Overview of Chapter 21 of ASCE 7-10

Site-Specific Risk-Targeted Ground Motion Procedures

Jorge F. Meneses, PhD, PE, GE, D.GE, F.ASCECarlsbad, California

consulting engineers and scientists

AEG Inland Empire Chapter Continuing Education SeriesMay 31, 2014

• Overview • Site-specific procedures• Risk coefficient• NGA Relationships• Deaggregation• Examples• Performance Based EE• Summary

Outline

Source, Path and Site

Evaluating Seismic Hazard and Ground Motions

• 2103 CBC, 1616.10.2, 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.”

SITE-SPECIFIC STUDY

• 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)

Site Response Analysis

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 11.4.7, p.67)

SITE-SPECIFIC STUDY (cont’d)

SITE RESPONSE ANALYSIS

(ASCE 7-10, Section 21.1, p.207)

GroundSurface

Rockbase

SITE-SPECIFIC GROUND MOTION PROCEDURE

(Sections 21.2, 21.3, and 21.4)

• 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)

• 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

Risk coefficient

Risk Coefficient


Prob MCER Det MCER

MCER Spectrum

DESIGN Spectrum

SITE-SPECIFIC DESIGN SPECTRUM

General DESIGN Spectrum

General MCE Spectrum

Site Coord Site Class

1% Prob. of collapse 50 yr(direction of max horiz resp)

Lesser of PSHA and DSHA

2/3 MCE Spectrum2/3 MCE Spectrum

> 80% General Design Spectrum

84th percentile(direction of max horiz resp)

(Sections 21.2, 21.3, and 21.4)


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 21.2.2, p. 209)

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?


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.


Distance from Source

Grou

nd M

otio

n Pa

ram

eter

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.


Modern attenuation relationships have terms that deal with such complexities as:

1) Fault type

2) Fault geometry

3) Hanging wall/Foot wall

4) Site response effects

5) Basin effects

6) Main shock vs. After shock effects

Pretty complex …. Hard to do by hand!!


Ideally, every geographic area that experiences EQs would have its own set of attenuation relationships. WHY?

Not enough recorded data!

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?

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

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

NGA=Next Generation “Attenuation” Relations

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 -Chiou & Youngs-Campbell & Bozorgnia

-Idriss-Boore & Atkinson

(rock only)

NGA-West 1: 2008NGA-West 2: 2014

NGA-West

Data set No. EQs No. Rec Sa Type Damping(%)

Periods (sec)

NGA-West 1

173 3,551 AR, GMRotI50

5 0.01 - 10

NGA-West 2

610 21,331 AR, RotDnn 0.5 - 30 0.01 - 20

AR= as-recorded

Rotate horizontal components, at each period compute:• RotD50 = 50 percentile• RotD100 = max• RotD00 = min

RotDnn

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

• 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)

NGA West-2 ranges of applicability

Parameter AS BSSA CB CY IMagnitude Mw Mw Mw Mw Mw

Top of rupture Ztor Ztor Ztor

Style of faulting RV, NM, SS RV, NM, SS RV, NM, SS RV, NM, SS RV, NM, SSDip Yes Yes YesDowndip fault width Yes YesClosest distance to rupture

Rrup Rrup Rrup Rrup

Hor. dist. to surface proj. Rjb Rjb Rjb Rjb

Hor. dist. Perpendicular to strike

Rx, Ry Rx Rx

Hanging wall model Yes (Rjb) Yes YesVs30 Vs30 (760m/s) Vs30, (Sj) Vs30 Vs30≥450Depth to Vs Z1.0 Z2.5 Z1.0

Hypocentral depth Hhyp

Vs30 for reference rock (m/s)

1,100 760 1,100 1,130

Horizontal NGA-West 2 GMPEs parameters

• Abrahamson-Silva-Kamai (ASK)• Boore-Stewart-Seyhan-Atkinson (BSSA)• Campbell-Bozorgnia (CB)• Chiou-Youngs (CY)• Idriss (I)

NGA West 2 Five models

NGA Distance Notations

𝑅𝑅𝑢𝑝= Closest distance to rupturing fault plane𝑅 𝐽𝐵= Boore − Joyner distance

𝑅𝑋= Closest horizontal distance to the top of rupture

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

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>150

0760-1500360-760180-360<180

Hard RockFirm RockSoft RockRegular SoilSoft Soil

ABCDE

NGA West 2 Excel spreadsheet

http://peer.berkeley.edu/ngawest2/databases/

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 USGSBoore and Atkinson (2008)Campbell and Bozorgnia (2008)Chiou and Youngs (2008)

• Not an average velocity in upper 30 m

• Ratio of 30 m to shear wave travel time

What is Vs30?

(Stewart 2011)



What is Vs30?

(Stewart 2011)



• Emphasizes low Vs layers

What is Vs30?

(Stewart 2011)

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

Risk-Targeted MCER Probabilistic Response Spectrum

CRS = 0.941CR1 = 0.906

Deterministic MCER Response Spectra

Site-Specific MCER Response Spectrum

Design Response Spectrum

Site-Specific Response Spectra at Ground Surface

Sa (0.2s)= 1.42 Sa (1.0s)= 1.2Sa peak= 1.67 Sa (2.0s)= 0.740.9*Sapeak= 1.503 2*Sa(2s)= 1.48

SDS = 1.503 SD1 = 1.48SMS= 2.255 SM1= 2.22

SMSgen= 2.262 SM1gen= 1.6240.8*SMSgen= 1.810 0.8*SM1gen= 1.299

DESIGN ACCELERATION PARAMETERS

Site-specific MCE geometric mean (MCEG) PGA

PROB MCEG PGAThe probabilistic geometric mean PGA shall be taken as the geometric mean PGA with a 2% PE in 50 years

DETERMINISTIC MCEG PGACalculated 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 PGALesser of probabilistic and deterministicMCEG PGA ≥ 0.80 PGAM

(Section 21.5)


Site-specific Probabilistic MCER

(1% probability of collapse in 50 years)

METHOD 1CR * Sa (2% PE 50 year)

METHOD 2From iterative integration of a

site-specific hazard curve with a lognormal probability density function

representing the collapse fragilityCR = 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 a) 10% Prob. of collapse; b) logarithmic std dev of

0.6

(Section 21.2.1)

PSHA Review…..

Risk is computed using a . Do you remember the concept of probabilistic seismic hazard analysis?

All possible magnitudes are considered - contribution of each is weighted by its probability of occurrence

All possible distances are considered - contribution of each is weighted by its probability of occurrence

All possible effects are considered - each weighted by its conditional probability of occurrence

Basic equation:

All sources andtheir rates ofrecurrence are considered

Performance-Based Earthquake Engineering

*1 1 1

[ * | , ] [ ] [ ]S M R

k ky i j ji j k

N N NP Y y P M P Rm mr r

Probabilistic framework

Pacific Earthquake Engineering Research Center (PEER) developed a probabilistic framework for considering the engineering effects from

EQ ground motions:

DV IMG DV DM dG DM EDP dG EDP IM d

Intensity measure,

IM

Engineering demand parameter

, EDP

Damage measure

, DM

Repair CostLives Lost

Down Time

Pile Deflection CrackingCollapse Potential

FSliq

Lateral Spread

SettlementStory Drift

PGAPGV

IACAV

Decision variable,

DV

Performance-Based Earthquake Engineering

dIMEDPdGEDPDMdGDMDVG IMDV

0

1

0.0P[D > 3.0 | PGA=0.3g]

P[D > 1.0 | PGA=0.3g]

P[D > 2.0 | PGA=0.3g]

0.3g

Example of Fragility curves

P[D > di | PGA]

3.0cm2.0cm

1.0cm

PGA

EDP = Displacement = DIM = PGA

Fragility 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!!

Seismic hazard

curve for IM (from

PSHA)

Fragility curve – EDP

given IM

Fragility curve – DM given EDP

Fragility curve – DV given DM

Risk curve – DV vs

DV

dIMEDPdGEDPDMdGDMDVG IMDV

Fragility Curves and Seismic Hazard Curves

Hazard curve

PGA

PGA

PGA

P[D>di| PGA]1.0

0.0

DPGA

D proportional to sum of thick

red lines

Fragility curve for D > 2.0cm

**

1

|N

EDP i IMi

P EDP EDP IM im

D


Hazard curve

PGA

IM

IM

1.0

0.0

DPGA

D proportional to sum of

probabilitiesFragility curve

D

D

Seismic hazard curve

for Displacemen

t

**

1

|N

EDP i IMi

P EDP EDP IM im

D


D=2.0cm

PGA

PGA

P[D>di| PGA]

Risk-targeted ground motions

(Luco 2009)

Risk-targeted ground motions

(Luco 2009)

Risk-targeted ground motions - Example

Summary of differences

ASCE 7-05 ASCE 7-10Name 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 0.72gAverage SD1 0.38g 0.40g

(Luco 2009)

Contact Jorge F. Meneses, PhD, PE, GE, D.GE, F.ASCE

[email protected](760)795-1964

For further information

Documents

Overview of Chapter 21 of ASCE 7-10