Study Of Methods Of Design Of RC Frame Structures For Progressive Collapse Based On Nonlinear Static Analysis

It is the main target of structural engineers to ensure the safety of engineering structures under specific economic constraints.Thanks to the probabilistic limit state design method,currently the safety of structures under normal condition has been generally satisfied.Structures may,however,see disproportionate progressive collapse under abnormal conditions.To prevent disastrous consequence,it is necessary to design important structures for progressive collapse.Among several usual design methods,the alternative path method enjoys the most popular adoption because of its maturity,as extensive experiments have been conducted,and simplified analytical and numerical models have been proposed.It is also the only method specified in the General Service Administration code in the United States,which provides detailed design procedures for progressive collapse.Reinforced concrete frame structure is a kind of mostly adopted structural type.To design reinforced concrete frame structures for progressive collapse,there are several difficulties that need to solve when applying the alternative path method,such as huge numbers of column removal scenarios,extensive computation cost,etc.The design target for progressive collapse could also be explored.Focused on these issues in the field of progressive collapse design for reinforced concrete frame structures,the major contributions of the dissertation are:(1)To determine the critical column removal scenarios is the premise of the alternative path method.For large reinforced concrete frame structures especially with irregular planer and elevation configuration,there are a huge number of column removal scenarios to check and design.When nonlinear finite element analysis is adopted,the design time expense would be significantly high and unacceptable.A novel index,namely irregularity index,is thus proposed to recognize the weakest column removal scenarios to facilitate this process.The index is easy to calculate,and it is derived by considering the effect of flexural yielding mechanism,tensile membrane action,unevenness and boundary conditions.The effectiveness and reliability of the index in terms of sorting of collapse resistances and identification of weak column removal scenarios are verified by finite element model calibrated by several experimental tests and sensitivity analysis.(2)A structure need to be enhanced if the collapse resistance of the structure design based on general codes is not satisfied.Since structural collapse analysis involves nonlinear finite element model,traditionally “trial-and-error” method is required in the design procedure,which costs huge computational resources or lead to conservative design.Hence,a novel method,namely virtual thermal collapse analysis method is proposed to design structures for progressive collapse without any iteration.Reinforcement amounts in beams and slabs are represented by a temperature,with higher temperature representing lower reinforcement amount.Given specified performance target for progressive collapse,the proper reinforcement amount in a structure could be easily obtained by virtual thermal analysis.No iteration is required in the process,significantly reducing computational cost during progressive collapse design.(3)Based on the virtual thermal analysis method,the local and global enhancement strategies are further studied through extensive parameter analysis.For two-span beam-column and beam-slab sub-assemblages,local enhancement strategies at compressive arch(membrane)and tensile catenary(membrane)stages are propose,including the reinforcement configuration at the end,middle,top and bottom of beams and slabs.These local enhancement strategies could help in the virtual thermal analysis.On the other hand,for vertical regular frame structures a global enhancement strategy is proposed such that only one virtual thermal analysis is enough to design all single column removal scenarios along a column axis at all stories.For vertically irregular frame structure such as that with a transfer storey,two enhancement strategies are proposed.Either the transfer storey could be enhanced,or several storeys selected among the structure are enhanced.(4)Facing the uncertainty loads,material strengths,geometrical dimensions,conditional probabilistic and risk-based design theory could be adopted if deterministic design theory is not enough for progressive collapse under the column removal scenario.According to the principle of the virtual thermal collapse analysis,Monte Carlo simulation is adopted to conduct uncertainty analysis,and a series of virtual thermal pushdown curves can be obtained.Based on these curves,fragility curves which represent the relationship between failure probability of various limited states and the virtual temperature or enhancement degree can be constructed,from which the design result can be readily determined.Associating the consequences and failure probabilities of various limite states,the relationship between total risk and virtual temeperature or enhancement degree is built,based on which the design result using risk-based desige philosophy is found.To further properly consider the effect of the probability of abnormal actions and initial structural damage,three design theories,namely expected risk,expected utility and cumulative prospect theory,are adopted to design structures for progressive collapse,and the difference is discussed.A numerical example shows that when expected risk and expected unitility theory are adopted,there is no need to enhance the structure for progressive collapse.By contrast,when cumulative prospect theory is adopted and the extra cost is low,the structure should be enhanced for progressive collapse.(5)To evaluate the property of structural progressive collapse,the concept of disproportion index is proposed.For multi-storey building,the disproportion index is defined as the ratio of total loss to initial damage degree to quantify the consequence of progressive collapse.The index is adopted to assess the disproportion progressive collapse of nine historical building collapse cases and the factors affecting the disproportion index is regonized.Based on these collapse cases and several progressive collapse design codes and guidelines,the preliminary allowable disproportion index is proposed.Based on the allowable disproportion index,the design performance targets for progressive collapse can be determined.(6)In the end,the systematic progressive collapse design method presented in the dissertation is shown and validated through a four-story large reinforce concrete frame structure.In the meantime,linear static design method is compared with the proposed nonlinear static design method.The increased amount of reinforcement for progressive collapse by linear static design method is huge,while that by the nonlinear static method locates in reasonable region,showing the necessity of nonlinear analysis for design.