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?
