Safety Margin Analysis And Design Optimization Of Braced Steel Frame Structures Subjected To Strong Earthquakes

In an earthquake,the major casualties are caused by building collapse,which means focusing on the prevention of structural collapse is to save lives.Besides the natural disasters,the structural collapse might also be the result of inadvertent collapses and intentional destruction.Collapse-proof building comes at a cost that only few can afford.Therefore,solutions that are affordable,practical,and efficient to prevent structural collapse are highly desirable.The safety margin of the steel structures subjected to earthquakes is quantifiable.However,its incorporation into the structural design remains an open question that this dissertation has attempted to investigate through the optimization of the bracing systems.Furthermore,the expansion of this safety margin into the progressive collapse field is an exciting topic that provides new perspectives to understand the demand-to-capacity ratio(DCR)in term of its value and its relation to the behavioral state of the structural members.These ideas have an effect on the design of new and existing steel structures,especially,the insertion of the safety concept in the bracing systems.They have widened these systems7 initial role as dissipaters of lateral loads to be among the primary structural members that fully participate in preventing structural collapse.This dissertation has addressed these subjects as follows:1)The seismic collapse margin of steel frame structures with symmetrically placed concentric braces was assessed.The investigation has revealed that the braces5 layout and sections govern the efficiency of the concentrically braced frames to improve the structural safety of the building.The numerical example showed that the optimum retrofit scheme did not necessarily have the highest structural safety nor have the cheapest bracing systems,but instead,had the best mix between safety and cost.Despite the consideration of all three parameters for the selection of the best scheme,the best results from the structural performance and cost rate of a given structure frequently match up with a desirable value of collapse margin ratio(CMR).In other words,both desirable cost rate and better structural performance level to the selected target give higher CMR.2)A new methodology incorporating ground approach optimization with probabilistic analysis using multi-element removal has been introduced to improve the safety design of steel structures with concentric braces.It considered the braces location and section as variables and optimized the structure according to the safety index.The results from the numerical examples highlight that symmetrical brace layouts are safer than the asymmetrical ones and fully braced structures do not have high safety index.The proposed methodology has proved to be a useful tool for the understanding of the optimal placement for the bracing systems regarding the seismic collapse margin of steel structures.3)A quantifiable approach measuring the seismic structural collapse under sudden column removal is proposed using the interaction of the vertical collapse margin ratio(CMR_V),the DCR and the robustness(R).This investigation used eighteen archetypes with 119 scenarios of sudden column removal.From the regression analysis,the influence of DCR on CMRv depends on the ultimate capacity(?u)of the structure under column removal.The CMRV formulation combines the influence of the seism through vertical-to-horizontal ratio(V/H)and vertical maximum considered earthquake;and the gravity load using both general and local behaviors of the structure under sudden column loss.The proposed values of DCR can be used in progressive collapse design to reduce the structural cost while preserving its strength.The evaluation of CMRV using the vertical ground motions reveals that the increase of the V/H increases the potential of progressive collapse.4)The design of buckling-restrained braces based on the structural work from the influence of both horizontal and vertical loadings to resist seismic collapse is proposed by considering the percentage of brace utilization and the V/H ratios.In this investigation,the critical DCR is used as a selection criterion for the column removal.The results show that the best approach to strengthen the steel structures is to place the bracing systems along the middle of the structure rather than the sides.The integrity of the braced frame is significant to the behavior of the failing structure toward the vertical loading.If the column removal was part of a braced bay,the structure would quickly exhibit large deformation.The proposed brace design expands the role of the bracing systems from the protection of the structure against the lateral loading to the prevention of the vertical structural collapse.