This I-beam is used to support the first floor of a house. The web resists shear forces, while the flanges resist most of the bending moment experienced bs 7121 pdf download the beam.
American bridge and skyscraper work of the mid-twentieth century. These sections have parallel flanges, as opposed to the varying thickness of RSJ flanges which are seldom now rolled in the UK. Parallel flanges are easier to connect to and do away with the need for tapering washers. UCs have equal or near-equal width and depth and are more suited to being oriented vertically to carry axial load such as columns in multi-storey construction, while UBs are significantly deeper than they are wide are more suited to carrying bending load such as beam elements in floors.
However, there has been some concern as to their rapid loss of strength in a fire if unprotected. Illustration of an I-beam vibrating in torsion mode. Tables are available to allow easy selection of a suitable steel I-beam size for a given applied load. To prevent failure, most of the material in the beam must be located in these regions.
Comparatively little material is needed in the area close to the neutral axis. The farther a given amount of material is from the neutral axis, the larger is the section modulus and hence a larger bending moment can be resisted. When designing a symmetric I-beam to resist stresses due to bending the usual starting point is the required section modulus. However, these ideal conditions can never be achieved because material is needed in the web for physical reasons, including to resist buckling.
Though I-beams are excellent for unidirectional bending in a plane parallel to the web, they do not perform as well in bidirectional bending. These beams also show little resistance to twisting and undergo sectional warping under torsional loading. These beams have flanges in which the planes are nearly parallel. ASTM grades A572 and A36. Like most steel products, I-beams often contain some recycled content.