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[CtrlAltDel]

As seen in this news report, part of the ramp from I-75 North to I-24 West (I think) has collapsed, and as you can imagine, traffic has snarled. It seems no one has died, though, which is good. The bridge is undergoing emergency repairs, but the whole thing was supposed to be torn down anyway as part of a reconstruction project that went to bid last fall.

[ilpt4u]

According to Google Maps, Thru I-75 South thru the Y interchange is closed. Thru I-75 South occupies the bridge that partially collapsed

The ramp from I-75 North to I-24 West is what was collapsed upon

[CtrlAltDel]

The ramp from I-75 North to I-24 West is what was collapsed upon

Yes. Thanks for the correction. Here is the status of things on Google Maps:


On a not completely unrelated note, though, why does I-75 swing to the west as much as it does? It seems like that turn should have been a bit more fluid. Something like this perhaps:

Does any one know why the actual configuration was selected?

[Duke87]

Does any one know why the actual configuration was selected?

Yes, because the routing of the highways was selected so that the most direct paths match the trips that people are taking, not to make a smooth path for the "through route" that happens follow the same route number.

More people are exiting to I-24 than are staying on I-75, so the configuration favors that.

2017 AADT figures:
I-75 directly north of interchange: 129,749
I-75 directly south of interchange: 108, 848
I-24 directly west of interchange: 134,737

Do the math and this gives us the following for each movement pair:
Between I-24 and I-75 to the north = 77,819
Between I-24 and I-75 to the south = 56,918
Thru traffic on I-75 = 51,930

[1995hoo]

My brother drove through there this weekend. Glad he got through when he did!

[wriddle082]

Interchange built in the very early 60’s and has been mostly unchanged since that time, though I suspect most of the ramps started out as one lane and were re-striped to two lanes as traffic warranted.  These bridges also originally featured Tennessee’s two-pipe railing, which was very unsafe for mainline freeways, and probably sometime in the 80’s nearly all bridges in the state with this railing were augmented either with metal guardrail or Jersey parapets, which this one had.  And that is what appears to have fallen off the bridge.

I believe the interchange rebuild isn’t going to change the overall configuration of the interchange, but every bridge is slated to be replaced.  Around 15 years ago, GDOT did something similar with the nearby 24/59 interchange at Wildwood.  Replaced all of the bridges but did not change the overall interchange configuration.

[CtrlAltDel]

Does any one know why the actual configuration was selected?

Yes, because the routing of the highways was selected so that the most direct paths match the trips that people are taking, not to make a smooth path for the "through route" that happens follow the same route number.

That makes sense, thank you.

Also, one lane of I-75 South has reopened to traffic, although things are still slow through the interchange.


Here is also a good picture of what happened.

[Tom958]

http://bridgereports.com/1502495 dates the bridge as 1959, along with several others on both 75 and 24. It also descibes the structure as "Prestressed Box beam or girders," but it looks like cored slabs to me. Streetview of its sister structure over the 24 to 75 ramp, since there isn't one of the collapsed span.

[roadman65]

I have driven twice over the bridge in 1995 and 96 and under that structure twice in 1993 and again in 2001.

[Tom958]

I'm not gonna try to quote posts on my phone, but...

Per https://www.tn.gov/tdot/projects/region-2/i-75-interchange-at-i-24.html

"The planned design consists of widening the existing roads and ramps, increasing ramp radii, reconfiguring I-24 ramps to enter and exit I-75 from the right side, shifting the interchange to the west, and modifying the Welcome Center area traffic circulation."

Also, I think it's obvious that the current interchange layout was chosen because it was the cheapest of any reasonable alternatives. In particular, moving the centroid of the interchange to the west at the expense of tight curve radii on the I-75 mainline made it possible to rationalize not making any special provisions for weaving movements between the US 41 interchange, the welcome center, and the 75-24 interchange itself. I also suspect that the desire to tighten up the layout is part of the reason why they went with cored slabs for the bridges, to reduce structure depth and thus ramp lengths, instead of a more standard design.  Admittedly, though, there are plenty of other such bridges at nearby sites where reducing structure depth doesn't appear to have been especially important.

[Tom958]

Apparently an illegal, overheight load hit the bridge, severing five of the 23 prestressing strands in the affected beam. "The standard minimum height for a bridge in Tennessee is 16 feet and 6 inches. The lowest point of the I-75 bridge is 16 feet, 10 inches." I neglected to mention that the bridgereports.com entry gave the bridge a score of 72/100 in 2016.

https://www.timesfreepress.com/news/breakingnews/story/2019/apr/02/heres-what-caused-railing-collapse-i-75-bridge-chattanooga/491807/

OT, but I was surprised to find clearances of 13'-8" on a state highway running under a section of I-24 that was built in the mid '60's. I'm just sayin.'

[Brian556]

Apparently an illegal, overheight load hit the bridge, severing five of the 23 prestressing strands in the affected beam. "The standard minimum height for a bridge in Tennessee is 16 feet and 6 inches. The lowest point of the I-75 bridge is 16 feet, 10 inches." I neglected to mention that the bridgereports.com entry gave the bridge a score of 72/100 in 2016.

https://www.timesfreepress.com/news/breakingnews/story/2019/apr/02/heres-what-caused-railing-collapse-i-75-bridge-chattanooga/491807/

OT, but I was surprised to find clearances of 13'-8" on a state highway running under a section of I-24 that was built in the mid '60's. I'm just sayin.'

Hmmm...I've noticed that Texas puts clearance signs on all bridges, but TN and several other states don't.
I googled standard minimum clearance for interstate highways, and it said 16 ft. In Texas, there are lots of bridges under that. Corinth Pkwy over I-35E in Corinth TX was 14-8, and it got hit all the time. It was built in 1961. It is weird that TX didn't follow the rules, and drivers have had to deal with the consequences of tons of bridge strikes as a result.  Sounds like TN was smarter in this regard

[SteveG1988]

Went over it today, they got the new railing in place. still single lane under it.

[sparker]

Apparently an illegal, overheight load hit the bridge, severing five of the 23 prestressing strands in the affected beam. "The standard minimum height for a bridge in Tennessee is 16 feet and 6 inches. The lowest point of the I-75 bridge is 16 feet, 10 inches." I neglected to mention that the bridgereports.com entry gave the bridge a score of 72/100 in 2016.

https://www.timesfreepress.com/news/breakingnews/story/2019/apr/02/heres-what-caused-railing-collapse-i-75-bridge-chattanooga/491807/

OT, but I was surprised to find clearances of 13'-8" on a state highway running under a section of I-24 that was built in the mid '60's. I'm just sayin.'

Hmmm...I've noticed that Texas puts clearance signs on all bridges, but TN and several other states don't.
I googled standard minimum clearance for interstate highways, and it said 16 ft. In Texas, there are lots of bridges under that. Corinth Pkwy over I-35E in Corinth TX was 14-8, and it got hit all the time. It was built in 1961. It is weird that TX didn't follow the rules, and drivers have had to deal with the consequences of tons of bridge strikes as a result.  Sounds like TN was smarter in this regard

The original '56-'57 standards for vertical clearance over an Interstate carriageway specified 14' only; over the years it was raised 6" at a time to the current 16'.  Out here in CA up until a few years ago everything under 15' was signed; presently Caltrans is signing (often with advanced notice if under 15'6") overcrossings below 16'.   Besides some of the older urban freeways that have several overcrossings with less than 15' of clearance, one of the prime examples of Caltrans signage is on CA 99 in the San Joaquin Valley -- a mixture of construction dating in sections back to 1952-53 interspersed with more recent deployment.  Pretty much everything built from 1970 to today exceeds 16' clearance, starting with the Turlock bypass that opened in 1973.  But besides the really early stuff, some of the segments built in the early/mid '60's featured decidedly low clearances (including 14'6" on the Crows Landing Road overcrossing in Modesto, which opened in 1965); low clearances seemed to be pervasive on construction from 1959-66 statewide (also see US 395/current I-215 between Riverside and Colton as well as I-10 through Yucaipa and Mentone -- the latter since rectified).   

[MCRoads]

Wow, this reminds me of the OKC partial collapse in 2016, where an oversight truck (seriosly people!) hit the bridge, and caused one section of the deck to come down.

[SteveG1988]

Wow, this reminds me of the OKC partial collapse in 2016, where an oversight truck (seriosly people!) hit the bridge, and caused one section of the deck to come down.

The Department of Oversight was not doing their job.

[DJStephens]

Apparently an illegal, overheight load hit the bridge, severing five of the 23 prestressing strands in the affected beam. "The standard minimum height for a bridge in Tennessee is 16 feet and 6 inches. The lowest point of the I-75 bridge is 16 feet, 10 inches." I neglected to mention that the bridgereports.com entry gave the bridge a score of 72/100 in 2016.

https://www.timesfreepress.com/news/breakingnews/story/2019/apr/02/heres-what-caused-railing-collapse-i-75-bridge-chattanooga/491807/

OT, but I was surprised to find clearances of 13'-8" on a state highway running under a section of I-24 that was built in the mid '60's. I'm just sayin.'

Hmmm...I've noticed that Texas puts clearance signs on all bridges, but TN and several other states don't.
I googled standard minimum clearance for interstate highways, and it said 16 ft. In Texas, there are lots of bridges under that. Corinth Pkwy over I-35E in Corinth TX was 14-8, and it got hit all the time. It was built in 1961. It is weird that TX didn't follow the rules, and drivers have had to deal with the consequences of tons of bridge strikes as a result.  Sounds like TN was smarter in this regard

The original '56-'57 standards for vertical clearance over an Interstate carriageway specified 14' only; over the years it was raised 6" at a time to the current 16'.  Out here in CA up until a few years ago everything under 15' was signed; presently Caltrans is signing (often with advanced notice if under 15'6") overcrossings below 16'.   Besides some of the older urban freeways that have several overcrossings with less than 15' of clearance, one of the prime examples of Caltrans signage is on CA 99 in the San Joaquin Valley -- a mixture of construction dating in sections back to 1952-53 interspersed with more recent deployment.  Pretty much everything built from 1970 to today exceeds 16' clearance, starting with the Turlock bypass that opened in 1973.  But besides the really early stuff, some of the segments built in the early/mid '60's featured decidedly low clearances (including 14'6" on the Crows Landing Road overcrossing in Modesto, which opened in 1965); low clearances seemed to be pervasive on construction from 1959-66 statewide (also see US 395/current I-215 between Riverside and Colton as well as I-10 through Yucaipa and Mentone -- the latter since rectified).   

There were numerous cast in place concrete culverts used for underpasses during original construction of both I-10 and I-25 in southern New Mexico.  Late fifties to late sixties.  They were 13'-6" to 14'-0" vertical clearance inside them.  One or two asphalt overlays on the floor, and it reduced it even more.  Was a fairly common occurrence for a way-ward semi to get stuck, and have to have the tire pressures reduced, in order to back it out of the situation.   

[RoadMaster09]

In general, I'd sign anything below 18 feet on an Interstate, 16 feet on a freeway or expressway (non-Interstate), 15 feet on an arterial road or truck route and 13' 6" (or the legal maximum without permit) otherwise.