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Why LL is 1.6 and DL is 1.2
2

Why LL is 1.6 and DL is 1.2

Why LL is 1.6 and DL is 1.2

(OP)
Why LL is 1.6 and DL is 1.2? Is it just because LL is unpredictable. Or bcoz its nature is dynamic?

RE: Why LL is 1.6 and DL is 1.2

Unpredictable.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Why LL is 1.6 and DL is 1.2

Load factors are based on statistical reliability.
The more uncertain the value of the load, the higher the safety factor required to provide similar levels of risk.

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RE: Why LL is 1.6 and DL is 1.2

So if you design a swimming pool on the top floor of a building, is the water considered a live or a dead load?

BA

RE: Why LL is 1.6 and DL is 1.2

I would say dead load, but design it as if it's filled to the brim.

RE: Why LL is 1.6 and DL is 1.2

(OP)
So, it is no where the dynamic effect or impact effect like people walking and moving things on the slab is considered?

RE: Why LL is 1.6 and DL is 1.2

Load factors for water are a problem, especially when the water is in soil, but in the case of a swimming pool I would suggest that 1.2 combined with a conservative estimate of maximum level and something for dynamic loads would be appropriate (but I have never designed a swimming pool on top of a building).

For live loads a separate dynamic load factor is often applied, or the base load might include a dynamic allowance, but I would suggest that part of the load factor is due to uncertainty in dynamic effects.

Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/

RE: Why LL is 1.6 and DL is 1.2

(OP)
BS says that water tank is a dead load. But tank should b considered full. This is not true when performing earthquake analysis because half filled tank when building is subjected to earthquake loads will intensify vibrations bcoz of oscillations water will hit the side walls.

RE: Why LL is 1.6 and DL is 1.2

But, in an earthquake zone, doesn't the "very live" excited movement at the top of the building's axis (long moment arm) of the water mass mean it has to be considered a live load?

RE: Why LL is 1.6 and DL is 1.2

From My Understanding, You can specify the dead load of the pool as mass... then the software will do the rest.. although it will not consider the movement of the fluid during Seismic events...

RE: Why LL is 1.6 and DL is 1.2

There's not any rule that prohibits you increasing the factors if you think appropriate.

RE: Why LL is 1.6 and DL is 1.2

But there is a rule about not hijacking threads to talk about swimming pool water when the question was about load factors. (just kidding!)

Isn't there a separate "F" load for fluids in ASCE 7 (vs. dead or live?)

And no, the 1.6 is not intended to deal with dynamic or impact effects.

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RE: Why LL is 1.6 and DL is 1.2

(OP)
JAE. Then there is no where the dynamic effect of people walking and things moving on the slab is considered? There are many cases that the slab failed when people dance on it. Because in the case of dancing, just increasing the load or using a higher factor of load comb will not work. Slab fails because of resonance(Natural frequency of slab matches induced vibrations bcoz of dancing or any other dynamic load).

RE: Why LL is 1.6 and DL is 1.2

what is bcoz?

RE: Why LL is 1.6 and DL is 1.2

Vibration issues may be partly addressed by specifying deflection limits as well.
"Just increasing the load" likely will work, although maybe not the best solution. But, I see in ASCE 7-05, "Dance halls and ballrooms" and gymnasiums get a live load of 100 psf, stage floors get 150 psf. I would think the 100 psf would be people packed in like sardines already.

RE: Why LL is 1.6 and DL is 1.2

In the AASHTO LRFD code, the load factors are calibrated based on probabilities of failure. The 1.2 and 1.6 from ASCE 7 are not calibrated for probabilities of failure, but have worked well in the past and there have been slight modifications when required.

Edit: As indicated by JAE, this is not true.

RE: Why LL is 1.6 and DL is 1.2

(OP)
bcoz = because

RE: Why LL is 1.6 and DL is 1.2

Code specified loads are minimums. Where designers have identified a need to design for criteria not set out in the code, such as possible resonance, it is incumbent upon them to design accordingly.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Why LL is 1.6 and DL is 1.2

Quote:

The 1.2 and 1.6 from ASCE 7 are not calibrated for probabilities of failure, but have worked well in the past and there have been slight modifications when required.

That isn't quite true. Bruce Ellingwood, and others, have written hundreds of papers over the years on developing the LRFD load factors based on statistical probabilities of failure. The older ACI factors of 1.4 and 1.7 were re-calibrated some years ago to better tie-in the failure probabilities to the variabilities of the various loads along with the applied phi factors (which are independent of the loads).

Basically they calibrated the various factors to essentially find a similar failure probability to what had been done in the past.

While the numbers aren't perhaps based on strict probability targets (AASHTO certainly isn't either) it is incorrect to suggest AASHTO is somehow more statistically based than others.

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RE: Why LL is 1.6 and DL is 1.2

JAE,

Thanks for schooling me; I knew of the re-calibrations but I didn't know it was probability based. That's good to know for the future and thanks for the reference for the papers.

RE: Why LL is 1.6 and DL is 1.2

Quote:

Code specified loads are minimums. Where designers have identified a need to design for criteria not set out in the code, such as possible resonance, it is incumbent upon them to design accordingly.

While that’s certainly the case the problem then becomes one of telling junior engineers that they are to second guess their engineering “betters” who write the codes. Not everyone in the position to need to do that necessarily has the confidence to do so. It’s akin to the situation of the junior copilot not having enough confidence to correct the senior pilot who’s flying the plane into the ground.

And when load factors are based on “sophisticated” statistical analysis the potential for problems increases. If nothing else it presents the moral hazard of built-in overconfidence in their validity…after all, they were developed by very “sophisticated” people…

We don’t have to go far to find an example of that occurring. Last week on this site we discussed a situation where using the code-proscribed loads would have materially under-designed a beam holding a roof snow load on one side and a Christmas party on the other. Some said it was realistic to consider that occurrence while others said it wasn’t. Meanwhile the owner presumably didn’t want to pay any more than he had to to have it built legally and safely, thus putting the engineer a difficult position of being arbiter of what constitutes that. (Pop says yes, Mom says no, make up your mind ‘cause I gotta go…)

The problem I have with statistics is not the math itself but the assumptions that go into formulating the pool from which the samples are drawn. I’m told that the math is ironclad and I don’t dispute that; what I question is who gets to decide from which pools and on what basis the samples are drawn. That always requires human judgement and that judgement call is virtually uncontestable by the end user because he’s too far removed from process to do so.

No situation is fool-proof and no law of man will ever be perfect but the custom-tailored “sophisticated” load factors provide one more opportunity for unsophisticated rube-like lesser engineers such as me to get it wrong. I miss ASD.*


*(Real ASD, that is, not the hoodwinking con that is Allowable Strength Design.)

RE: Why LL is 1.6 and DL is 1.2

Quote:

what I question is who gets to decide from which pools and on what basis the samples are drawn.

ASD's 0.6 factor (or 0.66 or 0.75, or somewhere in between in some cases) all were based on actual experience over time based on what worked.
Engineers sometime in the past started using the 0.6 safety factor on yield, we used it over time with success, and voila! we had a consensus on a safety factor.

So ASD, whether we like it or not, was based on a limited pool of knowledge, limited tests and measures (if any), and probably many anecdotal experiences.
We could perhaps realize that it was extremely uncertain, unsophisticated and a very inaccurate way to ensure safety...but it worked over time.

So ASD is also based on statistics...just done in a very crude way.

LRFD was simply keyed/calibrated to ASD failure probabilities for typical ranges of dead and live loads. This isn't a real earth shattering concept in my view.
While we might consider the use of statistics to be "sophisticated" they are simply a good way to understand and visualize measures of safety.

I think your main point seems to be that with separate dead and live load factors that there is a vast pool out there of rube-like engineers who just won't properly understand it and screw it all up when confronted with a gray area in the codes. I guess I don't think a pair of load factors are all that difficult to work with when compared with 0.6Fy but that's just my opinion.

PS - I'm not trying to be a cheerleader here for LRFD vs. ASD - I started my career with ASD and use both today...(and I certainly hope and pray that, Lord help us!, we don't fall into a debate here on the merits and evils of the two systems).

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RE: Why LL is 1.6 and DL is 1.2

>>>So ASD is also based on statistics...just done in a very crude way.<<<

Yes, certainly, I'm not saying otherwise. My point, as you noted, had more to do with the idea that the more we try to refine, segregate and statistically model the likelihood of various loads occurring -- and assigning them each their own statistically-likely load factor -- the more possibilities there become for getting them wrong while having a false sense of security about it. I contend that the assumed loads we work with are too approximate to merit all the various load combinations and, more importantly, said combinations can get in the way of each other, as we saw last week. I know we're not going back to how it used to be but I nevertheless reserve the right to my dinosaur opinion about it, statistical outlier though I may be. winky smile

RE: Why LL is 1.6 and DL is 1.2

(OP)
So there is nothing like 100% safe. We take chances like in earthquake and wind combination.. They never comes together because the probability that both occur together is very less but not 0 still.

RE: Why LL is 1.6 and DL is 1.2

Archie - yep - I am approaching dinosaur status myself - lots of empathy there.

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RE: Why LL is 1.6 and DL is 1.2

Quote:

While that’s certainly the case the problem then becomes one of telling junior engineers that they are to second guess their engineering “betters” who write the codes. Not everyone in the position to need to do that necessarily has the confidence to do so. It’s akin to the situation of the junior copilot not having enough confidence to correct the senior pilot who’s flying the plane into the ground.

I actually agree with your general point (in fact last week I presented a paper at a conference making much the same points), but I don't agree that returning to ASD is the way to solve the problem (even if it is "real ASD" whatever that is).

The point is that specific provisions in codes only (and can only) deal with predictable events. All codes have general provisions intended to reduce the risk of failure as a result of unexpected or unpredictable events. Junior engineers need to be aware that when they consider events not covered by the specific provisions of the code they are not "second guessing" the code writers, they are doing their job as engineers, and following the requirements of the code (although in some cases those requirements are pretty well hidden).

In my opinion code requirements for this sort of analysis should be amplified and made clearer, and having an explicit third "collapse" limit state, in addition to serviceability and ultimate limit states, would be the best way to achieve that. Nonetheless, with the codes we have all engineers should be aware that their is more to their job than working through the detailed provisions of the code and ensuring that all individual members have adequate strength under the specified loads.

Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/

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