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An I-beam, also known as H-beam, W-beam (for　wide flange ), Universal Beam (UB), Rolled Steel Joist (RSJ), ordouble-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 theoryshows 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.

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