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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

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TTSS LD

a

9

“1g” Building

W

V b=

1g

M

10

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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49

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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