Ep 28 Floating Bridges of Washington State

Engineering News – Champlain Tower Collapse in Miami Florida (0:24)

This week's episode is about the Floating Bridges of Washington State that sank (12:00). Two floating bridges, the Hood Canal Bridge (26:30) and the Lacey V Murrow Bridge (28:30) sank during storms that occurred while each bridge was under major reconstruction. Luckily, Washington learned from its mistakes and rebuilt the Evergreen Point Floating Bridge before it was too late (38:35).


Engineering News - Champlain Tower Collapse

Floating Bridges of Washington

Episode Summary

Hi and welcome to Failurology; a podcast about engineering failures. I’m your host, Nicole, and I’m from Calgary, Alberta. Joining me again this week is Brian. Welcome Brian! Thanks for having me again, I had so much fun being part of the Comet episode!

This week’s engineering news is about Champlain Towers

  • June 24 at 1:25am eastern a large section of the south tower collapsed

  • As of this recording, death toll is up to 97, 11 injured and 8 are still missing

  • Construction was completed in 1981 – 48m tall or 12 floors and 136 units – the un-collapsed portion was demolished on July 4th

  • Surfside Florida just north of Miami beach

  • There are 3 towers, the north and south were both built in 1981, and then there was another in the middle, all 12 stories

  • A large north central section of the building collapsed first, isolating and destabilizing the NE corner which then collapsed 9 seconds later – 12 seconds total

  • A resident filed a lawsuit the day of the collapse against the condo board for acts and omissions and failure to protect the lives and property of residents

Does the the City of Surfside require inspections or audits on buildings over a certain age for high rise towers or other structures?

  • June 26 – mayor ordered an audit of all high rise buildings over 40 years old as well as all buildings by that developer – this was to take place over 30 days

  • Crestview Towers in North Miami Beach were closed and evacuated due to structural and electrical concerns on July 2nd. The Miami-Dade County Courthouse was closed on July 9th due to safety concerns. - there have been others as well

  • KCE Structural Engineers was engaged on June 27th to study and assess the Champlain collapse – they were involved in assessing the Pentagon after 9/11 and the Florida International University pedestrian bridge collapse in 2018

  • Possible causes

    • As we talked about last episode, a 2018 inspection report called out sloping errors in the waterproofing layer of the pool deck; the layer was not sloped and water would sit on it until it evaporated, decreasing the lifespan of the membrane.

    • The report stated “failure to replace waterproofing in the near future will cause the extent of the concrete deterioration to expand exponentially”

    • In Oct 2020, the repairs couldn’t be completed because the deterioration was so bad that repairs risked destabilizing the area.

    • On April 9, 2021 residents received a letter outlining a 15 million dollar remedial works program – the roof repairs were underway when it collapsed, but not the concrete work

      • Assuming no money in the reserve fund, which I hope was not the case, this would be $110,000 per unit. Which, I don’t know about you, but I don’t have 110k just lying around.

    • A pool contractor was on site 36 hours before and has shared photos of the pool equipment room, which is underground and there is significant infiltration of salt water resulting in more spalling and corroded, exposed rebar

    • 7 minutes before the collapse, a bystander recorded video of water pouring into the parkade from above near the entrance door, and concrete rubble lying on the ground

    • NY Times reported that investigators found less rebar in the footing and columns than specified on the drawings. It’s possible the requirements changed during construction, with oversight from the engineer. And it’s possible the contractor cut corners and no one caught it. We don’t know.

    • Data from Florida International University indicates the building had been singing at a rate of 2mm per year, while the 97% of Miami Beach had been stable. This is only a likely cause if one part of the building was sinking faster than another causing tensions which would weaken the structure over time

    • Based on the consensus of 6 engineering reports, the Miami Herald reported that based on publicly available evidence, experts believe that a column or slab under the pool deck gave way first, causing the pool to collapse into the garage, de-stabilizing the tower which then collapsed

  • information for this segment came from Wikipedia. they have 176 resources linked to this one topic alone which in itself is just really impressive.

Now on to this week’s engineering failure; the floating bridges of Washington state, which sank. Not one bridge sank, but two. One bridge sinking isn’t something that should happen, two is definitely something that shouldn’t happen!

  • What is a Floating Bridge? - Floating bridges are made of large water-tight pontoons connected rigidly end-to-end, upon which the roadway is built. Despite their heavy concrete composition, the weight of the water displaced by the pontoons is equal to the weight of the structure (including all traffic), which allows the bridge to float.

  • In British Columbia, Canada, the William R Bennett Bridge - connecting downtown Kelowna to West Kelowna is the 9th longest bridge in the world - 690m

  • Washington state is the floating bridge capital of the world - 4 of the 5 longest floating bridges

  • The 4 longest pontoon bridges are located in North America, so you may have driven over one!

History of Pontoon Bridges

  • Pontoon bridges were likely the first attempts of humans to cross waterways.

  • First detailed description of a pontoon bridge is from the 5th century BC. A bridge over top of boats was used to cross the Hellespont by Persian king Xerxes

Construction Techniques

  • factors to consider when building a floating bridge

    • floating elements (pontoons) - most important part of floating bridges

      • can be made of wood, steel, concrete, rubber, plastic

      • they are often made elsewhere and then towed into place - very slow speed so the hydrodynamic shape of the pontoon is less important

      • there are usually multiple pontoons grouped together to support a section of the bridge deck

    • bridge deck

      • beams in between the pontoons to support the deck

      • place a metal mesh or metal sheet to build the deck on - olden times they used wood

      • the bridge moves, so there needs to be some flexibility between the sections.

    • anchors (How are they anchored?)

      • keeps the bridge in position

      • anchor to a fixed point with tension cables

      • fluke anchor - similar to a traditional boat anchor - flat plate and what looks like a wireframe pyramid that sits in the lakebed with a cable up to the bridge - deep water, soft soils, flat areas - up to 100 tons

      • gravity anchor - weights that the bridge is tethered to - solid soils, sloped topography, near shore - up to 500-600 tons

      • drilled shaft anchors - cylindrical anchors that are drilled into the lakebed - solid soils near shore where gravity anchors might be hard to navigate around

      • Washington bridges and most of the floating bridges around the world use a combination of anchor types depending on the lakebed soil composition, depth, water traffic, etc.

    • passages for ships (if applicable) (How do ships pass by?)

      • opening the bridge for ships to pass

      • build the passage into a fixed portion of the bridge - kelowna - both of the bridges that we’re going to talk about that sank had retractable sections

    • current and wind forces

      • weight of the bridge deck - static load

      • a number of dynamic loads

      • traffic loads

      • Archimedes’ thrust - “if a solid body floats or is submerged in a liquid - the liquid exerts an upward thrust force - a buoyant force - on the body equal to the gravitational force on the liquid displaced by the body” - thrust forces

      • the drag force from the current and waves

      • the reactions of the anchors

      • loads are variable - several pontoons and anchor points all playing on each other

    • approaches

      • these are the ramps used to get on and off the bridge

      • usually one fixed point and one rolling point that allows the bridge to move - if both were fixed and the bridge moved, something would fail

      • the amount the bridge moves is typically based on the difference between low and high tide

      • must move on its own without mechanical or human intervention

Hood Canal Bridge

  • Route 104 across Hood Canal of Puget Sound - connects Olympia and Kitsap peninsula

  • 2.398km long - floating portion 1.988km

  • third longest floating bridge in the world and the longest saltwater floating bridge in the world

  • Opened in 1961 - second concrete floating bridge constructed in Washington (first is Lacey V Murrow which we will talk about shortly) - cost $26.6 million

  • Pontoon bridge with retractable draw span - so boats can pass through

  • Depth below the bridge is 25-105m with tidal swings of 5m

  • Two pontoons sank when they were building the bridge - they built the pontoons in a nearby river and intended to tow it to the final location - sea conditions were too severe and they were returned for plan B - a new contractor was hired and the design was revised

  • Feb 13 1979 - overnight the bridge had sustained winds of 137kph and gusts of 190kph - sank 730am on Feb 13

  • Draw span was open to relieve lateral pressure - western half broke loose and sank - blown open hatches allowed flooding of the pontoons

  • Re-opened on Oct 24, 1982 for a total costs of $143 million ($100 million in federal emergency bridge replacement funds) - was toll bridge until 1985

Lacey V Murrow Bridge

  • Interstate 90 across Lake Washington (freshwater) to connect Seattle to Mercer Island - carried east bound traffic only, westbound traffic uses the Homer M Hadley Memorial Bridge

  • Second longest floating bridge in the world - 2,020m

  • Original opened in 1940 - named Lake Washington Floating Bridge - renamed in 1967 to current name - cost of $9 million

  • Original bridge closed in 1989 for major repair work and re-opened in 1993

  • design information

    • 62m movable span that could retract into a pocket in the centre of the fixed span to allow large boats to pass

    • design caused a bulge that required vehicles to swerve across polished steel joints

    • reversible lane system with arrows and X signs - alleviate rush hour traffic - many serious collisions

    • a parallel bridge was completed in 1989, allowing the Lacey V Murrow to be closed for renovation work

      • called Homer M Hadley Memorial Bridge - named after the engineer of the Lacey V Murrow - 1,772m span - 5th longest bridge in the world

  • needed resurfacing and widening by adding cantilevered sections

  • Nov 25, 1990 - Sank while being refurbished and repaired

  • used hydrodemolition (high pressure water) to remove unwanted material (sidewalks)

    • water was considered contaminated - could not flow into Lake Washington

    • engineers analyzed the pontoons, they were over designed and could be used as temporary storage for the contaminated water

    • holes were cut into the sides of the pontoons - watertight doors hadn’t been installed yet - leaked in the holes and through door frames where temporary doors were installed

    • because of contaminated water, the pontoons were lower in the water than normal - increased risk of taking on more water - rain and wave water

    • large storm on Nov 22-24 (American Thanksgiving) filled some of the pontoons with rain and lake water

    • on Nov 24 - works noticed the bridge was going to sink and started pumping water out of the pontoons

    • on sun nov 25 - 850m section sank - dumped contaminated water and tons of bridge material into the lake

    • one pontoon section had filled and sank, since they sections were cabled together and pulled the rest down

    • a dozen cables of the parallel bridge were damaged and it had to be temporarily closed

Recommendations - specifically from the Murrow Bridge but applicable to both

  • marine contractors should be involved in bridge reconstruction or major maintenance projects

  • third party should review ongoing operations and maintenance and emergency preparedness

  • continued flotation during reconstruction is imperative - they adopted a standard highway approach that didn’t consider the marine requirements and vulnerability to sinking

  • electronic monitoring of water levels in the pontoons

  • severe weather plans are necessary

  • staff continuity to implement the plans

  • automated bridge barricades to prevent the public from being on the bridge during risky or severe conditions

Evergreen Point Floating Bridge - Governor Albert D Rosellini Bridge

  • 10 min drive from Lacey V Murrow Bridge and 90 minutes from the Hood Canal Bridge

  • longest floating bridge in the world (2,310m) until it was replaced in 2016 - replacement was 40m longer

  • opened aug 28, 1963

  • over Lake Washington - SR-520 (state route)

  • didn't sink, but after the Hood Canal and Lacey V Murrow, the state commissioned a study

    • would only last until 2017

    • had to be closed during high winds

    • even with a seismic retrofit in 1999 it was at risk of collapse during an earthquake

    • bridge sat 30cm lower over the water than when it was originally built

    • vibrations from storm surges and strong winds could compromise the drawspan, anchor cables and pontoons

  • construction method for building the SR-520 Evergreen Point Floating Bridge that was replaced in 2016

    • build the pontoons, anchors and roadway sections elsewhere

    • drive temporary piles to hold the first pontoon section in place

    • install anchors

    • tow the pontoons to the bridge location

    • construct bridge piers

    • install cross pontoon at the temporary piles - perpendicular to the bridge orientation - end of the floating sections - one on each end of the bridge

    • assemble pontoons from the cross pontoon out

    • construct superstructure on top of the pontoons (bridge deck)

    • repeat in sections until all of the bridge deck is in place and then shift into final position

So there you have it, two floating bridges in Washington State that didn’t remain floating. Risk mitigation is an important part of every engineering design and project and it may not have been adequate for Hood Canal Bridge and the Lacey V Murrow Bridge. From the recommendations of the Governor’s Blue Ribbon Panel that was convened after the first two bridges sank, a third bridge was likely spared from sinking and appropriate mitigations (as well as a new bridge) were put into place.

For photos, sources and summary from this week’s episode head to Failurology.ca. If you’re enjoying what you’re hearing, please rate, review and subscribe to failurology, so more people can find it. If you want to chat with me, my twitter handle is @failurology, you can email me at thefailurologypodcast@gmail.com, 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 to the next episode of Failurology where Brian and I will talk about the Kursk, the Russian Submarine that sank. Yes, submarines can do that. But more on that next time. Bye everyone, talk soon!