Dry Flood Proofing and Buoyancy
Dry Flood Proofing and Buoyancy
(OP)
Dear Friends and Technical Experts.
I was assigned a task to dry flood proof an existing one storey building. I have calculated the dead weight of the building which counteracts the floatation forces. But the issue I have is the slab on grade.
with 2' of flood water the buoyancy forces are high on the slab. The slab is 4" thick and is not connected to the exterior walls. The client does not want to increase the slab thickness as they have to move all the interior walls and kitchen equipment and misc electric systems.
I have 4" slab weighing 49.5psf and I have 124.8 psf buoyancy force. I need to counteract 75.3 psf upward force.
Has any one used floor anchors before and if so can you suggest a manufacturer or a detail?
Wet flood proofing is not an option for the client.
Thank you all in advance.
I was assigned a task to dry flood proof an existing one storey building. I have calculated the dead weight of the building which counteracts the floatation forces. But the issue I have is the slab on grade.
with 2' of flood water the buoyancy forces are high on the slab. The slab is 4" thick and is not connected to the exterior walls. The client does not want to increase the slab thickness as they have to move all the interior walls and kitchen equipment and misc electric systems.
I have 4" slab weighing 49.5psf and I have 124.8 psf buoyancy force. I need to counteract 75.3 psf upward force.
Has any one used floor anchors before and if so can you suggest a manufacturer or a detail?
Wet flood proofing is not an option for the client.
Thank you all in advance.






RE: Dry Flood Proofing and Buoyancy
RE: Dry Flood Proofing and Buoyancy
Ask the owner which is cheaper, replacing all his kitchen equipment each tome it floods, or correcting the flooding issue.
Mike McCann, PE, SE (WA)
RE: Dry Flood Proofing and Buoyancy
RE: Dry Flood Proofing and Buoyancy
There would be some existing cracking in the concrete that would leak and the perimeter expansion joint that the OP sort of hinted at would allow water through as well.
The only way to dry proof the slab would be some kind of underfloor membrane or Volclay panels and the slab tied into the walls with waterproofed (waterstopped) joints.
The only way to do that is remove and replace the slab - thus a thicker slab would work (10" thick without any safety factor - 15" with a 1.5 SF).
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RE: Dry Flood Proofing and Buoyancy
Mike McCann, PE, SE (WA)
RE: Dry Flood Proofing and Buoyancy
thanks for your suggestions. In the book "FEMA 259 Eng Principles and Practices for Retrofitting Flood Prone Residential" by FEMA. in appendix C page C-7 it was mentioned to use floor anchors. I was checking if any of you had experience with anchoring the floor.
See attached screenshot.
http://www.fema.gov/media-library-data/20130726-15...
Unfortunately the client does not want to increase the thickness of the slab but will try to convince.
RE: Dry Flood Proofing and Buoyancy
Anchoring the floor will not solve the water penetration issue that the owner will have to contend with. Make sure he understands this or it will come back on you. He may think he is getting something he is not.
Mike McCann, PE, SE (WA)
RE: Dry Flood Proofing and Buoyancy
You might as well increase the floor thickness without using anchors. If headroom is a concern, you will need to go deeper with a new slab rather than topping the existing slab.Try tying the slab into the walls with some waterstop. Then, hope the walls had already been waterproofed. A cheaper solution may be to install some French drains in the basement and connect them to one or more sump pumps.
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RE: Dry Flood Proofing and Buoyancy
Thank you so much for your suggestions. I got a better understanding of the suggestions and the concept. This is my first time using this website after starting my career 8 years ago. Your immediate responses and your time is really appreciated.
I will contribute to other's questions on this website with the best of my knowledge.
Finally. We informed the client of the potential issues of the water seeping and buoyancy forces and cost of demolition and new construction.
Will wait for their response or decision and will keep you posted.
thank you all again.
RE: Dry Flood Proofing and Buoyancy
The client agreed to provide new slab in the building. Now I got one more confusion. When I calculated my initial buoyancy I used 62.4*2'-0". (2' being the flood water height0
I achieved that a 10" thick slab is required. Since I am using a 10" thick slab should I again reconsider using 62.4*2'-10" for my buoyancy? Because water pressure acts at the underside of the slab and not at finish floor.
If I do that, then the iteration ended at requiring 16" thick slab = 200 psf dead load. + misc interior wall non load bearing loads
flood load buoyancy = 2' flood water height + 1.33' thickness of slab = 62.4*3.33=207.7.
Is this correct or am I going crazy. I have never used a 16" thick as slab but used it like a raft foundation on a very small building.
Please suggest.
RE: Dry Flood Proofing and Buoyancy
Instead consider the elevations of both the water surface and finish floor. If you do that, then the flood water height and thickness of the slab are tied to each other. The "answer" for the minimum concrete thickness (to prevent floatation) then is defined by the simple relationship shown in the following sketch:
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RE: Dry Flood Proofing and Buoyancy
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RE: Dry Flood Proofing and Buoyancy
I HAVE CONSIDERED THE ABOVE EQUATION AND THE SLAB THICKNESS CAME UPTO BE 16" THICK.
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IT IS WEIRED HOW FEMA TOOK THE BUOYANCY FORCE HEAD TO TOP OF SLAB ONLY. IN THE ABOVE ATTACHED LINK IN THE THREAD.
MY BUILDING ITSELF WITH PERIMETER WALLS, FOUNDATIONS AND ROOF CAN WITH STAND THE BUOYANCY. BASED ON THE ABOVE CALCS I HAD TO MAKE THE SLAB 16" THICK SO THAT THE SLAB DOES NOT FLOAT.
THE 16" THICK BY ITSELF WILL HAVE TO RESIST MOST BUOYANCY FORCES.
I WILL CALL FEMA TO VERIFY AND MAKE CORRECTIONS.
RE: Dry Flood Proofing and Buoyancy
RE: Dry Flood Proofing and Buoyancy
Well, it's not that the slab will float but that the overall building would not float.
MotorCity above already pointed out that concrete doesn't float.
So the question is - did you include the weight of the building itself in your calculations?
If the slab is adequately attached to the perimeter building walls, then the buoyancy pressure would push up on your thick slab which would be held down by its own weight and also the load on the perimeter of the thick slab. So the slab would then try to span between these heavy walls, bowing upward in bending. You can design the slab to resist this upward bending if the span isn't too long.
With a 16" slab I would think that it could span up to 20 to 30 feet. The perimeter shear connections might be tricky.
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RE: Dry Flood Proofing and Buoyancy
I HAVE INCLUDED MY WEIGHT OF BUILDING INTO MY CALCUALATIONS. THE EXSITING WALLS ARE JUST 8" EXISTING CMU AND I COULD NOT RELY ON THEM HOLDING ( TAKING THE REACTION FROM) THE SLAB. THE SPAN OF THE (MINIMUM WIDTH) BUILDING IS 48FT. SO I HAD TO RESIST THE FULL BUOYANCY WITH SLAB FIRST AS I CANNOT SPAN THE SLAB THAT LONG.
THE EXSITING BUILDING ( WALLS, FOUNDATION, ROOF) BY ITSELF WITH OUT THE SLAB CAN HOLD THE BUOYANCY FORCE.SO I WAS LEFT WITH THE FINAL OPTION TO MAKE THE SLAB THICK.
RE: Dry Flood Proofing and Buoyancy
RE: Dry Flood Proofing and Buoyancy
RE: Dry Flood Proofing and Buoyancy
I am proposing to remove the existing slab completely because of the ceiling and elevations issues.
Once again thank you all for a very quick and knowledgeable responses.
RE: Dry Flood Proofing and Buoyancy
logically i don't see how the building will be water tight and there will be pumps for handling the intruding water and if these are taking water from the FFE i would therefore not have the added displaced water from the Mat thickness. That gets me back to a 3.74ft thick mat but with no factor of saftey.
Any thoughts
RE: Dry Flood Proofing and Buoyancy
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RE: Dry Flood Proofing and Buoyancy
My current approach is to use the 1.5 Fa but with 1.0 Dead giving a FS = 1.5 and using the displaced water depth to top of the Mat Fndn rather than the bottom.
RE: Dry Flood Proofing and Buoyancy
Concerning your question about "additional displaced water", I suggest disregarding incremental changes in foundation depth. Take a look at my sketch in this thread from 9 December. The needed total slab thickness, including safety factor, can be calculated using algebra, straight from Archimedes Principle.
In general, don't depend on (electric) pumps to help provide floatation "protection". Consider that a likely "perfect storm" of trouble is a combination high water table during heavy rains from tropical storms / hurricanes AND the increased probability of simultaneous wide spread, long duration, electrical power outage for the same reason.
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RE: Dry Flood Proofing and Buoyancy
RE: Dry Flood Proofing and Buoyancy
Replace the floor but place it on a granular layer of open graded gravel laid on a well graded filter. Install perforated pipes leading to a sump. Install a submersible pump. Run the permeability calculations to see what quantity of collected water might be per hour for pump sizing. This way you can allow for a significantly different flood height, providing your walls can hold back that pressure of flood water. Replace the slab.
Rather than a bentonite -soil perimeter trench, you also might look at sheet piling, some of which could be relatively light sections.
This was done for a bowling alley in Dubuque,Iowa about 1950's. I think temporary berm was also used around the building and some sumps were installed inside. Site as all sand I think.
Given some careful planning of all details, a higher flood crest can be accounted for with wall reinforcing, etc. I'd also have a portable generator handy..
RE: Dry Flood Proofing and Buoyancy
RE: Dry Flood Proofing and Buoyancy
Good move on your part to extend the mat beyond the building.
If I wanted to get "picky" would mention that 150 PCF is not conservative for uplift (even for reinforced concrete)... I would use 145 PCF. Also 62.4 PCF is not conservative for seawater... 64 PCF is more typical. But I will not quibble over the values you're using... just a thought.
oldestguy - Hate to disagree with you on your suggestion, but the Coastal A-Zone is almost certainly within sight of the ocean. Per FEMA: "In a Coastal A Zone, the principal source of flooding will be astronomical tides, storm surges, seiches or tsunamis, not riverine flooding. During base flood conditions, the potential for breaking wave heights between 1.5 feet and 3.0 feet will exist".
Here is a link to: FEMA Design and Construction in Coastal A-Zones
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RE: Dry Flood Proofing and Buoyancy
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RE: Dry Flood Proofing and Buoyancy
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RE: Dry Flood Proofing and Buoyancy