handout george christian (1).pdf

8/20/2019 handout George Christian (1).pdf

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Bridge Failures - Lessons learned

George A. Christian, P.E.Director, Office of Structures

New York State Dept. of Transportation

Bridge Engineering Course

University at Buffalo

March 29, 2010


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Bridge Failures – Lessons Learned

Outline

– Overview of Bridge Failures

• Historic Failures in North America

• Recent U.S. failures that impacted bridge engineering

practice

• Lessons and Response

o

Recent NYSDOT BridgeFailure Investigations

oDealing with a failure

Part 1:

Part 2:


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My general lessons from bridge failures

• Bridges can, and will fail, if not properly designed,

constructed and maintained

• We may think we know everything to prevent

failures, but we do not.• In hindsight, most failures could have been

prevented (but not all).

• Failures generally result from a confluence ofcontributing events and/or underlying causes.

• When it comes to underlying causes, history can

repeat itself.


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―Honest human error in the face of the

unforeseen—or the unforeseeable—isultimately what brings bridges down.‖

J.Tarkov, “Human Failure In, Bridge Failure Out ”,

Engineering Case Library report “ECL 270”, Carleton

University, CA

Two “Historic” Bridge Failures


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Quebec Bridge1800 ft. main span, collapsed Aug 29, 1907

Buckling Failure of compression

chord (A9L) –inadequate latticing


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Quebec Bridge Collapse -Findings

• Higher allowable stresses specified

• Underestimated dead load ( 18% +/-)

– Decision to lengthen span by 200 ft.

– Error discovered but accepted

• Financial pressures

• Project Management issues

– Ceding to Consulting Engineer reputation

– Lack of experience on site

– Communication failures


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Second Quebec Bridge - 1917

construction collapse Sept 1914


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Tacoma Narrows Bridge collapse- 1940


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Advancements in suspension bridge analysis (deflection theory)

Williamsburg Bridge

-1903

1600 ft. span, 40 ft.

deep stiffening truss

(Depth: span = 1:40)

Manhattan Bridge -1909

1470 ft. span, 27 ft. deep

stiffening truss (1: 54)


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1920’s -- Highway suspension bridges become practical

Bear Mountain Bridge -1924

1632 ft. spanWurts Street Bridge,

Kingston, NY -1921

705 ft. span


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1930’s--“Landmark” Bridges

Golden Gate Bridge - 19374200 ft. span, d:s = 1: 168

George Washington Bridge -1931

3500 ft. span, d:s = 1: 120

Originally opened with upper level

roadway only, no stiffening truss

d:s = 1: 350


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1930’s: maximize structural efficiency, economy, aesthetics

Plate girder in place of truss for deck stiffening


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Bronx Whitestone

Bridge -1939

-- 2300 ft. span

-- 11 ft. girder

-- d:s = 1: 209

--77 ft. wide, w:s = 1:31

--BWB and other new

suspension bridges withshallow stiffening girders

exhibit wind-induced

Vertical oscillations

--Early retrofits

implemented


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Tacoma-Narrows Bridge--1940

--2800 ft. span--8 ft. girder

--d:s = 1: 350

--39 ft. width, w:s = 1:72

Problem with vertical

oscillations-

Retrofits:

Clamp cable to girder @midspan

Side span tiedowns

Wind tunnel studies initiated


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Lessons Learned

• Lack of understanding of aerodynamics effects

• Extrapolated past design successes

• Economic pressures affecting design

• Emphasis on structural efficiency

• Lack of emphasis on designing to avoid failure

• Inadequate regard to failures of 19th century flexible

suspension bridges


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Impacts of TNB failure

• Intensive research on aerodynamic behavior – Still no unanimous consensus on actual cause

• Buffeting, Vortex shedding, Torsional flutter…

• Wind tunnel tests during design for all cablesupported structures (suspension and cable stayed)

• Ended use of stiffening plate girders

• Stiffening trusses continued to be used until 1970’s


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“Post-Tacoma” new bridges

Tacoma-Narrows Bridge

Replacement - 1950 Mackinac Straits Bridge -1954


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Thousand Islands Bridge -Retrofits


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Deer Isle Bridge retrofits


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Bronx-Whitestone Bridge

retrofits•Tower stays

•Stiffening truss retrofit

•Tuned mass Damper

at midspan


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Bronx-Whitestone Bridge --second retrofit 2007

•Replaced Concrete

deck with Orthotropic

steel deck

•Removed Stiffening

Trusses

• Added lateral bracingto lower flanges

• Added wind fairings

on stiffening girders

•Diagonal stays and

tuned mass damper

remain

Reduce Dead load,

improve torsional stiffness,

improve aerodynamic behavior


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“Recent” U.S. bridge Failures of significance(and one less significant failure)

• Last 30 years

• Had Significant impact on Federal and State agencybridge management and safety practices

• NTSB findings and recommendations


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Silver Bridge collapse

• Collapse initiated by eyebar fracture

– Initiated at a crack• Stress corrosion cracking

– High residual stress

– corrosion fatigue

At time of design these phenomena werenot known to occur with materials and

conditions present.

– Higher traffic loads than when

originally designed – New high strength steel had low

toughness

• Flaw was inaccessible to inspection

• Lack of Redundancy


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Silver Bridge Collapse

consequences

• Burning Question : How many other bridges canhave a similar fate??

• Resulted in Federal National Bridge Inspection

Standards regulations – National bridge inventory

– Biennial inspections

– Inspector qualifications – Reporting requirements

• New research: fracture mechanics, materials…


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Mianus River Bridge collapse

• Failure of pin and hanger assembly supporting suspended span – Hanger displaced laterally, worked off the pin

– Transferred (eccentric)load to other hanger

– Hanger worked outward, fractured pin

• Underlying causes

– Corrosion- unmaintained

drainage system

– Lack of redundancy

– Skew


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Mianus Bridge Collapse

Consequences

• Fracture Critical Inspection requirements

– Visual “hands on” every 2 years

– NDT methods

• Pin and Hanger inspection NDT methods

improved


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Mianus Bridge Collapse Consequences

New York DOT Response

• Add redundancy to all 2 and 3girder Pin and Hanger bridges

(approx. 24 bridges)

• Over time, these bridges (or

superstructures) have beenreplaced or made redundant /

continuous


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Mianus Bridge Collapse Consequences

New York DOT Response

• Detailed Inspections of 3 and 3 welded girderbridges (hands-on and NDT)

– Found many fatigue prone details, cracks

– Removed flaws, tab plates, drilled out cracks

– Some prioritized for replacement

Lesson – in 1960’s welding

became popular and economical,

however effects of fatigue andunintended structural participation

was not fully recognized.


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A near collapse

Hoan Bridge, Milwaukee, WIBuilt 1970, Failure on Dec. 13, 2000

Brittle fractures that originated

at a lateral bracing system

connection to the girder, where a

horizontal shelf plate intersects atransverse connection plate with

intersecting and overlapping

welds.

2 of 3 girders completely

fractured full depth


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Hoan Bridge Failure

• Connection detail provided high tri-axial

constraint at the web, resulted in very highstress concentration (1.6 x Fy).

• Very small initiating crack in web,

critical crack size not detectable.

• Cold weather contributed to

brittle behavior of steel.

• Steel toughness met spec.

requirements

Hoan Br idge Forensic I nvestigation,

Failur e Analysis F inal Report;

Federal Hwy. Admin. and Wisconson DOT,

2001


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(The one less significant failure)

New York County Road Bridge Failure -1986

• Significant section losson trusses ( up to 50%)

• Lack of redundancy

• Excessive dead load:

– Timber deck replaced bya steel pan deck with

asphalt

– 50 psf from 20 psf

• Shows importance

of load ratings

• Bridge should have

been closed

200 ft. deck truss span – one lane bridge

Load posted for 8 tons

Failure initiated by 16 ton truck crossing

the bridge


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Schoharie Creek BridgeNYS Thruway over Schoharie Creek

Built 1954, Collapsed April, 1987


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Schoharie Creek Bridge failure(NTSB Findings)

• Caused by scour undermining pier foundation

– 50 year flood event

– Spread foundations on dense glacial till

– Inadequate rip rap protection

• Inadequate rip rap size

• Damage from prior flood events

• Rip rap not maintained


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Schoharie Creek Bridge failure

• Contributing

causes- Lack of:

– Redundancy

– ductility in piers

– resiliency

f


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Follow Up Actions in NY

– Improved hydraulic and scour evaluations

• Post flood inspections

• Flood warning action plan

– Bridge Safety Legislation• Uniform Code of bridge inspection

– Codified inspection requirements

– Structural integrity evaluations

• NYSDOT oversight of Authorities, local owners• NYSDOT authority to close unsafe bridges

– Priority given to bridge inspection program

S h h i C k B id f il


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Follow Up Actions in NY

Bridge Safety Assurance (BSA) Initiative

– Program of assessment of bridges’ vulnerability

to structural failure due to their inherent

characteristics or due to extreme events – Assessments are made for individual failure

modes

“Identify causes of failure beyond condition”

(Why do Bridges Fail?)

Bridge Failures in the US: 1966 2005


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Bridge Failures in the US: 1966-2005

“Cause of Bridge Failures from 1966 to 2005”

Figure courtesy of J-L Briaud, Texas A&M University


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NYSDOT Bridge Failure Database


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• Sytematic evaluations of bridges based on individual

failure modes.

Hydraulics Steel Details

Overload Concrete Details

Collision Earthquake

• Evaluate statewide bridge population:

Screen Assess Classify

• Vuln. Classifications consider failure likelihood andconsequence.

• Evaluation data needs collected during bridge

inspections

NYSDOT Bridge Safety Assurance Initiative

Vulnerability Assessments


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• Scour repairs

• Steel Detail Retrofits

• Add Redundancy

BSA Retrofits

Vulnerability score may

influence rehab / replace

decision


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I-35W over Mississippi RiverBuilt 1967 , collapsed Aug 1, 2007


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• Inadequate load capacity of gusset plates at U10 joints,

attributed to design error• Substantial increases in weight of the bridge from prior

modifications

• Concentrated construction loads combined with traffic

I-35W over Mississippi River

NTSB Findings



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Inadequate Gusset plate thicknesses at U10 and L11

(NTSB) Contributing Cause: Failure of designer Quality

Control Procedures

Deficiency seems “evident” in hindsight.

Lesson: Design errors can slip through.

NTSB


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Bowed gusset plates suggested problem for further investigation.

NTSB

(NTSB) Contributing cause: Inadequate attention to gusset plates by

transportation agencies during inspections.


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Response by DOT’s and FHWA

• Inspections of all non-redundant deck truss bridges

(How many other bridges can have a similar fate?)

• Guidance on construction loads and stockpiling on bridges

• Gusset plate analysis

– Include gusset plate analysis in load capacity evaluations

– Evaluate gusset plates on all bridges that have undergone a substantial

change in load.

• Gusset Plate Analysis Research – NCHRP 12-84

• FHWA Advisory on non-destructive testing of gusset plates



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I 35W over Mississippi River

NYSDOT actions

• Inspected 50 deck truss bridges in NYS

• Analyzed Gusset Plates on 133 Trusses that had undergone asubstantial change in load.

• Developed analytical tools for gusset plate design and load

capacity checks (LFD and LRFD)

•Did not find design errors

similar to I-35W

•Found problems due to

deterioration

•Developed gusset repair and

replacement procedures

•Closed / replaced 1 bridge

due to gusset evaluations


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Failures Caused by Extreme Events


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Failures Caused by Extreme Events• Earthquakes

• Collisions

– Vessel – Vehicle

• Storm surge

• Fire


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Failures Caused by Extreme Events

Lessons learned result in improved design

specifications, detailing practices

– Seismic research,

– AASHTO seismic specifications

– AASHTO Guide specs. for Vessel Collision

– AAHSTO Guide specs. For Bridges Vulnerable to Coastal Storms

--NCHRP 12-85:Highway Bridge Fire

Hazard Assessment

--NCHRP 12-72:

Blast Resistant Highway

Bridges- Design and

Detailing Guidelines


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Failures during Construction

When a bridge may be

most at risk to a

structural collapse.



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Rt 470 / I-70 overpass, Golden CO; May 15, 2004

Probable Cause of Failure (NTSB Report):

―Failure of temporary bracing system due toinsufficient planning….‖

Contributing causes:

--girder installed out of plumb.

--inadequate standards for temporary bracing

--inadequate oversight

“Only ifs “ ---Problem reported by passerby, but miscommunication occurred.

---Subsequent girder erection was delayed

(NTSB) Recommendations / Lessons:

Improve standards for temporary works and erection procedures (FHWA, State

DOT, AASHTO, OSHA)

-Prequalification

-Submit written plan, dwgs.

-Certified by a P.E

Failures during construction


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Potential Issues

• Bridges are often in their most failure vulnerable

state during construction

• Considering construction states during design

– Design focuses on completed structure in service

– Specs may be vague in addressing construction states• Division of responsibility between designer and

contractor/erector.

– Designer responsibility for a constructible bridge

– Contractor responsible for means and methods for

construction.

l d


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Lessons

• Must provide a constructible design – Contract documents show one feasible method of

construction (plans or notes)

• Design specs shall address constructability

– Design loads, limit states during construction

• Structural construction operations shall be designed,

certified by a P.E., submitted for approval

– Temporary structures, temporary works – Erection Drawings

– Structural lifting


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Questions?


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Bridge Failures - Lessons learned

Recent NYSDOT Bridge Failure Investigations

George A. Christian, P.E.

Director, Office of StructuresNew York State Dept. of Transportation

Bridge Engineering Course

University at Buffalo

March 29, 2010


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4

Structure Layout (looking east)


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5

Overview of Failure


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6

High Rocker Bearings

History of Misalignment of High


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7

History of Misalignment of High

Rocker Bearings at Pier 11

1987Inspection

Temp. @ 45 °

1999Inspection

Temp. @ 70 °


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8

2003 Inspection – Span 12 East Bearing

Temp @ 45 F

Lifted 0.25 ft.(3 in.) - Eccentricity = 3.4 in.

How did the bearings get misaligned?


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9

How did the bearings get misaligned?

Superstructure Displacements:

• Survey of adjacent piers (w/ fixed bearings) – Pier 10 displaced north 1.6 inches.

– Pier 12 displaced north 1.0 in (avg.) 1.7 inches oneast side.

• History of Pier 13 joint

– Joints ‘reset’ (vertical) in 1990

– Joint was closed in 1990

– Closed in 1995 thru present

• Longitudinal forces due to braking, centrifugal force


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10

Condition of Rocker Bearings• Susceptible to corrosion, debris when continually

tilted

• Corrosion, debris prevents rocking back towardvertical

– Under rocker – Pin corrosion

• Contact surfaces flatten or “dish”

• Result: Bearings become resistant to horizontal

movement, especially back toward being plumb .Transfers longitudinal forces to substructure


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11

Corrosion &

Flattening


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13

Rocker and Pintle Corrosion – Span 11

Bearing

Pier 11


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14

Pier 11

• Height: 82.3 feet

• 13.9’ x 6.44’ at

base

• 9’ x 4’ at top of

stem

• Stem rebar: 46 -

# 8 bars


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15

Pier 11: Lack of Elastic Range

• Cracks 40 ft. upnorth face

• Rebounded51/2 in. when‘released’

• The pier failed

in flexure


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16

Comparison of Adjacent Piers

PIER

NO.

HGT. BASE REINFORCING STEEL

No.-Size Bars Area

BEARINGS

FIX OR EXP

9 67.47’ 131.4” x 71.2” 42 - # 8 33.18 Fix - Exp

10 72.79’ 132.6” x 72.6” 36 - #11 56.16 Fix

11 82.31’ 166.6” x 77.3” 46 - # 8 36.34 Exp - Exp

12 83.38’ 155.3” x 77.6” 42 – #11 65.52 Fix

13 84.75’ 156.4” x 84.2” 42 - #11 65.52 Exp - Exp


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17

Pier 11 Design Check

• Designed per 1965 AASHO code

• Meets strength req. for code assumptions

– Allowable / Actual ratio = 0.98 =1.0 (OK)

• No provision for large flexural displacements

• Equivalent Column with 1% reinforcing.

R lt f Pi A l i


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18

Results of Pier Analysis

• Limited elastic range - yields at 5.5” deflection

• Cracking at 2.5” deflection

• No capacity increase beyond cracking

Old Pier 11 (AC-12)

0

20

40

60

80

100

120

140

160

0 5 10 15 20

Longitudinal Pier Cap Displacement [in.]

L o n g i t u d i n a

l P i e r C a p

F o r c e [ k i p s ]

f’c=9

210psi


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19

Forces Needed for Failure

• Thermal: limited by resistance of bearing

– Up to 0.58 x Dead Load if sliding assumed: approx.

200 kip from Span 12 only

– Limited range of movement• Corrosion Build-up:

– Develops horizontal component of vertical dead, live

load reactions

– Larger range of movement

Probable Failure Sequence


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20

Probable Failure Sequence

• Bearings tilted to north > 20 years ago

• Bearings begin to resist horizontal mov’t.

• Superstructure longitudinal displacements began tomove pier instead of Span 12 bearings

• Bearings resist movement moving back toward vertical – Increased southward tipping of Span 12 and 11 bearings

• Instability point reached – bearings tipped

• Forces (displacements) sustained to deflect pier 16 +

in. (bearings tipping and spans falling on tippedbearings)


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21


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22


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25


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26


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32

U d l i C f F il


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33

Underlying Cause of Failure

1. Rocker bearings becoming misaligned

2. Rocker bearing not functioning properly

3. Pier 11 was flexible in direction longitudinal

to the bridge

4. Pier 11 stem was “lightly” reinforced, and

not elastically ductile

• 1, 2 and 3 were required for failure to occur.

4 may have been required, but contributed

to extent of failure


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35

Follow up actions


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36

Follow up actions

• Reviewed all high rocker bearings with lowinspection ratings (CR 3 or less)

• Inspected those overextended

• Preventive interim retrofits – bolsters

• Technical Advisory: INSP 05-001

Follow up actions


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37

Follow up actions

• Bolsters installed as an extraordinaryprecautionary measures on 10 bridges

• Alerted other owners of bridges not underDOT’s inspection jurisdiction

• Corrective action: – Dunn Complex, bearing replacements

Marcy Pedestrian Bridge Collapse


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y g pOctober 2002

South Abutment

North Abutment

Bracket

Field Splice

Span = 171 ft.


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Acknowledgements

• Sponsored by New York State DOT

• P.I.—Weidlinger Associates, Inc.

• Material testing and weld inspections byATLSS Research Engineering Center, Lehigh

University


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Tub Girder Cross Section


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Tub Girder Cross Section

Intermediate

Diaphragm

14.0 ft (4.27 m)

6 . 3

f t ( 1 . 9

3 m )

4.3 ft (1.3 m)

Collapsed Bridge


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Collapsed Bridge

North Abutment

South Abutment

Screed

Collapsed Bridge


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Collapsed Bridge

South Abutment

Exp. Bearing

Collapsed Bridge


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Collapsed Bridge

North AbutmentFixed Bearing


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2. Review of Bridge Design

Objective: Evaluate the adequacy of the

bridge design

2 1 D i C d


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2.1 Design Codes

• NYS Standard Specifications for Highway Bridges withprovisions in effect as of April 2000.

• AASHTO Standard Specifications for Highway Bridges, 16th

Ed. LFD (1996) with 1997, 1998 & 1999 interim

• · AASHTO Subsection 10.51 Composite Box Girders (LFD) ….

“This section pertains to the design of … bridges of

moderate length supported by two or more single cell

composite box girders…..”

2 3 Finished Bridge


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2.3 Finished Bridge

• Design assumption: Two I-girders

• Conclusions: The bridge, as designed, would have been

sufficient to resist its design loads if it had survived itsconstruction.

2 4 During Construction


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2.4 During Construction

Failure Modes:

• b/t of top flange;

•

Top flange buckling(between

intermediate diaphragms)

• Global Torsional buckling

Intermediate

DiaphragmTop

Flange


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3. Analysis of Bridge Failure

Objective: To find and prove the cause offailure

3.1 Deck Construction Facilities:


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Metal Form

Angle

Bottom Flange

Bracket

(3' apart)

Form/Catwalk

Tie-rod

(4' apart)

Web Concrete

Top Flange

Web

Hanger West

Operator

Drum

C.L. Bridge

Engine

Screed

East

3.3 Elastomeric Bearings


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Top

Top Steel Plate

Bottom Steel Plate

Elastomer Steel

Plates

a). Expansion Bearing

Top Steel Plate

Bottom Steel Plate

b). Fixed Bearing

Steel Pin

k ESk ES

k EC

X

Y

Z k BRG=k ES+k PIN

k FS

k EC

X

Y

Z

FD

Nonlinear Spring

k PIN


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3.5 Global Model


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End

Diaphragm

Top Flange

Top Flange

Web

Diaphragm

Strut


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SW+DL (rebar, form, etc.)Concrete Screed

Steel girder

North

Abutment

-10.0

-9.0

-8.0

-7.0

-6.0

-5.0

-4.0

-3.0

-2.0

-1.0

0.0

0.0 10.0 20.0 30.0 40.0 50.0

Concrete Pour Length, m

G i r d e r R o

t a t i o n ,

D e g r e e

As-built

As-designed (ideal)

Y

Rotation

42'

105'

3.6 Analysis Results


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3 9 Form Connection Model


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F

Fixed Boundarya

Weak Form: a=1/2" (12 mm)Fy = 40 ksi (275 MPa)

Strong Form: a=3/4" (20 mm)Fy = 45 ksi (310 MPa)

3.9 Form Connection Model


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Y

3 11 F R t ti C


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-10.0

-9.0

-8.0

-7.0

-6.0

-5.0

-4.0

-3.0

-2.0

-1.0

0.0

0.0 5.0 10.0 15.0 20.0 25.0 30.0 35.0 40.0 45.0 50.0

Concrete Pour Length, m

G i r d e

r R o t a t i o n ,

D e g

r e e

As-designed (ideal),

No form

As-built

No form

Strong form,

as-built

Weak

form,

as-built

42' 82'95' 105'

Rotation

3.11 Force-Rotation Curve

4. Demolition


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4. Demolition

• Remove debris safely;

• Sample materials;

• Preserve evidence

Temp. Support

Cut Location

Objectives

5. Laboratory Testing


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5. Laboratory Testing

• Verify Analysis Assumptions

• Check whether materials conform to contract specifications

Objectives:

5 1 Form Connection Tests


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0

2000

4000

6000

8000

10000

12000

0 2.5 5 7.5 10 12.5 15 17.5 20 22.5 25

Lateral Deformation, mm

F o r c

e ,

N

Force-Deformation Curve

of Form Connections

Strong Form Model

Weak Form Model

Lab Results

5.1 Form Connection Tests

Form connection test

Gage 12

Steel PL

Form

Screw

5 1 Form Connection Tests


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0

2000

4000

6000

8000

10000

12000

0 2.5 5 7.5 10 12.5 15 17.5 20 22.5 25

Lateral Deformation, mm

F o r c

e ,

N

Force-Deformation Curve

of Form Connections

Strong Form Model

Weak Form Model

Lab Results

5.1 Form Connection Tests

Form connection test

Gage 12

Steel PL

Form

Screw


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b. Bearing Model

Fixed Boundary

a. Damaged Fixed Bearing

5.2 Bearing Inspection

6. Conclusions


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• The bridge failed in a global torsional mode;

• Stay-in-place forms greatly delayed thecollapse, but were not strong enough to

prevent it;

• Progressive failure of form connections thatinitiated the failure sequence

• The bridge would have buckled even if thetwo deck haunches were identical

7. Recommendations


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• Clarify applicable codes;• Add a new code provision that requires full

length lateral bracing to be installed between

top flanges unless proven unnecessary byanalysis

8. NYSDOT Actions


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• Reviewed similar ongoing projects in NYS.

• Required bracing systems for similar bridges in

NYS—(NYSDOT “Blue Page”)

• Sought recommendations from AASHTO

regarding code revisions.

AASHTO LRFD Specs. – 3rd Edition (2004)


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• Art. 6.11: Provisions for single or multipleclosed-box or tub girders

• Art. 6.7.5: Lateral Bracing

– 6.7.5.3: Top lateral bracing shall be providedbetween flanges of individual tub sections. Theneed for a full-length system shall be

investigated…

– If a full length lateral bracing system is not provided, the local stability of the top flanges andglobal stability of the individual tub sections shall

be investigated for the assumed construction

sequence


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Application to I-Girder Bridges


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Application to I Girder Bridges

Application to I-Girder Bridges


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Twin I-Girders: No bottom lateral bracing


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Iyy = 15,470 in^4 Izz = 472 in^4

Twin I-Girders: With bottom lateral bracing


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Centroid @ +19.96”

Shear Ctr. @ -19.06” Izz = 472 in^4Iyy = 296,426 in^4




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LTB with Non-linear Plate Model

0.0

0.5

1.0

1.5

2.0

2.5

0.0 20.0 40.0 60.0 80.0

Transverse Displacement at Midspan [in.]

D

e a d

L o a d

F a c t o r

Twin I Girders: With bottom lateral bracing

6



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LTB with Non-linear Plate ModelNo Lateral Bracing

0.0

0.5

1.0

1.5

2.0

0.0 20.0 40.0 60.0 80.0 100.0

Transverse Displacement at Midspan [in.]

D e a d

L o a d

F a

c t o r

11

Twin I Girders: No bottom lateral bracing


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Twin I-Girder Behavior--summary


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• More stable than open tub girder.• Lateral or lateral-torsional behavior (vs. global torsional)

• Bottom lateral system effective for lateral resistance

• Consider top and bottom laterals for long, narrow spans

• No “spec-ready” equations for checking global behavior(Single Tubs or Twin-I systems)

Dealing with a bridge failure


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• Expect your inspection program to come underscrutiny

• Expect safety of other bridges to be questioned

• Expect requests for data on failed bridge and otherbridges

• Establish point of contact for all media questions.

• Make “public” info. Easily available –

Dealing with a bridge failure


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• Work with your lawyers, (but do not expect them toalways have the same priorities).

• Establish protections for privileged material, e.g.

ongoing investigations.

One Final Lesson


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The Paradox of Failure

“When it comes to bridge

design, collapse is a most

reliable teacher.”

Henry Petroski

“Success Through Failure; The

Paradox of Design”

Questions?


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handout george christian (1).pdf

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