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TOPIC in Earthquake Engineering # 7
CONCEPT OF DESIGN CODE FOR SEISMIC PERFORMANCE
By Valentin Shustov
Basic concepts of earthquake engineering,
born almost 70 years ago, have grown obsolete.
They assume that a building may survive The Big One though with partial
destruction. Drawing an analogy with a human body, it
will have dislocated joints, fractured ribs, traumatized spine and knocked
out teeth but be alive and, therefore, quite O.K. according
to seismic codes.
The present prescriptive design standards are a major barrier to innovations
and limit competition in the construction industry. A lack of performance
testing requirements may create a nutrient medium for less than desirable
techniques of seismic protection (SEAOC,1999).
Replacement of the contemporary earthquake
codes is imminent because they fit a single performance level when the
demand equals the ultimate
capacity. Besides, the current codes have other flaws
which make the seismic risk management more an art than a science.
| Unlike the contemporary
seismic codes (e.g. UBC, 1997), the
future code of performance should incorporate the following assumptions:
1. Demand and capacity are separated
and determined according to different procedures.
2. Performance criteria are identical
for designed, upgraded and existing structures.
Story Performance Rating R
may be used as a fundamental criterion: R = v/ve
is an actual or calculated inter-story drift and ve
is an inter-story drift at the assumed elastic limit of deformation. The
ultimate allowable value of R will occur when R =
Rw = vu/ve
where Quality Factor Rw
is understood as the ratio of the ultimate allowable story drift
vu that can be tolerated by the structure
without a collapse to the maximum elastic story drift ve.
| Ratio R
that may be called Seismic Performance Ratio
should be chosen as a primary parameter which would control anticipated
losses due to seismic exposure (Shustov,
A real time-history can not be "real" for a hypothetical earthquake specified
by a building code. Seismic inputs for the purpose of a code should possess
only essential features of a design earthquake like the imitating regime
Cone shown on the right (Shustov,
1993). The Cone, being
based on the effective peak velocity values, will ring up, in a sequence,
all building's modal frequencies in a transient process.
| The targeted code of performance
should incorporate neither the static shear force
nor the response spectrum analysis, which are the corner stones
of contemporary codes approximations.
A practical tool of the code, the Charts of Seismic
Performance (CSP) technique, is
a new way to organize valuable information on the anticipated performance
of the buildings without significant structural irregularities. CSP
is a graphical environment that introduces a more streamlined approach
to structural analysis for the needs of earthquake engineering. These Charts
represent families of contours connecting the points of equal Seismic
Performance Ratio R/Rw. The CSP
are built up in a domain of Ke /m and
Rw coordinates and they may depend upon
both the number of stories and earthquake
Thus, a system of properly arranged CSP
is needed to account for a wide variety of design configurations. Such
a system constitutes a matrix called Chart of
Seismic Performance System shown in Topic
6. In this matrix the rows correspond to the different numbers
of stories while the columns are associated with the different earthquake
of the matrix, or the individual CSPs,
are related to buildings with a certain story numbers N and to a
certain intensity earthquake as measured by peak ground velocity. Unlike
the HAZUS 97, the future code of performance
will introduce different building types with the help of the parameter
Rw that should be understood here
as the ductility component of
the Structural Response Modification Factor.
| The rectangle on the plane Ke/m
- Rw shown below accounts for possible deviations
in the structure's stiffness Ke and mass
m characteristics, as well as for an uncertainty in the structural
identification parameter Rw. This rectangle
may be called a structure's footprint.
Projected on the proper Chart level, the footprint
screens the area of the anticipated performance of the structure.
basic values of R/Rw. Any specific features
should be introduced by the Correction Factors
(CF). Thus, a corrected Seismic
Performance Ratio is:
= R/Rw . CFw.
where Correction Factors CFw,
CFt, CFa, CFd,
and CFs account, correspondingly, for
weak links in the building design, technological innovations, age of
the building, construction quality, and soil or site conditions.
| The code of performance should provide the
administrative, analytical and technical means to achieve the targeted
building performance goals expressed in terms of the Seismic
Performance Ratio R/Rw. The minimum
allowable level of performance is associated with the R/Rw
=1 which corresponds to the minimum requirements of the building codes
of today, whereas the lower the R/Rw
value, the better is the building's seismic performance.
| Shustov, V.,
Isolation: Fresh Insight", Technical Report to NSF BCS-9214754,
SRE, Los Angeles, CA.
Shustov, V., 1994, "Energy
Absorbing Technique: Challenge of Proportioning", Proc. 3rd Int'l
Conf. on Structures under Shock and Impact, Madrid, Spain.
Shustov, V., 1997, "Future
Seismic Codes and Earthquake Insurance", Proc. 66th Annual
SEAOC Convention, San Diego, CA.
Lateral Force Requirements and Commentary", Seismology
Committee of SEAOC, Sacramento, CA.
This page was last updated on
10 March 2000.