No. R736, January, 1997
Wheeler, AT, Clarke, MJ and Hancock, GJ
Bending Tests of Bolted End Plate Connections in Cold Formed Rectangular Hollow Sections
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Abstract
The research report describes an experimental investigation into the flexural behaviour of bolted end plate connections joining cold formed square and rectangular hollow sections. A total of twenty six (26) specimens, joined at midspan using a bolted end plate connection then loaded under four point bending, were tested in a servo-controlled testing machine. The primary aim of the investigation was to evaluate the effect of various end plate parameters on the connection strength and serviceability. The results of the bending tests are tabulated in the report, and graphs detailing the behaviour of each specimen are also presented. The experimental data will facilitate the development and verification of design models for the bolted moment end plate connection joining rectangular section tubes, and will provide data on the non-linear moment rotation characteristics of the connection.
No. R737
Pi, YL and Trahair, NS
Out-Of-Plane Inelastic Buckling and Strength of Steel Arches
Abstract
An arch loaded in-plane may suddenly deflect laterally and twist out of the plane of loading and fail in a flexural-torsional buckling mode. This paper investigates the out-of-plane inelastic flexural-torsional buckling and strength of steel arches using a finite element model in a three-dimensional nonlinear inelastic analysis. The three-dimensional elastic-plastic behaviour of an arch is determined by taking into account the effects of the in-plane curvature of the arch, subtended angle, large deformations, material inelasticity, initial crookedness and residual stresses. The strengths of steel arches in uniform compression and in uniform bending are studied. It is found that the design rules for steel columns and beams cannot be used directly for the design of steel arches. Design rules for steel arches in uniform compression and in uniform bending are developed based on the results of the finite element analyses. The effects of initial crookedness and twist, residual stresses, and yield stress on the strengths of arches are also investigated.
No. R738
Thorne, CP
Drag Anchor Capacity
Abstract
Marine drag anchors have developed significantly over the last few decades and are capable of providing anchorage loads of in excess of 1000 tonnes. In spite of this progress, prediction of the load capacity of such anchors has remained almost entirely empirical. This paper presents a method of analysis of the penetration depth and load capacity of drag anchors in normally consolidated clays based on geotechnical principles. First, equations are developed to represent the equilibrium forces acting on an anchor for a particular depth below the seabed and anchor orientation. These are based on the proposition that no movement will occur until the soil forces acting parallel to the shank are overcome. Predictions made using these equations show good agreement with full scale results. A theory is then explained which models the movement of the anchor in the soil as the chain or cable is tensioned. Examples of application of this theory show good agreement with six full scale tests covering two different sites and four different anchor types.
No. R739
O’Shea, MD and Bridge, RQ
Behaviour of Thin-Walled Box Sections With Lateral Restraint
Abstract
This report presents the results from an experimental study performed to investigate the behaviour of thin-walled box sections with internal lateral restraint. Two series of tests were conducted examining the influence of the width to thickness ratio and the length to thickness ratio on the buckling strength of the tube. The material properties, residual stresses and geometric imperfections were carefully measured. Three different axial compression tests were conducted which included: steel loaded only; steel loaded with internal concrete restraint; steel and concrete loaded together. In each test, load, axial shortening and lateral deformation was recorded.
It was found that the use of unbonded concrete infill in square tubes provides restraint to local buckling which can enhance the strength when compared to the bare steel tube. Current design standards can provide an estimate of the buckling strength with both conservative and unconservative results depending upon the standard used. A conservative estimate of the local buckling strength can be obtained using the proposed design method.
No. R740
O’Shea, MD and Bridge, RQ
Local Buckling of Thin-Walled Circular Steel Sections With Or Without Internal Restraint
Abstract
In this report a comprehensive series of tests are described and presented. The tests were performed to examine the behaviour of short thin-walled circular steel tubes with or without internal restraint. The tubes had diameter to thickness ratios of between 55 and 200 and a length to diameter ratio of 3.5. The tests included bare steel tubes loaded both axially and at small eccentricities, and axially loaded steel tubes with an internal restraint medium. The material properties have been measured including residual stresses and geometric imperfections. The test strengths have been compared to strength models in design standards and specifications. The design standards were found to be conservative for eccentrically loaded circular steel tubes especially those using a linear interaction between the axial load and moment.
No. R741
Fernando, NSM and Carter, JP
Elastic Analysis of Buried Pipes Under Surface Patch Loadings
Abstract
A parametric study was undertaken to assess the behaviour of buried pipes under uniformly distributed vertical patch loads applied to the surface of a layer of soil. Predictions of the maximum circumferential and longitudinal bending moments and hoop thrusts acting on the buried pipes are presented in the form of non-dimensionalised design charts.
Fourier transforms were used to model the three-dimensional behaviour under the patch loading and conventional finite element analysis was used to approximate the field quantities in the two-dimensional transform plane. The soil has been idealised as a homogeneous linear elastic continuum and the buried pipe has been represented by a linear elastic plate bending element capable of transmitting axial thrusts.
The results of the parametric study are relevant to the design of buried pipes under the action of vertical live loads applied to the surface of a soil layer.
No. R742
Fernando, NSM and Carter, JP
Elastic Analysis of Buried Box Culverts Under Surface Patch Loadings
Abstract
A parametric study was undertaken to assess the behaviour of buried single box culverts under uniformly distributed vertical patch loads applied to the surface of a layer of soil. Predictions of the maximum in-plane and out-of-plane bending moments acting on the buried culverts are presented in the form of non-dimensionalised design charts.
Fourier transforms were used to model the three-dimensional behaviour under the patch loading and conventional finite element analysis was used to approximate the field quantities in the two-dimensional transform plane. The soil has been idealised as a homogeneous linear elastic continuum and the buried culvert has been represented by a linear elastic plate bending element capable of transmitting axial thrusts.
The results of the parametric study are relevant to the design of buried box culverts under the action of vertical live loads applied to the surface of a soil layer.
No. R743
Fernando, NSM and Carter, JP
Validation of the Fourier Integral Technique for Predicting the Effects of Live Loading On Reinforced Concrete Box Culverts
Abstract
In this paper the Fourier integral technique combined with finite element method has been used to predict the stresses, moments, forces and displacements of a buried culvert due to the effect of wheel loads. The formulation has been encoded in a computer program, AFENA, and the computational method has been verified using experimental data from a full-scale load test. The proposed method is capable of assessing both the stresses induced by the wheel loads as well as their effects on the culvert, and there is shown to be excellent correlation with the field data.
The soil has been idealised as a homogeneous linear elastic continuum and the buried culvert has been represented by linear elastic plate bending elements capable of transmitting axial thrusts. Fourier transforms were used to model the three-dimensional behaviour under the surface patch loading and conventional finite element analysis was used to approximate the field quantities in the two-dimensional transform plane.A comparison has also been made between the predictions of the Fourier integral method and those of a more conventional plane strain analysis, in which the vehicle loading is represented as an equivalent strip load applied to the soil surface.
No. R744, June, 1997
Wilkinson , T and Hancock, GJ
Tests for the Compact Web Slenderness of Cold-Formed Rectangular Hollow Sections
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Abstract
This report describes a series of bending tests to examine the influence of web slenderness on the rotation capacity of cold-formed rectangular hollow sections (RHS) for use in plastic design. The results indicate that the plastic web slenderness limits in design standards, which are based on tests of I-sections, are not conservative for RHS. There is considerable interaction between the webs and flange which influences the rotation capacity which is shown by approximate isorotation curves. A proposal for a linear interaction formula between the web and flange slendernesses for the Compact limits of RHS is given.
No. R745, June, 1997
Wheeler, AT, Clarke, MT, Hancock, GJ and Murray, TM
Design Model for Bolted Moment End Plate Connections Using Rectangular Hollow Sections
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Abstract
The report presents a model for the determination of the ultimate moment capacity of bolted moment end plate connections utilising rectangular hollow sections and two rows of bolts. The model considers the combined effects of prying action due to flexible end plates and the formation of yield lines in the end plates, enabling the design of the connection with a row of bolts above and below the section.
The model has been calibrated and validated using experimental data from an associated test program. The design model constitutes a relatively simple method for predicting the ultimate moment capacity for the particular type bolted moment end plate connection described herein.
No. R746
Hasham, AS and Rasmussen, KJR
Member Capacity of Thin-Walled I-Sections in Combined Compression and Major Axis Bending
Abstract
This report describes a series of tests performed on thin-walled I-sections fabricated from high strength steel plates by welding. The objective of the test program was to determine the member capacity for I-section beam-columns and investigate member behaviour with interaction of local and flexural-torsional out-of-plane buckling of three different lengths when subjected to compression and major axis bending. The cross-section had slender flanges and web which underwent local buckling.
The tests were conducted in a purpose-built test rig capable of applying axial load and bending moment in a constant ratio. The tests were performed using different axial load to moment ratios, as well as pure axial load and pure moment, hence allowing the moment-axial force interaction curve to be constructed for the three lengths.
Detailed measurements of material properties, residual stresses and geometric imperfections were performed prior to the tests. The test results are compared with the rules of the Australian Steel Structures Standard AS4100 [1], the American Institute of Steel Construction Load and Resistance Factor Design Specification [2], and Eurocode 3: Part 1.1 [3].
No. R747
Liu, MD and Carter, JP
Virgin Compression of Structured Soils
Abstract
This paper describes a study of the virgin compression behaviour of both naturally and artificially structured soils under one-dimensional compression or isotropic compression. It is proposed that during virgin compression, the additional voids ratio sustained by soil structure is inversely proportional to the current mean effective stress. The proposed formula has been verified for twenty different naturally and artificially structured soils, and it is seen that the proposal describes well the behaviour of all these soils. Finally, a general discussion on soil structure and its features is presented, and the proposed compression equation is extended to describe the compression of structured soil along a general stress path.
Keywords: clays, structure of soil, compressibility.
No. R748
Pi, YL, Put, BM and Trahair, TS
Lateral Buckling Strengths of Cold-Formed Channel Section Beams
Abstract
This paper is concerned with the inelastic lateral buckling strengths of cold-formed channel section (CFC) beams. The monosymmetrical nature of the cross-sections of these beams introduces characteristics that are not encountered in I-section beams. Firstly, the effective section becomes asymmetrical after yielding, so that a CFC beam under in-plane bending about the major axis is subjected to bending about the minor axis and twisting, unless it is restrained at frequent intervals. Secondly, the distributions of the minor axis bending strains and the warping strains are related to the twist rotation and minor axis deflection directions, so that the strengths of CFC beams are affected not only by the magnitudes of the initial crookedness and twist, but also by their directions. The cold-formed nature of CFC beams also introduces some considerations that are not encountered in hot-rolled I-section beams, as the stress-strain curves, residual stresses, initial imperfections, and lipped flanges are all different. When a CFC beam has a very slender web, distortion may reduce the resistance of the beam to flexural-torsional buckling. For all these reasons, code rules for designing steel beams against lateral buckling which are based on data for hot-rolled I-section beams may be unsuitable for CFC beams.This paper develops a realistic finite element model for the analysis of CFC beams which accounts for the effects of web distortion and the asymmetrical nature of the effective section after yielding. The model is used to investigate the elastic lateral-distortional buckling, inelastic behaviour, and strengths of CFC beams with residual stresses and initial imperfections. The results of the study are used to develop improved design rules which are suitable for CFC beams. The effects of moment distribution and load height on the lateral buckling strengths are also studied.
No. R749
Rogers, CA and Hancock, GJ
Bolted Connection Tests of Thin G550 and G300 Sheet Steels
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Abstract
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 (SA/SNZ, 1996)) allows for the use of thin (t < 0.9mm), high strength (fy = 550MPa) sheet steels in all structural sections. However, due to the low ductility exhibited by sheet steels which are cold reduced to thickness the engineer must use a yield stress and ultimate strength reduced to 75% of the minimum specified values. The American Iron and Steel Institute (AISI) Design Specification further limits 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 high strength G550 steels available today.
A previous research report entitled Ductility of G550 Sheet Steels in Tension – Elongation Measurements and Perforated Tests (No. R735) detailed the basic material behaviour of G550 sheet steels. It was concluded 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. Furthermore, the G550 sheet steels tested for this project do not meet the Dhalla and Winter material requirements regardless of direction, except for the uniform elongation of longitudinal coupon specimens.
This report details the findings of bolted connection tests using G550 and G300 sheet steels which range in base metal thickness from 0.42 to 0.60mm. Test specimens were milled from the longitudinal, transverse and diagonal directions of the sheet to determine the degree of anisotropy and its effect on connection capacity and failure type. All specimens failed in one of three distinct modes; end pull-out, bearing or net section fracture.
The results of tests completed for this report indicate that 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 or resistance of bolted connections fabricated from thin G550 and G300 sheet steels. It is necessary to incorporate a variable bearing resistance equation which is dependent on the thickness of the connected material, similar to that found in the Canadian CSA-S136 Design Standard. Calculation of the ultimate tensile strength of a bolted connection 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 indicates that adequate ductility exists in the tested G550 sheet steels.
No. R750
Hasham, AS and Rasmussen, KJR
Nonlinear Analysis of Locally Buckled I-Section Beam-Columns
Abstract
The objective of this report is to compare test results with results obtained using a nonlinear finite element analysis. The report aims at validating the use of a non-linear analysis in predicting the behaviour of thin-walled beam-columns subjected to compression and major axis bending.The test program was described in two previous research reports by the authors. It involved tests on slender I-sections fabricated from high strength steel plates by welding. The objective of the test program was to determine the section capacity of two different I-sections when subjected to compression and major axis bending, and the member capacity of two different lengths of a slender I-section. The tests showed that the interaction curve may be convex for thin-walled
I-sections subjected to combined compression and major axis bending.
As shown in the present report, the observed test behaviour could be closely simulated using a nonlinear finite element analysis. The finite element analysis included the effects of material and geometric nonlinearities, residual stresses and the interaction of local and overall buckling modes. As part of developing the finite element model, a study was conducted to quantify the sensitivity of the results to changes in material properties and residual stresses. Furthermore, different models were investigated for including geometric imperfections in the finite element analysis.
Having validated the use of the nonlinear finite element analysis to predict the behaviour of fabricated thin-walled I-section beam-columns subjected to compression and major axis bending, the analysis was used to determine the maximum load and moment capacities, as well as the corresponding maximum second-order elastic moments. The interaction curves resulting from this numerical investigation were compared with the test interaction curves.
No. R751
Xu, KJ, Liu, MD, Carter, JP and Poulos. HG
Explicit Stress-Strain Equations for Geo-Materials
Abstract
General and explicit stress-strain equations are proposed, providing a complete stress-strain model for many geo-materials. The model contains only four parameters and the constitutive equations are valid for monotonic shearing. Predictions are made using the proposed equations for the stress-strain behaviour, the peak strength, and the distortional moduli for geo-materials such as clays, sands, clay-silt mixtures, compacted fill materials, and soft rocks. It was found that the proposed equations describe very satisfactorily the behaviour of many types of soils and soft rocks.
No. R754
O’Shea, MD and Bridge,RQ
Tests On Circular Thin-Walled Steel Tubes Filled With Very High Strength Concrete
Abstract
This report presents the results from an experimental study performed to investigate the behaviour of short circular thin-walled steel tubes filled with very high strength concrete. Five different circular steel tubes were examined with a diameter to wall thickness ratio ranging from fully effective to slender where local buckling of the steel tube dominates the behaviour. A nominal concrete strength of 100 MPa was used to fill these steel tubes. The material properties, residual stresses and geometric imperfections were carefully measured. Three different axial compression ‘tests were conducted which included: concentrically loading the steel and concrete simultaneously; concentrically loading only the concrete (steel unbonded and used only as confinement) and eccentrically loading the steel and concrete simultaneously at eccentricities of D/20 and D/10, where D is the external diameter of the steel tube.The ductility of the specimens was found to improve for increasing wall thickness and/or eccentricity of loading. However enhancement of the concrete strength by the steel tubes only occurred for concrete only loaded axial tests. Even for these the full confining action of the steel tube was unable to be mobilised due to the limited expansion of the very high strength concrete. From the tests conducted on concentrically loaded concrete filled steel tubes with the steel and concrete loaded simultaneously two different failure modes for the steel tube were observed. In the first type, bond between the steel tube and the internal concrete is maintained with the axial strength of the steel tube calculated from the full plastic capacity (with a reduction for the influence of tensile hoop stresses resulting from confinement). For the second type, bond between the steel and the concrete is not maintained with local buckling of the steel tube occurring. Although this reduced the axial capacity of the steel tube, enhanced confinement of the concrete can occur allowing a greater overall section strength.
The ultimate strength of the eccentrically loaded specimens was conservatively predicted using a cross-sectional analysis program using the stress-strain response of the steel and concrete. Thus for thin-walled steel tubes filled with very high strength concrete, the interaction curve can be derived once the stress-strain curves of the materials are known.
No. R755
O’Shea, MD and Bridge,RQ
Tests On Circular Thin-Walled Steel Tubes Filled With Medium and High Strength Concrete
Abstract
This report presents the results from an experimental study performed to investigate the behaviour of short circular thin-walled steel tubes filled with medium and high strength concrete. Five different circular steel tubes were examined with a diameter to thickness ratio ranging from fully effective to slender where local buckling of the steel tube dominates the behaviour. Two nominal concrete strengths of 50 MPa and 80 MPa were used to fill these steel tubes. The material properties, residual stresses and geometric imperfections were carefully measured. Three different axial compression tests were conducted which included: concentrically loading the steel and concrete simultaneously; concentrically loading only the concrete (steel unbonded and only used as confinement) and eccentrically loading the steel and concrete simultaneously at eccentricities of D/20 and D/10, where D is the external diameter of the steel tube.
Enhancement of concrete by the steel tube was found to occur for all concentrically loaded specimens filled with medium strength concrete. However for concentrically loaded specimens filled with high strength concrete, confinement of the internal concrete only occurred for the thicker tubes. In all cases, loading only the concrete resulted in a greater enhancement of the concrete strength and higher overall ultimate strength than loading the steel and concrete simultaneously. From the tests conducted on concentrically loaded concrete filled steel tubes with the steel and concrete loaded simultaneously two different failure modes for the steel tube were observed. In the first type, bond between the steel tube and the internal concrete is maintained with the axial strength of the steel tube calculated from the full plastic capacity (with a reduction for the influence of tensile hoop stresses resulting from confinement). For the second type, bond between the steel and the concrete is not maintained with local buckling of the steel tube occurring. Although this reduces the axial capacity of the steel tube, enhanced confinement of the concrete can occur allowing a greater overall section strength.
At small eccentricities the response of the specimens can be accurately predicted using the confined concrete stress-strain curve (obtained from concentric axial tests) while at high eccentricities a conservative results can be obtained from the unconfined concrete response.
Both thick and thin-walled steel tubes were found to improve the ultimate strength and ductility of the medium strength concrete under concentric axial loading and at small applied eccentricities. For high strength concrete, confinement effects resulting in improved strength and ductility only occurred for the thicker tubes under concentric axial loading and at small applied eccentricities.
No. R756
Wang, JC and Booker, JR
A Laplace Transform Finite Element Method (LTFEM) for the Analysis of Contaminant Transport in Porous Media
Abstract
This paper develops a finite element method for the analysis of contaminant transport in porous media. The method does not use the conventional time marching approach which tends to induce artificial oscillations in the solution but transforms the equations by taking a Laplace transform with respect to time. The Laplace transform removes the time derivative and consequently the transformed equation contains only spatial derivatives. The transformed equation is then approximated using the finite element method and solved to give the values of the nodal variables in Laplace transform space. The actual solution in the time domain is found by numerical inversion of the Laplace transform. Other investigators have used a variety of methods which require the use of complex values of the Laplace transform parameter to do this. This necessitates the use of complex arithmetic thus involving additional storage and an increase in computational cost. In this paper the disadvantages of using complex arithmetic in the analysis of solute transport are overcome by using a numerical Laplace transform inversion technique due to Stehfest (1970) which uses only real arithmetic and thus provides a significant reduction in storage together with an increased efficiency of computation. The method is validated against a number of closed form solutions and used to investigate the effectiveness of contaminant remediation using an extraction well.
Keywords: contaminant transport, Laplace transform, Finite element, Stehfest algorithm.
No. R757
Tabesh, A and Poulos, HG
A Review of Dynamic Analysis of Pile Foundations
Abstract
The main aim of this research report is to present a review of some of the outstanding works reported in the literature on the dynamic analysis of pile foundations with an emphasis on pile seismic response. This review introduces the reader to the problems involved in the dynamic analysis of pile foundations, and to some methods developed for their treatment. A number of these methods have been discussed in detail, while some others have been mentioned briefly.
This review is by no means comprehensive, both because of the sheer number and size of the publications available, and due to the availability of a state of the art report on the subject by Novak (1990). The emphasis has been on the more recent publications, although older works have also been reviewed.
The need for publication of this report was felt warranted partly due to rapid developments in recent years in the field of dynamic pile response analysis and design, and partly because the available publications are rather scattered and have different emphases on various aspects of the problem.
The thrust of the report is on the concepts behind different available methods and their formulation, although some of the results obtained from some of the methods have also been discussed.
No. R758
O’Shea, MD and Bridge, RQ
Design of Thin-Walled Concrete Filled Steel Tubes
Abstract
This report presents several design methods which can be used to conservatively estimate the strength of thin-walled steel tubes filled with medium to very high strength concrete under different loading conditions. The loading conditions examined include: axial loading of the steel only; axial loading of the concrete only and simultaneous loading of the concrete and steel both axially and at small eccentricities. The methods have been compared to test results recently completed at the University of Sydney.The strength of untilled steel tubes has been found to significantly affected by local buckling. For square tubes, unbonded concrete infill restrains the formation of local buckles and forces a higher buckling mode to be obtained while for circular steel tubes, the predominantly outwards buckle is unaffected. However both box and circular tubes can be designed using modifications to current codes.For circular concrete filled steel tubes with only the concrete loaded, equations have been developed to predict the ultimate strength and stress strain response of the confined concrete under axial load. For circular concrete filled steel tubes with the concrete and steel loaded simultaneously, local buckling of the steel tube does not occur if there is sufficient bond between the steel and concrete and the member is subjected to short term static loads. The existing provisions in Eurocode 4 (1992) have been found to accurately estimate the confined strength of axially loaded specimens with concrete strengths up to 80 MPa while beyond this negligible confinement of the concrete occurs. For eccentrically loaded specimens, the provisions in Eurocode 4 (1992) can be used for concrete strengths up to and including 50 MPa with no reduction for local buckling. For concrete strengths beyond this the simplifying assumptions of a fully plastic section are not valid. Instead a rigorous analysis of the cross-section should be performed using the actual material stress-strain curves.
No. R759
Young, B and Rasmussen, KJR
Design of Cold-Formed Singly Symmetric Compression Members
Abstract
This report presents a comparison of test strengths of cold-formed plain and lipped channels with the design rules of the Australian/New Zealand, American (AISI) and European specifications for cold-formed steel structures. The tests were previously reported by the authors, comprising different geometries of cross-section fabricated by brake-pressing from high strength zinc-coated Grade G450 (nominal yield stress of 450MPa) structural steel sheet. Tests were performed over a range of lengths such that the complete column curves were obtained for four cross-sections. The failure modes involved pure local buckling and distortional buckling as well as overall flexural buckling and flexural-torsional buckling. Both pin-ended and fixed-ended tests were performed.
The comparison with the Australian/New Zealand, American and European specifications for cold-formed steel structures showed that a fixed-ended channel column can be designed using an effective length of half of the column length and the applied force can be assumed to act at the centroid of the effective section. This result complies with the conclusion drawn previously by the authors that the shift of the effective centroid does not induce overall bending in a fixed-ended channel column.
The comparison also showed that the current design rules lead to very conservative predictions for pin-ended columns. The shift of the effective centroid, as predicted by the Australian/New Zealand and American specifications, was shown to be inaccurate compared to the tests for the lipped channel with slender flanges. This led to the abnormal result of an increase in column strength with increasing length.
No. R760
Young, B and Rasmussen, KJR
Bifurcation of Locally Buckled Channel Columns
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Abstract
The report derives the governing equations for the fundamental and bifurcated states of members with singly symmetric cross-sections that locally buckle in the fundamental state. The members are subject to pure compression and assumed to be geometrically perfect in the overall sense. The method of analysis uses elastic and inelastic geometric nonlinear finite strip buckling analyses to determine the tangent rigidities of the locally buckled section. These rigidities are substituted into the equations for overall flexural and flexural-torsional buckling to obtain the bifurcation loads. The effect of yielding is highlighted through a parametric study. The overall bifurcation loads are compared with a series of tests of fixed-ended plain channel columns. The variation of the bifurcation loads with the length is shown to be in good agreement with the tests. The bifurcation loads are shown to be sensitive to the magnitudes of local and overall geometric imperfections.
No. R761
Rogers, CA and Hancock, GJ
Screwed Connection Tests of Thin G550 and G300 Sheet Steels
Full Report in PDF
Abstract
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 Design Standard for cold formed steel structures (AS/NZS 4600) allows for the use of thin
(t < 0.9mm), high strength (fy = 550MPa) sheet steels in all structural sections. However, due to the low ductility exhibited by sheet steels which are cold reduced to thickness the engineer must use a yield stress and ultimate strength limited to 75% of the minimum specified values. The American Iron and Steel Institute (AISI) Design Specification further limits 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 high strength G550 steels available today.
A previous research report entitled Ductility of G550 Sheet Steels in Tension – Elongation Measurements and Perforated Tests (No. R735) detailed the basic material behaviour of G550 sheet steels. It was concluded 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. Furthermore, the G550 sheet steels tested for this project do not meet the Dhalla and Winter material requirements regardless of direction, except for the uniform elongation of longitudinal coupon specimens. An additional research report entitled Bolted Connection Tests of Thin G550 and G300 Sheet Steels (No. R749) concluded that a modification of the bearing coefficient provisions for thin G550 and G300 sheet steels is necessary to account for the reduced bearing resistance of the connected materials.
This document reports on the testing of single overlap screwed connections concentrically loaded in shear, and fabricated with multiple point fasteners using G550 and G300 sheet steels which range in base metal thickness from 0.42-1.0mm. Test specimens were milled from the longitudinal, transverse and diagonal directions of the sheet to determine the degree of anisotropy and its effect on connection capacity and failure type. All specimens tested for this report failed in the combined bearing/tilting mode. The results of additional screwed connection specimens, which were mainly composed of single point fasteners, and tested by the Australian Commonwealth Scientific and Industrial Research Organisation (CSIRO) Division of Building, Construction and Engineering, were also included as data for this report.
The results of all tests included in this report indicate that 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 fabricated from thin G550 and G300 sheet steels. The AS/NZS 4600 and AISI Design Standards could be used to predict the bearing/tilting failure mode of screwed connections if the stress reduction factor provision was removed from the net section fracture formulation. The Eurocode 3 Design Standard provides conservative estimates of the load carrying capacity of screwed connections using G550 sheet steels, whereas the AS/NZS 4600, CSA-S136 and AISI Design Standards provide accurate load predictions 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 using the AS/NZS 4600, CSA-S136 and AISI Design Standards becomes unconservative. It is necessary to reduce the bearing coefficient contained in these standards to accurately design thin G550 and G300 sheet steel screwed connections. Furthermore, all of the design standards are reliable when the 0.75¦ u material reduction requirement is used, although when the full material properties are used in design the reliability of the CSA-S136 and Eurocode 3 Design Standards decreases. It is also noted that the thread profile of screw fasteners has a significant effect on the load carrying ability of connections, where screws with a coarse thread pattern outperform their fine threaded equivalents. The proposed method of analysis for screwed connections loaded in shear can be used to improve the accuracy of predicted load resistance when two different thickness sheet steels are joined. It is recommended that the variable bearing coefficient formulation and the unreduced net section resistance be used in the design of screwed connections.