Research On Seismic Behavior Of Skewed Integral Abutment Bridge

With the continuous development of traffic network in China,the construction of small and medium-span bridges will continue to increase;and due to the restriction of the road or river direction below the bridge,a considerable proportion of the small and medium-span bridges are skewed.The seismic responses of the skewed bridges is stronger compared with orthogonal bridges,and the superstructure of the skewed bridges is more likely to have large displacement or even fall from the abutment during the earthquake.Therefore,skewed integral abutment bridges,in which the superstructure is connected monolithically with the abutments,will have remarkable anti-seismic advantages considering displacement-related seimic damage.On the other hand,the coupling effects of the longitudinal and transverse seismic responses of skewed integral abutment bridges are obvious,as well as the torsional effects,due to the influence of abutment-soil interaction,which makes the seismic responses more complex.Therefore,the dynamic properties,seismic responses and applicability of seismic design method of skewed integral abutment bridges are studied in this paper.The main research work and outcomes are as follows:(1)The fundamental mode of skewed integral abutment bridges are translational mode,which gradually changes from transverse translation mode to a coupled longitudinal-transverse translational mode as the skew angle increases.The other mode shapes also gradually change from a single direction vibration to bi-directional coupled vibration,with the increase of skew angle.The influence of skew angle and backfill compactness on the natural vibration period of the fundamental mode is small,while the influence of pile type and layout is realtively large.(2)The maximum displacement of the skewed integral abutment bridge occurs at the acute corner of the deck under earthquake,with a certain degree of in-plane rotation.Both the deck rotation and transverse displacement increases with the increase of skew angle,but increase first and then decrease with the increase of the backfill compactness.The longitudinal displacement increases with the increase of the skew angle,and decreases with the increase of backfill compactness.When the skew angle is greater than 15°and the backfill is dense,the maximum displacement of the superstructure is approximately along the axis of the abutment.(3)The maximum stress in steel piles occurs at the pile top.The internal force responses of steel piles increase with the increase of skew angle.When the skew angle equals to 45°,the internal force can reach to 1.4~4.0 times of the orthogonal bridge case.When the skew angle is less than 15°,the increase of backfill compactness can significantly reduce the internal force and plastic development of the steel pile,with the internal force decreasing by 38%~78%from non-backfill to dense backfill.(4)Skewed integral abutment bridge with small skew angle is recommended in engineering.Dense backfill is recommended when the skew angle is no more than 15°.For areas with high seismic intensity,when the skew angle is 45°,it is suggested to separate the abutment from the backfill.When using H-shaped steel pile,it is not recommended to align its web perpendicular to the axis of abutment.(5)When the skew angle is greater than 15°,bi-directional seismic effects should be adopted when calculating the seismic response of skewed integral abutment bridges;moreover,in addition to the S_X+0.85S_Y and S_Y+0.85S_X cases in the current code,two other cases S_X-0.85S_Y and S_Y-0.85S_X need to be included as well.The envelope value of seismic responses calculated from these four cases is then taken as the maximum seismic response.