Chattanooga 1-75 N to I-24 W Overpass Partial Collapse 6

Officials have reported that that the railing on 75 above may have been struck by a truck causing it to displace and fall on 24 below.

Interesting to see a tree growing on a concrete bridge. Im no specialist on green engineering but I don't think tree roots on top of an abutment located in a cracked in the concrete is appropriate.

The interesting part I got from the news is this bridge was scheduled for 2019 rehabilitation. So the owner new it was bad and they made a plan to fix it but it failed before it was fixed. So is there any real discipline for the owner, unlikely as this is public infrastructure. If this was a private bridge crossing from industry would it be the same outcome, i think not. I believe if this was a private bridge crossing there would be civil and criminal investigations launched with financial penalties and charges against executives and the company.

I-75 is part of the interstate system, so ultimately, the 'owner' is the taxpayer, be they state or federal, or more likely, a combination.

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Fortunately the driver of the blue car was not injured. The attached article has a link to the TnDOT inspection report for the subject bridge.

The Tennessee Department of Transportation says an oversized load is likely what led to the collapse of part of an I-75 overpass late Monday morning.

According to TDOT Chief Engineer Paul Degges, 5 steel cables were cut on the bridge Monday, which he says is the main evidence for their prediction of what happened.

...

TDOT says when the structure came down, they saw the steel cables were not rusted underneath, but several were cut around the same area. They said it seems improbable that all five failed at the same time.

TDOT says the driver of a blue car collided with the concrete barrier. He is now listed in critical condition at a local hospital.

Click to expand...

On the link is the inspection report from 2018.

I think that guy in Spartan5's linked video did a great job explaining things to the reporters, even in the face of some fairly dumb/trivial questions from particular reporters...

Rather than being scheduled to be fixed, it was actually scheduled for demolition due to the interchange being reconfigured.

Thanks for the links to the news reports and TDOT report.

The theory is that an oversized load impacted the overpass at some undetermined time prior to the collapse. The evidence supporting this is that five of the tension cables along the base of the beam are cleanly cut in a straight line.

Here are some screen captures of the cut cables.

It is common for precast/prestress spans to be damaged by over-height vehicles and then repaired many times over, maybe too many times. When the strands are broken or badly damaged they will be cut out and replaced with a section of cable using couplers. Sometimes the span will be preloaded during the repair.

In the street view shots it looks like the span may have been previously repaired based on the irregular concrete lines. From the incident shots it looks like the tendons are cleanly cut which speaks of a repair, perhaps improperly executed. The failure near mid-span (directly above the left lane) also suggests a pre-existing weakness in the tendons at that point. The bridge file of record should show the repair history.

Interesting that the clearance was not posted on the span.

Edit: Here is a link to the location. The irregular concrete lines could just be distortion where the street view photos are stitched together.


Another edit, sorry:

Is it not irresponsible to say on the one hand that you have no idea why something happened and then simultaneously express confidence that it cannot happen again?

It's still not clear what caused the bridge's concrete railing to come crashing down, according to Tennessee Department of Transportation officials. However, they are confident the bridge, which was built in the 1950s, is structurally sound and its condition will not lead to further injuries.

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Also what is the point of this kind of amateur damage control by TDOT? Their own spokesperson doesn't seem to understand how the bridge is constructed and the significance of the failure.

TDOT spokeswoman Jennifer Flynn clarified that the bridge did not fall: "The bridge didn't collapse," she wrote in an email. "The railing fell off for some reason.

Click to expand...

Maybe the tree roots pushed it off...

Google Maps Street View link. You can change between June 2011, June 2014, November 2017, and June 2018.
Nothing stands out to me, but my specialty is electrical, not civil.

Did the oversized load hit the bottom or the top?
Suppose an oversized load on top contacted the railing and forced it off of the support on the left.
Could the damage to the span and railing have happened when one end of the span hit the ground?

Bill
--------------------
"Why not the best?"
Jimmy Carter

Something going under struck it and they believe it cut 5 out of 24ish strands based on chief engineer at dot in the video at Spartans link.

Looking at the 2018 inspection report page 20 of 33 they call out an area with rebar and strands being exposed. Would this area line up with this or is that a different span? I can’t tell how to figure which beam is which.

Reading charliealphabravo‘s post got me wondering if a truck hitting the beam would cut the strand cleanly like they show in the picture. If the strand they used in the 50’s is like what we have now I would expect it to bend before breaking. Prestressing plants regularly harp the strands a considerable amount with no problems. Plus to cut the strand the impact would have to cut through the concrete. I would imagine the concrete would take the brunt of the impact.

The engineer in the video seemed to think impacts like this were quite common and had happened before (i.e. damage to the strands was consistent with his previous experience of people impacting their bridges and damaging strands is what I took away from what he was explaining). A truck with a load going 65 miles an hour that just impacts/clips the load into the structure would quite easily slice through the concrete and steel if said strands were close to the surface I would suspect.

5 out of 24 strands cut but no corrosion, and a sudden failure?
I am not abandoning my thought that a wide load may have tagged the railing and pushed it off.
I may be wrong so I'll wait and see.

Bill
--------------------
"Why not the best?"
Jimmy Carter

Something just doesn't sit right w/ me that a truck or load impacted that beam with enough force to perfectly cut those strands and then drive away leaving a fallen girder in its wake.

They thought it hung in there and collapsed at a later time based on the video. They have video monitoring, but are unable to record it (legally).

The cable cuts are in the middle area where it broke?