i-beam

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

I-beams (also known as W-beams or double-T esp. in Polish and German) are beams with an I- or H-shaped cross-section ("W" stands for wide flange). 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. 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 deeper.

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.)

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, which has generally replaced the older ASTM grades A572 and A36.

Rusty steel I-beam

Ranges of yield strength (where 1 ksi = 1,000 pounds per square inch):

A36 - 36 ksi

A572 - 42 ksi to 60 ksi (50 ksi most common)

A588 - Similar to A572

A992 - 50 ksi to 65 ksi

Wide-flange shapes are produced by the electric arc furnace method and generally contain more than 95% recycled content.

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 inches in depth (height when the I-Beam is standing on its flanges) and weighs approximately 22 pounds per linear foot.

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 250 mm in depth (height when the I-Beam is standing on its flanges) and weighs approximately 33 kg/m (kilogrammes per linear metre)