Blast Loads/Pressures

[Structures33]

Hey all - I am looking over some design information provided as a preliminary phase of a project. Part of the design requirements is blast resistance. Some initial blast analysis has been done and the incident and reflected pressures are stated/provided. Can someone explain to me what I'm looking at with these pressures? I'm assuming that they are not the applied pressures.
I need to know what the applied pressures are. How do I get them from these values?
Thanks

 

[kslee1000]

Try do a key word search on web. There are plenty of such info out there.

 

[InDepth]

FEMA and USDoD have several online pdf documents that are available for free.

 

[Structures33]

Thanks - I've been reading through documents, publications and websites all day but don't have the answer. I'd appreciate some direction from someone who knows about this rather than wrongly interpreting the technical information. What I'm finding is that the incident pressure is the immediate force but not the applied because it dissipates over time and distance. I still don't know how to determine/calculate the applied.

 

[Gumpmaster]

The incident and reflected pressures are possibly dynamic loads. To use them, you'll have to do a dynamic analysis. I most often use a single degree of freedom analysis. If this is DoD work, then you most likely don't need to have the members remain elastic. Depending on the member type, member response far into the plastic range is acceptable. There are a number of Unified Facilities Criteria (UFC) which outline how to analyze dynamic blast loads and acceptable response criteria.

Is this a job that follows the UFC?

 

[Structures33]

I already have the incident pressure (3 psi) and the reflected pressure (6 psi). Incident impulse is 15.5 psi-ms and reflected impulse is 30 psi-ms.
These were determined using a blast analysis program. What I don't know is how to convert to applied pressure on the walls of the structure. Thanks

 

[IRstuff]

?? Your specification should spell that out clearly, i.e., something like:
The gunfire overpressure levels shall be a 20psi, 4.0msec duration, triangular pulse.

that's what's applied to your system.

TTFN

FAQ731-376

 

[Gumpmaster]

Those are the applied loads, but they're dynamic. I don't think you can really convert them over to static loads.

There are some blast analysis programs that give equivalent static loads, such as equivalent static reactions of glass against a window frame. If your element goes plastic, such as after the glass breaks and the laminate takes over, I think the equivalent static reactions become less relavant. Also, I don't know if there are any programs which will give you an equivalent static load for a whole structure. I don't think these things scale too well.

Can you not perform a dynamic analysis on your structure? When your dealing with blast loads, member response is definately dynamic.

 

[kslee1000]

See link below for included example.

http://www.structuremag.org/article.aspx?articleID=426

 

[Gumpmaster]

That's a good example and a good overview of the subject.

FYI, you probably won't be able to do the SDOF analysis by hand.