Blast load conversion -STAAD time history

[VIVEKSHINE]

Hi engineers,

Can any one suggest a method for converting the blast pressure into force (Time history)

The Explosion overpressure of 10kn/m2 (0.1 barg) combined with pulse duration of 0.2s.

In STAAD time vs force how to convert the pressure into force.

Size of the frame is 3m x 3m x 8m(height) all columns/beams 250mm sq tube.It is a open steel structure.

 

[WARose]

I'm not sure I've ever used STAAD for this purpose. Usually the time-history function in STAAD is for sinusoidal loads (from machinery in operation). It would probably be more worthwhile to (on paper) figure the response of your structure and load it that way in a static analysis. In that regard, STAAD can be helpful (not just for the static analysis). For example, you could do a modal analysis of the frame and if the results show a great deal of the mass participating in a single mode (i.e. 90% laterally; without seeing your frame, I will just assume this is a blast load adjacent to it), you could reduce the response model to a single degree of freedom system (i.e. SDOF). With that, you could then easily figure the dynamic load factor (DLF) based on charts that are in any Structural Dynamics text book. In my favorite text: 'Structural Dynamics: Theory and Computation', 2nd Edition, by: Mario Paz, this chart (for a triangular loading) is on p. 69. And the response factor varies (for a undamped system) from as little as 0.2 to just less than 2.0. Two is the maximum DLF for any system IIRC.

Of course, this is a pretty deep topic. With blast loading one of the first questions (for the client) is: what sort of damage are you willing to take? Do you want the structure just to survive without collapsing....or do you want it to come through with it largely intact? The cost difference between the two can be enormous. That question becomes important after the steps I discussed in the previous paragraph because then you get into design.....and then the question becomes: do you go elastic or plastic in your design? So like I said: there is a lot to it. A good book I read on this once had a flow chart to get through it.

I hope this helps.

 

[haynewp]

Assuming this is a simple pulse, the load can be broken up into very small time steps down to zero in the form of a triangle. You have to figure out the appropriate time step required for the load. This triangle will give you the positive phase of the blast load only, of course. You need to be using the reflected and side-on pressures where each is applicable.