FONT & AUDIO
The Morrison Bridge's new deck
The steel grating on the Morrison Bridge has always been a little disconcerting to drive over — at least for the past 50 years — but now it's a lot smoother thanks to Hamilton Construction and Knife River sand and gravel supplier.
It hasn't escaped commuters' notice that the Morrison Bridge was closed over the last two weeks, and the $10.4 million project also faced criticism when the fiberglass deck, a $5 million project, failed. Approximately 90 percent of the funds are federal, with 10 percent from the county.
The failing deck of the Morrison Bridge lift span was replaced with a solid new deck this construction season. The project began in early April of this year. It is slated for completion by the end of October, with the final overlay of epoxy slated to be applied on the last weekend of the month.
Ken Huntley, structural engineer for Multnomah County, spoke with the Business Tribune about the engineering and physics that goes into updating an existing bridge.
"For the Morrison Bridge we had put the fiberglass deck on there that began to fall apart almost immediately, so we had to come up with a replacement deck as an emergency project," Huntley said. "We spent a lot of time looking at various deck options and alternatives to determine the best replacement for filling the fiberglass deck — we were looking at aluminum deck systems, different fiberglass and going back to the original deck, which was a steel grating."
The county analyzed different sandwich plate systems among other options to
determine the best one.
"That's how we, after about a year of studying and developing the different alternatives, came up with the deck option we have now: going back to steel grating, but filling it with lightweight concrete so we have a solid riding surface. We didn't want to go back to the open grate steel deck that would reintroduce all the problems we had before — primarily, safety," Huntley said. "It became slick. Speed turns into a lot of problems leading to accidents."
In between closures that allow the concrete to cure, they've been working on the fiberglass deck and putting in steel grating deck panels.
When the fiberglass, or fiber reinforced polymer (FRP), deck failed, the county fielded a lot of criticism from the public, even though the Broadway Bridge still has the same type of FRP deck that is functioning well.
"This one on Morrison failed for various reasons: installation, manufacturing, design. There's only one company in the country that made it and we had to go with them at the time," Huntley said. "It wasn't real simple to choose the new deck for Morrison: nobody wanted to go back to the same company and do the same FRP that had failed."
The old FRP deck now pulled off the bridge was so deteriorated that for the last three years the speed limit was lowered from 35 to 25, and the weight limit from 40 ton vehicles down to 10 tons.
"That will change at the end of the month when we reopen the bridge to traffic: we no longer have that weak deck system," said Mike Pullen, public information officer with the county.
But commuters didn't always follow the lower, safe speed limits — or the no-merge lane lines — and the bridge's configuration is partly at fault, as it's an on- and off- ramp for the freeway.
"Even though we signed and striped it for no lane changes, people were needing to change lanes to get where they're wanting to go: changing lanes is still very common on the bridge," Huntley said. "One of the main reasons, when looking at deck alternatives, we needed to have a solid riding surface that would be safe."
The total project budget is $10.4 million. The project spent $2 million in design and $8 million in the construction phase, spread out over the contractor, consultant, ODOT for managing all the contracts, inspectors, project managers and county staff who run the project.
"If you look at what we did for the fiberglass deck, that project only ran us about $5 million — about half what this cost us," Huntley said. "The FRP, even though it was a whole lot less expensive, with its failure it cost twice as much to replace, but half the cost of this project was to go in and strengthen the bridge to handle all this extra weight."
The layer of concrete gives wheels better traction, and there will be another overlay of epoxy that looks like asphalt for durability.
Huntley said a big issue is the lightweight deck system, which weighs 20 pounds per square foot — very light, comparatively.
"Even though the concrete fill is a lightweight fill, it still more than doubles the weight of the (former) deck, so it's going from 20 pounds per square foot to over 45 pounds per square foot," Huntley said. "So a lot of the design process in putting this new deck in was to determine whether or not the bridge could handle that much more weight."
The Morrison Bridge is a bascule bridge, or drawbridge, that moves with the help of counterweights that continuously balance the leaf's upwards swing. Huntley compared it to a teeter-totter: you have to balance out the weight on the other side. And if it's uneven, like for example a kid playing with a much heavier adult, then you have to adjust the weight by moving the heavier part closer to the trunnion pin — the pivot point.
"That's the way a bascule leaf operates. If you're going to add all the extra weight out there on the leaf of the bridge, you have to have a huge counterweight underneath on the opposite side that has to counterbalance that weight," Huntley said. "Because the counterweight is so much closer to the centerpoint, we have to add three times more weight there — if you add 100,000 pounds out on the deck, you've got to put 300,000 pounds of additional counterweight to balance that off."
In analyzing the engineering, he found that when the bridge is lifting, all the weight of both the leaf and counterweight are resting on the trunnion pin and its collar pieces — totalling an additional 500,000 pounds.
"We have to make sure it can handle that load, and we found the collars that were on there weren't sufficient to handle that load, so we had to change the collars out," Huntley said. "Which means that we have to jack up the whole bridge, the counterweight, the bascule leaf, everything and pick it up off that pin and change out the collars. That was very expensive and technically precise work we had to do to make that happen."
All told, they had to jack up about 3 million pounds of weight.
"It took a total of 16 jacks under the counterweight to distribute the load out on the pit floor to jack that up," Huntley said. "Even with that, we were pushing the limits of what the pit floor could handle with all that weight."
Analytics also take into account live weight — or, traffic using the bridge.
Since everything on the bridge was designed for the old deck in 1950, there wasn't really room to increase the multi-million pound concrete counterweight.
"A counterweight is about 10 feet thick, about 20 feet tall, and then about 50 feet wide — the width of the bridge — it's a huge, huge concrete block," Huntley said. "Inside are two rooms, an upper room and a lower room. Inside these rooms we stacked 80-pound concrete blocks, designed so we can adjust the weight of the counterweight depending on how much weight we have on the leaf itself."
The rooms are each seven feet wide, seven feet tall and 25 feet long. Calculating it out, Huntley's team needed to add 400,000 pounds to each counterweight.
"The number of blocks it would've taken far exceeded the capacity of those rooms to handle all the concrete weight," Huntley said. "Instead of putting concrete in the rooms, we made these huge one-inch thick steel plates that hung off the back of the counterweight, and each one of these counterplate weights weighs about 1,500 pounds."
Ultimately they hung somewhere between 80 to 90 of these plates on the back of the counterweight.
"The further back we hung the weight, the less weight we had to add because it's further away from the centerpoint and made it easier to balance," Huntley said. "They had cranes pick them up, drop them down into a pit, then have a hoist down there that would pick them up, turn them and manually guys would put them in position and set them on the hangers."
One plate at a time, the team hung 30 plates a day.
"That's a long, hard day's work," Stuart said. "Just watching them do it got me tired."
Hamilton Construction was awarded a $6.5 million contract to install the new deck this year.
Evan Stuart, Hamilton's project manager on the Morrison Bridge, told the Business Tribune the concrete pours went fine, and were completed within the expected four hours. (Hamilton is also the company that installs the area transit messaging, the light-up billboards over the freeways that broadcast commute times.)
"We got most of the work wrapped up this week, we're starting to finish painting and that sort of stuff," Stuart said. "An older, mechanical, moveable bridge is just a different challenge, it's just nice to be able to do something different every once in awhile."
Hamilton often works on concrete bridges for highways.
"We had some mechanical ... problems with getting the bolts in that we needed to and getting the weight we were after," Stuart said. "The concrete, I mean it was difficult to get it mixed — they asked for high-performance, lightweight, dense concrete — it's counterintuitive, lightweight concrete is not usually dense, but they had asked for a dense mix."
Concrete supply subcontractor Knife River made 40 trial batches of concrete before getting it right. Usually, they're able to chose from about five pre-approved mixes — but this was innovative.
"They weren't even sure at the time they could develop this mix design, in talking with ODOT," Stuart said. "It's kind of a pilot project for them, looking for lighter-weight overlays they can use — this works out, they're getting the strength they want and it's two-thirds the weight of normal concrete."
The material uses shale from Utah that's baked and "when it bakes it almost pops like popcorn. It still maintains most of its strength even though it's expanded."
The innovation is driven by the cost of freight: importing aggregate multiplies the cost by four.
"Normal concrete, you're paying $120-150 a yard. This stuff is approaching $400 a yard," Stuart said. "It's considerably more expensive if you have to truck or ship it via train a thousand miles away."
"When you look at a typical bridge deck that doesn't lift, those most commonly are concrete poured decks, usually anywhere from 6-8 inches in thickness and have a lot of rebar in them to make them strong. It's what most bridge decks are made of," Huntley said. "But when you look at a moveable bridge that has to lift — and particularly lift and rotate the way these do — we can't do a typical 6-8 inch thick concrete deck."
That's why steel grating is commonly used on moveable bridge deck systems — same strength, but lightweight.
"The concrete portion of this deck is not the same structurally as a typical 6-8 inch concrete deck," Huntley said. "This concrete is only two inches thick, so the strength of this deck still comes from steel grating that was put down — the concrete is simply there to give us a solid riding surface to provide the safety feature we need on the bridge."
The concrete is HPC, high-performance concrete, which is specially made to be lightweight.
"It has lightweight aggregate in it," Huntley said. "Typically, concrete weighs 150 pounds per cubic foot. This comes in about 107. It doesn't seem like a huge reduction, but actually is quite significant when you take into account the weight over the entire bridge."
The original steel grating deck lasted for 50 years.
"The new steel grating deck is essentially the same deck system," Huntley said. "There's no reason this deck structurally can't last another 50 years like the last one, even though it's running at a heavier weight. Most moving components, we analyze them to make sure they have infinite lifecycle."
For example, if you bent a paperclip all the way back and forth, it would snap after five or six bends. But, if you only bend it a little bit, it would take several times longer to break.
"You can get that movement in bending something like that down so low, you can bend it forever and it will never break," Huntley said. "That's why we had to replace those collars — to get us back to infinite lifecycle on trunnion pins."
Without replacing the collars, the analysis showed the bridge's lifespan would have been reduced to below 50 years — which Multnomah County considers unacceptable.
"If the weather's decent, the last weekend in October we're going to close the bridge and apply this top overlay, epoxy layer that looks like asphalt, and present the end of the project with all six lanes back open that Monday, the 30th," Huntley said.
If it rains out this weekend, they'll do the last step in the Springtime.
By Jules Rogers
Reporter, The Business Tribune
Follow Jules on Twitter
Visit the Business Tribune on Facebook and Instagram
Subscribe to our E-News