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1
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
2
Continuity Plates
Forces Acting on a Continuity Plate
3
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
5
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
21
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)
7
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
8
Controlled Damage Ductility Cost
Location of Structural Damage:
• Anywhere?• At Selected Locations?
Challenge 1
• For Non‐seismic Loadings
Plastic Design
9
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
10
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
11
Seismic Compactness Requirements
Note: Not Intended to Preclude Local Buckling
Courtesy: A. Chen (Thornton Tomasetti, Los Angeles)
12
Courtesy: P. Lee/R. Garai(SOM, San Francisco)
13
Courtesy: P. Lee/R. Garai(SOM, San Francisco)
RBS Connection with Built-up Box Column
14
Electroslag Welding of Diaphragm Plates
15
Notch Condition at ESW Joint
Buckling-Restrained Braced Frame (I. Aiken, SEI)
16
Wilshire Grand Center (I. AIken, SEI)
Near-Field Ground Motion Effect
17
AISC Research on Shallow (W14) Columns
NIST/ATC Research on Deep (W24) Columns
Column Behavior
18
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
19
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
20
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.
21
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
22
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
23
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)
24
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
25
• 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”
26
“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]
27
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
28
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
29
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
30
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
31
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
32
Critical Non-Design Issues
Detailing Welding QC/QA