site-specific design requirements and parameters

EERI Seminar on Next Generation Attenuation Models

SITE-SPECIFIC DESIGN REQUIREMENTS AND PARAMETERS

Charlie Kircher Kircher & Associates Palo Alto, California

BSSC Colloquium: Seismic Design Technology for New Buildings – February 11,2015

Presenter

Presentation Notes

New seismic design maps are based on new hazard maps that Mark described, which in turn are based on the NGA models, and now I’ve met the requirement of saying “NGA” at least once during my presentation.

Kircher Presentation – Site-Specific Design Requirements and Parameters

Topics

• Summary of New Requirements – Section 11.4.7 Site Specific Ground Motion Procedures – Section 21.4 Design Acceleration Parameters

• Impact on Design – Design Examples Spreadsheet

• The Problem • The Solution • Investigations and Findings

– FEMA-funded BSSC study

• Conclusion – Long-Term Solution


Acknowledgments

• FEMA (NEHRP) funded BSSC study

• Study Advisors and Contributors: – Nico Luco (USGS) – Sanaz Rezaeian (USGS) – C. B. Crouse (URS) – Jonathan Stewart (UCLA) – Kevin Milner (SCEC) – David Bonnevile (Degenkolb) – BSSC PUC Chair – John Hooper (MKA) – ASCE 7-16 SSC Chair

• PEER Center - Next Generation Attenuation Relations – Linda Al Atik (PEER NGA West1 GMPEs spreadsheet) – Emil Seyhan (PEER NGA West2 GMPEs spreadsheet)


Design Response Spectrum (Figure 11.4-1, ASCE 7-10 with annotation)

Acceleration Domain

Velocity Domain

Displacement Domain

SDS = 2/3 x SMS = 2/3 x Fa x Ss

TS = SD1/SDS

SD1 = 2/3 x SM1 = 2/3 x Fv x S1

Cs = SDS/(R/Ie) T ≤ Ts

Cs = SD1/T(R/Ie) Ts < T ≤ TL


Changes to the Site-Specific Procedures of Section 21.4

• Derivation of Design Acceleration Parameters SDS and SD1 from a site-specific design spectrum: – Base SDS on 90 percent of the peak acceleration of the

site-specific design spectrum (periods of 0.2s to 1s) Consistently define domain of constant acceleration in terms of 90 percent of peak acceleration response regardless of the period of peak response (within the range 0.2s to 1s).

– Base SD1 on 100 percent of T x site-specific design spectrum at the period of peak velocity response (periods 1s to 5s) Extend period range from 2s to 5s for Site Class D and E sites to avoid underestimating response in the domain of constant velocity


Illustration of the New Criteria of Section 21.4 Site Class DE, M8 at R = 8.5 km (PEER NGA-West1 Relations)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

0.1 1.0 10.0

Spec

tral A

ccel

erat

ion

(g)

Period (seconds)

MCEr - BC (Vs,30 = 2,500 fps)

MCEr - DE (Vs,30 = 600 fps)

Design DE (Vs,30 = 600 fps)

ELF DE (Vs,30 = 600 fps)

SDS = Max(0.9 x Sa[T ≥ 0.2s])

SD1 = max(T x Sa[1s ≤l T ≤ 5s])

ELF “Design Spectrum” Cs x (R/Ie) = min[SDS, SD1/T]


New Requirements for Site-Specific Analysis - Section 11.4.7

• Require site-specific ground motion procedures for: – structures on Site Class E sites with SS greater than or equal to 1.0. – structures on Site Class D and E sites with S1 greater than or equal to

0.2.

• Permit ELF (and MSRA) design using conservative values of seismic coefficients: – Structures on Site Class E sites with SS greater than or equal to 1.0,

provided the site coefficient Fa is taken as equal to that of Site Class C. – Structures on Site Class D sites with S1 greater than or equal to 0.2,

provided the value of the seismic response coefficient Cs is increased by up to 50 percent at periods greater than Ts

– Structures on Site Class E sites with S1 greater than or equal to 0.2, provided that T is less than or equal to Ts and the equivalent static force procedure is used for design.


Design Example – 4-Story Steel Special Moment Frame Building Site Class C

0.000

0.020

0.040

0.060

0.080

0.100

0.120

0.140

0.160

0.1 1 10

Des

ign

Coe

ffici

ent,

Cs

(g)

Period (seconds)

ASCE 7-10Design Value at TASCE 7-16 w/NSCDesign Value at TASCE 7-16 w/008Design Value at TASCE 7-16 w/SSAFDesign Value at T

Material Steel Site Class CSystem MF S s 1.50

Detailing Special S 1 0.60Floors 4 T L (s) 12

Height (ft) 45 SDC DT a (s) 0.59 R 8


Design Example – 4-Story Steel Special Moment Frame Building Site Class D

0.000

0.020

0.040

0.060

0.080

0.100

0.120

0.140

0.1 1 10

Des

ign

Coe

ffici

ent,

Cs

(g)

Period (seconds)


Material Steel Site Class DSystem MF S s 1.50




Design Example – 4-Story Steel Special Moment Frame Building Site Class E

0.000

0.020

0.040

0.060

0.080

0.100

0.120

0.140

0.160

0.1 1 10

Des

ign

Coe

ffici

ent,

Cs

(g)

Period (seconds)


Material Steel Site Class ESystem MF S s 1.50




Design Example – 20-Story Steel Special Moment Frame Building Site Class C

0.000

0.020

0.040

0.060

0.080

0.100

0.120

0.140

0.160

0.1 1 10

Des

ign

Coe

ffici

ent,

Cs

(g)

Period (seconds)


Material Steel Site Class CSystem MF S s 1.50




Design Example – 20-Story Steel Special Moment Frame Building Site Class D

0.000

0.020

0.040

0.060

0.080

0.100

0.120

0.140

0.1 1 10

Des

ign

Coe

ffici

ent,

Cs

(g)

Period (seconds)


Material Steel Site Class DSystem MF S s 1.50




Design Example – 20-Story Steel Special Moment Frame Building Ss = 1.5, S1 = 0.6, Site Class E, TL = 12s – SDC D

0.000

0.020

0.040

0.060

0.080

0.100

0.120

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0.1 1 10

Des

ign

Coe

ffici

ent,

Cs

(g)

Period (seconds)


Material Steel Site Class ESystem MF S s 1.50



Site-Specific Analysis Required (Ta > Ts)


Root Cause of the “Problem”

• Section 11.4 of ASCE 7-10 (ASCE 7-16) - Use of only two response periods (0.2s and 1.0s) to define ELF (and MRSA) design forces is not sufficient, in general, to accurately represent response spectral acceleration for all design periods

– Reasonably Accurate (or Conservative) – When peak MCER response spectral acceleration occurs at or near 0.2s and peak MCER response spectral velocity occurs at or near 1.0s for the site of interest

– Potentially Non-conservative – When peak MCER response spectral velocity occurs at periods greater than 1.0s for the site of interest (e.g., soil sites whose seismic hazard is dominated by large magnitude events)


Example ELF “Design Spectrum” based on ASCE 7-16 Seismic Criteria M7.0 earthquake ground motions at RX = 6.5 km, Site Class C

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

2.6

0.1 1.0 10.0

Res

pons

e Sp

ectra

l Acc

eler

tatio

n (g

)

Period (seconds)

MCEr Multi-Period Response Spectrum - Site Class BCMCEr Multi-Period Response Spectrum - Site Class CDesign Multi-Period Response Spectrum - Site Class CELF Design Spectrum (Cs x R/Ie) - Current ASCE 7-16 Criteria


Example ELF “Design Spectrum” based on ASCE 7-16 Seismic Criteria M7.0 earthquake ground motions at RX = 6.5 km, Site Class D

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

2.6

0.1 1.0 10.0

Res

pons

e Sp

ectra

l Acc

eler

tatio

n (g

)

Period (seconds)

MCEr Multi-Period Response Spectrum - Site Class BCMCEr Multi-Period Response Spectrum - Site Class DDesign Multi-Period Response Spectrum - Site Class DELF Design Spectrum (Cs x R/Ie) - Current ASCE 7-16 Criteria


Example ELF “Design Spectrum” based on ASCE 7-16 Seismic Criteria M7.0 earthquake ground motions at RX = 6.5 km, Site Class E

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

2.6

0.1 1.0 10.0

Res

pons

e Sp

ectra

l Acc

eler

tatio

n (g

)

Period (seconds)

MCEr Multi-Period Response Spectrum - Site Class BCMCEr Multi-Period Response Spectrum - Site Class EDesign Multi-Period Response Spectrum - Site Class EELF Design Spectrum (Cs x R/Ie) - Current ASCE 7-16 Criteria


Long-Term Solution (ASCE 7-21)

• Develop and adopt multi-period design spectrum approach

– Not feasible in current code cycle (ASCE 7-16)

• Multi-period spectrum approach will require:

– Major reworking of seismic design requirements and criteria now based on two response periods (e.g., Tables 11.6-1/2, Seismic Design Categories, etc.)

– Development of new ground motion design values maps (by the USGS) for each new response period of interest

– Development of new site factor tables for each new response period of interest (or site effects embedded directly in ground motion design values maps)


Short-Term Solution (ASCE 7-16)

• Two Options:

– Re-formulate seismic parameters to eliminate potential non-conservatism in ELF (and MRSA) seismic forces

– Require site-specific analysis when ELF (and MSRA) seismic forces could be potentially non-conservative

• Perform a study to develop the technical approach and basis for proposing changes to current seismic criteria – Develop new values of re-formulated parameters (Option 1) – Develop criteria for requiring site-specific analysis (Option 2)

• Develop conservative values of current seismic parameters for design using ELF (and MSRA) methods in lieu of site-specific analysis


Tentative Re-Formulation of Seismic Parameters • Add two new “spectrum shape adjustment” factors, Ca and Cv, to

Eqs. 11.4-1 and 11.4-2 of Section 11.4.3 which would define values of SMS and SM1, as follows:

SMS = CaFaSS (11.4-1) SM1 = CvFvS1 (11.4-2)

• Use the new values of seismic coefficients Fa and Fv

• Calculate values of spectrum shape adjustment factors Ca and Cv: Ca = CFa /F0.2 (e.g., F0.2 = Fa) Cv = CFv /F1.0 (e.g., F1.0 = Fv)

• Derive values of the parameters CFa and CFv from representative “target” MCER spectra using the new criteria of Section 21.4:

CFa = 0.9 max{SaM[T ≥ 0.2s]}/Ss CFv = 1.0 max{T x SaM[5s ≥ T ≥ 1s]}/S1


Example Derivation of Parameters, CFa and CFv using the New Criteria of Section 21.4 - Site Class DE, M8 at R = 8.5 km (NGA-West1 Relations)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

0.1 1.0 10.0

Spec

tral A

ccel

erat

ion

(g)

Period (seconds)

MCEr - BC (Vs,30 = 2,500 fps)

MCEr - DE (Vs,30 = 600 fps)

Design DE (Vs,30 = 600 fps)

ELF DE (Vs,30 = 600 fps)

SDS = Max(0.9 x Sa[T ≥ 0.2s])

SD1 = max(T x Sa[1s ≤l T ≤ 5s])

ELF “Design Spectrum” Cs x (R/Ie) = min[SDS, SD1/T]

Derivation of Parameters CFa and CFv CFa = SDS/(2/3 x Ss) = 0.9 x 0.87g)/(2/3 x 1.56g) CFv = SD1/(2/3 x S1) = 4s x 0.47g/s/(2/3 x 0.69g)

“Derived” CFa = 0.78 CFv = 3.81 Ts = 2.25s


Example Plots of Parameters CFa and CFv Derived from Deterministic M6.0 and M8.0 MCER Response Spectra

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2

Der

ived

Fac

tor,

CFv

1-Second Spectral Acceleration Parameter, S1 (g)

Site Class A (vs,30 = 5,310 fps)Site Class AB (vs,30 = 5,0000 fps)Site Class B (vs,30 = 3,000 fps)Site Class BC (vs,30 = 2,500 fps)Site Class C (vs,30 = 1,600 fps)Site Class CD (vs,30 = 1,200 fps)Site Class D (vs,30 = 870 fps)Site Class DE (vs,30 = 600 fps)Site Class E (vs,30 = 155 fps)

0.5

0.6

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1.0

1.1

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2.0

0 0.25 0.5 0.75 1 1.25 1.5 1.75 2 2.25 2.5

Der

ived

Fac

tor,

CFa

Short-Period (0.2s) Spectral Acceleration Parameter, Ss (g)


0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

1.3

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1.8

1.9

2.0

0 0.25 0.5 0.75 1 1.25 1.5 1.75 2 2.25 2.5

Der

ived

Fac

tor,

CFa

Short-Period (0.2s) Spectral Acceleration Parameter, Ss (g)


0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2

Der

ived

Fac

tor,

CFv

1-Second Spectral Acceleration Parameter, S1 (g)


CFa - M6.0 CFa – M8.0

CFv – M8.0 CFv – M6.0


Site-Specific Values Earthquake Magnitude - Map of TL Regions (Figure 22-12, ASCE 7-10 based on Figure 1 and Table 1, Crouse et al., 2006)


Example Values of Short-Period Spectrum Shape Adjustment Factor Ca (from Table 11.4-3 of Proposal BSSC PUC IT11-006)

Table 11.4-3 Short-Period Spectrum Shape Adjustment Factor, Ca

Site Class

Mapped Risk-Targeted Maximum Considered Earthquake (MCER) Spectral Response Acceleration Parameter at Short Period

SS ≤ 0.25 SS = 0.5 SS = 0.75 SS = 1.0 SS = 1.25 SS ≥ 1.5

A 0.9 0.9 0.9 0.9 0.9 0.9 B 0.9 0.9 0.9 0.9 0.9 0.9 C 0.9 0.9 0.9 0.9 0.9 0.9 D 1.0 1.0 1.0 1.0 1.0 1.0 E 0.95 1.0 1.1 1.15 1.2 1.25 F See Section 11.4.7

Selected Values of F a - Table 11.4-2, ASCE 7-16Site Class S s = 1.0 S s = 1.25 S s = 1.5

C 1.2 1.2 1.2D 1.1 1.0 1.0E 1.1 1.0 0.8


Example Values of Short-Period Spectrum Shape Adjustment Factor Cv (from Table 11.4-4 of Proposal BSSC PUC IT11-006)

Table 11.4-4 Long-Period Spectrum Shape Adjustment Factor, Cv, for sites with TL greater than or equal to 12 s

Site Class

Mapped Risk-Targeted Maximum Considered Earthquake (MCER) Spectral Response Acceleration Parameter at 1-s Period

S1 ≤ 0.1 S1 = 0.2 S1 = 0.3 S1 = 0.4 S1 = 0.5 S1 ≥ 0.6

A 1.0 1.0 1.0 1.0 1.0 1.0 B 1.0 1.0 1.0 1.0 1.0 1.0 C 1.0 1.05 1.05 1.05 1.0 1.1 D 1.0 1.2 1.3 1.35 1.45 1.5 E 1.0 1.3 1.5 1.75 1.9 2.0 F See Section 11.4.7


Conclusion

• Temporary Solution. The new site-specific design requirements of Section 11.4.7 provide a short-term solution that can and should be replaced by a more appropriate long-term solution in the next Code cycle

• Multi-Period Design Spectra. A long-term solution would necessarily include seismic criteria described by multi-period MCER response spectra

• Design Spectrum Shape. Ideally, multi-period design spectra would directly incorporate site, basin and other effects that influence the shape (i.e., frequency content) of the design spectrum

site-specific design requirements and parameters

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