Simplified design of bridges for multiple-support earthquake excitation

Abstract

This paper presents a novel, bridge-dependent approach for quantifying the increase of design quantities due to spatially variable earthquake ground motion (SVEGM). Contrary to the existing methods for multiple support bridge excitation analysis that are either too complicated to be applied by most practitioners or oversimplied (e.g. Eurocode 8, Annex D provisions), this method aims to strike a balance between simplicity, accuracy and computational efficiency. The method deliberately avoids generating support-dependent, acceleration or displacement, asynchronous inputs for the prediction of bridge response. The reasons behind this decision are twofold: (a) first, the uncertainty associated with the generation of asynchronous motion scenarios, as well as the exact soil properties, stratification and topography is high while, (b) the response of a bridge is particularly sensitive to the above due to the large number of natural modes involved. It is therefore prohibitive to address SVEGM effects deterministically in the framework of a design code. Instead, this new method is based on two important and well-documented observations: (a) that SVEGM is typically globally beneficial but locally detrimental [1], and (b) that the local seismic demand increase is very closely correlated with the excitation of higher modes, which are not normally activated in the case of uniform ground motion [2,3]. Along these lines, a set of static analyses are specified herein to complement the standard, code-based response spectrum analysis. These static analyses apply spatially distributed lateral forces, whose patterns match the shape of potentially excited antisymmetric modes. The amplitude of those forces is derived as a function of the expected amplification of these modes according to the process initially proposed by Price et al. [4]. Two real bridges with different structural configurations are used as a test-bed to demonstrate the effectiveness of the new method. Comparison of the results with those obtained through rigorous response history analysis using partially correlated, spatially variable, spectrum-compatible input motions [5] shows that, the simplified method presented herein provides a reasonably accurate estimation of the SVEGM impact on the response of the bridges examined at a highly reduced computational cost. This is essentially an elastic method that is found to be simple, yet precise enough to consist an attractive alternative for the design and assessment of long and/or important bridge structures in earthquake-prone regions.

Developments Relating to Seismic Action in the Eurocode 8 of Next Generation

“…( 12), where Lkl is the distance between supports k and l and Lg,k and Lg,l are characteristic lengths given in Table 11 as functions of the site category of the considered supports k and l, respectively. This formula is derived from analyses carried out by Papadopoulos and Sextos [13].…”

On the Susceptibility of Reinforced Concrete Beam and Rigid-Frame Bridges Subjected to Spatially Varying Mining-Induced Seismic Excitation

“…The spatially varying ground motion phenomenon is well recognized in regions with natural seismicity [12][13][14], but, to the best of the authors' knowledge, it is rarely referred to in the context of mining-induced tremors [15][16][17]. Also, most studies of bridges and other multiple-supports structures focus on their responses to spatiotemporal variation in ground motion resulting from natural earthquakes [18][19][20][21][22][23][24][25][26][27][28][29].…”

“…Studies by Parvanehro et al, Papadopoulos et al, Kumar, and Bakhshizadeh et al [18][19][20][21] illustrate various approaches used to estimate the responses of bridges to spatially varying kinematic excitation. The authors explored the main phenomena contributing to the nonuniformity of ground motion, such as the wave passage effect, coherency loss, and the site effect.…”