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TRANSCRIPT
Structural Steel Design
by Rafael Sabelli, S.E.
Instructional Material Complementing FEMA 1051, Design Examples
Disclaimer
Part 1: Background/Theoretical
• Context in Provisions• Steel behavior• Reference standards and design strength• Moment resisting frames• Braced frames• Other topics• Summary
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Steel Design: Context in Provisions
• Design basis: Strength limit state• Using the 2015 NEHRP Recommended Provisions, Refer to
ASCE 7 2016:– Chap. 11: Seismic Design Criteria– Chap. 12: Seismic Design Requirements (Buildings)– Chap. 13: Nonstructural components – Chap. 14: Design of steel structures
• Refers to AISC Specification (AISC 360-16)• Refers to AISC Seismic (AISC 341-16)
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Seismic Resisting Systems• Unbraced Frames• Connections are:
– Fully Restrained Moment-resisting– Partially Restrained Moment-resisting
• Seismic classes are:– Special Moment Frames– Intermediate Moment Frames– Ordinary Moment Frames – Systems not specifically detailed for seismic response
• Braced Frames• Ordinary Concentric Braced Frames• Special Concentric Braced Frames• Eccentrically Braced Frames• Buckling Restrained Braced Frames• Special Plate Shear Walls• Systems not specifically detailed for seismic response
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Monotonic Stress-Strain Behavior
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Plastic Hinge Formation
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Plastic hinge in beam
Behavior Modes For Beams
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Moment
Deflection
Mp
Mr
O
ML
AI
G
D JF
B
CE
KH
elastic LTB
inelastic LTB
strain hardening
idealized behavior
Flexural Ductility of Steel MembersPractical Limits
• Lateral torsional buckling– Provide sufficient lateral or torsional
bracing• Local buckling
– Limit width-to-thickness ratios for compression elements
• Fracture– Avoid by proper detailing to allow inelastic
strain
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Lateral Torsional Buckling
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0
Me
My
Mp
D
C
B
A E
L
u
Laterally unbraced length
Resisting moment
inelastic buckling
elastic buckling
buckling with
strain-hardening
Local Buckling
ycr tbEk
22
2
)/)(1(12
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yFkE
tb 95.0
b Classical plate buckling solution:
Substituting = 0.3 and rearranging:
t
Width-thickness ratios
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M
Mp
Increasing compactness(decreasing element slenderness)
Slender elementsElastic local buckling
Noncompact elementsInelastic local buckling
Compact elementsYielding
Highly compact elementsSignificant strain
Restraint
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Highly restrained
Example: Beam flange to (thick) column flange T joint
Toughness and energy absorption
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X
Displacement
Load
Low toughness High toughness
Shaded area is the energy absorbed
X
Ductile yielding (or even achieving full strength) can be precluded by failure of low-toughness material
Minimum toughness is required for seismic design at critical locations
The “k region”
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Rotary straightening of flanges strain-ages this area of the web, leading to reduced toughness.
Welds in this area have led to fracture during fabrication.
Welds to this region are proscribed in seismic design
Variability and predictability
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1.0
1.2
1.4
1.6
1.8
2.0
0 10 20 30 40 50 60 70b / t
Rat
io o
f Act
ual t
o M
inim
um
Spec
ified
Yie
ld S
tres
s A500 Gr. BMeanlp
p ps
Specified minimum yield
Measured yield
Liu et al.
Expected strength
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• RyFy
• Expected (mean) yield strength for material
• NOT maximum yield strength
• Determined by AISC using industry-wide data
Welded Beam to Column Laboratory Test - 1960s
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Pre-Northridge Standard
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Backing
• Backing used to support and retain molten filler metal
• Backing – also called “weld backing,” “backing bars,” “back-up bars”, and “backing strips”
• Fusible backing– weld is intended to bond to backing (e.g. steel backing)
• Non-fusible backing– weld not intended to bond to backing (e.g. ceramic and copper)
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Northridge Failure
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ColumnflangeColumnflange
Backup barBackup bar
Beam flangeand webBeam flangeand web
Following the 1994 Northridge earthquake, numerous failures of steel beam-to-column moment connections were identified. This led to a multiyear, multimillion dollar FEMA-funded problem-focused study undertaken by the SAC Joint Venture. The failures caused a fundamental rethinking of the design of seismic resistant steel moment connections.
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Northridge Failure
Northridge Failures
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Column Flange Heat Affected Zone Lamellar Tear
Weld Weld Fusion Column Divot
Flexural Mechanics at a Joint
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12
12
Beam Moment Fw
Fy
21 ZFZF yw
21 1 2
Cross Sections
Fw
Fy
Welded Steel Frames
• Northridge showed serious flaws. Problems correlated with:– - Weld material, detail concept and workmanship– Beam yield strength and size– Panel zone yield
• Repairs and new design– Move yield away from column face (cover plates,
haunches, reduced beam section)– - Verify through tests
• SAC Project: FEMA Publications 350 through 354• AISC 358
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Reduced Beam Section (RBS) Test SpecimenSAC Joint Venture
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Graphics courtesy of Professor Chia-Ming Uang, University of California San Diego
Extended Moment End-Plate Connection Results
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Photo courtesy of Professor Thomas Murray, Virginia Tech
Ductility of Steel Frame Joints
• Welded Joints– Brittle fracture of weld– Lamellar tearing of base metal– Joint design, testing, and inspection
• Bolted Joints– Fracture at net cross-section– Excessive slip
• Joint Too Weak For Member– Shear in joint panel
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Limit States
Multistory FrameLaboratory Test
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Axial StrutLaboratory test
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45Lr
Cross Braced FrameLaboratory test
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Eccentrically Braced FrameLab test of link
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NEHRP Recommended ProvisionsSteel Design
• Context in Provisions• Steel behavior• Reference standards and design strength
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Steel Design Specifications
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AISC 360: Specification for Structural Steel Buildings
AISC 341: Seismic Design Provisions for Structural Steel Buildings
Using Reference StandardsStructural Steel
• Both the AISC LRFD and ASD methodologies are presented in a unified format in both the Specification for Structural Steel Buildings and the Seismic Provisions for Structural Steel Buildings.
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Cold Formed Steel Standard
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Other Steel Members
• Steel Joist Institute• Standard Specifications, 2010
• Steel Cables• ASCE 19-2010
• Steel Deck Institute• Diaphragm Design Manual, DDM04, 2015
Instructional Material Complementing FEMA 1051, Design Examples Steel Structures - 36
NEHRP Recommended ProvisionsSteel Design
• Context in Provisions• Steel behavior• Reference standards and design strength• Moment resisting frames
Instructional Material Complementing FEMA 1051, Design Examples Steel Structures - 37
Steel Moment Frame Requirements
System Test or Prequalification
i Details and other
requirements Special Required 0.04 Many
Intermediate Required 0.02 Moderate
Ordinary Not required N.A. Few
Instructional Material Complementing FEMA 1051, Design Examples Steel Structures - 38
Moment frames
• Encourage– Flexural hinging in
beams• Avoid
– Flexural hinging in columns
• (occurs at base)
– Connection failure– Excessive column
panel-zone yielding
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Fuses
Encourage
Avoid
Panel Zones• Special and intermediate moment
frame:• Shear strength demand:
– 1.1 to 1.4RyMp of beams– Thickness (for buckling)– Use of doubler plates – (not economical, try to increase col.
size instead)
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Steel Moment Frames
• Beam shear: 1.15RyMp/Lh + gravity• Beam local buckling
– Smaller b/t than for plastic design• Continuity plates for smaller columns• Strong column - weak beam rule
– Prevent column yield except in panel zone– Exceptions: Low axial load, strong stories,
top story, and non-SFRS columns
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Steel Moment Frames
• Lateral support of column flange requirements– Top of beam if column elastic– Top and bottom of beam otherwise– Amplified forces for unrestrained
• Lateral support of beams requirements– Both flanges– Spacing < 0.095ryE/RyFy
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Prequalified Connections
• ANSI/AISC 358, Prequalified Connections for Special and Intermediate Steel Moment Frames for Seismic Applications– Reduced Beam Section Connections– Bolted Stiffened and Unstiffened Extended Moment End
Plate Connections– Bolted Flange Plate– Welded Unreinforced Flange-Welded Web (WUF-W)– Kaiser Bolted Bracket– CONXL
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NEHRP Recommended ProvisionsSteel Design
• Context in Provisions• Steel behavior• Reference standards and design strength• Seismic design category requirement• Moment resisting frames• Braced frames
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Concentrically Braced FramesBasic Configurations
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X Diagonal K
V(Chevron)
Inverted V(Chevron)
K
Concentrically Braced Frames
• Special AISC Seismic R = 6• AISC 341 Section F2
• Ordinary AISC Seismic R = 3.25• AISC 341 Section F1
• Not Detailed for Seismic R = 3• AISC 360
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Concentrically braced frames
• Encourage– Yielding of braces– Buckling of braces
• Avoid– Flexural hinging in
columns (story mechanisms)
– Buckling of beams or columns
– Connection failure
Fuses
Instructional Material Complementing FEMA 1051, Design Examples Section Name 1 - 47
Eccentrically Braced Frames
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Link
Brace
Beam
Eccentrically braced frames
• Encourage– Yielding of link
• Avoid– Flexural hinging in
columns (story mechanisms)
– Buckling of braces, beams or columns
– Connection failure
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Fuses
Buckling-Restrained Braced Frames (BRBFs)
• Type of concentrically braced frame• Beams, columns and braces arranged to form a
vertical truss. Resist lateral earthquake forces by truss action
• Special type of brace members used: Buckling-Restrained Braces (BRBs). BRBS yield both in tension and compression - no buckling !!
• Develop ductility through inelastic action (cyclic tension and compression yielding) in BRBs.
• System combines high stiffness with high ductility
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Buckling- Restrained Brace:Steel Core
+Casing
Casing
Steel Core
Buckling-Restrained Braced Frames (BRBFs)
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Bracing Configurations for BRBFs
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Single Diagonal Inverted V- Bracing V- Bracing
X- Bracing Two Story X- Bracing
Buckling restrained braced frames
• Encourage– Yielding of braces
• Avoid– Flexural hinging in
columns (story mechanisms)
– Buckling of beams or columns
– Connection failure
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Fuses
NEHRP Recommended ProvisionsSteel Design
• Context in Provisions• Steel behavior• Reference standards and design strength• Moment resisting frames• Braced frames• Other topics
Instructional Material Complementing FEMA 1051, Design Examples Steel Structures - 54
Special Truss Moment Frame
• Buckling and yielding in special section
• Design to be elasticoutside special section
• Deforms similar to EBF• Special panels to be
symmetric X or Vierendeel
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Special Truss Moment Frame
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General Seismic Detailing
• Materials:– Limit to lower strengths and higher
ductilities
• Bolted Joints:– Fully tensioned high strength bolts– Class-A (or better) faying surface)– Limit on bearing
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General Seismic Detailing
• Welded Joints:– AWS requirements for welding procedure specs– Filler metal toughness
• CVN > 20 ft-lb @ 0°F– Warning on discontinuities, tack welds, run offs,
gouges, etc.• Columns:
– Strength using o – Splices: Requirements on partial penetration welds
and fillet welds
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Qn = 0.5 Asc ( fc’ Ec)1/2 Rg Rp Asc FuRg = stud group adjustment factor
Rg = 1.0 Rg = 1.0* Rg = 0.85 Rg = 0.7
*0.85 if wr /hr < 1.5
Shear Studs Group Adjustment Factor
Quality and Welding• AISC 341
– Appendix Q• Basic Quality Assurance Plan• Inspection tables
–Welding–Bolting
– Appendix W• Welding quality
–Details–Inspection
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Quality and Welding
• AISC 341– Addresses:
• Quality Control - Fabricator/Erector• Quality Assurance - Owner• Non-Destructive Testing
– Uses:• Observe and Perform (AISC Seismic
Provisions)
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Inspection and TestingShop Certification
• Domestic:– AISC– Local jurisdictions
• Foreign:– No established international criteria
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Questions
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