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 10^{9} N / m^{2} 
structural steel 
200 
aluminum 
70 
concrete 
30 
wood (Douglas Fir) 
13 


As you
can see a concrete column will have about two and onehalf times the load
bearing 
capacity
of a wooden column with the same dimensions, but only about onesixth 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 H^{3} 
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 

