What is the water density required for structural design by AWWA Standard? 1

[chrislaope]

Good morning, Everybody:

Does anybody happen to Possess AWWA Standard or know the water density required for structural design by AWWA Standard? I know it varies with temperature but will not exceed 62.5 pcf and we usually base on 65pcf to design water tank structure. However this water container structure need to meet AWWA Standard. I looked at website and find AWWA Standard costs over 10,000$ which apparently beyond our budget.

Thank you very much in advance for your help.

 

[NCStructuralEng]

I have limited experience with water towers. However, the universal standard for water as I understand it and have used is 62.4 pcf for fresh water and 64 pcf for seawater. 62.4 is actually the maximum density of water (at 4 deg. C) and can vary to ~ 60 pcf at 100 deg. C. Density of seawater is variable, but 64 is on the high side. AISC actually backs up these values in the Table 17-12 Densities of Common Materials. Unless you are looking at a solution or mixture, these are the values I would stick to for just plain water.

 

[chrislaope]

NCStructuralEng:

Thank you very much for your input. The reason I asked this question is I have studied a competitor's design, and found their water tank design is nearly 40%---50% overkill in hoop tension capacity than necessary by using 65pcf density. According to customer, the water tanks were required to meet AWWA Standard, which makes me wonder if AWWA Standard requires a higher than 65pcf density in water tank structure design.

 

[BAretired]

Check the price of AWWA Standard again. I think you can purchase the design standard for $112.

BA

 

[fel3]

What type of water tank are you looking at?

I have done the civil (not structural) design for about a dozen ground-level, welded steel tanks for potable and reclaimed water. As I recall, all of the structural designs for these tanks used either 62.4 pcf or your slightly more conservative 65 pcf. Also, all of my tanks have been in California, so seismic was a big deal. Is seismic a factor for your tank?

However, if you are looking at a reinforced concrete tank for a waste water treatment plant, ACI 350 mandates higher load multipliers for moment calculations than we use for standard Ultimate Strength design (IIRC, these higher multipliers do not apply to shear and I don't know about hoop tension; unfortunately, I don't have a copy handy to verify). A higher density using standard load multipliers could have the same effect that you are seeing, at least for moments.

Finally, AWWA D100 (2005), which I have on this computer, says, under Sec. 3.1 Design Loads: "3.1.2 Water load. Water load shall be the weight of all of the water when the
tank is filled to the TCL. The unit weight used for water shall be 62.4 lb/ft³ (1,000 kg/m³). The weight of water in a wet riser, which is supported directly on
foundations, shall not be considered a vertical load on the riser." BTW, "TCL" = Top Capacity Level.

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"Is it the only lesson of history that mankind is unteachable?"
--Winston S. Churchill

 

[JedClampett]

Chris, you seriously need someone to guide you. These are questions that should be answered by your supervisor, not posted on line. The fact that you're designing water containing vessels and can't afford AWWA tells me someone's priorities are way out of whack. Steel and prestressed tanks are designed to AWWA. It's very unlikely that you're designing either one of those as they are almost exclusively delegated to specialty firms that do this a lot. Reinforced concrete tanks are designed to ACI 350.
If you think that your competitors tank was overdesigned, have you thought that they're designing to code?
Here's the thing. Do you want to go to court defending your leaky tank and testify that buying the AWWA standards was too expensive? How much money do you think that's going to cost your company?
I do this a lot. It worries me that people who need guidance on the weight of water are attempting this.

 

[woodman88]

BAretired
It is the full set of AWWA standards that cost $6k for single user and 10K plus for 2 or more users. A single volume out of the set costs from $100 and up.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[TLHS]

I'm not sure why you'd jump to this being a density of water issue rather than someone using a different factor of safety than you were assuming...

 

[BAretired]

woodman88,
Yes, I assumed the $10K was for the full set but I didn't think the full set would be needed for the structural design. I don't have any experience with AWWA standards, so I don't really know but I did check on the internet and found that one of the documents was for structural design and it was available for $112.

BA

 

[dhengr]

They assumed the water had a high iron content so they used 62.4002pcf in their calcs. Then there are the light water reactors where the water is so light that they have to tie the reactors down or they’ll float way.

 

[JStephen]

AWWA D100 assumes 62.4 pcf.

 

[JStephen]

Note that AWWA D100 use considerably different allowable stresses and some different design methods from API-650. If you need to meet the standard, you need the standard in question, it's not just a matter of tweaking one number. Generally, the design is more conservative. I don't know if that carries over to the bolted and concrete tank specifications or not.