I-beam

This I-beam is used to support the first floor of a house.

I-beams (also known as H-beams, W-beams (for "wide flange"), rolled steel joist (RSJ), or double-T (especially in Polish, Spanish and German)) are beams with an I- or H-shaped cross-section. The horizontal elements are flanges, while the vertical element is the web. The Euler-Bernoulli beam equation shows that this is a very efficient form for carrying both bending and shear 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.

Illustration of a vibrating I-beam.

Overview

There are two standard I-beam forms:

• Rolled I-beam, formed by hot rolling, cold rolling or extrusion (depending on material).
• Plate girder, formed by welding (or occasionally bolting or riveting) plates.

I-beams are commonly made of structural steel but may also be formed from aluminium or other materials. A common type of I-beam is the rolled steel joist (RSJ) - sometimes incorrectly rendered as reinforced steel joist. British and European standards also specify Universal Beams (UBs) and Universal Columns (UCs). These sections have parallel flanges, as opposed to the varying thickness of RSJ flanges. UCs have equal or near-equal width and depth, while UBs are significantly deeper than they are wide.

I-beams engineered from wood with fiberboard and/or laminated veneer lumber are also becoming increasingly popular in construction, especially residential, as they are both lighter and less prone to warping than solid wooden joists. However there has been some concern as to their rapid loss of strength in a fire if unprotected.

Design

I-beams are widely used in the construction industry and are available in a variety of standard sizes. Tables are available to allow easy selection of a suitable steel I-beam size for a given applied load. I-beams may be used both as beams and as columns.

I-beams may be used both on their own, or acting compositely with another material, typically concrete. Design may be governed by any of the following criteria:

• deflection - the stiffness of the I-beam will be chosen to minimise deformation
• vibration - the stiffness and mass are chosen to prevent unacceptable vibrations, particularly in settings sensitive to vibrations, such as offices and libraries
• bending failure by yielding - where the stress in the cross section exceeds the yield stress
• bending failure by lateral torsional buckling - where a flange in compression tends to buckle sideways or the entire cross-section buckles torsionally
• bending failure by local buckling - where the flange or web is so slender as to buckle locally
• local yield - caused by concentrated loads, such as at the beam's point of support
• shear failure - where the web fails. Slender webs will fail by buckling, rippling in a phenomenon termed tension field action, but shear failure is also resisted by the stiffness of the flanges
• buckling or yielding of components - for example, of stiffeners used to provide stability to the I-beam's web

Wide-flange steel materials and rolling processes (U.S.)

Rusty steel I-beam

In the United States, the most commonly mentioned I-Beam is the wide-flange (W) shape. These beams have flanges in which the planes are nearly parallel. Other I-Beams include American Standard (designated S) shapes, in which flange surfaces are not parallel, and H-piles (designated HP), which are typically used as pile foundations. Wide-flange shapes are available in grade ASTM A992,^[1] which has generally replaced the older ASTM grades A572 and A36. Ranges of yield strength:

• A36: 36,000 psi (250 MPa)
• A572: 42,000–60,000 psi (290–410 MPa), but 50,000 psi (340 MPa) is the most common
• A588: Similar to A572
• A992: 50,000–65,000 psi (340–450 MPa)

Wide-flange shapes are produced by the electric arc furnace method and generally contain more than 95% recycled content.^{[citation needed]}

The American Institute of Steel Construction ("AISC") publishes the "Steel Construction Manual" for designing structures of various shapes. It documents the common approaches, ASD and LRFD, (as of 13th ed.) to creating such designs.

Designation and terminology

• In the United States, steel I-Beams are commonly specified using the depth and weight of the beam. For example, a "W10x22" beam is approximately 10 in (25 cm) in depth (height when the I-Beam is standing on its flanges) and weighs approximately 22 lb/ft (33 kg/m).

• In Canada, steel I-Beams are now commonly specified using the depth and weight of the beam in metric terms. For example, a "W250x33" beam is approximately 25 cm (10 in) in depth (height when the I-Beam is standing on its flanges) and weighs approximately 33 kg/m (22 lb/ft).^[2] I-Beams are still available in U.S. sizes from many Canadian manufacturers.

• In India I-beams are designated as ISMB, ISJB, ISLB, ISWB. ISMB :Indian Standard Medium Weight Beam, ISJB : Indian Standard Junior Beams, ISLB : Indian Standard Light Weight Beams, ISWB : Indian Standard Wide Flange Beams. Beams are designted as per respective abbreviated reference followed by the depth of section such as for example ISMB 450, where 450 is the depth of section in millimetres (mm). The dimensions of these beams are classified as per IS:808. (as per BIS)

• In the United Kingdom, these steel sections are commonly specified with a code consisting of the major dimension (usually the depth)-x-the minor dimension-x-the mass per metre-ending with the section type, all measurements being metric. Therefore a 152x152x23UC would be a column section (UC = universal column) of approximately 152mm depth 152mm width and weighing 23kg per meter length.^[3]

See also

• Beam (structure)
• Structural steel
• T-beam
• I-joist

References

External links

• Canadian Institute of Steel Construction website
• American Institute of Steel Construction website
• Wood I-joists