Structural Steel Design - cdn.ymaws.com · Flexural Ductility of Steel Members ... of the design of...

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

Instructional Material Complementing FEMA 1051, Design Examples Steel Structures - 2

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)

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

Monotonic Stress-Strain Behavior

Plastic Hinge Formation

Plastic hinge in beam

Behavior Modes For Beams

Moment

Deflection

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

Lateral Torsional Buckling

Laterally unbraced length

Resisting moment

inelastic buckling

elastic buckling

buckling with

strain-hardening

Local Buckling

ycr tbEk

)/)(1(12

tb 95.0

b Classical plate buckling solution:

Substituting = 0.3 and rearranging:

Width-thickness ratios

Increasing compactness(decreasing element slenderness)

Slender elementsElastic local buckling

Noncompact elementsInelastic local buckling

Compact elementsYielding

Highly compact elementsSignificant strain

Restraint

Highly restrained

Example: Beam flange to (thick) column flange T joint

Toughness and energy absorption

Displacement

Low toughness High toughness

Shaded area is the energy absorbed

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”

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

0 10 20 30 40 50 60 70b / t

s A500 Gr. BMeanlp

Specified minimum yield

Measured yield

Liu et al.

Expected strength

• 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

Pre-Northridge Standard

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)

Northridge Failure

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.

Northridge Failure

Northridge Failures

Column Flange Heat Affected Zone Lamellar Tear

Weld Weld Fusion Column Divot

Flexural Mechanics at a Joint

Beam Moment Fw

21 ZFZF yw

21 1 2

Cross Sections

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

Reduced Beam Section (RBS) Test SpecimenSAC Joint Venture

Graphics courtesy of Professor Chia-Ming Uang, University of California San Diego

Extended Moment End-Plate Connection Results

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

Limit States

Multistory FrameLaboratory Test

Axial StrutLaboratory test

Cross Braced FrameLaboratory test

Eccentrically Braced FrameLab test of link

NEHRP Recommended ProvisionsSteel Design

• Context in Provisions• Steel behavior• Reference standards and design strength

Steel Design Specifications

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.

Cold Formed Steel Standard

Other Steel Members

• Steel Joist Institute• Standard Specifications, 2010

• Steel Cables• ASCE 19-2010

• Steel Deck Institute• Diaphragm Design Manual, DDM04, 2015

• Context in Provisions• Steel behavior• Reference standards and design strength• Moment resisting frames

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

Moment frames

• Encourage– Flexural hinging in

beams• Avoid

– Flexural hinging in columns

• (occurs at base)

– Connection failure– Excessive column

panel-zone yielding

Encourage

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)

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

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

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

• Context in Provisions• Steel behavior• Reference standards and design strength• Seismic design category requirement• Moment resisting frames• Braced frames

Concentrically Braced FramesBasic Configurations

X Diagonal K

V(Chevron)

Inverted V(Chevron)

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

Concentrically braced frames

• Encourage– Yielding of braces– Buckling of braces

• Avoid– Flexural hinging in

columns (story mechanisms)

– Buckling of beams or columns

– Connection failure

Instructional Material Complementing FEMA 1051, Design Examples Section Name 1 - 47

Eccentrically Braced Frames

Eccentrically braced frames

• Encourage– Yielding of link

– Buckling of braces, beams or columns

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

Buckling- Restrained Brace:Steel Core

+Casing

Casing

Steel Core

Buckling-Restrained Braced Frames (BRBFs)

Bracing Configurations for BRBFs

Single Diagonal Inverted V- Bracing V- Bracing

X- Bracing Two Story X- Bracing

Buckling restrained braced frames

• Encourage– Yielding of braces

– Buckling of beams or columns

• Context in Provisions• Steel behavior• Reference standards and design strength• Moment resisting frames• Braced frames• Other topics

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

Special Truss Moment Frame

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

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

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

Quality and Welding

• AISC 341– Addresses:

• Quality Control - Fabricator/Erector• Quality Assurance - Owner• Non-Destructive Testing

– Uses:• Observe and Perform (AISC Seismic

Provisions)

Inspection and TestingShop Certification

• Domestic:– AISC– Local jurisdictions

• Foreign:– No established international criteria

Questions

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