Compressive tests on stiffened panels of intermediate slenderness Jose ´ Manuel Gordo, C. Guedes Soares n Center for Marine Technology and Engineering (CENTEC), Technical University of Lisbon, Instituto Superior Te´cnico, Portugal article info Article history: Received 24 October 2010 Received in revised form 2 February 2011 Accepted 2 February 2011 Keywords: Ultimate strength Stiffened plates Tests Uniaxial compression Buckling abstract Test results are presented of eight stiffened panels subjected to axial compression until collapse and beyond. The specimens are three-bay stiffened panels with associated plate made of very high tensile steel S690. The use of this very high strength steel led to the unconventional solution of using U stiffeners and this paper aims at understanding the difference of performance of this stiffener type as compared with the conventional ones. Four different configurations are considered for the stiffeners, which are made of mild or high tensile steel for bar stiffeners and mild steel for ‘L’ and ‘U’ stiffeners. The influence of the stiffener’s geometry on the ultimate strength of the stiffened panels under compression is analyzed. & 2011 Elsevier Ltd. All rights reserved. 1. Introduction New, fast, large, efficient and safe ship structures require the adoption of new structural concepts and materials with high strength weight ratio. The application of Very High Tensile Steel may be considered as a solution but it requires explicit considera- tion of the failure mechanisms, primarily fatigue and buckling. The adoption of very high strength steels allows the use of thinner plates, with the corresponding weight reduction, which is very important for high-speed vessels. However thinner plating raises important concerns about the elasto-plastic buckling strength and to circumvent this constraint, new shapes of stiffen- ers of U shape have been considered [1]. In view of this novelty, a test program was planned so that the performance of the new U configuration could be compared with the ones of traditional solutions of bar and L stiffeners. Although several test programs have been made in the past on stiffened panels under compression [2–5] no results were found for the specific shape of U stiffeners. The review of theoretical and design methods for stiffened panels did not identify either results for U stiffeners of the type used in the present panels [6,7]. Furthermore, most of the tests reported in the mentioned references have been made on one stiffened panel that in real structure would be limited by transverse frames on the two longitudinal extremes. However this approach raises difficulties in reproducing in the experiments adequate boundary conditions at the loaded edges. Furthermore it has been shown that when analyzing only one plate field one does not properly take into consideration the interaction effects with adjacent plate fields as shown in [8,9]. To circumvent this problem the test series was planned using specimens with three bays longitudinally. The use of three-bay panels instead of one single bay panels [2–4] allows to have more realistic results by avoiding boundary condition problems for the central plates related to eccentricity of load and to include the interference between adjacent panels [5]. The first problem, related to the eccentricity of the applied load in the tops of the panel, does not affect the behavior of the panel since the middle panel is not affected by it because of the lateral reaction on the supports of the intermediate frames. Thus the middle bay of the panel, where one expects the failure to be developed, is always under axial compression without eccentri- city during the entire loading path, even when the plate effec- tiveness reduces and the neutral axis shifts. The interference between adjacent panels may have different consequences, by reducing or increasing the strength of the panel when compared to a similar one made of a single bay. The continuity of a three-bay panel ensures that the supporting conditions near the transverse frames are as close as possible to the simply supported ones, but, on the other hand, it is expected some reaction from the outer bays when the buckling begins to develop out of plane deformations on the middle bay panel. These reactions reduce the out of plane deformations on the middle panel for the same load, reducing the bending moment due to the induced deformations at every point of the middle panel. How- ever, if the plate induced failure and the stiffener induced failure loads are very close for a particular geometry of the panel, then the overall collapse may be very sudden due to statistical Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/tws Thin-Walled Structures 0263-8231/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.tws.2011.02.004 n Corresponding author. E-mail address: guedess@mar.ist.utl.pt (C. Guedes Soares). Thin-Walled Structures 49 (2011) 782–794