Optimal Performance- Based Seismic Design - Structural Seismic Design Optimization and Earthquake Engineering - page 178

Table 1. A sample of desired performances for different buildings

Earthquake hazard level OP IO LS CP

The most severe in 72 years
The most severe in 225 years
The most severe in 474 years
The most severe in 2475 years

Unimportant building such as a sunshade
Ordinary buildings such as residential buildings
Important buildings such as hospitals

Table 2. Performance objectives (FEMA356, 2000)

Earthquake hazard level OP IO LS CP

50% per 50 years A B C D

20% per 50 years E F G H

10% per 50 years I J K L

2% per 50 years M N O P

the structure has to tolerate for the specified per-
formance level. Some publications in the literature
claim that once the target displacement is known,
it is possible to determine the design base shear
and consequently to design the structure for the
base shear. A topic by Priestley et al. (2007), that
instructs the direct displacement-based seismic
design methodology, and research works by Fajfar
(2000) that propose the N2 method, and research
works by Aschheim (1999) that suggest YPS
method may be consulted in this regard. Since
these methods have not yet been adopted by design
codes, the iterative performance-based design is
often used. The iterative process consists of assum-
ing a trial design, finding its performance level,
modifying the design and repeating the process
again and again until the required specifications
that meet the performance criteria are obtained.

and the demand that an earthquake imposes to it,
in respect to its strength and ductility character-
istics. This comparison is made in member level
as well as structure level.
The capacity of a structure is a resultant of
capacity of its components. If a force-deformation
relation is defined for elements of a structure, the
capacity of structure can be found from nonlinear
analysis. Since computer programming for non-
linear analysis is difficult and is time consuming,
engineers prefer to conduct a linear analysis. Four
methods of analyses are accepted for seismic
evaluation of a structure:

1. Linear static analysis
2. Linear dynamic analysis
3. Nonlinear static analysis
4. Nonlinear dynamic analysis

How to Find the Performance
Level of a Structure

If a static type of analysis is performed, a
modification factor has to be applied to the results
to make them reflect the nonlinear behavior. This
will be discussed later. The spirit of almost all seismic evaluation methods

is a comparison between the capacity of structure