linear static analysis procedure can be very
the given floor level measured perpendicular to the
inaccurate when applied to buildings with highly
direction of the applied load.
irregular structural systems, unless the building is
capable of responding to the design earthquake(s) in
For buildings in Seismic Design Categories C, D, E,
a nearly elastic manner.
Therefore, linear static
and F, where torsional irregularity exists as defined
analysis procedures should not be used for highly
in Table 5.2.3.2 of FEMA 302, the effects of the
irregular buildings, except wood frame structures.
irregularity shall be accounted for by multiplying the
sum of Mt and Mta at each level by a torsional
b.
Limitations on Use of the Procedure. The
amplification factor, Ax, determined from:
linear elastic static procedure may be used unless one
or more of the following conditions apply, in which
2
δ ax
Ax = m
(5-1)
case the linear elastic dynamic procedure, described
1.2δvg
a
in Paragraph 5-3, shall be used:
where:
The building height exceeds 100 feet.
δ ax = maximum displacement at Level x
m
The ratio of the building' horizontal
s
δvg = average of the displacements at the
dimensions at any story to the corresponding
a
dimensions at an adjacent story exceeds 1.4
extreme points of the building at Level x.
(excluding penthouse).
The torsional amplification factor, Ax is not required
The building is found to have a severe
to exceed 3.0.
torsional stiffness irregularity in any story. A severe
torsional stiffness irregularity may be deemed to
exist in a story if the diaphragm above the story is
5-2.
Linear Elastic Static Procedure.
not flexible, and the results of the analysis indicate
a.
General. This procedure, also known as the
that the drift along any side of the structure is more
"Equivalent Lateral Force (ELF) Procedure," will be
than 150 percent of the average story drift.
the procedure most widely employed for one-story
buildings, and can be utilized for all regular
The building is found to have a severe
buildings of two to six stories, and is the preferred
vertical mass or stiffness irregularity.
A severe
procedure for structures of wood frame or light metal
vertical mass or stiffness irregularity may be deemed
frame construction. The required calculations are
to exist when the average drift in any story (except
relatively simple and can be performed by hand,
penthouses) exceeds that of the story above or below
although a number of computer programs are
by more than 150 percent.
available to facilitate the analysis. The results of the
5-2
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