Geometric And Material Nonlinear Analyses For Steel Framed Structures Of Tall Buildings

There are many advantages in steel structures, such as lighted-dead weight, excellent performances of ductility and seismic resistance, rapid construction speed and significant integrated economic benefits with increasing of the quantity and quality of the steel products and improving of the manufacturing and assembling technique of the steel tall-buildings. More and more steel and composite steel-concrete structures are established in our country. Since the structural members and influence factors of tall-buildings are more than common structures, the structural design of tall-buildings is more complicated and time-elapsed, and full structural analyses are needed. There is practical significance to establish a rational reliable theoretical analysis model and process an accurate analysis. In recent specifications for steel structures, limit-states design method has been widely used. This has resulted in more rational consideration of the effects of inelasticity and stability at maximum design load level. At the state-of-the-art, however, there is no practical and mature analysis method that may be used in limit-states design of common structures. A real limit-states design is used only in the design of small-scale structures and individual members. Most current specifications still use elastic internal force to perform structural design in virtue of effective length factor to check the stability of individual members, however, there is some limits in the method of effective length factor. First, the method cannot accurately account for the interaction between structural system and its individual members. Second, it does not consider the redistribution of inelastic internal force and cannot predict the instability mode. And it requires individual member capacity checks, including calculation of the effective length factor K. Therefore, the method that can describe various nonlinear performances exactly as well as individual member capacity checks is a current research work. Advanced analysis appeared recently is just this kind of method. The advanced analysis is practically a kind of second-order elasto-plastic analysis method. It refers to any method that captures the strength and stability of a structural system and its individual members in such a way that separate member capacity checks are not required. To realize accurate structural analysis, the various nonlinear factors such as geometric nonlinearity, material nonlinearity, initial imperfection, residual stress, connection nonlinearity and second-order effect of member load etc. must be considered. The purpose of the thesis is to build up an analysis model that meets the above requirements mentioned, to analyze the ultimate behavior of steel tall-buildings accurately. Second-order elastic analysis is already suggested in some advanced codes and specifications. An accurate and efficient four-termed beam-column element that adopts co-rotational formulation is proposed. The tangent stiffness matrix of the element is presented using the principle of stationary potential energy. The accuracy of the four-termed element is identical with stability function and the element overcomes the difficulty that the formulae of the stability function are different when the sign of axial force changes. The formulated element can be used for geometric nonlinear analysis by 1 element/member as well as describe the effect of axial-torsion buckling (Wagner effect) exactly. So far, there is no comparatively refined analysis method suitable for composite steel-concrete beam-columns with cracks. A simplified method that assumes the cross-section of the members remaining plane after deformation is proposed for composite steel-concrete beam-columns. The contribution of the concrete in the cracked zone is ignored and the member is modeled by stepped beam-columns. The stiffness matrix is derived and can be used for second-order elastic analysis conveniently. The plastic-zone analysis is the most accurate method for second-order inelastic structural analyses. It can trace the progress of the plasticity along member length and cross section and process initial imperfection directly. However, the plastic-zone method is very complicated and overloaded with details and not caters for practical application. The plastic-hinge method is a qualified model to the structural analysis of tall-buildings, but the classical plastic-hinge method cannot allow for initial imperfection and predict the possible local buckling. Moreover, the assumption of lumped plasticity is not applicable when the base columns of high-rise buildings and the predominant axial force members yield along the whole length. Most of the researchers assume the plastic-hinge forming at member ends and the inner plastic-hinge case is not considered. Therefore, based on M-P-F relation of eccentrically compressive members, the varying discipline of the member stiffness from elastic to plastic state is studied by virtue of equivalent column method. An efficient pseudo-spring model of gradual plastic-hinge is presented on the basis of lumped plastic assumption. The stiffness of the members under different load level can be simulated by modifying the stiffness of the pseudo-spring.The model can accurately depict the effects of inner plastic-hinge and gradual plastification, and the residual stress and the effect of transverse member load are considered automatically. The elastic and inelastic analysis can be implemented using same model by the condensation technique that eliminates the internal freedoms. The behavior of the connection of steel beam-columns is nonlinear and the semi-rigidity of the connection must be considered in the limit-states analysis. The geometric imperfection is another important factor that affects the load-bearing capacity of the structural system and its members. It will result in the load-bearing capacity decreasing significantly for the structure sensitive to imperfection. Based on the inner plastic-hinge model, a spring-in-series model combined the nonlinear behavior of connection and material is proposed. The intricate calculation and numerical instability are avoided and the incremental stiffness matrix is derived. The formulation of the model is consistent with the requirements of the advanced analysis and suitable for advanced analysis of the planar steel frames. Several methods processing geometric imperfection are discussed. It is indicated that the geometric imperfection is not always harmful to the structural bearing capacity and the direct analysis method is an accurate method to consider the geometric imperfection. The stiffness formula for the direct analysis of beam-column element that considers the transverse member load is obtained. On the basis of formulation of the four-termed element, using the flow theory and the Orbison's yielding surface, the second-order elasto-plastic stiffness matrix of the 3D beam-column element is suggested. Plastic-hinge can form at arbitrary position along the element and the basic plastic-hinge method is improved. The proposed formulation can be used for accurate entire process analysis of space steel frame of tall buildings and the structure of the analyzing program need not be modified.