4/16/2008
Design
g of Concentricallyy
Braced 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
braces
„
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 Frames
Column Axial Load Distribution
Special Concentrically Braced Frames
Column Axial Load Distribution
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Special Concentrically Braced Frames
Beam Design – Axial Load
Special Concentrically Braced Frames
Beam Design: Flexure
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Special Concentrically Braced Frames
Basic Design Procedure
„
„
„
„
Calculate the demand based on ASCE 7
Analyze the structure; find brace forces
Size the fuses i.e. braces
Capacity design other non yielding members
Special Concentrically Braced Frames
Basic Design Procedure
4 Capacit
4.
Capacity design other members
„
„
„
Use expected brace capacity
Eliminate conservative design assumptions
Do not use φ factors for expected strength
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Requirements for Member Design
Slenderness
„
„
„
KL / r ≤ 4 E / Fy
Bracing
B
i member
b slenderness
l d
Braces with 4 E / Fy ≤ KL / r ≤ 200 is permitted in
frames where columns are designed for Ry times
nominal strength of the brace elements
This load need not exceed the axial loads from
inelastic analysis or the max load that can be
developed by the system
Special Concentrically Braced Frames
Slenderness
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Requirements for Member Design
Brace Effective Length
Requirements for Member Design
Brace Effective Length
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Requirements for Member Design
Brace Effective Length: End Fixity
Requirements for Member Design
Brace Effective Length
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Requirements for Member Design
Required Strength
„
If UAnt<Agross
‰
‰
then Fu(UAnt)>RyFyAg
Max load indicated by analysis that can be
transferred to the brace by the system
Requirements for Member Design
Lateral 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
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 Design
Lateral Force Distribution
0.3V ≤ Tension ≤ 0.7V
0.3V ≤ Tension ≤ 0.7V
0.3V ≤ Compression ≤ 0.7V
0.3V ≤ Compression ≤ 0.7V
OK
OK
Requirements for Member Design
Width-Thickness Limitations
„
Members to be seismically compact. Follow
requirements of Table II-8-1
81
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Requirements for Member Design
Local Buckling
Design of SCBF Connections
„
Connections to be designed for expected
yield 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 fifixed)
d)
φRn ≥ 1.1R y M p
„
Design compressive strength of the
connection if pinned along with proper
detailing
φRn ≥ 1.1R y Pn
Design of SCBF Connections
2t Offset
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Design of SCBF Connections
2t Offset @ Concrete Filled Deck
Design of SCBF Connections
Tearing of Gusset: No Hinge Zone
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Design of SCBF Connections
Folding of Gusset: Hinge Zone
Design of SCBF Connections
Gusset Compression
„
Estimate the max. compression
p
force from
the brace:
„
„
„
„
Consider true brace length
Consider connection fixity
Consider material overstrength
Shortc t Tension strength is always
Shortcut:
al a s greater
than compression strength
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Design of SCBF Connections
Gusset Compression
Design of SCBF Connections
Gusset Compression
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Design of SCBF Connections
Gusset Compression
Design of SCBF Connections
Gusset Compression
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Design of SCBF Connections
Design of gussets
Design of SCBF Connections
Design of gussets: Uniform Force Method
θ
Ruc
Pu
β
β
wp
eb
Aub
Pusinθ-Aub
Rub
Ruc+Rub-Pucosθ
ec
α
α
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Design of SCBF Connections
Design of gussets: Uniform Force Method
P
ec
β
u
Vuc =
Vuc
Huc
H ub =
θ
Hub
Vub
α
r
Pu
H uc =
Pu
r
e
Vub = b Pu
r
where
Vub
Rub -Vub
Pu
r
Pu sinθ -Aub
r=
(α + ec )2 + (β + eb )2
Pu sinθ -Aub -Hub
Rub
Design of SCBF: Specials for Chevron
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4/16/2008
Design of SCBF: Specials for Chevron
Design of SCBF: Specials for Chevron
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