Readers in Edmonton and Halifax commented on an item I posted last Friday about the Edmonton bridge failure – and my initial hypothesis as to the cause of the failure. (Ref. 1)
I concluded the bridge failed because a crane supporting a middle section of one of the bridge beams moved in the wind pulling the beam sideways and causing it to buckle. Cross-bracing was not adequate enough to prevent this movement.
Gary in Halifax
Gary in Halifax wondered why I didn’t give links to the photographs and video that I studied and mentioned in my blog. He thought this would make it easier for the reader. He is quite right because we learn more visually than we do verbally or from text.
Basically, I was anxious to get the item out there while it was current news – and while some readers might still have the newspapers around that carried the story.
Googling “Edmonton bridge failure” quickly took me to a good number of sites with text, photographs and at least one good video. I looked at a number of them in forming my initial hypothesis.
It would have been an exercise in itself reviewing all this material and selecting three or four links, and in the process possibly omit some that might have been included. It seemed easier to let readers google. But, I might have suggested readers google in lieu of providing specific links rather than unconsciously assuming they would. I’ll do that next time.
Albert in Edmonton
Albert in Edmonton noted the lack of comment in my blog about the possibility of foundation failure causing the beams to buckle. He knew that as a civil engineer I had specialized in geotechnical and foundation engineering for a number of years. Why didn’t I mention the foundations?
Why I didn’t mention the foundations
Typical structural damage due to foundation failure
I didn’t mainly because I’ve seen a lot of structures damaged by foundation failure and what I saw in the pictures and video didn’t fit what I’ve learned over the years.
Most foundation failures – not all, mind you, result in typical damage to the structure above. You come to know the character of this damage after seeing quite a lot of it. Also, after seeing the different structures, foundations and sub-surface soils and rocks involved.
This damage is due to marked vertical movement of parts of the structure – inches, and a little horizontal movement – fractions to maybe inches. It’s characterized by cracking and distortion of the structure. The damage is differential in nature – more here and less there, not uniform throughout the structure.
(For certain, there are exceptions to this typical failure and distortion of a structure. I investigated a 54 foot long structure one time that had “settled” – moved vertically, 11 inches from one end to the other. Turned out it was mistakenly built this way)
Edmonton bridge damage
The middle sections of three of the Edmonton bridge beams moved sideways several feet, not inches. The end sections where the foundations are located did not move sideways hardly at all. And all of this movement was quite uniform, not differential. Quite unlike the typical damage associated with foundation failure.
Did the two end sections of each beam move towards one another and cause the beams to buckle and to do this uniformly? If so, where did the force come from to push on the ends of the beams? There’s nothing showing in the pictures and video.
Did the foundations beneath the two end sections move towards one another causing the beams to buckle, uniformly? I can’t see the foundations but I’ve never known foundations to move sideways several inches causing the structure above to move sideways too. Landslides and retaining wall failures possibly excepted, but that’s not the situation here. And where did the forces come from to push on the sides of the foundations? To the extent you can see the soil near the level of the foundations in some of the pictures, there’s nothing there.
Intuition? Engineering experience?
Sub-conscious thoughts like these would have been running through my head as I looked at the pictures. They resulted in me not considering the foundations as the cause of the problem. Intuition? Engineering experience?
But Albert in Edmonton made me think and next time I’ll draw attention to the fact that something in what I was seeing didn’t fit, didn’t look right.
Similarly, the structural engineer I chatted with about this bridge failure quickly dismissed wind forces on the sides of the beams as being anywhere near strong enough to cause the beams to buckle. That would be his intuition, his engineering experience kicking in.
Initial hypothesis of bridge failure validated to some extent by others
Barry Belcourt, manager of Edmonton’s road design and construction branch, was reported recently in the Globe and Mail as saying it was a construction procedure failure. This would suggest the failure is not due to inadequate design. I don’t know what Mr. Belcourt is basing that opinion on but he’s on the ground in Edmonton and all I’ve got are pictures and video.
Mr. Belcourt’s comments to some extent validate my initial hypothesis of the cause of the bridge failure.
Albert, who is also a civil engineer, also considered “careless handling during construction” as one of several possible causes
Reader’s questions are important
Questioning is how we revise hypothesis so keep them coming.
References
- Wind, construction crane and inadequate cross-bracing caused Edmonton bridge failure: An initial hypothesis http://www.ericjorden.com/blog/2015/03/27/wind-construction-crane-and-inadequate-cross-bracing-caused-edmonton-bridge-failure-an-initial-hypothesis/