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4/16/2008 1 Design of Concentrically Braced Frames Anindya Dutta, Ph.D., S.E. Example Configurations X-Braced Inverted V (Chevron) 2 Story X-Braced

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Page 1: Design of Concentrically Braced · PDF fileSpecial Concentrically Braced Frames ... detailing φRn ≥1.1RyPn ... Microsoft PowerPoint - Design of Concentrically Braced Frames.ppt

4/16/2008

1

Design of Concentrically g yBraced Frames

Anindya Dutta, Ph.D., S.E.

Example Configurations

X-Braced Inverted V (Chevron) 2 Story X-Braced

Page 2: Design of Concentrically Braced · PDF fileSpecial Concentrically Braced Frames ... detailing φRn ≥1.1RyPn ... Microsoft PowerPoint - Design of Concentrically Braced Frames.ppt

4/16/2008

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Example Configurations

V (Inverted Chevron) Zipper

Special Concentrically Braced Frames

Primary location of energy dissipation are the bracesbraces

Braces dissipate energy by tension yielding and compression buckling

Page 3: Design of Concentrically Braced · PDF fileSpecial Concentrically Braced Frames ... detailing φRn ≥1.1RyPn ... Microsoft PowerPoint - Design of Concentrically Braced Frames.ppt

4/16/2008

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Special Concentrically Braced Frames

Special Concentrically Braced Frames

Page 4: Design of Concentrically Braced · PDF fileSpecial Concentrically Braced Frames ... detailing φRn ≥1.1RyPn ... Microsoft PowerPoint - Design of Concentrically Braced Frames.ppt

4/16/2008

4

Special Concentrically Braced FramesColumn Axial Load Distribution

Special Concentrically Braced FramesColumn Axial Load Distribution

Page 5: Design of Concentrically Braced · PDF fileSpecial Concentrically Braced Frames ... detailing φRn ≥1.1RyPn ... Microsoft PowerPoint - Design of Concentrically Braced Frames.ppt

4/16/2008

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Special Concentrically Braced FramesBeam Design – Axial Load

Special Concentrically Braced FramesBeam Design: Flexure

Page 6: Design of Concentrically Braced · PDF fileSpecial Concentrically Braced Frames ... detailing φRn ≥1.1RyPn ... Microsoft PowerPoint - Design of Concentrically Braced Frames.ppt

4/16/2008

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Special Concentrically Braced FramesBasic Design Procedure

Calculate the demand based on ASCE 7Analyze the structure; find brace forcesSize the fuses i.e. bracesCapacity design other non yielding members

Special Concentrically Braced FramesBasic Design Procedure

4 Capacit design other members

Use expected brace capacityEliminate conservative design assumptionsDo not use φ factors for expected strength

4. Capacity design other members

Page 7: Design of Concentrically Braced · PDF fileSpecial Concentrically Braced Frames ... detailing φRn ≥1.1RyPn ... Microsoft PowerPoint - Design of Concentrically Braced Frames.ppt

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Requirements for Member Design

B i b l d

Slenderness

Bracing member slenderness Braces with is permitted in frames where columns are designed for Ry times nominal strength of the brace elementsThis load need not exceed the axial loads from inelastic analysis or the max load that can be

yFErKL /4/ ≤

200//4 ≤≤ rKLFE y

inelastic analysis or the max load that can be developed by the system

Special Concentrically Braced FramesSlenderness

Page 8: Design of Concentrically Braced · PDF fileSpecial Concentrically Braced Frames ... detailing φRn ≥1.1RyPn ... Microsoft PowerPoint - Design of Concentrically Braced Frames.ppt

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Requirements for Member Design

Brace Effective Length

Requirements for Member DesignBrace Effective Length

Page 9: Design of Concentrically Braced · PDF fileSpecial Concentrically Braced Frames ... detailing φRn ≥1.1RyPn ... Microsoft PowerPoint - Design of Concentrically Braced Frames.ppt

4/16/2008

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Requirements for Member DesignBrace Effective Length: End Fixity

Requirements for Member DesignBrace Effective Length

Page 10: Design of Concentrically Braced · PDF fileSpecial Concentrically Braced Frames ... detailing φRn ≥1.1RyPn ... Microsoft PowerPoint - Design of Concentrically Braced Frames.ppt

4/16/2008

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Requirements for Member DesignRequired Strength

If UAnt<Agrossthen Fu(UAnt)>RyFyAgMax load indicated by analysis that can be transferred to the brace by the system

Requirements for Member DesignLateral Force Distribution

All compression or tension system (generally not allowed)

Sum of horz. Comp. on either compression or tension ≥0.7V NG

• Along any line of bracing at least 30% but not more than• Along any line of bracing at least 30% but not more than 70% of the force is to be resisted by brace in tension

• Exception allowed when compression braces are designed for Amplified (Ω) load combinations of ASCE 7

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Requirements for Member DesignLateral Force Distribution

0.3V ≤ Tension ≤ 0.7V 0.3V ≤ Tension ≤ 0.7V

0.3V ≤ Compression ≤ 0.7V

OK

0.3V ≤ Compression ≤ 0.7V

OK

Members to be seismically compact. Follow requirements of Table I 8 1

Requirements for Member DesignWidth-Thickness Limitations

requirements of Table I-8-1

Page 12: Design of Concentrically Braced · PDF fileSpecial Concentrically Braced Frames ... detailing φRn ≥1.1RyPn ... Microsoft PowerPoint - Design of Concentrically Braced Frames.ppt

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Requirements for Member DesignLocal Buckling

Design of SCBF Connections

Connections to be designed for expected yield strength of member in tension RyFyAgyield strength of member in tension RyFyAg

This force need not exceed the max load indicated by analysis that can be transferred to the brace by the system

Page 13: Design of Concentrically Braced · PDF fileSpecial Concentrically Braced Frames ... detailing φRn ≥1.1RyPn ... Microsoft PowerPoint - Design of Concentrically Braced Frames.ppt

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Design of SCBF Connections

Design of SCBF Connections

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Design of SCBF Connections

Pin ended

Fixed ended

Design of SCBF Connections

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Design of SCBF Connections

Design flexural strength of the connection (if fi d)(if fixed)

Design compressive strength of the connection if pinned along with proper

pyn MRR 1.1≥φ

connection if pinned along with proper detailing

nyn PRR 1.1≥φ

Design of SCBF Connections2t Offset

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Design of SCBF Connections2t Offset @ Concrete Filled Deck

Design of SCBF ConnectionsTearing of Gusset: No Hinge Zone

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Design of SCBF ConnectionsFolding of Gusset: Hinge Zone

Estimate the max. compression force from

Design of SCBF ConnectionsGusset Compression

pthe brace:

Consider true brace lengthConsider connection fixityConsider material overstrengthShortc t Tension strength is al a s greaterShortcut: Tension strength is always greater than compression strength

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Design of SCBF ConnectionsGusset Compression

Design of SCBF ConnectionsGusset Compression

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Design of SCBF ConnectionsGusset Compression

Design of SCBF ConnectionsGusset Compression

Page 20: Design of Concentrically Braced · PDF fileSpecial Concentrically Braced Frames ... detailing φRn ≥1.1RyPn ... Microsoft PowerPoint - Design of Concentrically Braced Frames.ppt

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Design of SCBF ConnectionsDesign of gussets

Design of SCBF ConnectionsDesign of gussets: Uniform Force Method

P

Rucθ

Pu

Pusinθ-Aub

β

eb

Rub

wp

Aub

β

ec α α

Ruc+Rub-Pucosθ

Rub

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Design of SCBF ConnectionsDesign of gussets: Uniform Force Method

Pu

Vuc uc

ucuuc PeHPV ==β

θHub

Vub

Huc

Vub

ub

ubuub

uucuuc

PreVP

rH

rr

==α

where

Rub

Pusinθ-AubRub-Vub

Pusinθ-Aub-Hub

( ) ( )22bc eer +++= βα

Design of SCBF: Specials for Chevron

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Design of SCBF: Specials for Chevron

Design of SCBF: Specials for Chevron