Elevated Storage Tank

[BButler505]

I am doing an analysis of an elevated chemical storage tank. Should the liquid be defined as a live load or dead load? Secondly I need some insight into the seismic design of the elevated tank due to sloshing of liquid inside (ASCE chapter/section reference would help). Thanks

 

[BUGGAR]

Can I look that up for you?

 

[BButler505]

I just found the seismic stuff but could not find if the liquid is live or dead load

 

[SKJ25POL]

BButler505 (Structural,
Liquid in containers are Fluid Load "F".
The procedure is same as any tank regardless if is on ground or elevated.
Draw a pic with dimension and information if need more assiatance

 

[UtahEngineer]

Assuming the tank is used for the same kind of chemical throughout its life I would consider it as dead load. Since the weight of the chemical isn't variable, you wouldn't need to consider variability like there is with live load. I would also do two analyses of the tank, one empty (low gravity load, low period = high seismic load), and one full (high gravity load, higher period = potentially lower seismic load).

 

[UtahEngineer]

I edit my response to second SKJ25. Note that ASCE says to include fluids with the same load factor as dead load, so the logic is there.

 

[PittEng88]

Check out Section 15.7 of ASCE7-05 for the seismic design of the tank. If this is a reinforced concrete tank I'd also suggest checking out ACI 350 and PCA's Design of Liquid-Containing Concrete Structures for Earthquake Forces. As for the liquid itself SKJ25POL is right its a "Fluid Load," which gets its own load factors. The seismic forces that are induced by the sloshing fluid, should be classified as an earthquake load.

Hope this helps!!

 

[BButler505]

Thanks guys. I have analyzed the tank with its full design load. The tricky part will be the analysis of the elevated tank for sloshing under seismic loads. The tank and contents weight = 400kips and is elevated 16 ft.

 

[JStephen]

For foundation design of water tanks, AWWA D100 specifies that water weight is to be considered a live load. I don't think that's a different approach so much as an attempt to make sure the design is more conservative than usual.

And, for elevated water tanks, AWWA D100 does not consider sloshing effects. I think there are two issues there. One is, that ground storage tanks are vertical cylinders, there's been a lot of work done on dynamic effects of that shape, and so it's easily codified. But elevated tanks are a variety of spherical/cylindrical/conical/toroidal shapes, and combinations thereof, and so it's not as easy to work with. The second thing is that on a larger ground storage tank, consideration of sloshing effects leads to a reduction in the forces, not an increase, so neglecting sloshing is conservative in those cases.

 

[racookpe1978]

You've got four places to look at the "live" (changing) loads - compared to a "dead" (never-changing, very predictable) load. Make sense?

The fluid level can be assumed empty or full. Pretty easy.

But, the changing loads hit you four places:
The tank-to-support welds and their fasteners.
The tank-support stress levels. (middle of the post stresses)
The tank-support-to-base-plate loads,
then the base-plate-to-concrete pads loads, and the subsequent pads-to-dirt loads underneath the concrete)

A dead load (in civil terms) is easier to predict than a live load, seismic loads at all four places are the hardest of all. So, the difference in codes tends to give the biggest margins to the hardest-to-predict stresses. That's the reason for the difference in terms for the tanks - how well can somebody predict what is going to happen in worse-case conditions.

 

[steuey]

Great thread, thanks for all the responses~