flexural rigidity is relatively small, and the hydroelastic responses are more important than the rigid-body motions.In a real situation, this "sheet-like" structure could be excited in various ways. The most probable and important one is wave excitation, and many studies have been carried out on wave-induced hydroelastic responses in regular waves (e.g., see a recent review by Kashiwagi 1 ). The structure under consideration will also respond flexurally even under moving loads such as those imparted by an aircraft during landing or take-off. A huge mass impact would occur if an aircraft crashed onto the airport, or a VLFS might be used as a platform for a spacecraft launch. These transient phenomena must be studied to assess the safety of a floating airport.Only a few studies of transient problems have been reported to date. Using a FEM program, Watanabe and Utsunomiya 2 gave numerical results of the elastic responses when an aircraft lands on a circular VLFS. Kim and Webster 3 and Yeung and Kim 4 also studied transient phenomena on an infinite elastic runway by means of the double Fourier transform with respect to horizontal spatial variables.Ohmatsu 5 presented a numerical method based on the Fourier transform, utilizing the frequency-domain responses of elastic deflection from the various excitations considered. In his method, the infinite integral with respect to the frequency was truncated at some finite frequency and contributions from higher frequencies were completely neglected.In the meantime, Endo et al. 6,7 had reported another method, in which the so-called memory-effect function for hydrodynamic forces was computed using frequency-domain results, and then the differential equations for elastic motions were solved directly in the time domain. Their calculation method for the structural deflection is based on a FEM, and thus the unknown deflections are defined at a large number of discrete nodes over an elastic plate. In the present Abstract A time-domain calculation method is described for elastic responses to arbitrary time-dependent external loads, on the basis of a general differential equation of second order including the convolution integral related to memory effects in the hydrodynamic forces. The time-dependent elastic deflection of a structure is represented by a superposition of mathematical modal functions, and a Galerkin scheme is employed to obtain a linear system of simultaneous differential equations for the amplitude of modal functions assumed. Special care is paid to numerical accuracy in computing the memory-effect function and the added mass at infinite frequency. The validity of the numerical results was confirmed through a comparison with time histories of the vertical deflection measured in an impulsive weight-drop test conducted at the Ship Research Institute and a comparison with existing numerical results for the same problem. To check the necessity of memory-effect terms, computations using a constant value for the hydrodynamic damping coefficient were also performed, and pr...
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