CMU Blast wall design for HUD Construction 1

[keen_mentee]

I am working on a Department of Housing and Urban Development (HUD)project where I need to design a 6 ft high concrete masonry unit (CMU) blast walls. The requirement of the blast wall (and its material) is as follows, "Thermal heat flux exposure threshold of 450 BTU/hr/ft2 for people in open spaces where people congregate, such as parks and playgrounds and blast overpressure threshold of 0.5 psi as the maximum allowable pressure that can be measured at a distance from an explosive hazard applicable to outdoor unprotected facilities." I designed a cantilever cmu wall for the 0.5 psi (=72psf) and got a solid grouted 8-inch CMU wall with #4 rebars @16 inch O.C. vertical rebars to work for 72 psf pressure. This does not seem like a strong wall so I am not sure if there is more to the loading that I am missing. I would like the expertise of the community on this and also what I could do for the 450 BUT/hr/ft2 requirement. Any relevant information and design examples would be greatly appreciated.
#learningalittleeveryday

Thank you.

 

[Lutfi]

Very specialized field. My advice is to hire an expert in this field. I have done a few of these designs in the past. The 0.5 psi seems low but it depends on the TNT equivalent weight and its locations form the structure. Dr. Sam Kiger with University of Missouri used to teach a week long class on blasts, blast effects and designing for it. Besides his expertise, he has a line up of other experts that teach the course as well. I think Dr. Kiger might be retired by now but you may want to see if you can find and attend the course. There is proprietary software that you may or may not be able to get a copy of unless you are working with a government agency.

The loading on the wall will be pressure and impulse.



Regards,
Lutfi

 

[ThomasH]

First I think the load description may be incomplete. You have an overpressure but a blast load also requires a duration for the load. If we assume a triangular impulse, then the load is defined.
The impulse that Lutfi mentions is the area of the triangle.

I don't know exactly how far you want to push the analysis but for me the next parameter is the location of the explosion compared to the wall. Often the load given is a "side-on" pressure. If the source for the blast is perpendicular the wall then you will get a reflection factor of ~2. The load doubles.

Next is the dynamics of the wall or how the wall will respond to the impulse. To cover this in a few sentences of difficult but since you mention masonry the ductility of the wall will probably not be good. That means that you will probably work with linear response. And that may mean another doubling of the load but that depends on the relation between the duration and the natural frequency / period for the wall.

It may also be so that the load you have, 0.5 psi, is a static equivalent load for the described impulse. The problem is that you can't determine that load without knowing the dynamic properties of the wall. So somebody must in the case have made assumptions.

This became longer then I first intended . But I think that first you neeed to figure out the load. And then you may need to check a book on dynamics and transient analysis, that will help to figure out how the wall will respond to the load.

 

[bones206]

I think the design criteria is unclear as written. I'd ask the author for clarification and elaboration.

 

[bones206]

Also, CMU may be a poor choice of material for a blast wall, since it can generate shrapnel and make a blast even deadlier than it would be otherwise. A 6 ft high wall is probably only going to shield people from blast pressure who are standing close to the wall. The pressure wave will roll over it like a speed bump. An analogy is snow drift behind a parapet wall. If you are inside the "snow drift", you won't get blasted by the pressure wave, but then you might get shredded by the CMU shrapnel.

I've often found myself frustrated with clients who ask for blast-resistant structures, but don't have a comprehensive security plan in place. They often back into design criteria based an arbitrary target level of robustness, but is not really rational from a security perspective. For example, I had a client who asked for a 0.5 psi peak pressure for a CMU wall building. I explained one possible scenario that would cause that pressure on the front wall is a "small" truck bomb from the end of their driveway. But I pointed out that they had no gate, no perimeter security fence etc, so there would be nothing to stop that hypothetical truck from parking right next to the building. Total deer in headlights staring back at me. In the end they said just design it for the truck bomb at the end of the driveway.

I feel like the OP's design criteria is could be a similar situation. The design standoff distance and TNT weight for the 0.5 psi may be totally arbitrary, when in reality there may be no actual reason the bad actor couldn't set a charge off at a closer distance resulting in a much greater overpressure on the people in the playground. I think in these situations it is much preferable for a security consultant to do their rational security analysis first, then give the design criteria to the structural engineer in terms of standoff and TNT weight.

 

[ThomasH]

I agree with bones206, ask for clarificationb regarding the design criteria.

The overpressure 0.5 psi is not someting I would worry about if it happened to a person outside. It would depend a bit on the duration but I would not consider it dangerous. My idea was that the purpose for the wall was to shield from a thermal load. But when I read the description again, maybe that was an incorrect asumption.

 

[phamENG]

Bones - what do you use for the analysis for those sorts of jobs? All my blast experience is DoD, so we had USACE programs to use. Not supposed to use them for non-DoD work, though, if I remember correctly.

 

[bones206]

I've used SBEDS (USACE Excel Program), which is single degree of freedom nonlinear dynamic analysis and also wrote my own excel software when I lost access to SBEDS. This has some SBEDS design examples for anyone curious:

https://www.nwo.usace.army.mil/Port...oblems_DistribA.pdf?ver=2018-11-01-091704-817

I believe UFC blast design documents have an equivalent static load procedure, but it's been a while and I'm not as familiar with that.

 

[phamENG]

SBEDS was what I used as well. Unless they've changed course, the last time I tried (4 years ago?) to get a copy they required a copy of a current contract with with a DoD organization requiring its use.

 

[keen_mentee]

Thank you, everyone, for your responses. (I love this community!)

To add to the topic, this is for a housing project. Typically they would not require a blast wall. However, they need a blast wall because this is HUD-funded housing. The wall is trying to provide cover from a gas pipe blast in the ground at a depth of ~5 feet and at a distance of ~12 feet. The 0.5 psi refers to the incident pressure. I have attached a section for reference.

Lutfi: Thank you for directing me to some of the professors, I will reach out to them.

ThomasH: Yes, the 0.5 psi is an incident pressure (maybe this is a static equivalent) and I will increase it 2 times for reflected pressure. I think the thermal requirement is to ensure the wall can function during the high temperature in case of a pipeline blast.

bones206: I will try to research more on the design criteria. I have found some documents specific to HUD but still need clarity. The situation you referred to is similar to what I am in. The design requirement is "The blast wall to be designed for peak incident pressure of 0.5 psi and a thermal heat flux of 450 BTU/SF-HR". However, in my case instead of a truck bomb, the blast wall is to resist a gas pipe (embedded into the ground) explosion. So I can justify the 6 feet wall can shield the blast debris. You mentioned you did some blast walls and expressed that CMU may be a bad material. In my research I had a lot of people claiming that a hollow/partially grouted CMU is bad for blast wall design and a fully grouted CMU solves the problem of CMU shrapnel. What material did you use in your case?

Thank you.

 

[phamENG]

Hmm...is this your source document? Barrier Design Guidance for HUD Assisted Projects Near Hazardous Facilities

If so, I'm not sure you're reading it correctly. The way I read it, that heat flux and pressure are the maximum that can be experienced without a barrier - not the design levels for a barrier. You have to put up a barrier if those are going to be exceeded, and then design the barrier for the anticipated blast based on the distance from the explosion to the barrier.

Also, there is a nice exception on page 4:

Pipelines, such as high pressure natural gas transmission pipelines or liquid petroleum pipelines-Pipelines that transmit hazardous substances are not considered a hazard under 24 CFR Part 51 Subpart C if they are located underground or if they comply with applicable Federal, State or local safety standards.

 

[bones206]

You mentioned you did some blast walls and expressed that CMU may be a bad material. In my research I had a lot of people claiming that a hollow/partially grouted CMU is bad for blast wall design and a fully grouted CMU solves the problem of CMU shrapnel. What material did you use in your case?

In my case I had a cavity wall with brick veneer, but I did fully grout the CMU. The added mass helps the dynamic response as well as reducing fragmentation.

Sorry I originally read this as some kind of anti-terror protection. I think from a holistic point of view, I would approach this pipeline protection problem as if the CMU wall was a ductile "net" spanning horizontally between soldier columns that can take the reactions back into the earth. I wouldn't feel comfortable with cantilever wall action. Just my 2 cents.

 

[keen_mentee]

phamENG: I am using the document that you referenced " Barrier Design Guidance for HUD Assisted Projects Near Hazardous Facilities" as well. However, most of the documents (including the drawing provided by the client) that I am referring states to design the blast wall for 0.5 psi and 450 BTW/ft^2-hr. I will check this again. Don't you think if the wall can handle 0.5 psi pressure then it would resist the pressure and there would be no blast pressure beyond the wall? The exception you provided is going to be a big help. According to page 4, I would not require a blast wall in my case. I will discuss this with my client and see what we can do. I appreciate the help phamENG!

Thank you.

 

[phamENG]

That document states that those are the thresholds above which serious damage is likely to occur to a typical building. You only need to install a barrier if the setback to the hazard cannot be established to keep the impacts on the building below those values. The goal of these design criteria is rarely to keep the building in pristine shape - it's even okay if it has to be torn down afterward. Just so long as occupants are not seriously injured or killed when the extraordinary event occurs, the structure has done its job.

If you need to design it to resist a blast form a gas main 50 feet away...good luck. Those values will be significantly higher. Like an order of magnitude or two higher. The trouble you'll find, I think, is determining the volume of gas you need to account for in the explosion.

 

[dik]

For info and a caution... I did a report a couple of decades back on a curtain wall that was damaged by a propane tank explosion. Most of the damage to the connections was caused by the vacuum created by the reflected shock wave.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[bones206]

I'm not familiar with HUD requirements, but I feel like a properly designed buried gas pipeline would be exempt from the blast setback criteria. It's just not practical to expect houses to have secondary protection structures for gas pipeline infrastructure, which is ubiquitous.

 

[LittleInch]

I've never heard of anything like this for a buried pipeline.

The key risk mitigation for anything on a pipeline is that people can move away rapidly from any release / fire etc. To build a wall simply means anyone between the damage and the wall will be much more likely to die or be injured.

Apart from which the risk is linear, it is also very very low. So not like a Propane tank or similar.

You don't see buried pipelines encased in two blast walls anywhere.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[ThomasH]

Yes, the 0.5 psi is an incident pressure (maybe this is a static equivalent) and I will increase it 2 times for reflected pressure

If 0.5 psi is described as "incident pressure" it is the same thing as I called side-on pressure. I have seen both terms used. But it is not the same as static equivalent, in theory in can be the same value but I would not assume that. You need an impulse or duration for the load to calculate a static equivalent. Or a reference that has made a calculation based on a structure with the same dynamic behaviour as your structure.

Maybe I wasn't completely off with the guess of an explosion followed by a thermal load. I have no experience with gas explosions in underground piping but I have experience with vapor cloud explosions. I noticed that some of the material you hace recieved is based on TNT explosions. In my experience vapor clouds have lower peak pressure and longer duration than TNT explosions. Of course, the same dynamics apply, but the resulting response in the structure can be very different.

Also, is the wall intended to protect the housing from the blast load? I hope you don't expect the wall to function as a barrier and "remove" the overpressure.

 

[bones206]

I just pulled up that HUD guidance document that pham referenced and it looks like they specifically exclude buried gas pipelines from the requirement to protect with a blast wall. See section III subpart e).

https://www.hud.gov/sites/documents/BARRIER_DESIGN_GUIDANCE.PDF

 

[masonrygeek]

In regard to masonry being a poor choice, I believe that should be clarified to be UNreinforced masonry, not reinforced masonry walls. NCMA TEK 14-21A clearly states that unreinforced walls are subject to brittle failure (and shrapnel) while a fully grouted reinforced block wall can minimize projectiles from the back side:

https://ncma.org/resource/design-of-concrete-masonry-walls-for-blast-loading/.

A brick veneer may also provide more resistance.