The modulus of elasticity, E, of a material (sometimes referred to as Young's modulus or the

elastic modulus) is a measure of how stiff it is.  It is an experimentally determined measure

of how much tensile stress the material can withstand and still return to its original shape

after the stress has been removed, like stretching a rubber band.  A column made of a

material with a high modulus of elasticity is stiffer and therefore will bear a greater load

than one made from a material with a lower E.  The following table lists the modulus of

elasticity for a few common structural materials.

.

 Material Young's modulus 109 N / m2 structural steel 200 aluminum 70 concrete 30 wood (Douglas Fir) 13

As you can see a concrete column will have about two and one-half times the load bearing

capacity of a wooden column with the same dimensions, but only about one-sixth the load

bearing capacity of one made from steel.

The moment of inertia, Ι, of a structural member such as a column or beam is a measure of

its resistance to bending due to the geometry of its cross section, that is, the distribution of

its mass relative to the center of its longitudinal axis.  The moment of inertia of a solid

column with a square or rectangular cross section is given by the formula:

 I = B H3 where  Ι = moment of inertia 12 B = width of cross section H = height, or thickness of cross section

In the case of a column with a rectangular shaped cross section, the smallest dimension is

designated as H so as to yield the smallest moment of inertia.

For example, consider two wooden columns, a) and b), with dimensions that are commonly

used in light wood frame construction.  Column a) is a 2 in. x 4 in. wooden stud used in wall

framing.  Column b) is a 4 in. x 4 in. wooden column like that used for framing large

openings in walls.

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 Page 86 - Building stability - Columns
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