Blast load calculation
Blast load calculation
(OP)
I am in the process of designing a facility for exterior blast loads. Based on my requirements given to me, I have 55# of TNT at a standoff distance of 25 ft. I am finding extremely high pressures due to these requirements (Pr= 70 psi, Pa for the roof = 18.4 psi). Does anybody have experience with this that could say if these are appropriate values? Is concrete required on the roof to resist these blast loads? Thanks for any help you can provide!






RE: Blast load calculation
RE: Blast load calculation
RE: Blast load calculation
RE: Blast load calculation
Is the only logical thing i could find... i'd agree with the 1~2psi assessment but I can't say firmly what it is. Only work I've done in these situations is with non structural components.
RE: Blast load calculation
"25 lbs (11.3 kg) of TNT at a standoff distance of 50 ft (15.2 m) produces a blast pressure of 365 psf (17.5 kPa) at Point A. Larger standoff distances also produce more uniform pressure distributions on the structure" so... more like 10 to 15 psi?
And Fema426 Ch.4 discusses this topic in great depth... also references GSA Security Reference Manual: Part 3, which I cannot currently find online, as having a "Table of pre-determined values" for "design purposes". It also makes reference to two software's, ATBLAST from the GSA, and CONWEP from US Army Research and Development Center.
Good luck
RE: Blast load calculation
We have done something like this- refinery blasts and the highest over pressure stated by user was 5 psi - still 720 psf - PRETTY HIGH!!
RE: Blast load calculation
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RE: Blast load calculation
Structurally, there is a big difference between steady state pressure (say a 3 second gust) and a short duration shock wave. If your structure is fairly ductile, the blast should blow over before your structure gets really affected.
sam
cse
RE: Blast load calculation
Imagine a coffee cup full of pencils coming at you!!
RE: Blast load calculation
Second, the blast pressure goes up geometrically as the standoff distance get's less.
Third, the blast pressure is a dynamic load, occuring over a very short period of time, not a static load. The reaction of the building to the overpressure is related to the type of material of the structure, concerning ductility and such. So your 70 psi pressure may be occuring over 5 milliseconds. It's not the same load condition as a static load. The wall may not finish bending before the load has past; it just depends on the material.
The Army Corps of Engineers Protective Design Center offers a program called SBEDS, but as you noted, you have to get their permission to download it.
RE: Blast load calculation
RE: Blast load calculation
Team up with an engineering firm who does dynamic blast design as part of their services. A static design would be a killer with these high side-on and reflected pressures.
RE: Blast load calculation
RE: Blast load calculation
55# of any explosive is a significant quantity and in the wrong hands is definitely lethal. And I say that having personally regularily detonated blasts of more than 7000#., and some a LOT larger
RE: Blast load calculation
RE: Blast load calculation
I work as a structural engineer for the Air Force. I am the base Anti-Terrorism/Force Protection expert. I can tell you that, so far, this post hasn't been a problem. We deal with civilian contractors, both architect/engineer firms and construction contractors, all the time with this kind of information. None of these contractors are required to have clearances. The charge weight information is For Official Use Only (FOUO) but it only becomes sensitive when you tie all the information together with a specific facility and vulnerability; otherwise, it's just an academic discussion. I recommended a DoD regulation, UFC 4-010-01, to the original poster as a helpful source of information. This regulation is available to the general public via the web, so nothing said here so far has been beyond access of anyone, including terrorists.
RE: Blast load calculation
The pressure is pretty meaningless without the time of application. The pressure is reduced as time goes on. The meaningful value is the impulse, which is the area under the pressure/time curve. As lsmfse's post shows, your peak reflected pressure (you should understand what that means before applying it) is 70 psi. Your impulse is 112 psi-msec.