berkley - steel failure presentation - ce227sac

7/28/2019 Berkley - Steel Failure Presentation - CE227SAC

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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


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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:


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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

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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


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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


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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


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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


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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


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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


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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


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#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


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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


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SAC vs. 1994 UBC Designs


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%


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%


Reliabilities for different age building


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


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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


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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


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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.


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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