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What Seismic Steel Design Is All About

Chia-Ming UangUniversity of California, San Diego

34th annual SFNE Steel Design Conference, WPI

June 5, 2015

2015 AISC T.R. Higgins Lecture

AISC Engineering Journal (2013)

“A Flexibility‐Based Formulation for the Design ofContinuity Plates in Steel Special Moment Frames”

ANDY T. TRAN, PATRICK M. HASSETT and CHIA‐MING UANG

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Continuity Plates

Forces Acting on a Continuity Plate

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Scope of Presentation

Motivation of Presentation

1990

1992

1997

2002

2005

2010

0

50

100

150

200

250

No. of P

ages

Seismic ProvisionsCommentary

No.

of P

ages

4

Loadings Code     (Demand Side)ASCE 7: Minimum Design Loads for

Buildings and  Other Structures

Materials Code    (Capacity Side)AISC 360: Specification for Structural Steel

Buildings

Non‐Seismic Steel Design

Loadings Code                     (Demand Side)• ASCE 7

Materials Code (Capacity Side)• AISC 360

Seismic Steel Design

• AISC 341 (AISC Seismic Provisions)• AISC 358 (Prequalified SMF/IMF Connections)• AWS D1.8

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Loadings Code                     • ASCE 7

Materials Code• AISC 360• AISC 341 • AISC 358• AWS D1.8

Coupling between ASCE 7 & AISC 341

Coupled by R‐Factor (Response Modification Factor)

• Equivalent Lateral Force (ELF) Procedure

• Design Earthquake

R‐Factor in ASCE 7

To TS 1.0 TL

Period (sec)

Spe

ctra

l Acc

eler

atio

n, S

a(g

)

SD

1S

DS

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T

TSS LD

a

6

Elastically Designed “1g” Building

W = 1g×MV

b=

WW = 1g×M

Vb = W

“Understanding Seismic Design through a Music Analogy,” by Gilsanz and Vancura (March 2015)

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Seismic Design & Music Analogy

Seismic Spectrum 

Soil

Building                               

Score

Musician      

Instrument                

• Design for a Reduced Seismic Base Shear to Achieve 

Economy

• Expect Structural Damage in a Controlled Manner  While Achieving Life Safety

Conventional Design: A Compromise

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Controlled Damage             Ductility             Cost

Location of Structural Damage:

• Anywhere?• At Selected Locations?

Challenge 1

• For Non‐seismic Loadings

Plastic Design

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Target Plastic Mechanism for Each Seismic Force‐

Resisting System Is Pre‐Determined in AISC 341

Consistent with R Value Assigned to Each System

Seismic Design

R

Special Moment Frames 8

Ordinary Moment Frames 3½

Special Concentrically Braced Frames 6

Buckling‐Restrained Braced Frames 8

Target Plastic Mechanisms

SMF SCBF/BRBF EBFF

Target Yield Mechanism

Flexural Yielding Tensile Yielding/Buckling Shear Yielding

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Global vs. Partial Plastic Mechanism

Target                                 To Avoid

Deformation-Controlled Element (DCE)

Stringent Ductility Requirements• Limit Fy• Limit Slenderness Ratios to Delay Buckling and Reduce Rate of Postbuckling Strength Degradation (AISC 341 Section D) 

SMF SCBF/BRBF EBFF

DCE

DCE

DCE

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Seismic Compactness Requirements

Note:  Not Intended to Preclude Local Buckling

Courtesy: A. Chen (Thornton Tomasetti, Los Angeles)

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Courtesy: P. Lee/R. Garai(SOM, San Francisco)

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Courtesy: P. Lee/R. Garai(SOM, San Francisco)

RBS Connection with Built-up Box Column

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Electroslag Welding of Diaphragm Plates

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Notch Condition at ESW Joint

Buckling-Restrained Braced Frame (I. Aiken, SEI)

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Wilshire Grand Center (I. AIken, SEI)

Near-Field Ground Motion Effect

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AISC Research on Shallow (W14) Columns

NIST/ATC Research on Deep (W24) Columns

Column Behavior

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Shallow Column Cyclic Behavior

• W14×175 (P = 0.75Py)• At 10% Interstory Drift 

Angle

Motivation for Using Deep Column

To Meet Code‐Specified Story Drift Limit Example:

Sectionweight(lb/ft)

rx

(in.)ry

(in.)Ix

(in4)Iy

(in4)

W14×605 605 7.8 4.55 10800 3680

W27×258 258 11.9 3.36 10800 859

d

bf

tw

tf

W27×258

d tw t

f

bfW14×605

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NIST/ATC Project

0 2 4 6 8 100

10

20

30

40

50

60

W24x176W24x131W24x104W24x84W24x55

Flange Width‐Thickness Ratio, bf/2tf

Web Slenderness Ratio, h/t

w

For P/(cPy)= 0.20.40.6

Gr. 1

Gr. 2

Gr. 3Gr. 4

Gr. 5

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Group 2 Specimens

-6 -2 0 2 4 6-1500

-1000

-500

0

500

1000

1500

-6 -2 0 2 4 6-1500

-1000

-500

0

500

1000

1500

-6 -2 0 2 4 6-1500

-1000

-500

0

500

1000

1500

-6 -2 0 2 4 6-1500

-1000

-500

0

500

1000

1500

SDR (%)

SDR (%)

SDR (%)

SDR (%)

Lateral Force (kN)

Specimen 2Z

Specimen 2L

Specimen 2M

Specimen 2H

Increasing Axial Load

Group 2 Specimens

0

2

4

6

8

2Z 2L 2M 2H

Plastic Rotation (×0.01 rad.)

Increasing Axial Load

0.03 rad.

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Group 5 Specimens

0 2 4 6 8 100

10

20

30

40

50

60

W24x176W24x131W24x104W24x84W24x55

Flange Width‐Thickness Ratio, bf/2tf

Web Slenderness Ratio, h/t

w

Gr. 1

Gr. 2

Gr. 3Gr. 4

Gr. 5

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Group 1 Specimens

0 2 4 6 8 100

10

20

30

40

50

60

W24x176W24x131W24x104W24x84W24x55

Flange Width‐Thickness Ratio, bf/2tf

Web Slenderness Ratio, h/t

w

Gr. 1

Gr. 2

Gr. 3Gr. 4

Gr. 5

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Nonlinear Analysis Is Required Because Member  Yielding/Buckling Is Expected

SOLUTION: R‐Factor Based Equivalent Lateral Force(ELF) Procedure

Challenge 2

Multistory Frames

Vb

E

F1

F2

F3

ib FV

Pushover Analysis

SFirst Significant Yield

(Design EQ Level)

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Physical Meaning of R-Factor

VS

Vb

E

S

S

Ve

o

RIII

I

II

R

• It Greatly Simplifies the Design Process

• Design Base Shear (V) Does not Represent the Real Earthquake Loading.

•

Note on R-Factor Based ELF Procedure

oR R

Redundancy Factor in ASCE 7

Counting on AISC 341

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• How to Ensure a Target Yield Mechanism?

• SOLUTION: Capacity Design Concept

Challenge 3

SMF SCBF/BRBF EBFF

Force‐Controlled Element (FCE) vs. 

Deformation‐Controlled Element (DCE)

FCE vs. DCE (ASCE 41 Terminology)

SMF SCBF/BRBF EBFF

DCE

DCE

DCE

FCE to Remain Essentially Elastic under 

“Seismic Overload” 

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“Seismic Overload”

Seismic Force Level III

VS

Vb

E

S

Ve

o

RIII

I

II

R

“Seismic Overload”: Code Language

“Amplified Seismic Load” “Seismic Load Effect Including (System) 

Overstrength Factor”[Emh for the Horizontal Component]

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Two Pillars in Seismic Design

Component Seismic Force Level

AISC 341 Coverage

Ductility Design

DCE II

Capacity Design

FCE III More

Problem in Capacity Design

How to Avoid Nonlinear Analysis?

VS

Vb

E

S

Ve

o

RIII

I

II

R

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The horizontal seismic load effect with over‐strength factor, Emh, shall be determined as follows: 

Emh = ΩoQE (12.4‐7)

EXCEPTION: The value of Emh needs not exceed the 

maximum force that can develop in the element as 

determined by a rational, plastic mechanism analysis or 

nonlinear response analysis utilizing realistic expected

values of material strengths.

Two Approaches for Capacity Design

Global Approach

Local Approach

(ASCE 7 Sect. 12.4.3.1)

When the DCE Is Next to a FCE Apply Statics at “Local” Level Seismic Force Level II not Needed Use the Expected Material Strength of DCE 

to Compute the Demand (i.e., Required Strength) of FCE

Local Approach

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Expected Material Strength

• AISC 341 Sect. A3.2

• Expected Yield Stress:  yyye FRF

Example 1: SCBF Brace Connection

1.14

y y g

cre g

T R F A

C F A

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Example 2: SCBF Beam Design

T

0.3C

Unbalanced Beam Vertical Force in Post‐Buckling Region:

T

C

Global Approach

Use It When Local Approach Cannot Be Applied Easily

An “Elastic” Approach

Use o to Amplified Seismic Force Levels II to III

VS

Vb

E

S

Ve

o

RIII

I

II

R

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Example 1: Column Design in SMF

oF3

oF2

oF1

Pu

o

SMF 3

SCBF 2

Example 3: SCBF Beam Design

oF

Beam Unbalanced Force Will not Be Captured Think beyond Elastic Mentality

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Critical Non-Design Issues

Detailing Welding QC/QA