Berkley - Steel Failure Presentation - CE227SAC
Content
FEMA Program to Reduce Earthquake Hazards in Steel Moment-Frame Structures A New Paradigm for Design and Evaluation of Steel Moment Frame Buildings Stephen Mahin
Nishkian Professor of Structural Engineering University of California at Berkeley Chair, Project Management Committee
Steel Moment Frames
Widely used
– Construction ease – Architectural and functional versatility – Introduction of welding increased their efficiency and economy – Considered one of the best earthquake resisting systems available
Typical Steel Moment Frame Structures
Brittle Fractures Detected in Connections Following 1994 Northridge Earthquake
The Pre-Northridge Connection
Beams welded and bolted to columns
Brittle Connection Damage
• Generally confined to vicinity of welds of beams to column connections • Occurred in many types of welded steel moment frame buildings – New and old buildings – Tall and short buildings – Conventional and important buildings • Contrary to intent of modern building codes
Complex joint
Lateral loads resisted by moments developed in frame
A New Solution Approach
! Brittle nature of damage invalidated basic assumptions used to design and evaluate steel moment frames ! To find a reliable and practical solution to this problem, a new approach was utilized: • Integrating directed research, guideline development and training • Implemented performance-based engineering framework • Mobilized expertise and resources covering an unprecedented array of disciplines from throughout the US
FEMA Program to Reduce the Earthquake Hazards of Steel Moment-Frame Structures
Goal: Develop reliable, practical and cost-effective guidelines and standards of practice for:
– the design and construction of new steel momentframe buildings, – the identification, inspection, evaluation and retrofit of existing at-risk welded steel moment-frame buildings, and – the identification, evaluation, repair or upgrading of damaged buildings following earthquakes.
The FEMA/SAC Steel Project
FEMA-350: Materials and Fracture Issues
Other
The Guidelines
Recommended Seismic Design Criteria for New Steel Moment-Frame Buildings. Recommended Seismic Evaluation and Upgrade Criteria for Existing Welded Steel Moment-Frame Buildings. Recommended Post-earthquake Criteria for Welded, Steel Buildings. Recommended Specifications Assurance Frame Construction for Seismic Policy Guide for Steel Frame
L RA DE FE
State of the Art Reports
Capacity
Welding, Joining and Inspection Analysis and Testing of Connections Earthquake Performance Simulation of Seismic Response
Reliability Framework for Performance Prediction and Evaluation
Seismic Design Criteria
Building Codes
FEMA-351: FEMA-352:
Social, Economic and Policy Issues
FEMA-353:
of azards Ea es he tructur rthquake H ce t e Se e Ea ed u amuc th uctures ed R Fr Str to mR to ent nt Frame mMo og me o mra Pr a M el r l Ste g Stee Pro Program to Reduce the Earthquake Hazards of
Hazards of
Demand
Steel Moment Frame Structures
Results synthesized as State of the Art Reports
FEMA-355A: FEMA-355B: FEMA-355C: FEMA-355D: FEMA 355E: FEMA-355F: Base Metals and Fracture Welding and Inspection Systems Performance Connection Performance Past Performance of Steel Moment-Frame Buildings in Earthquakes Performance Prediction and Evaluation
Why did this happened?
Early assertions suggested brittle fractures were due to: ! Ground motion characteristics ! Inadequate workmanship ! Uncertain material properties ! Heavier and deeper sections ! Less redundant systems Investigations showed connection not well understood, with many contributors to poor performance
Plus more than 80 detailed technical reports
Program to Reduce the
Steel Moment
Frame Stru
Earthquake
ake
Trial Designs
Cost Analysis
Loss Analysis
and Quality t d eisende ing Seismic Design Criteria sis Recommended d Sm eom Reom t-Resist ec nd s en n t- M eR g eew Guidelines for Steel for New Moment-Resisting MomentN rm dine Buildings mm M foo il o c u am Steel Frame Buildings BFr Re New St ee el m a r for New Mo fo el Fr Applications. ment-Resisting S te Steel
Recommended
99 , 19 July,2000 July CY 352 50 NT AGEN FEMA Y A 3EMERGENCY FEDERAL MANAGEMENT AGENCY ME FEMA 351 July, 1999 C EM NAGE E FN GMA Y A NC ERGE NT eria RAL EM EME FEDERAL FEDE G ritNCY EME RGE C NA MANAGEMEN a MA ign T AGE teri CY NCY FEM A 350 July, EN D es ign Cri 1999 es RG D ic E ic g ism m EM in Se
Evaluation and Repair Moment- Frame
of
Seismic Desi
gn Criteria
FEMA-354:
zar
Frame Buildin
ds
Construction
ctures
gs
qu rth
Ha
Assertion: Damage due to unusual severity of ground shaking?
While ground motion was severe, it was not greater than anticipated in design of many damaged buildings. ! Most buildings were substantially (two to three times) stronger than minimum Undamaged Damaged code forces. 100% ! Many fractures occurred in 80% buildings that should have 60% 40% responded elastically 20% ! Typically, D/C < 2
!
0% 0 0.2 0.4 0.6 0.8 1
Assertion: Damage was due to inadequate workmanship?
! In many cases, workmanship was inadequate and
some construction practices led to poor quality welds
! Test results indicate that improved workmanship
was generally insufficient by itself to achieve reliable performance. ! All pre-Northridge connections tested by SAC failed brittlely, reflecting all of the fracture modes seen in the field.
Demand/Capacity
Identical Lab and Field Damage
Assertion: We can predict damage locations by computer analysis
• Only modest correlation of local damage location to computer predictions – Fracture criteria unknown? – Sensitive to modeling assumptions Regions (floors) with higher D/C ratios tend to have higher damage
•
13 Fractured Connections
6 Highest D/C Ratios
Pre-Northridge Welded Connections
Behavior of Pre-Northridge welded steel moment connections influenced by many interacting factors, including:
! Load
A closer look
Weak section at face of column Numerous stress risers in typical joint detail
transfer mechanism "Frame configuration "Basic geometry of connection "Shear transfer mechanism "Panel zone deformations, etc. ! Quality of Welds ! Fracture sensitivity of typical connection
High force transfer at connection
Weld quality issues
Welds difficult to make, especially on bottom flange ! Backing bar " Makes visual inspection of root pass impossible " Results in inconsistent ultrasonic test results ! Welding practices oriented towards high productivity ! Weld consumables " Selected based on strength " Notch toughness not normally specified.
!
What forces should the welds resist?
• Changing steel properties • Typical beam design assumptions flawed near connection (St. Venant 1855)
M V
Backing Bar
Tf = Fypr Aflange
Non-uniform distribution of axial stresses in beam flange at column face
Beam flanges carry considerable shear
Distance from Bottom (x/D)
1.0
D/2 from column face
0.5
At column face
High triaxial tension
0
0
1
2
Normalized Shear Stress
Local Flange Deformation
Panel Zone Yielding
“Kinked” column causes high local bending in column and beam flanges
Strong Panel Zone
-------------- Weak Panel Zone --------------
Why did These Connections Fracture?
CVN or KI
Fracture Susceptibility
Material Fracture Toughness
Temperature
Highly variable stress and strain distributions develop. But, if steel is ductile,why didn’t it just yield?
Weld root defect
For many typical pre-Northridge connections, the combination of: – joint geometry – imperfections – KIc for the weld metal were such that Fcritical ! fy In such cases, the joint would likely fracture brittlely before yielding and forming a plastic hinge in the beam
Backing Bar
Fcritical = KIc/[(!Ci)("a)1/2]
Some Alternatives Considered
Welded Connections
• “Improved” unreinforced connections. • Reinforced connections • Welded flange plate connections • Reduced beam section connections
Some Welded Connections
‘Plan B’ - Bolted Connections
Bolted Connections
• Tee-stub Connections • Bolted flange plate connections • End plate connections
Flange Plate Tee- Stub End Plate
Gravity Connections
• Simple connections with and without slabs
Slab Shear Tab
Some Bolted Connections
Theoretical & Experimental Verification Required
Case in point: Fracture control strategy for unreinforced connections
Rotations developed in Stage I unreinforced connections
Notch Tough Specimens
Fcritical = KIc/[(!Ci)("a)1/2]
One might control fracture by: ! Using notch toughness weld metals ! Controlling imperfections ! Improving the joint geometry However, ! We enter into another range of fracture mechanics related to plastic initiation, and plastic crack growth under cyclic loading, and ! There may be other failure mechanisms.
High toughness welds
Remove backing bar and reinforce root pass
PreNorthridge Detail
Other Locations have High Stresses
Computed Critical Plastic Strains at Large Drift
High stress at toe of weld cope High triaxial tension
Elastic range
Bi-directional bending in beam flange at toe of weld access hole
Behavior sensitive to shape and finish of weld access hole
Weld at column face protected by improved design, but failure shifts to next weakest link
Plastic Crack Initiation and Gradual Growth Under Cyclic Loading
T T T T
Eccentric shear link action
Continued refinement leads to “prequalified” connections
Identify and characterize all local failure modes; Specify design method that controls connection behavior
#pmax = 5.0% rad.
Systems Approach
Need method to relate capacities and demands Built upon transparent reliability framework •Utilization of engineering knowledge •Manage risk and uncertainties •Performance-based engineering concepts D(1+$d%d) > C(1-$c%c)
Probability Load and Resistance Factor Design Format
Improved Weld Access Holes
D
C Capacity
&('D) < (C
Demand
Specimen C2 upon completion of testing
Lateral load-displacement relationship
Performance Parameter
Performance-Based Engineering
Recent approaches
Stipulate performance desired for given earthquake hazard. For example, • Immediate occupancy is assured for an earthquake that has a 50% chance of occurring in 50 years • Collapse will be prevented for earthquake with 2% chance of occurring in 50 years Problem: This implies a warranty that performance will be achieved and is unrealistic given the uncertainties involved.
New SAC Approach
Re-phrases statement as:
I am highly/moderately/not confident that a stated performance level will be achieved for a given seismic hazard; for example,... I am 50% confident that the structure will not collapse if subjected to an earthquake with a 2% probability of occurring in 50 years. SAC targets for new construction (2% in 50 year event) • 90% confidence for global collapse • 50% confidence for local damage leading to local collapse
System Demand Estimates
M
Such “demand” analyses used to:
#
Probability
2% in 50 yrs.
Sa
– Develop analysis adjustment factors to account for:
• Simpler analyses procedures • Modeling simplifications
– Understand effect of:
• Ground motion intensity and characteristics • Structural configuration • Connection fracture • Deterioration of connection hysteretic loop characteristics • Alternative connection types • Aftershocks • Prior damage or defects
Sa1
Accel, g.
1
T, sec.
Response Parameter 1
– Develop cyclic loading histories for testing
21 19 17 15 13 11 9 7 5 3
time
Probability
D %d
Floor Level
20-STORY
time
Demand
time
Performance Parameter 1
1 0.00
0.01
0.02
0.03
Story Drift Angle
0.04
0.05
0.06
0.07
0.08
0.09
“Collapse Prevention” capacity evaluation
• Ductile modes associated with
– Local failure of plastic hinge (from tests) – Dynamic instability of system as a whole (analysis)
Representative confidences of not exceeding performance criteria in Los Angeles for 2% in 50 year earthquake hazard
For New Construction
• Brittle modes effecting vertical stability, particularly column failure modes
M #IO #CP Drift Local Connection Performance Global System Performance Probability
Performance Criteria Building Height 3 stories 9 stories 20 stories Global Stability 99% 99% 96% Local Stability 99% 95% 96%
%c
Csystem Capacity
SAC vs. 1994 UBC Designs
Representative confidences of not exceeding performance criteria in Los Angeles for 2% in 50 year earthquake hazard
Reliabilities for different age building
Representative confidences of not exceeding performance criteria in Los Angeles for 2% in 50 year earthquake hazard
Performance Criteria Building Height 3 stories 9 stories 20 stories Global Stability SAC 99% 99% 96% 1994 88% 57% 57% Local Stability SAC 99% 95% 96% 1994 22% 29% 39%
Performance Criteria Building Height 3 stories 9 stories 20 stories Local Stability SAC 99% 95% 96% 1994 1985 1973 22% 29% 39% 9% 21% 42% 2% 7% 2%
Reliabilities for different age building
Representative confidences of not exceeding performance criteria in Los Angeles for 50% in 50 year earthquake hazard
•
Design Provisions for New Buildings
Use NEHRP provisions for structure • analysis and proportioning:
– Definition of design earthquake – Analysis procedures and modeling – Force reduction factors, redundancy factors, drift limits, etc. – Proportioning (strong column-weak girder, etc.)
Use “prequalified” connections:
– Explicit design calculations – Limits on range of materials, sizes, relative strengths, details, etc. that can be used
Performance Criteria Building Height 3 stories 9 stories 20 stories Local Stability SAC 99% 99% 99% 1994 1985 1973 99% 99% 99% 99% 99% 99% 99% 99% 99%
•
•
Welding specifications and QA/QC more clearly articulated
New criteria for P-) effects
Not much different than 1997 UBC in application
Key Analysis Parameters Evaluated
• • Reduced forces at plastic hinge locations used to select beam sizes Interstory drift demand estimated based on unreduced design forces – Limited in a absolute sense (generally controls member sizes in moment frames). – New P-) effects criteria – Drifts used to select type of connection (SMF or OMF)
• global stability • local connection integrity
Prequalified Connections deemed to satisfy requirements of code Acceptance Criteria:
OMF: !SD=0.02, !U=0.03 SMF: !SD=0.04, !U=0.06 Detailed Design and Construction Requirements Specified for Prequalified Connections Welded Unreinforced Flange-Welded Web (WUF-W) Connection
•
Column axial force demand checked based on capacity of beams framing into column – compression (buckling) !tension (splice failure)
Prequalification Data WUF-W Connections
General: Applicable s ystems Hinge location dis tance Critical Beam Parameters: Maximum depth Minimum span -to -depth ratio SMF: 7 Flange thickness OMF: 1 -1/2Ó and less SMF: 1Ó or less Perm issible material specification s Critical Column Parameters: Depth OMF: Not Limited SMF: W12, W14 Perm issible material specification s Beam/Column Relations: Panel Zone strength Column/beam bending strength Connection Details Web connection Continuity plate thickness Flange weld s Weld ing parameters Weld access holes Special Connect ion DSee Fig. 3 -8; Welding QC/QA Category BM Section 3.3.3.1 Section 3.3.2.5 Section 3.3.2.4 , 3.3.2.5, 3.3.2.6 Section. 3.3.2.7 SMF: Section 3.3.3.2 A572 Grade 50; A913 Grade 50 and 65; A992 A572 Grade 50, A992, A913 Grade 50/575 W3 6 and shallower sh OMF, SMF d c /2 + d b/2
Welded Joints
• Provisions for welding and inspection are nearly identical to current provisions in AWS D1.1 • FEMA 353 has been formulated to gather requirements in one place and to tabulate these for the convenience of designers. • Some changes are highlighted in document
– Weld material properties and acceptance criteria – Weld demand and inspection requirements listed on drawings – Weld wire storage/exposure requirements
OMF: 5
Section 2.9.1
Filler Metal Toughness AWS A5 Certification 20 ft-lbs at 0oF
2.4.1.1 Appendix A
Weld QC / QA
QC/QA requirements are given for each weld of the Prequalified Connections. The form of notation is: “QC/QA Category DC/L”, where
– D indicates the Demand Category, – C is the Consequence Category and – L is the Primary Loading Direction.
SAC Heat Input Qualification (WPS) Test 40 ft-lbs at 70oF
(or Lowest Anticipated Service Temperature for cold exposure)
Notation required to be included on structural drawings
Process & Visual Welding Inspection Categories
Category W Inspection Tasks Prior During After QC QA 1 QC 2 Inspector QA
Table 6-2
Evaluation of Existing Buildings
Full PBE format used for existing buildings, or new buildings with special performance objectives. – User may select any performance objective – Documents describe conditions for collapse prevention and immediate occupancy – Immediate occupancy includes significant structural damage - but none that would reduce reliability of structural system – Confidence associated with attaining performance computed and discussed with owner – Suggested:
• 90% confidence for global behaviors • 50% confidence for local behaviors
3 QC QA
H O H O H O H O H O H O
Analysis Methods & Adjustment Factors
Structural Characteristics Analytical Procedure Linear Dynamic Nonlinear Static Performance Level Immediate Occupancy Fundamental Regularity Ratio of Period, T Column to Beam Strength T < 3.5Ts1 Regular or Any Irregular Conditions T > 3.5 Ts1 Regular or Any Irregular Conditions T < 3.5Ts1 Regular2 Strong Column3 Weak Column3 Irregular2 T > 3.5Ts Regular Any Conditions Strong Column3 Weak Column3 Irregular
2
Summary
Powerful performance evaluation method developed, evaluated and implemented for: ! evaluating and upgrading existing buildings, ! assessing repair or retrofit strategies, and ! designing new structures to special performance levels. " Incorporates system level capacity evaluation including instablility due to fracture and other forms of deterioration. " Rational implementation of complete PBE framework " Provides uniform reliability for various types of analysis " Used to manage risks and uncertainties, and to communicate these to owners, tenants and others (confidence levels).
Linear Static
Nonlinear Dynamic
Permitted Not Permitted Permitted Not Permitted Not Permitted Not Permitted Not Permitted Not Permitted
Permitted Permitted Permitted Not Permitted Not Permitted Permitted Not Permitted Not Permitted
Permitted Not Permitted Permitted Permitted Permitted Not Permitted Not Permitted Not Permitted
Permitted Permitted Permitted Permitted Permitted Permitted Permitted Permitted
Collapse Prevention
Any Conditions
Summary
!
Summary
• A systematic "approach to developing performance-based design methods for steel moment frame structures has been demonstrated to be highly effective and successful. – Integrated research, guideline development and training – Focussed substantial resources and expertise to solve complex technical, social and economic problems associated seismic loss reduction. – Widespread review by independent technical experts, design professionals, building officials, contractors, fabricators, and manufacturers. • But, many problems remain unresolved….
Details used for welded steel moment frame structures prior to 1994 have been shown to be vulnerable to brittle fracture contrary to the intent of building codes. New details, with simple design methods and stringent limitations on ranges of parameters that can be used, have been identified that are believed to satisfy building code life safety objectives. Methods have been developed for qualifying welded and bolted connections with • parameters outside the prequalified range, • having different configurations, or • requiring higher performance capabilities.
!
!
FEMA publications
Getting More SAC Information WWW Site
FEMA reports available for free from the FEMA Printing Office. Call 1-800-480-2520
www.sacsteel .org www.sacsteel.org
Nishkian Professor of Structural Engineering University of California at Berkeley Chair, Project Management Committee
Steel Moment Frames
Widely used
– Construction ease – Architectural and functional versatility – Introduction of welding increased their efficiency and economy – Considered one of the best earthquake resisting systems available
Typical Steel Moment Frame Structures
Brittle Fractures Detected in Connections Following 1994 Northridge Earthquake
The Pre-Northridge Connection
Beams welded and bolted to columns
Brittle Connection Damage
• Generally confined to vicinity of welds of beams to column connections • Occurred in many types of welded steel moment frame buildings – New and old buildings – Tall and short buildings – Conventional and important buildings • Contrary to intent of modern building codes
Complex joint
Lateral loads resisted by moments developed in frame
A New Solution Approach
! Brittle nature of damage invalidated basic assumptions used to design and evaluate steel moment frames ! To find a reliable and practical solution to this problem, a new approach was utilized: • Integrating directed research, guideline development and training • Implemented performance-based engineering framework • Mobilized expertise and resources covering an unprecedented array of disciplines from throughout the US
FEMA Program to Reduce the Earthquake Hazards of Steel Moment-Frame Structures
Goal: Develop reliable, practical and cost-effective guidelines and standards of practice for:
– the design and construction of new steel momentframe buildings, – the identification, inspection, evaluation and retrofit of existing at-risk welded steel moment-frame buildings, and – the identification, evaluation, repair or upgrading of damaged buildings following earthquakes.
The FEMA/SAC Steel Project
FEMA-350: Materials and Fracture Issues
Other
The Guidelines
Recommended Seismic Design Criteria for New Steel Moment-Frame Buildings. Recommended Seismic Evaluation and Upgrade Criteria for Existing Welded Steel Moment-Frame Buildings. Recommended Post-earthquake Criteria for Welded, Steel Buildings. Recommended Specifications Assurance Frame Construction for Seismic Policy Guide for Steel Frame
L RA DE FE
State of the Art Reports
Capacity
Welding, Joining and Inspection Analysis and Testing of Connections Earthquake Performance Simulation of Seismic Response
Reliability Framework for Performance Prediction and Evaluation
Seismic Design Criteria
Building Codes
FEMA-351: FEMA-352:
Social, Economic and Policy Issues
FEMA-353:
of azards Ea es he tructur rthquake H ce t e Se e Ea ed u amuc th uctures ed R Fr Str to mR to ent nt Frame mMo og me o mra Pr a M el r l Ste g Stee Pro Program to Reduce the Earthquake Hazards of
Hazards of
Demand
Steel Moment Frame Structures
Results synthesized as State of the Art Reports
FEMA-355A: FEMA-355B: FEMA-355C: FEMA-355D: FEMA 355E: FEMA-355F: Base Metals and Fracture Welding and Inspection Systems Performance Connection Performance Past Performance of Steel Moment-Frame Buildings in Earthquakes Performance Prediction and Evaluation
Why did this happened?
Early assertions suggested brittle fractures were due to: ! Ground motion characteristics ! Inadequate workmanship ! Uncertain material properties ! Heavier and deeper sections ! Less redundant systems Investigations showed connection not well understood, with many contributors to poor performance
Plus more than 80 detailed technical reports
Program to Reduce the
Steel Moment
Frame Stru
Earthquake
ake
Trial Designs
Cost Analysis
Loss Analysis
and Quality t d eisende ing Seismic Design Criteria sis Recommended d Sm eom Reom t-Resist ec nd s en n t- M eR g eew Guidelines for Steel for New Moment-Resisting MomentN rm dine Buildings mm M foo il o c u am Steel Frame Buildings BFr Re New St ee el m a r for New Mo fo el Fr Applications. ment-Resisting S te Steel
Recommended
99 , 19 July,2000 July CY 352 50 NT AGEN FEMA Y A 3EMERGENCY FEDERAL MANAGEMENT AGENCY ME FEMA 351 July, 1999 C EM NAGE E FN GMA Y A NC ERGE NT eria RAL EM EME FEDERAL FEDE G ritNCY EME RGE C NA MANAGEMEN a MA ign T AGE teri CY NCY FEM A 350 July, EN D es ign Cri 1999 es RG D ic E ic g ism m EM in Se
Evaluation and Repair Moment- Frame
of
Seismic Desi
gn Criteria
FEMA-354:
zar
Frame Buildin
ds
Construction
ctures
gs
qu rth
Ha
Assertion: Damage due to unusual severity of ground shaking?
While ground motion was severe, it was not greater than anticipated in design of many damaged buildings. ! Most buildings were substantially (two to three times) stronger than minimum Undamaged Damaged code forces. 100% ! Many fractures occurred in 80% buildings that should have 60% 40% responded elastically 20% ! Typically, D/C < 2
!
0% 0 0.2 0.4 0.6 0.8 1
Assertion: Damage was due to inadequate workmanship?
! In many cases, workmanship was inadequate and
some construction practices led to poor quality welds
! Test results indicate that improved workmanship
was generally insufficient by itself to achieve reliable performance. ! All pre-Northridge connections tested by SAC failed brittlely, reflecting all of the fracture modes seen in the field.
Demand/Capacity
Identical Lab and Field Damage
Assertion: We can predict damage locations by computer analysis
• Only modest correlation of local damage location to computer predictions – Fracture criteria unknown? – Sensitive to modeling assumptions Regions (floors) with higher D/C ratios tend to have higher damage
•
13 Fractured Connections
6 Highest D/C Ratios
Pre-Northridge Welded Connections
Behavior of Pre-Northridge welded steel moment connections influenced by many interacting factors, including:
! Load
A closer look
Weak section at face of column Numerous stress risers in typical joint detail
transfer mechanism "Frame configuration "Basic geometry of connection "Shear transfer mechanism "Panel zone deformations, etc. ! Quality of Welds ! Fracture sensitivity of typical connection
High force transfer at connection
Weld quality issues
Welds difficult to make, especially on bottom flange ! Backing bar " Makes visual inspection of root pass impossible " Results in inconsistent ultrasonic test results ! Welding practices oriented towards high productivity ! Weld consumables " Selected based on strength " Notch toughness not normally specified.
!
What forces should the welds resist?
• Changing steel properties • Typical beam design assumptions flawed near connection (St. Venant 1855)
M V
Backing Bar
Tf = Fypr Aflange
Non-uniform distribution of axial stresses in beam flange at column face
Beam flanges carry considerable shear
Distance from Bottom (x/D)
1.0
D/2 from column face
0.5
At column face
High triaxial tension
0
0
1
2
Normalized Shear Stress
Local Flange Deformation
Panel Zone Yielding
“Kinked” column causes high local bending in column and beam flanges
Strong Panel Zone
-------------- Weak Panel Zone --------------
Why did These Connections Fracture?
CVN or KI
Fracture Susceptibility
Material Fracture Toughness
Temperature
Highly variable stress and strain distributions develop. But, if steel is ductile,why didn’t it just yield?
Weld root defect
For many typical pre-Northridge connections, the combination of: – joint geometry – imperfections – KIc for the weld metal were such that Fcritical ! fy In such cases, the joint would likely fracture brittlely before yielding and forming a plastic hinge in the beam
Backing Bar
Fcritical = KIc/[(!Ci)("a)1/2]
Some Alternatives Considered
Welded Connections
• “Improved” unreinforced connections. • Reinforced connections • Welded flange plate connections • Reduced beam section connections
Some Welded Connections
‘Plan B’ - Bolted Connections
Bolted Connections
• Tee-stub Connections • Bolted flange plate connections • End plate connections
Flange Plate Tee- Stub End Plate
Gravity Connections
• Simple connections with and without slabs
Slab Shear Tab
Some Bolted Connections
Theoretical & Experimental Verification Required
Case in point: Fracture control strategy for unreinforced connections
Rotations developed in Stage I unreinforced connections
Notch Tough Specimens
Fcritical = KIc/[(!Ci)("a)1/2]
One might control fracture by: ! Using notch toughness weld metals ! Controlling imperfections ! Improving the joint geometry However, ! We enter into another range of fracture mechanics related to plastic initiation, and plastic crack growth under cyclic loading, and ! There may be other failure mechanisms.
High toughness welds
Remove backing bar and reinforce root pass
PreNorthridge Detail
Other Locations have High Stresses
Computed Critical Plastic Strains at Large Drift
High stress at toe of weld cope High triaxial tension
Elastic range
Bi-directional bending in beam flange at toe of weld access hole
Behavior sensitive to shape and finish of weld access hole
Weld at column face protected by improved design, but failure shifts to next weakest link
Plastic Crack Initiation and Gradual Growth Under Cyclic Loading
T T T T
Eccentric shear link action
Continued refinement leads to “prequalified” connections
Identify and characterize all local failure modes; Specify design method that controls connection behavior
#pmax = 5.0% rad.
Systems Approach
Need method to relate capacities and demands Built upon transparent reliability framework •Utilization of engineering knowledge •Manage risk and uncertainties •Performance-based engineering concepts D(1+$d%d) > C(1-$c%c)
Probability Load and Resistance Factor Design Format
Improved Weld Access Holes
D
C Capacity
&('D) < (C
Demand
Specimen C2 upon completion of testing
Lateral load-displacement relationship
Performance Parameter
Performance-Based Engineering
Recent approaches
Stipulate performance desired for given earthquake hazard. For example, • Immediate occupancy is assured for an earthquake that has a 50% chance of occurring in 50 years • Collapse will be prevented for earthquake with 2% chance of occurring in 50 years Problem: This implies a warranty that performance will be achieved and is unrealistic given the uncertainties involved.
New SAC Approach
Re-phrases statement as:
I am highly/moderately/not confident that a stated performance level will be achieved for a given seismic hazard; for example,... I am 50% confident that the structure will not collapse if subjected to an earthquake with a 2% probability of occurring in 50 years. SAC targets for new construction (2% in 50 year event) • 90% confidence for global collapse • 50% confidence for local damage leading to local collapse
System Demand Estimates
M
Such “demand” analyses used to:
#
Probability
2% in 50 yrs.
Sa
– Develop analysis adjustment factors to account for:
• Simpler analyses procedures • Modeling simplifications
– Understand effect of:
• Ground motion intensity and characteristics • Structural configuration • Connection fracture • Deterioration of connection hysteretic loop characteristics • Alternative connection types • Aftershocks • Prior damage or defects
Sa1
Accel, g.
1
T, sec.
Response Parameter 1
– Develop cyclic loading histories for testing
21 19 17 15 13 11 9 7 5 3
time
Probability
D %d
Floor Level
20-STORY
time
Demand
time
Performance Parameter 1
1 0.00
0.01
0.02
0.03
Story Drift Angle
0.04
0.05
0.06
0.07
0.08
0.09
“Collapse Prevention” capacity evaluation
• Ductile modes associated with
– Local failure of plastic hinge (from tests) – Dynamic instability of system as a whole (analysis)
Representative confidences of not exceeding performance criteria in Los Angeles for 2% in 50 year earthquake hazard
For New Construction
• Brittle modes effecting vertical stability, particularly column failure modes
M #IO #CP Drift Local Connection Performance Global System Performance Probability
Performance Criteria Building Height 3 stories 9 stories 20 stories Global Stability 99% 99% 96% Local Stability 99% 95% 96%
%c
Csystem Capacity
SAC vs. 1994 UBC Designs
Representative confidences of not exceeding performance criteria in Los Angeles for 2% in 50 year earthquake hazard
Reliabilities for different age building
Representative confidences of not exceeding performance criteria in Los Angeles for 2% in 50 year earthquake hazard
Performance Criteria Building Height 3 stories 9 stories 20 stories Global Stability SAC 99% 99% 96% 1994 88% 57% 57% Local Stability SAC 99% 95% 96% 1994 22% 29% 39%
Performance Criteria Building Height 3 stories 9 stories 20 stories Local Stability SAC 99% 95% 96% 1994 1985 1973 22% 29% 39% 9% 21% 42% 2% 7% 2%
Reliabilities for different age building
Representative confidences of not exceeding performance criteria in Los Angeles for 50% in 50 year earthquake hazard
•
Design Provisions for New Buildings
Use NEHRP provisions for structure • analysis and proportioning:
– Definition of design earthquake – Analysis procedures and modeling – Force reduction factors, redundancy factors, drift limits, etc. – Proportioning (strong column-weak girder, etc.)
Use “prequalified” connections:
– Explicit design calculations – Limits on range of materials, sizes, relative strengths, details, etc. that can be used
Performance Criteria Building Height 3 stories 9 stories 20 stories Local Stability SAC 99% 99% 99% 1994 1985 1973 99% 99% 99% 99% 99% 99% 99% 99% 99%
•
•
Welding specifications and QA/QC more clearly articulated
New criteria for P-) effects
Not much different than 1997 UBC in application
Key Analysis Parameters Evaluated
• • Reduced forces at plastic hinge locations used to select beam sizes Interstory drift demand estimated based on unreduced design forces – Limited in a absolute sense (generally controls member sizes in moment frames). – New P-) effects criteria – Drifts used to select type of connection (SMF or OMF)
• global stability • local connection integrity
Prequalified Connections deemed to satisfy requirements of code Acceptance Criteria:
OMF: !SD=0.02, !U=0.03 SMF: !SD=0.04, !U=0.06 Detailed Design and Construction Requirements Specified for Prequalified Connections Welded Unreinforced Flange-Welded Web (WUF-W) Connection
•
Column axial force demand checked based on capacity of beams framing into column – compression (buckling) !tension (splice failure)
Prequalification Data WUF-W Connections
General: Applicable s ystems Hinge location dis tance Critical Beam Parameters: Maximum depth Minimum span -to -depth ratio SMF: 7 Flange thickness OMF: 1 -1/2Ó and less SMF: 1Ó or less Perm issible material specification s Critical Column Parameters: Depth OMF: Not Limited SMF: W12, W14 Perm issible material specification s Beam/Column Relations: Panel Zone strength Column/beam bending strength Connection Details Web connection Continuity plate thickness Flange weld s Weld ing parameters Weld access holes Special Connect ion DSee Fig. 3 -8; Welding QC/QA Category BM Section 3.3.3.1 Section 3.3.2.5 Section 3.3.2.4 , 3.3.2.5, 3.3.2.6 Section. 3.3.2.7 SMF: Section 3.3.3.2 A572 Grade 50; A913 Grade 50 and 65; A992 A572 Grade 50, A992, A913 Grade 50/575 W3 6 and shallower sh OMF, SMF d c /2 + d b/2
Welded Joints
• Provisions for welding and inspection are nearly identical to current provisions in AWS D1.1 • FEMA 353 has been formulated to gather requirements in one place and to tabulate these for the convenience of designers. • Some changes are highlighted in document
– Weld material properties and acceptance criteria – Weld demand and inspection requirements listed on drawings – Weld wire storage/exposure requirements
OMF: 5
Section 2.9.1
Filler Metal Toughness AWS A5 Certification 20 ft-lbs at 0oF
2.4.1.1 Appendix A
Weld QC / QA
QC/QA requirements are given for each weld of the Prequalified Connections. The form of notation is: “QC/QA Category DC/L”, where
– D indicates the Demand Category, – C is the Consequence Category and – L is the Primary Loading Direction.
SAC Heat Input Qualification (WPS) Test 40 ft-lbs at 70oF
(or Lowest Anticipated Service Temperature for cold exposure)
Notation required to be included on structural drawings
Process & Visual Welding Inspection Categories
Category W Inspection Tasks Prior During After QC QA 1 QC 2 Inspector QA
Table 6-2
Evaluation of Existing Buildings
Full PBE format used for existing buildings, or new buildings with special performance objectives. – User may select any performance objective – Documents describe conditions for collapse prevention and immediate occupancy – Immediate occupancy includes significant structural damage - but none that would reduce reliability of structural system – Confidence associated with attaining performance computed and discussed with owner – Suggested:
• 90% confidence for global behaviors • 50% confidence for local behaviors
3 QC QA
H O H O H O H O H O H O
Analysis Methods & Adjustment Factors
Structural Characteristics Analytical Procedure Linear Dynamic Nonlinear Static Performance Level Immediate Occupancy Fundamental Regularity Ratio of Period, T Column to Beam Strength T < 3.5Ts1 Regular or Any Irregular Conditions T > 3.5 Ts1 Regular or Any Irregular Conditions T < 3.5Ts1 Regular2 Strong Column3 Weak Column3 Irregular2 T > 3.5Ts Regular Any Conditions Strong Column3 Weak Column3 Irregular
2
Summary
Powerful performance evaluation method developed, evaluated and implemented for: ! evaluating and upgrading existing buildings, ! assessing repair or retrofit strategies, and ! designing new structures to special performance levels. " Incorporates system level capacity evaluation including instablility due to fracture and other forms of deterioration. " Rational implementation of complete PBE framework " Provides uniform reliability for various types of analysis " Used to manage risks and uncertainties, and to communicate these to owners, tenants and others (confidence levels).
Linear Static
Nonlinear Dynamic
Permitted Not Permitted Permitted Not Permitted Not Permitted Not Permitted Not Permitted Not Permitted
Permitted Permitted Permitted Not Permitted Not Permitted Permitted Not Permitted Not Permitted
Permitted Not Permitted Permitted Permitted Permitted Not Permitted Not Permitted Not Permitted
Permitted Permitted Permitted Permitted Permitted Permitted Permitted Permitted
Collapse Prevention
Any Conditions
Summary
!
Summary
• A systematic "approach to developing performance-based design methods for steel moment frame structures has been demonstrated to be highly effective and successful. – Integrated research, guideline development and training – Focussed substantial resources and expertise to solve complex technical, social and economic problems associated seismic loss reduction. – Widespread review by independent technical experts, design professionals, building officials, contractors, fabricators, and manufacturers. • But, many problems remain unresolved….
Details used for welded steel moment frame structures prior to 1994 have been shown to be vulnerable to brittle fracture contrary to the intent of building codes. New details, with simple design methods and stringent limitations on ranges of parameters that can be used, have been identified that are believed to satisfy building code life safety objectives. Methods have been developed for qualifying welded and bolted connections with • parameters outside the prequalified range, • having different configurations, or • requiring higher performance capabilities.
!
!
FEMA publications
Getting More SAC Information WWW Site
FEMA reports available for free from the FEMA Printing Office. Call 1-800-480-2520
www.sacsteel .org www.sacsteel.org
