Abstract
In earthquake engineering, acceleration has played a major role, while wave energy has rarely been considered as a demand in design. In order to understand earthquake damage mechanism in terms of energy, the demand in terms of wave energy in surface soil layers is studied here, assuming one-dimensional SH wave propagation by using a number of vertical array records during nine strong earthquakes in Japan. A clear decreasing trend of the energy demand with decreasing ground depth and decreasing surface soil stiffness has been found as well as a propensity of incident energies calculated at bedrocks being roughly compatible with empirical formulas. How the energy demand is correlated with structural damage is also discussed in simplified models to show that induced structural strain is governed by upward energy flux, degree of structural resonance, and impedance ratio between structure and ground and structural stiffness. In low-damping brittle superstructures, wave energy flux in resonance and associated predominant frequency are decisive in determining the damage, while cumulative wave energy determines the damage in high damping ductile soil and massive concrete structures. The trend of lower energy demand in softer soil sites may not be contradictory, with a widely accepted perception that softer soil sites tend to suffer heavier earthquake damage as far as geotechnical damage is concerned.
Highlights
Seismic design in practice is based on inertia force given by acceleration or seismic coefficients
As for the wave energy carried by the upward SH wave passing through a horizontal
In order to evaluate how the upward wave energy tends to decrease as it approaches the ground surface, an empirical formula was developed, wherein ratios of upward energies between layers are correlated to corresponding impedance ratios using the dataset of vertical array records addressed above [12]
Summary
Seismic design in practice is based on inertia force given by acceleration (e.g., maximum or equivalent acceleration) or seismic coefficients. The concept of seismic coefficient was probably first started by Sano in 1917 [1] and was employed in developing building codes after the 1923 Great Kanto earthquake, which killed about 140 thousand people in metropolitan Tokyo This was followed by force-based design methodologies using peak values of accelerations or their spectral intensities, such as by Housner (1952) [2]. Kokusho and Motoyama (2002) [9] performed a basic study on the energy de demand of seismic waves in surface layers based on one-dimensional multi-reflection mand of ofSH seismic surface layers based on one-dimensional multi-reflection theory waveswaves using in vertical array records during the 1995 Kobe earthquake, which theory of SH wavesbyusing vertical array records during the 1995etKobe earthquake, which wa was followed theoretical study on the same topic by Kokusho al., (2007). The displacement u in SH wave propagating to the positive direction of z-axis as
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