Dry Flood Proofing and Buoyancy

[Bala0404]

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.

 

[MotorCity]

124.8 psf is the weight of the water, it is not the buoyant force. Solid concrete objects due not float since the unit weight of concrete is (usually) more than the unit weight of water. Usually you only get uplift on concrete structures if they are in the form of a tank or other hollow shape. Calculate the weight of the slab and the weight of water displaced by the slab, then compare the two and you will see that your slab will not float.

 

[msquared48]

Since the basement is not sealed, there is no bouyancy, only an uplift pressure. The water will merely seep through existing cracks unto the 2 foot level of water is seen inside the structure as well as outside, relieving that pressure.

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)

 

[jtreybal]

Bala0404 said it was to be DRY flood-proofed, not wet flood-proofed. Therefore no water in the structure and net buoyancy if the water level is two feet above the slab.

 

[JAE]

An older 4" thick basement floor slab won't prevent water from seeping through.
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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[msquared48]

Yes, the INTENT is dry flood, but NOT the construction. Reality is far different.

Mike McCann, PE, SE (WA)

 

[Bala0404]

Hi All,

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/20130726-1506-20490-2593/fema259_complete_rev.pdf

Unfortunately the client does not want to increase the thickness of the slab but will try to convince.

 

[msquared48]

You are not addressing the primary issue here either with adding concrete, or adding screw anchors.

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)

 

[PEinc]

msquared48 is correct. You probably do not have a water-tight basement now. Therefore, if the water cannot leak into the basement fast enough, the 75.3 psf of net uplift will lift or crack and lift the 4" slab until the water freely flows into and floods the basement. If you install tiedown anchors to hold the slab down, you still need to worry about seepage into the basement and you could get slab cracking in the slab span between the closely spaced anchors. If you install tiedown anchors, you will need to cover them over with a thickened slab in order to restore a smooth, flat slab without tripping hazards (the tiedown anchor heads).

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.

http://www.PeirceEngineering.com

 

[Bala0404]

Hi All,

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.

 

[Bala0404]

HI all,

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.

 

[SlideRuleEra]

Bala0404 - Don't worry, you are not "going crazy". You are making the problem appear more difficult than it is, by using terms like "flood water height" and "thickness of slab". The trouble is those terms are relative, not definitive. As floor thickness changes it is not clear to us what happens to the flood water height.

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:

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[JAE]

...with a safety factor included of course.

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[Bala0404]

DEAR SIDERULE AND JAE THANK YOU FOR YOUR SUGGESTIONS.
I HAVE CONSIDERED THE ABOVE EQUATION AND THE SLAB THICKNESS CAME UPTO BE 16" THICK.

http://www.eng-tips.com/

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.

 

[Bala0404]

mY BUILDING SECTION AND PLAN.

 

[JAE]

BASED ON THE ABOVE CALCS I HAD TO MAKE THE SLAB 16" THICK SO THAT THE SLAB DOES NOT FLOAT.

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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[Bala0404]

aGREE.
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.

 

[jayrod12]

Can the 8" CMU walls resist the hydrostatic pressure of being submerged? I think your bigger issue would be ensuring that wall is water tight and strong enough to resist the lateral water pressure.

 

[oldestguy]

While you are at it with the thicker slab, why not throw in some added resistance. For instance, roughening up the existing surface, placing a "bonding agent" of cement slurry immediately before placing concrete works well for rehabilitating slabs that have had salt problems removed by jack hammers in parking garages. Also re-bars in the slab possibly even attached to walls may provide that extra benefit if flood elevation goes higher than predicted.

 

[Bala0404]

8" cmu walls are verified and am reinforcing them to with stand lateral forces.
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.