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Seismic action and wind excitation are the main sources of excitation to civil engineering structures. The analytical structural responses are similar for both cases, but the simplified formula in design codes on the displacement response under these excitations is quite different. This paper re-visits this difference from under stationary random excitation. The power spectrum density function of the above excitations contains several parameters which define the excitations in frequency domain. The simplified formulas of the displacement variance under different excitations are derived by adjusting these parameters. The responses from these formulas always include the resonance component of the response, whereas the presence of the background component depends on the ratio between the predominant frequency of excitation and the natural frequency of the structure. The influence of this ratio on the displacement covariance and the modal combination rules is then further discussed.

The present study deals with the parametric instability of woven fiber laminated composite plates subjected to periodic dynamic loadings in adverse hygrothermal environment. The effects of various parameters like increase in number of layers and lamination angle of simply supported composite plates at elevated temperatures and moisture concentrations on the principal instability regions are investigated using finite element method. The effects of transverse shear deformation and rotary inertia, is used to study the anti-symmetric angle-ply square plates. The results show that instability of woven fiber laminated composite plates occurs for different parameters with increase in adverse temperature and moisture environment.