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.

 

[cab7320]

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

 

[SlideRuleEra]

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.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[cab7320]

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.

 

[SlideRuleEra]

cab7320 - Which of the ASCE 7 load combinations are you considering? Unless I'm missing something, note that paragraph 2.4.2 adds 1.5F[sub]a[/sub] 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.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[cab7320]

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)

 

[oldestguy]

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..

 

[darthsoilsguy2]

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.

 

[SlideRuleEra]

cab7320 - No, IMHO, F[sub]a[/sub] is vertical... but keep in mind that F[sub]a[/sub] is a negative (uplift) force. When ASCE 7 says "0.6 D + 1.5 F[sub]a[/sub]" it really means is "0.6 D - 1.5 F[sub]a[/sub]". 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

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[oldestguy]

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.
.

 

[SlideRuleEra]

OG - In the Coastal A-Zone the water table is almost guaranteed to be within inches of the ground surface 100% of the time.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[oldestguy]

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