Showing papers in "Thin-walled Structures in 2004"

Experimental and numerical investigation of static and dynamic axial crushing of circular aluminum tubes

Abstract: A comprehensive experimental and numerical study of the crash behavior of circular aluminum tubes undergoing axial compressive loading is performed. Non-linear finite element analyses are carried out to simulate quasi-static and dynamic test conditions. The numerical predicted crushing force and fold formation are found to be in good agreement with the experimental results. A summary of available analytical solutions is presented in order to estimate the mean crushing load and establish a comparison between these analytical loads and the experimental one. Some observations are made on the influence of geometrical imperfections and material strain rate effect.

Concrete-filled double skin (SHS outer and CHS inner) steel tubular beam-columns

Abstract: A series of tests on concrete-filled double skin steel tubular (CFDST) stub columns (14), beams (four) and beam-columns (12) were carried out. The specimens had square hollow section (SHS) as outer skin and circular hollow section (CHS) as inner skin. A mechanics model is developed in this paper for the CFDST stub columns, columns and beam-columns. A unified theory is described where a confinement factor (ξ) is introduced to describe the composite action between the steel tubes and the sandwiched concrete. The load versus axial strain relationship for CFDST stub columns is predicted. Simplified model is derived for section capacities of CFDST. The predicted beam-column strength is compared with that obtained in beam and beam-column tests. The load versus mid-span deflection relationship for CFDST beams and beam-columns is predicted. A simplified model is developed for calculating the member capacity of the CFDST beams. Simplified interaction curves are derived for CFDST beam-columns.

Performance of wall-stud cold-formed shear panels under monotonic and cyclic loading part i: experimental research

Abstract: The ever-increasing need for housing generated the search for new and innovative building methods to increase speed, efficiency and enhance quality, one direction being the use of light thin steel profiles as load bearing elements and different materials for cladding. The same methodology can be employed to build small steel structures for offices, schools or other purposes. Earthquake behaviour of these structures is influenced, together with other parameters, by the hysteretic characteristics of the shear wall panels. Results of a full-scale shear test programme on wall panels are presented, together with some numerical results concerning expectable earthquake performance of this structural typology.

CFRP strengthened butt-welded very high strength (VHS) circular steel tubes

Abstract: This paper investigates the behavior of carbon fibre reinforced plastics (CFRP) strengthened butt-welded very high strength (VHS) circular steel tubes. The VHS steel has a yield stress of 1350 MPa and an ultimate strength of 1500 MPa. Three types of epoxy resins with different lap shear strength were used. Tests were conducted to determine the lap shear strength between CFRP and VHS steel tubes. A total of 21 butt-welded VHS tubes strengthened with CFRP were tested in axial tension. Three kinds of failure modes, i.e. adhesive failure, fiber tear and mixed failure were observed. The suitable epoxy adhesive for strengthening VHS tubes was recommended. A significant strength increase was achieved using CFRP–epoxy strengthening technique. A theoretical model was developed to estimate the load carrying capacity of butt-welded VHS tubes strengthened using CFRP.

Experimental behaviour of thin-walled hollow structural steel (HSS) columns filled with self-consolidating concrete (SCC)

Abstract: In modern building construction, thin-walled hollow structural steel (HSS) sections are often filled with concrete to form a composite column. In recent years, the use of self-consolidating concrete (SCC), or self-compacting concrete, in such kinds of columns has been of interest to many structural engineers. Due to its rheological properties, the disadvantage of vibration can be eliminated while still obtaining good consolidation. Apart from reliability and constructability, advantages such as elimination of noise in processing plants, and the reduction of construction time and labor cost can be achieved. It is expected that SCC will be used in concrete-filled HSS columns in the future because of its good performance. However, the composite members are susceptible to the influence of concrete compaction. The lack of information on the behavior of HSS columns filled with SCC indicates a need for further research in this area. The present study is an attempt to study the possibility of using thin-walled HSS columns filled with SCC. New test data on 38 HSS columns filled with SCC to investigate the influence of concrete compaction methods on the member capacities of the composite columns are reported. The specimen tests allowed for the different conditions likely to arise in the manufacture of concrete: cured, well compacted with a poker vibrator, well compacted by hand, and self-consolidating without any vibration. The main parameters varied in the tests are: (1) column section type, circular and square; (2) tube diameter (or depth) to thickness ratio, from 33 to 67; and (3) load eccentricity ratio (e/r), from 0 to 0.3 mm. Comparisons are made with predicted column strengths using the existing codes such as AISC-LRFD-1999, AIJ-1997, BS5400-1979, EC4-1994, DL5085/T-1999 and GJB4142-2000.

Ultimate shear strength of plate elements with pit corrosion wastage

Abstract: The aim of the present paper is to investigate the ultimate strength characteristics of steel plate elements with pit corrosion wastage and under in-plane shear loads. A series of the ANSYS nonlinear finite element analyses for plate elements under in-plane shear loads are carried out, varying the degree of pit corrosion intensity and the plate geometric properties. Closed-form design formulae for the ultimate strength of pitted plates under edge shear, which are essentially needed for the ultimate limit state based risk or reliability assessment of corroded structures, are derived by the regression analysis of the computed results. The insights developed from the present study will be very useful for damage tolerant design of plated structures with pit corrosion wastage.

Elasto-plastic buckling of perforated plates under uniaxial compression

Abstract: The finite element method (FEM) has been employed to determine the elasto-plastic buckling stress of uniaxially loaded square and rectangular plates with circular cutouts. Plates with simply supported edges in the out-of-plane direction and subjected to uniaxial end compression in their longitudinal direction are considered. Much attention was placed on studying the elasto-plastic buckling behavior of perforated square plates, since understanding their behavior is the key to understand that of rectangular plates. Curves representing both elastic and elasto-plastic buckling stresses versus the plate slenderness ratio for different grades of steel were plotted in order to determine the governing failure mode as a function of the plate slenderness ratio, which is crucial for the design of such perforated plates. The center of perforation was chosen at different locations along the principal major axis of the plate in order to evaluate the effect of hole location on the failure stress of the plate. The study was extended to discuss the inelastic behavior of rectangular perforated plates with aspect ratio of 2. The study shows that the critical buckling stress for perforated plates always decreases as the plate slenderness ratio increases and that this decrease becomes steeper for large values of plate slenderness ratio, especially for small hole sizes where the failure changes from elasto-plastic into pure elastic. It also concludes that the critical stress decreases as the hole size increases, and that their values depend on the yield point of the steel used, especially for thick plates where the failure becomes elasto-plastic buckling for all hole sizes. The study further concludes that the behavior of perforated rectangular plates with aspect ratio of 2 is similar to that of a square plate with the same perforation and side length equal to the short side of the rectangular plate, but with slightly larger values of the critical failure stresses. Finally, the study recommends avoiding punching the hole near the plate edge since this decreases considerably the critical buckling stress, especially when the failure occurs in the elasto-plastic buckling mode.

Performance of wall-stud cold-formed shear panels under monotonic and cyclic loading. Part II: Numerical modelling and performance analysis

Abstract: The main components to provide earthquake performance of a light-gauge steel house are the shear walls. Understanding shear wall behaviour and finding suitable hysteretic models is important in order to be able to build realistic finite element models and assess structural performance in case of earthquake. As for any building structure expected to exceed its elastic behaviour-range in case of earthquake, the interaction of design capacity, load bearing capacity and structural ductility will influence the performance. In this paper alternative design methods and hysteretic modeling techniques are presented. Based on tests described in Part I, a numerical equivalent model for hysteretic behavior of wall panels working in shear was built and used in 3D dynamic nonlinear analysis of cold-formed steel framed buildings. Preliminary conclusions refer to the effect of over-strength and ductility upon possible earthquake load reduction in case of light-gauge shear wall structures.

A semi-analytical model for global buckling and postbuckling analysis of stiffened panels

Abstract: A computational model for global buckling and postbuckling analysis of stiffened panels is derived. The loads considered are biaxial in-plane compression or tension, shear, and lateral pressure. Deflections are assumed in the form of trigonometric function series, and the principle of stationary potential energy is used for deriving the equilibrium equations. Lateral pressure is accounted for by taking the deflection as a combination of a clamped and a simply supported deflection mode. The global buckling model is based on Marguerre’s nonlinear plate theory, by deriving a set of anisotropic stiffness coefficients to account for the plate stiffening. Local buckling is treated in a separate local model developed previously. The anisotropic stiffness coefficients used in the global model are derived from the local analysis. Together, the two models provide a tool for buckling assessment of stiffened panels. Implemented in the computer code PULS, developed at Det Norske Veritas, local and global stresses are combined in an incremental procedure. Ultimate limit state estimates for design are obtained by calculating the stresses at certain critical points, and using the onset of yielding due to membrane stress as the limiting criterion.

Flexural–torsional behavior of thin-walled composite beams

Abstract: This paper presents a flexural–torsional analysis of I-shaped laminated composite beams. A general analytical model applicable to thin-walled I-section composite beams subjected to vertical and torsional load is developed. This model is based on the classical lamination theory, and accounts for the coupling of flexural and torsional responses for arbitrary laminate stacking sequence configuration, i.e. unsymmetric as well as symmetric. Governing equations are derived from the principle of the stationary value of total potential energy. Numerical results are obtained for thin-walled composites under vertical and torsional loading, addressing the effects of fiber angle, and laminate stacking sequence.

Deflections of highly inflated fabric tubes

Abstract: Inflatable beams made of modern textile materials with important mechanical characteristics can be inflated at high pressure. The aim of the paper is to present experimental, analytical and numerical results on the deflections of highly inflated fabric tubes submitted to bending loads. Experiments are displayed and we show that tube behaviour looks like that of inflatable panels (Thin-Walled Struct. 40 (2002) 523–536). Equilibrium equations are once again written in the deformed state to take into account the geometrical stiffness and the following forces. The influence of the shear stress cannot be neglected and Timoshenko’s beam theory is used. A new inflatable tube theory is established and simple analytical formulas are given for a cantilever-inflated tube. Comparisons between analytical and experimental results are shown. A new inflatable finite tube element is constructed by use of algebraic operations, because the compliance matrix of the cantilever beam is not symmetric. Comparisons between experimental, analytical and numerical results prove the accuracy of this beam theory and on this new finite element for solving problems on the deflections of highly inflated tubes.

Section slenderness limits of very high strength circular steel tubes in bending

Abstract: This paper investigates the bending behavior of very high strength (VHS) circular steel tubes. The tested VHS tube has a normal yield stress of 1350 MPa and an ultimate strength of 1500 MPa. A total of 12 specimens, with the normal diameter ranging from 31.8 to 75 mm and the tube thickness between 1.6 and 2.0 mm, were tested. Large bending deflection was achieved for VHS tubes with the diameter to thickness ratio up to 24. Full plastic moment was reached for tubes with the section slenderness of 197 or less. New plastic slenderness limits of 117 and 153 were proposed based on the beam plastic rotation capacity of 4 and 3. A yield slenderness limit of 264 was proposed for VHS tubes.

Distortional buckling formulae for cold-formed steel C and Z-section members: Part I—derivation

Abstract: This paper presents the derivation of generalised beam theory (GBT)-based fully analytical formulae to provide distortional critical lengths and bifurcation stress resultant estimates in cold-formed steel C and Z-section members (i) subjected to uniform compression (columns), pure bending (beams) or a combination of both (beam–columns), (ii) with arbitrary sloping single-lip stiffeners and (iii) displaying four end support conditions. These formulae incorporate genuine folded-plate theory, a feature which is responsible for their generality and high accuracy. After a brief outline of the GBT fundamentals and linear stability analysis procedure, the main concepts and steps involved in the derivation of the distortional buckling formulae are described and discussed. Moreover, the paper also includes a few remarks concerning novel aspects related to the distortional buckling behaviour of Z-section beams and C-section beam–columns, which were unveiled by the GBT-based approach. Finally, note that, in a companion paper [Thin-Walled Struct., 2004 doi: 10.1016/j.tws.2004.05.002], the formulae derived here are validated and their application, accuracy and capabilities are illustrated. In particular, the GBT-based estimates are compared with exact results and, when possible, also with values yielded by the formulae developed by Lau and Hancock, Hancock, Schafer and Teng et al.

Buckling of elastic cylindrical shells considering the effect of localized axisymmetric imperfections

Abstract: The effect of localized axisymmetric initial imperfections on the critical load of elastic cylindrical shells subjected to axial compression is studied through analytical modeling. Some classical results regarding sensitivity of shell buckling strength with respect to distributed defects having axisymmetric or asymmetric forms are recalled. Special emphasis is placed after that on the more severe case of localized defects satisfying axial symmetry by displaying an analytical solution to the Von Karman–Donnell shell equations under specific boundary conditions. The obtained results show that the critical load varies very much with the geometrical parameters of the localized defect. These variations are not monotonic in general. They indicate, however, a clear reduction of the shell critical load for some defects recognized as the most hazardous isolated ones. Reduction of the critical load is found to reach a level which is up to two times lower than that predicted by general distributed defects.

The Computational Post Buckling Analysis Of Fuselage Stiffened Panels Loaded In Compression

Abstract: Fuselage panels are commonly fabricated as skin–stringer constructions, which are permitted to locally buckle under normal flight loads. The current analysis methodologies used to determine the post buckling response behaviour of stiffened panels relies on applying simplifying assumptions with semi-empirical/empirical data. Using the finite element method and employing non-linear material and geometric analysis procedures, it is possible to model the post buckling behaviour of stiffened panels without having to place the same emphases on simplifying assumptions or empirical data. Investigation of element, mesh, idealisation, imperfection and solution procedure selection has been undertaken, with results validated against mechanical tests. The research undertaken has demonstrated that using a commercial implicit code, the finite element method can be used successfully to model the post buckling behaviour of flat riveted panels. The work has generated a series of guidelines for the non-linear computational analysis of flat riveted panels subjected to uniform axial compression.

Lateral–torsional buckling of cold-formed zed-purlins partial-laterally restrained by metal sheeting

Abstract: This paper presents an analytical model for predicting the lateral-torsional buckling of cold-formed zed-purlins partial-laterally restrained by metal sheeting for both down and uplift loadings. The critical load is determined by using energy methods. The focus of the study is to investigate the individual influences of restraints provided by the sheeting and by interval anti-sag bars, the variation of moment distribution along the longitudinal axis, and boundary conditions on the lateral-torsional buckling behaviour of the purlin.

Buckling analysis of cracked plates using hierarchical trigonometric functions

Abstract: The present paper deals with the estimation of buckling loads of plates with cracking damages. The hierarchical trigonometric functions are used to define the displacement function of the cracked plate. Selective choosing of the trigonometric functions satisfies the various boundary conditions of a plate bounded by support members in a continuous plated structure. Moreover, the analysis of the cracked plate can be carried out with a minimum number of equations accurately. In the present paper, the buckling loads of plates with various types of cracks, such as edge crack and central crack, are estimated under uniaxial compressive load, biaxial compressive load and in-plane shear load. The results are found to correlate well with those obtained using a finite element method.

Improved prediction of simultaneous local and overall buckling of stiffened panels

Abstract: In this paper, improved expressions for elastic local plate buckling and overall panel buckling of uniaxially compressed T-stiffened panels are developed and validated with 55 ABAQUS eigenvalue buckling analyses of a wide range of typical panel geometries. These two expressions are equated to derive a new expression for the rigidity ratio (EIx/Db)CO that uniquely identifies “crossover” panels—those for which local and overall buckling stresses are the same. The new expression for (EIx/Db)CO is also validated using the 55 FE models. Earlier work by Chen (Ultimate strength analysis of stiffened panels using a beam-column method. PhD Dissertation, Department of Aerospace and Ocean Engineering, Virginia Polytechnic Institute and State University, Blacksburg, VA, 2003) had produced a new step-by-step beam-column method for predicting stiffener-induced compressive collapse of stiffened panels. An alternative approach is to use orthotropic plate theory. As part of the validation of the new beam-column method, ABAQUS elasto-plastic Riks ultimate strength analyses were made for 107 stiffened panels—the 55 crossover panels and 52 others. The beam-column and orthotropic approaches were also used. A surprising result was that the orthotropic approach has a large error for crossover panels whereas the beam-column method does not. Some possible reasons for this are suggested.

Effects of imperfections of the buckling response of composite shells

Abstract: The results of an experimental and analytical study of the effects of initial imperfections on the buckling response and failure of unstiffened thin-walled compression-loaded graphite-epoxy cylindrical shells are presented. The shells considered in the study have six different shell-wall laminates two different shell-radius-to-thickness ratios. The shell-wall laminates include four different orthotropic laminates and two different quasi-isotropic laminates. The shell-radius-to-thickness ratios includes shell-radius-to-thickness ratios equal to 100 and 200. The numerical results include the effects of traditional and nontraditional initial imperfections and selected shell parameter uncertainties. The traditional imperfections include the geometric shell-wall mid-surface imperfections that are commonly discussed in the literature on thin shell buckling. The nontraditional imperfections include shell-wall thickness variations, local shell-wall ply-gaps associated with the fabrication process, shell-end geometric imperfections, nonuniform applied end loads, and variations in the boundary conditions including the effects of elastic boundary conditions. The cylinder parameter uncertainties considered include uncertainties in geometric imperfection measurements, lamina fiber volume fraction, fiber and matrix properties, boundary conditions, and applied end load distribution. Results that include the effects of these traditional and nontraditional imperfections and uncertainties on the nonlinear response characteristics, buckling loads and failure of the shells are presented. The analysis procedure includes a nonlinear static analysis that predicts the stable response characteristics of the shells, and a nonlinear transient analysis that predicts the unstable response characteristics. In addition, a common failure analysis is used to predict material failures in the shells.

Knowledge-based global optimization of cold-formed steel columns

Abstract: Cold-formed steel member cross-section shapes are difficult to optimize because of the nonlinear behavior of such members under buckling loads Traditional gradient-based optimization schemes, employing deterministic design specifications for the objective function, are inefficient and severely limited in their ability to search the full solution space of member cross-sections Herein, a new global optimization approach that is well suited for optimization of such cross-sections is introduced There are two distinguishing characteristics of this approach: (1) it operates within a low-dimensional expert-based feature space rather than the high-dimensional design space of cross-section parameters; and (2) it uses a numerical implementation of the direct strength method (DSM) for the objective function Through the use of Bayesian classification trees, the most significant coordinates of the expert-based feature space are defined; these coordinates are of low dimension and are in terms of features which provide insight into structural behavior The classification trees are then used to efficiently generate candidate member cross-section prototypes for subsequent refined local optimization Optimization results are presented for three structurally distinguishable length regimes to provide proof-of-concept of the proposed scheme It is demonstrated that an expert-based feature space and its associated classification tree can effectively encapsulate the knowledge gained in the design optimization process and can be subsequently used as a starting framework for related design optimization problems This is, in essence, a highly efficient knowledge transfer mechanism that is absent in most optimization schemes Optimization of thin-walled members stands to benefit greatly from the combination of more flexible and general design methodologies (eg, the DSM) and novel, emerging, optimization schemes such as the one presented herein

A numerical and experimental investigation on composite stiffened panels into post-buckling

Abstract: The structural behaviour of composite stiffened flat panels under axial compression is here investigated up to collapse. The panel configuration is designed to buckle once the limit load is reached and to work in post-buckling until the ultimate load. The design phase is based on the use of four different kinds of finite element analyses: eigenvalue, non-linear static with modified Riks’ method and both implicit and explicit dynamic analyses. Once the final configuration is identified, two specimens are manufactured. The initial geometrical imperfections are measured and analyzed, then axial compression tests are performed until collapse. As foreseen by the numerical analyses, experimental results prove the ability of the panels designed to work in the post-buckling field until collapse which takes place due to the failure of the stiffener blades. Finally, the measured initial imperfections are included in the model significantly increasing the numerical–experimental correlation.

Distortional buckling formulae for cold-formed steel C- and Z-section members: Part II—Validation and application

Abstract: General GBT-based fully analytical formulae have been derived in a companion paper, which provides distortional bifurcation stress estimates in cold-formed steel C- and Z-section members acted by arbitrary applied stress distributions and displaying four end support conditions This paper (i) addresses the implementation of the above general formulae, (ii) illustrates their application in detail, (iii) validates them, by means of comparisons with exact results, and (iv) compares their estimates with values yielded by other formulae available in the literature After considering a wide range of (i) cross-section geometries and lengths and (ii) applied stress distributions, it is concluded that the GBT-based formulae are both accurate and universal

Experimental investigation into the structural behaviour of lapped connections between cold-formed steel Z sections

Abstract: This paper presents an experimental investigation on the structural behaviour of lapped cold-formed steel Z sections. A total of 26 one point load tests on lapped connections between Z sections with various lap lengths and test spans were carried out, and both the strength and the deformation characteristics of these connections were examined in detail. Among all tests, section failure at the end of lap under combined bending and shear was always found to be critical in the connected Z sections. Moreover, the moment resistances of lapped connections with lap lengths equal to 1.2 times section depth were found to develop only 80% of the moment capacities of connected sections. For lapped connections with lap lengths equal to six times section depth, their moment resistances were found to be significantly increased to about 140% of the moment capacities of connected sections. Similar results in the flexural rigidities of the lapped connections were also found. Consequently, it is shown that the degree of structural continuity in lapped connections against bending depend on not only the load levels, the lap length to section depth ratios, but also the lap length to test span ratios. Hence, the widely adopted assumption of full strength and stiffness connections in lapped sections is not always correct. The research work aims to provide understanding to the structural performance of lapped connections between cold-formed steel Z sections, and hence, to develop a set of rational design rules for multi-span purlin systems with overlaps in modern roof construction. The analysis and design method will be fully presented in a complementary paper.

Load-capacity estimation and collapse analysis of thin-walled beams and columns—recent advances

Abstract: The present paper is devoted to the recent results of research in the area of load-capacity and post-failure behaviour of thin-walled beams and columns (among them thin-walled cold-formed profiles). It deals with ultimate load and collapse of box-section girders (tubes) of different cross-sections under bending, as well as of lipped and plain channel-section beam-columns. The paper contains the presentation of theoretical analysis and experimental investigation of plastic mechanisms of failure and collapse behaviour of these thin-walled sections. The short review of results obtained in recent years in general precedes those obtained by the Department of Strength of Materials and Structures, TUL. The problem of post-failure behaviour is solved using the rigid-plastic theory adopted and modified for the purposes of the solution taking into consideration strain-hardening of the member’s material. On the basis of experimental investigations theoretical models of plastic mechanisms of failure are produced for different sections. Theoretical analysis is based on the principle of virtual velocities. The problem is solved in an analytical–numerical way. The particular attention has been paid to the influence of the strain-hardening of the material after yielding upon the collapse structural behaviour and also to the influence of cross-section shape and dimensions on the character of collapse. The upper-bound estimation of the load-carrying capacity of analysed thin-walled sections by combining results of non-linear, elastic post-buckling analysis with the results of plastic mechanism analysis is carried out. Results are presented in diagrams showing post-failure curves as well as curves representing structural behaviour in the whole range of loading up to and beyond the ultimate load. Some results are compared with experimental results and those obtained from FE analysis. A comparison of lower- and upper-bound estimation of the load-carrying capacity is discussed and illustrated in diagrams. Conclusions dealing with the influence of strain-hardening phenomenon displayed by the material upon the load-carrying capacity and collapse behaviour of examined sections are derived. Also conclusions concerning different upper- and lower-bound estimations of the load-carrying capacity of analysed sections are presented.

Non-uniform torsional behavior and stability of thin-walled elastic beams with arbitrary cross sections

Abstract: In this paper, the analysis of the behavior of thin-wa lled beams, derived from Prokic’s work, is carried out by using beam theory with a single warping function valid for arbitrary form of cross sections, without any distinction between open and closed profiles and without using sectorial coordinates. The finite element method is used and numerical examples show the accuracy of the solution by comparison with other numerical or analytical results. For the stability analysis, analytical and numerical calculations of critical loads are given for beams submitted to bending moment and centrally applied forces. Equilibrium equations are established from the principle of virtual work. Critical loads are calculated by considering that a structure already in equilibrium reaches instability if there is one or more than one equilibrium position for the same loading. Results with this formulation are compared to those obtained with classical warping functions.

Behaviour of high strength rectangular concrete-filled steel hollow section columns under eccentric loading

Abstract: This paper presents an experimental study on the behaviour of 12 high strength rectangular concrete-filled steel hollow section columns subjected to eccentric loading. The primary test parameters were the cross-sectional aspect ratio, slenderness and load eccentricity. The specimens with cross-sectional aspect ratios of 1.0, 1.5 and 2.0 were fabricated from high strength materials (fy=550 MPa; fcu=70.8 and 82.1 MPa). The slenderness ratios of the specimens were 20 and 50, while the load eccentricity ratios (e/B; e is the load eccentricity, B is the breadth of cross-section) varied between 0.17 and 0.40. Favourable ductility performance was observed for all specimens during the test. The experimental ultimate capacities of the specimens were compared with the design strengths predicted by the codes. Comparison of results showed that Eurocode 4 overestimated the ultimate capacities of the columns with a difference of 3%. ACI and AISC, on the other hand, conservatively predicted the failure loads by 11% and 25%, respectively.

Minimum weight of cold-formed steel sections under compression

Abstract: This paper presents a combined theoretical and experimental study on the minimum weight and the associated optimal geometric dimensions of an open-channel steel section with given length subjected to a prescribed axial compressive load. Sections both with and without lips are analyzed. The results obtained using a nonlinearly constrained optimization method are compared with those estimated from a simple-minded optimization procedure that assumes the simultaneous occurrence of all failure modes in a minimum weight structure. The types of failure mode considered include yielding, flexural buckling, torsional–flexural buckling, and local buckling. The failure criterion is based purely on compressive strength, with other possible design constraints (e.g. bending stiffness, minimum gauge and cost) ignored. The effects of end support conditions and restraint on torsional buckling are examined. The load capacity of a C-section calculated according to the 1998 British Standard Institution’s specifications on Structural Use of Steelwork in Building is used to check the validity of theoretical predictions. Finally, two new C-sections with lips were designed and manufactured based on the optimal results, and tested. Test results confirm the analytical predictions, with the optimal C-sections performing much better than the existing ones.

A numerical imperfection sensitivity study of cold-formed thin-walled tubular steel columns at uniform elevated temperatures

Abstract: A numerical study is carried out on cold-formed rectangular hollow section columns to evaluate the sensitivity of column failure strength to initial imperfections, stress–strain relationships and to assess the existing design methods. It is shown that the magnitude of initial local buckling imperfection has a significant effect on the ultimate strength of short columns where failure is predominantly local buckling. Its effect on long columns is relatively small. Similarly the magnitude of initial global imperfection has more influence on the ultimate strength of a long column, whose failure is governed by global buckling, than on short columns, where local buckling controls. The shape of the stress–strain relationship of cold-formed steel will have noticeable effect on the column failure load. Current design methods, for high temperatures in ENV1993-1-2 and for ambient temperature in ENV1993-1-3, can provide a valid basis of calculation but modification will be necessary, depending on the exact model of stress–strain relationship of cold-formed steel at elevated temperatures.

Delamination buckling and postbuckling in composite cylindrical shells under external pressure

Abstract: Composite cylindrical shells and panels are widely used in aerospace structures. These are often subjected to defects and damage from both in-service and manufacturing events. Delamination is the most important of these defects. This paper deals with the computational modelling of delamination buckling and postbuckling of laminated composite cylindrical shells subjected to external pressure. The use of three-dimensional finite elements for predicting the delamination buckling and postbuckling of these structures is computationally expensive. Here, the combined double-layer and single-layer of shell elements are employed to study the effect of delamination on the global load-carrying capacity of such systems under external pressure. It is shown that through-the-thickness delamination can be modelled and analysed effectively without requiring a great deal of computing time and memory. A parametric study is carried out to investigate the influence of the delamination size, orientation and through-the-thickness position of a series of laminated cylinders. The effects of material properties and stacking sequence are also investigated. Some of the results are compared with the corresponding analytical results. It is shown that ignoring the contact between the delaminated layers can result in wrong estimations of the critical buckling loads in cylindrical shells under external pressure.

An Experimental Study on the Axial Behaviour of Cold-Formed Steel Wall Studs and Panels

Abstract: A full-scale experimental study on the structural performance of load-bearing wall panels made of cold-formed steel frames and boards is presented. Six different types of C-channel stud, a total of 20 panels with one middle stud and 10 panels with two middle studs were tested under vertical compression until failure. The measured stud failure load agrees well with analytical prediction. The load carrying capacity of a stud increases significantly when one- or two-side sheathing is used, although the latter is more effective. It is also dependent upon the type of board used. Whereas panels with either OSB (orient strand board) or CPB (cement particle board) sheathing have nearly identical load carrying capacity, panels with CSB (calcium silicate board) sheathing are considerably weaker. Screw spacing affects the load carrying capacity of a stud. When the screw spacing on the middle stud in panels with one-side sheathing is reduced from 600 to 300 mm, the stud load carrying capacity increases by 14.5, 20.6 and 94.2% for OSB, CPB and CSB sheathing, respectively.