STEEL FRAMED ROCKING STRUCTURAL SYSTEMS FOR MODERATE SEISMIC ZONES

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STEEL - FRAMED ROCKING STRUCTURAL SYSTEMS FOR
MODERATE SEISMIC ZONES
ASCE Structures Congress - May 2, 2009 - Austin,
Texas

• Matt Eatherton Graduate Research Assistant,
  University of Illinois
• Jerome F. Hajjar Professor, University of
  Illinois
• Gregory G. Deierlein Professor, Stanford
  University
• Xiang Ma Graduate Research Assistant, Stanford
  University
• Sarah Billington Professor, Stanford University
• Helmut Krawinkler Professor, Stanford University

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Acknowledgments
Gregory G. Deierlein, Sarah Billington, Helmut
Krawinkler, Xiang Ma, Alejandro Pena, Eric
Borchers, Stanford University Jerome F. Hajjar,
Matthew Eatherton, Noel Vivar, Kerry Hall,
University of Illinois Toru Takeuchi, Tokyo
Institute of Technology Tsuyoshi Hikino, Hyogo
Eq. Engineering Research Center Mitsumasa
Midorikawa, Hokkaido University David Mar,
Tipping Mar Associates and Greg Luth,
GPLA In-Kind Funding Tefft Bridge and Iron of
Tefft, IN, MC Detailers of Merrillville, IN,
Munster Steel Co. Inc. of Munster, IN,
Infra-Metals of Marseilles, IN, and
Textron/Flexalloy Inc. Fastener Systems Division
of Indianapolis, IN.
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General Overview

• Conventional braced frame absorbs seismic energy
  through compression buckling and tension yielding
  of braces.
• Buckled braces exert unbalanced loads in chevron
  configurations.
• Also, there is concern about brace fracture due
  to low cycle fatigue and fracture at connection
  regions.
• Allowing uplift can limit the amount of force
  applied to braces and connections and can
  preclude undesirable limit states.
• Goal of this study is to examine uplift and
  rocking for application in moderate seismic
  zones.
• This study is related to an investigation on
  rocking of steel frames applied to high seismic
  zones to achieve higher performance goals
  involving repairability.

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Literature Review
From Clough and Huckleridge 1977
From Wada et al. 2001
From Pollino and Bruneau 2007
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Literature Review
From Takamatsu et al. 2006
From Midorikawa et al. 2006
From Ikenaga et al. 2006
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Prototype Structure

• SAC Configuration
• Site was chosen in Boston, MA
• Site Class D
• Codes used ASCE 7-05, AISC 341-05, and AISC
  360-05

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Frame Design
Ordinary Concentrically Braced Frame R3.25 for
Ordinary Concentrically Braced Frames (OCBF) Top
floor is chevron configuration requiring
substantial roof beam to support unbalanced loads
Rocking Frame Design Rocking frame would have
more ductility Use same member designs as OCBF
(Assume higher R factor and amplified seismic
loads approximately cancel each other
out). Columns and connections could be reduced
compared to the OCBF Design yielding element for
R5 (Assumed) Gravity load greater than yield
force keeps frame from stepping up
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Designed Frames
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Computational Model of Braces
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Computational Model of Frames
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Additional Info on Modeling

• Beams and columns are elastic beam-column
  elements
• Braces are fiber sections with steel material
• Gap elements are stiff in compression and zero
  stiffness in tension

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Pushover Analysis Comparison
1st Floor Tension Brace Yields
2nd Floor Tension Brace Yields
R3.25 Design Force
1st Floor Comp. Brace Buckles
3rd Floor Comp. Brace Buckles
R5 Design Force
Corner of Rocking Frame Uplifts
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Computational Model of Frames
Rocking Frame Peak 1st Floor Tension Brace Force
Rocking Frame Peak 1st Floor Comp. Brace Force
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Deformed Shapes / Configurations
Yielded Braces
Elastic Elements
Buckled Braces
Yielding Elements
Ordinary Concentrically Braced Frame
Rocking Frame
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Related Controlled Rocking Project for High
Seismic
Replaceable Fuse Elements Absorb Majority of
Damage and Dissipate Energy
Post-Tensioning Provides Self-Centering
Capabilities by Bringing the Frame Back Down
after Rocking
Braced Frames Remain Essentially Elastic and are
Allowed to Uplift at the Base
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Controlled Rocking Project Phases

• Schematic design to define feasible
  configurations and schematic construction
  details.
• SDOF study to examine the characteristics of the
  flag-shaped hysteresis loop and study the
  proportioning of the system.
• Parametric study using an MDOF model to identify
  key variables and their effect on the system
  response.
• Fuse development through analysis and large-scale
  testing.
• Large-scale quasi-static cyclic and hybrid
  simulation tests of the rocking frame.
• Large-scale shake table testing.
• Development of design recommendations to enable
  practical implementation in practice.

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Half Scale Cyclic and Hybrid Simulation Tests at
UIUC
Loading and Boundary Condition Box (LBCB)
Biaxial Load Cell Pins
Replaceable Energy Dissipating Fuses
Steel Frames that Remain Elastic, but are Allowed
to Uplift
Post-Tensioning Strands Provide Self-Centering
Bumpers Restrain Horizontal Movement
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E-Defense Tests in August 2009
Single Frame Configuration Compact Fuse Frame at
Base Final Test Uses Buckling Restrained Brace as
Replaceable Fuse Element
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Conclusions

• Pushover analyses were conducted to demonstrate
  that rocking can protect other members and
  connections from undesirable limit states.
• Uncertainty related to premature fracture is
  avoided.
• Uplifting braced steel frames have more ductility
  than OCBFs.
• Higher ductility can mean larger response
  modification factor. Although braces might be
  designed to preclude buckling, other members and
  connections might be reduced compared to OCBFs.
• Yielding elements at the uplifting base dissipate
  energy. Several examples of uplifting bases can
  be found in the literature.
• If the gravity load is greater than the yielding
  element strength then the frames will also
  self-center after the earthquake.
• Similar work being conducted for high seismic
  zones highlights the potential for rocking
  systems.