رویکرد نظام مند به سمت رفتار سازه سیستم پوسته به سمت یک عرشه بسیار سبک وزن

Lightweight structures are increasingly used for high-speed ships. This paper presents a systematic approach to analyse the structural behaviour of a lightweight deck–side shell system using high strength steel. An analytical model of the deck–side shell system was first given, which includes the effects of stiffeners for the deck and side shell, the support conditions of the centreline girder (CL-girder), the influence of transverse beams, and the interaction between the side shell and the lightweight deck as parts of problems to the solution. By changing several geometric parameters, the sensitivity of both overall and local stress and deflection for the deck–side shell system was investigated. The different geometric parameters analysed comprise the influence for variation in the thickness of the web for transverse beams, longitudinal stiffeners and the CL-girder, the thickness of lower flange for the transverse beam and, the thickness for the panel. Furthermore, the influence of the lightweight deck and loads from the deck above on the side shell, the effects of the side shell and loads from top deck on the deck, the support conditions for the CL-girder, and the influence of deck loads on the eigenmodes were also analysed. By evaluating the results obtained from FE simulation, the support conditions of the CL-girder, the thickness of the panels and the lower flange of the transverse beams were found to be the most relevant parameters affecting both the stress and the deflection distribution of the structure. The dynamic characteristics of the structure were also analysed. The FE analysis concerning buckling of the structure was present. The results enable naval architects and structural engineers to design new extreme lightweight deck structure more reliable and economical. And some suggestions for future research are also given.

1.1. Review During the last two decades, lightweight ship's deck–hull structure is widely used in high-speed ships and the research concerning this type of structures has attracted a lot of research efforts recently. When lightweight ships are developed, different kinds of questions and problems have appeared. Complex structures and the use of non-conventional materials are factors that make the structural analysis of these types of ships more challenging. It is expected that the natural frequencies of the ship deck will decrease and new problems of vibration and damping can be expected. There are mainly three types of lightweight deck–side shell system, the first type is to use the aluminium for the material of panel, which can significantly reduce the weight, but the cost for construction and material will be higher than the conventional steel deck–side shell. The second type of deck–side shell structure is characterized by employing composite material (such as FRP) to fabricate the structural members and the panels for the deck and side shell. The third type belongs to those that the high tensile steel is employed to form stiffened plates. Although lightweight deck–side shell structure is widely used, the information available in respect of the design recommendations is still under development. Smith [1] has carried out considerable research on the deck and side shell structure using composite material such as FRP, and gave some guidelines on the design and fabrication of composite deck and side shell structure. Because of the cost he concluded that structure using FRP seems unlikely to become competitive with steel for construction of ships over about 40 m in length. However, the production of sandwich structure may change this situation in particular, if problems concerning fire and smoke are solved. Based on an FE analysis of a deck structure using box shape aluminium panel, Jia and Ulfvarson had presented the static and dynamic behaviour of a lightweight ship deck on a PCTC (pure car truck carrier) vessel [2] and [3]. By varying special parameters, such as material in the panel, numbers and locations of loaded cars, the speed of running cars on the deck during loading and the frequencies of the propeller excitation, they contributed to the understanding of how a conventional steel structure is improved by introducing lightweight material. They also made both theory study and finite element simulation to show that the chassis of the car parked on the deck influence the dynamic behaviour of the loaded structure. With the recent trend to widely use high tensile strength steel in decks, much effort has been given to study the buckling of the structure. By conducting parametric analysis for stiffened plate, Sheikh et al. concluded that the plate transverse flexural slenderness is the most significant parameter affecting the behaviour of stiffened steel plates for the buckling failure modes under combined compression and bending. He also pointed out that the plates failing by either plate buckling or overall buckling showed a stable post buckling behaviour, and the tripping failure of the stiffeners is not a concern [4]. It is found by Grondin that both the magnitude and the shape of the initial imperfection have significant influence on the stiffened plates failing by plate buckling [5]. By performing a parametric study of the post-buckling behaviour of a stiffened plate, Mateus [6] present that the slender plates have their collapse mechanism dominated by elastic effects. Only few studies have been found on optimisation and structural behaviour of the deck and side shell structure seen as a system. Thus it was considered worth to make a parametric study of the influence between different parts of structural members. When very high tensile steel is employed the buckling modes stay elastic for much higher loads than usual. This can be utilised to the benefit of light weight design if care is taken in the post buckling assessment. 1.2. The aim The aim of the study in this paper is to understand the structure of the target ship in order to make comments on further improvement of lightweight design, in particular with regard to dynamic response. The current paper presents and discusses the results of a parametric investigation into the structural behaviour of a lightweight deck–side shell system using high tensile steel. The parameters include the thickness of the web for transverse beams, longitudinal stiffeners and the middle support beam, the thickness of lower flange for transverse beams, the thickness for the panel; the effects of the lightweight deck and loads from other stories of deck on the side shell, the influence of the side shell and loads from other stories of deck to the deck, and the support conditions for the middle beam. The dynamic characteristics of the structure were also analysed. Generally speaking, a parametric study is an important part of a structural design analysis. One benefit of such a study is the identification of the parameters that have the most significant effects on the structural behaviour. The other benefit is to enable the identification of non-sensitive parameters, i.e. those parameters that can be taken as fixed values and need not be considered in depth. Despite being an important aspect affecting structural behaviour of the deck–side shell system, the influence of initial imperfections for the structure was not simulated since the work was aimed at achieving the general influences between different structural parameters. It is believed that this study is needed to obtain a comprehensive understanding of the structural behaviour. A marginal influence from initial deflection can be emphasized and will be discussed in the critical analysis of results. Some questions can be put forward: Do these lightweight ship decks have similar performances as the traditional ship decks? Can such decks be safely designed with simplistic criteria, i.e. with quasi-static load cases? 1.3. Demarcations This study has limited scope in the sense that it does not try to compare the results with classification rules. The conclusions will be more general in order to give guidance for further optimisation.

The structure for analysis in this paper is a typical lightweight deck–side shell system using the stiffened plate. This enables the results and analysis to be widely used in the application of deck–side shell structure using stiffened plate. By doing the parametric study, we know the sensitivity of structure due to changes of geometric and boundary condition parameters. In the future, we can use those results to design a more economical structure. The results obtained from FE analysis show that the structure using very high tensile steel will exhibit large local deformation in the deck panel as the panels are thinner than usual for ships. In this structure, the overall stress is more critical than the deflection on the deck. While the buckling of the panels between each two adjacent longitudinal girders may be the most critical factor. So if the stiffness of the stiffeners is increased, or the distance between each two adjacent stiffeners is decreased, the capacity of the structure may be increased further. And from the linear buckling analysis we found that the local panel buckling between longitudinal stiffeners appears prior to the overall buckling of the deck or local stiffener buckling. The analytical model gives the relations between different components of the structure in such a way that the analyst can easily judge the effects of changes for structural parameters. In particular it will show how deck and side structures interact. It has been shown from the simulation results that the support condition of the CL-girder, the thickness of the panels, and the thickness of the lower flange of transverse beams are the most relevant parameters efficiently affecting the structural behaviour of the structure. From the analysis, it is found that resonance will not occur. So, for the deck–side shell structure we have, the structure can be safely designed with simplistic criteria, i.e. with quasi-static load cases. However, the fatigue of the structure must be considered. Due to corrosion, the total thickness of the deck panels may not be fully utilized. Thus in realistic design, the thickness of the panels needs to be slightly thicker if we wish to obtain the same structural behaviour for the deck unless we manage to improve the corrosion protection system. The dynamic effect such us whipping or non-uniform distributed cargo load operation needs to be investigated. With the development of more powerful computer, finite element analysis of the whole deck–side shell structure is possible. However, to perform FE analysis is still time-consuming especially when creating the complete deck–hull model with fine mesh. Therefore, a common simplified guideline to design lightweight deck and side shell should be developed.