Blast Load Service Level or Ultimate?

[swearingen]

ASCE 59 gives the limit state load combination of 1.0DL + 1.0LL + 1.0BL as the only combination to be used. It then says we are to adjust live loads and dead loads based on whether they help resist the blast load or add to it. That I understand. What I am trying to nail down is whether this combination is intended to be a service level or ultimate level combination.

With ASCE's (and the structural engineering industry's) drift toward all ultimate loads over the past 20+ years, I would assume the blast load calculated from ASCE 59 would be an ultimate load. It seems odd that they would then combine an ultimate load with 1.0DL and 1.0LL which are normally taken as service level dead and live. That may be intentional, but I cannot find anything definitive on the subject.

Here is the question in a nutshell - which of these is true:

1.0DL + 1.0LL + 1.0BL is an ultimate case which means,
0.6DL + 0.6LL + 0.6 (or 0.7?)BL is the service level

OR

1.0DL + 1.0LL + 1.0BL is a service level case which means,
1.2DL + 1.6LL + 1.5(?)BL is the ultimate level.

I suspect it's the first scenario, but then, if I am working at service level, what factor do I use for BL (or DL and LL for that matter)? I would think it would be 0.7 for BL since it is more of a rare and variable loading (hence the 0.6 for wind and 0.7 for seismic).

Thanks in advance for any insight on this one.

-5^2 = -25 ;-)

 

[JStephen]

I suspect the first approach is correct, but the first load combination doesn't necessarily imply the second. That is, they may be specifying design with a factor of safety of 1.0 rather than 1.2 or 1.6 or whatever.

Anyway, some googling didn't turn up any definitive answers, although it did turn up a similar question elsewhere on this site (

https://www.eng-tips.com/viewthread.cfm?qid=456360).

I did run across this pdf, where they show loads with a factor of 1.0 for LRFD, but it's not necessarily comparable loads, either- but see Page 30 of the pdf file, Slide 118:

https://www.nrc.gov/docs/ML1214/ML12146A122.pdf

 

[jayrod12]

In the event of a blast, do serviceability requirements matter? I would argue no. The only goal in that scenario is to make sure things don't fall down. If they become distorted or damaged, that's generally considered an acceptable outcome in my mind. Similar to fall arrest, in the event of the full load being seen by a fall arrest system, there is allowed to be permanent deformation of the system, just not failure. It's also why the systems are not tested to the full amount, they're tested to 50% (I believe) and are expected to have no permanent deformation at the test load.

 

[canwesteng]

IMO - it is an ultimate limit state. The 1.0 factors for dead and live are meant to account for the likelihood of dead or live exceeding design estimates in the case of a blast, which would be different to the case of these exceeding design values over the life of the building.

 

[haynewp]

I've never seen blast treated as anything except as ultimate level.

 

[Once20036]

I don't want to sidetrack the conversation, but I`ve been wondering the same thing within the context of ASCE 7-16.
Section 2.5.2.1 says,
"For checking the capacity of a structure or structural element to withstand the effect of an extraordinary event, the following load combination shall be considered
(0.9 or 1.2)D + Ak + 0.5L + 0.2S"

To me, this indicates that the blast loads are being considered at an "Ultimate" level.
I agree with jayrod that deflection checks probably don't matter, but it gets weird to me that you can't use an "Allowable Strength Design" methodology.
I can't think of any other load scenario where you are required to use LRFD due to the lack of ASD style load combinations.

 

[human909]

To me, this indicates that the blast loads are being considered at an "Ultimate" level.
I agree with jayrod that deflection checks probably don't matter, but it gets weird to me that you can't use an "Allowable Strength Design" methodology.
I can't think of any other load scenario where you are required to use LRFD due to the lack of ASD style load combinations.

To me this indicates that some code writers are moving with the times and moving on from the aging and less suitable ASD approach.

Maybe someday other antiquations like slugs, pounds, feet and inches will die the death they deserve too.

 

[Once20036]

Its an interesting thought. The equation first appeared in the ASCE 7-10.
Both the 02 and 05 versions stated, "...strength and stability shall be checked to ensure that structures are capable of withstanding the effects of extraordinary events..." without providing any specific load combination.

 

[ThomasH]

I have done several blast load designs and that has been ultimate load, in Eurocode terms accidental load which is ultimate load but the partial coefficient for the load is 1.0.

A typical load combination would be 1.0 DL (typically self-weight) + X LL + 1.0 BL, X for live load can be 1.0 but is usually reduced to 0.7 or something like that. You usually don't have to combine the full live load with an accidental load.

 

[haynewp]

There actually are deformation related limits with blast design.

 

[ThomasH]

There actually are deformation related limits with blast design.

There can be deformation limits, but I would still not consider the blast load a service load.
When designing for blast it is not uncommon to go into plastic response. That can result in large deformations. And it can ultimately mean that a structural component does not function as intended.

 

[haynewp]

There can be deformation limits, but I would still not consider the blast load a service load.
When designing for blast it is not uncommon to go into plastic response. That can result in large deformations. And it can ultimately mean that a structural component does not function as intended

I wasn't suggesting to treat it as a service load.

 

[ThomasH]

I wasn't suggesting to treat it as a service load.

I wasn't sure what you meant but my intention was not to offend you. Thats why I tried to give an explanation.

But do you have other types of deformation limits for blast loads?

 

[haynewp]

End rotation and ductility ratio limits are what I've used before based on DoD requirements.

 

[ThomasH]

End rotation and ductility ratio limits are what I've used before based on DoD requirements.

Now I better understand your comment. Of course it is a valid comment but those deformations are related to the material and its failure. I was more into deformations in the SLS context. I misunderstood your comment.

 

[haynewp]

No problem, I didn't take offense.