Bobcat punches through ped overpass deck, lands on van 1

[ACtrafficengr]

Apparently it wasn't designed for vehicle loads. I thought it was common practice to design them for H-10 loads for maintenance vehicles and ambulances.

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[IRstuff]

Usually, you would think there would be barriers installed to prevent vehicles getting on something that is not rated for vehicles

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[StrucDesignPE]

I thought it was common practice to design them for H-10 loads for maintenance vehicles and ambulances.

Judging the width of the bridge to be maybe 6 or 7 feet, it doesn't look like a vehicle larger than the skid steer could actually fit on it. The pedestrian bridges I have worked on have always just been people loads.

 

[MintJulep]

I'll guess that "I've driven over that bridge hundreds of times and it never collapsed before" will come out in the investigation.

 

[JoshPlumSE]

Am I the only one who read this title thinking I'd see a video of an actual Bobcat (i.e. the animal) landing on some guys car?

 

[MintJulep]

@JoshPlum
No, you're not.

 

[JStephen]

The little bridges on the local bike trail (Continental, I think) have tags indicating 5,000 lbs capacity. They're not too wide, either, but I've seen police cars drive over them, so I guess that's why the rating. A Bobcat is a little more concentrated than a police car, though.

 

[TenPenny]

Looking at that, I have to wonder: how did the designers expect maintenance or snow removal to happen?

 

[SuperSalad]

As the article says, the bridge was owned (and likely the design was approved) by NJDOT. Maintenance was the responsibility of Pasaic county. Since it is the NJDOT, I think it is a safe assumption that the maintenance details were of no concern to the NJDOT.

But really, I am positive they assumed a worker with a shovel/snowblower would be out there every day of snow (disregarding the fact that each shovelful would need to be carried to the end of the bridge, since you can't throw it over the side). Yeah, ok........good luck with that.

Andrew H.

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[itsmoked]

I'd think maybe 4 feet of snow might even trump a Bobcat?

Keith Cress
kcress -

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[JohnRBaker]

Yes, but at least the load would be spread out fairly evenly.

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[Comcokid]

Yes, but still, most structures, other than aircraft, are designed with Factor-of-Safety of 8 to 10. The Bobcat was suppose to weigh 4000 lbs. Are we to believe the design load was then 400 lbs?

Reminds me of a old 4 ton rated bridge that collapsed years ago when being re-paved. It had a dump-truck full of asphalt, a paving machine, a asphalt roller, pickup truck, and work crew on it when it collapsed. Total load at collapse was around 45 tons.

 

[GeoEnvGuy]

To me this is another example of bureaucratic processes failing. The NJDOT pays for the cheapest capital bridge design and doesn't care about maintenance costs or methods as that is the county's budget. County is looking for cost effective measures and assumes bridge should be able to support the weight of the largest vehicle that it can drive on the bridge.

 

[JohnRBaker]

Bridges have a unique place in the world, and that's because virtually ALL of them are built with 'public' funds, which means that they are being paid for with taxpayer's dollars and therefore the people responsible for writing the specs, letting the bids, reviewing the bids, making the final selection as to who actually builds the bridge, supervising its construction and inspecting/maintaining it afterwords, are under enormous pressure to spend as little of those taxpayer's dollars as possible. And on the other side, you have contractors who want to win the bid and still make a profit for their owners. When you look at in that light, it's a wonder that we don't lose a bridge a week.

For some very good insight into this 'bridge' phenomenon, I can highly recommend some books written by Professor Henry Petroski, including 'To Engineer Is Human: The Role of Failure in Successful Design', 'Design Paradigms: Case Histories of Error and Judgment in Engineering', 'Success Through Failure: The Paradox of Design', to name a few.

John R. Baker, P.E. (ret)
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[gte447f]

most structures, other than aircraft, are designed with Factor-of-Safety of 8 to 10

Most building structures and bridges are not designed with a factor of safety of 8 to 10. Not even close.

 

[JohnRBaker]

Most building structures and bridges are not designed with a factor of safety of 8 to 10. Not even close.

This is one of the issues that Professor Petroski covers in his books that when it comes to bridges, because of who pays for them and how contractors have to win the bid and still make a profit, this tends to over time force the designers to keep trimming the safety factor for subsequent bridge designs based on the success of the last design. This continues until someone goes one step too far and there'a a failure. It's the old adage that "You don't know what's enough until you know what's too much".

Petroski uses the history of suspension bridge designs as an example. He starts with something like the Brooklyn Bridge, often credited with being the first modern suspension bridge, erected in 1883, and points out how the safety factor was very high because it was the first of it's kind and you can literally look at the bridge and get that feeling. However, based on the success of the Brooklyn Bridge, as subsequent suspension bridges were designed and constructed, the safety factor was continued to be tightened-up until we reached the spectacular disaster of the Tacoma Narrows Bridge (AKA Galloping Gertie) that collapsed a few months after it opened in 1940. Since then, there has not been a failure of a large suspension bridge built after this date anywhere in the world. If you look at other bridge designs, such as cantilevered or truss, you would see the same history of early over-designed examples that lead to more elegant structures until a failure occurs and then everyone simply uses the last successful example as the standard for all future bridges of that design.

Anyway, it's stuff like this that Professor Petroski does a good job of explaining and something that all engineers need to think about. His primary thesis is that we learn very little when there are no failures. That it's only when a design fails that we have an opportunity to learn something about the assumptions that we made with respect to that design in the first place.

John R. Baker, P.E. (ret)
EX-Product 'Evangelist'
Irvine, CA
Siemens PLM:
UG/NX Museum:

The secret of life is not finding someone to live with
It's finding someone you can't live without

 

[BridgeSmith]

It was probably too narrow for typical vehicular traffic (full-sized pickup), so it was not designed for vehicle wheel loads. Likely, it was designed for distributed loads only.

From the news video, you can tell it's a thin slab with small welded wire fabric reinforcing. Not surprising the skidsteer broke through.

 

[JAE]

A university of Nebraska professor, some years ago, was given free reign to field test a three span slab bridge in western Nebraska as the bridge was no longer being used since the highway had been re-routed elsewhere.

They installed apparatus to apply downward load on the bridge to see where its load capacity actually was compared to a standard AASHTO truck loading (which was the basis of the bridge’s original design).

Upon loading they determined that the bridge held a capacity of about 8-10 times the design load. Reason: the bridge’s side rails participated in the bridge flexural moment capacity and shear wasn’t a problem.

So depending on the quirks and unique features of a structure you can have very high “real” safety factors... or not. This prof was involved in the Florida International University pedestrian bridge that recently collapsed.

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[BridgeSmith]

A fairly extensive research project some years ago discovered that typical concrete highway bridge decks fail at somewhere around 5 times the anticipated capacity, and when they did it was not a flexural failure, but a punching shear failure. That is what led to the inclusive of what is referred to as empirical deck design in the AASHTO spec. that has less than half the reinforcing of traditional designs.

As far as the testing JAE referred to, it's not surprising to see that FOS in the 8-10 range, assuming the side rails were concrete. While the target reliability index (roughly analogous to an overall FOS) for highway bridges is calibrated to be about 3.5, it's considerably higher for non-ductile (brittle) failures. While we don't want bridges to fail, what we really don't want is for them to fail suddenly, without warning.

 

[JAE]

Yes the side rails were concrete.

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