Study On Seismic Ductility Calculation Of Rectangular Reinforced Concrete Bridge Pier

Bridge ductile seismic theory and design method have been widely accepted at home and abroad, and a considerable number of research results have been achieved in this field. However, the effect studies of design parameters to ductile performance of bridge structure and the determination of related design parameters are not yet detailed and systematic for the railway bridge of our country. Based on the principle of philosophy of capacity design and elastic-plastic time-history analysis method, the calculation studies are carried out on the effect of the main design parameters to ductile performance of bridge structure and some valuable results are acquired. The research works and principle results are as follows:1. The fundamental theory of ductile seismic design on bridge structures is systematically reviewed firstly. Based on the software SAP2000, considering the factors including axial compression ratio, reinforcement ratio and shear span ratio of bridge, a finite element calculation model of single pier with a hinge at the bottom is established and extensive calculated analyses of elastic-plastic earthquake responses on bridge pier have been done.2. According to the moment curvature analysis, skeleton curve parameters of reinforced concrete piers' hysteretic curve model are obtained. Through a large number of calculations, factors which influence the curvature ductility are analyzed, such as axial compression ratio, longitudinal reinforcement ratio and stirrup volume ratio. Some conclusions are acquired as follow: (1) with the increasing of pier's axial compression ratio, yield moment and yield curvature of the piers grow, and the curvature ductility of cross-section decreases as a concave curve. (2) Enlarging the pier's cross-section reinforcement ratio will increase the yield moment of piers, but reduce the cross-section curvature ductility. Increasing of the stirrup volume ratio significantly improves the sectional ultimate curvature and curvature ductility. When the reinforcement ratio is constant, increasing of the stirrup volume ratio can linearly increase the curvature ductility of piers. But when the stirrup volume ratio exceeds a certain critical value, the curvature ductility does not increase basically. This critical value means that the failure mode of pier has been changed from the failure mode caused by core concrete achieving its ultimate compressive strain to the failure mode caused by the reinforcement of tension zone achieving its ultimate tensile strain. 3. Considering different shear span ratios, site conditions, seismic fortification intensities and bridge spans, some study conclusions on effect of main parameters to bridge structures ductility performance are obtained by calculation and analyses as follow: (1) For a determined bridge superstructure and the pier height, adjusting the pier cross-section size (changing the shear span ratio) can change the displacement ductility of the bridge. Too small or too large shear span ratio will lead to lower ductility and energy dissipation capacity of the bridge structure. At the same time, the selecting of shear span ratio should make the main vibration period of the bridge structure avoid the predominant period of the site. (2) With the increasing of seismic fortification intensity (peak acceleration of the earthquake increased), the displacement ductility coefficient of piers increases. This paper suggests that different displacement ductility demands should be adopted for different seismic fortification intensity. (3) As the mass increasing of superstructure, the yield moment and displacement of the pier are increased, and the displacement ductility coefficient is yet increased. This is probably because the increasing of superstructure mass leads to enlarge maximum displacement of the pier larger than to increases the yield displacement. For the same cross-section and height of the pier, the calculation results show that the larger the bridge structure' span is, the larger the displacement ductility coefficient of the pier.