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.
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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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.
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.
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?
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...
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).
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.
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.
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.
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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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.
