Use of Concrete Filled Tube Vertical Braces in a ModerateHigh Seismic Area By: John W' Oleksik, P'E'

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Use of Concrete Filled Tube Vertical Braces in a
Moderate/High Seismic AreaBy John W. Oleksik,
P.E.,S.E.Timothy M. Gilbert, P.E.,S.E.Louis
Perry Associates, Wadsworth, OH Sanj R.
Malushte, PhD, P.E., S.E. Bechtel Power
Corporation, Frederick, MD

• SEI 2009 Structures Congress
• 2009 May 01

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Outline

• Motivation for Using CFT Braces
• Vertical Brace Shape Comparison
• Code Interpretations and Design Choices
• 3D Analytical Model
• Summarize Advantages of CFT Braces

SEI 2009 Structures Congress 2009 May 01
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Introduction

• Boiler Support Structure
• 200 x 300 x 280 tall
• Seismic Design Category D
• Total Weight 125,000 kips
• 640 Vertical Brace Members

SEI 2009 Structures Congress 2009 May 01
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Motivation for CFT Braces

• Brace size is controlled by compression demand,
  but its strength in tension is typically much
  larger
• Brace connections need to be designed for
  expected tensile strength of the brace (high Ry
  values exacerbate this problem further)
• The wide disparity between tension and
  compression strength results in inefficient brace
  sizing and difficult brace connections
• CFT braces were an attractive alternative as they
  have good tension-compression strength parity

SEI 2009 Structures Congress 2009 May 01
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Wide Flange Vertical Brace
Design Load 1930 kips Design Length 467
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Built-up Tube Vertical Brace
Design Load 1930 kips Design Length 467
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CFT Vertical Brace
Design Load 1930 kips Design Length 467
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Built-up Steel Tube versus CFT Braces
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Code Interpretation

• Project Codes
• Project designed per 2003 IBC and AISC 341-02
• Frame System SCBF (R6) with CFT Braces rather
  than OCBF or C-CBF
• Design Preferences might vary under Current Codes
  - 2006 IBC and AISC 341-05
• Frame System SCBF and C-CBF (height limits)
  OCBF (NL with R1.5)

SEI 2009 Structures Congress 2009 May 01
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CFT Brace Design Choices

• Project Choices
• Nominal Compressive Strength AISC LRFD 99
• Minimum Wall Thickness (b/t) AISC 341-02
• Slenderness Limits AISC 341-02

• Current Choices
• Nominal Compressive Strength AISC 360-05
• Minimum Wall Thickness (b/t) AISC 341-05
• Slenderness Limits AISC 341-05

SEI 2009 Structures Congress 2009 May 01
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Compressive vs. Tensile Stiffness
Stiffness of brace varies with direction of
load. Concrete adds to stiffness in compression

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Compressive vs. Tensile Stiffness

• Design challenges of stiffness difference
• Analysis software did not consider variable
  stiffness with dynamic analysis
• Develop procedure to use constant stiffness
  braces for dynamic analysis

SEI 2009 Structures Congress 2009 May 01
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Compressive vs. Tensile Stiffness

• Design Method
• Part I Static analysis of single column lines
  where
• Co-linear members are used for braces
• A steel member resists tension and compression
• A concrete member resists compression only
• Determine portion of load carried by concrete and
  portion by steel

SEI 2009 Structures Congress 2009 May 01
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Compressive vs. Tensile Stiffness

• Design Method (continued)
• Part II Static analysis of single column lines
  where
• Single members are used for braces
• Determine load carried by each brace

SEI 2009 Structures Congress 2009 May 01
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Compressive vs. Tensile Stiffness

• Part III Compare Results
• Deflections of two models are within 1
  differential deflections not significant
• Composite braces carry larger compression
  load
• Establish overload factor how much more
  compression composite braces resist

SEI 2009 Structures Congress 2009 May 01
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Compressive vs. Tensile Stiffness

• Part III Compare Results (continued)
• On average composite brace carried 7 more load
  with 13 standard deviation. Use 1.20 factor
• A second test model created
• One line of full 3-D (static) model is replaced
  with co-linear braces
• Results agree with 2-D modeling results

SEI 2009 Structures Congress 2009 May 01
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Compressive vs. Tensile Stiffness

• Part IV Design
• Design brace for 3-D model load amplified by
  overload factor
• Design steel shell for full load in tension
• Design composite section for full load in
  compression using AISC I2
• Design shear studs to transfer compression to
  concrete from connection plate

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CFT Brace Construction
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CFT Brace Advantages vs. HSS
Less steel in shell and connections
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CFT Brace Advantages vs. HSS

• Improved Strength and Stiffness
• Confinement due to steel shell improves concrete
  resistance to load
• Concrete fill reduces potential for local
  buckling of shell

SEI 2009 Structures Congress 2009 May 01
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CFT Brace Advantages vs. HSS

• Improved Fracture Resistance and Ductility
• Concrete fill reduces potential for inward
  buckling of shell
• Near equal compression and tensile strengths
  improve behavior under dynamic load

SEI 2009 Structures Congress 2009 May 01
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CFT Brace Advantages vs. HSS

• Erection Efficiency
• Fewer bolts less erection time
• Concrete fill can be done on or off site

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CFT Brace Advantages vs. HSS

• Improved Fire Resistance
• Concrete fill reduces need for fire proofing
  material
• Might not apply to all projects

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Thank you for your attention!
SEI 2009 Structures Congress 2009 May 01