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Building Seismic Safety Council Annual Meeting ATC-84 Project Improved Structural Response Modification Factors for Seismic Design of New Buildings – Phase I Charlie Kircher Kircher & Associates Project Director January 20, 2010 ATC-84 Project Organization Review Panel (PRP)

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Building Seismic Safety Council Annual MeetingATC-84 ProjectImproved Structural Response Modification Factors for Seismic Design of New Buildings – Phase I

Charlie Kircher

Kircher & Associates

Project Director

January 20, 2010

ATC-84 Improved Seismic Performance Factors for Seismic Design of New Buildings


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ATC-84 Project Organization

Review Panel (PRP)

Bob Bachman (REBC)

Kelly Cobeen (WJE)

Ron Hamburger (SGH)

Greg Kingsley (KL&A)

Richard Klingner (UT)

Phil Line (URS Corp.)

Jim Malley (Degenkolb)

Bonnie Manley (AISI)

Jack Moehle (UCBerkeley)

Laurence Novak (PCA)

Charles Roeder (UW)

Kurt Stochlia (ICC-ES)

NIST (NEHRP)

John (Jack) Hayes (NIST)

John (Jay) Harris (NIST)

Technical Committee (PTC)

Charles Kircher (K&A) Chair

Finley Charney (VPI)

Greg Deierlein (Stanford)

James Harris (JRH&Co.)

William Holmes (R&C)

John Hooper (MKA)

Laura Lowes (UW)

Working Groups

TBD

ATC-84 Improved Seismic Performance Factors for Seismic Design of New Buildings


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ATC-84 Project Objectives

  • Develop a framework for alternative formulations of seismic performance factors (e.g., R, Cd, WO) that consider inter-dependency of:

    • Period, Ductility, Overstrength

    • Site Classification

    • Seismic Design Category (and/or Occupancy)

  • Perform analyses to define SPFs in variable terms in order to achieve uniform risk

  • Identify research to improve alternative SPFs

ATC-84 Improved Seismic Performance Factors for Seismic Design of New Buildings


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Black Jack Numbering Scheme (?)

  • ATC-63 – Quantification of Building Seismic Performance Factors (FEMA P695, June 2009)

  • 63 Plus 21 = 84

  • ATC – 84 – Improved Structural Response Modification Factors for Seismic Design of New Buildings – Phase I

  • 84 Minus 1 = 83 - Number of different SFRSs currently in Table 12.2-1 (ASCE 7-05)

ATC-84 Improved Seismic Performance Factors for Seismic Design of New Buildings


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ATC-63 Project Objectives

  • Primary – Create a methodology for determining Seismic Performance Factors (SPF’s) “that, when properly implemented in the design process, will result in the equivalent earthquake performance of buildings having different structural systems” (i.e., different lateral-force-resisting systems)

  • Secondary – Evaluate a sufficient number of different lateral-force-resisting systems to provide a basis for Seismic Code committees (e.g., BSSC PUC) to develop a simpler set of lateral-force-resisting systems and more rational SPF’s (and related design criteria) that would more reliably achieve the inherent earthquake safety performance objectives of building codes

ATC-84 Improved Seismic Performance Factors for Seismic Design of New Buildings


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Background and Overview Material

Methodology Guidelines

Supporting Material

FEMA P695 Report

  • Introduction

  • Methodology

  • Required System Information

  • Archetype Development

  • Nonlinear Model Development

  • Nonlinear Analysis

  • Performance Evaluation

  • Documentation and Peer Review

  • Example Applications

  • Supporting Studies

  • Conclusions and Recommendations

    Appendices, References

ATC-84 Improved Seismic Performance Factors for Seismic Design of New Buildings


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Seismic Performance Factors (FEMA P695)

  • Methodology determines appropriate values for the following seismic performance factors:

    • Response modification factor (R factor)

    • Over-strength factor (WO factor)

    • Deflection amplification factor (Cd factor)

  • Methodology could be used to evaluate other performance-related design criteria:

    • Drift limits

    • Height limits

    • Other design requirements (e.g. weak-beam/strong-column requirements, seismic detailing, etc.)

ATC-84 Improved Seismic Performance Factors for Seismic Design of New Buildings


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Scope and Basis of the FEMA P695 Methodology

  • New Buildings – Methodology applies to the seismic-force-resisting system of new buildings and may not be appropriate for non-building structures and does not apply to nonstructural systems.

  • NEHRP Provisions (ASCE 7-05) – Methodology is based on design criteria, detailing requirements, etc. of the NEHRP Provisions (i.e., ASCE 7-05 as adopted by the BSSC for future NEHRP Provisions development) and, by reference, applicable design standards

  • Life Safety – Methodology is based on life safety performance (only) and does not address damage protection and functionality issues (e.g., I = 1.0 will be assumed)

  • Structure Collapse – Life safety performance is achieved by providing an acceptably low probability of partial collapse and global instability of the seismic-force-resisting systemfor MCE ground motions

  • MCE Ground Motions – MCE ground motions are based on the spectral response parameters of the NEHRP Provisions (ASCE 7-05), including site class effects

ATC-84 Improved Seismic Performance Factors for Seismic Design of New Buildings


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Ground Motions

Analysis Methods

Design Information Requirements

Test Data Requirements

Peer Review Requirements

Elements of the FEMA P695 Methodology

Methodology

ATC-84 Improved Seismic Performance Factors for Seismic Design of New Buildings


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Notes

Develop Test Data

Develop Design Rules

“Homework” phase

Characterize System Behavior

Define Archetypes

Design archetypes (w/trial of R Factor)

Develop Archetype Models

Perform Pushover and NDA

Analyze Archetype Models

Evaluate CMR values (and overstrength)

Evaluate System Performance

P[Collapse] < Limit

No

Trial value of the R factor acceptable?

Yes

Peer Review applies to total process

Review and Documentation

Notional Flowchart of FEMA P695 Process

ATC-84 Improved Seismic Performance Factors for Seismic Design of New Buildings


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FEMA P695 RC SMF Example:Conclusions

  • Value of current SPFs (R=8) provide an acceptable level of collapse safety

  • The Methodology is reasonably well-calibrated to current design provisions

    • RC SMF systems didn’t fail miserably

    • RC SMF systems didn’t pass easily

  • This was true of all systems tested

ATC-84 Improved Seismic Performance Factors for Seismic Design of New Buildings


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FEMA P695 ExamplesObservations and Findings

  • Methodology was developed considering:

    • special concrete moment frames

    • ordinary concrete moment frames

    • special steel moment frames

    • wood shear walls

  • Some trends in our current design process have become apparent…

ATC-84 Improved Seismic Performance Factors for Seismic Design of New Buildings


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Ongoing Synergistic Activities

UC San Diego

Benson Shing

Ioannis Koutromanos

  • NIST funded ATC-76-1, ATC-76-4 Projects

  • Beta testing the Methodology on the following systems:

    • Reinforced Masonry Shear Wall

    • Reinforced Concrete Shear Wall

    • Special Steel Concentric Braced Frame

    • Special Steel Moment Frame

UCLA

John Wallace

Aysegul Gogus

UC Berkeley

Stephen Mahin

Chui-Hsin Chen

Stanford/UC Irvine

Helmut Krawinkler

Farzin Zarein

ATC-84 Improved Seismic Performance Factors for Seismic Design of New Buildings


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FEMA P695 Sections (90% Draft Report)

  • Introduction

  • Methodology

  • Required System Information

  • Archetype Development

  • Nonlinear Model Development

  • Nonlinear Analysis

  • Performance Evaluation

  • Documentation and Peer Review

  • Example Applications

  • Supporting Studies

  • Conclusions and Recommendation

    Appendices, References

Chapter 11. Conclusions and Recommendations

11.2 Observations and Conclusions

11.2.1 Generic Findings

ATC-84 Improved Seismic Performance Factors for Seismic Design of New Buildings


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Systems Approach

  • Collapse Performance and associated seismic performance factors must be evaluated in terms of the behavior of the overall seismic-force-resisting system:

    • Collapse failure modes are highly dependent on configuration and interaction of elements within a seismic-force-resisting system

  • Seismic performance factors apply to an entire system, not elements comprising it.

ATC-84 Improved Seismic Performance Factors for Seismic Design of New Buildings


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Precision of Seismic Performance Factors

  • In general, there is no practical difference in the collapse performance of systems designed with fractional differences in the response modification factor, R, for example:

    • Essentially the same performance, R = 6 and 6.5

    • Modest difference in performance, R= 6 and 8

    • Significant difference in performance, R = 3 and 6

  • Current values of R (Table 12.2.1 of ASCE/SEI 7-05) reflect a degree of precision not supported by results of example collapse evaluations

ATC-84 Improved Seismic Performance Factors for Seismic Design of New Buildings


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Spectral Content of Ground Motions

  • Consideration of spectral content (spectral shape) of ground motions can be very important to the evaluation of collapse performance of ductile seismic-force-resisting systems

    • Epsilon-neutral earthquake records scaled to represent very rare ground motions significantly overestimate demand on ductile systems

  • Methodology incorporates a spectral shape factor (SSF) to adjusts calculated response (margin) to account for spectral content of rare ground motions and avoid overestimation of nonlinear response

ATC-84 Improved Seismic Performance Factors for Seismic Design of New Buildings


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Short-Period Buildings

  • In general, results of example evaluations found smaller values of collapse margin ratios for short-period buildings

    • True for all types of seismic-force-resisting systems

    • Consistent with findings of previous research (Newmark & Hall, 1973, Nassar and Krawinkler, 1991, Miranda and Bertero, 1994, etc.)

  • Findings suggest possible need for short-period buildings:

    • Period-dependent R-factors?

    • More liberal collapse performance objectives?

ATC-84 Improved Seismic Performance Factors for Seismic Design of New Buildings


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Comparison of Strength Reduction Factors (Rm) (Figure 8. Strength Reduction Factors for Earthquake-Resistant Design, Miranda & Bertero, Earthquake Spectra, Vol. 10, No. 2, 1994)

ATC-84 Improved Seismic Performance Factors for Seismic Design of New Buildings


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Effective R Factor(Based on C1Factor of ASCE 41-06)

ATC-84 Improved Seismic Performance Factors for Seismic Design of New Buildings


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Governing Seismic Design Category

  • In general, results of example evaluations found smaller values of collapse margin ratios for seismic-force-resisting systems designed and evaluated for seismic criteria of Seismic Design Category (SDC) D than for the same system designed and evaluated for SDC C, all else equal

    • Trend attributed to increasing role of gravity loads in the strength of systems as the level of seismic load decreases (i.e., increased overstrength)

  • Findings suggest possible need for load-dependent (SDC dependent) seismic performance factors

ATC-84 Improved Seismic Performance Factors for Seismic Design of New Buildings


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Overstrength

  • In general, example evaluations found values of collapse margin ratio strongly related to calculated values of overstrength. For larger values of overstrength, large values of collapse margin ratio were observed

    • True for all types of seismic-force-resisting systems

    • Consistent with findings of previous research

  • Calculated values of overstrength of different archetypes vary widely, depending on configuration and seismic design criteria

    • Current values of WO (Table 12.2-1 of ASCE/SEI 7-05) are not representative of actual seismic-force-resisting system overstrength

ATC-84 Improved Seismic Performance Factors for Seismic Design of New Buildings


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Distribution of Inelastic Response

  • In general, example evaluations found values of collapse margin ratio strongly related to the distribution of inelastic response over the height of structure archetype

    • True for all types of seismic-force-resisting systems

    • Consistent with findings of previous research

  • Values of collapse margin ratio are significantly larger for systems with more evenly distributed inelastic response over height

ATC-84 Improved Seismic Performance Factors for Seismic Design of New Buildings


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Effect of Minimum Design Base Shear Strength

OUTCOME: Minimum base shear requirement from ASCE 7-02 (V/W = 0.04) reinstituted in ASCE 7-05

ASCE 7-05

Perimeter

ASCE 7-02

Space

ATC-84 Improved Seismic Performance Factors for Seismic Design of New Buildings


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FEMA P695 - Conclusion

  • Methodology provides a powerful tool for investigating importance of design requirements (not just the R factor) to collapse performance of seismic-force-resisting systems

ATC-84 Improved Seismic Performance Factors for Seismic Design of New Buildings


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But, if all else fails .......

  • Rely on traditional methods

R

3/8RW

RW

0.7R

W0

Cd

I

ATC-84 Improved Seismic Performance Factors for Seismic Design of New Buildings


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