Written by John Boyle, Asheville Watchdog.
A new round of questions, smart-aleck replies, and the real answers with Asheville Watchdog’s John Boyle.
Question: In regard to the new Blue Ridge Parkway bridge over I-26, there have been several non-technical articles in the Citizen-Timesand now the Watchdog, but they leave tech nerds dissatisfied. For instance, what design/engineering challenges were there in ensuring the two ends would be at the same elevation — or what was the allowable deviation? — when they met? How are the sections connected to each other? What provides vertical shear resistance between modules? Are the “tension cables” just there to prestress the structure prior to pouring the road bed? Will the road bed be structural or just a passive layer? The public needs to know.
My answer: Man, I hate to get the tech nerds riled up. Next thing you know they’re pounding on your front door with a slide rule and insulting your mother with impenetrable equations only they can understand.
Real answer: Luke Middleton, resident engineer for the I-26 widening project in Buncombe County, took this on, and I’ve got to say I’ll think it will scratch my tech nerd’s itch.
First, let’s lay out the basics of the bridge, which has a $14.5 million price tag. It will be 605 feet long, with two travel lanes and a sidewalk on the north side. The width is 36 feet, and the bottom of the bridge will be 85 to 90 feet above the I-26 roadway, the top 105 to 115 feet.
The 76 concrete pieces of the bridge were precast in Wilmington, starting in April 2021. Of the pieces, 62 are for the bridge superstructure and 14 for the two main piers.
The bridge was built in segments, starting from either side of the interstate. Regarding the middle of the bridge where there will be a “closure pour” to finish joining the two sides — no, this is not last call in your local taproom — Middleton said via email that an enormous amount of work goes into this type of construction.
“A massive amount of survey review takes place throughout this work, along with checking and comparing the designed plans and individual as-built segments,” Middleton said. “The surveying starts before any segments go up and continues on from the excavation of the footer, to the top of the footer, to setting of pier segments, to setting of the bearings — and that’s just the pier.”
The piers are the bases on either side of the interstate.
“For a majority of the segment pairs that are set, we probably survey 90 percent of them,” Middleton said. “This is performed so that if a problem is identified, measures can be put into place to make adjustments.”
When surveying, Middleton said the addition of weight is taken into account when evaluating elevations.
“Also, the bridge is constructed in a manner where there is some play in making adjustments, meaning there are 1,000-ton jacks on top of a temporary tower that’s supporting the cantilevers as they are constructed, and these jacks can also be used to make adjustments,” Middleton said.
Additionally, jacks support the three abutment pieces, and the abutments and these segments can be adjusted.
“The adjustments can be made with the jacks prior to the pouring of the high strength grout at the bearings finalizing the location,” Middleton said. “This is completed after all of the segments have been set on that particular half of the bridge.
“Even after the bearings have been addressed, there is still some room for adjustment when the jacks are released,” Middleton continued. “Even at the casting yard in Wilmington, there’s survey stations set up to ensure that the right dimensions are being constructed. The pieces are matchcast so there are very few, if any, issues with the segments lining up.”
The true engineering geeks probably want more right now, and Middleton said the information above is just scratching the surface of what has to be monitored in order to attempt to control the structure as it’s being built.
“It’s all a challenge (because) it is a dynamic situation, especially with the distribution of weight,” he said. “There is very little room on the tolerance end of the review; however that being said, this is a dynamic situation and we are still working towards the final goal of finishing the remaining closure pours.”
That’s still a few weeks out, by the way. When you drive by the bridge, you can notice just a small gap remains. Just glance, please, and don’t cause an accident with your gawking.
This is a good time to note that we’ll all get to enjoy the never-ending I-26 widening for a good while longer, as I first noted a year ago. It started in 2019 and originally was supposed to wrap up this year, and then in 2025. But now the NCDOT is shooting to wrap up in early January 2026.
The addition of a new exit contributed to the delay, as well as other delays on the project, which spans Buncombe and Henderson counties.
Now, let’s move on to how the bridge sections are connected.
“After each pair of segments is in place, the contractor runs at least one pair of permanent tendons through ducts that run through all of these segments,” Middleton said, noting most of the tendons have 12-7 wire carbon steel strands. “Once a strand is in place for a pair, the contractor will stress the tendon, which makes the tendon even stronger and pulls all of the pieces between said pair even tighter.”
I had to ask what 12-7 meant, too. Middleton explained that, “In each tendon there are a bundle of 12 strands. Each of one of those strands has seven carbon steel wires.”
The bridge has close to 100 horizontal tendons running through the bridge, with a couple running through the majority of the length of the deck. Along with the tendons, each segment has a significant amount of rebar that you cannot see.
“On a temporary basis, there are post-tension bars that are also in place that are used to hold the pieces in place,” Middleton said. “Once all the tendons have been stressed, a majority of the temporary bars are released and removed.”
Man, we are definitely in sweet geek-a-rama world now, and we haven’t even gotten to what provides vertical shear resistance between modules.
No, I had no idea what the heck this means. Happily, RMIT University in Australia provided this fairly simple definition: “Vertical shear forces are generated in a beam by the applied loads and by the support reaction. The reactions push up while the load pushes down. In most cases the maximum value of shear occurs close to the supports.”
The aforementioned tendons that run through the bridge deck help hold the bridge up, Middleton said. So they’re fighting the vertical shear force.
Regarding the tension cables, the reader asked if they’re just to prestress the structure before pouring the roadbed.
“As described above, there are some temporary bars that are used to help hold the structure up as it’s being constructed, and there are permanent tendons that are installed that help hold the bridge together,” Middleton said. “Once the segmental work is completed, the prime contractor will work on completing the bridge. There will be a 2-inch concrete overlay on the bridge, but it’s not a structural component.”
Middleton said he wouldn’t really call the concrete roadbed a “passive layer,” but “it does help protect the segments below and because of that should help with the longevity of the bridge.”
Alrighty then, I am now officially geeked out!
Question: I’ve heard from Asheville Water Resources Department workers that some of the pipes in West Asheville, especially along Patton Avenue, are actually made of asbestos. They’re supposed to be very brittle because of their age and can actually shatter if you were to hit them with a hammer. Supposedly, the water department has to test the water in these areas every two weeks to make sure it’s OK to drink. When it reaches a certain parts per million level, the city replaces the pipes. How many asbestos pipes does Asheville have? Where are they? And isn’t this dangerous, considering asbestos is a known carcinogen?
My answer: On a positive note, if the water were to catch on fire, it would be quickly contained.
Real answer: This is indeed correct, at least as far as asbestos piping being in place.
“Asheville does have about 30,000 feet of water service line whose material is a mixture of cement and asbestos,” water department spokesperson Clay Chandler said via email. “The vast majority of this ‘AC pipe,’ as it’s commonly referred to, is located in West Asheville.”
Asbestos used to be a very common building material in the United States and worldwide, used in everything from floor tiles to siding, in part because it is a fire-resistant material.
The Centers for Disease Control & Prevention notes that “Asbestos is the name given to a group of six different fibrous minerals that occur naturally in the environment. All forms of asbestos are hazardous and can cause cancer.
“Asbestos fibers do not have any detectable odor or taste,” the CDC states. “They do not dissolve in water or evaporate and are resistant to heat, fire, chemical and biological degradation. Because of these properties, asbestos has been mined for use in a wide range of manufactured products, mostly in building materials, friction products, and heat-resistant fabrics.”
As asbestos fibers can be harmful, “all new uses of asbestos have been banned in the United States by the EPA,” the CDC states. Typically, the danger with asbestos comes from breathing in the fibers, often when the original materials start to crumble or otherwise fail.
This holds true with the city’s AC pipes.
“It’s important to note that, because the pipe is lined with cement, asbestos fibers only contact the water when the pipe crumbles, or otherwise fails,” Chandler said. “When that happens, Water Resources staff trained in the handling and removal of AC pipe respond and conduct appropriate repairs, which would include replacement of the failed portion of AC pipe with iron pipe.”
The water is tested regularly, but certainly not every two weeks.
“Testing of drinking water that traverses AC pipe is conducted every nine years, per rules made by federal and state regulatory agencies,” Chandler said. “We last tested ours in 2020, and found no asbestos in the water. We’ll test again in 2029.”
Asheville Watchdog is a nonprofit news team producing stories that matter to Asheville and Buncombe County. Got a question? Send it to John Boyle at [email protected] or 828-337-0941. His Answer Man columns appear each Tuesday and Friday. The Watchdog’s reporting is made possible by donations from the community. To show your support for this vital public service go to avlwatchdog.org/donate.