Steel - Concrete Composite Beam Stability Study

Lateral stability and local stability are two key factors that dominate the loadcarrying capacity of steel-concrete composite beams. The design methods to stability of composite beams in the present Chinese codes are basically steel beams, which differ from the stability behaviours of composite beams. For this, the research approach in stability of steel-concrete composite beams is summarized. And this dissertation studies the stability of composite beam in the following aspects:1) Based on the theoretical analysis of steel-concrete composite I-beams' lateral buckling, analytical model on the stability of composite I-beams is put forward, and the formula which is used to calculate critical bending moment in negative bending regions is deduced. By comparison with other stability theories and methods, this dissertation indicates the design correction and suggestion about the stability of composite I-beams in negative bending regions.2) The rotation stiffness of the concrete slab and the fastener is brought in to asses the influence of the distortional restraint of the slab and the fastener on the steel top flange. Based on the theoretical analysis of steel-concrete compositeΠ-beams' lateral buckling, computational model and simplicated computational model on the stability of compositeΠ-beams are put forward. Based on these two models the formulas which are used to calculate critical bending moment in negative bending regions are deduced. By comparison with other stability theories and methods, this dissertation indicates the design correction and suggestion about the stability of compositeΠ-beams in negative bending regions.3) The mechanical behavior of steel webs with negative bending will be studied with combined stress loads of flexure, axial pressure and shear. The elastic local buckling coefficient for complicated stress loads is calculated based on interaction equations relating eccentric compression and shear. The limiting height to thickness ratio was also determined for webs without stiffening ribs in the elastic stage based on the critical buckling load. The spacing of stiffening ribs is also determined in this dissertation to prevent local buckling when the webs are only stiffened with transverse stiffening ribs.