【正文】 umerical 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． It uses the profound knowledge of blast load distribution to define the blast loads and then directly apply them to the structure． This could dramatically decrease the size of finite element model and increase the efficiency of the simulation． To sum up， the direct simulation method can yield reliable predictions of structural collapse to blast and impact loads， but it is extremely time consuming， and requires a profound knowledge of structural dynamics， damage mechanics， dynamic material properties and putational skills． It is therefore not practical for mon engineering application． The practicability of this category of methods could be improved in the following way： 1)Developing new finite element， which is not only simple to be established and calculated， but also can represent the plex behavior of structural ponents such as nonlinear and dynamic effects； 2） Proposing new method of acc