whose
cross sections have unequal dimensions, such as the one modeled in Fig.
134, the |
SR of
the narrower face, e.g. 2 inches, is the determining factor. |
|
Building
codes specify the maximum permissible SR of load bearing columns for
common |
structural materials. This is to establish minimum uniform building
standards for structures. |
The SR
for three common structural materials are listed here: |
. |
Material |
Slenderness ratio |
structural steel |
> 150 |
aluminum |
> 55 |
wood |
< 50 |
|
|
 |
Finally, regardless of a column's geometry or the material it is |
|
made
of, another factor affecting its load bearing capacity is its |
|
end
condition. That is, whether its ends are fixed or whether it |
|
is
freestanding. Euler's equation for the critical buckling load |
|
of
column's presented earlier presumes they are freestanding. |
|
However the ends of most structural columns are fixed in place |
|
to
the foundation they rest on and to the load they support. |
|
Thus
Euler's equation needs to be modified by adjusting the |
|
Fig. 135 - Column with |
length factor, L, with an "effective length" factor, Leff,
for those |
|
fixed ends |
columns with fixed ends. Basically the effective length of such |
|
|
columns is one-half the length of a
freestanding column of the |
|
|
same
length. Since the critical buckling load of a column is indirectly
proportional to its |
length
squared, fixing the ends of a column increases its strength by a factor of
four. |
|
Beams |
 |
|
Beams are called by different names, such as beam, girder, |
strut, brace, or joist, depending on their function or location |
in
the structure. However they all share similar characteristics. |
A
beam is one of the simplest structures in design but one of |
the
most complex to analyze in terms of the external and |
|
|
internal
forces acting on it. The complexity of its behavior under load
depends on how it |
is
supported - at one or both ends - and how its ends are attached to the
supports - fixed, |
hinged,
or roller. Three basic beam types are the hinged cantilever beam,
the fixed |
cantilever beam, and the simply supported beam. |
|
Back to
Knowhere |
 |
Page 89 -
Building stability - Beams |
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