An Analytical Technique for the Seismic Response Assessment of Slender Bridge Piers

Abstract

Abstract. This work proposes an analytical technique for the analysis of the effects of axial loads on the dynamic behaviour and seismic response of tall and slender bridge piers. The pier is modeled as a linear elastic INTRODUCTIONA large number of bridges over mountain areas and deep valleys are built on tall piers. Because of the rugged topography of these areas, the height of bridge piers could even reach 200 m. The seismic assessment and design of slender piers usually differs from that of short piers because of the influence of axial load effects on the dynamic behaviour.It is well known that axial load effects induce in general a reduction of the transverse bending stiffness and this may have different consequences on the transverse structural response depending on the type of loading experienced by the system. In the case of static loadings, the axial load effects usually increase the response beyond the values obtained by first order analysis. On the other hand, in the case of dynamic loadings such as earthquake-induced loadings, the interaction between axial loads and transverse displacements induces changes in the system dynamic properties and elastic periods elongation [1,2], which in turn may result on either a decrease or an increase of displacements and internal action demands.In bridge engineering-design practice, the axial-load effects are usually taken into account in a simplified manner by introducing an amplification factor for the piers seismic moments [3] evaluated via first-order analysis. Many studies in past years have been devoted to the calibration of expressions for the amplification factor [1,[4][5][6][7][8][9], on the basis of simple singledegree-of-freedom (SDOF) models. These models often consisted in a rigid inverted pendulum with an elasto-plastic rotational restraint at the base, and a tip mass at the free end with a concentrated weight force. Recent works [10,11] extended the application of the amplification factor method also to the direct displacement based design of bridge piers. Other alternative methods were defined to account for axial load effects in a simplified way, without recourse to the amplification factor. In this context, the method proposed in [12] introduced the concepts of effective height and yield point spectra for the design of piers behaving as SDOF system. The main limitation of the SDOF model employed in these studies is that it is adequate to represent only short bridge piers, for which the inertia is concentrated at the top whereas the distributed pier mass and the relevant variation of axial loads can be neglected. Slender piers, as those considered in this study, are on the contrary characterized by significant distributed masses and the analysis of their dynamic behavior should also consider higher modes effects [13]. Only few studies analyzed the axial-load effects on slender piers by employing refined multi-degree-of-freedom (MDOF) models. In this context are the work of [14] and [15], analyzing the evolution of plastic hinges formation in tall p...