Evaluation of seismic response of bridges under asynchronous excitation and comparisons with Eurocode 8-2 provisions

Abstract

The paper aims to evaluate the way Eurocode 8 treats the consideration of asynchronous earthquake ground motion during the seismic design of bridges, and to discuss alternative solutions for cases wherein existing provisions do not lead to satisfactory results. The evaluation of EC8-2 new provisions and simplified methods is performed through comparison with a more refined approach whereas an effort is made to quantitatively assess the relative importance of various design and analysis assumptions that have to be made when spatial variability of ground motion is taken into consideration, based on the study of the dynamic response of 27 different bridges. It is concluded that, despite the complexity of the problem, there are specific cases where EC8 provisions can be safely and easily applied in practice, while in other cases ignoring the effect of asynchronous excitation or performing simplified calculations can significantly underestimate the actual seismic demand.

Excitación sísmica asíncrona en puentes: patrones de asincronismo, métodos de análisis y tipologías estudiadas

“…Apart from that, the Eurocode 8 (EC8) (European Committee for Standardization, 2012) proposes considering the asynchronism only if: i) there are geological discontinuities, near faults or abrupt topographic characteristics; ii) the length of the bridge exceeds 600 m. The second consideration has been called into question through studies such as that carried out by (A. S. and E. G., 1994), which emphasizes the importance of carrying out asynchronous analyses in metal arched bridges with a span greater than 400 m. Following the same line, (Álvarez et al, 2012) found out that in the case of concrete arched bridges, the asynchronous movement generates an increase in the rotation demand of the arch springs by bending and in the axial load fluctuation in bridges larger than 400 m. However, proposals based on the research of the EC8 have been made in order to include lower limits with regard to the total length of the bridge, depending on the type of soil it is supported (Sextos and Kappos, 2009).…”

“…There are regulations and design guidelines such as AASHTO (American Association of State Highway and Transport, 1996), ATC (Applied Technology Council, 1996), ATCM (Applied Technology Council and Multidisciplinary Center, 2003), the report on soil-structure interaction presented by the advisory committee of CALTRANS (Caltrans, 1999), and DSHB (Japan Road Association, 2000) that limit the asynchronous analysis on bridges only if the total length exceeds 600 m (Sextos and Kappos, 2009). Apart from that, the Eurocode 8 (EC8) (European Committee for Standardization, 2012) proposes considering the asynchronism only if: i) there are geological discontinuities, near faults or abrupt topographic characteristics; ii) the length of the bridge exceeds 600 m. The second consideration has been called into question through studies such as that carried out by (A. S. and E. G., 1994), which emphasizes the importance of carrying out asynchronous analyses in metal arched bridges with a span greater than 400 m. Following the same line, (Álvarez et al, 2012) found out that in the case of concrete arched bridges, the asynchronous movement generates an increase in the rotation demand of the arch springs by bending and in the axial load fluctuation in bridges larger than 400 m. However, proposals based on the research of the EC8 have been made in order to include lower limits with regard to the total length of the bridge, depending on the type of soil it is supported (Sextos and Kappos, 2009).…”

“…In a study carried out by (Sextos and Kappos, 2009) (Mehanny et al, 2014) (b) (a) Figure 8. Variation of the effect of 27 bridges subjected to asynchronous seismic excitation, taking the wave passage effect and the loss of correlation into account (Sextos and Kappos, 2009) …”

Anti-symmetric mode excitation and seismic response of base-isolated bridges under asynchronous input motion

“…In the second case, numerous studies investigated both the linear and/or the non-linear response of different types of bridges, namely: (a) straight bridges on uniform [44][45][46][47] or varying soil profiles, ignoring [45,47,48] or accounting for the soil-structure interaction (SSI) effects [49,50], (b) curved bridges [47,51,52], (c) skewed bridges [45,53], and (d) isolated bridges [54][55][56][57]. An extensive comparative study of 27 different structural bridge systems was presented by Sextos & Kappos [58]. Sensitivity of cable-stayed bridges in terms of SVEGM has been studied analytically [34,59] and based on existing measurements.…”

“…However, the provisions of these codes aim at capturing solely bridges' distress due to the pseudo-static response component ignoring the potential impact of higher anti-symmetric modes' excitation. What's worse, these simplified methods have a minor effect on the predicted design quantities when compared to more sophisticated ones and, quite naturally, they are not applicable to bridges which are insensitive to statically imposed displacements, such as for instance seismically isolated ones [58]. Overall, in all cases, modern seismic codes in the U.S., Europe and Asia are very reluctant to provide a detailed framework for considering the SVEGM effect in the design and assessment of bridges.…”