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Underlying Concepts in Seismic Design Codes
Chia-Ming UangProfessor
University of California, San Diego
2009 NASCC: The Steel Conference
Recorded video at http://www.aisc.org/content.aspx?id=26268
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Seismic Loadings Codes
• 1985 UBC (K Factor)• 1988 UBC (Rw Factor)• 1997 UBC (R Factor)• ASCE 7, IBC (R Factor)
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Steel Materials Codes
• 1985 UBC• 1988 UBC
<1994 Northridge Earthquake>• 1997 UBC• FEMA 350• Seismic Provisions (AISC 341)• Prequalified Connections (AISC 358)• AWS D1.8
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Objective of Presentation
• Fundamental Concepts Underlying theSeismic Provisions
• Why and How These Concepts Are Implemented in AISC Seismic Provisions
• not to Elaborate Detailed Design Provisions of any Particular System
• Some Popular Systems Will be Used to Demonstrate the Concepts
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Basic Load Combinations (ASCE 7-10)
1. 1.4D2. 1.2D + 1.6L + 0.5(Lr or S or R)3. 1.2D + 1.6(Lr or S or R) + (L or 0.5W)4. 1.2D + 1.0W + L + 0.5(Lr or S or R)5. 1.2D + 1.0E + L + 0.2S6. 0.9D + 1.0W7. 0.9D + 1.0E
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Earthquake “Load”
• Earthquake-Induced Inertia Effect on Structures
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Elastic Response Spectra
Late
ral D
ispl
., S
dS
pect
ral A
ccel
., S
a
Period (sec)
d
2
a ST2S
Sd
M
K
Max. Member Force = M Sa
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Design Basis Earthquake (ASCE 7)
TSS D
a1
To TS 1.0 TL
Period (sec)
Spe
ctra
l Acc
eler
atio
n, S
a(g
)S
D1
SD
S
21
TTSS LD
a
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“1g” Building
W
V b=
1g
M
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Resort to DUCTILITY
(or Trade Ductility for Strength)
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Ductility Factor
M
K
e
Ve = W(Sa)
y m
Vy
y
m
FactorDuctility
Base Shear, V
K1/R
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Newmark-Hall Ductility Reduction Rule
my
Vy
Base Shear, V
Ve
1/R
Equal Displacement Rule
Ductility Reduction Factor:
R
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Seismic Design Concept 1Ductility Design
• A Reduced Design Seismic Force Can Be Used IF Sufficient Ductility Is Built into the Structure
• But Only a Certain Elements Are Strategically Designated to Serve as Structural Fuses, i.e., Deformation-Controlled Elements (DCE)
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Example
• Diagonal Braces as Structural Fuse
• Braces to Buckle Out of Plane
• To Achieve This, More Effort Is Needed to Make It Happen!
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Seismic Design Concept 2”Capacity Design”
• Remaining Part of the Structure Is Designed to Remain Elastic, i.e., Designed These Elements as Force-Controlled Elements (FCE)
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Two Key Concepts in AISC Seismic Provisions (AISC 341)
Ductility Design Provisions +
Capacity Design Provisions
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Ductility vs. Capacity Design
RequiresUnderstanding/Judgment
Easier(Straightforward)
DesignEffort
MoreResearchEffort
Capacity Design(Force-Controlled
Elements)
Ductility Design(Deformation-Controlled
Elements)
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SCBF Design Provisions (AISC 341-05)
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SCBF Design Provisions (AISC 341-10)F2. Special Concentrically Braced Frames (SCBF) . . . . . . . . . . 9.1–217
1. Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. .9.1–217
2. Basis of Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. .9.1–217 3. Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. .9.1–219 4. System Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9.1–222
4a. Lateral Force Distribution . . . . . . . . . . . . . . . . . . . . . . . . . . .. .9.1–2224b. V- and Inverted V-Braced Frames . . . . . . . . . . . . . . . . . . . . ..9.1–222 4c. K-Braced Frames . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9.1–223 4d. Tension-Only Frames . . . . . . . . . . . . . . . . . . . . . . . . . .. . . .. . .9.1–223
5. Members . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9.1–2235a. Basic Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9.1–223 5b. Diagonal Braces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9.1–2245c. Protected Zones . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . .9.1–225
6. Connections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. .9.1–2256a. Demand Critical Welds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9.1–225 6b. Beam-to-Column Connections . . . . . . . . . . . . . . . . . . . . .. .. . .9.1–2276c. Required Strength of Brace Connections . . . . . . . . . . . . . . . .9.1–2286d Column Splices 9 1–230
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AISC 341-05
• Moment Frames (Sections 9, 10, 11)• Special Truss Moment Frames (Section 12)• Concentrically Braced Frames (Sections 13,14)• Eccentrically Braced Frames (Section 15)• Buckling-Restrained Braced Frames (Section 16)• Special Plate-Shear Walls (Section 17)
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AISC 341-10
E. MOMENT-FRAME SYSTEMSE1. Ordinary Moment Frames (OMF) E2. Intermediate Moment Frames (IMF) E3. Special Moment Frames (SMF) E4. Special Truss Moment Frames (STMF)E5. Ordinary Cantilever Column Systems (OCCS)E6. Special Cantilever Column Systems (SCCS)
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AISC 341-10
F. BRACED-FRAME AND SHEAR-WALL SYSTEMSF1. Ordinary Concentrically Braced Frames (OCBF) F2. Special Concentrically Braced Frames (SCBF)F3. Eccentrically Braced Frames (EBF) F4. Buckling-Restrained Braced Frames (BRBF)F5. Special Plate Shear Walls (SPSW)
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Ductility Design Concept
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Target Yield Mechanism
Moment FrameMoment Frame Concentrically Concentrically Braced FrameBraced Frame
Eccentrically Eccentrically Braced FrameBraced Frame
F
Target Yield Mechanism
Flexural Yielding Tensile Yielding/Buckling Shear Yielding
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Ductility Requirements
Code Implementation Example 1:Special Moment Frame (SMF) Design
(Courtesy:M.D. Engelhardt)
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Steel Moment Connections
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RBS Moment Connection
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RBS Moment Connection
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Dynamic Testing of Pre-Northridge Moment Connection
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RBS Moment Connection Response
Load
(kip
s)
-5 0 5
-300
-200
-100
0
100
200
300
Deflection (in)
-0.04 -0.02 0.0 0.02 0.04Story Drift Ratio
Deflection (in)
Story Drift RatioLo
ad (k
ips)
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Local Buckling Control
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Local Buckling Control (AISC 341-05)
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Local Buckling Control (AISC 341-10)
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Lateral-Torsional Buckling
yyb FErL /086.0
AISC SP §9.8:
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Panel Zone
90/)( zz wdt
AISC SP §9.3:
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Protected Zone (AISC SP §9.3)
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Ductility Requirements
Code Implementation Example 2:Special Concentrically Braced Frame (SCBF) Design
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Target Yield Mechanism
(Courtesy: K.C. Tsai, NCREE)
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Bracing Ductility Requirements
• Bracing Buckling (SP §13.2a)
y
s
FE
rKL 4
max
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Bracing Ductility Requirements
• Local Buckling (SP §8.2b): Seismically Compact
(Courtesy: K.C. Tsai)
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Gusset “2t” Requirement
(Courtesy: K.C. Tsai)
SCBF
>2t
OCBF
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Gusset “2t” Requirement
>2t
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Ductility Requirements
Code Implementation Example 3:Eccentrically Braced Frame (EBF) Design
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EBF Configuration
Structural Fuse: Links
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Link Ductility Requirement
e
L
p
heL p
p
h
Plastic Deformation Demand
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Link Ductility Requirements
• Link Deformation Capacity Depends on (Seismically) Compactness Length Link Stiffeners
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Link Length Effect
(Courtesy: M.D. Engelhardt)(Courtesy: M.D. Engelhardt)
(AISC SP §15.2c)
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Capacity Design Concept
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Ductility vs. Capacity Design
Requires Understanding/Judgment
Easier(Straightforward)
DesignEffort
MoreResearchEffort
Capacity Design(Force-Controlled
Elements)
Ductility Design(Deformation-Controlled
Elements)
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ASCE 7 Seismic Performance Factors
3 Mysterious Factors: R, Cd, and o
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Newmark-Hall Ductility Reduction Rule
my
Vy
Base Shear, V
Ve
1/R
Equal Displacement Rule
Ductility Reduction Factor:
R
M
K
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Multistory Frames
F1
F2
F3
ib FV
Vb
E
S
Pushover Analysis
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Multistory Frames
y m
VS
Vb
E
S
S
Vy
Ve
Y
o
R
R
R = Ro
Cd
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Capacity Design Seismic Forces
VS
Vb
E
S
S
Ve
o
RIII
I
II
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Seismic Load Combinations (IBC)
• §16.5.2.1 Basic Seismic Load Combination:1.2D + f1L + f2S + 1.0E
• §1605.4 Special Seismic Load Combination:1.2D + f1L + 1.0Em
Seismic Force Level II Force for Deformation-Controlled Elements (Ductility Design Needed)
Seismic Force Level III Force for Force-Controlled Elements (Capacity Design Needed)
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Internal Force Distribution
• At Seismic Force Level II (Basic Load Combination)Use Elastic Structural Analysis to Determine Internal Force Distribution
• At Seismic Force Level III (Basic Load Combination)Internal Force Re-distribution Occurs due to Nonlinear Response
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Example
FCheck as Compressive Member
Check as Beam-Column
(a) Seismic Force Level II (b) Seismic Force Level III
oF
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Capacity Design
• Think beyond Elastic Response Mentality• Use Expected Material Strength for Estimate
Maximum Force Developed in Structural Fuse(Note: Structural Fuse Material Strength too High Is not Desirable for Seismic Design)
• Two Methods to Calculate Seismic Force Level III for Capacity Design
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Expected Material Strength
• AISC 341-10 §A3.2• Expected Yield Stress, yyye FRF
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Capacity DesignMethod 1
• When the Structural Fuse Is Next to Force-Controlled Element
• Apply Statics at “Local” Level• Seismic Force Level II not Needed• An Upper-Bound Estimate of Seismic Force
Level III
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Example 1: SCBF Bracing Connection
• Bracing is Structural Fuse
• AISC 341-10 §F2.6 Bracing Connection Design
ny
gyy
PRC
AFRT
1.1
Don’t Oversize Structural Fuse!
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Example 1: SCBF Beam Design
• AISC 341-10 §F2.3: Beam Design for V-Type Bracing
n
gyy
PC
AFRT
3.0
T C
Check as Beam-Column
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Example 2: EBF Column Design
• Links Are Structural Fuse
• AISC 341-10 § F3.3 for Column Design
nyVR1.1
Pbr
Pu
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Example 2: EBF Brace Design
• Links Are Structural Fuse
• AISC 341-10 § F3.3 for Beam/Bracing Design
1.25RyVn
e
Don’t Oversize Links
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Example 3: SMF• AISC 358-10 (CPRP)
eyyprpr ZFRCM fM
*pbM
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Capacity DesignMethod 2
• An Approximate (or “Lazy”) Method:o (Seismic Force Level II)
• Use It When Method 1 Cannot Be Applied Easily• Usually Applied at the “Global” (or System)
Level• Can Be Dangerous If Not Properly Applied
Elastic AnalysisASCE 7
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Example 1SCBF Column Design
Pu = ?
Method 1
oF3
oF2
oF1
Pu = ?
Method 2
0.3Pn
0.3Pn
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Example 1SCBF Column Design
oF3
oF2
oF1
Pu 0!Method 2
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ExampleSCBF
FCheck as Compressive Member
Check as Beam-Column
(a) Seismic Force Level II (b) Seismic Force Level III
(Method 2 Will not Work)
oF