Moment of Inertia - I-Beam (Ideal cross section)
An I-beam, also known as H-beam, W-beam (for “wide flange”), Universal Beam (UB), Rolled Steel Joist (RSJ), or double-T (especially in Polish, Bulgarian, Spanish, Italian and German), is a beam with an I- or H-shaped cross-section. The horizontal elements of the “I” are known as flanges, while the vertical element is termed the “web”. I-beams are usually made of structural steel and are used in construction and civil engineering.
The web resists shear forces, while the flanges resist most of the bending moment experienced by the beam. Beam theory shows that the I-shaped section is a very efficient form for carrying both bending and shear loads in the plane of the web. On the other hand, the cross-section has a reduced capacity in the transverse direction, and is also inefficient in carrying torsion, for which hollow structural sections are often preferred.
For a beam of cross-sectional area a and height h, the ideal cross-section would have half the area at a distance h/2 above the cross-section and the other half at a distance h/2 below the cross-section.
For this cross-section the moment of inertia is shown here.
|moment of inertia of the beam (m4)
|cross-sectional area (m2)
|cross-sectional height (m)