I am designing an aquatic center on the top floor of a multi-story building. The critical beams and columns to be designed are supporting a 25 m lap pool and I am wondering whether to apply a live-load or a dead-load safety factor to the water load acting on the structural elements.
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$\begingroup$ A slight piece of terminology: dead load is normally (in my experience) the weight of structural elements; any other permanent loading is termed superimposed dead load or SDL. $\endgroup$– AndyTOct 7, 2016 at 13:15
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$\begingroup$ I can't decide if this is a duplicate or just very similar: engineering.stackexchange.com/questions/5622/… $\endgroup$– hazzey ♦Oct 7, 2016 at 14:46
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1$\begingroup$ No need for that tone. Also, his argument is entirely valid. Both questions, at their core, are whether water loads should be considered live or dead loads. Your question has more specifics and the fact of the water being on the top floor of a building adds some additional wrinkles to the considerations, but fundamentally the questions are the same. That being said, I do believe this is, as @hazzey put it, just very similar. But the question he raised is 100% justified. $\endgroup$– Wasabi ♦Oct 9, 2016 at 2:52
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$\begingroup$ I've edited your title to make it more clearly distinct from the related question. Please review our "be nice" policy and conduct yourself accordingly in the future. $\endgroup$– AirOct 10, 2016 at 16:10
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2 Answers
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Given that the water level will probably remain almost entirely stable, dead loading is the way to go.
In fact, the only time I imagine the water level would change is if the pool is emptied for cleaning, but that is quite a rare event and, in fact, would reduce the applied load, so no worry. Sure, when the pool is then being refilled there would be some dynamic action, but filling up a pool takes a lot of time, so the "dynamic factor" should be negligible.
Also, the reason safety factors are greater for live loads is due to their inherent variability. You may design a bridge for a 45-ton truck, but who knows how heavy trucks will be in 40 years' time? Meanwhile, dead loads are far more consistent: in 40 years' time, the bridge will weigh the same as it does now. And unless your material is defective or your construction is stupendously shoddy, the self-weight you use in your analysis will probably be within a few percentage points of the true weight. You can therefore place a lower safety factor for dead loads, while adopting large ones to cover the uncertainty in live loads.
As @AndyT mentioned in a comment, there may be elements that are relieved by the pool. An empty pool in such cases would not have this relieving effect. Relieving dead loads are usually fully considered (maybe slightly reduced). Relieving live loads are ignored, causing no relief. For these elements, one may decide to consider the empty-pool case as well.
As @Ethan48 mentioned in a comment, it may be relevant to calculate the structure with the pool both empty and full. The empty pool could be relevant for the columns, when checking the "maximum bending moment (due to wind, for example) and minimal axial compression" case.
So, perhaps the most reasonable approach is to consider the load as a hybrid: with a dead load factor when unfavorable but a live load factor (0.0) when favorable.
This mostly applies to somewhat "simple" structures. If your building requires more complex calculations, then things might get messy.
For instance, I'd guess this may have an important effect in seismic calculations, since the water (or lack thereof) would represent a large mass (or lack thereof) at the very top of the structure. As well, as mentioned by @CarlWitthoft in a comment below, if the building is slender and therefore vulnerable to wind-induced dynamics, the dynamic motion of the water could come into play and therefore act more like a live load.
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3$\begingroup$ If the pool does not fill the entirety of the top floor, I assume you would need to consider load cases where the pool is empty as well as full, as there is potential to cause a moment in the columns below, no? $\endgroup$– Ethan48Oct 7, 2016 at 13:14
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$\begingroup$ I had a similar question at the office recently and we debated whether one should consider wave action or surges when people jump into the pool in an organised manner to create large waves. In our case we dismissed it because that was very unlikely to occur in our scenario. Any opinions? $\endgroup$ Oct 10, 2016 at 9:37
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1$\begingroup$ @NamSandStorm I'd probably dismiss it as well. I also believe (with nothing to back me up) that the load redistribution of water in such a case is probably negligible when compared with the self-weight loads, for instance. People jumping in a pool will create large waves, but it's not as if they're going to empty the pool on their side and double (or +50%) the water depth on the other side. $\endgroup$– Wasabi ♦Oct 10, 2016 at 10:19
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2$\begingroup$ I wouldn't worry about "people-induced waves," but reasonably tall buildings are subject to a decent amount of wind-induced torsion. If the building doesn't have anti-torsing systems, the water could well become a live load. Ditto for earthquakes and systems to reduce their effect. $\endgroup$ Oct 10, 2016 at 13:56
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$\begingroup$ On another note, here where I live (Namibia) we have circular open reservoirs up to 3m tall made from corrugated iron sheets which are bolted together with gaskets as seals. Usually they are built right next to wind driven pumps (wind mills). People love to climb up the wind mill towers and jump into the reservoirs. The farmers get very upset because the reservoirs start leaking at the joints due to strain on the structure. $\endgroup$ Oct 11, 2016 at 7:19
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You need to check with your local building codes. California has special codes for treating elevated water tanks and pools and containers. Not only they are treated with dynamic load factor but other considerations in structural assembly, detailing and use of facility apply.
Intuitively in case of an earthquake after part of pool water is spilled out, after a few seconds the left over will start to swing back and forth and collide to the walls of the pool and break on them in an explosive impact, much like the shore line retaining walls and piers.
So you need to be mindful of local seismic spectra and avoid resonance and in the absence of dynamic lateral analysis follow the prescribes scheme advised by you local building department.
