Research On Damage And Robustness Of Structures Under Unexpected Conditions

If important engineering structures, such as landmark building and nuclear facilities, aresubjected to malicious terrorist attacks, it will lead to serious consequence in life propertyand will produce severe political effects. The event of September11has focused the effortsof the structural engineering community on understanding the causes of collapse inengineering structures. Among the various collapse scenarios, the progressive collapse is avery special one. It is an incremental type of failure wherein the final state of failure isdisproportionately greater than the failure that initiated the collapse. Namely, small damagecan result, eventually, in the collapse of an entire structure or a disproportionately largepart of it. In fact, the unexpected conditions could be caused by the extreme earthquake aswell as man-made sabotage. In the present thesis, considering two extreme cases,earthquake and man-made sabotage, the studies focus on the following five aspects:(1) Under strong earthquake, in order to accurately model damage evolution ofstructures, the initial damage was studied through two series of laboratory tests and wasused to modify plastic-damage model by Jeeho Lee. The renewed model is applied toevaluate the earthquake response of a gravity dam and reproduce the damage process. Thecalculated result shows that it is feasible to use continuum assumption during structuraldamage analysis. In addditon, the explicit finite element method is adopted to deal withcontact and rigid body displacement during the collapse. Through the simulation ofcollapse of Arifiye bridge, it shows that structural integrity can effectively prevent theprogressive collapse due to near fault ground motions.(2) Structural collapse subjected to earthquake initiate from weak location, however, theprogressive collapse under the blast loads, as another unexpected condition, is triggered byfailure of the key elements. Therefore, the characterization of structural response underabove-ground explosion is studied using the combined numerical approach with bothLagrangian and Eulerian methods. The results show that the structural damage fromexplosion is local and instant. It gives the validity to the conceptual remove of some elements from the structures during structural robustness analysis. A two-step approach toprogressive collapse is presented, and the case study is provided.(3) Under subsurface blast, a coupled model with the smoothed particle hydrodynamicsand finite elements are build up to study the response of both foundations and structures.The validity of computational model as well as material parameters is examined by theexperiential equations. The structural damage due to underground explosion is simulated.The debris of medium can be observed. The damage to structures from the above andunder ground explosions is compared, and the vibrational environment can be obtained.The results provided helpful data for structures against unexpected conditions.(4) A simplified approach to analysis of progressive collapse is studied. On base of theenergy principle, characterization of location of initial damage initiated one dimensionprogressive collapse in vertical direction is studied, and the approximate assessment ofenergy dissipation for structures per unit height is obtained. In addition, the dynamicequation of entire overturn collapse is presented for the unsymmetrical destroy in the basesection, and the evaluation method on energy dissipation of beams during catenary stage isalso given. Finally, based on energy dissipation of beams and columns, the combination ofnonlinear dynamic analysis and energy assessment criteria is introduced to identify thepotential for progressive collapse as a simplified method.(5) A comprehensive discussion of practical approaches for mitigating progressivecollapse potential is included. The general strategy to enhance the structural robustness issummarized, and some considerations are also recommended. The effects of velocityvariety and locations on structural dynamic behaviour are given. Most importantly, basedon the sensitivity analysis approach, a quantitative evaluation index on structuralrobustness is proposed and a practical frame, from both qualitative and quantitative pointsof view, to enhance the structural robustness is presented.In this thesis, the structural collapse and robustness under the unexpected events arestudied. The results may supply not only a useful method for structural designers toprevent unpredicted damage but also a guide for determing the reasonable sensor locationsin health monitoring systems of important structures.