Abstract: Based on the wave motion principle, the impedance ratio and sublayer period ratio were extracted as basic variables to characterize the regularities of fundamental periods under different site conditions. The concept of sublayer contribution coefficient was put forward, which can be definitely determined by the two basic variables selected. Thus, a general relationship among the coefficient, impedance ratio and period ratio was established. A simple and practical method with clear physical significance was proposed to estimate the predominant period of stratified sites by using the sublayer periodic weighted cumulative technique. Taking the transfer function results as standard values, overall comparison of the proposed method with available simplified approaches was carried out under 240 calculation conditions, and the deviation analysis and applicability evaluation of different methods were given accordingly. Results show that the impedance ratio and sublayer period ratio were suitable as characteristic variables for site period research. The traditional method with simple summation of sublayer periods cannot reflect the effect of soil layer structure on the site period. Its relative deviation may reach more than 40% for the most common progressive hardening sites, and the deviations can easily be great than 20% for situations with extreme soft interlayer. Hadjian’s method exhibits an obviously higher accuracy than the classical method, while the method itself and its parameters lack clear physical meanings. The maximum relative deviations are always less than 8% for the proposed method under general and extreme situations, and the case proportions with deviation less than 5% and 2% are above 85% and 60% respectively, significantly higher than those of Hadjian’s method. The results strongly reveal that the new method fully meets the engineering accuracy requirements, and can be promoted in practical engineering.

Key words: layered soil site, fundamental period, simplified calculation method, period contribution coefficient, soil layer structure