-
7/28/2019 Berkley - Steel Failure Presentation - CE227SAC
1/16
FEMA Program to ReduceEarthquake Hazards in Steel
Moment-Frame Structures
A New Paradigm for Design andEvaluation of Steel Moment FrameBuildings
Stephen MahinNishkian Professor of Structural EngineeringUniversity of California at Berkeley
Chair, Project Management Committee
Steel Moment Frames
Widely used Construction ease Architectural and functional
versatility Introduction of welding
increased their efciencyand economy
Considered one of the bestearthquake resisting systemsavailable
Typical Steel Moment Frame Structures Brittle Fractures Detected in ConnectionsFollowing 1994 Northridge Earthquake
-
7/28/2019 Berkley - Steel Failure Presentation - CE227SAC
2/16
The Pre-Northridge Connection
Lateralloadsresisted bymomentsdevelopedin frame
Beams welded andbolted to columns
Complex joint
Brittle Connection Damage
Generally conned to vicinity of welds of beams to columnconnections
Occurred in many types of weldedsteel moment frame buildings New and old buildings Tall and short buildings Conventional and important
buildings Contrary to intent of modern
building codes
A New Solution Approach! Brittle nature of damage invalidated basic assumptions used to design
and evaluate steel moment frames! To nd 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 EarthquakeHazards of Steel Moment-Frame Structures
the design and construction of new steel moment-frame buildings,
the identication, inspection, evaluation and retrotof existing at-risk welded steel moment-framebuildings, and
the identication, evaluation, repair or upgrading of damaged buildings following earthquakes.
Goal: Develop reliable, practical and cost-effective
guidelines and standards of practice for:
-
7/28/2019 Berkley - Steel Failure Presentation - CE227SAC
3/16
State of the ArtReports
The FEMA/SAC Steel Project
Materials andFracture Issues
Welding, Joiningand Inspection
Analysis and Testingof Connections
EarthquakePerformance
Simulation of Seismic Response
Reliability Framework for
Performance Predictionand Evaluation
TrialDesigns
CostAnalysis
LossAnalysis
SeismicDesignCriteria
Social, Economicand Policy Issues
BuildingCodes
C a p a c i t y
D e m a n d
Other
The GuidelinesFEMA-350: Recommended Seismic Design Criteria for New Steel Moment-Frame
Buildings.
FEMA-351: Recommended Seismic Evaluation and Upgrade Criteria for ExistingWelded Steel Moment-Frame Buildings.
FEMA-352: Recommended Post-earthquake Evaluation and Repair
Criteria for Welded, Steel Moment- FrameBuildings.
FEMA-353: Recommended Specications and QualityAssurance Guidelines for Steel Moment-Frame Construction for Seismic Applications.
FEMA-354: Policy Guide for Steel Frame Construction
P r o g
r a m
t o R e
d u c e
t h e E
a r t h
q u a k
e H a z
a r d s
o f
S t e e
l M o m
e n t F
r a m
e S t r u
c t u r e
s
F E D E R A
L E M E R G
E N C Y M
A N A G E M
E N T A G E
N C Y F E M A 3 5
0 J u l y, 1
9 9 9
R e c o m m e
n d e d S e i s
m i c D e s i
g n C r i t e r
i a
f o r N e w M
o m e n t - R
e s i s t i n g
S t e e l F r a
m e B u i l d i
n g s
P r o g r a m t o R e d u c e t h e
E a r t h q u a k e
H a z a r
d s o f
S t e e l
M o m
e n t F
r a m e
S t r u c t u r e s
F E D E R A L E M E R
G E N
C Y M A N A G E M E
N T A G E N C Y F E M A 3
5 2 J u l y,2 0 0 0
R e c o m m e n d e d S e
i s m i c D e s i g n C r i t
e r i a
f o r N e w M o m e n t
- R e s i s t i n g
S t e e l F r a m e B u i l d
i n g s
P r o g r a m
t o R e d u c e
t h e
E a r
t h q u a k e
H a z a r
d s o f
S t e e l
M o m e n
t F r a m e
S t r u c t u r e s
FEDERALEMERGENCYMANAGEMENTAGENCYF
EMA351 July,1999
Recommended Seismic Design Criteriafor New Moment-ResistingSteel Frame Buildings
P r o g r a m
t o
R e d u c e t h e
E a r t h q u a k e
H a z a r d s
o f
S t e e l M o m e n t F r a m e S t r u c t u r e s
F E DE R AL E ME RGE NCY M AN AGE
ME NT AGE NCY F E M A 350 J ul y , 19 9 9
R ec o mmended Sei smi c D esi g n C r i t er i af o r N ew M o m ent -Resi st i n g Ste el F r ame Buildi n g s
Results synthesized asState of the Art Reports
FEMA-355A: Base Metals and Fracture
FEMA-355B: Welding and InspectionFEMA-355C: Systems Performance
FEMA-355D: Connection Performance
FEMA 355E: Past Performance of Steel Moment-Frame Buildings inEarthquakes
FEMA-355F : Performance Prediction and Evaluation
Plus more than 80 detailed technical reports
Why did this happened?
Early assertions suggested brittle fractureswere 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
-
7/28/2019 Berkley - Steel Failure Presentation - CE227SAC
4/16
Assertion: Damage due to unusualseverity of ground shaking?
! While ground motion was severe, it was not greater thananticipated in design of many damaged buildings.
! Most buildings were substantially (two to three times)stronger than minimumcode forces.
! Many fractures occurred inbuildings that should haveresponded elastically
! Typically, D/C < 20%
20%
40%
60%
80%
100%
0 0.2 0.4 0.6 0.8 1
Demand/Capacity
Undamaged Damaged
Assertion: Damage was due toinadequate workmanship?
! In many cases, workmanship was inadequate andsome construction practices led to poor quality welds
! Test results indicate that improved workmanshipwas generally insufcient by itself to achievereliable performance.
! All pre-Northridge connections tested by SAC failedbrittlely, reecting all of the fracture modes seen inthe eld.
Identical Lab and Field Damage Assertion: We can predict damagelocations by computer analysis
Only modest correlation of local damagelocation to computer predictions Fracture criteria unknown? Sensitive to modeling assumptions
Regions (oors) with higher D/C ratiostend to have higher damage
13 Fractured Connections
6 Highest D/C Ratios
-
7/28/2019 Berkley - Steel Failure Presentation - CE227SAC
5/16
Pre-Northridge Welded Connections
Behavior of Pre-Northridge welded steel momentconnections influenced by many interacting factors,including:
! Load transfer mechanism" Frame configuration" Basic geometry of connection" Shear transfer mechanism" Panel zone deformations, etc.
! Quality of Welds! Fracture sensitivity of typical connection
A closer look
High forcetransfer atconnection
Weak section atface of column
Numerousstress risers
in typical joint detail
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.BackingBar
What forces should the welds resist?
Changing steel
properties Typical beam design
assumptions awednear connection (St.Venant 1855)
Tf = F ypr A flange
MV
-
7/28/2019 Berkley - Steel Failure Presentation - CE227SAC
6/16
Non-uniform distribution of axialstresses in beam ange at column face
High triaxial tension
Beam anges carry considerable shear
D i s t a n c e
f r o m
B o t
t o m (
x / D )
0
0.5
1.0
0 1 2Normalized Shear Stress
D/2 fromcolumn face
At columnface
Local Flange Deformation Panel Zone Yielding
Strong Panel Zone -------------- Weak Panel Zone --------------
Kinked columncauses highlocal bending incolumn and beam
flanges
-
7/28/2019 Berkley - Steel Failure Presentation - CE227SAC
7/16
Why did These Connections Fracture?
Highly variable stress andstrain distributions develop.
But, if steel isductile,why didntit just yield?
For many typical pre-Northridgeconnections, the combination of: joint geometry imperfections K Ic for the weld metal
were such that F critical ! f y
In such cases, the joint would likelyfracture brittlely before yielding andforming a plastic hinge in the beam
Backing Bar
Weld rootdefect
Fcritical = K Ic/[(! C i)(" a)1/2 ]
Fracture Susceptibility
CVN or K I
Temperature
Material FractureToughness
Some Alternatives ConsideredWelded Connections Improved unreinforced
connections.
Reinforced connections Welded ange plate
connections Reduced beam section
connections
Some WeldedConnections
-
7/28/2019 Berkley - Steel Failure Presentation - CE227SAC
8/16
Plan B - Bolted Connections
Bolted Connections Tee-stub Connections Bolted ange plate
connections End plate connections
Gravity Connections Simple connections with
and without slabs
End Plate
Tee- Stub
FlangePlate
Shear Tab
Slab Some BoltedConnections
Case in point: Fracture control strategy for unreinforcedconnections
One might control fracture by:! Using notch toughness weld metals! Controlling imperfections! Improving the joint geometry
F critical = K Ic/[(! C i)(" a)1/2 ]
Theoretical & Experimental Verification Required
However,! We enter into another range of
fracture mechanics related to plasticinitiation, and plastic crack growth under cyclic loading, and
! There may be other failure mechanisms.
Hightoughness
welds
Removebacking bar and
reinforce rootpass
Rotations developed in Stage Iunreinforced connections
PreNorthridgeDetail
Notch ToughSpecimens
-
7/28/2019 Berkley - Steel Failure Presentation - CE227SAC
9/16
High triaxial tension
High stress attoe of weld cope
Other Locations have High Stresses
Elastic range
Computed Critical Plastic Strains at Large Drift
T
T
Eccentric shear link action
Behavior sensitive toshape and finish of weld access hole
Bi-directional bending in beam flangeat toe of weld access hole
T
T
Plastic Crack Initiation andGradual Growth Under Cyclic Loading
Weld at column face protected by improveddesign, but failure shifts to next weakest link
-
7/28/2019 Berkley - Steel Failure Presentation - CE227SAC
10/16
#pmax = 5.0% rad .
Specimen C2 upon completion of testing Lateral load-displacement relationship
Continued refinement leads toprequalified connections
Identify and characterize all local failure modes; Specifydesign method that controls connection behavior
Improved Weld Access Holes
Systems Approach
Need method to relate capacities and demandsBuilt upon transparent reliability framework
Utilization of engineering knowledge
Manage risk and uncertaintiesPerformance-based engineering concepts
Probability
Performance Parameter
Demand
D
D(1+ $ d%d) > C(1- $ c%c) Load and ResistanceFactor Design Format
&(' D) < ( CCapacity
C
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 beachieved and is unrealistic given the uncertaintiesinvolved.
Re-phrases statement as:
I am highly /moderately /not confident that a statedperformance level will be achieved for a given seismichazard; for example,...
I am 50% confident that the structure will notcollapse 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
New SAC Approach
-
7/28/2019 Berkley - Steel Failure Presentation - CE227SAC
11/16
System Demand Estimates
ResponseParameter 1
time
Accel, g.
Probability S a
S a1 1 T, sec.
2% in 50 yrs.
time
time
Probability
Performance Parameter 1
Demand
D
%d
M
#
Such demand analyses used to: Develop analysis adjustment
factors to account for: Simpler analyses procedures Modeling simplications
Develop cyclic loadinghistories for testing
Understand effect of: Ground motion intensity and
characteristics Structural conguration
Connection fracture Deterioration of connection
hysteretic loop characteristics Alternative connection types Aftershocks Prior damage or defects
1
3
5
7
9
11
13
15
17
19
21
0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09Story Drift Angle
F l o o r
L e v e l
20-STORY
Collapse Prevention capacity evaluatio Ductile modes associated with
Local failure of plastic hinge (from tests) Dynamic instability of system as a whole (analysis)
Brittle modes effecting vertical stability,particularly column failure modes
Global System Performance
Probability
Capacity
C system%c#CP#IO
M
Drift
Local Connection Performance
For New Construction
Performance Criteria
BuildingHeight
GlobalStability
LocalStability
3 stories 99% 99%
9 stories 99% 95%
20 stories 96% 96%
Representative confidences of not exceeding performancecriteria in Los Angeles for 2% in 50 year earthquake hazard
-
7/28/2019 Berkley - Steel Failure Presentation - CE227SAC
12/16
SAC vs. 1994 UBC Designs
Performance Criteria
GlobalStability
LocalStability
BuildingHeight
SAC 1994 SAC 1994
3 stories 99% 88% 99% 22%
9 stories 99% 57% 95% 29%
20 stories 96% 57% 96% 39%
Representative confidences of not exceeding performancecriteria in Los Angeles for 2% in 50 year earthquake hazard
Reliabilities for different age building
Performance CriteriaLocal StabilityBuilding
Height SAC 1994 1985 1973
3 stories 99% 22% 9% 2%
9 stories 95% 29% 21% 7%
20 stories 96% 39% 42% 2%
Representative confidences of not exceeding performancecriteria in Los Angeles for 2% in 50 year earthquake hazard
Reliabilities for different age building
Performance Criteria
Local StabilityBuildingHeight SAC 1994 1985 1973
3 stories 99% 99% 99% 99%
9 stories 99% 99% 99% 99%
20 stories 99% 99% 99% 99%
Representative confidences of not exceeding performancecriteria in Los Angeles for 50% in 50 year earthquake hazard Use NEHRP provisions for structure
analysis and proportioning: Denition of design earthquake
Analysis procedures and modeling Force reduction factors, redundancy
factors, drift limits, etc. Proportioning (strong column-weak
girder, etc.)
New criteria for P- ) effects
Use prequalied connections: Explicit design calculations Limits on range of materials, sizes,
relative strengths, details, etc. that canbe used
Welding specications and QA/QCmore clearly articulated
Not much different than1997 UBC in application
Design Provisions for New Buildings
-
7/28/2019 Berkley - Steel Failure Presentation - CE227SAC
13/16
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 momentframes).
New P- ) effects criteria
Drifts used to select type of connection (SMF or OMF) global stability local connection integrity
Column axial force demand checked based on capacity of beams framing intocolumn compression (buckling) ! tension (splice failure)
Welded Unreinforced Flange-Welded Web (WUF-W) Connection
Detailed Design andConstruction RequirementsSpecified for PrequalifiedConnections
Prequalified Connectionsdeemed to satisfyrequirements of code
Acceptance Criteria:OMF: ! SD =0.02, ! U =0.03SMF: ! SD =0.04, ! U =0.06
eneral:
Applicable systems ,
Hingelocationdistance s h critical Beam Parameters:
Maximum depth
Minimumspan -to-depthratio
:
Flange thickness : -:
Permissiblematerial specifications , ,
ritical Column Parameters:
Depth :
: ,
Permissiblematerial specifications ; ;
eam/Column Relations:
Panel Zone strength : . . .
Column/beam bending strength
onnection Details
Web connection . - ;
Continuityplate thickness . . .
Flangewelds . . .
Wel d in g pa ra me te rs . . . , . . . , . . .
Weld access holes . . . .
Prequalification Data WUF-W Connections
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
-
7/28/2019 Berkley - Steel Failure Presentation - CE227SAC
14/16
Filler Metal Toughness
AWS A5 Certification
SAC Heat Input Qualification (WPS) Test
20 ft-lbs at 0 o F
40 ft-lbs at 70 oF(or Lowest Anticipated Service Temperature
for cold exposure)
2.4.1.1Appendix A Weld QC / QA
QC/QA requirements are given for each weld of thePrequalied 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.
Notation required to be included on structural drawings
After
During
Prior
OHOHOHOHOHOH
QAQCQAQCQAQC
InspectorWInspection
Tasks
321Category
Process & Visual WeldingInspection Categories
Table 6-2
Full PBE format used for existing buildings, or new buildings with specialperformance objectives. User may select any performance objective Documents describe conditions for collapse prevention and immediate
occupancy Immediate occupancy includes signicant structural damage - but none that
would reduce reliability of structural system Condence associated with attaining performance computed and discussed
with owner Suggested:
90% condence for global behaviors 50% condence for local behaviors
Evaluation of Existing Buildings
-
7/28/2019 Berkley - Steel Failure Presentation - CE227SAC
15/16
Analysis Methods & Adjustment FactorsS tr uc tu ra l C har ac te ri sti cs A nal yt ic al P ro ce du re
PerformanceLevel
FundamentalPeriod, T
Regularity Ratio of Column toBeamStrength
Linear Static
Linear Dynamic
Nonlinear Static
Nonlinear Dynamic
T < 3.5 T s 1 Regular or Irregular
AnyConditions
Permitted Permitted Permitted PermittedImmediateOccupancy
T > 3.5
T s
1
Regular or Irregular AnyConditions NotPermitted Permitted NotPermitted Permitted
CollapsePrevention
StrongColumn 3
Permitted Permitted Permitted PermittedRegular 2
WeakColumn 3
NotPermitted
NotPermitted
Permitted Permitted
T < 3.5 T s 1
Irregular 2 AnyConditions
NotPermitted
NotPermitted
Permitted Permitted
T > 3.5 T s Regular StrongColumn 3
NotPermitted
Permitted NotPermitted
Permitted
WeakColumn 3
NotPermitted
NotPermitted
NotPermitted
Permitted
Irregular 2 AnyConditions
NotPermitted
NotPermitted
NotPermitted
Permitted
SummaryPowerful 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).
Summary! Details used for welded steel moment frame structures prior
to 1994 have been shown to be vulnerable to brittle fracturecontrary to the intent of building codes.
! New details, with simple design methods and stringentlimitations on ranges of parameters that can be used, havebeen identified that are believed to satisfy building code lifesafety objectives.
! Methods have been developed for qualifying welded andbolted connections with
parameters outside the prequalified range, having different configurations, or requiring higher performance capabilities.
Summary A systematic approach to developing performance-based design
methods for steel moment frame structures has beendemonstrated 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 associatedseismic loss reduction.
Widespread review by independent technical experts, designprofessionals, building ofcials, contractors, fabricators, andmanufacturers.
But, many problems remain unresolved.
-
7/28/2019 Berkley - Steel Failure Presentation - CE227SAC
16/16
Getting More InformationSAC WWW Site
www.www. sacsteel sacsteel .org .org
FEMA publications
FEMA reports availablefor free from the FEMAPrinting Office.
Call 1-800-480-2520