Studies On Ultimate Strength Of Reinforced Concrete Members Subjected To Combined Actions

Failures of reinforced concrete(RC)members in hazards of earthquake and hurricane are typically caused by different combinations of axial force,bending,shear and torsions.However,existing researches are far from conclusive and focus on two or three combinations of the loads.Some existing researches suggested the experimental equations based on regression analyses of experimental results.Statistical analysis methods require abundant experimental data to calibrate the equations.The lacking of sufficient experimental data lowers the accuracy of the methods.Some existing researches suggested the modified spatial truss models based on the three Navier equations.The introducing of compatibility condition makes the calculation based on these spatial truss models very difficult and not convenient for engineering.Therefore,the design and calculation theory of reinforced concrete members under combined loading actions of tensile/compressive axial force,bending,shear and torsion is relatively imperfect and not formed as the unified failure theory commonly used worldwide.This research studies the unified failure mechanism and ultimate bearing capacity model of reinforced concrete members under combined actions of tensile/compressive axial force,bending,shear and torsion,using experimental investigation,theoretical derivation and numerical analysis methods.Followings are the main research work and research results.1.This research carries out an experimental study on the ultimate capacities of RC members under different combined actions of compressive axial force,bending,shear and torsion,using self-designed experimental equipment.15 RC members are fabricated with same dimensions and same materials.These members are tested under different combined loadings of compressive axial force,bending,shear and torsion.The experiment focus on the influence of different load combinations on the geometric shape of warped failure surface and ultimate strength,especially the influence of axial force.The concept of "limit axial force under combined action" is proposed in this research.When the axial force is larger than the "limit axial force under combined action",the RC members will fail brittly.Otherwise the member will fail unbrittly.2.In this research,a calculation model is established using ultimate equilibrium theory based on the experimental research results.The influence of different load combinations on the shape and angles of warped failure surface is studied and then a new approach is proposed to determining the shape of the warped failure surface for rectangular section RC members.By assuming an appropriate stress distribution of concrete and steel on the failure surface section and considering the equilibrium conditions,the theoretical calculation model is established for ultimate strength of rectangular section RC members under combined actions of axial force,bending,shear and torsion.The calculation model can be degenerated into formulas for RC members subjected to a sole or two loading combinations of the four loading actions,which is compared with the related calculation methods of the Chinese National Regulation for RC Structural Design GB50010-2010.Finally,the proposed calculation model is compared with the existed experimental data.3.This research focuses on the ultimate capacities of box-section and I-shaped section RC members under combined actions of axial force,bending,shear and torsion using the modified calculation model of rectangular section members.These modified calculation model could be used in the simplified calculation of the ultimate capacities of box-section and I-shaped section RC members under combined actions.4.In this research,a new approach is suggested to determine the geometric shape of the warped failure surface for annular section RC members.Then a calculation model is established for ultimate strength of annular section RC members under combined actions of axial force,bending,shear and torsion.The calculation model is compared with the experimental data.5.In this research,the numerical method is used to simulate the different combination of actions.The numerical models and parameters are checked with the experimental results carried out by this research.The suggested theoretical model is checked with the results of the numerical research.In conclusion,this research carries out a series of experimental research of ultimate strength of RC members under different load combinations using a self-designed experimental equipment,proposed the concept of "limit axial force under combined action”,and suggests a theoretical calculation model of ultimate strenghth of RC members under combined actions of axial force,bending,shear and torsion based on the warped failure surface and ultimate equilibrium theory.The RC members are with rectangular section,boxed section,I-shaped section and annular section.The researches of this dissertation can be used as the references of a unified concrete strength theory and design codes.