ACI 350.3-06: What about dynamic earth pressure?
ACI 350.3-06: What about dynamic earth pressure?
(OP)
I'm designing a rectangular concrete tank and I'm working through the seismic loads per ACI 350.3-06. I have the various dynamic forces calculated from Section 4.4.1.....Pw', Pr, Pi and Pc. What is stumping me is the consideration of "Peg" in the base shear calculation. "Peg" is the lateral force on the buried portion of the tank due to the dynamic earth and groundwater pressure. Chapter 8 is referenced for Peg, but it doesn't appear to include any further details, except for resultant locations.
I know the static lateral pressure from the soil and groundwater. In this design, the tank is being designed for groundwater at the surface (21' depth). The lateral pressure will be due to the buoyant weight of the soil (120 pcf - 62.4 pcf = 57.6 pcf = 57.6 psf/ft) and the groundwater (62.4 pcf = 62.4 psf/ft). This will be assumed to produce a simple triangular pressure distribution (total of 120 psf/ft) over the 21' depth. This equates to a static pressure of 2,520 psf at the wall base.
Is Chapter 8 simply stating that you are to use the full static load as the dynamic load (Peg)?
I know the static lateral pressure from the soil and groundwater. In this design, the tank is being designed for groundwater at the surface (21' depth). The lateral pressure will be due to the buoyant weight of the soil (120 pcf - 62.4 pcf = 57.6 pcf = 57.6 psf/ft) and the groundwater (62.4 pcf = 62.4 psf/ft). This will be assumed to produce a simple triangular pressure distribution (total of 120 psf/ft) over the 21' depth. This equates to a static pressure of 2,520 psf at the wall base.
Is Chapter 8 simply stating that you are to use the full static load as the dynamic load (Peg)?






RE: ACI 350.3-06: What about dynamic earth pressure?
That said, there are a variety of methods and considerations to be taken into account for soil dynamics problems -- you probably want to consult a geotech.
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The name is a long story -- just call me Lo.
RE: ACI 350.3-06: What about dynamic earth pressure?
RE: ACI 350.3-06: What about dynamic earth pressure?
RE: ACI 350.3-06: What about dynamic earth pressure?
What is a quick characterization of your soils (e.g. risk of liquifaction as WARose mentioned)
What proportion and depth of the tank is buried? (e.g. what is the rough magnitude of lateral pressures from outside the tank vs inside?)
Where is the project located? (e.g. are dynamic effects due to earthquakes likely to be significant?)(I can't think of any other plausible dynamic drivers for most situations, but that's not to say there couldn't be others)
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The name is a long story -- just call me Lo.
RE: ACI 350.3-06: What about dynamic earth pressure?
It's been so long since I've used ACI 350.3-06, I cannot say. However, you do not ignore earth pressures in any design (static or dynamic).
RE: ACI 350.3-06: What about dynamic earth pressure?
RE: ACI 350.3-06: What about dynamic earth pressure?
But based on your first two data points -- buried full depth and silty sand -- my judgement would be that you absolutely should not neglect dynamic soil pressure.
The only other tidbit I can provide is to steer you toward a few methods (ideally, performed with the guidance of your friendly local geotech). The Mononobe-Okabe method is probably the most widely known and used (it's codified in AASHTO). Last I heard though, it was considered to be lagging behind the bleeding edge methods like Steedman-Zeng, Newmark(?) or FEA methods.
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The name is a long story -- just call me Lo.
RE: ACI 350.3-06: What about dynamic earth pressure?
RE: ACI 350.3-06: What about dynamic earth pressure?
Sitar (UCB) has argued for years that for low walls and moderate seismic, static design will govern. However, don't ignore dynamic forces. The dynamic event will of course create more demand on the retaining wall vs static only. If the static ground water is above the footing base (no drainage condition), then the lateral seismic pressure for saturated soil will be even greater.
Seismic wall pressure is very complex. The geo and structural community does not agree on how to calc seismic pressure distributions, but Seed-Whitman is probably the most accepted. Use y = 0.6*h per ACI 350.3 ch 8. [Sitar probably argues for y = 0.33*h or 0.5*h].
RE: ACI 350.3-06: What about dynamic earth pressure?
I do have access to a geo report that was originally completed about 10 years ago for this site. In that report, it is noted that groundwater was not encountered during the investigation, however, due to observations of wet areas within the boring and typical seasonal variations, the groundwater should be assumed to be 12' below grade for design. Obviously though,the project specifications, and ACI 350, override this recommendation......at least for the static condition.
The dynamic/seismic condition is a separate consideration in my mind. It seems completely unreasonable to assume the static flooded condition can occur simultaneously with a seismic event, especially considering the location (Ss = 0.139 & S1 = 0.052). To me, for the dynamic/seismic condition, I think that the groundwater level should be assumed as noted in the geo report (12' below grade). This produces a MUCH different, and MUCH lower magnitude, static lateral pressure distribution. The dynamic/seismic loads would add to this revised soil condition, however, based on what I've calculated so far in ACI 350.3, it seems very unlikely that it would be enough to approach the same magnitude of forces as the static condition, simply because of the groundwater level assumptions.
There is nothing of substance in the geo report dealing with the dynamic lateral soil load during seismic events, so maybe I can touch base with the geotech engineer and see if they could possibly add an addendum to it. I'll also have to look into the various methods listed above and see what I can track down. I just was a bit surprised at the lack of detail within ACI 350.3 on that subject....you're just left hanging.
RE: ACI 350.3-06: What about dynamic earth pressure?
Since seismic capacities don't include an environmental durability factor (Sd) penalty, static loads (hydro on the inside, soil on the outside) will govern your wall thickness and reinforcing design.