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SUMMARYThe paper presents a numerical investigation on the behaviour of reinforced concrete bridge piers subjected to horizontal seismic input. Scope of the investigations is to quantify the phenomenon of bending-induced axial vibrations. The results of a set of analyses conducted on single-column bent systems indicate that #exural cracking produces, in fact, signi"cant axial vibrations. This e!ect is particularly relevant in squat elements with low axial force where the sway of the cross-sectional neutral axis under alternate bending causes strong hammering impulses at crack closure. Quanti"cation of the e!ects related to this phenomenon can be determinant for the seismic assessment of existing bridges as well as for the design of new bridges. Likewise, performance and design forces of bearings and other anti-seismic devices can be estimated with more accuracy, based on the expected level of combined vertical and horizontal acceleration response on decks. The pier overall #exural response is not signi"cantly altered by the #uctuation in axial force associated to these impulses, although local moment}curvature behaviour is, due to axial}bending interaction. Shear resisting mechanisms should be more sensitive to these vibrations and shear failure anticipated when a reduction in the axial contribution to the section shear capacity occurs. A tentative equation for the prediction of this #exural-induced vertical acceleration component is proposed based on simpli"ed section kinematics and elastic impact analysis.

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