Wall or column: the 6.5 ft rule
ACI Contractor's Guide (3rd ed.) Ch. 5, from ACI 347, has two lateral pressure formulas, chosen by plan dimensions:
- Column: no plan dimension larger than 6.5 ft.
- Wall: at least one plan dimension greater than 6.5 ft.
One dimension is enough. Height over 8 ft is the OSHA masonry bracing threshold, and R is a variable in both formulas; neither is the test.
The formulas and their limits
Variables: p lateral pressure (psf), R rate of placement (ft/hr), T concrete temperature (°F), w unit weight (pcf), h depth of fluid concrete (ft).
- Column: p = Cw·Cc[150 + 9000R/T], maximum 3000·Cw·Cc, minimum 600·Cw, never more than wh.
- Wall: p = Cw·Cc[150 + 43,400/T + 2800R/T], maximum 2000·Cw·Cc, minimum 600·Cw, never more than wh.
Compute, then apply the lowest cap. Wall at R = 4, T = 70°F, 12 ft: 150 + 620 + 160 = 930 psf, under 2,000 and under wh = 1,800, so 930 governs. Wall at R = 20, T = 40°F, 16 ft: 2,635 psf, but the wall maximum is 2,000 psf, lower than wh = 2,400, so 2,000 governs. Column 10 ft high at R = 10, T = 60°F: 1,650 psf, but wh = 150 × 10 = 1,500 psf governs.
Traps: dropping the 150, the wrong formula, forgetting wh, and the 3,000 column maximum on walls.
Unit weight coefficient Cw
- 140 to 150 pcf: 1.0
- above 150 pcf: w/145
- below 140 pcf: 0.5(1 + w/145), but not less than 0.80
For 80 pcf: 0.5(1 + 80/145) = 0.776, below the floor, so Cw = 0.80. The 0.78 answer forgets the floor.
Pumping from the bottom
For forms filled by pumping from the bottom, design for full hydrostatic wh plus at least 25% for pump surge. A 12 ft wall of 150 pcf concrete: 150 × 12 = 1,800 × 1.25 = 2,250 psf.