Load Factor for Blast Loading?

[GalileoG]

I have a project spec that requires blast loading of 5 kPa to be accounted for in the design. The NBCC does not appear to provide a load factor for blast loading, so I am curious as to what factor you would use? Someone in the office suggested 1.5 (similar to live load) whereas someone else suggested 1.0 due to the fact that it is an "accidental" load and that the specified blast load itself is highly variable.

Clansman

If a builder has built a house for a man and has not made his work sound, and the house which he has built has fallen down and so caused the death of the householder, that builder shall be put to death." Code of Hammurabi, c.2040 B.C.

 

[ishvaaag]

I agree. Earthquakes and explosions are loads expected hopefully be met if anytime once in the life of the buildings. Hence the usual approach of the codes is to factor their load as 1 and use statistical averages for other concurrent loads as the hypothesis to be checked, ultimate (factored) level. With the bigger knowledge of Earthquakes and target behaviours to be met, the thing may be becoming more complicated for earthquakes, but that is the classical approach.

In Spain for eartquake hypothesis you would take dead load, earthquake, 30% of live load and 0 wind load and 0 snow load unde 1000 meters over sea level and 20% snow load over 1000 m over sea level (just one of many hypotheses to be checked).

However some code could be aberrant in this aspect, so give a look. I haven't found any yet, but, there may be.

 

[lsmfse]

If that is the given load, my opinion would be to use 1.0. Was there a code or manual referenced in the spec? A good reference is the Army TM5-1300 for designing the structure or components.

 

[hokie66]

I think 5.0 kPa came off the top of someone's head, so 1.0 comes off the top of mine.

 

[GalileoG]

Well said Hookie! I was thinking the same..

Clansman

If a builder has built a house for a man and has not made his work sound, and the house which he has built has fallen down and so caused the death of the householder, that builder shall be put to death." Code of Hammurabi, c.2040 B.C.

 

[youngstructural]

Gentlemen;

While I haven't been online in a while, I have been working as a Blast Hazard Engineer (with more training comming shortly). I have learnt to repect all of you in my time participating here, but if you are asking how to factor a blast load, you are not applying it correctly. Ask how and why the load has been developped; Blast loading is complex (as in three weeks of reading before I calculated my first) and I'm sure there is no need to point out that it is important...

Clansman: I know you're Canadian, so I can help you directly... I am now a public servant working at NRCan CANMET-CERL. I'll apologise for the acronym after the fact and spell it out: Natural Resources Canada, etc, Canadian Explosives Research Laboratory. We've been a lab for around a hundred years, and are now into certification and retrofit.

I am not yet able to really be of service, but I'm in an office that definitely can be... I'll be happy to help. Let me know if you want a hand. Might need to contact me here first, if you haven't caught my name on any previous posts.

Cheers,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

 

[concretemasonry]

There is a big difference between public or general facilities and unique facilities when it come to blast. - You must design for the possibly foreseeable blast and not just the minimum codes or guidelines.

My first structural design was a reinforced concrete block instrumentation/observation center in the zone of the discharge from the direct blast from rocket engines engines (that ended up in the Saturn series)and from the flame deflector that directed the vertical blast 90 degrees toward the instrumentation building.

All perameters were for 50% above the current projected thrust/observed pressure plus the percentage over-rides for larger engines. It was also designed for projectiles (rocks, wrenches, scraps, etc.) that could be picked up and sent toward the center. The roof was designed for the blast and anticipated newer engines(+50% capacity/pressure)for down load and uplift. The first few tests went well and the 8" projectile resistant windows suffered hit, but no damage. - After that, the unforeseen occurred.

After several days of periodic testing the 18,000 gallon LOX/RP1 and LH2 tanks were purged with nitrogen, which is a common and necessary practice. The atmospheric conditions were very unique and a "bubble" of gas formed and drifted directly over the center. After a few hours, a static charge ignited the the "bubble" and blew the over-designed wood roof system onto the floor of the center and destroyed all the computers and electronics requiring a several month delay in rocket engine testing and development. - A strange effect for your first design!!! You have to look at all the possibilities if you are away from the normal world of conditions. Fortunately, the explosion occurred when no one was around and the property was government and corporation owned and no one was injured.

Look at all the possibilities. - This occurred after living daily with the daily problems of unloading LOX anywhere near any asphalt pavement (1-2 hour road closings) and the requirements of having two people when walking on a structure that had any hydrogen piping because of the possibility of invisible flames from friction caused ignition from a leak.

Anything is possible!!! - Look at all possibilities.

Dick

 

[ishvaaag]

Well, respecting the informed opinion of the contributors indicating that blast can vary very wildly, and that this means that they consider such thing as a "blast" factor is some kind of ill conceived concept, I think all others are answering as well correctly the question.

I mean, if you have really a specific situation where the exposure is, say, to rocket or bomb explosion, and you have not something on what you are to cover as a structural designer you better call someone that knows what to expect. But, in the other hand, some codes simply are adding some clauses to enhance the response to explosion of some buildings of rooms in generic way, and if the code is not talking of some specific safety factor, we are answering correctly, the classical assumption is to take coeff 1.0 for all accidental loads.

In reality I see your observations as informed individuals as a worry or comment on that the mere inclusion of such simplistic clauses are tremendously deficient respect the potential outcomes of explosions, and would love them developed to a more comprehensive chapter of code or code itself. We have accepted more or less such thing for fires, so why not for explosions, the complexity of projects makes nothing but to grow. But to say that would mean that maybe buildings and facilities of every kind should resist tornadoes etc ... and unfortunately what coded tries but falls short of it. And structural design under whatever the chapter will continue serving the safety as by law mandated. In the end, we were not saying that we should calculate for a blast in such and such way or magnitude, but just saying what is our opinion on safety factors for accidental exposures.

And from an european perspective, it continues surprising us to what extent the love for wood construction in the USA continues having such negative impact on fire hazards at homes ... reading what happened at the saturn rocket site I think would put most of us wondering ... I think scarcely one architect in Spain would have thought a minute of making a roof for such use in wood.

 

[KootK]

YoungStructural,

You can't just tell us that we're doing it all wrong and then not tell us how to do it propely? C'mon, out with it!

And welcome back to the mother ship.

 

[youngstructural]

Hey KootenayKid;

Lol... I've done all that I can with so little information! The field isn't exactly standardised (think of a nightmare version of ASD vs. LSD) and without knowing more there really isn't anything else I can do...

I should, however, point out that just because you're asking how to factor a blast load, this doesn't automatically mean you're applying it incorrectly. It just means that chances are you are guessing, and that the probability falls on the side of screwing it up... Think of blast loading as similar to vibration, at least in so far as the empiracle nature of the works necessitates an understanding of what the client is trying to achieve and the nature of the mission.

Most private industry blast loads are much as Hokie has guessed: Just a number chosen for arbitrary reasons, however many are specific with either epiracle real-world testing or computational fluid dynamics underlaying their development. It's important to know which you are working with before you apply them.

Off to the US next week for more training... Given time I'll actually be able to do my own job!

Cheers,

YS


B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...