رفتار سازه یک عرشه فولاد کششی بالا با استفاده از سخت کننده های ذوزنقه ای و پویایی تعاملات خودرو عرشه

As an early part of a large design and fabrication-oriented project FasdHTS funded by the GROWTH programme of the European Commission, an exotic concept ship was designed in very high tensile steel (EHS690) with the purpose of finding out consequences for design and production. The project has already produced a considerable bank of knowledge for design and shipyard production in this material. This paper presents analysis and discussions on static and dynamic behaviour of a high tensile steel deck designed with trapezoidal stiffeners. First, a finite element model of the deck structure is created. The influence of support condition for the longitudinal girders, and the contact area between the vehicle tyre and panel were analysed. The results from modal analysis of the structure under different load conditions are presented. The different load conditions comprise the unloaded and loaded deck, and the load type, i.e. cargo loads or vehicle loads (car loads or truck loads). From the frequency response analysis under harmonic excitation, it shows how the locations and numbers of cars parked on the deck influence the dynamic response of the structure. Furthermore, by studying the car–deck interaction, it is found that the effects of normal cargo loads are quite different from the vehicle loads due to the spring/damping effects of the vehicles. It is suggested that the carloads have a similar mechanism to that of tuned mass dampers. Finally, two transient analyses of the structure due to excitations transferred from deck supports and lorry braking-induced loading are performed. It is suggested that the deck structure and vehicle design could have more interactions with each other.

With the growing demands for fast ships, weight is a key factor in the structural design. Thus, the lightweight ship's deck structure is widely used in high-speed ships and the research concerning this type of structure has recently attracted a lot of effort. The classification societies do not normally require a dynamic analysis of a deck structure in ships. The design is based on a quasi-static model of analysis. It is expected that a very lightweight deck structure has a somewhat higher natural frequency in the non-loaded condition and a relatively lower frequency in the as-loaded condition compared to a conventional deck structure. The reason is of course that the designer makes an effort to reduce the mass of the structure, while limiting the deformation. It is inevitable that stress levels increase and that the deformations for the as-loaded condition increase. The consequences of working with different stiffness and thus frequencies are not known from experience and need to be better researched before entering production. Complex structures and the use of non-conventional materials are factors that make the analysis of these types of ships more challenging. It is expected that the natural frequencies of the ship deck may decrease and new problems of vibration and damping can be expected. With the increase of the design speed of the ship, the speed of cargo loading and discharge will also increase. When combined with lightweight design, the dynamics of the cargo loading may be more important to study. It is known that the behaviour and the strength considerations of the structural elements under dynamic loading are quite different from those under static or quasi-static loading conditions [1]. There are mainly three types of lightweight deck structures, the first type is to use aluminium for the panel, which can significantly reduce the weight, but the cost of construction and material will be higher than the conventional deck. The second type of deck structure is characterised by employing composite material (such as FRP) to fabricate the structural members. The third type is to employ very high tensile steel to form ship plating. In the FasdHTS-project, the target ship is designed with EHS690 steel. The Poisson's ratio for that is 0.3, the modulus of elasticity is 210 GPa, the density is 7800 kg/m3, and the yield stress is 690 MPa. Although lightweight deck structures are widely used, the information available in respect of the design recommendations is still under development. Based on FE analyses of a deck structure using box shape aluminium panels, Jia and Ulfvarson presented the static and dynamic behaviour of a lightweight ship deck on a pure car truck carrier (PCTC) 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 a theoretical study and a finite element simulation to show that the chassis of the cars parked on the deck will have influences on the dynamic behaviour of the structure. With the recent trend to use high tensile strength steel in decks, much effort has been given to studying the limit load and behaviour of the structure using T-shaped stiffeners. By conducting parametric analysis for stiffened plate, Sheikh et al. [4] concluded that the plate transverse flexural slenderness, i.e. the inverse of bending stiffness, is the most influential parameter affecting both the strength and behaviour of stiffened steel panels for the buckling failure modes under combined compression and bending. It is found by Grondin [5] that both the magnitude and the shape of the initial imperfection have significant influences on the stiffened plates failing by plate buckling. By performing a parametric study of the post-buckling behaviour of a stiffened plate, Mateus [6] showed that for slender plates the collapse mechanism is dominated by elastic effects. By doing a series of experiments to test the dynamic ultimate compressive strength of the ship plating, Paik and Thayamballi [1] found that the in-plane stiffness and ultimate compressive strength of steel plates both tend to increase with the increase in the speed of load application (the speed of loading ranges from 0.05 to 400 mm/s, which corresponds to strain rates in the range from 10−4 to 0.8 s−1). They also showed that as the speed of load application increases, the response of steel plates becomes more unstable after the ultimate strength is reached, i.e. there are less post-buckling margins of safety. By studying the behaviour of one longitudinal trapezoidal stiffener, Garbatov and Soares [7] had investigated the influence of transient-induced stress cycles on the fatigue life of the structure. They also present a simplified approach for analysing the structural response due to wave-induced and slamming-induced load [7]. By performing a systematic study for a lightweight deck-hull structure using high tensile steel, Jia and Ulfvarson [8] presented the static and dynamic analysis of a lightweight ship deck. By analysing the analytical model and observing the FE results, they concluded that large deformations between each two adjacent longitudinal stiffeners occur if the deck is extreme lightweight. The local panel buckling between longitudinal stiffeners appears prior to the overall buckling of the deck or local stiffener buckling. The support conditions of the CL-girder (centre line girder), the thickness of the panels and the lower flange of the transverse beams close to the CL-girder were found to be the most relevant parameters affecting the stress and the deflection distribution of the structure. Although trapezoidal stiffeners have been used for many years in the field of civil engineering, this type of structure is not much used in naval architecture. Therefore, very little research has been carried out so far on the dynamic behaviour of trapezoidal stiffeners in ships. Since the structure may be manufactured more lightweight and slender than the conventional deck, it may be more vulnerable to dynamic loads. Thus, the current paper presents the discussions on both static and dynamic characterization of this type of deck using high tensile steel. As part of the vehicle–deck interaction analysis, the effects of the vehicles parked on the deck are studied. Compared to the vehicle deck models the authors have presented previously [3], suspension stiffness and damping are included in order to study the dynamic response of the vehicle–deck system. The knowledge contributed by this paper enables naval architects and structural engineers to make improvements on the design of the deck structures using trapezoidal stiffeners.

By analysing all the results present in this paper, the following conclusions and suggestions can be drawn: • The support conditions for the longitudinal girders significantly influence the structure behaviour of the deck. • Large stresses were observed in some structural discontinuities, such as the connection between the longitudinal girders and the transverse deck beam. The structure is based on the strength-driven design. Thus it is recommended to locally strengthen those areas. Local strength gives margins for further weight reductions. • To simplify the FE modelling, it is suggested that the FE model of deck structure could be created in a way that the boundary conditions from the supporting structure of the deck can be calculated by using an analytical approach. • The structure should be modelled in detail, thus the stresses in local area, which may be critical for design, could be found and improved. • The contact area between the vehicle tyre and the deck has some influence on the static behaviour of the deck structure. The relation between tyre pressure, contact area and the stiffness of the tyre should be appreciated in the design analysis. • The vehicles parked on the deck have some influences on the dynamic behaviour of the structure. Compared to cargo loads, the carloads may decrease the dynamic response in a specific range of excitation frequency. • The deck structure is quite stiff due to the contribution of trapezoidal-shaped stiffeners. It is obvious that the stiffeners can be further optimised. The structure is safer if the design is based on a static analysis combined with a dynamic as well as stability control, considering plasticity of the structures. • Optimisation of the web plates against buckling criteria must be made. Then we will see thinner plates between the deck horizontal plates and the flanges of CL-girders and other beams. • In the dynamic analysis, it is quite important to consider both the excitation frequency and the ratio of effective mass in each direction; thus a suitable number of eigenmodes would be extracted for analysis. • It is believed that the FE analysis could predict accurate results for static, quasi-static and modal analysis of the structure. Harmonic analysis and transient analysis depend on many factors, such as the damping of the structure, the loading type, and the solution method, etc. Thus in situ measurements on a real ship's deck are recommended for the validation of the results predicted by FE analysis. • It is noticed that the stresses in the deck panel are not high. • It is observed that the stresses in the panels are much lower than the stresses in the supporting beams. In an effort to optimise the total deck structure with regard to yielding (permanent deformation) an improvement can be obtained by making the panels and the beams approach the yielding together with less margins between them. The separation should be chosen to fit carefully selected safety considerations. • Since the merits of the lightweight structures would be lost if the associated connections were not properly designed, thus it is worthy to study in which way could the joints be designed to ensure the structural redundancy and system ductile characteristics most efficiently. • By studying the vehicle dynamics, tyre pressure and the operation of vehicles on deck, it is suggested that the deck structure and vehicle design should have more interaction with each other. By combining vehicle dynamics, wave induced ship motions and propeller excitations, the authors are currently engaged in developing a code to calculate the vehicle cargo securing on board. • It is strongly advised to go back to the drawing board and intensively interact with the customers (users, stake holders) of the ship and develop the logistics of cargo loading in such a way that lightweight structure design can be optimised further. The relation between pillar support and lightweight design is shown to be so strong that these steps of optimisation must be taken in order to fulfil the requirement of lightweight design for the target ship.