performance- and risk-based design approaches for seismic

EERI Seminar on Next Generation Attenuation Models

Performance- and Risk-based Design Approaches for

Seismic Safety

Nicolas Luco, Research Structural Engineer

U.S. Geological Survey Golden, Colorado

2012 Indo-American Frontiers of Engineering (IAFOE) Symposium


Outline of Presentation

• Past/present design approaches for seismic safety

• Quantification of seismic risk/performance

• Recent/future risk-based seismic design approaches

• Implications for designing against hazards, in general

• Outstanding issues

2012 IAFOE Symposium, Session on “Engineering Large Infrastructure for Natural Hazards”

“Performance- and Risk-based Design Approaches for Seismic Safety,” N. Luco, USGS March 1, 2012


Past Seismic Design Approaches

• Past and most present design approaches for seismic safety are prescriptive, based on following rules rather than explicitly quantifying performance/risk.

e.g., “If at least seven ground motions [(time series)] are analyzed, the design member forces … are permitted to be taken … as the average of the … values determined from the [building response] analyses …” – ASCE 7 Standard

• For earthquake loads/demands, past approaches typically designed against …




Deterministic Earthquake Scenarios



e.g., “ShakeMaps”


Probabilistic Seismic Hazard Maps



e.g., ground motion intensity with a uniform1/2500 annual probability of being exceeded

EERI Seminar on Next Generation Attenuation Models2012 IAFOE Symposium, Session on “Engineering Large Infrastructure for Natural Hazards”


Past Seismic Design Approaches

Although seismic safety was implied by these past design approaches, …

… the seismic risk for the resulting structures …

(e.g., annual probability of seismically-induced failure)

… was not explicitly quantified in the design process, or in the development of the design process by regulators.


Meanwhile, for decades now (e.g., ATC 3-06, 1978) …

Seismic Risk Quantification



EarthquakeScientists

EarthquakeEngineers

SeismicRisk

Probabilistic SeismicFragility Curves

Probabilistic SeismicHazard Curves


Probabilistic Seismic Hazard CurvesResults of Probabilistic Seismic Hazard Analysis (PSHA), described in preceding presentation by Dr. Goulet.

Notes: Hazard curves are interpolated to derive aforementioned “hazard maps”.

Shapes of hazard curves can vary significantly with location.



Ground Motion Intensity

Ann

ual P

rob.

of E

xcee

danc

e


10-2 10-1 100 1010

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Ground Motion Intensity, IM = z

Pro

b [ C

olla

pse

| IM

= z

]

Design IM = 1.38gDesign IM = 0.96g

Based on the performance of similar structures in past earthquakes, and/or numerical simulations.

Probabilistic Seismic Fragility Curves

Notes: Fragility curve depends on the ground motion intensity for which the structure was designed (“Design IM”).It also depends on the prescriptive requirements that are used to design the structure (e.g., those for buildings vs. nuclear power plants).




“Risk Integral”

Combination of such hazard and fragility curves is an application of the total probability theorem …



d |d d

∞

HazardRisk Fragility


“Risk Integral” (Example)



Haz

ard

Ris

kFr

agili

ty

Ground Motion Intensity, IM=z

P[ C

olla

pse

| IM

=z]

P[ I

M>

z]

Inte

gran

d



Risk-Based Design Approaches

• Risk-Targeted Maximum Considered Earthquake (MCER ) Ground Motions for designing new buildings and other structures, 2012 International Building Code

• A Performance-Based Approach to Define the Site-Specific Earthquake Ground Motion, U.S. Nuclear Regulatory Commission (NRC) Regulatory Guide (RG) 1.208 (2007)

• Development of Next-Generation Performance-Based Seismic Design Procedures for New and Existing Buildings, ongoing Applied Technology Council (ATC) Project #58, funded by U.S. Federal Emergency Management Agency (FEMA)




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4.9

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REFERENCES

Building Seismic Safety Council, 2009, NEHRP Recommended Seismic Provisions for New Buildings and Other Structures: FEMA P-750/2009 Edition, Federal Emergency Management Agency, Washington, DC.Huang, Yin-Nan, Whittaker, A.S., and Luco, Nicolas, 2008, Maximum spectral demands in the near-fault region, Earthquake Spectra, Volume 24, Issue 1, pp. 319-341.Luco, Nicolas, Ellingwood, B.R., Hamburger, R.O., Hooper, J.D., Kimball, J.K., and Kircher, C.A., 2007, Risk-Targeted versus Current Seismic Design Maps for the Conterminous United States, Structural Engineers Association of California 2007 Convention Proceedings, pp. 163-175.Petersen, M.D., Frankel, A.D., Harmsen, S.C., Mueller, C.S., Haller, K.M., Wheeler, R.L., Wesson, R.L., Zeng, Yuehua, Boyd, O.S., Perkins, D.M., Luco, Nicolas, Field, E.H., Wills, C.J., and Rukstales, K.S., 2008, Documentation for the 2008 Update of the United States National Seismic Hazard Maps: U.S. Geological Survey Open-File Report 2008-1128, 61 p.

DISCUSSION

Maps prepared by United States Geological Survey (USGS) incollaboration with the Federal Emergency Management Agency(FEMA)-funded Building Seismic Safety Council (BSSC) andthe American Society of Civil Engineers (ASCE). The basis isexplained in commentaries prepared by BSSC and ASCE and inthe references. Ground motion values contoured on these maps incorporate: • a target risk of structural collapse equal to 1% in 50 years based upon a generic structural fragility • a factor of 1.1 to adjust from a geometric mean to the maximum response regardless of direction • deterministic upper limits imposed near large, active faults, which are taken as 1.8 times the estimated median response to the characteristic earthquake for the fault (1.8 is used to represent the 84th percentile response), but not less than 150% g. As such, the values are different from those on the uniform-hazard 2008 USGS National Seismic Hazard Maps posted at:http://earthquake.usgs.gov/hazmaps. Larger, more detailed versions of these maps are not providedbecause it is recommended that the corresponding USGS webtool (http://earthquake.usgs.gov/designmaps orhttp://content.seinstitute.org) be used to determine the mappedvalue for a specified location.

E

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6.3

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16.9

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50° 90° 80° 70°

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100 0 100 200 300 400 500 Miles

100 0 100 200 300 400 500 Kilometers

Explanation

Contour intervals, %g

3002001501251009080706050403530252015105

Areas with a constant spectral response acceleration of 150% g

10101010

E16.9

Point value of spectral response acceleration expressed as a percent of gravity

Contours of spectral response acceleration expressed as a percent of gravity. Hachures point in direction of decreasing values

2012 “International” Building Code

FIGURE 1613.3.1 RISK-TARGETED MAXIMUM CONSIDERED EARTHQUAKE (MCER ) GROUND MOTION …




2012 International Building CodeH

azar

dR

isk

Frag

ility



DD

Ground Motion Intensity, IM=z

P[ C

olla

pse

| IM

=z]

P[ I

M>

z]

Inte

gran

d



U.S. NRC RG 1.208 (2007)“Target Risk” = 10-5 annual probability of Onset of Significant Inelastic Deformation (conservative w.r.t. structural failure)



Ground Motion Intensity, IM

Ann

ual P

rob.

of E

xcee

danc

e Design IM presented as …

where Design Factor is

and

}6.0,0.1max{DF 8.0RA

)10(/)10( 45 IMIMAR

DF)10(Design 4 IMIM


ATC-58 Project (funded by FEMA)

• In addition to building collapse, these new design procedures target annual probabilities of earthquake-induced …

– casualties (Deaths), – repair costs (Dollars), and – loss of use (Downtime)

… via generalizations of the risk integral.

• Furthermore, risk quantification is “high-resolution,” i.e., …

… fragility curve depends on details of individual elements of the building, not just the design ground motion intensity




Risk-Based Design Approaches



Seismic Risk Quantificationfor Existing Structures

Seismic Designof New Structures

Risk-Based Design


Implications

• Tolerable seismic risk has now been specified quantitatively (vs. qualitative “safe”), which in turn impacts, e.g., …

– evaluation and retrofit of existing structures,

– design for other hazards (e.g., wind, rock falls), and

– prescriptive design procedures (e.g., selection and modification of ground motion time series for structural response simulations).

• Risk-based approaches might eventually allay the need for prescriptive design procedures altogether.




Outstanding Issues

• Acceptable/tolerable risk level?

– once in terms of deaths/dollars/downtime, might be borrowed from other hazards?

– for individual structures and aggregation/region?

– must it be geographically uniform?

• Validation of risk modeling/quantification (vs. reality)

– hazard and fragility components/curves

– data very scarce at large ground motion intensities



performance- and risk-based design approaches for seismic

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