BY IAN HARVEY - Structural steel in bridge design has one fatal flaw - bridges are usually over water, often over salt water, making failure through corrosion almost inevitable.
Even more frustrating, steel bridges usually corrode and weaken at the one place where inspection is costly and difficult, the gusset.
>New York-based, HNTB Corp. national chief bridge engineer Ted Zoli had a better idea.
Instead of doing what every engineer does and just focus on materials, he thought, why not focus on the design and try to eliminate some of the inevitable costs associated with bridge maintenance?
The result is the Memorial Bridge in Portsmouth, N.H., a refurbished three-span bridge, which was originally built in 1922 and is now the world’s first gusset-less bridge.
It sounds simple enough, but it took nearly 100 years, an innovative design approach and the right advances in fabrication technology to make it all possible, said Zoli, a nationally renowned engineer and an expert in making structures more resilient to fires, blasts and accidents.
“It’s been my experience we’ve had a tendency in the past to focus on optimizing materials when fundamentally fabrication techniques and in-field direction is what costs us money,” he said.
“With my work, I explore what would happen if we put material efficiency in the back seat and what we could do differently to come up with a better structural system.”
Case in point, he said, is eliminating gussets from trusses.
“I was really enthusiastic about trying to eliminate this real problem area we have in truss design and the problem is that they are difficult if not impossible to inspect,” he said.
“Also, all the bolts tend to be single shear, so I can’t make efficient use of bolts and then from a design perspective, I’m connecting members in a somewhat arbitrary way.”
Throwing that traditional premise changed everything.
“I can get bolts in double shear if I connect in “I” shaped sections and half the number of bolts, whether shop installed or field installed and that’s a big deal when you look at construction costs,” he said, noting the Memorial Bridge project allowed him to rehabilitate the splices in sections.
It’s a huge jump from traditional gusset plate rehabilitation which would require either over-spanning or underpinning and makes the work go faster and is much more cost effective, he said.
It also makes future inspections more simple
In the case of the Memorial Bridge, it was a rush job.
In 1923 it was one of the first centre-span, vertical lift, truss bridges in America.
The bridge was closed on an emergency basis and the replacement had to be in place as quickly as possible and it was budgeted at US$89 million.
Esthetically, it’s not much different from the original.
“Work started Jan. 15, 2013 and it opened Aug. 15.
“Everything went together like a dream even though we only used one barge,” he said.
His design moved the machinery and operator booth and made some other changes, but the missing gussets remain unseen.
His design uses three 65-foot sections with reinforcing plates fabricated out of cold bend steel.
The finished job was then electro-coated in zinc to give it up to a 60-year corrosion resistance cycle.
“(By doing) things in I-shape sections and double shear bolts, it allowed me to potentially rehabilitate the splice in pieces,” he said.
“This becomes a joint that is much more reinforcement or repair friendly. Then, finally, the biggest thing is once you orientate the chords and diagonals in I-sections in strong axis bending, if I lose one of the diagonals on the truss, I essentially put bending in on the parallelogram. I’m left with in diagonals and chords. The beauty of this system is that the diagonals and the chords we are left with are well capable of taking that bending.”
The big issue in truss field construction is that the diagonals have to be force-fitted, he said.
“By moving the diagonal connections away from the nodal points, it gives you enhanced flex and ease of fabrication,” he said.
“These trusses went together like a dream. Hats off to the fabricator, moving the connection away from the node points and using double shear connections gives you a lot more ability to make a truss connection which made it all so successful.”
The result is a more robust and redundant truss than the traditional design.
“It also gave me the ability to make a truss a much more safer structural system and, because we were competitively bidding on this, it would prove to be more cost effective and certainly quicker,” he said.
It also allows for a more efficient use of materials in fabrication and less waste.
“You’re also using thicker plate with more resistance to corrosion,” he said.
Another factor in Zoli’s interest was that so little had changed in bridge design.
“I wanted to try do something different in a way of pure exploration... I most certainly will do it again,” Zoli said.
He added he’s already getting interest from other engineers on his approach.
“This is an adaptation to a full welded truss, which is more popular in Europe, but there’s a tendency not to want to field weld in cold climates,” he said.
“For me, the idea of being able to cold bend plate accurately and make web cutouts in such a highly precise manner, allowed me to recognize we have some enhanced fabrication techniques and we’re able to bend things more effectively as we’ve enhanced the fracture toughness of modern steel.”>>