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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
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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
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Special Concentrically Braced Frames
Special Concentrically Braced Frames
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Special Concentrically Braced FramesColumn Axial Load Distribution
Special Concentrically Braced FramesColumn Axial Load Distribution
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Special Concentrically Braced FramesBeam Design – Axial Load
Special Concentrically Braced FramesBeam Design: Flexure
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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
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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
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Requirements for Member Design
Brace Effective Length
Requirements for Member DesignBrace Effective Length
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Requirements for Member DesignBrace Effective Length: End Fixity
Requirements for Member DesignBrace Effective Length
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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
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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
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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
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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
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