We'll watch this unfold. Montreals bridges and overpasses are very complex, convoluted, and in various states of deterioration. All built of reinforced concrete some 30 years ago and subjected to a very harsh environment of chloride based deicers. The Supreme Court of Canada ruled in the last several years that all construction in Quebec must be unionized to avoid the 'problems' with the construction industry of the past. We drive through Montreal often and wonder about its infrastructure. This problem will boil down to money and political will and lets hope finally will be put on the front burner. Five people dead.
Connect2, Are you familar with this kind of bridge? It appears to be a vaulted abutment that reaches out toward the roadway and has the concrete slab (or girders) bearing atop the abutment seat. It looks like a single span.
From the pictures I couldn't tell if the superstructure is concrete slab or slab on girder.
I'm very interested in learning more about this type of bridge.
Much Thanks
Regards,
Qshake
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"The incident occurred before 1 p.m. (1700 GMT) One witness told TVA he noticed that the road sunk by 1 to 2 inches (2.5-5 centimeters) when the traveled over the overpass minutes earlier and called emergency dispatchers at 12:30 p.m. (1630 GMT)."
A road crew cleaned up some pieces of concrete from the roadway below a couple of hours before the collapse.
The bridge does seem to have a cantilevered bearing seat which is common in the area. I think it's a two span bridge with simple spans. A quick glimpse I got from a video seemed to show that the bearing seat and a large portion of the abutment face sheared off diagonally. Some horizontal rebar are also seen extending from the top of the abutment indicating a possible lap/bond failure at the critical point. The abutment would have been simply reinforced, not prestressed, so it would have cracked, perhaps at the back of the bearing seat, early on in order to mobilize the top horizontal steel. This bridge would have also had an expansion joint directly above the bearing seat. After 36 years (built around 1970) of deicing salt and a leaking joint seal (inevitable) any significant cracking at the top would have lead to likely corrosion of the top rebar. A public inquiry has been called and it will interesting to see if an expanding crack at the back of the bearing seat and probably down the sides of the abutment have been overlooked during inspections over the years.
They've now closed a similar bridge and are looking at 45 more. The Quebec construction industry had problems with organized crime about the time of construction. We'll see if that played a part.
A local professor said that since the bridge survived 36 years it meant that the design/construction was OK and that it must have been simply poor maintenance. While it's true that poor maintenance is a likely contributing factor even a compromised design or construction mistakes/shortcuts can survive for a while until it's luck runs out due to it's reduced safety index.
In a way it's good that this didn't happen during construction with private contractors involved. Legal suits involving private interests tend to result in publication bans on engineering reports as part of any settlement. There have been many cases where the engineering profession can't learn from the failure due to these bans.
I see a parallel between falling concrete and industry where a part keeps failing and is no problem as long as there is a spare in the parts bin. Here the there is no problem until the cleanup crew doesn't have any concrete to cleanup.
To update my previous observations. It is a single span bridge with cantilevered bearing seats and a suspended span. Looks like a complete shear failure of one cantilevered end with a horizontal separation between the top of the slab and the top of the top rebar at the bearing seat (see 6th photo). I think there was a lap failure in the top rebar. The top rebar into the abutment isn't visible and is probably still embedded in the remnants of the top of the slab. No apparent stagger on the lap. Not much shear steel evident!
My previous comments about possible cracking at the top of the bearing seat don't seem to be correct.
Looks like a fairly standard 'half-joint' (or 'halving joint') bridge. Britain has quite a few similar bridges on their older motorways that are 30-40 years old.
"An inspector from Transport Quebec, the provinces transportation ministry, was sent to the site about 30 minutes before the tragedy but the overpass remained open."
Excellent link, cooperDBM. I'd also agree that the steel looks light, although it's hard to tell from photos.
I'm curious about how bridge inspection works in Canada. I would assume that they have something similar to the NBIS in the US, which calls for inspections every 2 years. In the US, a lot (I'm guessing a majority) of bridge inspectors are non-engineers from various maintenance and construction backgrounds that have taken a few weeks of class for the job, and the rest engineers that have taken the same class. This varies from owner to owner, since even though NBIS is a federal program it is ultimately performed on the state and local level. Can anyone in Canada compare and contrast?
I have to feel bad for the inspector that was sent to the bridge. I don't understand from the statement if he left his office 30 minutes before the collapse or if he was on site 30 minutes. If it is the former he would have probably arrived just in time to see the collapse. Even if it was the later, I think one would still be hard pressed to comprehend the problem and get a bridge closed in that much time unless it was pretty obvious.
I'm inclined to agree with cooperDBM. In 1984 I was inspecting four bridges in Staten Island NY - multiple simple spans, rolled beams, built in the 1960's - each pier consisted of four concrete columns with a concrete cap beam. The center portion of the cap was similar to the configuration of the Laval Bridge - supported by a cantilevered seat. All four cap beams were cracked above the seat to various degrees, with some being fully cracked, being held in place by friction or a miracle.
Our chief engineer believed it was a shear failure although we never were privy to the results of the owner's investigation. He also said it was "down and dirty" way to design a pier quickly.
Hardly ever hear a more subtle possible reason for the failure--poor inspection techniques (notice I didn't say 'poor inspectors'). I can only imagine if similar things were happening in the aircraft industry, what would happen if visual inspections were the SOP (standard operating procedure) but an aircraft crashed from damage caused, it was later determined, by defect that was not visible? Certainly NDIs (non destructive inspections) would be specified from then on (along with the usual visual inspections), would it not?
I do not think poor inspection was the reason for the collapse - it's combination of aging and design parameters of the bridge.
Halved seats were quite common in 70-ties and 80-ties. I have also design some of these.
It's my understanding that the code applicable at the time of design would allow for the design for shear by combination of concrete and steel capacity - while for example DIN, or PN (Polish Standard) disallowed such provisions, when the allowable shear was exceeded. This was based on the assumption, that stressed concrete would develop micro cracks, thereby loosing the capacity to carry the stresses.
In very light and short spans, the “square” seated joint could be acceptable by some, but my mentor always requested a 45 degree transition, and all shear to be carried by steel alone, with ample provision for corrosion, regardless of the concrete capacity.
Judging form the photographs it’s rather difficult to assess if the failure was related to the inadequate design. But the rebars look thin, there are not severely corroded, leading me to the conclusion that the issue of the shear design was the culprit.
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