【正文】 ing AUTODYN shell solver and Lagrange solver． Fig． 3 gives the AUTODYNgenerated 3一 dimensional finite element model of the WTC North Tower and the Boeing 767 airplane． The plete simulation consists of four stages： static equilibrium calculation to get the initial stresses and strains from gravity； simulation of a Boeing 767 airplane impacting the tower； the reduction of the strength and stiffness of the tower resulting from the postimpact fuel fire； and finally， the progressive collapse of the Tower driven by gravity． Fig． 4 gives one of the typical results， which presents the material location of the tower at 6． 5 s after the impact． As can be seen， the building had already collapsed to half of its height． The numerical results showed that a scientifically substantiated plausible scenario for the impact and collapse event of the North Tower could be obtained through the numerical simulation． In the progressive analysis， a reasonable simplification in the modeling such as using simple elements to represent plex behavior of structural members will not affect the accuracy of the results， while this might dramatically reduce the size of the mode1． This could save lots of puter memories and puting time， especially for the plex structures， such as tall buildings． Of course， certain finite elements are still needed to be developed DYN method in progressive analysis of RC flame Hao et ai proposed a DYN method based on the continuum damage mechanics theory． The main difference between this method and other direct simulation methods is that the explosion initialization and blast wave propagation were not directly modeled． Instead， blast loads acting on the structure were calculated first using the formulae given in TM5—1300 and Ref.． Then they are directly applied on front， side，roof and rear of the building structure． This method was used to analyze structural progressive collapse of a threestorey and twospan reinforced concrete（ RC） frame structure． Computer software LS． DYNA with user—defined subroutines was used to perform the analyses． Fig． 5 gives the configuration of the RC frame and the explosive location． In the analysis， the blast was assumed to be detonated on ground surface． The blast weight was chosen to be 1 000kg equivalent of TNT， while the scaled distance used was 1． 7 m／ kg ， at which only the first floor middle column would collapse due to blast effects． Fig． 6 shows the numerical results of the structural progressive collapse obtained from DYN method analysis． As can be seen， DYN method gives reasonable prediction of the RC frame collapse under critical blast load． In their study， parisons were also made between results from DYN method， GSA and DoD guidelines based alternative load path methods． It was found both the GSA and DoD based alternative load path methods may not give reliable prediction of structural progressive collapse and usually underestimate the stress and strain response at the supporting joint above the blast damaged columns. As mentioned above， the great improvement of the DYN method is that， interaction between the blast wave and building structure， blast initial and blast wave propagation are not needed to be directly simulated．