G550 Steel Research Articles

Structural performance of cold-formed steel built-up section beams under non-uniform bending

The experimental investigation and numerical analyses are presented in this paper on cold-formed steel built-up open and closed section beams under either uniform or non-uniform minor axis bending. To the best knowledge of the authors, this is the first study devoted to exploring the effect of non-uniform bending on the behaviour of cold-formed steel built-up section flexural members. In the experimental investigation, the built-up section specimens were assembled by fastening two individual channels as press-braked from G500 and G550 steel sheets. Four-point bending tests of eighteen specimens with varying moment distributions were conducted to examine the flexural performance of the built-up section members experiencing uniform bending and moment gradients. In the numerical simulation, the finite element models were established for the built-up section flexural members and calibrated against the experimental results in terms of moment-deflection response, failure mode and moment capacity. Based on the validated finite element models, additional 300 numerical data of the built-up section beams under uniform and non-uniform minor axis bending were generated through a parametric study. Subsequently, the comparison of moment capacities obtained from the 18 tests and 300 numerical predictions with nominal strengths was carried out to evaluate the appropriateness of the current direct strength method as prescribed in the North American Specification for the built-up section flexural members. Aimed to achieve more suitable strength predictions, the modified design rules were recommended in this study and proven to be generally reliable as well as accurate for the design of cold-formed steel built-up section beams.

Effect of manufacturing process on microstructures and mechanical properties, and design of cold-formed G450 steel channels

Cold-formed steel members are manufactured from cold-reduced steel coils by either press-braking or cold-forming. During manufacturing processes at ambient temperature, two stages, namely cold-reducing and cold-forming, create significantly large deformation on the cross-sections. While the first stage involves cold-reducing hot-rolled steel coils to thinner desired thicknesses resulting in higher yield strength and less ductility, the second stage including rolling and bending processes causes strain hardening of the material, which affects the local mechanical properties especially at the bending corners of the cross-sections. This paper presents an experimental study on the effect of the latter manufacturing process on the microstructure of flat and corner regions of C-shaped cold-formed high strength G450 steel channel sections with different sizes and thicknesses. Optical microscopy, scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD) were used for microstructure analyses. Tensile coupon and Vikers hardness tests were performed for mechanical properties of cold-formed steel material including yield strength, ultimate strength and Vickers hardness at both flat and corner regions. Understanding the relationship between the changes in the microstructure of the high strength G450 steel material provides insights into the mechanical and structural behaviour of cold-formed channel members. In addition, the experimental yield strengths at corner regions are compared with those from the AISI S100-16 and AS/NZS 4600:2018 design specification/standard, showing conservative predictions of the design codes over the measured corner yield strengths, and especially providing inconsistent predictions in thin sections. Hence, a new suggested modification of design equations in the current specification/standard is proposed in this paper to better predict the yield strengths at corner regions of high strength G450 steel cold-formed channels.

Local and Distortional Interaction Buckling of Cold-formed Thin-Walled High Strength Lipped Channel Columns

In this paper, a series of axial compression tests were conducted on lipped channel columns constructed from cold-formed high strength Grade G550 steel sheet of thickness of 1 mm in order to investigate the local and distortional interaction buckling performance. The failure modes, ultimate loads and load–displacement curves of the specimens were reported and analyzed. The experiment revealed the deformation mechanism of specimens failing in local-distortional interaction or distortional mode. Besides, a finite element model was developed to predict the buckling performance of the specimens. The effect of different imperfection shapes and material models on numerical results was investigated so that the reasonable finite element analysis can be carried out. It was found that the local buckling mode with an odd number of waves as the initial imperfection can produce practical deformation process. Meanwhile, the material model of Ramberg–Osgood can be applied for the nonlinear finite element analysis of the specimens. The parametric analysis was also performed to study the relationship between the sensitivity of specimens to local and distortional imperfections as well as the variation of bearing capacity with the geometric size. It was found that the lip width has great influence on the bearing capacity of specimens because of the significant restriction of lip on distortional deformation.

A combined theoretical-experimental approach for modelling ductile fracture of cold-reduced G450 steel sheet

A thermodynamics-based coupled damage-plasticity model for high strength cold-reduced steel is developed on the basis of a generic form, allowing the use of any yield criterion and damage evolution rule. The model possesses essential features of ductile failure, including large plastic deformation prior to fracture and the effects of both triaxiality and Lode angle on the failure. In conjunction with theoretical development, experimental work for both furthering the understanding of ductile fracture in high strength cold-reduced steel and validation of the proposed model under different loading conditions is also performed. In particular, advanced non-contact measurement techniques based on Digital Image Correlation (DIC) are used not only for characterising the material failure/data in both pre- and post-peak phases but also for calibration of parameters and validation of the proposed model. While the pre-peak hardening responses with homogeneous deformation can be described by a nonlinear hardening rule, in the post-peak regime, localisation of deformation before complete fracture requires the use of energy in both characterisation of the material failure and calibration of the model. In this sense, the essential work of fracture obtained from the experiments is linked with parameters controlling the fracture of the material and also used for a simple regularisation technique based on the concept of smeared deformation. The model responses are validated against experimental data in terms of both macro responses and evolving failure patterns, demonstrating good potential for applications in failure analysis of structures made of high strength cold-reduced steel.

Block shear strength and behaviour of cold-reduced G450 steel bolted connections using DIC

This paper presents an application of the Digital Image Correlation (DIC) based technique to investigate the block shear failure mode of bolted connections made of high strength cold-reduced G450 sheet steel. In conjunction with the conventional testing approach, the DIC provides detailed failure information on connection behaviour and its correlation with macroscopic material tests. A total number of 12 tests of 3 mm thickness specimens were tested at the University of Sydney. The experimental results provide a better understanding on how the failure of the bolted connections progress. The features of the new block shear equations recently adopted in the North American Specification for Design of Cold-Formed Steel Structural Members (AISI S100:2016) and the Australian/New Zealand Standards 4600:2018 Cold-Formed Steel Structures have been clarified.

Retracted] Examining Dent Formation Caused by Hailstone Impact

Hailstorms pose significant risk for exposed building cladding materials. Steel sheeting is the most important cladding material used. The understanding of steel sheets behavior under hail impact loading is not sufficient for the manufacturing of hail‐resistant sheets. With the purpose‐built equipment, artificial hailstones of different sizes were launched to impact at steel sheets of different thicknesses and yield stresses as targets. A theoretical approach for the problem of predicting the dent size due to hailstone impact was developed and compared to the test results. The expressions developed in the theory can predict the dent depth before the impact, assuming the ratio between the dent depth and dent diameter is constant. The expression is not able to predict the depth of dents smaller than 0.75 mm and cannot predict whether the denting will occur or not. All hailstone sizes lead to visible dent on steel sheet of thicknesses 0.35 mm, 0.42 mm, and 0.55 mm. Visible denting was also obtained for the 0.75 mm steel samples with 45 mm and 55 mm hailstones; however, no denting occurred using 40 mm hailstones. It was found that the dent depth was inversely proportional with thickness and yield stress, while the dent diameter was found to be proportional to yield stress. As the yield stress of the steel sheet increased, the dent depth decreased for G300 and G550 steel. The dent diameter however increased as the yield stress increased. When the artificial hailstone shatters on impact, significant energy is lost and less energy is available to cause plastic deformation of the impacted material.

Numerical prediction of structural steel flat and slant fracture modes using phenomenological shear fracture model

This paper presents the finite element (FE) predictions of structural steel flat and slant fracture modes using the phenomenological shear fracture model that employs the shear stress ratio instead of the lode angle parameter employed in literature for predicting structural steel ductile shear fracture. A992, 350W and G550 steel grades solid tension coupons (STCs) and perforated tension coupons (PTCs) flat fracture and S690 and S235 steel grades STCs and PTCs slant fracture are predicted. It is demonstrated that the phenomenological shear fracture model can be employed for predicting the displacement at fracture or fracture strain of structural steel that exhibits the flat or slant fracture mode. Thus, the phenomenological shear fracture model can be employed for predicting civil engineering structural steel complete fracture behaviors.

09.07: Observations on fracture toughness measurement: At the corners of G450 cold‐formed steel channel sections subjected to Tension

ABSTRACTHigh strength G450 steel sheets are manufactured from hot‐rolled coil steel using a cold‐reduction process which involves cold‐rolling the steel under a series of rollers until a suitable thickness is achieved. Cold‐formed sections are subsequently manufactured by bending flat sheets at ambient temperature to achieve desired shapes. The plastic deformation associated with bending process results in strain hardening of the material and this in turn affects the properties. Due to the small difference between the yield point and tensile strength, fracture toughness becomes an important parameter in ultimate strength analysis as cold‐reduced steels are more brittle compared with hot‐rolled steels. Recent work to investigate the fracture behaviour of G450 sheet steels to AS1397 subjected to tension at ambient temperature (approximately 22°C) has been performed for different thicknesses (1.9, 2.4 and 3.0 mm). This paper describes the experimental investigation and observations of the corner effect on the fracture toughness of G450 steel. A series of six corner coupon tests of each thickness was cyclically loaded to produce a fatigue crack and then monotonically loaded in tension (ASTM E399) until fracture. A combined image processing technique was used to monitor the test from the two sides of the corner, where a grid of targets was initially marked at the surface and used for scaling and orientating all images. A digital correlation procedure was then applied to track the displacement of the tips of the notch of each crack and quantify the opening during the test. By correlating load with the different stages of propagation, the critical moments could be identified together with the fracture toughness. Results are thoroughly presented and discussed.

Failure modes and buckling coefficient of partially stiffened cold-formed sections in bending

Flange edge stiffeners increase the ultimate moment capacity of cold-formed channel sections. At the same time, they cause complexity to the buckling failure mode of the section. There is a lack of experimental research on the failure mode of sections with a partially stiffened element, such as channel sections with edge stiffeners, in which a distortional buckling mode can be observed. The focus of recent studies is mainly on the behaviour of the whole section as one member under bending without any concern about the relationship between the web and flange ratio.In this study, an extensive experimental analysis of 42 cold-formed channel sections was used to explore the failure behaviour of cold-formed channel sections under pure bending. The sections were made from cold-formed G450 steel with a nominal thickness of 1.6mm. The results of the pure bending experimental investigations are used to describe the relationship between the web and flange ratio and the failure deformations. It is also shown that the current international cold-formed steel specifications over-predict the buckling coefficient of partially stiffened elements with high aspect ratio values. The experimental results are used to propose revisions to current international cold-formed steel specifications.

Section classifications for cold-formed channel steel

The inelastic reserve capacity, which is the additional capacity of a member beyond the first yielding, is very important for limit-state design of frame structures. The aim of this work was to investigate the inelastic bending capacity of cold-formed channel sections according to European standards and to propose revisions to current design rules. An extensive experimental and analytical analysis of 42 cold-formed channel sections was conducted. Material properties of the tested sections were examined using tension tests on metal coupons. The sections were cold-formed from G450 steel with a nominal thicknesses of 1·6 mm, and varying theoretical buckling stresses ranging between elastic and seven times the yield stress. The results from the pure bending experimental investigations and the European design standards for steel structures were compared. It is concluded that the section classifications defined in Eurocode 3 are not accurate for cold-formed channel sections. Therefore, modifications to the section classifications that have been derived for hot-rolled sections are required in the case of cold-formed sections to maintain accuracy (based on the presented test series). Design rules are developed to account for such behaviour.

Effect of Loading Direction on the Bearing Capacity of Cold-Reduced Steel Sheets

AbstractThis study is concerned with double-shear bolted connections in cold-reduced steel sheets that undergo the pure bearing failure mode of the inside sheet. Compared to the published test results of bolted connections failing in the net section fracture, those involving the bearing failure mode had very wide scatter in the ultimate test loads of specimens having seemingly similar configurations. This technical note presents the laboratory test results of 51 specimens composed of G2 and G450 steel sheets, which have very different ductility properties. One new and significant finding is that the absolute bearing capacity can be considerably higher in the rolling direction of the cold-reduced steel sheet than in the perpendicular direction, even though the tensile strength has the opposite trend. Another result is that material ductility has a much greater effect on the bearing capacity than on the net section tension capacity. It was also found that snug tightening had little effect on the bearing cap...

Mechanical properties of G550 cold-formed steel under transient and steady state conditions

Material properties at elevated temperatures are the important factors in the fire safety design and numerical analysis of cold-formed steel structures. Most of the previous research on material properties at high temperatures has adopted the steady state test method. However, the transient state test method is more realistic for actual fire conditions. This paper presents a detailed experimental investigation of G550 steel with a thickness of 1mm under both transient and steady state test methods. The test results obtained from transient and steady state methods are discussed, and the results show that the steady state method is not equivalent to the transient state method for G550 steel. The steady state test results of G550 may result in an overestimate of the fire resistance of cold-formed steel structures. In addition, the test results were also compared with those obtained from other researchers and the current design rules. The comparison shows that the yield strength predicted by BS5950 agrees well the transient state test results of G550 and is conservative for the steady state test results. However, BS5950, AS4100, and Eurocode 3 provide nonconservative predictions in other cases. Finally, a unified equation for the reduction factors, including the yield strength, elastic modulus, and ultimate strength of G550 at elevated temperatures, is proposed by the numerical fitting technique. A stress–strain expression of G550 at elevated temperatures is also given based on the Ramberg–Osgood model. The proposed equations are in good agreement with test results and meet the requirements for engineering.

Inelastic behaviour and design of cold-formed channel sections in bending

The aim of this paper is to investigate the inelastic bending capacity of cold-formed channel sections, and provide design rules to account for such behaviour. An extensive experimental and analytical analysis of 42 cold-formed channel sections in three different geometric categories is conducted, including simple channel sections, channel sections with simple edge stiffeners and channel sections with complex edge stiffeners. The sections were cold-formed from G450 steel with nominal thickness of 1.6 mm, and varying theoretical buckling stresses ranging between elastic to seven times the yield stress. The results of the pure bending experimental investigations show that current international cold-formed steel specifications are conservative, for channel sections with low slenderness values. The experimental results are used to propose revisions to current international cold-formed steel specifications.

Block Shear Capacity of Bolted Connections in Cold-Reduced Steel Sheets

AbstractThis paper examines the mechanisms for block shear failures of bolted connections in steel plates postulated in the design equations specified in the North American, European, and Australian steel structures codes. It explains that there is only one feasible mechanism for the limit state of conventional block shear failure, that which involves tensile rupture and shear yielding, regardless of the steel material ductility. It describes the fundamental shortcomings of various code equations for determining the block shear capacity of a bolted connection. Based on the tensile rupture and shear yielding mechanism, an in-plane shear lag factor, and the active shear resistance planes identified in the present work, this paper proposes a rational equation that is demonstrated to provide more accurate results than all the code equations in predicting the block shear capacities of bolted connections in G450 steel sheets subjected to concentric loading. The resistance factor of 0.8 for the proposed equation...

Axial capacity and design of thin-walled steel SHS strengthened with CFRP

Carbon fibre reinforced polymer (CFRP) strengthening of structures has been gaining increasing interest, traditionally applied to concrete structures, and more recently applied to steel structures. This paper describes 20 experiments on short, axially compressed square hollow sections (SHS) cold-formed from G450 steel and strengthened with externally bonded CFRP. The SHS were fabricated by spot-welding and had plate width-to-thickness ratios between 42 and 120, resulting in plate slenderness ratios between 1.1 and 3.2. Two different matrix layouts of the CFRP were investigated. It is shown that the application of CFRP to slender sections delays local buckling and subsequently results in significant increases in elastic buckling stress, axial capacity and strength-to-weight ratio of the compression members. The experiments are an extension of a previous study [Bambach MR, Elchalakani M. Plastic mechanism analysis of steel SHS strengthened with CFRP under large axial deformation. Thin-Walled Structures 2007;45(2):159–70] in which 25 commercially produced SHS with plate slenderness values between 0.3 and 1.6 were strengthened with CFRP in the same manner. A design method is developed whereby the theoretical elastic buckling stress of the composite steel–CFRP sections is used to determine the axial capacity, and is shown to compare well with the 45 test results. A reliability analysis shows the method to be suitable for design.

Compression Strength of Unstiffened Elements in Cold-Reduced High Strength Steel

This paper presents research on unstiffened elements made from high strength G550 steel and subjected to uniform compression. The elements are 1 mm or less in thickness and prone to local buckling. Current design rules for these steels specify additional reduction factors to compensate for their reduced strength, which has previously been attributed to the lack of strain-hardening. The paper presents tests and advanced finite-elements analyses of unstiffened elements in compression and identifies appropriate reduction factors for this type of element. The analytical studies demonstrate that the effect of gradual yielding may be the main reason for the relatively low strength of G550 steels in the intermediate slenderness range, rather than the lack of strain hardening. Tests are performed on short single-angle stub columns and on double-angle stub columns. A comprehensive series of material tests of tension and compression coupons is also presented. The test strengths are compared with current American an...

Strength of Welded Connections in G450 Sheet Steel

G450 steel to AS 1397 is a cold-reduced sheet steel with in-line galvanizing. Its grade is 450MPa yield and 480MPa tensile strength. It is widely used in Australia for purlins, and is being used to fabricate lightweight portal frames, often by welding. The effects of welding on G450 sheet steel in the heat affected zone with respect to the connection strength were unknown. In the present work, transverse and longitudinal fillet, and flare-bevel welded connections in 1.5 and 3.0mm G450 steel were tested to failure. The test results are used to check the relevant design rules in the North American Specification for Cold-Formed Steel Structural Members and the Australian/New Zealand Standard for Cold-Formed Steel Structures AS/NZS 4600. It was found that the current design rules are not adequate for certain connection configurations in G450 steel to achieve the target safety index of 3.5. Reduced resistance (capacity) factors are proposed for some connections. It was also found that the current design rule for longitudinal flare-bevel welded connections is unnecessarily conservative. The quality of welded connections in thin sheet steels produced by industry fabricators was investigated. The need to complement the macrotest with the destructive prying test as part of the prequalification procedure for such connections is demonstrated. Part of the results of the present work has recently been incorporated into the North American Specification for Cold-Formed Steel Structural Members with a lowering of the thickness to 2.5mm above which weld throat failure should be checked.

Compression Tests of Cold-Reduced High Strength Steel Sections. I: Stub Columns

This paper describes a series of compression tests performed on stub columns fabricated from cold-formed high strength steel plates with nominal yield stress of 550MPa. The steel is classified as G550 to Australia Standard AS1397. The test results presented in this paper are the first stage of an Australian Research Council research project entitled “Compression stability of high strength steel sections with low strain-hardening.” The tests include lipped-square and hexagonal sections, including 94 box-shaped fix-ended stub columns. The purpose of these tests was to determine the influence of low strain hardening of G550 steel on the compressive section capacities of the column members. The results of the successful stub column tests have been compared with the design procedures in the Australian/New Zealand Standard for Cold-Formed Steel Structures and recent (1999) amendments to the American Iron and Steel Institute Specification. As expected, the greatest effect of low strain hardening was for the stoc...

Compression Tests of Cold-Reduced High Strength Steel Sections. II: Long Columns

This paper describes a series of compression tests performed on long columns fabricated from cold-formed high strength steel plates with nominal yield stress of 550 MPa. The steel is classified as G550 to Australia Standard AS1397. The test results presented in this paper are the second stage of an Australian Research Council research project entitled “Compression stability of high strength steel sections with low strain-hardening.” A total of 28 long columns, which were made from two thicknesses of sheet steel (0.42 and 0.6 mm), were tested. A box shaped section was tested between pinned ends over a range of lengths. This paper shows the comparison of loads obtained experimentally with those predicted on the basis of AS/NZS 4600 and the AISI specification including Supplement No. 1, 1999. The finite element program, \IABAQUS\N, was also used to simulate the column behavior. For sections which undergo local instability at loads significantly less than the ultimate loads, the column design rules in AS/NZS 4600 and the AISI Specification are unconservative if used in their current form for G550 steel. Proposals for improved column design of high strength slender sections are proposed in this paper. A first companion paper (Part I) describes a series of compassion tests on stub columns.

Stability and ductility of thin high strength G550 steel members and connections

High strength cold-reduced steel is typically of stress grade G550 (550 MPa nominal yield and tensile strength) and less than 1 mm thick. The steel has been used for many years for sheeting and decking but is now being used for structural members such as roof trusses and stud walls of steel framed houses. The paper summarises a major research program on the stability and ductility of this steel which has been proceeding for several years at the University of Sydney. The paper relates the Sydney research to the work of others being undertaken around the world.