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CONCEPT OF DESIGN CODE FOR SEISMIC PERFORMANCE
By Valentin Shustov 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. |
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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. |
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| 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. |
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Story Performance Rating R
(Shustov, 1994)
may be used as a fundamental criterion: R = v/ve
where v 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 / Rw that may be called Seismic Performance Ratio should be chosen as a primary parameter which would control anticipated losses due to seismic exposure (Shustov, 1997). |
| 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. |
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| 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. |
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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 intensity. |
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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
intensities. Elements
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. |
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| 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. |
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Projected on the proper Chart level, the footprint
screens the area of the anticipated performance of the structure.
CSP provides 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)cor = R/Rw . CFw. CFt. CFa. CFd. CFs 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. |
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| 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.,
1993, "Base
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. SEAOC,1999, "Recommended Lateral Force Requirements and Commentary", Seismology Committee of SEAOC, Sacramento, CA. |
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Our address is: CME, 18111 Nordhoff Street, Northridge, California 91330-8347 |