Study Of Advanced Analysis And Performance Based Plastic Design For Eccentrically Braced Steel Frames

The advantage of advanced analysis method has been emphasized due to the insufficientof current analysis and design method for steel structure. Eccentrically braced frames (EBFs),which combine the advantages of moment resisting frames (MRFs) and concentrically bracedframes (CBFs), can exhibit both adequate ductility and lateral stiffness during rare earthquake.Advanced analysis of steel frames has been a subject of extensive research over the pastdecade, but little research has been done in the EBFs. In the Code for Seismic Design ofBuildings, the design method about EBFs are mainly derived from AISC, few studies focus onthe mechanical properties of links that use Q235or Q345steel, so the performance of links inEBFs can’t be well predicted.In the current design method of EBFs, the elastic static analysis is utilized to distributebase shear and storey shear force along the height, then shear force and sizes of all the linkshave been obtained, so the other members can be determined by using the links and relativeamplification factors. However, when the structures designed by current elastic static analysisundergo large inelastic deformations under rare earthquakes, the yield mechanisms andinterstory drift may not meet the expected requirements.According to the current disadvantages of advanced analysis and EBFs, this papercontains following subjects:(1) Based on the section assemblage concept and section spring stiffness, an advancedanalysis method for steel frames using element with internal plastic hinge is proposed. In theproposed method, the stability function is used to consider the geometric nonlinearity andshear deformation, and the residual stress is considered in the initial yield function. Also, thereduced tangent modulus method is used to consider the influence of initial imperfection. Theelement subjected to distributed loads is derived by assuming a spring with zero-length at theend and internal of an element to consider the cross-sectional stiffness. It is demonstrated thatthe plastic hinge at the internal of the element can be well predicted, based on the proposedmethod, by only using one element per member.(2) An advanced analysis method for semi-rigid steel frames using element with internal plastic hinge is also proposed. Element with springs-in-series at each end is presented toconsider the effect of semi-rigid connections and material nonlinearity. Based on the proposedmethod, it is demonstrated that internal plastic hinge can be also well predicted by only usingone element for the members subjected to distributed loads.(3) Flange width-thickness ratio of links is so conservative in the current specificationthat many rolling sections cannot be used as links. In order to reevaluate the value,70linksare conducted to investigate the effect of different flange width-thickness ratio and length,which are all based on the Q235steel. The finite element model, verified by usingexperimental data during cyclic loading, indicates that the flange width-thickness ratio can berelaxed to10235/fy for shear links and flexural links. Also, the failure modes, overstrengthfactors and hysteresis curves can be obtained.(4)50links are also conducted to investigate the influence of stiffeners spacing,stiffeners thickness and placing on side(s), and36links are designed to consider the effect ofaxial loads. Finite element analysis is used to investigate the influencing factors on therotation capacity of links during cyclic loading. Then an optimum and practical designmethod of links with varying length is proposed.(5) According to the multilinear force-displacement curves of links obtained by finiteelement analysis, and yield functions derived by using section assemblage concept andaccounting for strain hardening, the advanced analytical model of links is proposed by usingthree rotational subprings and three translational subsprings elements with zero-length,respectively, at each element end to simulate the flexural and shear yielding behavior andstrain hardening effect, and the inner element should be constrained to remain elasticallyunder any condition. Moreover, semi-rigid connection is also considered based on the linksmodel. For the other elements in EBFs, a spring with zero-length is provided at each end toconsider yielding on the cross-sectional level, it is as same as the conventional model. So asimple and practical advanced analysis method for EBFs is proposed.(6) Based on the pre-selected target drift and yield mechanism under the inelastic seismicbehaviors and the Code for Seismic Design of Buildings, performance-based plastic design (PBSD) methods for moment resisting frame, D-, K-and V-EBF are proposed. In the PBSDmethods, the base shear and lateral force distribution are derived from modified energybalance equation under rare earthquake, and the reduced energy dissipation coefficient η isalso important to the proposed method. Numerical analysis shows that the frames withdifferent seismic precautionary intensity designed by the proposed method can exhibitexpected functions under frequent and rare earthquake without complicated iteration andcalculation.(7) A30-storey K-EBF designed by the proposed PBPD method is evaluated by theperformance-based seismic design method. The performance objective D is selected for thisEBF, and three earthquake records meet the specification requirement are chosen. Thecomponent performance levels, based on related specifications, are defined to consider theeffect of inelastic deformations. Both interstory drift and component deformations areincluded in the acceptance criteria.