Elsevier

Structures

Volume 26, August 2020, Pages 996-1009
Structures

Full-scale collapse testing of a steel stiffened plate structure under cyclic axial-compressive loading

https://doi.org/10.1016/j.istruc.2020.05.026Get rights and content

Abstract

Plate panels of ships and floating offshore structures are likely subjected to cyclic loads arising from waves at sea. Depending on sea states, e.g., whipping in harsh sea states, the maximum amplitude of the cyclic loads may reach over 70% of ultimate loads. Of concerns is how the cyclic loads will affect the ultimate strength compared to a case of monotonically increasing loads. The aim of this paper is to experimentally investigate the ultimate strength characteristics of a steel stiffened plate structure under cyclic axial-compressive loading. A full-scale collapse testing in association with bottom structures of an as-built 1,900 TEU containership was conducted. It is concluded that the effects of cyclic loading on the ultimate compressive strength of steel stiffened plate structures are small as far as fatigue damages are not suffered due to the small number of load cycles and/or local structural members do not reach the ultimate strength during cyclic axial-compressive loading. Details of the test database are documented, which will be useful to validate computational models for the ultimate strength analysis.

Introduction

Stiffened panels are used in naval, offshore, mechanical, aerospace and civil engineering structures as primary strength members of ships, ship-shaped offshore installations, fuselages and bridges. The ultimate limit states are primary criteria for structural design and safety assessment, and they are usually evaluated considering that the external forces are increased monotonically until or after the maximum load-carrying capacity is unveiled [1], [2]. In reality, however, ship and floating offshore structures while in service are likely subjected to cyclic loads arising from waves at seas and the maximum amplitude of cyclic loads may reach over 70% of ultimate loads in whipping [3], [28]. Even if the structures may or may not reach the ultimate strength solely by cyclic loading, it is considered that the loading history and load effects associated with plastic deformations or local instability may reduce the maximum load-carrying capacity.

A number of studies on these issues are available in the literature. Yao and Nikolov [4] showed that cyclic loading may cause local failure in steel plates and thus the ultimate strength of plates can be reduced by the accumulated damages due to cyclic loading. Goto et al. [5] investigated the plastic buckling behaviour of steel plates under cycling loading. During late 1990s until recent years, a number of similar studies have been continued [6], [7], [8], [9], [10], [11], [12]. Very recently, Li et al. [13] proposed an analytical method to predict the ultimate strength behaviour of steel plates and stiffened panels under various patterns of cyclic loading. Jagite et al. [14] investigated dynamic ultimate strength behaviour of steel stiffened plate structures under more realistic scenarios of cyclic loading. In fact, this problem may also be associated with shakedown limit states [15], [24]. Wave-induced hull girder loads depending on sea states can be predicted by probabilistic approaches [16] and used for structural response analysis [18]. Cyclic extreme loads can reduce welding-induced residual stresses [25], and they are associated with moving ice loads [26] or seismic loads arising from earthquake in jacket offshore structures [27].

Most of previous studies in the literature have been made by theoretical or numerical methods. Experimental studies have used small-scale models of plates or stiffened panels which cannot convert directly to full-scale prototypes in the ultimate strength behaviour for many reasons of scale effects such as welding-induced initial imperfections and other nonlinear effects associated with multiple physical processes, multiple scales and multiple criteria. Most of all, previous studies do not clearly resolve the issues – some studies argue that the effects of cyclic loading on the ultimate strength are negligible, while others indicate that cyclic loading can reduce the ultimate strength.

The objective of the present paper is to contribute to developing test database on the ultimate strength behaviour of a full-scale steel stiffened plate structure under cyclic axial-compressive loading. A full-scale substructure of an as-built containership carrying 1,900 TEU was tested. The test structure was constructed in a shipyard using exactly the same technique of welding as used in today’s shipbuilding industry.

Section snippets

Design of a full-scale steel stiffened plate structure

Ship hull structures are repeatedly subjected to hogging or sagging in waves, and subsequently plate panels of hull structures develop axial compressive or tensile loads, as shown in Fig. 1. In harsher sea states, the hogging or sagging moments may become large, e.g., in whipping at harsh sea states, and the plate panels may or may not buckle locally even if they may not reach the ultimate limit states. Having recognized that containerships in full load condition are in hogging [16] and

Fabrication of the test structure

The test structure was fabricated by a shipyard in Busan, South Korea, which usually builds small and medium sized ships for trading cargoes and patrolling along coastlines. After material procurement of high tensile steel with grade AH32 was completed, tensile coupon test specimens were extracted from the steel sheet as per the specification of ASTM (American Society for Testing and Material) E8 [18], as shown in Fig. 6.

Fig. 7 shows the engineering stress-engineering strain curves of the

Measurements of welding-induced initial imperfections

Welding induces initial imperfections in the form of initial deformations and residual stresses which significantly affect the ultimate limit states of structures, and thus their magnitudes and shapes were measured after completing the construction of the test structure. Details of the measurement results together with methods of numerical predictions are reported in Yi et al. [20], [21]. In this paper, the results are briefly presented.

Modern technologies for measuring the initial

Test frame and jigs

Fig. 11 shows the layout of test set-up with a specially designed rig which makes possible to perform a large scale physical model testing. The test was conducted at the ICASS/KOSORI test site (www.icass.center) in Hadong, South Korea. As shown in Fig. 12, the axial compressive loads were provided by two hydraulic actuators (among three at the test facility) which were fixed on a reaction wall. Each loading actuator can carry up to 1,000 tons in compression. The ‘rigid-body’ jig helped achieve

Test results and discussion

Fig. 15 shows the axial-compressive load versus time history (for a load ratio R of tension to compression to tension with R = 0) and the axial shortening versus time curve during the testing. Table 7 provides details of the load application at each step which was recorded by a personal computer.

By combining both Fig. 15(a) and (b) at identical time steps, the axial compressive load versus axial shortening curve was obtained as shown in Fig. 16. Table 7 provides in-plane stiffness of the test

Concluding remarks

The aim of the paper was to obtain the test database on the ultimate limit states of a full-scale steel stiffened plate structure under cyclic axial-compressive loading. Based on the studies, the following conclusions can be drawn.

  • (1)

    A collapse testing was conducted on of a full-scale steel stiffened plate structure under cyclic axial-compressive loading in association with bottom plate panels of an as-built 1,900 TEU containership in whipping condition with a load ratio of tension to compression

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This study was undertaken at the International Centre for Advanced Safety Studies/The Korea Ship and Offshore Research Institute (www.icass.center) which has been a Lloyd’s Register Foundation Research Centre of Excellence since 2008. Part of the work was supported by the Swedish Research Council by the project “Fundamental research on the ultimate compressive strength of ship stiffened plate structures at Arctic and cryogenic temperatures”, contract no. 2018-06864.

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