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Hydrostatic Pressure Live Load Factoring

Hydrostatic Pressure Live Load Factoring

Hydrostatic Pressure Live Load Factoring

I am assessing a tank that will be filled and unfilled on a daily basis with water. I'm using FEA and have modeled the hydrostatic pressure. I usually work to Eurocodes but have yet to find anything useful in the various tank and silo codes. Just vague notes that I should assess hydrostatic pressure, and no notes on live load factoring. Poor really , unless I've missed something definitive.

Given the load is variable, I'm thinking a live load factor of 1.5 would apply. Does this sound over the top - or - not enough? Hopefully, somewhere between :)

RE: Hydrostatic Pressure Live Load Factoring

Sorry, I don't know about Eurocode. Maybe ASCE 7-05 can give a hint. These are the load combinations from there, where H is the load due to lateral earth pressure, ground water pressure, or pressure of bulk materials. It considers H and L together.

Newer versions of ASCE 7 also have this, but it's a bit less decipherable. I can't judge if the newer version took a step back or not, but ASCE 7-05 laid it out very clearly and concisely. Maybe there's an analogue in Eurocode.

RE: Hydrostatic Pressure Live Load Factoring

The welded steel tank standards AWWA D100 and API-650 use allowable stress design for the tanks themselves. I think the bolted tank standards do as well. Ditto for the ASME pressure vessel code.
For the foundations, they defer to ACI-318. AWWA D100 specifies that water load is to be considered a "live" load, which leads to some inconsistencies in how loads are actually to be applied. I think the intent is simply to be more conservative in the design, though.
Cast-in-place tanks may fall under ACI-350, and I don't remember if loading varies from the load combinations above- but it is more conservative on design.
Wire-wound concrete tanks have their own AWWA standard, and I don't know how it compares.

RE: Hydrostatic Pressure Live Load Factoring

Us old-timers ignore ACI 318 for tank pressures. The old ACI 350-89 used 1.7 on water pressures and a 1.3 "Sanitary Factor" for bending on top of that. That's 2.21. Is it overdesigned? Possibly. But water pressure is relentless. It's not like an office building where the LL is a high guess, and then only part of the floor sees that.
If you get near the limit and your reinforcing yields, it will crack. And cracks leak. Then you have embarrassing visual evidence of a problem.
The good news is that I get to visit a lot of pretty nice places with leaky tanks for engineers who used a 1.4 LF.

RE: Hydrostatic Pressure Live Load Factoring

Thanks everyone, looks like I need to read up on the ASCE and ACI standards. Jed, I've read similar to what you've noted in other places, so I think I'll take a proper look at ACI 318. Thanks. I don't want a leaky tank glasses

RE: Hydrostatic Pressure Live Load Factoring

@WorcesterSorcerer (Mechanical);

Apparently your country is UK. ( Since ur internet country code GB).
I will suggest you to look Eurocode 1 part 4 .The recommendations in Annexes A und B at UK National annex.
For liquid LL , γq=1.2 may be taken for ULS and γq=1.0 for SLS .
Provided that , tank content is always water ( ρ =1.0) and the liquid height is the max. level dictated by overflow etc ( rather than level switches , level gages ..etc)

Just curious , can the hydrostatic pressure build up with some reason and reach γq=1.5 ??

Use it up, wear it out;
Make it do, or do without.


RE: Hydrostatic Pressure Live Load Factoring


I am indeed in the UK :)

Are you referencing part B.3 of Annex B? My understanding of the partial factors, is that they are applied to the yield or ultimate strength of the tank material, to determine the allowable stress.

1.2 would be applied to the yield strength? Given it is an SLS or 'Operational' case. I normally apply 1.25 to the yield for SLS. So the 1.2 is useful.

The 1.5 LL comes from Table 2, Table A.A1.2 (B)of NA to BS EN 1990:2002+A1:2005,’UK National Annex for Eurocode – Basis of Structural design’.

Only because it is the only definitive number i could find. I'm certainly happy to be corrected/enlightened :)

RE: Hydrostatic Pressure Live Load Factoring


The older sanitary factor comes out with reinforcement ratios pretty similar to what happens when you use some of the fancier new concrete tank or water structure codes with their reduced crack width requirements. They've just gone toward directly controlling strain rather than using a load/stress based factor that does the same thing. After doing it the fancy new way a couple of times I'm mostly just doing it the old way because it's way easier to do without iterating a bunch of times in the design.

RE: Hydrostatic Pressure Live Load Factoring

Dear WorcesterSorcerer (Mechanical),

EN 1990 Eurocode is Basis of structural design. You are expected to look BS EN 1991-4 (Eurocode 1 — Actions
on structures — Part 4: Silos and tanks )
I have copy and pasted relevant clause of BS EN 1991-4,

Quote (B.3 Partial factors for actions
(1)P The partial factors according to EN 1990 shall be applied to the actions B.2.2 to B.2.14.
(2) The recommended value of the partial factor for the liquid induced loads during operation (see B.2.1(1)) is
γF = 1,20.
(3) The recommended value of the partial factor for the liquid induced loads during test (see B.2.1(2)) is
γF = 1,00.
(4) For accidental design situations, the recommended value of the partial factor for the variable actions is
γF = 1,00)

γq=1.2 is partial factor for action , in this case hydrostatic loading . Partial safety factors for material resistance γm , for concrete γc=1.5 , for steel γs=1.15 etc. But this is basic knowledge ..

Use it up, wear it out;
Make it do, or do without.


RE: Hydrostatic Pressure Live Load Factoring

Ah, thank you- that's all clear now and makes sense.

Can I ask what specific clause you get the partial safety factors for material resistance from please?

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