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Performance-Based Earthquake

Engineering Design and Analysis

for Geotechnical Professionals

MARSHALL LEW

WOOD ENVIRONMENT & INFRASTRUCTURE SOLUTIONS, INC.

FEBRUARY 6, 2020

2

Performance-Based Earthquake Engineering

(PBEE) – What is it?

• Building codes intend to provide for safe buildings by

prescribing loads and material properties as well as

structural detailing; these codes generally address all

building types and construction. However, building codes

may not provide for the most economical, efficient, and

safe tall buildings because the codes are general and

prescriptive in nature. (The building codes are really

developed for the 98 to 99% of buildings that are not

“tall.”) Tall buildings are really a special class of buildings

that have unique qualities and characteristics. Tall

buildings need to be designed with a different approach

to meet safety and performance requirements, especially

in regions with high seismic activity.

PBEE for Structural Design and Analysis

3 A presentation by Wood.

• Moehle in Seismic Design of Reinforced Concrete

Buildings (2015)

– Most PBEE designs rely on the prescriptive building

code provisions, with specific exceptions to those

provisions that emphasize the unique aspects of the

proposed design. The performance evaluation can

then focus mainly on those aspects of the design that

are exceptions, greatly simplifying the process.




Buildings (2015)

– For buildings located in seismically active regions, PBEE

design generally involves a seismic hazard analysis to

determine site-specific shaking levels, and usually

includes the selection of representative earthquake

ground motions by which to “test” the structure. A

nonlinear computer model of the building is then

subjected to these ground motions to determine the

building response.




Buildings (2015)

– Key response quantities are analyzed to establish

whether the design meets the performance criteria that

have been adopted for the buildings.

– Performance criteria are contained in guidelines for

PBEE of tall buildings such as developed by TBI and

LATBSDC.





Add caption directly on to

solid colour parts of image

• PBEE gives the SE much greater flexibility in the choice of

the structural system and its design method. Such

designs, however, typically require additional design effort

and time, advanced engineering capabilities, and a

building official who is willing to accept designs not

conforming strictly to the prescriptive provisions of the

building code.

• Most building officials will not have the expertise

necessary to judge the adequacy of a design falling under

the alternative methods clause of the building code.

Therefore, an independent peer review is usually required

to advise the building official as to whether a design is

satisfactory.



Example of Structural Basis of Design (BOD)




Please do not distribute






Note: 2019 CBC

(based on 2018 IBC)

is now the law of the

land in California.

ASCE 7-16 replaces

ASCE 7-10 as the

seismic standard.






*


20 A presentation by Wood. Please do not distribute

Performance-Based Earthquake Design and Analysis


• What is the role of the Geotechnical Professional in PBEE Design and Analysis?

– Characterization of the site’s seismic hazard profile

– Characterize the site’s dynamic properties

– Characterize the ground motions for design and analysis

– Evaluate the performance of the soil-structure system



• Prescriptive Building Code requirements have

limitations on building systems and material properties.

However, alternative means of compliance to

demonstrate conformance to the intent of the Code is

allowed by means of PBEE design and analysis.

• TBI and LATBSDC adopt the concept of using nonlinear

response history analysis to demonstrate acceptable

strength, stiffness, and ductility to resist maximum

considered earthquake (MCER) shaking with acceptable

performance as defined in the ASCE Standard ASCE/SEI

7-16.



• The nonlinear response history analysis relies on a suite or suites of ground motions that satisfy a target response spectrum defined in ASCE 7-16. The target response spectrum is defined as the “Risk-targeted maximum considered earthquake (MCER) ground motion.” The determination or calculation of this MCER ground motion is through both Probabilistic Seismic Hazard Analysis (PSHA) and Deterministic Seismic Hazard Analysis (DSHA). The site-specific MCER spectral response acceleration at any period is taken as the lesser of the spectral accelerations from the PSHA and the DSHA. A suite or suites of ground motions are scaled or matched to the resulting MCER spectrum for the nonlinear response history analysis.

Development of Risk-targeted Maximum

Considered Earthquake (MCER) Ground Motions


• ASCE 7-16 allows two methods. The first is Site

Response Analysis

This is

rarely

used.

Development of Risk-targeted Maximum

Considered Earthquake (MCER) Ground Motions


• The second procedure allowed by ASCE 7-16 is a ground motion hazard analysis. The ground motion hazard analysis should account for:

– the regional tectonic setting, geology, and seismicity;

– the expected recurrence rates and maximum magnitudes of earthquakes on known faults and source zones;

– the characteristics of ground motion attenuation

– near source effects, if any.

– and the effects of the subsurface site conditions on ground motions.

Earthquake Forecast Models


• Earthquake fault rupture forecast (or source) models are a

key aspect of the hazard evaluation. The source models

should reflect global and regional earthquake observations,

widely accepted seismology-based principles, and scientific

analyses in the science and engineering communities.

• The U.S. practice generally follows the lead of the United

States Geological Survey (USGS) National Seismic Hazard

Mapping Project (NSHMP) in its use of earthquake fault

rupture forecast models (USGS, 2014). The NSHMP has

documented the earthquake fault rupture forecast models

for the identified seismic sources that affect the different

regions of the U.S., including California and the west coast.



• Uniform California Earthquake Rupture Forecast, Version 3

(UCERF3) fault model



• Uniform California Earthquake Rupture Forecast, Version 3

(UCERF3) fault model

– UCERF3 fault model allows for ruptures on multiple fault

segments, such as observed in Denali

Ground Motion Prediction Equations


• Because of the different seismic tectonic regions of the

US, different GMPEs are used regionally. For the

Western US (WUS), the currently accepted GMPEs for

crustal earthquakes are the NGA-West2 equations

developed by PEER:

– Abrahamson, Silva and Kamai (2014)

– Boore, Stewart, Seyhan and Atkinson (2014)

– Campbell and Bozorgnia (2014)

– Chiou and Youngs (2014)

– Idriss (2014)

Seismic Hazard Analysis


• Uniform Hazard Maximum Considered Earthquake

(MCE) Ground Motions (ASCE 7-05)

– This was the definition of the design ground motions in

ASCE 7-05 and was replaced in ASCE 7-10 by the MCER

– Defined as the spectral response acceleration from a 5%

damped acceleration response spectrum that has a 2%

probability of exceedance within a 50-year period; this

corresponds to a return period of about 2,475 years

– Computing by a PSHA analysis



• Uniform Hazard Maximum Considered Earthquake (MCE) Ground Motions (ASCE 7-05)

– ASCE 7-05 also required the determination of the deterministic MCE ground motions. The deterministic MCE is defined as the spectral response acceleration at each period calculated as an 150% of the median 5% damped spectral response accelerations. In addition, the deterministic MCE spectrum was not to be lower than the ordinates of the ASCE 7-05 minimum “code” spectrum; this would be equivalent to the spectrum used for prescriptive code design.

– The site-specific MCE spectral acceleration at any period is to be taken as the lesser of the spectral response accelerations from the probabilistic and the deterministic ground motions. This has sometimes been referred to as a “Deterministic Cap” to the MCE spectrum.



• Risk-Targeted Maximum Considered Earthquake (MCER)

Ground Motions

– ASCE 7-10 and ASCE 7-16 redefined what would be

the target ground motion response spectrum for the

nonlinear response history analysis; ASCE 7-10 and

7-16 introduced the new concept of the “Risk-

Targeted Maximum Considered Earthquake” or MCER.

– The MCER ground motions are related to the old

MCE ground motions with some additional

provisions. These additional provisions include the

concepts of “maximum direction ground motions”

and “risk targeting” of the ground motions.




Ground Motions

– Maximum Direction Ground Motions

• ASCE 7-10 and ASCE 7-16 require that the spectral

response accelerations represent the maximum

response in the horizontal plane. Current Ground

Motion Prediction Equations (GMPEs) predict the

“geomean” response accelerations; these geomean

responses are referred to as the RotD50 response

accelerations. ASCE 7-10 and 7-16 require what are

referred to as the RotD100 response accelerations.

GMPE equations are not currently available to predict

RotD100 response accelerations.




Ground Motions

– Maximum Direction Ground Motions




Ground Motions

– Shahi and Baker Factors for Maximum Direction Conversion




Ground Motions

– 1% probability of collapse within a 50-year return period



• Risk-Targeted Maximum Considered Earthquake (MCER) Ground Motions– In the U.S., the deterministic MCER ground motions may be

lower than the probabilistic MCER ground motions where there are regions where characteristic earthquakes with relatively shorter return periods (i.e., much less than 2,475 years) may dominate the seismic hazard. One such region is the San Francisco Bay Area where the San Andreas fault and related Hayward fault are close to the urbanized areas. Characteristic earthquake moment magnitudes may be on the order of 7.5 to 8.0 with recurrence intervals of several hundred years.

– The concept of risk targeting the ground motions may be appropriate for the U.S. where there are very distinct differences in the seismic environments in a very large country. Japan may be more homogeneous in terms of seismic environment than the U.S. and thus risk targeting as a tool may not be necessary. It is a concept that may warrant further study.



• Other Considerations for the Risk-Targeted MCERGround Motions– Directivity - Directivity effects were not included

explicitly in the NGA-West2 GMPEs. Using data from NGA-West2, Shahi and Baker (2014) found that a, the minimum angle between the strike of the fault and orientation of the maximum direction spectral acceleration, was closer to strike-normal orientation (a=90 deg) more often than the strike-parallel orientation (a=0 deg) for recordings located within 5 km of a fault for periods greater than 1 sec. Shahi and Baker found that about 60% of the time, a was between 60 and 90 deg; a was between 80 and 90 deg about 26% of the time. For recordings greater than 5 km and periods greater than 1 sec, Shahi and Baker found that a was almost uniformly distributed between 0 and 90 deg.



• Other Considerations for the Risk-Targeted MCER

Ground Motions

– Deep Basin Effects - Some urban areas are located

on deep sediment-filled basins. These basins consist

of alluvial deposits and sedimentary rocks overlying

geologically older rocks that have higher seismic

velocities.



• Other Considerations for the Risk-Targeted MCER

Ground Motions

– Deep Basin Effects –

• NGA-West2 GMPEs attempt to model basins using the

Z1.0 and Z2.5 depths.

– Z1.0 is the depth where the shear wave velocity is 1,000

m/sec

– Z2.5 is the depth where the shear wave velocity is 2,500

m/sec



• Available Software for PSHA Analysis in US

– Commercial Software

• EZ-Frisk

• Haz45

– Open Source Software

• OpenSHA

Ground Motions for Nonlinear Response History

Analysis (NLRHA)


• ASCE 7-16 allows Nonlinear Response History Analysis

(NLRHA) to demonstrate acceptable strength, stiffness

and ductility to resist MCER shaking with acceptable

performance when designing and analyzing structures

that are using PBEE principles and not adhering to

prescriptive building code design. The NLRHA should

include the effects of horizontal motion and may

include vertical motion where required by ASCE 7-16.


Analysis (NLRHA)


• Uniform Risk Spectrum as Target Spectrum

– This first option would require scaling or matching

the ground motions to the entire MCER spectrum,

which is the uniform hazard spectrum (UHS)

modified by the risk-targeting factors, for the period

range of interest; the modified UHS is a uniform risk

spectrum (URS).

– Period range of interest (ASCE 7-16) is maximum of

1.5 or 2.0 times the highest fundamental period of

the building (T1) and the minimum of 0.2 x T1 or the

period of higher modes capturing 90% of the mass

participation.


Analysis (NLRHA)


• Uniform Risk Spectrum as Target Spectrum


Analysis (NLRHA)


• Uniform Risk Spectrum as Target Spectrum– It has been long understood that matching or scaling ground

motions to a uniform hazard design spectrum from PSHA analysis is extremely conservative (Naeim and Lew, 1995). PSHA analysis considers all possible earthquake sources in the region surrounding a site. A site-specific design spectrum from a PSHA analysis represents the cumulative contribution of hazard from the seismic sources in the region for a given risk level. The UHS does not and was never intended to represent any single ground motion event. The UHS represents an envelope of the contributions of ground motion hazard from multiple events which correspond to the specified risk level. A ground motion time history that is matched or scaled to the UHS spectrum would contain energy over the whole range of structural periods that is not seen in actual individual recorded time histories. Thus, the UHS conservatively implies that large-amplitude spectral values will occur at all periods within a single ground motion and is inherently conservative.


Analysis (NLRHA)


• Conditional Mean Spectrum (Scenario Spectrum)


Analysis (NLRHA)


• Selection of Ground Motions

– ASCE 7-16 requires a minimum of 11 ground motion time

histories for each target spectrum. If there are multiple

CMS spectra used, there should be one suite of at least 11

ground motions for each CMS. ASCE 7-16 allows for

unacceptable response in one of the ground motions; this

would meet the goal of achieving a 10% target collapse

reliability. However, the LATBSDC guidelines do not allow

for any unacceptable response in any of the 11 ground

motions.


Analysis (NLRHA)



– Ground Motion Components

• Generally, it is only necessary to consider the

response to horizontal components of ground

motion in most structures. However, where

structures may be sensitive to vertical earthquake

effects, the vertical ground motion component

should be included in the analysis.


Analysis (NLRHA)



– Near-Fault Sites

• Near-fault sites are defined sites located within 9.3 miles (15

km) of the surface project of faults capable of producing

Mw7.0 or greater earthquakes and within 6.2 miles (10 km) of

the surface projection of faults capable of producing

earthquakes of Mw6.0 or greater, where the faults must meet

minimum annual slip rate criteria. The ASCE 7-16

Commentary states that such near-fault sites have a

reasonable probability of experiencing ground motions

strongly influenced by rupture directivity effects. These

effects can include pulse-type ground motions observable in

velocity histories and polarization of ground motions such

that the maximum direction of response tends to be in the

direction normal to the fault strike.


Analysis (NLRHA)


• Selection of Recorded Motions – Primary Considerations– Source Mechanism: Ground motions from differing tectonic regimes

(e.g., subduction vs. active crustal regions) can have different spectral shapes and durations.

– Magnitude: Earthquake magnitude is related to the duration of ground shaking and can have some correlation with the shape of the response spectrum.

– Site Soil Conditions: Site soil conditions can have a large influence on the spectral shape of the ground motions.

– Usable Frequency of the Ground Motions: The ground motions should have usable frequencies that are relevant to the building response.

– Period/Frequency Sampling: The ground motions should have a sampling rate that is adequate so that the important characteristics of the motion are not lost and building response is properly evaluated.

– Site-to-Source Distance: The distance is a lower priority parameter to consider in selecting ground motions and may not significantly affect demands on structures if not strictly met.


Analysis (NLRHA)


• Selection of Recorded Motions – Secondary Considerations– Spectral Shape: The shape of the spectrum should be a primary consideration

and the spectral shape should be similar to the target spectrum.

– Scale Factor: It is desirable to select motions that do not require a large scaling factor; it is common to limit the scaling to between 0.25 and 4.

– Maximum Number of Ground Motions from a Single Event: It is desirable to limit the number of ground motions from a single seismic event so that the ground motion suite is not unduly influenced by the single event.

• Additional insight on ground motion selection can be found in the document titled “Selecting and Scaling Earthquake Ground Motions for Performing Response-History Analyses (NIST, 2011).

• The ground motions should have an appropriate number of pulse-type ground motions. Criteria for evaluating pulse probabilities can be found in NIST (2011) and Shahi et al. (2011). Ground motion records with pulse-type characteristics are identified in the PEER Ground Motion Database.


Analysis (NLRHA)


• Selection of Recorded Motions – Secondary

Considerations

– the duration of strong shaking of the ground motion

time history should also be another consideration.

Especially with taller buildings that may have longer

fundamental periods, the duration strong ground

shaking should be long enough that the motion

would be able to develop response in the building.

One possible exception could be a recording that

has a significant velocity pulse with a shorter

duration of ground shaking; some recordings from

the 1994 Northridge earthquake are in this category.


Analysis (NLRHA)


• Ground Motion Modification– Amplitude Scaling - The procedure for amplitude scaling

in ASCE 7-16 differs from the procedure in earlier editions where scaling was based on the square root of the sum of the squares spectrum; the original target spectrum was the UHS spectrum. In ASCE 7-16, amplitude scaling is now based directly on the maximum direction spectrum to be compatible with the definition of the MCER. (Also referred to as the RotD100 spectrum.) The maximum direction spectrum is to be constructed from the two horizontal ground motion components. Each ground motion is to be scaled with an identical scale factor applied to both components, such that the average of the maximum-direction spectra from all the ground motions does not fall below 90% of the target response spectrum for any period within the period range of interest.


Analysis (NLRHA)


• Ground Motion Modification- Amplitude Scaling– The figure illustrates amplitude scaling for a CMS with a conditioning

period, T*, set at 1 sec.; the period range of interest is 0.2 to 2 sec. The

spectral acceleration of each of the selected ground motions at T* is

anchored to the spectral acceleration of CMS target spectrum at T*.


Analysis (NLRHA)


• Ground Motion Modification– Spectral Matching - Spectral matching is defined in ASCE

7-16 as the modification of a real recorded earthquake ground motion in some manner such that its response spectrum matches a desired target spectrum across a period range of interest. ASCE 7-16 requires that when spectral matching is used, the average of the maximum direction spectra of the matched motions must exceed the target spectrum by at least 110% over the period range of interest. The rationale behind it is based on the thought that spectral matching removes variability in the ground motion spectra and has the potential to predict lower mean response as reported by Luco and Bazurro(2007) and Grant and Diaferia (2012). The LATBSDC guidelines do not require the 110% increase in the target spectrum.


Analysis (NLRHA)


• Ground Motion Modification– Spectral Matching - As it is undisputed that the UHS-

based MCER spectrum is inherently conservative and

ground motions that are matched or scaled to the

URS will also be conservative (Naeim and Lew, 1995);

therefore, LATBSDC does not deem it necessary to

apply a penalty for spectral matching of ground

motion time histories. However, LATBSDC guidelines

do agree that the 110% penalty should apply if

ground motions are being spectrally matched to

CMS spectra.


Analysis (NLRHA)


• Ground Motion Modification– Spectral Matching - The most common and preferred

method of spectral matching of ground motion time histories was developed by Abrahamson (1992) with the program RspMatch. The program adjusts the time series in the time domain by adding wavelets to the initial time series. The method has good convergence properties and can preserve the nonstationary character of the initial time series. Enhancements to RspMatch were developed to correct for drift in the corresponding velocity and displacement time series (Hancock et al., 2006). Al Atikand Abrahamson (2010) introduced a new version of RspMatch with a new adjustment function that allows the use of an analytical solution in the spectral matching algorithm and that readily integrates to zero velocity and displacement without having to perform baseline correction.


Analysis (NLRHA)


• Ground Motion Modification– Spectral Matching - ASCE 7-16 does not specify how to

perform spectral matching. Both components of horizontal motion could be matched to a single target spectrum or the individual components could be matched to different spectra; the only requirement is that the average maximum direction spectra for the matched records meet the specified criteria. For ground motions that are deemed to be near-fault (i.e., within 5 km or 3 miles of the site), it would be expected that those horizontal ground motions are more likely to be strongly oriented with larger fault-normal (FN) components and less strong fault-parallel (FP) components. In these cases, it is common to match the FN component to the MCERspectrum (the maximum direction or RotD100 spectrum). For the FP component, it is common to conservatively match it with the geomean (or RotD50 spectrum).


Analysis (NLRHA)


• Ground Motion Modification– Spectral Matching - ASCE 7-16 does not allow spectral

matching for near-fault sites unless the pulse

characteristics of the ground motion are retained after the

spectral matching process has been completed. This may

be accomplished by “loose” spectral matching and not

trying to get a tight match with the target spectrum; this

can also be accomplished by other methods. For sites

that are not near-fault (i.e., site is greater than 5 km from

the fault), the maximum direction ground motion does

not align with particular orientation and should be

applied to the building in a random orientation.


Analysis (NLRHA)


• Practical Issues in Ground Motion Selection and Modification– The selection of ground motions for seismic hazard

assessment is not a trivial process. The PEER NGA-West2 database has over 8,600 three-component records from 334 shallow crustal events (Ancheta et al., 2012). Another database for ground motions is the Center for Engineering Strong Motion Data (CESMD, 2019). CESMD is a cooperative center established by the US Geological Survey (USGS) and the California Geological Survey (CGS) to integrate earthquake strong-motion data from the CGS California Strong Motion Instrumentation Program, the USGS National Strong Motion Project, and the Advanced National Seismic System (ANSS). The CESMD provides raw and processed strong-motion data for earthquake engineering applications.


Analysis (NLRHA)


• Practical Considerations for Development of CMS Target Spectra– In the ASCE 7-16 Commentary on ground motions in Section C16.2, it

specifically addresses CMS target spectra. This section goes into great detail in explaining the purpose of why the CMS or scenario spectra are not allowed to be less than 75% of the MCER spectrum for any period within the period range of interest. The primary purpose for this “75% floor” is to provide a basis for determining how many target spectra are needed for analysis. It follows that when there are small period ranges, few target CMS spectra are needed, and more target CMS spectra are needed where a wider range of periods are important to the structural response (such as in taller buildings).

– For a shorter building with shorter fundamental periods, two CMS spectra may suffice. In this case, two suites of at least 11 horizontal ground motion pairs would be needed as a minimum; this would require 22 nonlinear response history analyses to be performed. A taller building will have longer fundamental periods, then two CMS spectra may not be sufficient since the envelope of the two spectra may fall below 75% of the MCER spectrum within the period range of interest. This may require three or even four CMS spectra, resulting in three or four suites of at least 11 horizontal ground motion pairs and requiring 33 or 44 nonlinear response history analyses.


Analysis (NLRHA)


• Practical Considerations for Development of CMS Target Spectra– Another situation where more CMS spectra might be required is

where there is a large difference in the fundamental periods in the two translational axes of the building, where one axis may be significantly stiffer than the other orthogonal axis.

– U.S. structural engineers have resisted using more than two CMS because of the increase in nonlinear response history analyses that would be required, thus increasing computational effort and time to make the required analyses. To accommodate this desire by structural engineers, geotechnical engineers have broadened the CMS spectrum to capture two or more disparate fundamental periods. For the longer period CMS, this may be performed by developing separate CMS for the two distinct fundamental periods (Tx* and Ty*) in each direction of response (x- and y-axes of the building, respectively).


Analysis (NLRHA)


• Practical Considerations for Development of

CMS Target Spectra


Analysis (NLRHA)



CMS Target Spectra

Enveloping of the individual CMS spectra conditioned at periods of Tx*

and Ty* to create one single broadened CMS spectrum


Analysis (NLRHA)



CMS Target Spectra

Two hybrid broadened CMS spectra


Analysis (NLRHA) - Example



CMS Target Spectra





CMS Target Spectra




• Tight Spectral Matching

0.01

0.1

1

10

0.01 0.1 1 10

Spec

tral

Acc

ele

rati

on

(g)

Period (sec)

MCEr Spectrum -Fault Normal




• Scaling of Response spectra

0.01

0.1

1

10

0.01 0.1 1 10

Sp

ect

ral A

ccele

ratio

n [

g]

Period [s]

Period range of interest


Analysis (NLRHA)


Thank you!

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performance-based earthquake engineering design and ......• earthquake fault rupture forecast (or...

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