An evaluation of the behaviour of RC bridge piers under vertical and horizontal components of earthquake ground motion

Abstract

Reinforced concrete (RC) bridges with single piers have been used extensively in Turkish highways. During the past earthquakes in Japan, USA and New Zealand extensive damages occurred in the single piers indicates the inadequacy of seismic design. In the design of the concrete bridge piers, generally horizontal earthquake forces are taken into consideration. An additional P-∆ effect occurs when the horizontal earthquake effect and the vertical earthquake effect act together on the bridge piers with a large beam cantilever width. The effect of additional P-∆ effect under the effect of vertical earthquake may increase the bending moment and shear forces values in the columns and the piers supporting the bridge may experience damage. Another circumstance encountered in large-scale projects is the use of filler material as a dump / storage area under bridge piers. This causes a certain part of the bridge pier heights to remain under the soil filling material and shortens the free pier height. For a single column pier of a conventional bridge maximum moment and shear values occur at the bottom of the column under seismic forces. This region where the maximum stress occurs is designed to be the plastic hinge region of the column. Maximum shear and longitudinal reinforcement used in the plastic hinge region to provide the conditions is stated in the regulations. When the part of the column is buried under the soil, maximum moment and shear force values of the bridge columns may not occur at the bottom of the column but at the upper end of the buried height. Therefore, the critical section of the column will be above the bottom of the column which was not considered in the seismic design. The amount of longitudinal and shear reinforcement may also be inadequate than the required reinforcement to resist the seismic forces which may cause the flexural and shear failure. In this study, a single 30 m-tall RC pier of a conventional bridge is examined under horizontal and vertical components of three separate earthquake records for three different deck to pier flexural stiffness ratios. Conventional bridges are constructed with movement joints and connections. The pier has a monolithic 15m-wide cap beam. The height of the cap beam is increased to provide different flexural stiffness ratios between column and cap beam. As the flexural stiffness of the cap beam is increased, the change in the column shear force and moment due to additional P-Δ effect of vertical component of earthquake is investigated. As a result, only the horizontal impact of the earthquake and the effect of both horizontal and vertical earthquakes are compared for the column forces. Secondly, the single pier of the conventional bridge with the same geometric properties is investigated for the cases that it is buried under 5 m, 10 m and 15 m of earth fill. The horizontal components of the same earthquake records are used in the analysis. Forcedeformation relation of the soil is represented by non-linear p-y springs for the buried cases.