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. |
|
Back to
Knowhere |
 |
Page 86 -
Building stability - Columns |
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