Blast Loading: Structure as a whole.

[3doorsdwn]

To my knowledge there are two approaches to designing structures for blast loading: you can take it on a member by member basis (modeling each beam with a Single Degree Of Freedom [i.e. SDOF] there are some good texts on that), or you can have a program analyze the structure as a whole and include the effects of plastic deformation and so forth.

But consider this for a moment: if you have a structure (say a frame made up of all welded tubes) and you were only interested in an elastic response.......what would you think of using the period calculated by the program [of the structure as a whole] and using that to get your DLF [i.e. Dynamic Load Factor]? There are a lot of charts that plot the DLF vs. duration time [of the ramp/triangle loading].

My first reaction to this approach [at the risk of answering my own question] is: it's sort of (inappropriately) mixing a solution for a SDOF and the results from a Multi Degree of Freedom analysis (as the FEA model clearly is). I'm not sure if the chart I mentioned previously would be applicable from this type of analysis.......or that this result would make it appropriate to apply this load [modified by that DLF] to members in the frame.

Thoughts?

 

[sebivaq]

Dear 3doorsdwn,
in my opinion for a plane-frame the charts could be valid (a frame can easily be modeled as a SDOF, in the horizontal direction). For a more complex structure, I would model the hole structure and the explosion as a load (in function of time).

You should distinguish between two different cases:

1) Large-scale blast, where the structure is "engulfed" by the blast wave.
2) Small-scale blast, where only a member is affected.

For the first case I would model the whole structure. For thee second case, I would use the charts. Regards,

Sebastian

 

[3doorsdwn]

Thanks for the info (and sorry for the belated reply).

The thing I have always struggled with is the peak reflected load vs. over pressure. (I.e. I have never been sure when to use one vs. the other.) For most of the design examples I have seen, for a isolated [rectangular/square] building loaded by an external blast, the peak reflected pressure really isn't a consideration (except in calculating impulse time). Where I have seen the reflected pressure come into play is an internal blast or a building with re-entrant corners (among other features). This aspect of blast loading has always been a mystery to me. (All the more reason to call a consultant when it comes up.)