If the depth of a structural beam is doubled, what is the expected effect on bending strength?

If the depth of a structural beam is doubled, what is the expected effect on bending strength?

• A. It increases dramatically
• B. It increases significantly
• C. It remains the same
• D. It becomes unstable

Answer: (B)

Bending strength rises because when you change the beam’s depth, you change the section that resists bending. For a rectangular cross-section, the bending capacity is proportional to the section modulus S, which for a rectangle equals (width × depth^2) divided by 6. If the depth doubles while the width stays the same, the section modulus grows by a factor of four. Since the bending capacity M_allow is roughly sigma_allow × S, doubling the depth makes the bending strength about four times larger. That’s a substantial, but not unlimited, increase, so describing it as increasing significantly is the most accurate choice.

Bending strength rises because when you change the beam’s depth, you change the section that resists bending. For a rectangular cross-section, the bending capacity is proportional to the section modulus S, which for a rectangle equals (width × depth^2) divided by 6. If the depth doubles while the width stays the same, the section modulus grows by a factor of four. Since the bending capacity M_allow is roughly sigma_allow × S, doubling the depth makes the bending strength about four times larger. That’s a substantial, but not unlimited, increase, so describing it as increasing significantly is the most accurate choice.