Stainless Steel Circular Hollow Section Research Articles

Numerical study of fire resistance of stainless steel circular hollow section columns

Stainless steel has countless desirable characteristics for a structural material. Although initially more expensive than conventional carbon steel, stainless steel structures can be competitive due to their smaller need for fire protection material and lower life-cycle cost, thus contributing to a more sustainable construction. The most common stainless steel groups used in structural applications are the austenitic, ferritic and austenitic–ferritic (also known as Duplex grades). This work presents a numerical study on the behaviour of stainless steel circular hollow section members under axial compression at elevated temperatures, with different cross-section slenderness. The numerically obtained ultimate load-bearing capacities are compared with simplified calculation formulae from Eurocode 3 for columns under fire situation. A parametric study, considering different stainless steel grades from the aforementioned groups, cross-sectional classes and slendernesses, is here presented for different elevated temperatures. The numerical analyses were performed with the finite element programme SAFIR, with material and geometric non-linear analysis considering imperfections. Comparisons between the numerical results and the Eurocode 3 rules demonstrated that a specific design approach must be developed for stainless steel columns under fire situation.

Compressive testing and numerical modelling of concrete-filled double skin CHS with austenitic stainless steel outer tubes

A comprehensive experimental and numerical study of concrete-filled double skin tubular (CFDST) stub columns is presented in this paper. A total of 23 tests was carried out on CFDST specimens with austenitic stainless steel circular hollow section (CHS) outer tubes, high strength steel CHS inner tubes, and three different grades of concrete infill (C40, C80 and C120). The ultimate load, load-deflection histories and failure modes of the stub columns are reported. The test results were employed in a parallel numerical simulation programme for the validation of the finite element (FE) model, by means of which an extensive parametric study was undertaken to extend the available results over a wide range of cross-section slendernesses, inner tube strengths and concrete grades. The experimentally and numerically derived data were then employed to assess the applicability of the existing European, Australian and North American design provisions for composite carbon steel members to the design of the studied CFDST cross-sections. Overall, the existing design rules are shown to provide generally safe-sided (less so for the higher concrete grades) but rather scattered capacity predictions. Use of an effective concrete strength is recommended for the higher concrete grades and shown to improve the consistency of the design capacity predictions.

Experimental study of cold-drawn duplex stainless steel circular tubes under axial compression

Cold-drawing is one of the typical cold-forming techniques in producing stainless steel circular hollow sections (CHS). Compared with cold-rolling, cold-drawing can produce large scale stainless steel tubes which have wide applications in highly corrosive environment. Nevertheless, few researches have been carried out on the duplex stainless steel CHS columns. In this paper, cold-drawn duplex S22053 stainless steel CHS of three types of cross-sections were tested, including 8 material tensile coupon tests, 6 axially compressed stub column tests and 7 axially compressed long column tests. The material stress-strain curves of the cold-drawn CHS tubes, ultimate loads and failure modes of the stub and long columns were obtained. It has been revealed that material properties of cold-drawn stainless steel CHS tubes are close to those of the virgin plate in the annealed state. Moreover, all the stub columns show local buckling in “elephant foot” mode, while all the long columns undergo overall flexural buckling, among which a long column specimen with the nominal length of 700 mm（CHS-700） shows local buckling after the peak load due to the interaction of axial compression and second-order bending moment. Furthermore, comparing the test values with the predicted values based on the European, American, Australian/New Zealand and Chinese design codes, and the calculation methods available in literatures, it is concluded that the predicted values of the stub columns and the long columns with relatively large member slenderness are all lower than the test values, while the predicted values using the design methods except that in the Chinese design code are higher than the test values for the long columns with relatively small member slenderness. Chinese design code could provide safety predictions for cold-drawn duplex stainless steel CHS columns.

Experimental and numerical studies of austenitic stainless steel CHS stub columns after exposed to elevated temperatures

This paper presents a thorough testing and finite element modelling study of the material response and structural behaviour of austenitic stainless steel circular hollow section (CHS) stub columns after exposed to elevated temperatures. The testing programme was conducted on two cold-formed austenitic stainless steel circular hollow sections: CHS 73 × 3 and CHS 89 × 3, and involved 16 material tensile coupon tests and 16 stub column tests after exposure to eight levels of elevated temperatures ranging from 30 °C to 1000 °C. The experimental investigation was supplemented by a finite element modelling study, where numerical models were firstly developed and validated against the post-fire stub column experimental results, and afterwards utilised to carry out numerical parametric studies to extend the test data pool over a wider range of cross-section geometric dimensions. The measured post-fire tensile coupon test results indicated that the Young's modulus and the ultimate stress of austenitic stainless steel generally remained unchanged after exposure to elevated temperatures, while the post-fire 0.2% proof (yield) stress and ultimate strain were similar to the corresponding room temperature properties for temperatures up to around 600 °C–800 °C, but experienced a reduction and an increase after exposure to higher elevated temperatures, respectively. Given that there have been no established design standards for stainless steel structures after exposure to fire, the corresponding room temperature design rules, as set out in the European code, American specification and Australia/New Zealand standard, were evaluated for austenitic stainless steel CHS stub columns after exposed to elevated temperatures, revealing safe but conservative cross-section compression resistance predictions, owing principally to the adoption of the 0.2% proof (yield) stress as the design stress without considering strain hardening. The deformation-based continuous strength method accounts for material strain hardening in calculating cross-section capacities at ambient temperature. Its applicability to the design of austenitic stainless steel CHS stub columns after exposed to elevated temperatures was also assessed. The results of the assessment indicated that the continuous strength method leads to both precise and consistent compression resistance predictions of austenitic stainless steel CHS stub columns after exposed to elevated temperatures.

Full-Range Compressive Stress–Strain Curves for Cold-Formed 304 Stainless Steel Circular Hollow Sections After Exposure to Vacuum Brazing

With the rapid development of microscale cellular structures, the small-diameter cold-formed welded stainless steel tubes have recently been used for creating the metallic lattice topologies with high mechanical properties. In this paper, to obtain the accurate material properties of the circular hollow section (CHS) under pure compression, a series of concentric compression tests are conducted on the millimeter-scale cold-formed 304 stainless steel circular tubular stub columns after exposure to a vacuum brazing process. The tests cover a total of 18 small-diameter stub tubes with measured thickness-to-diameter ratios (t / D) from 0.023 to 0.201. A generalized three-stage nominal stress–strain model is developed for describing the compressive behavior of the post-brazing CHSs over the full strain range. This mechanical model is especially applicable to computer code implementation. Hence, an interactive computer program is developed to simultaneously optimize three strain hardening exponents $$(n_1 ,n_2 ,n_3 )$$ in the expression of the model to produce the stress–strain curve capable of accurately replicating the test data. To further reduce the number of the model and material parameters on which this model depends, this paper also develops five expressions for determining the 2.5% proof stress $$(\sigma _{p_2 } )$$ , $$n_2 $$ , the ultimate compressive strength $$(\sigma _{p_3 } )$$ , $$n_3 $$ , and the ultimate plastic strain $$(p_3 \% )$$ for given experimental values of three basic material parameters $$(E_0 ,\sigma _{0.01} ,\sigma _{0.2} )$$ . These expressions are validated to be effective for the CHSs with $$t{/}D\ge 0.027$$ . The analytically predicted full-range stress–strain curves have generally shown close agreement with the ones obtained experimentally.

Testing, simulation and design of cold-formed stainless steel CHS columns

Stainless steel tubular members are employed in a range of load-bearing applications due to their strength, durability and aesthetic appeal. From the limited existing test data on stainless steel circular hollow sections (CHS) columns it has been observed that the current Eurocode 3 provisions can be unconservative in their capacity predictions. A comprehensive experimental programme has therefore been undertaken to provide benchmark data to validate numerical models and underpin the development of revised buckling curves; in total 17 austenitic, 9 duplex and 11 ferritic stainless steel CHS column buckling tests and 10 stub column tests have been carried out. Five different cross-section sizes (covering class 1 to class 4 sections) and a wide range of member slendernesses have been examined. The experiments were initially replicated using finite element (FE) simulations; the validated FE models were then used to generate 450 additional column buckling data points. On the basis of the experimental and numerical results, new design recommendations have been made for cold-formed stainless steel CHS columns and statistically validated according to EN 1990 [1].

Design of cold-formed stainless steel circular hollow section columns using direct strength method

Cold-formed stainless steel circular hollow section (CHS) columns have been increasingly used in construction, due to its aesthetic appearance, long life-span and good ductility. It is shown that direct strength method (DSM) is capable of predicting cold-formed steel column strengths accurately. However, the DSM is developed for cold-formed steel sections with plate rather than curved elements, and thus its applicability for cold-formed stainless steel CHS is worth investigating. This paper presents a numerical investigation of cold-formed stainless steel CHS columns. A non-linear finite element model was developed and verified against column tests. Extensive parametric study of cold-formed duplex, lean duplex and ferritic stainless steel CHS columns has been performed to obtain column strengths. A total of 273 experimental and numerical cold-formed stainless steel CHS column strengths, which are obtained from previous researches and parametric study obtained from this study, are compared with the design strengths predicted by the current DSM. Reliability analysis was performed to evaluate the reliability of the design rules. It is shown that the current DSM provides unconservative and not reliable prediction for cold-formed stainless steel CHS columns. Therefore, modified DSM is proposed for cold-formed stainless steel CHS columns. It is shown that the modified design rule is more accurate than the current DSM, and the modified design rule is considered to be reliable.

Experimental and numerical investigations on flexural behaviour of CFRP reinforced concrete-filled stainless steel CHS tubes

An experimental investigation was conducted on CFRP reinforced concrete-filled stainless steel circular hollow section (CHS) tubes subjected to in-plane bending. A total of 36 specimens including concrete-filled stainless steel CHS tubes and CFRP reinforced concrete-filled stainless steel CHS tubes were tested. An extensive parametric study was carried out by using the verified finite element model to investigate the strengthening effect of CFRP on the flexural behaviour of concrete-filled stainless steel CHS tubes. It is shown from the comparison that the strengthening effect of CFRP improved with the increase of the strength ratio (β) of CFRP. Furthermore, the ultimate strength was enhanced proportionally with the increase of the layers of CFRP positioned in the longitudinal direction for reinforcement, but the ductility generally deteriorated. Finally, the flexural strength and flexural stiffness of concrete-filled stainless steel CHS tubes is improved by increasing the interface friction coefficient between stainless steel tubes and concrete up to 0.6.

Flexural behaviour of concrete-filled stainless steel CHS subjected to static loading

This paper presents a test program on flexural behaviour of concrete-filled stainless steel circular hollow section (CHS) tubes under in-plane bending. A total of 27 specimens including 18 concrete-filled stainless steel CHS flexural members and 9 empty stainless steel CHS flexural members were tested. The ultimate strengths, failure modes, flexural stiffness, ductility, bending moment-midspan deflection curves, overall deflection curves and strain distribution curves of test specimens are reported. It is demonstrated that the ultimate strength, initial stiffness and ductility of empty stainless steel CHS flexural members are significantly enhanced by filling the concrete in the specimen along its full length. The enhancement is increased with the increase of the thickness of the CHS tube. Furthermore, the concrete strength has little influence on the ultimate strength, initial stiffness and ductility of concrete-filled stainless steel CHS flexural members. The test flexural stiffness including both initial flexural stiffness and flexural stiffness at the serviceability limit state of concrete-filled stainless steel CHS tubes under in-plane bending were compared with the design flexural stiffness calculated using the current AIJ standard, BS 5400, Eurocode 4 and AISC specification for concrete-filled steel tubes. It is shown from the comparison that the current design rules are all unconservative for initial flexural stiffness and flexural stiffness at the serviceability limit state of concrete-filled stainless steel CHS tubes under in-plane bending with high scatter of predictions.

Bond-slip behaviour of concrete-filled stainless steel circular hollow section tubes

This paper presents the repeated push-out tests on concrete-filled stainless steel circular hollow section (CHS) tubes with different values of height-to-diameter ratio, diameter-to-thickness ratio and concrete strengths. The bond-slip behaviour of all specimens and the strain distribution on the exterior of stainless steel tubes along the longitudinal height direction were carefully investigated. It was found that the shear failure loads of bonding slip decreased successively with more loading cycles of the repeated push-out test employed in the same direction. Hence, the mechanical interlock force and friction force of the interface elements gradually decreased. Furthermore, the bond-slip failure of the interface elements between the inner concrete and outer stainless steel tube of the specimens consists of the adhesive stage, the sliding stage and the friction resistant stage. It can be generally concluded that 70% of the shear resistance of the bonding strength is taken by the interface friction force, while the remaining 30% of the shear resistance of the bonding strength is sustained by the chemical adhesive force and the mechanical interlock force. On the other hand, it was demonstrated that the height-to-diameter ratio (H/D) and the diameter-to-thickness ratio (D/t) of the stainless steel tube as well as the concrete strength (C) have insignificant influence on the shear resistance of the bonding strength of the interface elements. It was also shown from the comparison that the current design rules of concrete-filled carbon steel CHS tubes are inapplicable to the shear resistance of the bonding strength of concrete-filled stainless steel CHS tubes.

Testing and numerical modelling of austenitic stainless steel CHS beam–columns

This paper presents a comprehensive experimental and numerical study of the global stability of stainless steel circular hollow section (CHS) structural members subjected to combined axial load and bending moment. The experimental investigation, which was carried out on two grade 1.4301 austenitic stainless steel cross-section sizes (CHS 60.5×2.8 and CHS 76.3×3), included material coupon tests, initial geometric measurements, two column tests, and ten beam–column tests. The test results were employed in a parallel numerical simulation programme for the validation of finite element (FE) models, by means of which a series of parametric studies were carried out to extend the available results over a wider range of cross-section sizes, member lengths and loading combinations. The experimentally and numerically derived data were used to assess the structural performance of CHS beam–columns and to determine the accuracy of the current design provisions given in the European code, American specification, Australian/New Zealand standard, and other recent proposed approaches. Generally, all the existing design methods were shown to yield a high degree of scatter in the prediction of beam–column strength, with both conservative and over-predicted capacities. The scatter was attributed mainly to inaccurate predictions of the column buckling and bending end points of the design beam–column interaction curves (i.e. member strengths under pure compression and pure bending, respectively) and inaccurate interaction factors. The development of improved provisions for the design of stainless steel circular hollow section beam–columns is currently underway.

Structural performance of stainless steel circular hollow sections under combined axial load and bending – Part 1: Experiments and numerical modelling

A comprehensive experimental and numerical investigation into the structural performance of stainless steel circular hollow sections (CHS) under combined compression and bending moment has been performed and is fully reported in the present paper and its companion paper. The experimental programme employed four CHS sizes made of austenitic stainless steel, and included material tensile coupon tests, four stub column tests and twenty combined loading tests. The initial loading eccentricities for the combined loading tests were varied to provide a wide range of bending moment-to-axial load ratios. In conjunction with the testing programme, a numerical modelling programme was performed to simulate the experiments. The developed FE models were shown to be capable of replicating the key test results, full experimental curves including the post-ultimate range and deformed failure modes. Upon validation of the FE models, a series of parametric studies were conducted in the companion paper, aiming at extending the current test data pool over a range of cross-section sizes and combinations of loading. The experimental data, together with the generated parametric study results, were analysed and employed to evaluate the applicability of the codified provisions given in the European code, American specification and Australia/New Zealand standard for design of CHS under combined loading. Improved design rules were also sought through extension of the deformation-based continuous strength method (CSM) to the case of stainless steel CHS under combined loading.

Structural performance of stainless steel circular hollow sections under combined axial load and bending – Part 2: Parametric studies and design

This paper reports the second part of the study on the structural behaviour of stainless steel circular hollow sections subjected to combined axial load and bending moment. A series of numerical parametric studies is presented, using the validated finite element (FE) models from the companion paper, with the aim of generating further structural performance data over a wider range of stainless steel grades, cross-section slendernesses and combinations of loading. The considered parameters include the outer cross-section diameter, the ratio of outer cross-section diameter to thickness and the initial loading eccentricity. Both the experimentally and numerically derived section capacities were compared with the strength predictions determined from the current European code, the American specification and the Australian/New Zealand standard, allowing the applicability of each codified method to be assessed. The comparisons revealed that the current design standards generally result in unduly conservative and scattered strength predictions for stainless steel circular hollow sections under combined loading, which can be primarily attributed to the neglect of strain hardening in the determination of cross-section resistances and to the use of linear interaction formulae. To overcome these shortcomings, improved design rules are proposed through extension of the deformation-based continuous strength method (CSM) to the case of circular hollow sections subjected to combined loading. Comparisons between the proposals and the test and FE results indicate a high level of accuracy and consistency in the predictions. The reliability of the proposed approach was confirmed by means of statistical analyses according to EN 1990.

The continuous strength method for the design of circular hollow sections

Circular hollow sections (CHS) are widely used in a range of structural engineering applications. Their design is covered by all major design codes, which currently use elastic, perfectly-plastic material models and cross-section classification to determine cross-secti\\on compressive and flexural resistances. Experimental data for stocky sections show that this can result in overly conservative estimates of cross-section capacity. The continuous strength method (CSM) has been developed to reflect better the observed behaviour of structural sections of different metallic materials. The method is deformation based and allows for the rational exploitation of strain hardening. In this paper, the CSM is extended to cover the design of non-slender and slender structural steel, stainless steel and aluminium CHS, underpinned by and validated against 342 stub column and bending test results. Comparisons with the test results show that, overall, the CSM on average offers more accurate and less scattered predictions of axial and flexural capacities than existing design methods.

Compressive strength of girth-welded stainless steel circular hollow section members: Stub columns

In recent years, stainless steel circular hollow section members have gained more widespread usage as load bearing constituents in construction due to their combination of corrosion resistance, aesthetic appearance and structural efficiency. This paper investigated the compressive strength of girth-welded stainless steel circular hollow section stub columns by numerical method. At first, finite element simulation of the girth welding process was carried out to obtain the weld-induced imperfections such as residual stresses and deformations employing sequentially coupled three-dimensional thermo-mechanical finite element formulation. Then, nonlinear finite element analysis in which the failure mode, the ultimate load-carrying capacity and the full load-displacement response of the girth-welded circular hollow section stub columns were explored taking the residual stresses and distortions together with the initial global geometric imperfections into consideration was conducted. Seven finite element models with different diameter-to-thickness ratios were developed in order to examine the effects that the diameter-to-thickness ratio has on the compressive strength. Results have shown that the weld-induced imperfections should be taken into account in accurate assessment of the compression behavior of girth-welded stainless steel CHS stub columns.

Bending resistance of girth-welded stainless steel circular hollow sections

This paper investigated the bending strength of girth-welded stainless steel circular hollow sections by numerical method. At first, finite element simulation of girth welding process was carried out to obtain the weld-induced residual stresses and deformations employing sequentially coupled three-dimensional thermo-mechanical finite element formulation. Then, nonlinear finite element analysis in which the moment versus end-rotation response, the ultimate moment resistance and the failure mode of the girth-welded circular hollow sections under pure bending were explored incorporating the residual stresses and depressions along with the initial global geometric imperfections was performed. Feasibility of adopting the current codified provisions (Eurocode 3) for stainless steel circular hollow sections in bending as the cross-section slenderness limit for girth-welded stainless steel circular hollow section flexural members was also examined. Results showed that the flexural behavior of girth-welded stainless steel circular hollow sections always involves local buckling near the girth weld on the compression side, which significantly affects the moment versus end-rotation behavior. Moreover, the results demonstrated that the Class 3 slenderness limit of 280ε2 from Eurocode 3 for stainless steel circular hollow sections in flexure may be applied to girth-welded stainless steel circular hollow sections subjected to bending.

Structural design of stainless steel concrete filled columns

This paper presents the behaviour and design of axially loaded concrete filled stainless steel circular and square hollow sections. The experimental investigation was conducted using different concrete cube strengths varied from 30 to 100 MPa. The column strengths and load-axial shortening curves were evaluated. The study is limited to cross-section capacity and has not been validated at member level. Comparisons of the tests results together with other available results from the literature have been made with existing design methods for composite carbon steel sections — Eurocode 4 and ACI. It was found that existing design guidance for carbon steel may generally be safely applied to concrete filled stainless steel tubes, though it tends to be over-conservative. A continuous strength method is proposed and it is found to provide the most accurate and consistent prediction of the axial capacity of the composite concrete filled stainless steel hollow sections due largely to the more precise assessment of the contribution of the stainless steel tube to the composite resistance.

Resistance of stainless steel CHS columns based on cross-section deformation capacity

A conceptually new structural design approach has recently been proposed by the authors to predict the resistance of stainless steel members subjected to various types of loading with cross-sections formed from thin flat plates including angles, channels, lipped channels, I-sections and rectangular hollow sections (RHS). The proposed method does not follow the traditional cross-section classification approach, which primarily relies on the assumption of a bilinear, elastic–perfectly-plastic material model. Instead, deformation capacity of a cross-section is determined directly from the local buckling characteristics of the constituent plate elements. This is then used to obtain the corresponding local buckling stress utilising an appropriate material model. This basic concept is extended herein to predict compression resistance of stainless steel columns with circular hollow sections (CHS). Available test and finite element (FE) results have been used to develop the basic design equation to predict the compression resistance of cross-sections and to propose column curves to determine flexural buckling resistances. The predicted resistances have been compared to those obtained using the current Eurocode; the predictions are significantly more accurate and more consistent than those given by the existing Eurocode.

Behavior and Design of Seam-Welded Stainless Steel Circular Hollow Section Flexural Members

In an experimental study, a series of tests was performed on seam-welded structural stainless steel circular hollow sections (CHS) subjected to bending. The test program included various cross section geometries of CHS that covered a range of structural responses. Three-dimensional nonlinear finite-element (FE) models (ABAQUS) were established and verified with the experimental results accounting for material nonlinearity and manufacturing distortions. Close agreement was achieved between the test and finite-element analysis results in terms of load-deformation response and ultimate strength. The test and finite-element strength results with additional FE results generated for CHS models not considered in the test are compared with the strengths predicted using the American, Australian, and European specifications for cold-formed stainless steel structures. In light of the experimental/numerical results and code recommendations, strength and cross section classification criteria for stainless steel circular hollow section members in bending were examined. The results showed that significant improvements are possible for cross section slenderness limits provided by the current specifications.

Strength and stability criteria for thin-walled stainless steel circular hollow section members under bending

A series of tests consisting of various cross-section geometries were performed on structural stainless steel circular hollow sections (CHS) subjected to bending. The test program comprised a total of eight tests on CHS in two grades of stainless steel, namely 304 and Duplex 2205. For each grade four sections, each with a different slenderness, were tested, in order to cover a range of structural responses. Measurements of overall geometric imperfections and material properties were conducted. The test strengths are compared with the strengths predicted using the American, Australian and European specifications for cold-formed stainless steel structures. In the light of the test results and code recommendations, strength and cross-section classification criteria for stainless steel circular hollow section members in bending are examined.