Over many years, engineers have observed that some structural systems perform
better in earthquakes than others. Based on these observations, the Provisions
design criteria for building structures are based on the structural system used.
Structural systems are categorized based on the material of construction (e.g.,
concrete, masonry, steel, or wood), by the way in which lateral forces induced by
earthquake shaking are resisted by the structure (e.g., by walls or frames), and by
the relative quality of seismic-resistant design and detailing provided.
The Provisions recognizes six broad categories of structural system:
• Bearing wall systems,
• Building frame systems,
• Moment-resisting frame systems,
• Dual systems,
• Cantilever column systems, and
• Systems not specifically designed for seismic resistance.
In bearing wall systems, structural walls located throughout the structure provide
the primary vertical support for the building’s weight and that of its contents as
well as the building’s lateral resistance. Bearing wall buildings are commonly
used for residential construction, warehouses, and low-rise commercial buildings
of concrete, masonry, and wood construction. Figures 21, 22, and 23 show typical
bearing wall buildings.
Building frames are a common structural system for buildings constructed of
structural steel and concrete. In building frame structures, the building’s weight
is typically carried by vertical elements called columns and horizontal elements
called beams. Lateral resistance is provided either by diagonal steel members
(termed braces) that extend between the beams and columns to provide horizontal
rigidity or by concrete, masonry, or timber shear walls that provide lateral
resistance but do not carry the structure’s weight. In some building frame
structures, the diagonal braces or walls form an inherent and evident part of the
building design as is the case for the high-rise building in San Francisco shown in
Figure 24. In most buildings, the braces or walls may be hidden behind exterior
cladding or interior partitions.
Moment-resisting frame systems are commonly used for both structural steel and
reinforced concrete construction. In this form of construction, the horizontal
beams and vertical columns provide both support for the structure’s weight and
the strength and stiffness needed to resist lateral forces. Stiffness and strength are
achieved through the use of rigid connections between the beams and columns
that prevent these elements from rotating relative to one other. Although somewhat
more expensive to construct than bearing wall and braced frame structural
systems, moment-resisting frame systems are popular because they do not
require braced frames or structural walls, therefore permitting large open spaces
and facades with many unobstructed window openings. Figure 25 shows a steel
moment-resisting frame building under construction.
Dual systems, an economical alternative to moment-resisting frames, are commonly
used for tall buildings. Dual system structures feature a combination of
moment-resisting frames and concrete, masonry, or steel walls or steel braced
frames. The moment-resisting frames provide vertical support for the structure’s
weight and a portion of the structure’s lateral resistance while most of the lateral
resistance is provided either by concrete, masonry, or steel walls or by steel braced
frames. Some dual systems are also called frame-shear wall interactive systems.
Cantilever column systems are sometimes used for single-story structures or in
the top story of multistory structures. In these structures, the columns cantilever
upward from their base where they are restrained from rotation. The columns
provide both vertical support of the building’s weight and lateral resistance to
earthquake forces. Structures using this system have performed poorly in past
earthquakes and severe restrictions are placed on its use in zones of high seismic
activity.
In regions of relatively low seismic risk, the NEHRP Recommended Seismic
Provisions permits the design and construction of structural steel buildings that
do not specifically conform to any of the above system types. These buildings are
referred to as “structures not specifically detailed for seismic resistance.”
In addition to these basic structural systems and the primary materials of construction,
the Provisions also categorizes structural systems based on the quality
and extent of seismic-resistant detailing used in a structure’s design. Systems that
employ extensive measures to provide for superior seismic resistance are termed
“special” systems while systems that do not have such extensive design features
are typically called “ordinary” systems. The Provisions also includes design rules
for structural systems intended to provide seismic resistance that is superior to
that of “ordinary” systems but not as good as that of “special” systems; these systems
are called “intermediate” systems.
