Capacity-based inelastic displacement dual spectra for reinforced concrete bridges considering the basin effects

Structures at a sedimentary basin could suffer from amplified and prolonged ground shaking and severe damage under a strong earthquake, even though its epicenter is far from the construction site. In order to investigate the basin effects on the seismic response and damage accumulation of reinforced concrete bridges, the capacity-based inelastic displacement dual spectra that comprise an inelastic displacement ratio spectrum ($C_R$) and the corresponding damage index ($\mathrm{DI}$) spectrum were constructed specifically for the Taipei Basin. The computed dual spectra show very different spectral shapes between three seismic zones in the Taipei Basin, where the seismic zone I of the Taipei Basin has the largest spectral ordinates for the structural period ($T_n$) less than 1.8 s. It was found that as the longitudinal steel ratio of bridge columns increases, the $C_R$ would decrease, and on the contrary, the $\mathrm{DI}$ would increase. Meanwhile, the column aspect ratio was found to have minor influences on the computed dual spectra. Comparisons of the Taiwan code’s $R-\mu -T_n$ relation to the computed $C_R$ spectra indicate that the equal-energy principle could significantly underestimate the structures’ maximum inelastic displacement demand. In contrast, the $R_d$ factors suggested in FHWA and AASHTO specifications can provide a better estimation than the Taiwan code. Finally, the dual spectral formulae for the three seismic zones of the Taipei Basin were proposed, which can be applied to the damage-based seismic design and evaluation of reinforced concrete bridges.