The 'shape factor' for an I-section in bending is approximately:
Choose the correct answer
f = 2.0 (fully plastic vs elastic)
f ≈ 1.12–1.15 — flanges concentrate area away from NA, leaving small plastic reserve over elastic
f = 1.5 (same as rectangle)
f = 1.0 (no reserve)
Correct Answer
B. f ≈ 1.12–1.15 — flanges concentrate area away from NA, leaving small plastic reserve over elastic
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Shape factor (form factor) f = Zp/Ze (plastic section modulus / elastic section modulus). Rectangle: f=1.5; Circle: f=1.7; I-section: f≈1.12–1.15 (most material in flanges, NA close to centroid → flanges yield last, small additional plastic reserve). Hollow tube: f≈1.27. I-sections used in beams because f close to 1 but moment of inertia is very high (efficient use of material).
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