“We can’t just build everything we want to – we have to continue to reuse and reinvent ways of utilising our existing critical infrastructure.” So says Michael Miller, VP of engineering services at Exo, a Houston-based firm specialising in the inspection and remediation of US-based utilities.
The structural engineer – who was responsible for managing transmission line infrastructure projects across 25 states and 4 Canadian provinces when he worked at the US Department of Energy – admits that most of his 36-year career has been spent working on newer designs.
Joining Exo four years ago has enabled him to view infrastructure in a more circular way: What can be remediated? How long can the lifecycle of a structure be extended by?
As a case in point, during Bentley’s Year in Infrastructure conference in Vancouver this year, Exo was invited to talk about a powerline project that involved extending the life of two electricity towers.
Located on the Ohio River, the Ohio Falls Hydro station includes two towers that were built in 1927 and support a transmission line crossing the river.
Rather than building a new crossing in a different location, the utility clients – LG&E and Kentucky Electric – wanted to see if they could reuse these towers and hired Miller’s firm Exo to investigate.
“You would take one look at these towers and say, ‘Get rid of them, they’re too old!’ – that’s our instinct as structural engineers – we’re so used to building new. But looking at something like this and trying to delve into it was a real challenge,” Miller recalls.
The question at the centre of the investigation: What was the expected design life of this structure? Was it 50 years? 100 years? There were no designs or drawings that existed that detailed the components that were used or hinted at the structure’s estimated longevity.
The towers primarily comprised of weathering steel – a chemically formulated metal developed to eliminate the need for painting through its ability to form a stable layer of rust.
Because of its rusty orange patina, weathering steel can make structures seem unfinished or past their shelf life to the casual observer – but this doesn’t mean that they are. Some iconic buildings have purposely opted for this industrial look – including John Deere’s HQ in Illinois and, in the UK, Antony Gormley’s lauded Angel of the North sculpture in Gateshead is made from weathering steel, which gives it its distinctive orange colour.
“The main problem is the joints in structures like this tend to exhibit corrosion over time,” Miller explains.
“This can put tension in the tower’s rivets. So, one of the big pieces of work we needed to do was to map out this crevice corrosion,” explains Miller.
Another challenge was accessing two towers that were so precariously situated, at either side of the waterway – one by a dam, another across the river.
Exo decided the safest way to inspect the towers was virtually – using a drone – and feeding the resulting footage into a digital twin to create a detailed 3D model.
MicroStation vendor Bentley System’s iTwin software was used to process 250 gigabytes of high-resolution drone-captured photos into a reality mesh model.
“It took us a couple of days to get the drone footage, but it meant that we could then measure the dimensional characteristics of the tower so that we could do the modelling despite the lack of existing drawings,” says Miller.
“This also helps us look at how much corrosion has occurred in the towers’ crevices,” he adds.
Miller was impressed with the detail the engineers were able to view in the digital twin which allowed the team to virtually access and measure the degrees of corrosion.
“Once you have that 3D mesh model you can do almost anything you would have been able to do if you had hand climbed up that tower,” he says.
“You can see all that detail – we are getting down to one eight-inch tolerances on the detail – which means you can see faces and rivets and the condition of the services at these locations,” he adds.
Based on the digital twin, the team began the job of determining the structural stability of the towers. “One of the things that we looked at was rivet capacity,” says Miller. “Back in the 1920s there were lots of different strengths of rivets and we needed to figure out which ones were used.”
This was done using Bentley’s Power Line System, a piece of software that automates the calculation of design loads and checks strength according to international standards.
The rivets on the towers turned out to be strong – 24 kips [a ‘kip’ being a unit of force that’s equal to 1,000 pounds of force]. “That was good because the structures were also taking on all the tension from the crevice corrosion,” Miller adds.
Once the strength of the structure was established the next step was to examine how best to remediate the crevice corrosion.
Because there’s plenty of older infrastructure in the US facing the same challenges – and back then there was no unified standards on what materials to use, it varied from state-to-state and project-to-project – there have been a few ‘best practice’ studies to draw from.
Data that the Exo team drew from included a study undertaken at Purdue University which canvassed all the Department of Transports in the US, to find the best ways of remediating crevice corrosion.
Miller also found another Czech study which pointed to ways in which infrastructure which used a lot of weathering steel could be rectified -so long as it was not badly corroded.
The team also looked at structures from around the world, including the Eiffel Tower. Designed by Gustav Eiffel in 1889 for the World Fair, the wrought iron structure possesses a similar riveted construction to the towers in Ohio.
A symbol of Paris for more than a century, the tower was only meant to be temporary with an estimated life of 25 years. However, its puddle iron structure has stood the test of time – so long as it’s treated to a lick of paint every 7 years.
“These structures last a lot longer than we think they are going to, so long as they are well maintained. Steel towers don’t have an expiration date,” says Miller.
Drawing on this data and a variety of studies, the team produced a three-step process that will effectively treat and protect the Ohio towers for another 30 or 40 years of service life.
According to Exo, the decision to remediate rather than rebuild, made possible through a digital twin, has saved the utility company a potential ten years of permitting delays and approximately $80 million dollars building a new tower crossing.
The project also impressed the judges of Bentley Systems Year in Infrastructure and Going Digital Awards this year. Considered the ‘Oscars for Infrastructure’, Ohio Falls River Crossing beat all the competition in Transmission and Distribution to become the winner in its category.
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