Ep 46 Second Narrows Bridge
Engineering News – 2D Polymers (2:00)
This week's engineering failure is the Second Narrows Bridge (5:00). The cantilever bridge (7:50) collapsed during construction (12:30), with an unexpected cause (14:00). Decades later the bridge was renamed the Ironworkers Memorial Second Narrows Crossing (26:30).
Second Narrows Bridge
Hi and welcome to Failurology; a podcast about engineering failures. I’m your host, Nicole
And I’m Brian. And we’re both from Calgary, AB.
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This week in engineering news, a 2D polymer.
Chemical engineers at MIT created a new material that is stronger than steel, lighter than plastic and can be made in large quantities
Current polymers form 1 dimensional chains, but this new material can form 2 dimensional sheets which scientists previously thought was impossible
Polymers are chains of building blocks that grow by adding new molecules on their ends.
2 dimensional polymers grow in a sheet.
The product has a wide range of applications from cell phones to structures.
The material’s elastic modulus or the amount of force required to deform it, is 4 to 6 times greater than bulletproof glass. The yield strength, or how much force it takes to break the material is double what it is for steel.
It's also impermeable to gases which makes it an ideal material to provide a gas tight and watertight coating on various metals or structures. For example a coating on a car or steel structure.
If you want to read more on the 2D polymer, check out the links to sources on the web page for this episode at Failurology.ca
We’re going to take this opportunity to plug our April Fool’s special about the movie Airplane. The special will only be available on our Patreon, so head on over there and support our show. It’s going to be a really fun episode; I watched the movie last weekend, better than I expected.
Now on to this week’s engineering failure; the Second Narrows Bridge.
Second bridge constructed at the Second Narrows of Burrard Inlet in Vancouver BC - the original bridge was replaced in 1968 in the same location and now operates exclusively as a rail bridge
The Lions Gate Bridge at the First Narrows is 8km west of this bridge
Connects Vancouver to the north shore.
It was constructed next to the original bridge which has now been converted to a rail bridge
The original bridge was constructed in 1925 and was the first bridge to connect Vancouver to the North Shore over the narrows.
It was hit by a number of ships - the freighter Urana in 1927, Norwich City in 1928 and Losmer in 1930. In September 1930, the Pacific Gather got wedged under the bridge and ripped away the centre span.
In 1933 the provincial government bought the bridge, after being out of service for 4 years and installed a lift section in the middle span to allow ships to easily pass through.
The lift section remains up unless a train is crossing over the bridge
Bridge is a steel truss cantilever bridge designed by Swan Wooster Engineering Co Ltd
Construction started in November 1957 and the bridge opened August 25, 1960
It cost $15 million to build and tolls were charged until 1963 to help pay for it
The bridge is 1,292 metres long with a centre span of 335 metres and a vertical clearance of 44 m and is part of the Trans Canada Highway #1 which goes from coast to coast
120,000 people drive over the bridge every day. Although maybe that number is smaller due to covid.
Four northern approach spans leaving from a viaduct, a main cantilever section spanning 335m and two anchor spans of 140m
Spans were numbered 1 to 7 from north to south and the piers are numbered 1 to 17 from north to south
The bridge deck was about 25m wide with the intention of carrying 6 lanes of traffic and two sidewalks
The erection plan called for two temporary piers, called false bents (N4 and N5), to support the cantilever sections between pier 14 and pier 15.
The false bents were designed by an engineer but not the engineer of record, who failed to review the design before the false bents were constructed. Both of these engineers were lost in the collapse
Falsework is a temporary structure used in construction to support a permanent structure until its construction is sufficiently advanced to support itself.
Important to note. It’s easy to think that the section that collapsed was the long span, but it was the section north of the long span.
3:40 pm pacific time on June 17, 1958 - a crane was stretching from the north side to join two chords of an unfinished arch
South end of span 5 collapsed followed by the south end of span 4
Span 5 was first
Dropped ½ to 2 m based on eye witnesses, paused and then collapsed into the water
The north end of span 5 that remained on pier 14 was deflected to the south, which is what caused the south end of span 4 which was also supported by pier 14 to collapse
The beams at the base of false bent N4 collapsed to the north.
For the 20 or 30 minutes before the collapse of span 5, the locomotive, trucks and west bottom chord were in position and stationary on the deck.
Before we talk about what the cause was, let’s talk about what the cause wasn’t. The investigators, in an effort to ascertain the cause, began ruling out possible causes.
The design of the bridge itself was sound engineering practice and similar to structures designed around the US and UK.
There was no sabotage, explosion, extreme winds or earthquake that impacted the bridge. The tides were also normal and there was no ship collision.
The crane that was used to lift sections of the bridge into place was not defective
Span 4, the second span to collapse, was not structurally weak or insatiable such that it could have led to the collapse of span 5.
Pier 14, which supported spans 4 & 5 was constructed in accordance with design
Even span 5 was ok. There were no structural members that were over stressed during the erection process and the critical members were reinforced as required.
The collapse wasn’t due to careless or faulty workmanship on the erection crew
So what was it?!?
Remember those temporary piers that were installed to erect span 5; false bents N4 and N5. well it was N4.
Elastic instability of the webs of the stringer beams of N4 grillage, accentuated by the plywood packings between the beams and the omission of stiffeners and effective diaphragming in the grillage - this was caused by an error in the calculations
Grillage - stacking of beams in layers perpendicular the layer above and below them. So let's say the bottom layer runs left right, the next layer would run up down and then the third layer would run left right again. Grillage beams are used to disperse heavy point loads from the superstructure to an acceptable ground bearing pressure
And the diaphragms act as bookends to hold all of the beams in row in place
The use of plywood above and below the upper tier of grillage beams was a contributory cause of the failure because of the absence of stiffeners and effective diaphragming
The pressure on the plywood was 9,240 kpa under the west leg of bent N4. For reference, a car tire is around 240 kpa. The pressure on the plywood was almost 40 times as much.
Interestingly enough, the investigation determined what they believed to be the cause of the collapse, submitted this to the contractor and engineer, who both agreed without argument, and they presented a final conclusion at the hearing; rather than discussing and arguing findings at the hearing.
I have had clients decide part way through a repair that a temporary fix will become permanent for cost or schedule or what have you. Sometimes it’s even a matter of permanent repair parts being delayed in delivery. Where possible and within reason, I typically plan for temporary measures to be permanent so that if the plug gets pulled, I’m still covered.
79 workers fell 30m into the water, 18 were killed (14 ironworkers, 3 engineers and a painter), most died instantly. 4 other workers died throughout construction of the bridge.
A diver searching for victims also drowned during the recovery
The bridge was eventually completed and opened on August 25th 1960.
Renamed the “Ironworkers Memorial Second Narrows Crossing” in 1994 to honour those who died
Stompin’ Tom Connors sang about the bridge in The Bridge Came Tumbling Down in 1972.
So there you have it, plywood in between and no book ends of stacks of beams led to the collapse of two spans of the Second Narrows Bridge and the loss of 18 lives. Even though they were temporary, it was imperative that the false bents be correctly designed to support the structure until the permanent piers are constructed.
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