Blast load calculation

[EricPE]

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!

 

[JStephen]

Sorry, no experience with it. However, it does occur to me that if the pressure pulse was of sufficiently short duration, then modeling dynamic effects should lower the loading (ie, pressure resisted by inertia of the walls rather than as a static load.)

 

[steellion]

Eric, that does seem quite high. I would expect it be more on the order of 1-2 psi. If you are an approved government contractor for the military, you can get dynamic analysis tools for free to assist in blast design.

 

[EricPE]

The problem for us is that we cannot get clearance for the programs until we have a government contract, and right now we have always only made it to the prelim design stage and have never been granted the contract...

 

[jmcoope3]

http://engineeringcases.knovelblogs...resilient-design-of-concrete-frame-buildings/

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.

 

[jmcoope3]

From the National Concrete Masonry Association: (

http://www.ncma.org/CMDOnline/Issue...BulletResistantConcreteMasonryBuildings.aspx)

"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

 

[MiketheEngineer]

Possibly the ARMY or Homeland Security might be helpful.

We have done something like this- refinery blasts and the highest over pressure stated by user was 5 psi - still 720 psf - PRETTY HIGH!!

 

[IRstuff]

There's some interesting information here:

http://www.grahamwindows.com/blast/about_blast.html

and:

http://www.wbdg.org/ccb/DOD/UFGS/UFGS 08 39 54.pdf

TTFN
faq731-376

 

[samcse]

This is really too much. I once designed blast-proof for a gas field and the "company-specified" blast pressure was about 150psf. This was fairly close to a pressure vessel so the blast potential must have been fairly high.

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

 

[MiketheEngineer]

That was the number the customer dictated - 5 psi over pressure. Biggest problem was making sure it didn't slide down the road or over turn. I was also VERY worried about window A/C units "popping" in like corks gone wild or "things" that were set against the walls. Where would they end up and at what velocity??

Imagine a coffee cup full of pencils coming at you!!

 

[vmirat]

First, I would direct you to UFC 4-010-01 (recently updated in February), which dictates standoff distance for conventional construction based on construction type.

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.

 

[lsmfse]

I've attached a quick trial run based on your post.

 

[JLNJ]

The pressures are high, but not impossibly so.

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.

 

[lsmfse]

Agree with JLNJ, team with somebody with experience in blast design and dynamic analysis. Static analysis is way to conservative and cost prohibited.

 

[miningman]

Speaking as someone who has considerable experience with the practical usage of explosives........ and`who therefore could cause an awefull lot of structural damage to almost any building should I ever develop terrorist tendencies , I have to say that I have a problem with some of the info in this post. Please be a bit circumspect if you have knowledge that could be usefull to terrorists.

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

 

[cloa]

How about put a link post in Military Engineering- they might have some good ideas.

 

[vmirat]

miningman,
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.

 

[Gumpmaster]

If you want to look at a dynamic analysis by hand, without the use of SBEDS, UFC 3-340-02 (or the old TM 5-1300) provides a method for calculating the response. That should give you more insight on how the blast affects the structure.

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.