ACI 350--Durability Factor 1

[rholder98]

Does the durability factor S for liquid-containing structures apply to the "empty" load case? That is, do I apply it to the flexure imposed by the soil on the outside of the tank before it gets filled? Section 9.2.8 says "Required strength U shall be multiplied by the following environmental durability factors (S) in portions of an environmental engineering concrete structure. . ." 'U' is typically the controlling of several load cases, but in the Commentary, one specific load case is mentioned, which includes D + L + F, but does not include earth pressure effects. Is that just an example, or does it apply to all 'U'?

 

[JedClampett]

Short answer is: Some people do and some don't.
What I find is that our at-rest soil pressures are in the range of 60 pcf, very close to water pressure. I can apply the durability factor to the soil load and come out with the same design for both faces. If it's conservative at least it can't be questioned.

 

[miecz]

Good question. The reasoning given for the application of "S", durability, water tightness, would not seem to apply to load cases that don't include fluid pressure.

However, the code says it must be applied "in portions of an environmental concrete structure where durability, liquid tightness, or similar serviceability are considerations. It does not differentiate between load cases, except for earthquake loads. Because the code singled out earthquake loads, and not soil loads, I think the code committee intended "S" to apply to soil pressure.

 

[BSE05]

My experience with water and waste treatment facilities. (lots of large concrete tanks) We always took a full buoyant load on the outside of the tank. water + buoyant wt of soil and in most cases a 200 plf surface surcharge for truck/crane equipment.

Most tanks during their life are emptied for maintenance.
The exterior water level should be at the top of tank unless you have documented data. With water and waste treatment facilities they ususually are near water bodies and have high water tables.

Buoyancy (uplift) can also be a problem to investigate for the empty tank. We have installed relieving hatches in the base slab of some waste tanks.

Our criteria of course preceded "code". But it is good to be on the safe side! Does anyone have any tank horror stories?

 

[JedClampett]

The usual disaster that I've heard of is that a design assumes that the soil resists the water pressure load. For some reason, the earth is excavated away and the wall falls down. This isn't directly related to durability factors, but does relate to load cases considered.