(Forensic investigations are carried out by hypothesizing the cause of a failure or accident based on the evidence available at the time – as limited as this might be, and revising the hypothesis as more evidence comes in.  This successive hypothesizing and revising might be done several times during an investigation.  The following is an example of this process.

Counsel benefits from a process like this early in a case – ideally before deciding to take the case, when an expert studies the evidence, then, based on the available evidence, identifies and evaluates the technical issues and the cost to investigate these)

***

The cross bracing was inadequate.  I concluded that last March, a few days after the failure. (Ref. 1) I used the bridge failure to illustrate how a hypothesis – an idea, could be formed about the cause of an engineering failure based on very little evidence.  In this case, all I had were some on-line photographs .

But, inadequate for what?  To resist the service plus construction loads – weights and pressures on the bridge, as required by the Canadian Highway Bridge Design Code?  If there’s no bracing at all – none, these loads would cause the girders to buckle sideways in the order of 375 mm, not the approximately 1,000 mm or more seen in the photographs.

The 1,000 mm was determined from the photographs by scaling like we do on maps.  The known 3.0 metre depth of the girders at the middle section – see Sources below, is like a scale or ruler in the photograph.  This depth is about the same as the spacing between the girders, maybe a little less.  I decided the spacing was 3.5 metres.  I saw that the girders had buckled about 1/3 of the way into the 3.5 metre spacing – 1,000 mm or more.

The 1,000 mm buckling indicates a greater load was acting on the girders than perhaps was required to be resisted by the Code.  Where did the greater load come from to cause the 1,000 mm?

The only thing attached to the girder – at the top, that can be seen in the photographs is a sling at the end of a crane’s cable.  The cable is attached to a crane’s telescopic boom.  The boom would sway and flex a little in the wind that was blowing that night which would cause the sling below to tug on the girder – a point load in engineering.  Construction cameras show the girders intact at 2:00 in the morning and buckled at 2:15.

I hypothesized last March that this repetitive tugging caused the girders to buckle like they did.

The 1,000 mm magnitude of the buckling and the fact that crane booms sway in the wind supports this idea that tugging on the girders was the source of the greater load.

It would be of interest to know if the bracing that was in place – and seen bent in the photographs, was adequate to prevent buckling, except for the 1,000 mm due to the tugging.  There are simple calculations that bridge design engineers do to determine the bracing needed to prevent buckling in the order of 375 mm.

I did think about a sudden foundation soil failure causing the crawler crane to subside and the cable to tug on the girder as a result.  I dismissed this idea because the crane had been there a while lifting 40 tonne girders into place.  Foundation failure would have occurred some time before because of these heavy lifts if the foundation soils were inadequate.

The only way I would revise my hypothesis is to note that the crane operator did not contribute to the failure because he was not working.

The initial hypothesis

The bridge failed because the middle crane’s boom moved in the wind – possibly also due to the crane operator’s actions, causing the cable to periodically tug at the middle section of beam #6 and eventually cause it to bend.  This caused the middle sections of beams #5 and #4 to bend as well because they were connected to #6 by some cross-bracing.  The cross-bracing was inadequate to resist the force from the tugging indefinitely and eventually failed too.  The middle sections of beams #3, #2, and #1 did not bend and fail because they were adequately cross-braced.

Revised hypothesis

The bridge failed because the middle crane’s boom moved in the wind causing the cable to periodically tug at the middle section of beam #6 and eventually cause it to bend.  This caused the middle sections of beams #5 and #4 to bend as well because they were connected to #6 by some cross-bracing.  The cross-bracing was inadequate to resist the force from the tugging indefinitely and eventually failed too.  The middle sections of beams #3, #2, and #1 did not bend and fail because they were adequately cross-braced.

Sources

I studied various photographs on-line including construction photographs taken at the time of the failure.

I spoke with Barry Bellcourt, the Road Design and Construction Manager for the City of Edmonton, a few weeks ago, also Bryon Nicholson, Manager of Special Projects. Barry mentioned the litigation and the city’s position.

I also learned from him that the bridge consists of seven, 40-tonne girders.  Each girder consists of two 7.5 metre long end sections and a 43 metre middle section.  The end sections are 4.5 metres deep arching up to 3.0 metres at the middle section.  The sizes are approximate.

I saw and photographed the underside of the repaired bridge girders from Groat Road in early August when I was in Edmonton.

I understand it was windy the night the girders buckled and that was the reason workers were not on the job.

I spoke with four companies in Nova Scotia that operate cranes.  I learned that crawler crane booms move in the wind; flex and sway.  There is greater movement sideways because there is less strength that way.  Telescopic booms move more than lattice booms because of the greater surface area.  Booms are lowered to the ground in strong winds.  One company doesn’t operate its cranes in winds of 50 km/hr or more.

I also talked with Amjad Memon, a structural engineer with the Nova Scotia Department of Transportation, about the Canadian Highway Bridge Design Code.

References

1. Wind, construction crane and inadequate cross-bracing caused Edmonton bridge failure: An initial hypothesis.  Posted March 27, 2015
2. Why, in a recent blog, didn’t I seem to consider foundation failure as a possible cause of the Edmonton bridge failure?  Posted April 3, 2015
3. Bridge beams that fail are sometimes like balloons filled with water – squeeze them and they pop out somewhere else.  Posted May 20, 2015
4. Google: Edmonton bridge failure, Groat Road, Buckling, etc. to see photographs of the buckled girders.

I attended the regular quarterly meeting of CATAIR last Friday – this time at Dartmouth Crossing to enable some field testing, and learned a few things, both encouraging and disturbing.

1. I felt good learning that there are training and qualifying programs in Canada for traffic accident investigators.
2. Also, not surprisingly, that school buses have numerous safety features.
3. I was disturbed learning about the blind spots at the back of a school bus where the driver can`t see.  What he sees isn’t all of what might be there.

CATAIR along with ACTAR are two separate associations of traffic accident investigators.  The one is a forum for investigators to meet and share experiences and ideas.  The other is an accrediting organization for investigators. (Ref. 1)

It`s in order to take an interest in this field of practice considering the number of traffic fatalities in Atlantic Canada in a year, not a few of which result in charges under the law, civil litigation or insurance claims.

Encouraging

I suggested last week that it is important that your traffic accident expert is well trained, experienced and accredited.  That is still true.  ACTAR can perhaps be seen to be the ultimate and most demanding accrediting group.

However, I did learn at the meeting on Friday that there are qualifying programs in Canada that are demanding enough.  They vary across the country but generally require that traffic accident reconstructionists study and train and go through several levels of qualification.

A course for police officers comprises three main levels.  Two levels are done in the area in which the applicant serves and focuses on investigation of the traffic accident.  The third is completed at a Canadian Police College and covers reconstruction of the accident.  I understand that members of the public can take this course for a fee.

It’s important that an investigator reconstruct a traffic accident generally in accordance with the procedures his peers in the area would follow, and to have comparable qualifications.  That is, to measure up to the standard of care existing in his area of Canada at the time. (Refs 2, 3) That standard is certain to include the expectation that you went through a qualifying program of some sort, in view of the fact they do exist.

If charges or a dispute arises from the traffic accident the investigative procedures followed by the investigator may be evaluated by his peers at the report writing stage or the discovery and trial stages, according to the standard of care.

***

I mentioned a few days ago that the meeting on Friday would do the following things, and these got done:

1. See a demonstration of the latest school bus safety features,
2. Perform instrumented braking and acceleration tests,
3. Measure the bus’s turning radius and rear wheel off-tracking, and,
4. Examine sight lines/views obstructions.

There are school bus safety features too numerous to mention, but the bus driver did a good job briefing us on these.  Proper thing parents would say. These features include:

• Exacting bus driver training and qualification,
• Walk around safety checks,
• Knowing where the bus is at all times,
• Training students on how to exit the bus in an emergency – including through roof escape hatches,
• Front windows that pop out in an accident,
• Doors that can be opened easily both inside and out,
• Etc.

I was impressed to learn that these very large buses when empty, at a speed of 50 km/hour can be stopped within about 2/3 to 3/4 the length of a bus when the brakes are applied.  Dr. Stu Smith, C. Tyner and Associates, measured these speeds and stopping distances with a braking test computer.  Skid resistance or sliding resistance of the asphalt pavement was also measured by consultants using a drag sled, a test that is very similar to the coefficient of friction test in high school physics.

Disturbing

What disturbed me was the school bus driver’s blind spots at the back.  I sensed from the tone of the bus driver`s voice that these are worrying.  They just can`t see everything at the back of the bus from the driver`s seat regardless the number and size of the rear view mirrors.

I think it’s also going to be interesting to see the results of the rear wheel off-tracking measurements.  The rear wheels are in a different place to the front wheels when a bus is turning.  It`s in order for the bus driver to know where they`re at, a skill acquired by the time the driver gets his licence.  Not so easy dealing with the blind spots.  The wheel tracks were accurately located by RCMP Corporal Michel Lanteigne, Tracadie, NB using total stations land surveying equipment.

***

Why should you take an interest in all of this?  How about 18 traffic fatalities on Prince Edward Island Island last year, and possibly more in Nova Scotia, New Brunswick, and Newfoundland.  And all quite likely got investigated by traffic accident reconstructionists.  Some of these I’m sure resulted in charges and possibly disputes arose and civil litigation begun.

***

Ken Zwicker, the CATAIR regional director, organized a very instructive meeting and kept it “moving right along“ during the day.  Corporal Lanteigne – who travelled the farthest, a 9.5 hour round trip, was everywhere during our field work on Friday, including on Ken`s heels.  Others came from Fredericton, I think Saint John, and from Prince Edward Island.  Several of us travelled all of 20 minutes from Halifax.

References

1. Is your traffic accident investigator well trained, experienced and “accredited”?. Posted February 23, 2016
2. Garner, Bryan A., ed., Black`s Law Dictionary, 4th ed. 2011, Thomson Reuters, St. Paul, MN
3. How the standard of care is determined when a failure or accident occurs in the built environment.  Posted June 28, 2014