A question for the Florida Engineers
A question for the Florida Engineers
(OP)
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.






RE: A question for the Florida Engineers
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 ...
RE: A question for the Florida Engineers
Thanks for your input.
RE: A question for the Florida Engineers
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.
RE: A question for the Florida Engineers
RE: A question for the Florida Engineers
RE: A question for the Florida Engineers
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.
RE: A question for the Florida Engineers
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.
RE: A question for the Florida Engineers
RE: A question for the Florida Engineers
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.
www.SlideRuleEra.net
RE: A question for the Florida Engineers
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.
RE: A question for the Florida Engineers
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.
RE: A question for the Florida Engineers