Mars Rover Perseverance (4:25) - https://phys.org/news/2021-02-mars-team-awestruck-photo-descending.html
Hi and welcome to Failurology; a podcast about engineering failures. I’m your host, Nicole, and I’m from Calgary, Alberta.
I hope you all enjoyed the Failurology Origin Story Q&A episode last week. It was a lot of fun to make. I haven’t made a video recording like that before, and even with just that one, I’ve already learned a lot. I even started making little videos of mechanical systems for the linked in page of the company I work for. On that note, I’ve added my linked in to the show notes if you want to connect with me. I recorded the video a week before I released it, during the Family Day long weekend. And then spent a good portion of the week second guessing posting it, because being vulnerable is scary; but in the end I had to just go for it. And also, I wanted to give myself a good week off to relax. I love making this podcast, and I love my job. But I am very aware that burnout is real. And I had been spending the majority of my free time working on the podcast. Which again, I love doing. But a little break was just what I needed and I’m back, rejuvenated and ready to bring you more engineering failures. Just a reminder to all of you out there, especially if you are working from home and work and home start to blend together; it’s not only ok, but also required to take a break.
Also, I was notified last week that Failurology is the #4 physics podcasts in Canada over the last 30 days. This is amazing news! And I want to thank each and every one of you for downloading and listening to Failurology each week. I am beyond grateful for your support.
This week’s episode is the Sunshine Skyway Bridge in Tampa Bay Florida. I love bridges, almost as much as I love tunnels. There is this one bridge in Chatham Ontario that has an open grate deck and it made a noise when we drove over it. As a really small child, I affectionately named it the noisy bridge. Every time we went to Chatham I asked if we were crossing the noisy bridge. And admittedly, I would get really upset if we weren’t going to cross it. I also called convertibles cars with no lids. It was such a simple time then.
When I started this podcast, I set some ground rules for what failures would qualify for episodes. The rules were pretty simple, the accident needed to be a failure that resulted due to shortfalls in the engineering design. This week’s failure doesn’t quite follow that rule. But since I made the rule, I can break it, right?!? The Sunshine Skyway Bridge collapse wasn’t due to design flaw, it was hit by a ship during a squall. But the collapse led to design changes to future bridges to prevent this from happening again. So while previous episodes have been an engineering flaw that resulted in failures, this episode is a failure that resulted in engineering changes. But I still think this story is important, it demonstrates how failures impact and shape future designs. While the failure was not necessarily caused by design shortcomings, the bridge collapse directly resulted in a change to the new bridge design to prevent something like this from happening again.
Like the Sampoong Department Store Collapse, when I heard about the Sunshine Skyway Bridge, I immediately looked it up and read the entire Wikipedia article. I even bought a book on the bridge, called Skyway: The True Story of Tampa Bay’s Signature Bridge and the Man Who Brought it Down by Bill DeYoung. And then when I was watching the superbowl recently, Go Mahomes, which took place in Tampa, I was reminded of the bridge, and immediately looked it up again, as if history had changed since last time I read about it. It’s funny, I’ve seen many Batman movies, some of them multiple times. And every time I see that scene with Thomas, Martha and Bruce Wayne in the alley, I think, even if for a brief moment, maybe Thomas and Martha won’t get shot and die this time?
Alas, time travel is not yet possible and we can’t rewrite history. But we can definitely learn from it. So without further adieu, let’s get to the news.
This week in engineering news the NASA Perseverance rover touchdown on Mars on February 18th after launching July 30th of 2020.
The rover touched down near an ancient river delta, looking for signs of ancient life and collecting rock samples to bring back to Earth.
The 2012 Curiosity rover, which was roaming about 3,750km away from where Perseverance landed, caught stop motion video of the last two minutes of Perseverance’s descent.
The Perseverance rover has 25 cameras and 2 microphones, most of which were on and transmitting images, video and sound to NASA during the seven minute lunge through Martian atmosphere. And Perseverance is the fifth NASA rover to be placed on Mars.
Since 1962, humans from four space agencies have been trying to land robots on Mars to study it. There have only been nine successful landings, including this most recent one. What makes this landing so special is that Perseverance, for the first time ever, is a round trip journey. The rover has 43 sample tubes designed to be filled with rocks, dirt and air for eventual examination back on Earth. While we have been able to see these samples for the last few decades, this will be the first time they will be brought back to Earth.
If you want to read more about the Mars rover Perseverance, check out the link in the show notes, or head to Failurology.ca
Now on to this week’s engineering failure; the Sunshine Skyway Bridge Collapse.
Replaced ferry from Point Pinellas at the SE tip of St Petersburg to Piney Point just north of Port Manatee
8850m long – for scale Brooklyn Bridge in NYC 1055m and Golden Gate Bridge in San Francisco is 2740m
Highway US 19 was extended from St Petersburg to its current end north of Palmetto
The original bridge was aCantilever bridge – just like the Quebec Bridge – which spanned the shipping channel in and out of the Port of Tampa - The majority of the bridge was a few metres above the water surface. But where it crosses the shipping channel it rises up to 46 above the water. So it looks like the bridge has a hump in the middle of it
Opened sept 6th 1954 as a two lane bridge built by Virginia Bridge Company
Similar span was built in 1969 parallel and to the west of the original span – delayed opening due to reinforcing work on the south main pier, cracked due to insufficient supporting pile depth
Original span was used for northbound traffic and the 1974 span for southbound traffic
Connect St Petersburg Florida to Terra Ceia across the Lower Tampa Bay
Two major maritime disasters
Jan 1980 US coast guard cutter Blackthorn collided with the tanker Capricorn, near the bridge. The cutter sank and 23 crew died
The star of the episode - May 9th 1980, the freighter MV Summit Venture collided with a bridge support during a sudden squall and collapsed a portion of the southbound span, killing 35 people when their vehicles plunged into the Tampa Bay.
Harbor pilot – maneuvers the ships through dangerous or congested waters - #1 priority, don’t hit the bridge
Egmont Key pilot station outside the bay to meet incoming ships, or depart outgoing ones – worked on a rotation
In 1975 a law was passed that changed the hiring process for new pilots – was done by pilots themselves, now done by Florida’s Professional Board of Regulation – Lerro was first under new – forced on them, not their choice
When John Lerro, the 37 year old harbor pilot steering the ship through the shipping channel, realized they had veered out of the channel, he put the engines into full reverse and dropped the anchor, but it still hit to support piers of the bridge.
MV Summit Venture – 185m wide, 25m wide – made in a Japanese shipyard, flying a Liberian flag
May 9, 1980 – 7:33am – freighter MV Summit Venture collided with a pier / support column
The main pier, at the passage under the bridge through the channel withstood the ship’s contact without much damage, but the secondary pier, which was not designed to withstand the impact, failed catastrophically –
370m of the southbound (west / newer) span into Tampa Bay
The storm came up suddenly and caused torrential rains,110kph winds, and dense fog - reduced visibility to near zero and rendering the freights radar useless
6 cars, a truck and a greyhound bus fell 46m into the water – 35 people died
sidebar story time – I mentioned in the Flint episode that I grew up outside of Windsor Ontario. Being at the southern tip of the Ontario peninsula, surrounded by the great lakes, we got some interesting weather coming off the lakes. I still see weird weather in Calgary with the Chinooks, but that’s another story for another time. Anyways, we saw a lot of fog in Windsor. Like a lot. So much so that we used to have fog days where the school buses didn’t run in the morning, but would run in the afternoon after the fog lifted. I know what it’s like to drive down the road, windows down so you can hear, trying to see what’s in front of you, but so much fog you can’t see more than 10m. There was one fog day that was especially bad and has always stuck with me. It was Sept 3, 1999. The Windsor Airport Observation Station failed to detect foggy conditions. Around 8am, near the Manning Road overpass, one semi entered very dense fog and slowed down, causing the semi behind him to jack knife. Visibility was 1m. A chain reaction of crashes resulted in an 87 vehicle pile up, many burnt and destroyed. A lot of people were injured in their vehicles as cars kept crashing into the pile; some were even trapped. And some were injured trying to escape. 8 people died that day and 45 people were injured. Improvements have since been made to the 401 to try to prevent accidents like this one from happening again. But I`m telling you this story because imagine you are driving around, and can only see 1m in front of you. You`re probably questioning why you left the house that morning, or maybe you have an important deadline and are just trying to make it to work safely. You`re driving along and then all of a sudden, there is no road, or in the case of the Sunshine Skyway, no bridge. And you fall 46m in the water below. 46m is a long long way down. And breaking the surface tension of water is not forgiving. I believe the majority of those that drove off the Skyway bridge died on impact, and those that didn’t drowned. I can't even imagine how terrifying their last few seconds were.
the collapse section was on the descent of the hump over the channel
drivers had no idea that a section of the bridge was missing
there is a toll booth at the start of the bridge – they were contacted, but by the time they stopped traffic, there were already several people driving across
Dick Hornbuckle and three passengers were driving across – noticed the bridge was out – slammed the brakes, stopped 14in from the abyss – first ones to stop before driving off – helped prevent others from driving off
Wesley MacIntire’s truck flew off the bridge, into the side of the freighter before rolling into the water – he survived
Many other drivers were able to stop their vehicles before reaching the gap.
The Florida Department of Transportation realized it needed to improve the crossing to prevent this from happening again. Even with harbour pilots and protected main piers, there was still a risk
Southbound span was used as a fishing pier and the northbound span was converted to two way traffic until a new bridge was built – before the original spans were demolished, MacIntire was the last one to drive over it.
Demolished in 1993 and the approaches were made into the Skyway Fishing Pier State Park – the south approach, sitting 800m south and west of the current bridge is still there today, the north approach was demolished in 2008
Disassembly engineering challenges
All underwater piers and piles, surface roadway, girders and beams had to be dismantled
Order of disassembly
Safe method for detonating charges on concrete and steel members in a publicly open and difficult to control area
Safe removal in one piece of the bridges main span and concrete piers so as not to block the shipping channel
4x1:16 ratio pulley system – each of the four corners of the span were connected to two 25 ton winches, bolted to the pavement of the deck – 110m, 608 ton span lowered onto a barge 46m below
Real time, computerized, synchronized descent calculator and control program to ensure all winches lowered at the same rate of 9.1m per minute – completed in 2.5hrs despite adverse weather conditions
While today this might seem like an easy system to develop, for 1993 this was very impressive
It was determined that rebuilding the original spans would not improve shipping conditions, and that a tunnel was impractical due to the high water table, so a new bridge was built. – opened mid-1988
Cable stayed – there are two towers, each centred on the main piers across the channel, that cables span from to the bridge deck – only the channel crossing is cable stayed, the rest of the bridge is supported by closely spaced piers
Main span is 50% wider than the old bridge
Piers of the main span and the approaches for 500m in either direction are surrounded by concrete barriers – called dolphins – to protect the piers from collisions by tans, container ships and cruise ships.
36 dolphins total – 4 large ones protecting the two main piers supporting the cable stayed main span and 32 smaller ones protecting the bridge piers for 500m to either side of the main span.
Cost $41 million in 1987 - $94 million today
Piles driven into the seabed and connected to platform above the water
Can withstand an impact of an 87,000 ton ship
300 precast concrete segments were linked together with high strength steel cables to form the roadway
Exploit the elasticity of prestressed concrete to absorb and transfer impact loads – protected along entire length, from shore to shore
300m to the east to reduce the skew to the channel and provide more space between bridge and dogleg or offset in the channel. As if this isn’t complicated enough, the ships have to make a turn before they cross under the bridge; whether they are coming or going. Changing the bridge to be more perpendicular to the channel, results in better maneuverability to ship captains
The new bridge crossing is almost 7km long, the span across the shipping channel is 106m, and the clearance over the channel was increased from 46m with the original bridges to 54m. The new bridge is designed to handle a hurricane within 110km every other year.
Two separate parallel roadways merge into a single roadway on the main span.
Low level approaches across water, high level approaches up to main span, main span across channel
Low level approaches
Two parallel, two lane structures
Superstructure - Four-span continuous reinforced concrete deck slab supported on prestressed concrete girders
Substructure – reinforced concrete wall type piers on 51cm square prestressed concrete piles
Span is approx. 30m
Piers of parallel roadways are connected across between the two structures by precast prestressed concrete frangible struts – this link reinforced the spans together, they work together like interconnected springs – it’s weird to think about concrete and steel asking like a spring - to you and I, concrete and steel is a solid, non-compressible object. But every object has a breaking point and there is some ability to absorb force, depending on the composition of each material.
High level approaches
Two parallel, two lane structures
Single cell, precast post-tensioned, continuous concrete box girders, supported on concrete piers
Foundations – 61cm square, precast prestressed concrete piles
Superstructure - Single cell, precast post-tensioned, concrete box
Substructure – post-tensioned concrete piers, 61cm square prestressed concrete piles
Main piers support 131m cable stayed pilons (towers)
Main and flanking spans are cable stayed, single plane of stays at the centre of the two roadways
ship collision risk assessment study from COWIconsult of Denmark
main reasons for collision – human error, mechanical failure, environmental conditions, alignment of the bridge with respect to the entrance channel
practical protective measures suggested
design piers and pier shafts along entire length for ship impact
raise the roadway along the entire crossing
protect main and flanking piers with sand filled rock islands and dolphins
shift alignment further east to increase the distance from the 18 degree dogleg in the navigation channel – increase navigational horizontal and vertical clearance
consider aberrant barges as most likely to hit the bridge
impact criteria – impact loads were provided for each section
stiffer soil, smaller deflection of top of pile, less movement to absorb forces – piers very massive or slender and flexible
north – sand and shell layer, clay, intermittent silt and limestone pockets, predominates
south – sand and shell layer, clay, sandstone, dolomite lenses present in the clay
modelled the foundation system – pile depths varied based on soil conditions and depths
6 to 10 piles at a pier – piles at several angles and slopes – four a exterior with additional interior
advantages to prestressed concrete – high load resistance of the piles in compression and tension, ductility of the piles to transmit excess loads to adjoining elements, transfer connection, strut designed to fail before transmitting too much load (ie if the load is too great that one bridge span fails, it won’t take the other one with it)
Problems with the new Bridge
The new bridge hasn’t been without its own problems, although nothing as catastrophic as a collapse. Although, while they may seem like problems, they appear to me as general maintenance items that come along with a superstructure in a humid, salty climate
Corrosion – corrosion of steel in the precast concrete segmental columns of the high level approaches
Paint – paint touch ups in 1998, using a newer environmentally safe paint – faded faster than expected and began to show paint discoloration in 05/06
Fill repaint was done in 06-08 – included repainting the bridges 42 steel cables a consistent shade of yellow and rehabilitating the lighting system as the summit of the bridge – yellow is deliberate to reflect the location, the Sunshine State
2014 – Florida Department of Transportation noted low bridge clearance prevents larger vessels from using the Port of Tampa Bay terminals – most new cruise ships have an air draft (height from water surface to top of the ship) more than 55m.
Officially named the Bob Graham Sunshine Skyway Bridge after the former governor of Florida and then US senator who presided over the design and most of the construction.
So there you have it, the Sunshine Skyway Bridge in Tampa Florida, USA. While the original bridge design wasn’t directly responsible for the collapse, the failure led to significant design changes to the replacement bridge to prevent such a failure from happening again. While this is somewhat different from previous episodes, it's a really important story that demonstrates how failures impact future designs and how we can learn from each other and grow as an engineering community.
Henry Petroski said “failure is central to engineering. Every single calculation an engineer makes is a failure calculation. Successful engineering is all about understanding how things break and fail.”
The real trick is finding the failure point on paper, or computer, during design, before anything is built. This is why engineers love excel so much, you can play around with variables in the formula and compare multiple outcomes quickly.
For photos and sources from this week’s episode, head to failurology.ca; there’s a link in the show notes. On there you will find an image of the old and new bridges, the collapsed bridge, a map of the bridge in relation to the channel, and the concrete dolphins.
If you’re enjoying what you’re hearing, please rate, review and subscribe to failurology, so more people can find it. And tell all your friends! If you want to chat with me, my twitter handle is @failurology, you can email me at firstname.lastname@example.org, or you can connect with me on Linked In. Check out the show notes for links to all of these.
Thanks everyone for listening. And tune in next week to hear about the Hindenburg Explosion. Before there were airplanes, there were Zeppelins. Did you know they travelled around the world?!? I don’t know how I missed that part of the story until now. But more on that next week. Bye everyone, talk soon!