For a standard I-section (web: height h, thickness tw; flanges: width b, thickness tf), the second moment of area Ixx about the horizontal centroidal axis is approximately:
Choose the correct answer
b x h^3 / 12 (rectangle, overestimates)
(b x h^3 / 12) - 2 x ((b-tw)/2 x (h-2tf)^3 / 12) (gross minus web cut-outs) = bd^3/12 - (b-tw)(d-2tf)^3/12
Only tw x h^3 / 12 (web only)
b x tf^3 / 12
Correct Answer
B. (b x h^3 / 12) - 2 x ((b-tw)/2 x (h-2tf)^3 / 12) (gross minus web cut-outs) = bd^3/12 - (b-tw)(d-2tf)^3/12
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I_xx for I-section: I = (1/12)[b h^3 - (b-tw)(h-2tf)^3], computed as rectangle minus two rectangles (void area between flanges). For standard rolled sections, tabulated in SP 6(1). Alternatively: Ixx = 2 x [tf x b^3/12 (flange about own axis) + tf x b x (h/2 - tf/2)^2] + tw x (h-2tf)^3/12 (parallel axis theorem).
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