1999 Higher Degree Theses
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DOCTOR OF PHILOSOPHY
Colin Rogers, Behaviour of Thin Sheet Steels
Andrew Wheeler, The Behaviour of Bolted Moment End Plate Connections in Rectangular Hollow Sections Subjected to Flexure
Hossein Taiebat, Three Dimensional Liquefaction Analysis of Offshore Foundations
Xiao-Bo Yu, Fatigue crack propagation under mixed mode loads
MASTERS OF ENGINEERING (Research)
Michel Chaaya, Analysis of Thin-Walled Tanks and Differential Settlement and Internal Pressure
Nicholas Forde, Impact Strength and Safety of Architectural and Structural Glazing
Gemma Heddle, Satellite Remote Sensing of the Ocean
Chivaluck Pongburanakit, Control of Wind-Induced Vibrations by Liquid Column Vibration Absorbers (LCVAs)
Colin Rogers
Behaviour of Thin Sheet Steels
Supervisor: Prof Greg Hancock
Cold formed structural members are fabricated from sheet steels which must meet the material requirements prescribed in applicable national design standards. The Australian / New Zealand standard for cold-formed steel structures (AS/NZS 4600) allows for the use of thin (t < 0.9 mm), high strength (fy = 550 MPa) sheet steels in all structural sections. However, due to the low ductility exhibited by sheet steels that are cold reduced to thickness, engineers are required by standards and specifications to use a yield stress and ultimate strength limited to 75% of the minimum specified values. The American Iron and Steel Institute (AISI) Specification further restricts the use of thin, high strength steels to roofing, siding and floor decking panels. Sheet steels are required to have a minimum elongation capability to ensure that members and connections can undergo small displacements without a loss in structural performance, and to reduce the harmful effects of stress concentrations. The ductility criterion specified in the Australian / New Zealand and North American design standards is based on an investigation of sheet steels by Dhalla and Winter, which did not include the thin, higher strength G550 sheet steels available today.
This thesis details the findings of a research project into the ductile and connection behaviour of G550 sheet steels that range in base metal thickness from 0.40 mm to 1.0 mm. Test specimens were milled from the longitudinal, transverse and diagonal directions of the sheet to determine the degree of anisotropy and its effect on material properties and structural behaviour. Measurements of overall, local and uniform elongation were completed using a fine gauge length grid (2.5 mm c/c) marked on the surface of tensile coupons. Perforated coupon specimens were tested to determine the ability of the G550 sheet steels to redistribute stress concentrations throughout the cross-section. Test results indicate that the ability of G550 sheet steels to undergo deformation is dependent on the direction of load within the material, where transverse specimens exhibit the least amount of overall, local and uniform elongation. The G550 sheet steels included in this thesis do not meet the Dhalla and Winter elongation and ultimate strength to yield stress ratio requirements regardless of direction, except for the uniform elongation of longitudinal test specimens. However, perforated specimens are able to develop the full net section capacity under tensile load.
Single overlap bolted connection specimens loading in shear, that were composed of G550 and G300 sheet steels, all failed in one of three distinct modes; end pull-out, bearing or net section fracture. The current connection provisions set out in the AS/NZS 4600, AISI and Eurocode 3 design standards cannot be used to accurately predict the failure mode of bolted connections that are fabricated from thin G550 and G330 sheet steels. A gradated bearing coefficient method that is dependent on the thickness of the connected materials and the size of the bolt(s) used in the connection was developed; similar to that which is found in the Canadian CSA-S136 design standard. Calculation of the ultimate tensile strength of a bolted connection which fails by fracture of the net section, using the net cross-sectional area and the ultimate material strength, without a stress reduction factor, is accurate and reliable. Bolted connections composed of G550 sheet steels were able to displace to at least 90% of the distance measured for the nominally identical G300 test specimens; which is an indication that G550 sheet steel connections possess adequate ductility. The behaviour that is associated with the bearing and net section failure of bolted connections that are loaded in shear is also detailed. Recommendations concerning the procedure used to identify the net section fracture and bearing failure modes are made. In addition, a detailed discussion of the test data used in the development of the current AS/NZS 4600 and AISI design equations for net section fracture at connections is completed.
The G550 and G300 sheet steel screwed connection tests that were completed for this thesis and by the Australian Commonwealth Scientific and Industrial Research Organisation (CSIRO) were combined to provide a comprehensive listing of available data. The current connection provisions set out in the CSA-S136 and Eurocode 3 design standards can be used to predict the bearing/tilting failure mode of screwed connections that are fabricated from thin G550 and G300 sheet steels. The AS/NZS 4600 and AISI design standards can also be used to predict the bearing/tilting failure mode of screwed connections if the stress reduction factor provision is removed from the net section fracture formulation. The design standards provide accurate load predictions of the capacity of screwed connections when the two connected sheet steels are of a similar thickness. However, when the two connected sheet steels are of a different thickness the calculated connection capacity becomes unconservative. A method of analysis for screwed connections loaded in shear that can be used to improve the accuracy of the predicted load resistance when two different thickness sheet steels are joined is proposed.
This thesis also reports on the fracture properties of G550 sheet steels loaded in tension. Descriptions of failure surfaces which were observed through a scanning electron microscope are provided. The fracture resistance of G550 sheet steels is measured for a range of temperatures and a numerical study of the effect of cracks on structural performance in the elastic load range is completed using the FRANC2D finite element computer program.
Andrew Wheeler
The Behaviour of Bolted Moment End Plate Connections in Rectangular Hollow
Sections Subjected to Flexure
Supervisor: Dr Murray Clarke
The main objective of this thesis is to investigate the behaviour of bolted moment end plate connections joining square (SHS) and rectangular (RHS) hollow sections subjected to pure flexural loading. The studies carried out into this type of connection include experimental, theoretical and numerical and design aspects.
An extensive test programme into the behaviour of two types of bolted end plate connections was carried out. The experimental tests were performed on beam splice connections located at the midspan of a beam subjected to four point bending. The four-point bending arrangement ensured that the connection was subjected to flexural loading only.
The two types of connections tested were classified on the basis of the number of bolts used. The first type of connection contained eight bolts located symmetrically around the tubular section, while the second contained a row of bolts above and below the tubular section. For both types of connection, the effects of the end plate thickness and the position of the bolts relative to the section flanges and webs were investigated. These investigations revealed that the stiffness and strength of the connection were dependant on the strength of the bolts and the relative stiffness of the end plate.
For I-section moment end plate connections subjected to bending, the stub-tee analogy is generally used to determine the ultimate capacity of the connection, considering both the prying of the bolts and the formation of a plastic mechanism in the end plate. The model developed in this thesis for rectangular hollow sections utilises the principles of the stub-tee analogy and the yield line analysis, but is modified to consider the alternate mechanisms that form in the end plate as a result of the webs being a significant part of the rectangular hollow section. The predictions from the model are compared against the experimentally derived results and some additional results of similar connections tested by other authors.
The numerical analysis of bolted moment end plate connections joining tubular members utilises a commercially available finite element package and models the connections using three-dimensional solid elements. Both geometric and material non-linearities are included in the analyses. Each component contained in the connection is modelled as a separate identity and verification tests carried out to ensure each component behaves in the correct manner. The numerically predicted behaviour for each connection is compared with the experimental results. Of particular interest is the moment-rotation response of the connection and the forces induced in the bolts.
The design procedures from Annex J of Eurocode 3 are reviewed and compared with the tests results. New design procedures for both the four-bolt and eight-bolt connections for the strength and serviceability limit states are also presented, together with fully worked design examples.
Hossein Taiebat
Three Dimensional Liquefaction Analysis of Offshore Foundations
Supervisor: Prof John Carter
This thesis presents numerical techniques which have been developed to analyse three-dimensional problems in offshore engineering. In particular, the three-dimensional liquefaction analysis of offshore foundations on granular soils is the main subject of the thesis.
The subject matter is broadly divided into four sections:
1) Development of an efficient method for the three dimensional elasto-plastic
finite element analysis of consolidating soil through the use of a discrete
Fourier representation of field quantities.
2) Validation of the three dimensional method through analyses of shallow
offshore foundations subjected to three dimensional loading and
investigation of the yield locus for foundations on purely cohesive soils.
3) Formulation of governing equations suitable for three dimensional
liquefaction analyses of offshore foundations founded on granular soil,
presentation of a method for liquefaction analyses, and application of the
method in modified elastic liquefaction analyses of offshore foundations.
4) Application of a conventional elasto-plastic soil model in the
liquefaction analyses of offshore foundations using the three-dimensional
finite element method.
The finite element method developed in this thesis provides a rigorous and efficient numerical tool for the analysis of geotechnical problems subjected to three-dimensional loading. The efficiency of the numerical tool makes it possible to tackle some of the problems in geotechnical engineering which would otherwise need enormous computing time and thus would be impractical. The accuracy of the numerical scheme is demonstrated by solving the bearing capacity problem of shallow foundations subjected to three-dimensional loading. The generalized governing equations and the numerical method for liquefaction analyses presented in this thesis provide a solid base for the analysis of offshore foundations subjected to cyclic wave loading where they are founded on potentially liquefiable soil. The practicability of the numerical scheme is also demonstrated by a modified elastic liquefaction analysis of offshore foundations. The liquefaction phenomenon is redefined in the context of the conventional Mohr-Coulomb model, so that a relatively simple and practical model for elasto-plastic liquefaction analysis is presented.
The three-dimensional finite element method together with the numerical scheme for liquefaction analysis and the elasto-plastic soil model provide a suitable practical engineering tool for exploring the responses of offshore foundations subjected to cyclic wave loading.
Xiao-Bo Yu
Fatigue crack propagation under mixed mode loads
Supervisor: A/Prof Andrew Abel
The study presented in this thesis is focused on the intrinsic and extrinsic mechanisms of fatigue crack propagation under mixed mode loading conditions. Detailed review is provided, covering the historical aspects of fatigue research, the fracture mechanical approach to fatigue, fatigue crack growth behaviour, fatigue crack path prediction, fatigue crack growth micromechanism, and modelling and measurement of crack surface interference.
The thesis work involves: a) experimental study of fatigue crack growth behaviour, b) modelling analysis of crack surface interference (CSI), and c) experimental study of CSI. The experimental study was performed on notched thin-walled tubular specimens of mild steel.
The crack growth behaviour was investigated under three load conditions: i) cyclic mode I plus steady or low frequency mode II, ii) cyclic mode II plus steady mode I, and iii) cyclic mode I plus cyclic mode II with various loading paths. The crack growth direction, rate, and surface topography are evaluated for the above conditions. It is concluded that a fatigue crack in steel may propagate in a shear mode, in addition to the conventionally recognised opening mode. The shear mode propagation is promoted by non-proportional mixed loads. It is stable, associated with small scale yielding, and can be up to 6 times faster than an equivalent mode I propagation. In addition, sliding movement along the asperity facets is proposed, as a supplementary to the conventional roughness-induced crack closure mechanism.
The modelling analysis of CSI provided a quantitative description of the proposed mechanism of sliding movement along slanted asperity facets. The effects of plasticity/oxidation crack surface build-ups and unsynchronised CSI development were incorporated in the analysis. Two load conditions were considered: cyclic mode I plus steady mode II, and cyclic mode II plus steady mode I. The modelling predictions were related to the observations of fatigue crack growth behaviour.
The experimental study of CSI was designed to provide verification or otherwise of the validity of the proposed sliding movement mechanism. CSI was measured by near-tip strain gauges and a new method was developed for the uncoupling of mode I and mode II CSI from the raw strain readings. Two load conditions were tested and the experimental results agreed well with the modelling prediction.
The work described above leads to two significant conclusions in relation to mixed mode fatigue. The first lies in the intrinsic mechanism: it is strongly suggested that shear mode propagation in steel can take place in a stable manner and it is promoted by non-proportional mixed mode loads. The second is related to the extrinsic mechanism: it is concluded that sliding movement along asperity facets is possible and therefore the CSI-induced cyclic wedge opening and mode II stress intensities should also be considered in addition to the conventionally recognised shear attenuation and mode I crack closure.
Michel Chaaya
Analysis of Thin-Walled Tanks and Differential Settlement and Internal
Pressure
Supervisor: A/Prof Peter Ansourian
The objective of this thesis is to present the results of
an investigation into the behaviour of thin-walled tanks under differential
settlement and internal pressure.
The linear buckling of closed-top steel cylindrical shells under vertical
edge deformation (differential settlement) has been studied, followed by
non-linear analysis under combined internal pressure and vertical harmonic
settlement at n=2.
For this purpose, a finite element model has been established with a range
of values of r/t and l/r. Linear buckling solutions have been used to
calculate the maximum meridional stress at the base and to identify the
dominating eigenmode. A non-linear least squares approximation to the
computed buckling stresses has been derived, and a general expression of
maximum meridional membrane stress at the base has been given.
Since the mode of buckling was one of shear, the shear stresses relative to
the buckling mode were extracted and compared with the buckling shears
stresses of the cylinder under pure torsion.
In non-linear analysis, due to the significant effect of the
pressure/settlement ratio, a range of pressures has been considered to
determine the final nominal hoop tensile stresses near the base. The results
of non-linear analysis in upward and downward settlement zones have been
presented and compared with the classical buckling stresses of axially
compressed perfect cylinders.
Chivaluck Pongburanakit
Control of Wind-Induced Vibrations by Liquid Column Vibration Absorbers (LCVAs)
Supervisor: A/Prof Kenny Kwok
TThis thesis presents an experimental study of geometric
properties on the fundamental natural frequency and liquid damping ratio of
bi-directional Liquid Column Vibration Absorbers (LCVA) using free-vibration
experiments. Natural frequency of liquid oscillation of a LCVA was found to
be dependent on its geometric configuration and gravitational acceleration.
LCVA liquid damping ratio was found to be dependent on the amplitude of
liquid motion in the vertical columns, properties of the LCVA liquid, and
the geometric scale.
Empirical expressions to estimate LCVA natural frequency and liquid damping
ratio are proposed. These proposed empirical formulae were obtained from
free-vibration experimental results for various square-based bi-directional
LCVAs.
The effectiveness of a rectangular-based bi-directional LCVA, with a ratio
of horizontal column length of 2:1, was investigated by frequency-sweep
experiment along two orthogonal axes. The experimental results showed that
the rectangular based bi-directional LCVA can suppress vibration of a
pendulum structure along two orthogonal axes, and its effectiveness is not
diminished when subjected to simultaneous orthogonal bi-directional
excitations.
Many researchers reported a number of advantages of multiple tuned vibration
absorbers over single tuned vibration absorbers. Results of experiments
conducted on nested bi-directional Multiple Liquid Column Vibration
Absorbers (MLCVA) configuration are presented. Their effectiveness in
reducing the displacement of a pendulum structure subjected to sinusoidal
excitation was investigated by frequency sweep experiments for
uni-directional excitation, simultaneous orthogonal bi-directional
excitations, and for excitation at different orientations to the principal
axes of a LCVA. The effectiveness of a nested, square-based bi-directional
NILCVA is not diminished due to simultaneous bi-directional excitations or
directions of excitations, and is essentially the same as when excited along
one axis only.