Pure Axial Compression Research Articles

New Buckling Curve for a Compressed Member with Cold-Formed Channel Cross-Section

The verification of a column made from a lipped cold-formed channel section, subjected to pure axial compression relative to the gross cross-section, often results in a combined verification of bending and compression due to the appearance of a shift of the centroid of its effective cross-section. Following Eurocode 3 rules, this requires the determination of two distinct effective cross-sections and various interaction factors. This paper, based on an analytic approach, offers a modification to the actual buckling curve, based on Ayrton–Perry formulation, to include the second-order effects raised by the eventual shift of the effective centroid due to local buckling of the compressed web plate. This eliminates the need to use an interaction formula. The modified buckling curve is verified based on a GMNIA analysis performed on a numerical parametric model, which was previously validated by laboratory tests. In addition, the results are compared with strength results provided by appropriate Eurocode 3 formulas and AISI Direct Strength Method for global-local interaction and with classic experimental results.

Buckling strengths of cold-formed built-up cruciform section columns under axial compression

This paper describes experiments addressing the buckling and collapse behaviour of cold-formed stainless steel cruciform section columns. Doubly symmetrical flanged cruciform section columns were built using six individual press-braked plain channels made of austenitic grade EN 1.4307 assembled back-to-back. Three different column lengths were tested – namely, short (600 mm), intermediate (1200 mm) and long (2400 mm) lengths, and two channel geometries were used in all tests – with cross-section depths of 200 mm and 100 mm. Tests were carried out under pure axial compression and with fixed end support conditions. Tests were repeated two times for each length. It was observed that the buckling patterns were affected both by the global slenderness and by the spacing between the fasteners. Stocky columns experienced local buckling of the individual channel sections. The plastic failure mechanism was dependent on the fasteners spacing. Intermediate slenderness columns were characterised by interaction between global torsional buckling and local buckling whereas more slender columns failed predominantly through torsional buckling. For the latter, the spacing between the fasteners had a minor influence on the ultimate capacity.

Design procedures for cold-formed stainless steel built-up columns assembled from equal-leg angles

In this paper, the structural behaviour of cold-formed stainless steel (CFSS) built-up T- and cruciform-section columns is studied experimentally and numerically, and the effect of discrete fasteners on their flexural and torsional rigidity is considered. To build the singly symmetrical built-up T-section columns, two equal-leg angle sections made of lean-duplex grade EN 1.4162 were assembled back-to-back. The doubly symmetrical built-up cruciform-section columns were composed of four star-oriented equal-leg angle sections made of austenitic grade EN 1.4301. They were connected using M8 bolts of class 8.8. Two pin-ended built-up T-section columns with length of 1000 mm, and four built-up cruciform-section columns with lengths of 600 mm and 2400 mm with semi-rigid support conditions, were tested under pure axial compression. As observed in the tests, built-up T-section columns experienced flexural-torsional buckling failure, whereas built-up cruciform-section columns failed in torsional buckling mode. Finite element models were validated against the conducted experiments and used to perform a parametric study of the structural behaviour of CFSS built-up T- and cruciform-section columns. The study included the column length, cross-sectional dimensions and bolt spacing as variables. The experimental and numerical data, i.e., the column strengths and failure modes, were compared against the results obtained based on the European and North American Specifications alongside a recent method established for cold-formed carbon steel equivalents and based on the Direct Strength Method (DSM). It is shown that the DSM-based approach predicts the ultimate buckling resistances of the considered built-up columns more accurately than the available codified design methods.

Monotonic and cyclic axial compressive responses of concrete specimens externally confined with different types of FRP composites: experimental and analytical investigations

The main objective of this study was to investigate the monotonic and cyclic axial stress versus strain response of the concrete confined with different types of fiber reinforced polymer (FRP) composites such as cotton fiber rope reinforced polymer (CFRRP), glass fiber reinforced polymer (GFRP) composites and carbon fiber reinforced polymer (CFRP) composites. Another objective was to investigate the applicability of existing ultimate strength models to predict the ultimate strength of concrete confined with CFRRP, GFRP and CFRP composites. For this, a total number of 28 concrete cylinders were cast, strengthened and tested under pure axial compression. The concrete cylinders were strengthened with two and four layers of FRPs. The experimental results indicate that all kinds of FRPs are very useful to alter the ultimate strength and strain of the confined concrete. The highest increase in ultimate strength was recorded for CFRP composite confined specimens and the highest increase in ultimate strain was observed for CFRRP composite confined specimens. Further, both ultimate strength and ultimate strains are found higher in the case of cyclic loads compared with the monotonic load. In general, the stress versus strain curves of cotton fiber rope and glass FRPs are found trilinear. Whereas, stress versus strain curves of CFRP confined specimens are observed to be bilinear. Existing ultimate strength models developed for CFRRP and CFRP composites are found well accurate to predict the ultimate strengths of CFRRP and CFRP confined concrete specimens tested in this study.

Practical ANN prediction models for the axial capacity of square CFST columns

In this study, two machine-learning algorithms based on the artificial neural network (ANN) model are proposed to estimate the ultimate compressive strength of square concrete-filled steel tubular columns. The development of such prognostic models is achievable since an extensive set of experimental tests exist for these members. The models are developed to use the simplest possible network architecture but attain very high accuracy. A total dataset of 1022 specimens with 685 stub columns and 337 slender columns subjected to pure axial compression is collected from the available literature. This is significant for the development of the initial model considering that for this field it falls under the scope of big data analysis. The ANN models are validated by comparison with experimental results. The validation study has shown the superiority of surrogate models over the Eurocode 4 design code. The empirical equation derived from the best-tuned Bayesian regularization algorithm shows a better agreement with the experimental results than those obtained by the Levenberg–Marquardt algorithm, and Eurocode 4 design code. A similar conclusion applies to stub and slender columns independently. The Bayesian regularization-based model is negligibly slower than the one developed on the Levenberg–Marquardt algorithm but gives a better generalization even with simplified ANN. Generally, besides its high accuracy, one of the key benefits of the presented ANN model is its applicability to a broader range of columns than Eurocode 4 and other studies.

Continuum modeling of nonlinear buckling behavior of CNT using variational asymptotic method and nonlinear FEA

Carbon nanotubes (CNTs) exhibit a unique buckling behavior due to their slender tubular geometry and thin-walled circular cross section. This study aims to analyze effect of nonlinear cross-sectional deformation on buckling of CNTs. To accomplish this, CNTs are modeled as beam structures, and the analysis is conducted using the Variational Asymptotic Method (VAM) and a geometrically exact beam theory, as well as nonlinear finite element analysis (FEA). The study considers various loading cases, including pure axial compression and combined loading scenarios, such as bending-axial compression and torsion-axial compression. The results of the study indicate that inclusion of cross-sectional deformation-induced nonlinearity reduces the critical buckling load of CNTs. The reduction is 2–5% for pure axial compression and 10–40% for combined loading cases. The results are validated against existing literature and commercial finite element software, ABAQUS®. Additionally, parametric studies with different slenderness and radius-to-thickness ratios were carried out to further understand the impact of these parameters on buckling of CNTs. Finally, the study presents 3D deformed shapes of CNTs during buckling by combining the results of the 1D analysis and the 2D cross section analysis. The findings show that nonlinearity associated with radius-to-thickness ratio has a significant impact on the cross-sectional ovalisation of CNTs and is critical in evaluating their buckling behavior. This aspect of nonlinearity is often overlooked in continuum modeling methods of CNTs, making this study an important contribution to the field.

Cross-section resistance and design of stainless steel CHS and EHS at elevated temperatures

The structural behaviour and design of stainless steel circular hollow sections (CHS) and elliptical hollow sections (EHS) at elevated temperatures are investigated in this paper. Shell finite element models of stainless steel CHS and EHS are created and validated against experimental results from the literature, which are subsequently used to generate benchmark structural performance data. Parametric studies are performed on a large number of cold-formed and hot-rolled austenitic, duplex and ferritic stainless steel CHS and EHS subjected to (i) pure axial compression, (ii) pure bending, (iii) combined axial compression and bending and (iv) combined bending and shear at elevated temperatures; the studied cases comprise 24,495 stainless steel CHS and EHS in fire and cover a wide range of cross-section slendernesses and elevated temperature levels. Calibrated against the benchmark structural performance data obtained from the numerical parametric studies, new design proposals for predicting the cross-section resistances of stainless steel CHS and EHS in fire are put forward. The accuracy, safety and reliability of the new design proposals are assessed. It is shown that in comparison to the design provisions of the European structural steel fire design standard EN 1993-1-2, the proposed design methods provide more accurate and safe-sided cross-section resistance predictions for stainless steel CHS and EHS at elevated temperatures.

Test and direct strength method on slotted perforated cold-formed steel channels subjected to eccentric compression

This paper aims to investigate the buckling behaviors of cold-formed steel (CFS) channels with slotted web holes determined by the Steel Stud Manufacturers Association (SSMA) under eccentric compression, and propose the simplified direct strength method (DSM) formulas for engineering practicing. An experimental study on 20 specimens was conducted, including 18 slotted perforated specimens with nine eccentricities increasing from −20 mm to 20 mm and 2 unperforated specimens subjected to axial compression. Two kinds of nominal lip lengths for cross sections were selected as 15 mm and 20 mm. The experimental results indicated that the failure modes of slotted perforated specimens subjected to eccentric compression depended on the position of the eccentricity. Compared with the slotted perforated specimens subjected to pure axial compression, the ultimate loads of specimens subjected to positive eccentric compression decreased by 21.70 % on average, while they decreased by 16.38 % on average for those subjected to negative eccentric compression. Moreover, a nonlinear elasto-plastic finite element model (FEM) was developed and validated against the experimental results. Furthermore, combined with the experimental results, extensive FE parametric analyses were conducted to predict the eccentricity impact coefficient formulas. Finally, based on DSM of unperforated CFS members subjected to pure axial compression in AISI S100-16 and the eccentricity impact coefficient formulas, the suitable DSM design formulas for slotted perforated CFS channels determined by the SSMA under eccentric compression were proposed, without the need to calculate the elastic buckling critical stress for slotted perforated CFS channels.

Experiments and design of concrete-filled steel tubes with timber chips under axial compression

This study includes experimental and analytical sections, which discuss the structural response of concrete-filled steel tubes (CFST). In the experimental section, the authors investigate the CFST columns with different proportions of recycled redwood timber chips embedded inside the concrete. An analytical investigation was also carried out in this paper, which puts together equations proposed by four design codes of Eurocode-4, AISC-LRFD, the Chinese code of DL/T, and ACI to predict the axial capacity.The idea of embedment of timber inside aggregates was inspired due to the significant compression strength of timber compared to concrete. In addition, environmental and sustainability concerns advocate substituting conventional aggregates with recycled natural material. In this study, 18 stub columns (in two groups with different steel tubes) were tested under pure axial compression with various compositions of the aggregates. The failure, capacity, weight, and energy absorption of the specimens were studied to evaluate the effect of the weight reduction as a result of timber embedment into the concrete. The results showed that embedding waste timber inside concrete is promising in upcycling wood residue into a commercially viable product. An equation is developed in this paper that takes into account the effect of confinement based on the assumptions that have been made in the literature. 100 test samples were extracted from the literature with a relatively wide range of geometry and material features, and then the capacity of these columns was calculated by the four standards and the proposed equation in this study. The results of the comparisons were analysed and it was found that the proposed equation predicts the capacity accurately.

Experimental and numerical study on the structural behaviour of HST, RCC and CFST stub columns under pure axial compression

Experimental and numerical study on the structural behaviour of HST, RCC and CFST stub columns under pure axial compression

Compressive Behavior of Concrete Containing Glass Fibers and Confined with Glass FRP Composites

In this paper, numerous experimental tests were carried out to study the behavior of concrete containing glass fibers and confined with glass fiber-reinforced polymer (GFRP). Concrete specimens containing different fiber percentages ( 0.3 wt.%, 0.6 wt.%, 0.9 wt.% or 1.2 wt.%) and with different strengths of concrete (8.5 MPa, 16 MPa and 25 MPa) and different confinement levels (two, four and six layers of GFRP) were used as research parameters. The samples were tested to failure under pure axial compression. The results imply that the confinement effect with GFRP is relatively higher for concrete samples containing glass fiber (GFCC) with a percentage equal to 0.6 wt.%. The theoretical of stress ratios (fcc/fco) estimated by using existing ultimate strength models are found to be close to the experimental results for high strength of GFCC, but not close to the experimental results for low strength of GFCC.

Compression-torsion coupling behaviours in double-helix nanofibers assembled from negatively curved nanographenes

The recently synthesised one-dimensional (1D) double-helix nanofibers that are self-assembled from negatively curved nanographenes, i.e., 1-H nanofibers are expected to have new applications in materials science and biology. In this paper, the mechanical behaviours of 1-H nanofibers are investigated by using molecular dynamics simulations. 1-H nanofibers are found to possess a Young’s modulus comparable to that of conventional cup-stacked carbon nanofibers. Meanwhile, a large plastic deformation is observed in the axially stretched 1-H nanofibers before their final breaking. In addition, 1-H nanofibers possess a bending stiffness significantly larger than that of many other 1D nanomaterials, making 1-H nanofibers behave more like an elastic rod or beam. More importantly, a unique compression-torsion coupling behaviour is observed in 1-H nanofibers, which is strongly dependent on the rotation direction. Specifically, when 1-H nanofibers are rotated along the helix direction, a residual tensile stress or, equivalently, an axial contraction is generated. However, no significant compression-torsion coupling behaviour is observed in 1-H nanofibers rotated along the direction reverse to helix. Furthermore, due to the compression-torsion coupling effect, a unique helical buckling behaviour can occur in 1-H nanofibers under pure axial compression, which is totally different from the conventional Euler buckling behaviour observed in many other 1D nanomaterials.

Experiments on buckling behavior of thin-walled steel pipes subjected to axial compression and external pressure

Pipelines are exposed to various forces in order to develop relative compressive axial and external pressure load in some cases of fluid transmission. An experimental study was conducted to investigate the buckling behavior of thin-walled steel pipes when subjected to a combination of axial compression preloading and external pressure. In addition, the response of some specimens was separately investigated subjected to pure external pressure and axial compression load. A significant reduction was observed in the buckling load and the post-buckling capacity of the thin-walled pipes with increasing the value of axial compression preloading. The existing analytical models for predicting the buckling load of the pipelines were modified. The numerical models were developed and verified using the experimental results, and interaction between the axial load and external pressure was graphically demonstrated for different geometrical ratios.

Analysis of axially loaded columns strengthened with fibre-reinforced polymer

When estimating the rectangular stress block of a reinforced concrete flexural member, it is important to take into account different behaviours due to load eccentricity, the ratio and type of fibre-reinforced polymer strengthening, specimen height and confinement ratio. To investigate the impact of these key variables on a rectangular stress block, an extensive analysis of an experimental database of 84 specimens reported by several independent research groups was undertaken. The accuracy and conservativeness of several models under axial compression and flexural loading were first assessed. Comparisons revealed overestimations for specimens strengthened with carbon-fibre-reinforced polymer and underestimations for those reinforced with glass-fibre-reinforced polymer. Inaccurate predictions were also obtained even after more accurate models of concentrically loaded tests were analysed using existing concrete design codes. Expressions for columns strengthened with fibre-reinforced polymer under pure axial compression and modifications for the coefficients of the rectangular stress block were developed. The results indicate a good and safe correlation of predicted strength values.

Fire Behavior and Modeling of Short RC Columns in Pure Axial Compression: Role of Volume, Configuration, and Spacing of Lateral Reinforcement

AbstractThis paper studies the role of volume, spacing, and configuration of lateral reinforcement on the axial load resisting capacity of reinforced concrete (RC) columns at elevated temperatures....

Variational modelling of local–global mode interaction in long rectangular hollow section struts with Ramberg–Osgood type material nonlinearity

A variational model describing the nonlinear mode interaction in thin-walled box-section struts under pure axial compression made from a nonlinear material obeying the Ramberg–Osgood law is presented. The formulation combines continuous displacement functions and generalized coordinates, leading to the derivation of a system of differential and integral equations that describe the static equilibrium response of the strut. Solving the system of equations using numerical continuation techniques reveals the strongly unstable post-buckling response arising from combined geometrical and material nonlinearities during the interactive buckling of the global and local buckling modes—the resulting behaviour being more unstable with decreasing material hardening. A finite element (FE) model is also devised and reveals very similar post-buckling behaviour as highlighted in the variational model. The results compare very well in terms of the mechanical destabilization and the post-buckling deformation, which verifies the analytical model.

Design criteria for pin-ended hot-rolled and laser-welded stainless steel equal-leg angle columns

This paper explores an attempt to investigate the structural behaviour and design of equal-leg stainless steel angle columns fabricated by the laser-welding of individual hot-rolled plates and by hot-rolling process. The studies cover columns with pin-ended boundary conditions exposed to pure axial compression. An advanced and realistic finite element model able to replicate the buckling features of stainless steel angle columns is developed and validated against available experiments collected in the literature. Employing the validated finite element model, quantitative numerical parametric studies, considering minor-axis flexural buckling, as well as flexural–torsional buckling, are performed. Based on comprehensive benchmark data, the accuracy, safety and applicability of the existing column design procedures provided in the European and North American structural stainless steel codes are assessed. The evaluation results reveal that the North American codified design procedures lead to conservative and scattered resistance predictions, thereby demonstrating a need for their improvement. As a result of conducted studies, the new criteria for design of pin-ended hot-rolled and laser-welded stainless steel equal-leg angle columns are proposed. Their suitability for inclusion in the European structural stainless steel design code is confirmed by statistical reliability analyses in accordance with EN 1990.

Numerical study of fixed ended Lean duplex stainless steel (LDSS) flat oval hollow stub column with square perforation under pure axial compression

This paper presents the parametric study of single square perforated fixed ended LDSS (Lean Duplex Stainless Steel) flat oval section stub columns, by varying the square perforation size, location of perforation along transversely as well as longitudinally, using the Finite element software Abaqus (2009). Based on the Finite element study, a near-linear reduction in ultimate load capacity with perforation size has been observed. The reduction is found to be higher for the stockier cross-sections as compared to slender sections. The percentage drop in ultimate load capacity remains relatively unaffected for larger perforation when the section is slender and located at the flat element. It is seen that, in comparison to the slender section, perforated stocky sections showed a mild drop in ultimate load as the perforation is located toward the corner junction of flat and curve portion from the mid-flat region. Further, in general, it is observed that the reduction in load capacity to be highest when the perforation is located near the mid-height. A design approach based on modified ASDM (Automotive Steel Design Manual) has been proposed to estimate the design capacity of single square perforated flat oval LDSS hollow columns

Experimental study of laser-welded stainless steel angle columns

Substantial research has been conducted in the last years into the structural responses of cold-formed and hot-rolled stainless steel equal-leg angle columns, whereas there is no experimental data on laser-welded stainless steel angle columns despite these sections being increasingly used in the modern construction industry. To fill this gap and provide benchmarks for design, a comprehensive experimental investigation into the structural strength and stability of laser-welded austenitic stainless steel equal-leg angle columns under pure axial compression was performed and described in this paper. A series of laboratory tests, performing on two test groups of specimens with nominal dimensions of 60 ×60× 6 mm and 100 ×100× 10 mm and different lengths, involved 6 stub column tests and 20 global buckling tests together with measurements on material properties, initial geometric imperfections and residual stresses. The experimental observations reveal that the column ultimate capacity and prevalent failure mode – major-axis flexural–torsional buckling and minor-axis flexural buckling – are highly dependent on the column slenderness and initial imperfections. The full load-lateral displacement, load-torsional rotation and load-axial strain responses of the specimens were fully reported and discussed. The results were employed to evaluate the accuracy of the current European and North American stainless steel design standards. The evaluation results revealed that the codified design procedures yield conservative resistance predictions of laser-welded stainless steel equal-leg angle columns, showing a prospective for their modification and improvement by providing more optimized and realistic results in design considering the actual residual stress distributions, material nonlinearity and strain hardening.

Experimental response of hot-rolled stainless steel angle columns

This paper reports on experiments addressing the buckling and collapse behaviour of pin-ended hot-rolled stainless steel equal-leg angle columns under pure axial compression, aiming to provide reliable experimental data needed to develop explicit design guidelines for hot-rolled stainless steel angle columns currently absent in European standard. The systematic experimental programme included two test groups of specimens with nominal dimensions of 60 ×60× 6 mm and 100 ×100× 10 mm produced from austenitic grade EN 1.4301, and involved tensile material tests, geometric imperfection measurements, residual stress measurements, 6 stub column tests and 20 global compression tests. A wide range of column slenderness ratios was carefully selected to account for both elastic and inelastic failure characteristics including flexural and flexural–torsional modes and the influence of the legs’ width-to-thickness ratios. The test setup and procedure together with the key experimental results and characterised failure modes, were fully reported and discussed. The test results were compared with those predicted using the design methods for compressed stainless steel angle columns provided in European and North American standards. It was observed that the specifications offer conservative strength predictions of the tested columns, primarily due to the lower distribution of initial geometric imperfections of tested columns in comparison with the permissible fabrication tolerances that are usually used as the basis of stability checks in codified design.