ABSTRACT
An integral bridge (IB) is one in which the continuous superstructure, the abutments and the single row of flexible piles supporting the abutments are built monolithically to form a rigid frame structure. The most common types of piles used at the abutments are steel H-piles. The seasonal and daily temperature changes result in imposition of cyclic horizontal displacements on the continuous bridge deck of integral bridges and thus on the steel H-piles supporting the abutments. As a result, the piles may experience cyclic plastic deformations. These plastic deformations causes low cycle fatigue in steel H-piles of IBs. The service life of the IBs highly depends on these low-cycle fatigue effects due to temperature changes. In addition, under the effect of medium and large intensity ground motions, the seismically-induced lateral cyclic displacements in steel H-piles of integral bridges (IBs) could be considerable. Modern IBs are known to have performed well in recent earthquakes due to the increased redundancy, larger damping resulting from cyclic soil-pile-structure interaction, smaller displacements and elimination of unseating potential. The monolithic construction of IBs also provides better transfer of seismic loads to the back-fill and pile foundations. However, similar to their performance under thermal effects, the seismic performance of IBs may depend on abutment height and thickness, pile size and orientation, backfill compaction level as well as stiffness of the foundation soil. A comprehensive seismic research study on IBs has not been conducted yet to provide clear suggestions for the configuration and geometric detailing of IB structural components as well as appropriate backfill and foundation soil properties to enhance their seismic performance. In the last decades, many research studies have been conducted on the performance of IBs under thermal loads, live load distribution among components of IBs and soil-structure interaction effects in IBs (Dicleli 2005, Erhan and Dicleli 2009, Kalayci et al. 2012). However, research studies concerning the seismically induced low cycle fatigue effects in steel H-piles of IBs does not exist in the literature. Accordingly, in this study, low cycle fatigue in integral bridge steel H-piles is investigated under seismic effects. For this purpose, a two-span integral bridge is considered. Three dimensional nonlinear structural models of the IB including dynamic soil-bridge interaction effects are built. Then, time history analyses of the IB models are conducted using a set of ground motions with various intensities representing small, medium and large intensity earthquakes. In the analyses, the effect of various properties such as soil stiffness, pile size and orientation are considered. The magnitude of cyclic displacements of steel H piles are then determined from the analyses results. In addition, using the existing data from experimental tests of steel H-piles, a fatigue damage model is formulated. This fatigue damage model is used together with the cyclic displacement obtained from seismic analyses to determine the remaining service life of IBs under cyclic displacement due to thermal effects. The fatigue damage analyses results revealed that the calculated cumulative fatigue damage indices in the steel H-piles induced by seismic loadings are negligible.
