Rolled steel I-sections are most commonly selected for beams because they provide which combination of structural advantages?

Introduction / Context:
Beam selection aims to maximize bending stiffness and strength per unit weight while ensuring stability. The rolled I-section concentrates material in the flanges away from the neutral axis where it contributes most to bending resistance, making it the workhorse of steel framing.

Given Data / Assumptions:

• Comparison among generic rolled shapes for beams under vertical loads.
• Focus on inertia, section modulus (moment of resistance), and stability.

Concept / Approach:
For a given mass, shifting area to flanges increases second moment of area about the strong axis, boosting bending stiffness and section modulus. The web provides shear capacity with minimal added weight. This distribution also assists lateral stability for practical spans when bracing is adequate; torsional properties are acceptable for strong-axis bending with common restraints.

Step-by-Step Solution:
Relate flange area at large distance from neutral axis to increased I and Z. Note the web's efficiency in carrying shear with small thickness. Acknowledge improved stability compared with shallow/wide thin plates when braced. Conclude that I-sections offer all listed advantages for beams.

Verification / Alternative check:
Design tables show higher Z_x/I_x per unit weight for I-sections than many alternative shapes (e.g., solid rectangles), validating their efficiency for bending about the major axis.

Why Other Options Are Wrong:
Each single advantage is true; the most complete and therefore correct option is “All the above.”

Common Pitfalls:

• Ignoring lateral-torsional buckling; beams still require lateral restraint of compression flanges.
• Assuming I-sections are optimal for all loadings; for torsion or biaxial bending, other shapes may be superior.

Final Answer:
All the above.