Openings in the Skirt

For skirt compression, deduct the width of openings, G (ft), around the perimeter of the skirt. The following equations take into account the shift in the neutral axis as well as the reduction in cross section. The equation for maximum compressive stress f under combined axial and bending loads at openings can be written:

D = Diameter of skirt, ft
a and ß are coefficients depending on the ratio G/D. The expression for determining
a and ß is complex, but both can be approximated with slight conservatism
for ranges up to G/D = 0.7 (90° opening) by the following equation:

This equation is for a single opening and is conservative when G is the sum of several openings distributed around the circumference. When the skirt thickness obtained using this equation is large and when there are several distributed openings or one large opening with G > 0.7D, more exact methods of determining FB may be justified.

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