Llano River Bridge Collapse in Texas 21

[JAE]

Video of the collapse due to high river:

https://www.youtube.com/watch?v=EY45z_vXbSQ

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After:

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[BridgeSmith]

Incredible video. Not an engineering failure, per se. I'm fairly sure it wasn't designed to withstand that level of flooding.

 

[JAE]

...but it was a Disaster (per the forum title).


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[BridgeSmith]

...I thought in the forum title "engineering" applied to both "failures" and "disasters". Oh well, it still makes for some great video footage.

 

[dgallup]

That's just incredible to see the bridge structure and concrete slabs "flowing" downstream. Doesn't look like a lot of clearance under the bridge in the top picture so maybe a failure in planning? Wouldn't have cost much to raise it up 5 feet.

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[thebard3]

Washington Post reports this was the highest water level recorded there in 83 years. I am unable to determine when the bridge was built, but was probably designed to withstand a 100 year event.

Brad Waybright

It's all okay as long as it's okay.

 

[msquared48]

100 year storms now are not what they were 83 years ago... Computer models are always changing as more data enters the system.

Mike McCann, PE, SE (WA)

 

[BridgeSmith]

"Wouldn't have cost much to raise it up 5 feet."

How about 10 feet? Where do you draw the line? Most likely, it was adequate for a 500 year flood event. Depending on the foundation conditions, that extra 5 feet could cost quite a bit.

It also may not be as simple as just raising the bridge portion. What made the Sunshine Skyway bridge collapse much worse is that the bridge profile grade was on a crest, so drivers could not see that a span had collapsed in time to avoid driving off into the river.

 

[JohnRBaker]

It was reported that the Llano River had risen 35 feet prior to the bridge failing. I suspect that that went well beyond what engineers would have considered to have been a "100 year event".

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[IRstuff]

The video seems to show the upstream side falling, so it's more likely that the footings got eroded, rather that the bridge deck getting pushed off it supports. That would suggest that the height was irrelevant; it was simply the shear volume and speed of water running over the footings.

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[BridgeSmith]

I suppose all buildings along the Gulf Coast could be designed to withstand a Cat. 5 hurricane, too. At what point does spending an extra X% to protect a structure from an extremely unlikely event become an unjustified expense? When safeguarding lives is involved, quite a bit of extra expense is justified (not the case here, as there was plenty of time to close the bridge before collapse). When essential services would be interrupted, there is also justification to spend some extra cash to lessen the likelihood of failure. That may or may not be the case here; I don't know what other access routes are available, nor what level of event the bridge was designed for or had freeboard for.

 

[cranky108]

There is a point here that might be missed, but would the essential services people have to assume the reliability of the bridge, and it might be expected they might assume wrong?

 

[BridgeSmith]

There are categories for essential and critical bridges that require design for more extreme events. I don't have any information on whether that particular bridge qualifies for either of those categories, or what level of event this flood may have been.

 

[msquared48]

Well, I assume it will take at least two years to replace this bridge, not considering arguing about paying for it.

How much is the lack of usage going to cost the local economy?

Mike McCann, PE, SE (WA)

 

[stookeyfpe]

I've grown up in this area. I've hunted and fished in this area in the 1960s-1980s. This area and the surrounding Austin metro area has grown over the past 50 years. I doubt the designers realized the amount of impervious cover that was going to be created in the Austin and Hill Country region.

 

[IRstuff]

"At what point does spending an extra X% to protect a structure from an extremely unlikely event become an unjustified expense?"

That supposes that this latest hurricane was "an extremely unlikely" event. One issue that the likelihood statistic is only based on the last 200 years or so. Another is that it's well known that the requirements have invariably gotten watered down because of the gnashing of teeth at the cost to implement designs for the actual extreme requirements.

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[JohnRBaker]

It appears that some people here is under the impression that the flooding in the Llano, Texas area was somehow related to Hurricane Micheal. I was not aware that it had moved from Georgia and the Carolina's to Texas. My understanding was that while Michael was a near-category 5 hurricane, that it is was very compact and fast moving so I'm not sure how much of an impact it had on a location hundreds of miles from the Florida pan-handle.

John R. Baker, P.E. (ret)
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Irvine, CA
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UG/NX Museum:

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[BridgeSmith]

"That supposes that this latest hurricane was 'an extremely unlikely' event."

Not necessarily. I was attempting to shed some light, in general terms, on the decision-making process used to set the design criteria for civil infrastructure. I haven't seem anything thus far indicating how the level of flooding in this instance would be classified. Nor have I seen what level of flooding event the bridge was designed for. Without those two pieces of information, it's impossible to say whether this was a design failure, the result of inadequate design criteria, or an extremely unlikely event which was not reasonable to anticipate, nor spend hundreds of thousands or millions of dollars to mitigate a tiny risk.

In hindsight, it's easy to say they should have spent the money, since it will now cost millions more to replace it. However, spending an extra million or so, of a finite budget, to potentially save this bridge from an event it isn't likely to ever see within its useful life, would have meant other bridges didn't get safety upgrades that could save lives. Maintaining infrastructure on a limited budget requires tradeoffs and making hard decisions about how to allocate resources. It must be done prudently, weighing risk against cost, and placing a high premium on the safety of those using that infrastructure.

 

[IRstuff]

While it wasn't a hurricane, it was a boatload of rain; totaling 4 to 8 inches over 4 days, equivalent to the rainfall for a entire normal October. The net result was the Llano river level rising 37 ft in less than 2 days.

This type of storm, and the hurricanes, continue to get more frequent and more severe.

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[BridgeSmith]

"This type of storm, and the hurricanes, continue to get more frequent and more severe."

Yeah, I heard the same fear-mongering from the IPCC, too. The problem is that the evidence doesn't actually support that assumption.