response modification factor, R, are prescribed in
will be in the same proportion to the mass as the
forces on the structure, or F = ma (i.e., the
FEMA 302, and are incorporated by reference in this
document
for
compliance
with
Performance
accelerations of the elements will be about the same
Objective 1A (Life Safety).
These linear elastic
as the acceleration of the floor on which they are
analyses are also prescribed in this document for
supported). However, elements that are flexible and
Performance Objectives 2A, 2B, and 3B with
have periods of vibration close to any of the
modification factors, m, for deformation-controlled
predominant modes of the building vibration will
structural components or elements.
Nonlinear
experience accelerations substantially greater than
analyses can be either elastic or inelastic. Nonlinear
the accelerations on the structure (i.e., accelerations
elastic analyses, also known as "pushover" analyses,
of elements will be greater than floor accelerations).
subject an elastic model of the structure to a
The above actions are approximated by the design
predetermined
pattern
of
static
forces.
A
force equations in Chapter 6 of FEMA 302, and as
force/displacement curve is then constructed by
prescribed in Chapter 10 of this document for the
iterative analyses with yield "hinges" placed at the
various performance objectives.
yielding ends of the structural elements. Compliance
is determined by matching a target displacement
2-5.
Fundamentals of Seismic Design.
with acceptable inelastic deformation of the yielding
elements. Nonlinear inelastic analyses are usually
The type of structural system used will determine the
time-history dynamic analyses with predetermined
magnitude of the design lateral forces. The decision
elastic/inelastic characteristics for the structural
as to the type of structural system to be used will be
elements. Guidance on the use and limitations of the
based on the merits and relative costs for the
above analytical procedures is provided in Chapters
individual building being designed.
There are
4 and 5.
innovative systems available for particular structural
configurations and conditions, such as eccentric
c.
Response of Elements Attached to the
braced frames, seismic isolation, friction devices,
Structure.
Elements attached to the floors of the
and other response control systems. These systems
building or structure (e.g., mechanical equipment,
are described below.
ornamentation, piping, nonstructural partitions)
respond to floor motion in much the same manner
a.
Gravity-Load System. The basic elements of
that the building responds to ground motion;
a gravity load system are: (a) horizontal elements
however, the floor motion may vary substantially
(e.g., slabs, sheathing, beams, girders, or trusses)
from the ground motion.
The high-frequency
that collect the dead and live loads in various levels
components of the ground motion tend to be filtered
in the structure; (b) the vertical-resisting elements
out at the higher levels in the building, while the
(e.g., columns and bearing walls) that receive the
components of ground motion that correspond to the
gravity loads from the horizontal elements: and (c)
natural periods of vibration of the building tend to be
the foundations (e.g., footings, piers, piles) that
magnified. If the elements are rigid and are rigidly
receive the loads from the vertical elements and
attached to the structure, the forces on the elements
2-5
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