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**consulting engineers and scientists**

Site-Specific Risk-Targeted Ground Motion Procedures Jorge F. Meneses, PhD, PE, GE, D.GE, F.ASCE Carlsbad, California AEG Inland Empire Chapter Continuing Education Series May 31, 2014

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**Outline Overview Site-specific procedures Risk coefficient**

NGA Relationships Deaggregation Examples Performance Based EE Summary

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Source, Path and Site

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**Evaluating Seismic Hazard and Ground Motions**

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SITE-SPECIFIC STUDY 2103 CBC, , 1616A.1.3 “For buildings assigned to Seismic Design Category E and F, or when required by the building official, a ground motion hazard analysis shall be performed in accordance with ASCE 7 Chapter 21, as modified by Section 1803A.6 of this code.”

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**SITE-SPECIFIC STUDY (cont’d)**

Site Response Analysis Structures on Site Class F sites (Ts > 0.5 seconds) At least 5 recorded or simulated horizontal ground motion acceleration time histories (MCER spectrum at bedrock) Seismic Hazard Analysis Seismically isolated structures (S1 0.6) Structures with damping systems (S1 0.6) A time history response analysis of the building is performed (ASCE 7-10, Section , p.67)

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**SITE RESPONSE ANALYSIS**

Ground Surface Rock base (ASCE 7-10, Section 21.1, p.207)

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**Probabilistic ground motion **

SITE-SPECIFIC GROUND MOTION PROCEDURE Probabilistic ground motion Method 1: Uniform-hazard GM * Risk Coefficient Method 2: Risk-targeted probabilistic GM directly Deterministic ground motion 84th-%ile GM, but not < 1.5Fa or 0.6*Fv/T MCER = Min (Prob. GM, Det. GM) All GMs are max-direction spectral accelerations (Sa) (Sections 21.2, 21.3, and 21.4)

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Risk coefficient Risk coefficient: CR T ≤ 0.2 s; CR = CRS (Figure 22-17) T ≥ 1.0 s; CR = CR1 (Figure 22-18) 0.2 s ≤ T ≤ 1.0 s; CR linear interpolation of CRS and CR1

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Risk Coefficient

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**(direction of max horiz resp)**

SITE-SPECIFIC GROUND MOTION PROCEDURE Prob MCER Det MCER MCER Spectrum DESIGN SITE-SPECIFIC DESIGN SPECTRUM General DESIGN General MCE Site Coord Site Class 1% Prob. of collapse 50 yr (direction of max horiz resp) Lesser of PSHA and DSHA 2/3 MCE Spectrum > 80% General Design Spectrum 84th percentile (direction of max horiz resp) (Sections 21.2, 21.3, and 21.4)

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**SITE-SPECIFIC GROUND MOTION PROCEDURE**

Deterministic Lower Limit (DLL) on MCER Spectrum 1.5 Fa 0.6 Fa Sa = 0.6 Fv/T 0.08 Fv/Fa 0.4 Fv/Fa TL Period (seconds) Sa (g) Sa = 0.6 Fv TL/T2 (Section , p. 209)

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**Attenuation Relationships**

Several types of ground motions parameters can be calculated from a recorded EQ time history. But what do you do if you want to estimate what the ground motion parameters are going to be from an earthquake that hasn’t happened yet?

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**Attenuation Relationships**

ANSWER: Use the data that we’ve collected so far and fit equations to them for predicting future ground motions. These equations are often called attenuation relationships.

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**Attenuation Relationships**

Ground Motion Parameter Distance from Source Initial relationships were just based on Magnitude (M) and Distance (R), but equations become much more complex as researchers looked for ways to minimize data scatter.

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**Attenuation Relationships**

Modern attenuation relationships have terms that deal with such complexities as: 1) Fault type 2) Fault geometry Pretty complex …. Hard to do by hand!! 3) Hanging wall/Foot wall 4) Site response effects 5) Basin effects 6) Main shock vs. After shock effects

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**Attenuation Relationships**

Ideally, every geographic area that experiences EQs would have its own set of attenuation relationships. WHY? Scatter in the data could be minimized! …But we can’t really produce site-specific attenuation relationships for places other than those that have a lot of frequent earthquakes. WHY? Not enough recorded data! So we start combining earthquake records from geographically different areas with the assumption that the ground motions should be similar despite the differences in location. Ergodic Assumption

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**NGA=Next Generation “Attenuation” Relations**

Three NGA projects: For active crustal Eqs (California, Middle East, Japan, Taiwan,…): NGA-West For subduction Eqs (US Pacific Northwest and northern California, Japan, Chile, Peru,…): NGA-Sub Stable continental regions (Central and Eastern US, portion of Europe, South Africa,…): NGA-East

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**Attenuation Relationships (GMPEs)**

For crustal faults in the Western US and other high- to moderate- seismicity areas, most professionals currently use: Next Generation Attenuation Relationships (NGAs) NGA West 1: 5 separate research teams were given the same set of ground motion data and were asked to develop relationships to fit the data. Their results were published in 2008. -Abrahamson & Silva -Boore & Atkinson -Chiou & Youngs -Idriss (rock only) -Campbell & Bozorgnia

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**NGA-West NGA-West 1: 2008 NGA-West 2: 2014 Data set No. EQs No. Rec**

Sa Type Damping (%) Periods (sec) NGA-West 1 173 3,551 AR, GMRotI50 5 NGA-West 2 610 21,331 AR, RotDnn AR= as-recorded

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**Rotate horizontal components, at each period compute: **

RotDnn Rotate horizontal components, at each period compute: RotD50 = 50 percentile RotD100 = max RotD00 = min RotD50 is the main intensity measure PGA, PGV and Sa at 21 periods: 0.01, 0.02,……,5, 7.5, 10 sec No GMPE for PGD

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**NGA West-2 ranges of applicability**

Applicable magnitude range: M ≤ 8.5 for strike-slip (SS) M ≤ 8.0 for reverse (RV) M ≤ 7.5 for normal faults (NM) Applicable distance range: 0 – 200 km (preferably 300km)

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**Horizontal NGA-West 2 GMPEs parameters**

AS BSSA CB CY I Magnitude Mw Top of rupture Ztor Style of faulting RV, NM, SS Dip Yes Downdip fault width Closest distance to rupture Rrup Hor. dist. to surface proj. Rjb Hor. dist. Perpendicular to strike Rx, Ry Rx Hanging wall model (Rjb) Vs30 (760m/s) Vs30, (Sj) Vs30≥450 Depth to Vs Z1.0 Z2.5 Hypocentral depth Hhyp Vs30 for reference rock (m/s) 1,100 760 1,130

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NGA West 2 Five models Abrahamson-Silva-Kamai (ASK) Boore-Stewart-Seyhan-Atkinson (BSSA) Campbell-Bozorgnia (CB) Chiou-Youngs (CY) Idriss (I)

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**NGA Distance Notations**

𝑅 𝑅𝑢𝑝 = Closest distance to rupturing fault plane 𝑅 𝐽𝐵 = Boore−Joyner distance 𝑅 𝑋 = Closest horizontal distance to the top of rupture

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More on distances Geotechnical Services Design Manual, Version 1.0, 2009, Caltrans Development of the Caltrans Deterministic PGA Map and Caltrans ARS Online, 2009, Caltrans

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NGA Soil vs. Rock NGA equations don’t have a “trigger” for soil or rock. They just rely on the VS30, which is the average shear wave velocity in the upper 30 meters of the ground. VS30 (m/s) Type Site Class >1500 Hard Rock A Firm Rock B Soft Rock C Regular Soil D <180 Soft Soil E

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**NGA West 2 Excel spreadsheet**

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**2013 CBC, Section 1803A.6 Geohazard Reports**

The three Next Generation Attenuation (NGA) relations used for the 2008 USGS seismic hazard maps for Western United States (WUS) shall be utilized to determine the site-specific ground motion. When supported by data and analysis, other NGA relations, that were not used for the 2008 USGS maps, shall be permitted as additions or substitutions. No fewer than three NGA relations shall be utilized 2008 USGS Boore and Atkinson (2008) Campbell and Bozorgnia (2008) Chiou and Youngs (2008)

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**Not an average velocity in upper 30 m **

What is Vs30? Not an average velocity in upper 30 m Ratio of 30 m to shear wave travel time (Stewart 2011)

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**Not an average velocity in upper 30 m **

What is Vs30? Not an average velocity in upper 30 m Ratio of 30 m to shear wave travel time (Stewart 2011)

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**Not an average velocity in upper 30 m **

What is Vs30? Not an average velocity in upper 30 m Ratio of 30 m to shear wave travel time Emphasizes low Vs layers Travel time is proportional to slowness. (Stewart 2011)

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**Seismic Source Interpretation from PSHA Results**

Deaggregation: Break the probabilistic “aggregation” back down to individual contributions based on magnitude and distance. Provides: - Mean M,R: weighted average - Modal M,R: Greatest single contribution to hazard

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**Risk-Targeted MCER Probabilistic Response Spectrum**

CRS = 0.941 CR1 = 0.906

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**Deterministic MCER Response Spectra**

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**Site-Specific MCER Response Spectrum**

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**Design Response Spectrum**

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**Site-Specific Response Spectra at Ground Surface**

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**Site-specific MCE geometric mean (MCEG) PGA**

PROB MCEG PGA The probabilistic geometric mean PGA shall be taken as the geometric mean PGA with a 2% PE in 50 years DETERMINISTIC MCEG PGA Calculated as the largest 84th percentile geometric mean PGA for characteristic earthquakes on all known active faults. Minimum value 0.5 FPGA (FPGA at PGA=0.5g) SITE-SPECIFIC MCEG PGA Lesser of probabilistic and deterministic MCEG PGA ≥ 0.80 PGAM (Section 21.5)

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**Site-specific Probabilistic MCER **

SITE-SPECIFIC GROUND MOTION PROCEDURE Site-specific Probabilistic MCER (1% probability of collapse in 50 years) METHOD 1 CR * Sa (2% PE 50 year) METHOD 2 From iterative integration of a site-specific hazard curve with a lognormal probability density function representing the collapse fragility CR = risk coefficient (from maps) T ≤ 0.2s CR = CRS T ≥ 1.0s CR = CR1 0.2s < T < 1s Linear interp CRS and CR1 (i.e., probability of collapse as a function of Sa) Collapse fragility with 10% Prob. of collapse; logarithmic std dev of 0.6 (Section )

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**Performance-Based Earthquake Engineering**

Risk is computed using a Do you remember the concept of probabilistic seismic hazard analysis? Probabilistic framework All possible distances are considered - contribution of each is weighted by its probability of occurrence PSHA Review….. Basic equation: All possible magnitudes are considered - contribution of each is weighted by its probability of occurrence All sources and their rates of recurrence are considered All possible effects are considered - each weighted by its conditional probability of occurrence

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**Performance-Based Earthquake Engineering**

Pacific Earthquake Engineering Research Center (PEER) developed a probabilistic framework for considering the engineering effects from EQ ground motions: Engineering demand parameter, EDP Damage measure, DM Decision variable, DV Intensity measure, IM PGA PGV IA CAV Repair Cost Lives Lost Down Time Pile Deflection Cracking Collapse Potential FSliq Lateral Spread Settlement Story Drift

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**Fragility Curves Example of Fragility curves P[D > di | PGA] PGA**

EDP = Displacement = D IM = PGA P[D > di | PGA] 1 P[D > 1.0 | PGA=0.3g] 0.3g 1.0cm 2.0cm P[D > 2.0 | PGA=0.3g] 3.0cm P[D > 3.0 | PGA=0.3g] 0.0 PGA

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**Fragility Curves and Seismic Hazard Curves**

The PEER performance-based framework incorporates seismic hazard curves and fragility curves. Convolving a fragility curve with a seismic hazard curve produces a single point on a new hazard curve!! Risk curve – lDV vs DV Fragility curve – DV given DM Fragility curve – DM given EDP Fragility curve – EDP given IM Seismic hazard curve for IM (from PSHA)

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**lD proportional to sum of thick red lines**

Fragility Curves and Seismic Hazard Curves 1.0 P[D>di| PGA] Fragility curve for D > 2.0cm lD proportional to sum of thick red lines 0.0 PGA lPGA DlPGA Hazard curve PGA

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**Seismic hazard curve for Displacement**

Fragility Curves and Seismic Hazard Curves 1.0 P[D>di| PGA] lD proportional to sum of probabilities Fragility curve lD D IM Seismic hazard curve for Displacement 0.0 PGA lPGA D=2.0cm DlPGA Hazard curve IM PGA

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**Risk-targeted ground motions**

(Luco 2009)

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**Risk-targeted ground motions**

(Luco 2009)

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**Risk-targeted ground motions - Example**

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**Summary of differences**

ASCE 7-05 ASCE 7-10 Name MCE MCER Probabilistic GMs (objective) Uniform hazard (2%-in-50 yr Pr. of Exc.) Risk targeted (1%-in-50 yr Pr. of Collapse) Deterministic GMs 1.5*median 84%-ile (approx. 1.8*median) GM parameter Geometric mean, Sa Maximum direction, Sa USGS web tool Java ground motion parameter calculator Seismic design maps web application Average SDS 0.73g 0.72g Average SD1 0.38g 0.40g (Luco 2009)

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**For further information**

Contact Jorge F. Meneses, PhD, PE, GE, D.GE, F.ASCE (760)

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