Civil Engineering Reference
In-Depth Information
The 16-story Apoquindo Tower, Santiago, Chile. (Courtesy of Economy Forms Corporation.)
14.8
APPROXIMATE ANALYSIS OF CONTINUOUS FRAMES
FOR LATERAL LOADS
Building frames are subjected to lateral loads as well as to vertical loads. The necessity
for careful attention to these forces increases as buildings become taller. Buildings must
not only have sufficient lateral resistance to prevent failure, but also must have sufficient
resistance to deflections to prevent injuries to their various parts. Rigid-frame buildings
are highly statically indeterminate; their analysis by “exact” methods (unless computers
are used) is so lengthy as to make the approximate methods very popular.
The approximate method presented here is called the
portal method
. Because of its
simplicity, it has probably been used more than any other approximate method for de-
termining wind forces in building frames. This method, which was presented by Albert
Smith in the
Journal of the Western Society of Engineers
in April 1915, is said to be
satisfactory for most buildings up to 25 stories in height. Another method very similar
to the portal method is the cantilever method. It is thought to give slightly better re-
sults for high narrow buildings and can be used for buildings not in excess of 25 to 35
stories.
6
The portal method is merely a variation of the method described in Section 14.7 for
analyzing beams in which the location of the points of inflection was assumed. For the
portal method, the loads are assumed to be applied at the joints only. If this loading condi-
6
“Wind Bracing in Steel Buildings,” 1940,
Transactions of the American Society of Civil Engineers
, 105,
pp. 1723-1727.
