Ep 40 SPECIAL CN Tower
Engineering News – Underwater Adhesive (0:50).
This week's episode is about an engineering marvel, the CN Tower (4:10). Building the tower wasn't a simple feat, there were several construction and engineering issued they had to tackle (8:35). The owners have also made changes and updades to the tower over the years such as adding the skywalk (24:00).
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And I’m Brian. And we’re both from Calgary, AB.
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This week in engineering news, underwater adhesives.
Marine organisms, such as mussels, secrete adhesive proteins allowing them to stick to things underwater
It’s hard enough to find adhesives that work when wet, let alone underwater
McKelvey School of Engineering at Washington University in St. Louis
Engineered microbes produce the necessary ingredients for a biocompatible adhesive hydrogel that’s stronger than spider silk and as adhesive as mussel foot protein
They have experimented with mussel foot protein adhesives before but they quickly diffused underwater and were therefore hard to use. Even when they could get it to work, the adhesive would stick to the two surfaces but not to itself. Like separating an Oreo cookie and getting the creme filling on both sides.
The trick was the spider silk. When they mixed it with the mussel foot protein and synthesized a tri-hybrid protein that was as strong as spider silk and as adhesive as mussel foot protein. It’s also biocompatible and biodegradable.
The hydrogel is also slightly more dense than water so it’s easy to work with
The main application they are hoping to use the underwater adhesive for is tissue repair, specifically rotator cuff surgeries.
If you want to read more on this study, check out the link on the webpage for this episode at failurology.ca
Now on to this week’s marvellous episode, coincidentally an engineering Marvel, the CN Tower in Toronto, Ontario, Canada.
Located at 290 Bremner Blvd in downtown Toronto - built on former Railway Lands
CN refers to Canadian National, the company rail that built the tower, although it’s now owned by Canada Lands Company
World’s 9th tallest free-standing structure - was the tallest until 2007 when it was surpassed by the Burj Khalifa - it is still the tallest free-standing structure in the western hemisphere
One of the American Society Civil Engineer’s seven wonders of the modern world
Construction started in 1973 and finished in 1976, opened June 26, 1976
Cost 63 million CAD
457m to the roof and 553m to the Antenna spire - the equivalent of a 147 storey building
The Calgary tower is 191m to the top of the spire. It’s just a baby tower, we have office buildings that are taller than it.
Classified as mixed-use - observation, telecommunications, attraction, restaurant
Attracts more than 2 million international visitors annually
Construction and Engineering Issues they had to Tackle
The original plan was a tripod with three independent cylindrical pillars linked at various heights by structural bridges - this would have been a shorter tower and over time the vision evolved to the one we see today. Which is a hollow concrete hexagonal pillar containing stairwells, power and plumbing connections. There are 6 elevators, two each three of the 6 sides. The elevators cabs and shafts are lined with glass.
The SkyPod (originally called the Space Deck) was not part of the original concept but was added during construction as one engineer thought visitors would pay extra for a higher observation deck
The 15m deep foundation took about 4 months to dig and build back to grade.
They used a hydraulically raised slipform style concrete formwork that moved about 6m per day as the concrete set below. This is typically used for continuous pours, but they ran the pours from Mon to Fri with a small crew for about 9-10 months. The slipform style pour was a fairly new tech at the time.
The nature of the tower design meant that they had to sleeve the concrete, run the steel cable tendons through these ducts and post-tension the cables once the concrete was poured. There were not sufficient supports for the ducts in some locations, resulting in the displacement of said ducts and impairment of post-tensioning. Some ducts were blocked or resulted in kinked cable paths. Although there was only one tendon that was completely blocked and not used. That said, there were several tendons that had broken or lost strands.
The stress on both ends of one cable was so great, that it resulted in a loss due to grip failure on the far anchor head. The stress was so high, it lifted the stressing jack off its seat.
Because the construction proceeded through winter, the concrete and post-tensioning of the steel cable tendons had to be able to withstand severe weather conditions such as high winds and cold weather.
They also couldn’t grout the tendons in the winter and had to accommodate corrosion and made it hard to maintain uniform final stresses across the tower.
To protect the steel cable tendons against corrosion until they would be grouted, they blew about 190 l/s of oil-free air at ambient temperature into each duct to achieve 3 air changes per hour.
Also related to the grout, the water-cement ratio had to be just right. They started off with a thicker grout, but it was very hard to pump up the tower and set very quickly.
They mixed the concrete on-site to maintain batch consistency, which is not something you see very often, although I have to assume quality control measures for concrete production have improved over time.
They used massive plumb bobs hanging from the slip form, viewed from small telescopes on the ground, to check the tower’s vertical accuracy. It varies from true vertical accuracy by only 29mm throughout the tower. Which is impressive.
In August 1974, they raised 12 giant steel and wooden bracket forms with cables and jacks to the top of the tower to form the pod at the top which houses all of the observation decks and the restaurant.
The pod is supported by a ring beam that forms a 12 sided polygon. The tendon units in the ring beam were one of the more delicate design problems as it had to arrange all of the stress patterns symmetrically and all of the anchors had to be located towards the inside of the beam. Each tendon was placed in the identical position, reaching 180 degrees around the ring beam and offset 30 degrees from the adjacent tendons. All 24 anchors were stressed sequentially to ensure the closest approximation of symmetry at all times.
They had a crane on-site, which is handy to get material up to the top. But when the US Army sold one of their Sikorsky S-64 Skycrane helicopters to civilian operators, they used it to remove the crane and then fly up the antenna in 36 sections. The helicopter’s name was Olga.
Using the helicopter shortened the antenna install from 6 months to 3.5 weeks
On April 2, 1975, after 26 months of construction, the tower structure was complete.
There is apparently only one person who died during construction. Jack Ashton, a consultant for the concrete inspection company, was hit on the head by a falling piece of plywood and died on impact.
The average hourly wage was $8, which is equivalent to $42/hr today. There was also danger pay which equated to $1/hr for every 100m over 300m.
Two years into tower construction, plans for a Metro Centre were scrapped, which meant the tower is fairly isolated on the Railway Lands light industrial site, making it challenging for tourists to access. Now it’s near the Rogers Centre where the Blue Jays play and the Scotiabank Arena where the Maple Leafs lose all of their hockey games.
The 360 Restaurant completes a revolution every 72 minutes.
They have made changes and completed renovations on the tower over the years.
In 2008, they added glass panels to one of the elevators, reaching a world record for the highest glass floor panelled elevator in the world at 346m
In 2011 the EdgeWalk attraction opened, allowing people to go outside and walk around the roof of the main pod
There are 1,776 steps to the main deck and 2,579 steps to the SkyPod. They hold charity stair climb events twice a year and it takes the average climber 30 minutes to the main deck, but the record is 7 minutes and 52 seconds.
On April 16, 2018, falling ice punctured the roof of the Rogers Centre
So there you have it, the CN Tower. One of the American Society of Civil Engineer’s seven wonders of the modern world. All things considered and compared to many of the other dumpster fires we’ve covered on this show, the tower construction was a pretty cool endeavour. Not everything went to plan, but they got to work with some new tech and some old tech to make what is still the tallest free-standing structure in the western hemisphere.
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Bye everyone, talk soon!