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Dry Flood Proofing and Buoyancy

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.

RE: Dry Flood Proofing and Buoyancy

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.

RE: Dry Flood Proofing and Buoyancy

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)


RE: Dry Flood Proofing and Buoyancy

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.

RE: Dry Flood Proofing and Buoyancy

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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RE: Dry Flood Proofing and Buoyancy

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

Mike McCann, PE, SE (WA)


RE: Dry Flood Proofing and Buoyancy

(OP)
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/20130726-15...

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


RE: Dry Flood Proofing and Buoyancy

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)


RE: Dry Flood Proofing and Buoyancy

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.

www.PeirceEngineering.com

RE: Dry Flood Proofing and Buoyancy

(OP)
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.

RE: Dry Flood Proofing and Buoyancy

(OP)
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.

RE: Dry Flood Proofing and Buoyancy

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:

www.SlideRuleEra.net idea
www.VacuumTubeEra.net r2d2

RE: Dry Flood Proofing and Buoyancy

(OP)
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.

RE: Dry Flood Proofing and Buoyancy

Quote:

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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RE: Dry Flood Proofing and Buoyancy

(OP)
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.

RE: Dry Flood Proofing and Buoyancy

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.

RE: Dry Flood Proofing and Buoyancy

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.

RE: Dry Flood Proofing and Buoyancy

(OP)
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.

RE: Dry Flood Proofing and Buoyancy

I am working on a similar issue and have read thru the thread. My problem is a little worse though. The project is in the low country of south Carolina, Charleston county. I have a Gatehouse to a community to design and b/c its a gatehouse can not be elevated. I have a 9ft differential from FF to Dry Flood proofing elevation. I started by just taking the 9ft of displaced water and calcing a Mat thickness to keep the bldg from floating. 9' * 62.4lb/cf / 150 lb/cf = 3.74 ft conc mat. Now digging into it more i see the issue of adding the mat thickness to the displaced water and it gets worse now i am at a 6.4ft thick Mat, digging further the ASCE 7 load combo is 0.6D +1.5Fa and this is just too ridiculous to tell anyone about. There is a mention of Fluid loads being factored the same as dead loads and i was hopefully thinking this would be applicable, at least for the bouyancy issue.

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

cab7320 - Don't play games with these floatation calculations. Do what it takes to meet the requirements with dead weight, including the safety factor. After 40 years of working in the Charleston area, I can assure you that the calculated floatation numbers are "real". We assume the water table is at the surface of the ground and, under the right conditions - which happens frequently, it is.

www.SlideRuleEra.net idea
www.VacuumTubeEra.net r2d2

RE: Dry Flood Proofing and Buoyancy

SlideRule - would you use 0.6 Dead and 1.5 Fa when looking at bouyancy? this is a safety factor of 2.5 effectively. Also to confirm what you are saying, you do not take additional displaced water depth for the Foundation thickness?

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

cab7320 - Which of the ASCE 7 load combinations are you considering? Unless I'm missing something, note that paragraph 2.4.2 adds 1.5Fa only for the V-Zones and Coastal A-Zones. If the project is that close to the ocean (which it may be), the design needs to be conservative for many reasons.

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.

www.SlideRuleEra.net idea
www.VacuumTubeEra.net r2d2

RE: Dry Flood Proofing and Buoyancy

i am using 2.4.2 as you note which wants the 1.5 Fa to be added to combos 5, 6, and 7. Combo 7 has 0.6 D. We are coastal A Zone. I think the 1.5 Fa should be considered for the horizontal loads on walls etc. because the horizontal force has a dynamic component that is not completely predictable, but i think there is less uncertainty with the buoyancy issue. If you are designing to a certain water level with your dry flood proofing there is not much uncertainty on the max buoyancy force which is why i think the buoyancy should be considered more like a Fluid Load which gets added to load combos in the same magnitude as your dead load, this relieves the 0.6 D issue. I would still plan to have a factor of safety against buoyance just not 2.5 as the code would make you have believe. 1.5/0.6 = 2.5. I ran all my options and could calc a Mat thickness to resist buoyancy of anywhere from 2.76 ft up to 6.18 ft (i am using some extension of the Mat beyond my building in all cases)

RE: Dry Flood Proofing and Buoyancy

Here is another thought. Do a test boring to see what the building sits on. Consider then surrounding the building with a trench, backfilled with a bentonit - soil mix and connect the top of trench with a seal of soil and bentonite mix.

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

In my town and many towns in my state which participate in the FEMA National Flood Insurance Program, the local zoning administrator requires all construction projects within the defined 100-yr Floodplain to be designed for wet floodproofing or dry floodproofing. Typically what happens in our case is that the zoning admin reviews the details with a state regulator and they provide feedback and then a building permit is issued or denied. I would suspect that if the permitting process is similar in your region, you should find out how involved the regulators are and how much leeway is given to engineering. although you might be willing to stamp it, what really matters if it others will consider it being eligible for flood insurance. based on the conditions you describe, anchors to an existing unreinforced slab would be a non-starter with permitting in my area. deadweight overlay slabs and wet floodproofing approaches are easy for permitters. A new structural overlay/replacement slab designed to resist buoyant forces would have traction but you'll have to hold their hand and should engage them before talking it up too much with the Owner.

RE: Dry Flood Proofing and Buoyancy

cab7320 - No, IMHO, Fa is vertical... but keep in mind that Fa is a negative (uplift) force. When ASCE 7 says "0.6 D + 1.5 Fa" it really means is "0.6 D - 1.5 Fa". That is, 60% of the buildings dead load is offsetting the 150% of the building's buoyancy. Any additional uplift resistance has to come from the mat, which has a submerged weight of 87.6 PCF (150 PCF - 62.4 PCF).

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

www.SlideRuleEra.net idea
www.VacuumTubeEra.net r2d2

RE: Dry Flood Proofing and Buoyancy

Mr. Slide Rule: It is most unlikely that short term surges such as waves or storm surge will be dampened out due to the long flow path from outside to the drain system. Even an open graded drainage layer will absorb any peaks that get that far. Finally any sumps will absorb pressure peaks also, with average head outside controlling the main flow. With the floating slab method, certainly you are not expecting to design it according to outside wave or storm surge height either, with its even shorter "flow" path through the soil. If either method will have an extended surge period, the designs should allow for that higher head.
.

RE: Dry Flood Proofing and Buoyancy

OG here. No problem doing my suggestion. The trench would be filled with bentonite slurry and water may have to be added.

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