storm shelter buoyancy

[smvk3]

Section 303.3 of the ICC500 states: "Underground portions of storm shelters shall be designed for buoyancy forces and hydrostatic loads assuming that the ground water level is at the surface of the ground at the entrance to the storm shelter, unless adequate drainage is available to justify designing for a lower ground water level."

My interpretation of this is that if you are designing an underground storm shelter, you must assume the ground water level is at grade and subsequently you must design for the hydrostatic and buoyancy loads that come with this.

We are currently designing a storm shelter that is above-ground with shallow foundations. I am having a disagreement with my coworker who is saying that you must also design the foundations assuming ground water level is at finished grade (which is +/- 3' below finish floor). Is this interpretation of the code correct? I am getting insanely large footing sizes since the buoyancy force uplift is equivalent to the volume of the foundations displaced by water.

 

[SlideRuleEra]

...design the foundations assuming ground water level is at finished grade (which is +/- 3' below finish floor). Is this interpretation of the code correct?

Yes, that is my interpretation...

...unless adequate drainage is available to justify designing for a lower ground water level."

http://www.SlideRuleEra.net

 

[fel3]

The specific weight of reinforced concrete is about 2.4x the specific weight of water. So, for an above-ground structure, where only the foundation is below grade, the buoyancy force will be more than offset by the weight of the foundation alone, not to mention the weight of the structure above. Now, if you are displacing some of this notional groundwater with air (which would happen if the floor of the structure is somewhat below grade), then buoyancy might be as issue.

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

 

[smvk3]

For the dead load only load case, the buoyancy uplift is not giving me any stability issues.

It is the .6D +.6W + H (where H is the uplift due to buoyancy) load case that is giving me issues where the footing is subjected to overturning from in-plane shearwall loads. The buoyancy uplift loads are causing my load resultant to be off of the footing making it unstable. I can't make my footing any larger due to geometric restriction of the project.

 

[oldestguy]

What about storm shelter partly below nearby ground level and partly above, but there is adjacent fill sloped up to storm shelter roof elevation? I'd consider water table elevation only equal to ground elevation away from the sloped up fill against the shelter. In any case when considering resistance to uplift by earth, saturated earth probably which is submerged where its effective density is reduced due to buoyancy. A rough effective density number probably is 80 lb/cf or less. Go out from any support slab projection at 60 degrees to horizontal for calculating its total weight. With no projection I'd take skin friction against the shelter as low. In some cases may be near zero. Since there is no water inside the shelter all concrete is considered not submerged.

 

[smvk3]

The geotech report states that the long-term groundwater level is anticipated to be at or below the explored depth (14' below existing grade).

The geotech engineer for the project stated that in his opinion he would not design the foundations for buoyancy loads and tornado loads at the same times since it would be such a short-term event.

I'm thinking that the spirit of the code is that you should check the building against buoyancy forces but you would not have to do this check with tornado loads acting on it.

 

[SlideRuleEra]

The geotech report states that the long-term groundwater level is anticipated to be at or below the explored depth (14' below existing grade).

I'm thinking that the spirit of the code is that you should check the building against buoyancy forces...

The code is written that way because there are locations where storm shelters are used and the the water table can be at the surface of the ground... the location for your project is apparently not one of those locations. IMHO, go with the results of the geotech report (water table is not an issue).

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