Blast Resistant Design Question (typical peak side-on overpressures, Pso)

[Stenbrook]

So I am working on the design of a blast-resistant building at a chemical plant, however I have not been given much information to begin my design. Does anyone know a typical peak or incident side-on overpressure (in psi) that would be used for this type of design. The design guides I have looked at basically say that the owner or client would provide this design value, however I have not had any luck with this. I also am looking for some typical duration's for this load. From what I have read, the range of pressures vary from about 1.5 psi all the way to 15-20 psi and that the duration can be anywhere from 20 milliseconds to 200 milliseconds. My anticipated load would be from the exterior of the building about 200 feet away. It is a tilt wall structure with a concrete roof deck and spread footings for the foundation.

Any help would be much appreciated. I don't want to be overly conservative because cost is a concern for this building, but I also don't want to assume that the pressure is 1.5 psi and the duration is 200 milliseconds and have that be way lower than the actual load that could be applied.

Thanks!

 

[CELinOttawa]

Your task is hopeless. If the client will not provide the required info, feed back a standard to which you can prepare your values. If you cannot name such a standard, get the support of someone with more blast experience.

I cannot emphasize too strongly just how involved this work can be. It is not a task for someone without an experienced mentor assisting them through the first twenty or so designs...

No one can help you enough online.

 

[IRstuff]

You need to at least nail down what sort of overpressure the customer requires, i.e., grenade, RPG, TNT, ANFO, etc. Once there, there are design guides from FEMA and USACE that could help with the particulars.

TTFN
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[WARose]

Usually it starts with someone (typically the client or someone supplying them with the dangerous product) telling you that [whatever] will blow up with a charge that is X amount of equivalent pounds of TNT. From there, you can get pressures based on is it confined/unconfined, standoff distance, etc. So I'd find that out first.

 

[Stenbrook]

CELinOttawa,

Yeah, I figured that was the case. I have been reading the ASCE 59-11 as well as the UFC 3-340-02 and the Design of Blast-Resistant Buildings in Petrochemical Facilities by the ASCE to gain a complete understanding of blast loading. I feel like I have a general understanding, but I agree that having a mentor with some experience in the matter would be helpful. Any idea where to find one? It seems like not many people have any idea how to do blast and dynamic loading of a structure.

It does seem though that since there really isn't any standard loads that should be used for different situations given by the code, whatever load is acceptable to the client is acceptable for the project.

IRstuff,

I have read through a lot of those guides, and I understand how to convert the overpressures into a dynamic impulse load. And I understand how to determine an equivalent static load from my graduate school courses. I just don't have that initial design overpressure and duration to start with that would normally be provided from the client. So,as CELinOttawa has stated, it seems without that, my task is pretty hopeless.

Thanks for the help! I will continue to do research and ask questions and bug my client some to try and get the information I need.

 

[CELinOttawa]

Alright, well I think I may stand corrected on this one... If you've done quasi static load conversion from an overpressure blast curve, you're likely in a good place to muddke through this with our help online...

Note that for a gaseous event, you're going to have a relatively low peak pressure, but a fairly long duration. Your dynamic increase factors likely still apply, but a safer approach is to simply strip out the traditional reduction factors.

Go ahead and look up a standard gas explosion curve for a similar volume as your subject area. From that work out your quasi-static. Post it here for review and I guarantee you will get good quality reviews. I for one will be happy to help, and I've done a fair amount of this work.

 

[Stenbrook]

CELinOttawa,

I have looked around a little bit for some gas explosion values but haven't had a ton of time to locate any, do you have anywhere that would be a good place to look?
I have assumed a 1.5 psi peak over pressure with a 200 millisecond duration to be conservative for now. This would fall in line with your low pressure but high duration. After going through the calculation for the stiffness, mass, natural frequency, and period of the structure. Going through a design example in the Design of Blast-Resistant Buildings in Petrochemical Facilities, I was able to determine the Pr and te for the front wall, and the Pa and te for the roof and side walls. I Then found the t/T value to use in Figure 3-49 and/or Figure 3-52 from the UFC which gives the DLF( or maximum response) for an elastic, one-degree-of-freedom system for a triangular impulse load. I found that my structure has a relatively high period compared to the duration ( this seems like it would be expected due to the relatively short duration) thus giving me a DLF less than 1 (approximately 0.2-0.4) which reduces the load seen by the structure. We are not actually designing the tilt wall for this project. They are using a separate pre-cast concrete company to design these so I am primarily concerned with the roof. After all the calculations, I came up with a maximum pressure of 0.296 psi which is approximately 42.7 psf for the average pressure on the roof. (This number varies depending on what you are looking at on the roof, for instance the stiffness to mass ratio will impact your period T which will impact your DLF). After that I believe that is close to an equivalent load. We are only doing a pricing set right now so I just want to make sure that that procedure sounds about right for the design.

I wasn't going to consider any Dynamic increase factors or strength increase factors just to remain a little conservative. I have heard word though that I will be getting more information from the client eventually as to what kind of load we are dealing with, but probably not until way later.