Bridge Collapse
Bridge Collapse
(OP)
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RE: Bridge Collapse
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A 30-year-old bridge, it says.
RE: Bridge Collapse
RE: Bridge Collapse
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
Eng-Tips Forums:Real Solutions for Real Problems Really Quick.
RE: Bridge Collapse
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RE: Bridge Collapse
Hmmm...seems like abrupt failure with no evidence of the impending collapse during a previous bridge inspection.....shear failure is abrupt.
http://www.foxnews.com/story/0,2933,216926,00.html
RE: Bridge Collapse
"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)."
Was this a warning unheeded?
RE: Bridge 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.
Sorry for the long post.
RE: Bridge Collapse
RE: Bridge Collapse
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.
RE: Bridge Collapse
RE: Bridge Collapse
another link...
RE: Bridge Collapse
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"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."
RE: Bridge Collapse
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.
RE: Bridge Collapse
RE: Bridge Collapse
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.
RE: Bridge Collapse
RE: Bridge Collapse
RE: Bridge Collapse
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RE: Bridge Collapse
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.
RE: Bridge Collapse
34 collapses in 160 years seems QUITE low
RE: Bridge Collapse
If we just count all bridges on public roads with spans over 20' in the US, India, the UK, and Canada (i.e. where most of the people on this form are) we could name at least one collapse per year.
RE: Bridge Collapse
I think the shear failure started as a vertical separation of the top long. steel lap at the back of the bearing seat (along a horizontal plane). Long. steel on the bearing side of the failure went with the falling concrete while the long. steel on the abutment side stayed embedded in the renmants of the topping. Seems that there was no shear steel tied over the top of the top steel to prevent this separation.
If this is true it would have been quite sudden and prior inspections may not have seen much of a crack unless they could have seen the back face of the bearing seat (probably a hor. crack a couple of inches below the top of the deck). The inspector who looked at the bridge immediately before the failure wouldn't have had a change to make that observation. The span seems to be box beams placed side-by-side which wouldn't have allowed a view of that face.
Annual or biannual inspections in Ontario, and I believe Quebec, are carried out by trained technologists or maybe young engineers. Experienced engineers would inspect the bridge if a structural evaluation was required, say for a rehab. I don't think it mattered in this case since I think it was a sudden brittle failure due to bad design detailing. If I'm right I'm amazed it lasted as long as it did.
RE: Bridge Collapse
RE: Bridge Collapse
RE: Bridge Collapse
Hg
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RE: Bridge Collapse
Dik
RE: Bridge Collapse
RE: Bridge Collapse
Hg
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RE: Bridge Collapse
If you had inspected the bridge and it checked OK according to the inspection parameters and as you were walking back to your truck and turned your ankle on a chunk of concrete and at the same time a chunk fell on your head, thank goodness for hard hats, wouldn't one think something is amiss.
RE: Bridge Collapse
Hg
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RE: Bridge Collapse
equipment passed over this bridge in
the last month. Did the overhanging
walkways put too much tensile stress
on the reinforcing bars?
RE: Bridge Collapse
The bridge was 30 years old, right? I would think normally a bad design would be evident much sooner than 30 years. The fact that the bridge was erect for 30 years (assuming no problems encountered during that time) would suggest that the bridge was properly designed and constructed.
The statement was made by 'dik' that corrosion was ruled out (of course given the recent history of early news reports on various sensational cases in the States, I would tend to 'trust but verify' all such reports). After 30 years, some kind of age degradation is inevitable of course, the question is did the age degradation cause the failure?
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Failure due to inadequate design or construction doesn't mean that it didn't meet the required standards back in 1970.
RE: Bridge Collapse
I'm guessing that chloride corrosion was not given a lot of consideration when that bridge was designed, because nobody bothered to salt the roads then. When I lived near that latitude decades ago, salt was used only to keep the sandpiles from freezing solid in their shelters. Salt is just an expensive abrasive when the temperature is below 0F all the time.
Sand was applied after each snowfall to provide traction, with maybe a little extra salt in the spring, to help break up the accumulated foot of ice on which we had been driving all winter.
Maybe global warming made it less futile to salt the roads, and hence catalyzed increased salt usage, and eventual failure of bridges not designed to withstand salting. Speaking of which, are _any_ steel- reinforced concrete bridges actually designed to deal with heavily salted decks?
Mike Halloran
Pembroke Pines, FL, USA
RE: Bridge Collapse
The product was defective from day due to bad design/construction. The little bit of corrosion was simply the straw that broke the camels back.
RE: Bridge Collapse
As I understand it, shear reinforcement provisions in the codes have been increased since the fifties but I don't know a better date on that change. This is one of the problems I see in our industry (transportation infrastructure) that rehabilitation of existing structures is rarely undertaken to address code modifications. That is a major discussion in itself. Also, I believe bridges are rated based on their moment capacity alone. Shear is not part of the rating. This should be appropriate for 95% or more of the structures in the inventory but I am sure some, as possibly this one, are controled by their shear capacity.
I hope we can find a specific cause that can be addressed in the future.
RE: Bridge Collapse
RE: Bridge Collapse
Right, this is going to take a lot more analysis than can be had from a few pictures. Hopefully this all comes out in a form that we can all use on our own designs. It's fun to discuss now, though.
Shear is conisdered, and I find that shear often controls the ratings of concrete bridges. (PS and RC)
I will admit that we can be negligent on keeping a current rating on the substructures, which the failed portion of the subject bridge could be called, depending on your point of view.
I would also say that you have plenty of company (including mine, for what that's worth) on your dim view of drop in spans.
RE: Bridge Collapse
RE: Bridge Collapse
RE: Bridge Collapse
It would be my opinion (at this tme) that the failure is cracking deterioration due to corrosion, time dependant cracking of concrete and at the end of the day poor inspection and maintenance. Chunks of concrete falling of Montreal bridges is a fairly common occurance. We will see that a number of bridges in Montreal will be closed, already I know of three since the collapse. If you've driven in Montreal this is a problem for the 'public'.
The bridge remained safe and serviceable for 36 years. We now know alot more about time dependant effects on concrete, corrossion, we certainly have the technology to inspect, and we know the construction sector in Montreal at the time was circumspect. The government needs to put this on the front burner.
RE: Bridge Collapse
RE: Bridge Collapse
If it was found below the joint, then the support for the cantilever end was likely the first failed part. Corrosion could play a major role here. The cantilever part sat on the joint and it was a simple support beam until the moment that the support failed suddenly. If you look at the second and forth photo of http://www.keystonebridge.ca/, you can see the support for the cantilever was sheared off. Now the beam became suddenly a purely cantilever. The abutment side incurred much bigger moment and much bigger shear for which the beam was not designed. As a consequence, the bridge had a brittle failure because of one or both of the two reasons: 1) shear failure, 2) reinforcement details designed according the old code were not ductile as required by the current code. (I am not sure about the code development history.)
If it was found near abutment, it is difficult to understand. It could mean that the concrete crushed due to some reason. From the photos we see not much of reinforcements near the abutment. So it was not likely due to over reinforce if that is the case.
I found almost no top reinforcements on the face of abutment. Is this normal? Anybody can give an insight idea?
RE: Bridge Collapse
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RE: Bridge Collapse
From what I can see from photos and (to some extent) guess from comments, I would say the most likely scenario would be:
primary cause was the reo (reinforcing steel) near top of cantilever and extending back over the pier was not properly tied in with the reo around the seat for the suspended span. This reo is to carry tensile forces on the top surface of the cantiver and down to the seat. If this is the case then there has been a fundamental design failure (including failure to check design) OR a failure to comply with the design (with constructor and inspection both guilty) during construction. Would appear that a more-or-less horizontal crack (possible initially from drying shrinkage) has formed on the vertical face between the two sets of reo and has propagated as one would expect along the plane of the lower (seating) reo then from the end of the reo diagonally down to the top of the pier.
Other issues of spalling, salt, corrosion, etc would be incidental in this case. It is amazing that this disaster took so long to happen.
RE: Bridge Collapse
RE: Bridge Collapse
In many (most) design situations, concrete is assumed to have no ability to carry tension but in reality it can usually carry at least 5 megapascals IN THE ABSENCE OF CRACKS. So with quite a bit of luck we had concrete carrying quite a bit of tension for many years - then the luck ran out.
Maybe it was corrosion of reo that triggered the collapse by increasing the tensile forces in the concrete (due to expansion of rust) but the fact remains that there does not appear to be steel available to carry the forces.
Maybe spalling led to redistributions of forces in the failure zone but it still comes back to bad reinforcing.
RE: Bridge Collapse
You say "Whenever there is a disaster, whether you are down here in Australia, in North America or anywhere else you have a lot of people jumping to all sorts of conclusions which is a pity because aspertions are cast on many innocent people."
and then proceed to so exactly that yourself !
RE: Bridge Collapse
The time dependent component hopefully will be something more than "... then the luck ran out...". What about movement of soil bearing the footers? Was sufficient care taken in design and construction to assure minimal or at least symmetric foundation shifts?
Were any unusual loads applied to the bridge during the past year, any history of large oversize loads taken across the span recently, any history of flooding or freeze-thaw heaving?
RE: Bridge Collapse
Let's keep in mind that all this forum has for evidence is the posted photos and some unreliable media reports. As I've said the photos do seem to give good evidence, in my hypothesis, of the mode of failure (shear) and probable reason (lack of shear steel) but they provide no evidence for the trigger that caused the failure.
If my hypothesis is about right then the question "what kept it standing for 36 years" is the big unknown. We won't learn that from the evidence presented in this forum.
Lastly, keep in mind that in structural engineering we use a lower bound (or low percentile) approach to predict strength from a sometimes wide distribution of actual strength. A structure can have a lot of reserve and redundant strength that we don't count on.
DBM
RE: Bridge Collapse
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RE: Bridge Collapse
RE: Bridge Collapse