A question for the Florida Engineers 3

[aggman]

We have designed several material processing plants over the past few years in Florida and lower Alabama. As a rule, even if ground water is not an issue, we have always designed the foundations for buoyancy in the hurricane regions. Our justification is that during a hurricane there is always mass flooding to go along with it and the foundations will likely be submerged under water during the storm. I recently had another firm looking at a design we had done and they were questioning the inclusion of buoyancy for the foundation design when the water-table was four - six feet below the bottom of the foundations. We explained our reasoning and he felt that we were just killing it to be conservative. In the end we were signing off on the design so we stuck to our guns, but I am wondering what other opinions are on this. Thanks for your input.

 

[rb1957]

it sounds to me like you're being conservative, thorough, and reasonable !

you're designing a fouundation that will survive flooding, that can be expected ... sounds reasonable, and i think the owner's (and their insurance company) would appreciate this.

i don't imply that the other firm is wrong. i think they're approach is to consider the flooding to be a "failure" case, and so all bets are off (?)

thinking about the insurance angle, your design is more expensive (yes?), could you offer the owner (and his insurance company) so assurance as to the survivability of this foundation, so that maybe he could get a reduction in his insurance costs ? i appreciate that this is probably difficult to quantify, but ...

 

[aggman]

From the standpoint of cost, obviously it is more expensive to increase the foundation sizes. The buoyant weight of concrete is 87.6 pcf verses 150 pcf for regular weight concrete. Here is where we come from though. When something fails, we all get drug into it, right or wrong. Even if you did everything right and everyone knows this you still will have to spend hard earned money to prove that you are right. Also one particular plant took a direct hit from Wilma and took some pretty good hits over the last few years, only to come through literally unscathed while other buildings and structures at the same site have had significant damage. They were back up running within a few days producing stone which was going towards the rebuilding effort. We just feel that it is a valid design criteria (engineering judgement). We are also not saying the other firm was wrong, its more like a difference of opinion. Would I go to say that we will make assurances? Well I don't know because technically we are all making that assurance when we sign off on the project.
Thanks for your input.

 

[jike]

I may be missing something here and I apologize if that is the case. If it is a normal building with slab-on-grade and perimeter wall foundation and individual spread footings how can it possibly float? The concrete is significantly heavier than the water it displaces. I assume the building is not watertight and once it reaches grade it will fill up the building. What am I missing?

Certainly, if the structure is a pit (like a boat) then there is a definite possibility of it floating since the total concrete weight might well be less than the water it displaces.

 

[aggman]

Well partially they are not normal buildings with slab on grades, etc. They are proccessing equipment supports with and without building enclosures, conveyor bent supports, etc. So I guess I am talking about the typical lateral force resisting system foundations which are subject to significant uplift on the foundation, meaning the foundation weight compensates for the net uplift applied at the column base. So bouyancy does have a large effect. No it will not float, but the water pressure will reduce the net weight of the foundation by trying to push it out of the ground.

 

[jike]

So, you are talking about the combination wind uplift on the footings (that now weigh less due to buoyancy), therefore, under the flood scenerio, the structure no longer meets the 0.6D + W?

 

[aggman]

Yes. I can give an example for a conveyor bent. It is a 2D braced frame supported on a single combined footer. This isn't for Florida but it just so happens to be in front of me at this time so its easy to use.

Foundation Loads...
Dead Load = 11,418#
Dead Load Moment = 5532 #-ft
Wind Shear = 4837#
Wind Load OTM = 230710 #-ft

Width of Foundation = 16'

Thus Max OTM = 5532 + 230710 = 236,242 #-ft

Resisting Force required = 236,242*2/16 = 29,530#
FS = 1/0.6 = 1.667

Thus Preq'd = 29530# * 1.667 = 49,227#

Subtract out the DL and get the foundation weight required..

Pfoundation = 49227 - 11418 = 37,809#

Divide by 4050#/yd and we get 9.34 yds required.

If we were to include bouyancy in the foundation then the net weight of the concrete is 150pcf - 62.4pcf = 87.6pcf = 2365.2 #/yd instead of 4050#/yd. Thus to maintain our safety factor of 1.667 we will now require 16 yds of concrete. If we did not include the bouyancy effects but the foundation becomes submerged in water during a design wind load then the Resisting Moment = ((2365.2 * 9.34) + 11418) * (16'/2) = 268,072#-ft

FS = 268,072 / 236,242 = 1.13 (OK but nowhere near the code required 1.667).

Will it actually fail or not? Who knows. It will depend on a lot of things but one thing is for sure the code desired safety factor has not been met under this condition.

 

[denoid]

(I apologize in advance, but I can't resist)!

This reminds me of the David Letterman/Paul Schaefer segment "Will it Float"?

All you need is a hula hoop girl and a metal grinder girl.

 

[jike]

I agree with your approach.

 

[SlideRuleEra]

aggman - I, too agree with your approach. For electric power station construction in coastal South Carolina our design criteria documents define the water table as top of grade. All foundations and structures are designed using submerged concrete weight.

Without going into the details, our (then) newest plant (550 MW, coal fired) took a direct hit from Hurricane Hugo's (Category 4 Storm in 1989) combined wind and rains without significant damage. This proved to any doubters the value of this assumption.

I always liked to be even a little more conservative by ignoring the conventional 150 lb/cu ft concrete weight assumption and using a (typically) more accurate "true" weight for concrete (144 lb/cu ft) when considering uplift.

http://www.SlideRuleEra.net

 

[Ron]

aggman....while your approach is thoughtful and conservative, keep in mind that in most cases where the water table is 4-6 feet below grade in Florida, that the saturation condition above that during a storm event is fairly transient, particularly with respect to the "floating" occurring in concert with a maximum wind load.

Having surface water does not necessarily mean that the soils below are saturated, a condition necessary to achieve full buoyancy. Further, to cause a water table rise of 4 to 6 feet requires more than a single storm event. Depending on where you happen to be in Florida, the groundwater fluctuations are usually less than a couple of feet.

While I see nothing inherently wrong with your approach, keep in mind that the "local" engineer might have some site-specific insight that could be valuable to your design considerations, so don't discount them too readily.

 

[1254BW]

Imagine a multilevel underground structure with no super structure above ground level. The result of a buoyant effect is no way to resolve with underground building weight. Soils engineer come up with a solution to replace the site soil with non-permeable soil which he predicted 50 to 100 years flow time and adding counter weight. This is done circa 20 years ago.
I'd like hear from geologist, structural engineers any other options latest; specifically in Gulf coast area, any study or practical approach done. Thanks you aggman; your story is a rare kind.

 

[aggman]

Thanks to everyone for their insight. Ron your description of the bouyant effect is interesting in that I never really considered that the soil "under" the foundation must be completely saturated to create the buoyant effect. To me if it was submerged then its buoyant but I guess if the uplift pressure is not actually under the foundation then you don't have full buoyancy. I would expect that in most cases you are somewhere in between. I think I will do some further reading in this area to make sure I fully understand buoyant effects.