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Hydrostatic Pressure on Mat foundation

Hydrostatic Pressure on Mat foundation

Hydrostatic Pressure on Mat foundation

(OP)
I have a mat foundation that supports a 9 story hotel building in NJ. Besides supporting the building above, the mat/basement slab will also see a significant amount of hydrostatic pressure due to high ground water level in the area. What load combination should I use for overall stability of the building against uplift and design of mat foundation as a slab to support the hydrostatic pressure. Do you have any code reference?
Thank you in advance.

RE: Hydrostatic Pressure on Mat foundation

How about ASCE 7-10? Chapter 2.4 has load combinations. Combinations for hydrostatic are given in a footnote for using H, which is the hydrostatic pressure. These load combinations are more concerned with member design than the over all building. For stability (uplift, sliding or overturning) combine 1.0 H + .6D.
I would not ever use the load factors for fluids that they allow. They're too low. Treat it as at least a live load.

RE: Hydrostatic Pressure on Mat foundation

MDengr - A high water table does not necessary cause hydrostatic pressure on a foundation. It DOES cause buoyancy on a foundation. Is it possible you are using the wrong term? If a site did have issues with hydrostatic pressure, there would probably be seepage to the surface or even geysers nearby. If a soil boring is made, there would be water coming up near the surface or even out the out of the boring.

If it is buoyancy that is the concern, mat foundations are routinely constructed with all, or part, of it below the water table. The only effect is that the dead weight of the concrete mat below the water table is reduced by the weigh of water displaced. Normally not a concern, unless you are relying on the mat's dead weight to resist an uplift force.

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

RE: Hydrostatic Pressure on Mat foundation

Although at first, it may be difficult to fathom hydrostatic uplift pressure exceeding dead load of 9-story building with mat slab, it can happen.....and must be considered.
Analysis below is for non-factored loads.

Interestingly, Commentary to ASCE 7-05 and ASCE 7-10 does not address hydrostatic uplift directly.

For ASCE 7-05........load due to hydrostatic pressure was included "out front" in load combinations
Load Combination 7 (ASD): 0.6 DL + Wind + Hydrostatic
When using the 60-percent DL factor......there has always been question as to whether is should apply to full design dead load used for gravity load design or to actual weight (which is generally less and perhaps much less).

For ASCE 7-10.......hydrostatic load (H) is now added to any load combination as noted in "Exceptions".........and can easily cause confusion if not carefully considered.
For uplift condition, my interpretation of these ASCE 7-10 code provisions is that full dead load may be used along with hydrostatic load. However, I submit that dead load used must only be actual weight, not the greater-than-actual weight most often used for gravity load design.

Using ASCE 7-05......with full design DL.......4-inch thick concrete slab on steel deck (40 psf) plus steel framing (10 psf) plus partition walls (10 psf to be conservative).......and neglecting weight of roof.......uniform dead load for 9 floors is 540 psf. For 2-foot thick slab, weight of slab is 300 psf. For this rough estimate, weight of foundation walls is also not included.
Per ASCE 7-05; usng 60-percent DL factor for load combinations.....total effective weight is then about 500 psf so that height of unbalanced water would have to be just more than 8 feet to cause net uplift, without taking into account any frictional resistance along sides of basement foundation walls.

This also is based on soil-supported slab (not pile-supported) since slab could be tied to piles to provide much greater uplift resistance.

Net upward pressure against slab underside of slab (at 500 psf) results in net upward pressure of 200 psf for design of slab between columns and foundation walls.

Using ASCE 7-10.........and somewhat lesser DL of 55 psf.......uniform DL is 495 psf. With same 300 psf for slab, height of unbalanced water would have to be 12.8 feet to cause net uplift greater than 795 psf dead load. Net uplift against underside of slab is then 495 psf, much greater than for ASCE 7-05, due to much greater height of unbalanced water.

Of course, lateral hydrostatic pressure must also be considered against foundation walls.

Relief pipes / valves might be one way to limit height of unbalanced groundwater.....as long as basement can safely be flooded (and then drained afterwards!)

Per ASCE 7-05.......wind was clearly to be considered along with hydrostatic load. This does not change with ASCE 7-10 since H-load is to be added to each load combination as applicable. Considering overturning force due to wind against 9-story building........without large effective dead load resistance from building........is rather frightening!
Therefore.......unbalance height of water would almost certainly have to be much less than the height that would equal weight of building.







John F Mann, PE
www.structural101.com

RE: Hydrostatic Pressure on Mat foundation

SRE,
I think for buoyancy, 'hydrostatic pressure' is an acceptable term.

RE: Hydrostatic Pressure on Mat foundation

0.9DL+1.6H using ULS.
0.6DL+H using ASD.

I would like ASCE to have provisions similar to AS and BS standards where if the water pressure is well defined or cannot be exceeded than the load factors are akin to that of a dead load instead of a live load.

RE: Hydrostatic Pressure on Mat foundation

hokie - I'm not trying to play word games with the OP. But, IMHO there is a difference, I'll give my views below:

Hydrostatic pressure acts in all directions, up, down, horizontal, or at any angle in between. It is important when considering a "hollow" structure, not "solid" structures like a mat foundation.

Buoyancy may be considered a special case of hydrostatic pressure since it acts in only one direction, straight up, or for "negative buoyancy" straight down. It is important for both "hollow" and "solid" structures.

I'll give examples where hydrostatic pressure and buoyancy are treated completely separately, on the same structure:

For both a basement and a cofferdam, hydrostatic pressure is important for designing both a basement's walls and a cofferdam's sheeting, wales and struts.

In both the basement and cofferdam example, buoyance calculations are separate and have to do with keeping the entire basement or cofferdam from "floating".

To me, these items are completely separate. I did just ask the OP for clarification of exactly what type force he is considering - did not say he is wrong.

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

RE: Hydrostatic Pressure on Mat foundation

SRE,
I understand, and you are correct that buoyancy is appropriate for the global calculation, preventing 'floating'. But the buoyant pressure when resisted by the mat or lowest floor, then 'supported' by the columns and walls, is probably most commonly referred to as hydrostatic pressure.

RE: Hydrostatic Pressure on Mat foundation

Even if the mat, probably with walls to form a box, does not float, the slab and walls will see pressures associated with it, uplift and lateral, for which they must be designed.

Mike McCann, PE, SE (WA)


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