human factors and bridge failure

8/9/2019 Human Factors and Bridge Failure

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32 Civil Engineers Australia | August 2012

FEATURE Bridges

Human factors

and bridge failureby Sean Brady

While the vast majority ofbridge structures performsatisfactorily throughout

their lifetime, bridge failures stillcontinue to occur throughoutthe world, with the collapse oftemporary formwork during a bridge’sconstruction in Canberra being a

recent Australian example. Fortunately,the failure resulted in no fatalities,but it was the third such temporaryworks collapse in recent years – acable-stayed pedestrian bridge failedduring construction in the US in 2008,and a 20m-long section of formworkcollapsed during a concrete pour fora partially cable-stayed bridge in thePyrenees in 2009. In addition to theseconstruction failures, there have alsobeen a number of catastrophic bridgecollapses in service, such as the failure

of the I-35W in Minneapolis, US,and the De La Concorde Overpass inMontreal, Canada.

The engineering profession is well

aware that lessons learned from pastfailures can play an important role inpreventing reoccurrence. Ken Carper,the US forensic engineer, suggests thatfailures result from a variety of causesinvolving both technical and “human”factors (human error/proceduralissues). While knowledge of technical

factors is critical, it is often the humanfactors that allow these technicalfactors to culminate in failure. Attheir most basic, these human factorscan be summarised in what forensicengineer Neal FitzSimons described asthe “four horsemen of the engineeringapocalypse: ignorance, incompetence,negligence and avarice”. Themanagement of human factors is notonly critical during a bridge’s designand construction phases, but is equallyimportant throughout its lifetime.

However, managing these humanfactors is not a new challenge.

As far back as the 1700s, JohnSmeaton, the founder of the Society

While under-sized gusset plates,which were the result of a designerror, were found to be the causebehind the I-35W bridge collapsein 2007, it was human factorsthat permitted this error to gounnoticed for 40 years, despite

numerous opportunities for itsidentification.

PHOTO: FLICKR/SLKLUG,

USED UNDER CREATIVE COMMONS LICENCE


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of Civil Engineers, described a similarissue: “Stone, wood and iron arewrought and put together by mechanicalmethods, but the greatest work isto keep right the animal part of the

machinery.” In modern times, of course,the “animal part of the machinery”is typically managed through thecreation and implementation ofquality assurance programs aimed atidentifying human errors and omissionsbefore negative consequences ensue.Therefore, a review of recent bridgefailures provides a useful reminder ofthe typical human factors that may beencountered by engineers involved inthe design, construction, operation and

maintenance of bridges.I-35W highway bridge, US

Just after 6pm on Wednesday 1August 2007, the main span of theI-35W highway bridge in Minneapoliscollapsed into the Mississippi River,resulting in 13 fatalities. The subsequentNational Transportation Safety Board(NTSB) investigation identified under-sized gusset plates, in combination withincreases in dead and live load on thebridge, as the cause of collapse.

While the under-sized gusset plates,which were the result of a design error,initiated the bridge’s collapse, it washuman factors that permitted thiserror to go unnoticed for 40 years,despite numerous opportunities for itsidentification. Among the human factorsidentified by the NTSB investigationwere:• “Even though the bridge design firm

knew how to correctly calculatethe effects of stress in gusset plates,it failed to perform all necessarycalculations for the main truss gussetplates of the I-35W bridge, resultingin some of the gusset plates havinginadequate capacity.

• “The design review process usedby the bridge design firm wasinadequate in that it did not detectand correct the error in design of thegusset plates.

• “Neither federal nor state authoritiesevaluated the design of the gussetplates for the I-35W bridge in

sufficient detail during the designand acceptance process to detect thedesign errors in the plates, nor was itstandard practice for them to do so.”

• During the bridge’s lifetime a numberof load ratings were undertaken,however, the NTSB investigationfound that the capacity of the gussetplates was not evaluated as part of

these load ratings and concludedthat had the “American Associationof State Highway and TransportationOfficials guidance included gussetplates in load ratings, there wouldhave been multiple opportunities todetect the inadequate capacity of theU10 gusset plates of the I-35W bridgedeck truss”.

• And, finally, the potential forfailure was exacerbated “becausebridge owners generally consider

gusset plates to be designed moreconservatively than the othermembers of a truss”, an assumptionthat proved incorrect in this case.

De La Concorde overpass, Canada

A year prior to the I-35W failure, thecollapse of an overpass in Montreal,Canada, resulted in five fatalities. Theoverpass consisted of a drop-in spancomprised of prestressed concretebox girders, which were supported bycantilevered slabs, via halving joints.

The Commission of Inquiry into thecollapse concluded that the failure of theoverpass was a result of a shear failureof a cantilevered slab. The commissionconcluded that the shear failure wascaused by concrete deterioration,with a number of other contributingfactors playing a role in the collapse,such as a lack of shear reinforcementin the cantilevered slab, a lack ofwatertightness in the slab, and damagecaused in previous rectification works.

The commission found that whilethe bridge’s design met with thecode requirements at the time of itsconstruction, human factors during boththe bridge’s construction and operationphases contributed to the failure.Among the human factors identified bythe commission’s investigation were:• The commission blamed the

contractor for “failing to meettheir legal and contractualobligations”. It also blamed theprincipal subcontractor “for failing

to adequately control the qualityof the work by passing on theirresponsibilities to the workersand to the supervising consulting

“Stone, wood

and iron are

wrought and put together

by mechanical

methods, but

the greatest

work is to keep

right the animal part of the

machinery.”


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FEATURE Bridges

engineer”. The commission stated thatthis lack of quality control allowedthe faulty installation of some steelreinforcement in the cantilevered slab,“one of the main physical causes of

the collapse”.• The commission also blamed

the bridge designer, as well asits supervising engineer, for “notfulfilling their contractual obligationto exercise full-time supervision ofthe construction of the overpass andtherefore, for not preventing the faultyinstallation of the steel reinforcementthat resulted in a structure not inaccordance with the drawings andspecifications”.

•

With respect to the bridge operator,the MTQ, the commission found thatthe presence of the halving jointspresented inspection difficultiesthat were not adequately takeninto account by the MTQ: “In itsinterventions, the MTQ did notrigorously and effectively deployall the means at its disposal toproperly evaluate the condition ofthe overpass despite numerous signsof deterioration; it also failed tomaintain adequate records that could

have better guided its inspectors andmaintenance workers.”

• Further, the commission found that“the overpass inspections were attimes deficient, lacking adequatequantification of the deterioration,sometimes incomplete because notenough time was devoted to theinspections, and not thorough becausethe inspectors failed to look for thereasons behind the deterioration”.

Malahide Viaduct collapse, Ireland

In 2009, just outside Dublin, Pier 4 ofthe Malahide Viaduct collapsed intothe Broadmeadow Estuary, resultingin the partial failure of two spans. Aninvestigation by the Railway AccidentInvestigation Unit (RAIU) identifiedthe cause of the failure as scour actionundermining the rock weir thatsurrounded and supported Pier 4,leading to the pier’s failure.

While the technical cause offailure was straightforward, human

factors played a significant role in thecollapse, culminating in what the RAIUinvestigation termed as “corporatememory loss”. This corporate memory

loss occurred because “former IarnródÉireann staff left the Division, whichresulted in valuable information inrelation to the historic scouring andmaintenance not being available to the

staff in place at the time of the accident”.Specifically, these human factorsincluded the following:• “Iarnród Éireann’s likely failure to

take any action after an independentinspection carried out on theMalahide Viaduct in 1997 identifiedthat scouring had started at the baseof Pier 4 and that the rock armourweir was ‘too light for the job’.”

• Historically, in order to managethe risk of scour to the viaduct, it

appears a continuous maintenanceregime was required. But “the historicmaintenance regime for the dischargeof stones along the Malahide Viaductappears to have ceased in 1996,resulting in the deterioration ofthe weir which was protecting thestructure against scouring”.

• “A scour inspection undertaken in2006 did not identify the MalahideViaduct as a high-risk structure to theeffects of scouring.

• “An inspection carried out on the

Malahide Viaduct three days beforethe accident did not identify thescouring defects visible at the time.”

Closure

These cases illustrate the critical roleplayed by human factors in bridgefailures when quality assuranceapproaches become ineffective orneglected over the course of a bridge’slifetime. Not only were many of theabove failures avoidable, but numerousopportunities were missed to identify

the potential for failure. These casesremind engineers that ensuring qualityassurance approaches are effective, bothin their conception and implementation,is perhaps the best defence againstthe inevitable human factors that willcontribute to structural failure. n

Sean Brady is the managing director ofBrady Heywood (www.bradyheywood.com.

au), based in Brisbane. The rm providesforensic and investigative structural

engineering servicesand specialises in determining

the cause of engineering failureand non-performance.

“An inspection

carried out on

the MalahideViaduct three

days before the

accident did

not identify the

scouring defects

visible at thetime.”

human factors and bridge failure

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