Don't be shaky on this subject. In general, nearly all building design and construction
can be classified into one of three lateral-load-driven categories:
- Wind applications -- Wind controls over earthquake (with R taken as equal to or less
than 3 for design purposes) and the building is designed to meet the provisions in the
AISC LRFD Specification for Structural Steel Buildings using structural steel
systems of normal ductility.
- Low-seismic applications -- Earthquake (with R taken as equal to or less than 3 for
design purposes) controls over wind and the building is designed to meet the provisions in
the AISC LRFD Specification for Structural Steel Buildings using structural steel
systems of normal ductility.
- High-seismic applications -- R is taken greater than 3, and the building is designed to
meet the provisions in both the AISC LRFD Specification for Structural Steel Buildings
and the AISC Seismic Provisions for Structural Steel Buildings using structural
steel systems of high ductility. Note that it does not matter if wind or earthquake
controls in this case. The use of R greater than 3 in the calculation of the seismic base
shear requires the use of a seismically detailed system that is compatible with R even if
wind forces control.
Based upon these definitions, one can see that the only real difference between lateral
framing systems in wind applications and low-seismic applications is the type of lateral
load that controls the design. Other than that, design and construction in both
applications are based upon the code-specified forces distributed throughout the framing
assuming a nominally elastic structural response. The provisions in the AISC LRFD
Specification for Structural Steel Buildings are applicable and the building systems
that most everyone is familiar with can be used.
In contrast, high-seismic lateral framing systems are configured to be capable of
withstanding controlled ductile deformations to dissipate energy as they undergo strong
ground motions. Why? Because the code-specified base accelerations used to calculate the
seismic forces are not necessarily maximums -- they represent the intensity of ground
motions that have been selected by the code-writing authorities as reasonable for design
purposes. Accordingly, the provisions in both the AISC LRFD Specification for
Structural Steel Buildings and the AISC Seismic Provisions for Structural Steel
Buildings must be met so that the resulting frames can then undergo controlled
deformations in a ductile, well-distributed manner. A few examples:
- Special Moment Frames (SMF) -- SMF are generally configured so that any inelasticity
will occur by flexural yielding in the girders near, but away from, the connection of the
girders to the columns. The connections of the girders to the columns and the columns
themselves must then be proportioned to remain nominally elastic as they withstand these
deformations. Ordinary Moment Frames (OMF) are also configured to provide improved seismic
performance, although the avaiable ductility will be lower than that for SMF.
- Special Concentrically Braced Frames (SCBF) -- SCBF are generally configured so that any
inelasticity will occur by tension yielding and.or compression buckling in the braces. The
connections of the braces to the columns and beams and the columns and beams themselves
must then be proportioned to remain nominally elastic as they withstand these
deformations. Ordinary Concentrically Braced Frames (OCBF) are also configured to provide
improved seismic performance, although lower than that for SCBF.
- Eccentrically Braced Frames (EBF) -- EBF are generally configured so that any
inelasticity will occur by shear and/or flexural yielding in the link. The beam outside
the link, connections, braces and columns must then be proportioned to remain nominally
elastic as they withstand these deformations.
The design provisions for these and the other high-seismic systems are also intended to
result in distributed deformations throughout the frame, rather than the formation of
story mechanisms, so as to increase the level of available energy dissipation and
corresponding level of ground motion that can be withstood. As one example, SMF have
framing that satisfy the a strong-column/weak-beam concept. As another example, SCBF are
commonly configured with tension and compression bracing and/or secondary framing members
that tie braces together between stories (e.g., a zipper column).
What does all this mean? Well, it means that the member sizes in high-seismic frames
will be larger than comparable members in frames in wind and low-seismic applications. It
also means the connections will also be much more robust so they can transmit the
member-strength-driven force demands. Net sections will often require special attention so
as to avoid having fracture limit states control. Special material requirements, design
considerations and construction practices must be followed. In the end, it means
high-seismic design and construction will cost more than wind and low-seismic design and
construction. If permitted, the use of systems of normal ductility with R taken as equal
to or less than 3 may be the most cost effective approach.
For more specifics on the design and construction of high-seismic systems, see the AISC
Seismic Provisions for Structural Steel Buildings and the 1999 AISC Seismic
Provisions Supplement No. 1. Get them as free *.pdf downloads here and here,
respectively. If you want the paper versions, go here
(and scroll down to the bottom).
Miscellaneous links:
Many case studies of seismic design projects are also listed in AISC's Modern Steel Construction magazine.
- Airport Features a Seismic
Retrofit During Construction -- After the 1999 quake near Istanbul, technical team
evaluates the seismic resistance of Ataturk International Airport. From the August 2000
issue.
- Economical Health Care Design
in Seismic Zone 4 -- Three Rivers Community Hospital serves as an example of how a
team of professionals from all disciplines can work together to explore all options and
arrive at the most appropriate solution for the project. From the May 2000 issue.
- Isolated Problem -- A new
research center at Missouri's Botanical Garden in St. Louis utilizes base isolation for
earthquake protection. From the December 1999 issue.
- Oregon State Library Seismic
Upgrade -- Today, renovation of existing historic buildings often requires
construction methods to be less intrusive and construction materials to be smaller,
thinner, and more manageable in order to minimize any damage to the structure’s
appearance. From the January 2000 issue.
- Pacific Place -- A
straight-forward solution to the complex problem of eliminating several floors of columns
in an historic 10-story concrete-framed building. The massive loads from above plus the
requirements to upgrade to current standards resulted in a solution using
three-dimensional trusses. From the March 2000 issue.
- Performance Criteria For Bridge
Isolation Bearings -- A seismic analysis led to the use of isolation bearings for the
retrofit of California’s Three Mile Slough Bridge. From the September 1999 issue.
- Propped Shear
Walls -- The I. Magnin Building in Oakland, CA combines steel braces and concrete
shear walls for seismic strengthening. From the January 2001 issue.
- Public Market to
Ballroom -- A Cinderella transformation of the Public Market Building in Sacramento.
From the January 2001 issue.
- Renovating Concrete --
Structural steel moment resisting frames are proving to be an attractive alternative for
retrofitting concrete structures in seismic areas. From the March 1999 issue.
- Seismic Analysis Options For
Steel Truss Bridges -- Seismic retrofitting is becoming increasingly common during
major rehabilitation contracts. From the March 1999 issue.
- Steel Pyramid --
Fluid viscous dampers are designed to control this complex building's response during a
seismic event. From the November 1998 issue.
- Wings of Isolation -- San
Francisco International Airport's new terminal is protected by 267 steel seismic
isolators. From the October 1999 issue.
See also our features Organizations > Seismic Design
and Great References > Seismic Design.
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