The figure (right) shows the location of holes and clipped corners, which must be flush. Using the following formulas, the table below shows typical sizes of holes for square box sections only. For rectangular section, calculate the required area and check with your galvanizer for positioning of openings.
H + W = 24" (61 cm) or larger, the area of the hole, plus clips, should equal 25% of the cross-sectional area of the box (H x W).
H + W is < 24" (61 cm) but > 16" (40.6 cm), the area of the hole, plus clips, should equal 30% of the cross-sectional area of the box.
H + W is < 16" (40.6 cm) but > 8" (20 cm), the area of the hole, plus clips, should equal 40% of the cross-sectional area of the box.
H + W < 8" (20 cm), leave completely open, no end-plate or internal gusset.