Question 1

Why is depth more important than width for beams?

Accepted Answer

Moment of inertia scales with height³ but only linearly with width (I = bh³/12). Doubling the height increases I (and stiffness) by 8×, while doubling the width only doubles I. This is why beams are always tall and narrow, and why I-beams concentrate material in the flanges.

Question 2

What is the difference between Ix and Iy?

Accepted Answer

Ix is the moment of inertia about the X-axis (horizontal), resisting bending in the vertical plane. Iy is about the Y-axis (vertical), resisting lateral bending. For beams loaded vertically, Ix is the primary design value. Iy matters for lateral-torsional buckling checks.