AbstractThe basic equation of motion to analyse the interaction of a non‐linear structure and an irregular soil with the linear unbounded soil is formulated in the time domain. The contribution of the unbounded soil involves convolution integrals of the dynamic‐stiffness coefficients in the time domain and the corresponding motions. Alternatively, a flexibility formulation for the contribution of the unbounded soil using the dynamic‐flexibility coefficients in the time domain, together with the direct‐stiffness method for the structure and the irregular soil can be applied. The dynamic‐stiffness or flexibility coefficient in the time domain is calculated as the inverse Fourier transform of the corresponding value in the frequency domain. The dynamic‐stiffness coefficient's asymptotic behaviour for high frequencies determines the singular part whose transformation exists only in the sense of a distribution. As the dynamic‐flexibility coefficient converges to zero for the frequency approaching infinity, the corresponding coefficient in the time domain is simpler to calculate, as no singular part exists. The salient features of the dynamic‐stiffness and flexibility coefficients in the time domain are illustrated using a semi‐infinite rod with exponentially increasing area. The dynamic‐flexibility coefficients in the time domain are calculated for a rigid circular disc resting on the surface of an elastic halfspace and of a layer built‐in at its base. Material damping is also introduced using the three‐parameter Kelv
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