Shear resistance in rolled steel beams – which element takes most of the shear? In a typical rolled I- or H-beam under vertical loads, which part primarily resists the shear force?

Introduction / Context:Understanding how internal forces distribute within an I- or H-section helps in safe and economical design. While flanges carry most of the bending moment (tension and compression), the web is the main element resisting vertical shear in beams.

Given Data / Assumptions:

• Standard rolled steel I- or H-beam under vertical gravity loading.
• Plane sections remain plane (elementary beam theory).
• No unusual torsion or complex load paths.

Concept / Approach:From elementary beam theory, bending stress distribution is maximum at flanges and zero at the neutral axis. Shear stress distribution is parabolic with a maximum at the neutral axis, which lies in the web region. Since the web occupies most of the area near the neutral axis, it resists the bulk of the shear force, while flanges contribute little to vertical shear.

Step-by-Step Solution:

Verification / Alternative check:Design codes allow shear capacity checks using the web area (or effective web area) times an allowable shear stress, reflecting the web-dominant mechanism.

Why Other Options Are Wrong:Flanges only is incorrect since flanges mainly resist bending. Equal measure contradicts the well-known distribution. Shear negligible is unsafe. Flange-web junction only is not a recognized design assumption.

Common Pitfalls:Assuming large flange area also means large shear capacity; overlooking the parabolic shear stress profile that peaks in the web.

Final Answer:Web only