Internal Vibrator Spacing (ACI 309)
Spaces the internal-vibrator insertions that consolidate fresh concrete (ACI 309): max spacing = 1.5 x radius of action; edge distance <= 0.75 R; insertions = ceil(lift length x 12 / spacing). A 12 in radius head spaces at 18 in, stays within 9 in of the form, and a 20 ft lift takes 14 stabs; a smaller 8 in head tightens to 12 in and 20 stabs. The head's radius of action, not the wall, sets the count. ACI 309 governs.
Formula and source
max_spacing_in = 1.5 x radius_of_action_in; edge_max_in = 0.75 x radius_of_action_in; insertions = ceil(lift_length_ft x 12 / max_spacing_in).
ACI 309 internal-vibration spacing rule by name (spacing = 1.5 R; edge <= 0.75 R); first-principles count arithmetic.
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