A.E. Elwi

Buckling of stiffened steel plates—a parametric study

The stability of plates stiffened with tee-shape stiffeners was investigated using a finite element model. Four series of stiffened plate panels were modeled using a finite strain fournode shell element. The model was validated using the results of tests on full-size stiffened plate specimens and was subsequently used to perform the study of various parameters presented in this paper. The parameters investigated are: the shape and magnitude of initial imperfections in the plate; residual stress magnitude and direction of applied uniform bending; plate slenderness ratio; plate aspect ratio; and plate to stiffener cross-sectional area ratio. The effect of the investigated parameters on the axial load carrying capacity and the mode of failure of stiffened plates is investigated both in the elastic and inelastic ranges. A comparison of these results with design guidelines formulated by Det norske Veritas and the American Petroleum Institute indicates that the guidelines are generally conservative for cases where initial imperfection magnitudes do not exceed the guidelines’ prescribed maximum.

Stiffened steel plates under uniaxial compression

Abstract The stability of steel plates stiffened with tee-shape sections under uniaxial compression and combined uniaxial compression and bending was investigated using a finite element model. The emphasis of the work presented in this paper was to find the parameters that uniquely describe the strength and behaviour of stiffened steel plates. A finite element model, validated using the results of tests on full-size stiffened plate panels, was used to investigate the scale effect for five dimensionless parameters. The parameters investigated were: the transverse slenderness of the plate, the slenderness of the web and flange of the stiffener, the ratio of torsional slenderness of the stiffener to the transverse slenderness of the plate, and the stiffener-to-plate area ratio. Average magnitude residual stresses and initial imperfections were assumed for this study. A parametric study covering a wide range of dimensionless parameters indicated that stiffened steel plates do not fail by stiffener tripping unless a bending moment is applied to create flexural compressive stresses in the stiffener. Although plate buckling and overall buckling were found to lead to a very stable post-buckling behaviour, the interaction between these two buckling modes was found to give rise to a sudden loss of capacity following initial plate buckling. The plate transverse slenderness, the stiffener slenderness-to-plate slenderness ratio, and the stiffener-to-plate area ratio were found to have a significant effect on this behaviour. A comparison of the numerical analysis results with API and DnV design guidelines indicates that the guidelines predict stiffened steel plate capacity with various degrees of success, depending on the governing mode of failure. Neither guidelines address the potential interaction-buckling phenomenon.

Stiffened steel plates under combined compression and bending

Abstract This paper presents part of a series of investigations of the behaviour of steel plates stiffened with tee-shape stiffeners and loaded with axial compressive forces with or without bending moments. These elements typically form bridge decks, ship hulls, ship decks and heavy haul equipment walls. Earlier work by the authors validated a non-linear large deformation-finite strain elasto-plastic finite element model by comparison of the model with the results of sophisticated full-scale physical experimental trials under different load combinations. A parametric study carried out using the finite element model is presented in the following. The study deals with the response of stiffened plate elements under combined uniaxial compression and bending moment. The parameters investigated were the transverse slenderness of the plate, the slenderness of the web and flange of the stiffener, the ratio of torsional slenderness of the stiffener to the transverse slenderness of the plate, and the stiffener to plate area ratio. Average magnitude residual stresses and initial imperfections were assumed. The parametric study indicated that the plate transverse flexural slenderness is the most influential parameter affecting both the strength and behaviour of stiffened steel plates for all the failure modes observed under combined compression and bending. The ratio of stiffener torsional slenderness to plate transverse flexural slenderness, β 4 , affected both the strength and behaviour of only the stiffened plates failing by stiffener tripping. A comparison of the numerical analysis results with American Petroleum Institute and Det Norske Veritas design guidelines indicates that the guidelines predict stiffened steel plate capacity with various degrees of success, depending on the governing mode of failure.

Flexural tests of longitudinally stiffened fabricated steel cylinders

Abstract Fabricated steel cylinders with large R t ratios are often used in conveyor galleries, offshore platforms, storage tanks, towers, vessels, and the like. The use of longitudinal stiffeners in these large cylinders has proven to be an efficient way to promote structural stability. The increase of load capacity is significant. However, the attachment of the stiffeners complicates the problem by introducing additional buckling modes and new design parameters. Four large-scale specimens were tested. The collected data included the load versus deformation behavior, as well as the geometric imperfections and welding residual stresses of the specimens. The objective of the research program was t investigate the local buckling behavior and ultimate load capacity of longitudinally stiffened steel cylinders subjected to bending. The results showed two failure modes, a shell buckling mode and a general buckling mode. The occurrence of both modes and the corresponding capacity depend on the shell properties, as well as on the spacing and angle of arc covered by the stiffeners. The general buckling mode has strength and ductility much higher than those predicted by existing design guidelines.

Tests of fabricated steel cylinders subjected to transverse loads

Abstract The inelastic shear behavior of large-diameter cylindrical shells subjected to transverse loads was investigated. Two specimens of 1270 mm diameter were tested under different boundary conditions. The tests were preceded by finite element simulation in order to help plan the tests and to offer insight into the findings. Measurement of the geometric imperfections and the welding stresses was made prior to testing and the testing itself was carried out in both the pre-buckling and the post-buckling ranges. The finite element predictions and predictions made using other design equations compare favorably with the test results.

Ultimate shear strength of large diameter fabricated steel tubes

Abstract Large diameter fabricated steel tubes subjected to transverse shear forces and bending moments can fail either in a local compression buckling mode because of flexure or in a diagonal buckling failure mode as a result of shear forces. Drawing on recent work by the writers at the University of Alberta, the ultimate shear strength is investigated through a parametric study to determine the factors affecting failure. A nonlinear regression analysis is then carried out using the available experimental data to derive an empirical relation for the ultimate shear strength. The formula is compared to the current design approach and found to perform better throughout the range of applicability.

Longitudinally-stiffened large diameter fabricated steel tubes

Abstract Fabricated steel cylinders are often used as conveyor galleries, stacks or masts, or as members in offshore structures. The cylinders are fabricated by first cold rolling steel plates to form short cans and then joining these together by circumferential girth welds to yield long spans. Circumferential stiffeners are almost always present, in order that the circular shape of the tube is maintained and as an assistance when the cylinder is being handled. Longitudinal stiffeners may be used as well. If they are present, they will be welded to the cylinder surface, thereby improving the behaviour under axial compression or beam bending. The paper addresses the design of the type of cylinder commonly used as a conveyor gallery. In this application, diameters of from 2·5 to 4·0 m are typical and the radius to thickness ration, R/t, ranges from about 100 to 400. Spans can reach 60 m. The results of four large-scale flexural tests on large diameter fabricated steel cylinders that had longitudinal stiffeners and a parametric study of hypothetical cylinders of this configuration are used as the basis of the study. The paper reviews current design standards in the light of those test results, presents the results of the parametric study and makes recommendations for the design of longitudinally-stiffened fabricated steel cylinders.

Shear capacity of large fabricated steel cylinders: A truss model

Abstract Failure of large diameter steel cylinders fabricated from hot rolled sheet steel plates and subjected to transverse beam shear occurs when large diagonal buckles develop in the panels formed by the circumferential stiffeners. The load-carrying capacity then drops to a slightly lower level than the ultimate strength level. Whereas the ultimate strength has been shown to be significantly reduced in the presence of fabrication residual stresses, the postbuckling load level is not affected. Further, this load level is stable. It is advocated, therefore, that the postbuckling load be used as a conservative design base for calculation of shear capacity. A general truss model is developed herein as a description of the load-carrying mechanism in the postbuckling range. Predictions obtained using the model are compared with available test results and a design chart is developed that will enable designers to handle most practical cases.