Research On Design Methods Of Composite Frame Beams Considering The Out-of-plane Slab Composite Effect

The mechanical characteristic of the composite frame beam plays a significant role in the overall mechanical performance of the composite frame structure.Reasonable consideration of the composite effect between the concrete slab and the steel beam is always a difficult problem in the design of the composite frame beam.This dissertation focuses on the mechanical mechanism of the composite beam in the frame structure,and the methods for the internal force analysis,ultimate bending capacity calculation and deflection calculation of the composite frame beam is discussed.The main research works and results are as follows:(1)The stiffness amplification factor of the composite frame beam for internal force calculation is studied.A beam-shell mixed model of the composite frame beam is established,and is checked by different softwares.This model is used to study the main factors affecting the stiffness of the composite frame beam.Based on the parametric analysis,design formulas for the stiffness amplification factor are proposed for the composite frame beam under vertical loading and with asymmetric steel beam sections.Numerical results of composite frame systems using the proposed stiffness amplification factor are compared with the elaborate finite element numerical results which considers the slab spatial composite effect by using shell elements to verify the accuracy of the proposed stiffness amplification factor in the analysis of the composite frame system.(2)The effective flange width for calculating the bending capacity of the critical sections of the composite frame beam is discussed.The shell-solid elaborate model is used to find the key parameters of the beam-end bending capacity of the composite beam in the frame using columns with circular sections under lateral loads.The results show that the beam-end bending capacity of the composite frame beam is independent of the span length,and is closely ralated to the cross-sectional dimensions of the column,the steel beam and the slab.Based on the parametric analysis results,the design formulas of the effective flange widths for calculating the ultimate capacity are proposed for the beam end sections under the positive and negative bending moments.It is also found that the proposed effective flange width formulas under lateral loads are applicable for calculating the ultimate bending capacity of the composite frame beam-end section under vertical loads.(3)The equivalent stiffness for calculating the vertical deflection of the composite frame beam is studied.An equivalent stiffness analytical model which can consider the influence of the beam-end rotational constraint on the stiffness reduction at negative moment regions of the composite frame beam is established.The influences of the beamend constraint and the composite beam section characteristics on the equivalent stiffness of the composite frame beam is analyzed,and design formulas of the equivalent stiffness is proposed to calculate the deflection of the composite frame beam.The proposed formulas have greatly improved the accuracy of the exiting formulas in the current design codes and the literature.The accuracy of the proposed formulas is also validated by a series of numerical results of whole-process nonlinear analysis of a structural system.In addition,in order to make the practical design procedure more convenient,a simplified deflection modification method using uncracked internal force analysis results is proposed based on the equivalent stiffness analytical model.