Bridge Failures - Lessons learned

George A. Christian, P.E.

Director, Office of StructuresNew York State Dept. of Transportation

Bridge Engineering Course

University at Buffalo

March 29, 2010

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

oRecent NYSDOT Bridge

Failure Investigations

oDealing with a failure

Part 1:

Part 2:

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 of contributing events and/or underlying causes.

• When it comes to underlying causes, history can repeat itself.

―Honest human error in the face of the

unforeseen—or the unforeseeable—is

ultimately 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

Quebec Bridge 1800 ft. main span, collapsed Aug 29, 1907

Buckling Failure of compression

chord (A9L) –inadequate latticing

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

Quebec Bridge Collapse -Findings

• Lack of knowledge of behavior of large compression members.

Lattice bracing proved to be inadequate.

Second Quebec Bridge - 1917

construction collapse Sept 1914

Tacoma Narrows Bridge collapse- 1940

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)

1920’s -- Highway suspension bridges become practical

Bear Mountain Bridge -

1924

1632 ft. spanWurts Street Bridge,

Kingston, NY -1921

705 ft. span

1930’s--“Landmark” Bridges

Golden Gate Bridge - 1937

4200 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

1930’s: maximize structural efficiency, economy, aesthetics

Plate girder in place of truss for deck stiffening

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 with

shallow stiffening girders

exhibit wind-induced

Vertical oscillations

--Early retrofits

implemented

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

Torsional motion of Tacoma Narrows bridge prior to failureNov 7, 1940

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

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 cable supported structures (suspension and cable stayed)

• Ended use of stiffening plate girders

• Stiffening trusses continued to be used until 1970’s

“Post-Tacoma” new bridges

Tacoma-Narrows Bridge

Replacement - 1950Mackinac Straits Bridge -

1954

Thousand Islands Bridge -Retrofits

Deer Isle Bridge retrofits

Bronx-Whitestone Bridgeretrofits •Tower stays

•Stiffening truss retrofit

•Tuned mass Damper

at midspan

Bronx-Whitestone Bridge --second retrofit 2007

•Replaced Concrete

deck with Orthotropic

steel deck

•Removed Stiffening

Trusses

•Added lateral bracing

to lower flanges

•Added wind fairings

on stiffening girders

•Diagonal stays and

tuned mass damper

remain

Reduce Dead load,

improve torsional stiffness,

improve aerodynamic behavior

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

• Last 30 years

• Had Significant impact on Federal and State agency bridge management and safety practices

• NTSB findings and recommendations

Silver Bridge over Ohio RiverPoint Pleasant , WV – Gallipolis, OHBuilt 1928 , collapsed Dec. 15, 1967

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 were not 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

Silver Bridge Collapseconsequences

• Burning Question : How many other bridges can have 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…

Mianus BridgeI-95 over Mianus River, Greenwich, CT.

Built 1958 , collapsed June 1983

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

Mianus Bridge CollapseConsequences

• Fracture Critical Inspection requirements

– Visual “hands on” every 2 years

– NDT methods

• Pin and Hanger inspection NDT methods improved

Mianus Bridge Collapse Consequences

New York DOT Response

• Add redundancy to all 2 and 3 girder Pin and Hanger bridges (approx. 24 bridges)

• Over time, these bridges (or superstructures) have been replaced or made redundant / continuous

Mianus Bridge Collapse Consequences

New York DOT Response

• Detailed Inspections of 3 and 3 welded girder bridges (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 and

unintended structural participation

was not fully recognized.

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 a

transverse connection plate with

intersecting and overlapping

welds.

2 of 3 girders completely

fractured full depth

Hoan Bridge Failure

• Connection detail provided high tri-axial constraint at the web, resulted in very high stress 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 Bridge Forensic Investigation,

Failure Analysis Final Report;

Federal Hwy. Admin. and Wisconson DOT,

2001

(The one less significant failure)

New York County Road Bridge Failure -1986

• Significant section loss on trusses ( up to 50%)

• Lack of redundancy

• Excessive dead load:– Timber deck replaced by

a 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

Schoharie Creek BridgeNYS Thruway over Schoharie CreekBuilt 1954, Collapsed April, 1987

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

Schoharie Creek Bridge failure

• Contributing causes- Lack of:

– Redundancy

– ductility in piers

– resiliency

Schoharie Creek Bridge failure

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

Schoharie Creek Bridge failureFollow 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

“Cause of Bridge Failures from 1966 to 2005”

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

NYSDOT Bridge Failure Database

• 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 and consequence.

• Evaluation data needs collected during bridge inspections

NYSDOT Bridge Safety Assurance Initiative

Vulnerability Assessments

• Scour repairs

• Steel Detail Retrofits

• Add Redundancy

BSA Retrofits

Vulnerability score may

influence rehab / replace

decision

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

• 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

I-35W over Mississippi River

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

I-35W over Mississippi River

Bowed gusset plates suggested problem for further investigation.

NTSB

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

transportation agencies during inspections.

I-35W over Mississippi River

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

I-35W over Mississippi River

NYSDOT actions

• Inspected 50 deck truss bridges in NYS

• Analyzed Gusset Plates on 133 Trusses that had undergone a substantial 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

NYSDOT Gusset repairs

Failures Caused by Extreme Events• Earthquakes

• Collisions

– Vessel

– Vehicle

• Storm surge

• Fire

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

Failures during Construction

When a bridge may be most at risk to a structural collapse.

Failures during ConstructionRt 470 / I-70 overpass, Golden CO; May 15, 2004

Probable Cause of Failure (NTSB Report):

―Failure of temporary bracing system due to

insufficient 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 constructionPotential 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.

Failures during constructionLessons

• 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

Questions?