(Forensic investigations are carried out by hypothesizing the cause of a failure or accident and revising the hypothesis as the evidence comes in. This revising might be done several times during an investigation. The following is an example of how an initial hypothesis can be made of the cause of a failure based on information from the early stages of a forensic investigation – after reading documents or carrying out a visual “site” assessment)
Simple visual “site” examination – one of several initial steps in forensic work, identifies cause of failure
It was fairly easy to figure out the probable cause of this unusual bridge failure based on a simple visual “site” examination and knowing the weather at the time. This type of examination is one of the first steps in a forensic investigation. Also noting the fairly conventional design of the bridge and how such bridges are constructed. I did this in about an hour last Monday after some discussion with a structural engineer.
My “site” examination was of photographs and video in the Globe and Mail and on-line. (Ref. 1) Photographs can be thought of as documents in a forensic case the examination of which is another preliminary step in a forensic investigation.
Another basic step in a forensic investigation is comparing the failed structure to its condition before it failed. Parts of the bridge failed and some parts didn’t, as evident in the photographs, providing this before-after comparison.
The procedure followed in constructing such a bridge is also important information.
The bridge on 102 Avenue in Edmonton reportedly failed on Monday, March 16th when the middle section of three of the seven long, deep, parallel beams spanning the area below bent sideways into a curve. The beams support a concrete bridge deck (not in place at the time of the failure).
The bridge is constructed a little like the floor in your home
The view from above in a low altitude aerial photograph taken from a drone would be similar to that of the floor joists in your home supporting the floor you walk on. The beams are cross-braced horizontally and diagonally by pieces of steel. The bracing keeps the deep beams upright, apart, and parallel. Floor joists are also braced like this for the same reason.
The large beams – several 10s of feet long and several feet deep, are constructed away from the bridge site in two end sections and one middle section. The end sections are hoisted into place by cranes to start construction of a beam and supported there by the cranes, one at each end. The end sections are cross-braced horizontally and diagonally to a previously constructed beam.
A third crane lifts the middle section into place which is bolted to the end sections. The middle section is then cross-braced to the previously constructed middle section. The cables from the cranes can be seen on the right in many of the pictures and in a video.
How the bridge failed
The bridge failed when the middle section of three of the beams bent sideways – buckled is another way of describing what happened to the beams. The failure appears as smoothly curved beams in pictures on-line and in the newspaper. These would be failed beams #4, #5, and #6 counting from the left in many of the pictures. Cranes and cables can be seen in the pictures supporting the three sections of beam #6.
If you look closely at the pictures you can see that it is basically the middle sections that have buckled and curved. You can also see that the end sections are cross-braced but there is very little cross-bracing in the middle sections, and what bracing is there has also buckled and failed.
The beams reportedly failed without warning as crews worked on them. It was also reported that work on the beams was stopped the previous Saturday night due to extreme high winds.
The bridge beams could fail if the cable of the third crane connected to the middle section of beam #6 moved to the left pulling the top of the beam towards beams #5 and #4. Beams #5 and #6 would be pushed over by the small amount of cross-bracing at the middle sections until the pushing force needed was too great for the cross-bracing and it buckled and failed too. The force needed would increase because beams offer increased resistance as they are bent.
The middle sections of the three beams on the left, #1, #2, and #3, did not fail because they were adequately braced. Neither did the end sections of beams #4, #5, and #6 fail, like the middle sections did, because they were adequately braced. The end sections show a little movement as a result of being “dragged” along by the middle sections.
What caused the bridge failure?
What would cause the cable to move to the left initiating the failure of the middle section? Well, a crane consists of a large boom with a cable hanging down at the end with a hook or, in this case, a sling on it. A boom moves a little when the wind blows same as the cable does. We see cables moving often enough when we go by a crane and look up.
Although a boom moves a lot less in the wind than a cable, it still moves. If the boom moved back and forth a little in the extreme wind for hours on end this would cause the cable to periodically tug on the inadequately braced middle section. It’s not difficult to imagine the middle section being brought to the point of failure by this periodic tugging – a form of fatigue.
If the crane operator climbed up into his cab Monday morning, started his crane up, and moved his boom a little for whatever reason that could also cause the cable to tug on the middle section and contribute to the initiation of the failure.
It’s also possible the workers came in Monday morning to work on the bridge and found that the beams had failed, rather than came in, did some work, as stated in the newspaper, then the bridge failed. There are errors in the news reporting suggesting this possibility. Notably, the number of beams that failed, reported as four when only three appear in the pictures.
There would be tugging on the end sections of beam #6 as well because the crane booms would be moving in the wind at these locations too. However, the end sections did not buckle and fail because they were adequately cross-braced.
The bridge might not have failed if ….
If the crane’s cable was not connected and tugging on the middle-section, it’s likely the bridge would not have failed. The wind would not have been strong enough to push a large beam like this sideways even though it was inadequately braced.
An 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.
Revising the hypothesis
This hypothesis might be revised after more evidence came in from additional forensic investigation such as the following:
- Visually examining the site after putting “boots on the ground”,
- Determining the failed condition of the bridge – particularly the cross-bracing that was in place,
- Interviewing workers, including the crane operators,
- Determining the micro-weather at the site between Saturday and Monday, and,
- Reviewing the bridge design.
This is the process followed in all forensic engineering investigations
Hypothesizing then revising the hypothesis based on the evidence available at each stage is the basic process followed in all investigations. The process can sometimes identify the cause of a failure at an early stage. Additional investigation is carried out to corroborate the early findings.
It will be interesting to see what transpires during the investigation of the Edmonton bridge failure. To what extent my initial hypothesis based on a visual “site” examination might need to be revised.
- Globe and Mail page A8 Tuesday, March 17, 2015, “Buckled girders may delay Edmonton bridge a year“