The Failure Mode Of Frame Structure In The Strong Earthquake With Monolithical RC Slab In Consideration

In order to achieve the seismic fortification target of No Collapsing in the Strong Earthquake, a philosophy of Strong Column Weak Beam is recommended in the current codes. However, many actual structural failure modes in post earthquakes showed that, there exist deficiencies in the current design method which even make the structural failure mode change to Weak Column Strong Beam. The main cause lies in the beam over-strength. The provisions in the current codes can only delay the appearance of column hinges at a certain extent, but cannot realize the Strong Column Weak Beam philosophy substantiality. Currently, the slab contributions in the longitudinal beam over-strength are still localized in qualitative level and less convincing qualitative method developed.Aiming at making clear the influencing way of slabs which are monotonically connected to the longitudinal beams, quantifying the amount of slab reinforcement which is effective to the beam over-strength and giving an improving suggestion to the current design method, a series of research are expanded. The main contents and conclusions in this disquisition are listed as follows:1. Two series of 3D cast-in-place reinforce concrete (RC) frame specimens subjected to uniformly distributed vertical loading was investigated. Each series contain four kinds of different calculation models to calculate the reinforcement ratio, namely, the solid model, the truss model, the truss-shell model and the optimization model. The experiment results indicate that: (1) plastic hinge cannot first form at beam end by the truss or truss-shell model which is adopted as the main calculation model in current design. (2) the solid model shows the best failure mode in the four calculation models, and some plastic hinges can be found at beam end. (3) the slab participation in longitudinal beam strength should be taken into consideration in structural design. (4) because of the slab participation, the transverse beam should resist an additional torsional moment besides the bending moment. Moreover, the torsional moment in exterior beam is larger than that in interior beam.2. A nonlinear FEA software ABAQUS is adopted to conduct the specimen simulation. It indicates that: (1) the simulation results are in good agreement with the experiment results. (2) in the rectangular section of longitudinal beam, there exists not only bending moment but also the axial load which is caused by the slab. (3) the axial load appears compressive at beam end and tensile at beam midspan. Moreover, the direction of axial force varies along the beam longitudinal direction and the value of the axial load varies with the increase of load. 3. The comparison between frame structure with slab and without slab, and the comparison between frame structures with different axial compression ratios are discussed. It can be concluded that: (1) the existence of monolithical slabs delay the development of plastic hinges at longitudinal beam end. (2) the slab participation extent increases with the increase of lateral drift. However, when the story drift angle reachs 1/50, the increasing extent slows down and tends to a constant. (3) the slab participation extent increases with the decrease of axial compression ratio. (4) the main influence factors for the slab reinforcement contribution in the longitudinal beam over-strength are: lateral drift, slab reinforcement ratio, axial compression ratio and the type of joints.4. The FEM simulations of nine kinds of frame structures with different axial compression ratio and reinforcement ratio at longitudinal beam end subjected to a lateral load are carried out. The failure modes, the implement of Strong Column Weak Beam philosophy and the structural ductility are compared. It indicates that: (1) the Strong Column Weak Beam philosophy can be actualized if the frame structures are designed reasonably. (2) in the aspect of improving structural ductility and actualizing the Strong Column Weak Beam philosophy, the way of adjusting the axial compression ratio is more effectively than the way of adjusting the reinforcement ratio at longitudinal beam end. (3) when the axial compression ratio decreases from 0.9 to 0.6, the amplitude of structural ductility can be improved 31.5%; when the axial compression ratio decreases from 0.9 to 0.3, the amplitude of structural ductility can be improved 55.0%; and when the axial compression ratio decreases from 0.6 to 0.3, the amplitude of structural ductility can be improved 17.9%.5. The participation way of slab reinforcement in beam over-strength is analyzed in detail. It shows that: (1) the upper limit of the amount of slab reinforcement at top surface in resisting longitudinal beam negative moment is 0.50 times of transverse beam span for exterior joints and the whole span of transverse beam for interior joints; (2) the upper limit of that at lower surface is about 0.32 times of transverse beam span for exterior joints and 0.70 times of transverse beam span for interior joints.6. A concept of Bending Potential is adopted to describe the upper limit of slab ability in resisting negative moment at longitudinal beam end, and a concept of Bending Potential Usage is adopted to describe the real slab participation extent corresponding to some lateral drift. Then conclusions can be received: (1) when the transverse beam span, slab thickness and the reinforcement situation in slab are the same, the corresponding Bending Potential is the same for frames with different axial compression ratio in the same joint type. The difference lies in the rate of its usage, namely, the Bending Potential Usage. (2) the Bending Potential Usage is smaller at interior joints than at exterior joints. (4) in order to actualize the Strong Column Weak Beam philosophy, the actual column capacity should be always larger than the sum of actual column capacity and acting Bending Potential.7. A formula of effective flange width to consider the slab reinforcement participation is educed by the analysis of slab reinforcement stress distribution way. Moreover, a simplified formula is given corresponding to the story drift angle of 1/50 for the design use.8. At last, an improving suggestion which take the slab reinforcement parallel to the longitudinal beam in consideration is recommended. The seismic fortification target of No Collapsing in the Strong Earthquake can be more effectively to be carried out. Moreover,the feasibility of the recommendation is validated.