and Carter, JP
A General Method for Defining the Number of Cycles of Repeated Loading
Abstract
In geotechnical engineering designs and safety checks, the number of cycles of repeated loading N is frequently used as a parameter for calculating the response of soil structures under the repeated loading. However, there appears to be lack of a rational method for calculating N for repeated loadings along general stress paths. A new method for defining N is proposed in this paper, based on the assumption that a yield surface may be identified for the material. It is demonstrated that the proposed method provides a rational and useful means for engineering calculation of soil deformations under repeated loadings.
Keywords
Cyclic loading, number of cycles, pore pressure development.
No. R788
Taiebat, HA and Carter, JP
Numerical Studies of the Bearing Capacity of Shallow Foundations on Cohesive Soil Subjected to Combined Loading
Abstract
This paper presents the results of three-dimensional finite element analyses of circular foundations resting on the surface of homogeneous, purely cohesive soil. The predicted ultimate response of the foundations to combined vertical, moment, and horizontal loading is compared with other available theoretical predictions. A three-dimensional failure locus is presented for these foundations, based on the numerical predictions. An equation that approximates the shape of the failure locus is also suggested, and this provides a convenient means of calculating the bearing capacity of circular foundations resting on the surface of a uniform clay and subjected to combined loading.
No. R789
Carter, JP, Airey, DW and Fahey, M
A Review of Laboratory Testing of Calcareous Soils
Abstract
This paper presents a review of laboratory testing of calcareous soils. Special attention has been paid to the challenging problems of in situ density determination and laboratory sample preparation. Recently developed techniques for preparing reconstituted samples with relatively high voids ratios, representative of those measured for tube samples of carbonate “clays” recovered from the field, are described. The review also includes discussion of procedures used to prepare artificially cemented soil specimens, the mechanical behaviour of cemented calcareous sands, as well as some recent findings on the creep of calcareous sediments, the influence of the pore fluid on the response of these materials, and the behaviour of both uncemented and cemented calcareous soils under undrained cyclic loading. Some recent developments in laboratory apparatus and testing techniques are also described.
No. R790
Pi, YL and Trahair, NS
Distortion and Warping at Beam Supports
Abstract
This paper presents a review of laboratory testing of calcareous soils. Special attention has been paid to the challenging problems of in situ density determination and laboratory sample preparation. Recently developed techniques for preparing reconstituted samples with relatively high voids ratios, representative of those measured for tube samples of carbonate “clays” recovered from the field, are described. The review also includes discussion of procedures used to prepare artificially cemented soil specimens, the mechanical behaviour of cemented calcareous sands, as well as some recent findings on the creep of calcareous sediments, the influence of the pore fluid on the response of these materials, and the behaviour of both uncemented and cemented calcareous soils under undrained cyclic loading. Some recent developments in laboratory apparatus and testing techniques are also described.
No. R791
Radioactive Waste Disposal in Geological Media: Three-Dimensional Transient Thermo-Elasticity by Boundary Elements
Abstract
Where inertia effects and pore water pressures are negligible and no appreciable consolidation is expected, as in the case of seismically stable dry rock formations, the problem of thermal stresses in porous media can be represented by the uncoupled time-dependent thermo-elasticity equations. A combined boundary element Laplace transform solution of these equations in three-dimensional homogeneous media is proposed. The method is an extension of an earlier, two-dimensional formulation. Temperature and heat flux are computed first at any specified time station. Displacements, stresses and strains are derived next. Only boundary discretisation and no time stepping are required. The method is based on a Laplace transform of the governing equations of temperature and a fundamental solution in the Laplace domain. Computed temperatures are inverted numerically into time domain. A thermal vector is assembled in the thermal phase of the solution and added to the standard boundary element matrices of elastostatics. This treatment thereby extends to transient problems the elegant steady-state boundary element solution. Domain integrals are avoided without resort to any computationally costly transformation technique. The validity and accuracy of the method are demonstrated through a comparison of its predictions to analytical results, and its scope of application is illustrated with two examples.
Keywords
Boundary Elements, Radioactive Waste Disposal, Thermoelasticity, Time-Dependent, Laplace Transform.
No. R792, October, 1999
Wilkinson, T and Hancock, GJ
Finite Element Analysis of Plastic Bending of Cold-Formed Rectangular Hollow Section Beams
Full Report in PDF
Abstract
This report describes the finite element analysis of rectangular hollow section (RHS) beams, to simulate a series of bending tests. The finite element program ABAQUS was used for the analysis. The main aims of the analysis were to determine trends, to understand the occurrence of inelastic instability, and to attempt to predict the rotation capacity of cold-formed RHS beams.
The maximum loads predicted were slightly lower than those observed experimentally, since the numerical model assumed only three distinct material properties in the RHS. In reality, the variation of material properties around the RHS cross-section is gradual.
Introducing geometric imperfections into the model was essential to obtain results that were close to the experimental results. A perfect specimen without imperfections achieved rotation capacities much higher than those observed experimentally. Introducing a bow-out imperfection, constant along the length of the beam, as was measured (approximately) experimentally, did not affect the numerical results significantly. To simulate the effect of the imperfections induced by welding the loading plates to the beams in the experiments, the amplitude of the bow-out imperfection was varied sinusoidally along the length of the beam. The size of the imperfections had an unexpectedly large influence on the rotation capacity of the specimens. Larger imperfections were required on the more slender sections to simulate the experimental results. The sensitivity to imperfection size increased as the aspect ratio of the RHS decreased.
The finite element analysis determined similar trends as observed experimentally, namely that the rotation capacity was a function of both the flange and web slenderness, and that for a given aspect ratio, the relationship between web slenderness and rotation capacity was non-linear, and the slope of the line describing the relationship increased as the web slenderness decreased.
Keywords
Cold-formed steel, hollow sections, RHS, finite element analysis, ABAQUS, plastic design, bending, rotation capacity, local buckling.
No. R793
Trahair, NS
Lateral Buckling Design of Portal Frame Rafters
Abstract
This paper is concerned with the lateral buckling design of portal frame rafter segments between the knee and the first fly-brace. The lowest tier design method of AS 4100 often predicts low moment capacities for these segments, while the more economical method of design by buckling analysis is usually considered to be too difficult to use, because of a lack of easily available design data.
The effects of moment distribution, load height, and intermediate and end restraints on the elastic buckling of rafter segments are studied, and approximations are developed for use in the method of design by buckling analysis. A method of design is developed and illustrated by a worked example.
The design lateral buckling capacities fMbx are investigated for three different rafter sections, a 360UB56.7, a 250UC72.9, and a 550LB29.8 Industrial Light Beam (ILB). These sections have comparable design section moment capacities fMsx. It is concluded that the 250UC72.9 has low in-plane stiffness, that the 360UB56.7 has low lateral buckling capacities, and that the low weight 550LB29.8 has high in-plane stiffness and superior lateral buckling capacities.
No. R794
Young, B and Hancock, GJ
Design of Cold-Formed Unlipped Channels Subjected to Web Crippling
Abstract
The report presents a series of tests on cold-formed unlipped channels subjected to web crippling. The tests were conducted under the four loading conditions specified in the Australian/New Zealand Standard (AS/NZS 4600, 1996) and the American Iron and Steel Institute (AISI, 1996) Specification for cold-formed steel structures. The four specified loading conditions are the End-One-Flange (EOF), Interior-One-Flange (IOF), End-Two-Flange (ETF) and Interior-Two-Flange (ITF) loading. The loading of concentrated load or reaction forces were applied by means of bearing plates which acted across the full flange widths of the channels. The web slenderness values of the channel sections ranged from 15.3 to 45.
The test strengths are compared with the design strengths obtained using the AS/NZS 4600 and the AISI Specification. It is demonstrated that the design strengths predicted by the standard and the specification are generally unconservative for unlipped channels. Test strengths as low as 37% of the design strengths were obtained. For this reason, new web crippling design equations for unlipped channels are proposed in this report. The proposed design equations are derived based on a simple plastic mechanism model, and the web crippling strength is obtained by dispersing the beating load through the web. The proposed design equations are calibrated with the test results. It is shown that the web crippling strengths predicted by the proposed design equations are generally conservative for unlipped channels with web slenderness values of less than or equal to 45. The reliability of the current design rules and the proposed design equations used in the prediction of web crippling strength of cold-formed channels are evaluated using reliability analysis. The safety indices of the current design rules for different loading conditions are found to be much lower than the target safety index specified in the AISI Specification, while the safety indices of the proposed design equations are generally higher than the target value.
Keywords
Cold-formed channels, Design strength, Plastic mechanism model, Proposed design equations? Reliability analysis, Steel structures, Structural design, Test program, Test strength, Web crippling and Web slenderness.
No. R795
Elzein, A and Booker, JR
Organic Contamination of Stratified Porous Media: Two-dimensional Transport with Non-Equilibrium Partitioning by Boundary Elements
Abstract
Transport of pollutants in soil and groundwater often occurs in stratified media under non-equilibrium conditions. Confined aquifers are usually bounded by low-permeability layers of soil which have been shown to exert a significant influence on the fate of contaminants in groundwater. Numerical solutions of transport equations have usually been restored to single layers and have included single sources of non-equilibrium processes or none at all. The effect of soil stratification itself has sometimes been reduced to a transport-based non-equilibrium process.
A boundary element solution of the transport equations in the Laplace domain is extended to include multiple sources of non-equilibrium processes in saturated media under the assumption of rate-limited mass transfer. Green functions accurately model infinite and semi-infinite domains such as soils and Laplace transforms remove the need for time-stepping and the associated numerical complexity. The proposed numerical technique is validated by comparing its results to analytical solutions. Its scope is illustrated through a case study of a sand aquifer bounded by less permeable layers of silt, and infiltrated by pollutants from a neighbouring lake.
Keywords
Contaminant Transport, Non-Eqnilibrium Sorption, Dispersion-Advection, Boundary Elements, Laplace Transform, Stratified Porous Media.
No. R796
Elzein, A and Booker, JR
Organic Contamination of Stratified Porous Media: Three-dimensional Transport with Non-Equilibrium Partitioning by Boundary Elements
Abstract
Industrial contaminants and landfill leachates, particularly those with high organic content, may migrate into groundwater streams under conditions of non-equilibrium partitioning. These conditions may either be induced by time-dependent sorption onto the soil skeleton and intra-sorbent diffusion in the soil matrix, or by heterogeneous advective fields within the pore. These processes are known as chemical and physical non-equilibrium processes respectively, and may result in significant deviations from the paths predicted by steady-state partitioning assumptions. In addition, multi-directional soil properties, soil stratification and complex geometries of the pollution source may require a full three-dimensional analysis for accurate contamination prediction.
A three-dimensional boundary element solution of the time-dependent diffusive/advective equation in non-homogeneous soils with both physical and chemical non-equilibrium processes is developed. Saturated conditions and rate-limited mass transfer are assumed. The Laplace transform remove the need for time-stepping and the associated numerical complexity, and the use of Green’s functions yields accurate solutions of infinite and semi-infinite domains such as soils as well as media with finite dimensions. The solution requires boundary discretisation only and can therefore be a valuable tool in bio-remediation and landfill design where different geometries, soil properties and pollutant loads may be analysed at low cost. The proposed technique is validated by comparing its predictions to analytical solutions obtained for different types of soil and contaminant sources. The scope of the method is illustrated by analysing the contamination of multi-layered soils by a neighbouring river and a surface source.
Keywords
Contaminant Transport, Non-Equilibfium Sorption, Dispersion-Advection, Boundary Elements, Laplace Transform, Stratified Porous Media.
No. R797
Deng, W and Carter, JP
Predictions of the Vertical Pullout Capacity of Suction Caissons in Fine-grained Soils
Abstract
Suction caissons are being used increasingly for the anchorage of large compliant offshore structures. The response of these caissons to vertical pullout loads is a basic and very important issue in these applications. This paper presents the results of a finite element study into the deformation, failure mechanisms and ultimate capacities developed under vertical uplift. Included in the analyses are investigations of the effects of different loading rates and drainage conditions, the effects of the strength properties of the seabed soil and the influence of the caisson aspect ratio on the pullout capacity. The finite element predictions have been verified by comparing them to experimental results. Simplified methods for the estimation of vertical uplift capacity are developed from the results of the numerical study.
Keywords
Suction caissons, vertical uplift capacity, bottom resistance, finite element analysis.
No. R798
Deng, W and Carter, JP
Analysis of Suction Caissons in Uniform Soils Subjected to Inclined Uplift Loading
Abstract
Suction caissons are being used increasingly for the anchorage of large compliant offshore structures. Uplift capacity of the suction caissons under inclined loading is a critical issue in these applications, and reliable methods of predicting the capacity under this form of loading are required in order to produce reliable designs. An extensive theoretical investigation has been carried out of suction caissons in uniform soils subjected to inclined uplift loading for cases where the behaviour of the seabed soil is undrained. A brief review of previous research on the uplift behaviour of suction caisson under uplift loading is included, and results of finite element modelling are presented. The finite element predictions have been validated by existing rigorous solutions obtained independently using the limit analysis technique from the theory of plasticity. A simplified method for the estimation of the uplift capacity for suction caissons is described, based on the results of the finite element study. The failure mechanisms observed in the FE modelling are also discussed. The expressions developed in this paper take into account the influence of the aspect ratio of the caisson, the point of application and angle of inclination of the loading, and the undrained shear strength of the soil.
Keywords
Suction caissons, inclined uplift loading, uplift capacity, yield locus, finite element analysis, upper bound limit analysis, and uniform soils.