Fluid and hydrostatic pressure

Loads on a rectangular tank or pit:

• the liquid inside as the fluid load F;
• the backfill and groundwater outside as H;
• the uplift under the base slab.

The calculation gives the wall strip moments and shears in the governing load combinations of ASCE/SEI 7-22 2.3.1 (or the 2.3.7 alternative) and 2.4.1. It also checks flotation with the tank empty: the factored uplift against 0.9D (0.6D in ASD), and your safety factor.

Inputs of the fluid pressure #

• Tank or pit:
  • The inside length and width, the wall height and thickness.
  • The base slab thickness, and its projection past the walls (it engages the soil above against flotation).
  • Open or covered (top slab thickness).
  • The concrete unit weight.
  • Any other permanent dead load that is always present when the tank is empty.
• Liquid (F): water (62.4 pcf), seawater (64.0 pcf) or a unit weight you enter, and the maximum depth (2.2: a fluid of well-defined maximum height). The wall top is free or pinned by the top slab.
• Backfill and groundwater (H):
  • The backfill height above the base slab.
  • The lateral earth pressure coefficient K, from the geotechnical report (3.2.1; at rest where the wall top is held).
  • The moist and saturated soil unit weights.
  • The highest groundwater below grade, which sets the uplift (3.2.2).
  • Whether the backfill is permanent, so that it may resist the liquid (2.3.1 and 2.4.1: 0.9 or 0.6 where permanent, else 0).
  • The groundwater load factors: H with 1.6 (2.3.1), or the 2.3.7 / 3.3 alternative with the groundwater Hw at 1.0.
• Flotation:
  • Whether the soil over the projection counts.
  • A safety factor on the unfactored loads, beside the load combination. This is your criterion; 0 means none.

Auto-design varies the base slab thickness (12 to 48 in.) and the projection (0 to 8 ft) for the least slab concrete. The tank, the liquid, the soil, the groundwater and the factors are kept.

Checks performed #

The drawing shows a section with the liquid, soil and groundwater pressures and the uplift.

Limitations #

• Wall model. One-way vertical strips only. Plate action of walls with a low aspect ratio (the PCA rectangular tank coefficients), circular tanks, wall reinforcement, crack control, joints and waterstops are not covered: see the liquid-containing tank wall type.
• Resisting case. The resisting backfill is taken without groundwater (H at its minimum).
• Load cases not covered. Seismic sloshing (ASCE 7 15.7) and the base slab's bending are not covered.
• Flotation weight. Only the walls, the slabs, the soil over the projection and the permanent load you enter count. Side friction is not counted.
• Safety factor. The usual 1.1 to 1.5 is from knowledge (to confirm against the code text).

Verification examples #

Verification status: 17 values compared in 5 examples (0 published, 5 hand calculations from the cited equations): 17 within tolerance, 0 recorded deviations. No published example: every value is a hand calculation. That checks the program against this reading of the code, not the reading itself. Code basis: the provisions it uses were read in the target edition.

No published flotation or tank-wall example is on disk. Hand calculations check:

• A cantilever wall: γd³/6 and γd²/2, in LRFD and ASD.
• A propped wall: w0 L²/15, 0.0298 w0 L², w0 L/10 and 2 w0 L/5.
• The soil-only case: K γ H³/6.
• The default pit's uplift and dead load, with the flotation demands of 2.3.1, 2.3.7 and 2.4.1.