It’s Saturday, and my wife and my son and I are standing on a deck overlooking the Ohio River. My son is fascinated by the barges working up and down the river, and he’s running back and forth pressing his face up against the rail and staring. Then he looks up at the Daniel Carter Beard Bridge, known locally as the “Big Mac Bridge”, and just stares. He’s seen the bridge before, but never this close from below.
“Daddy? How did they build that?”
I’ve got nothing, really. So, I turn to Google and start researching. Here’s my disclaimer: I was originally, before I started researching, planning on making this all about the different types of bridges and how they get built. And then I started finding out about all the different types of bridges, and realized that I’d have to write a whole book to do it successfully. And that seems a touch… extreme. So we’ll just focus on this specific type of bridge.
According to Wikipedia, the bridge is a “twin span steel tied arch bridge”, which is a type of bridge with one or more arches braced at either end into a foundation, with the bridge itself passing beneath the arch. Cables or other supports then connect the bridge to the arch along the length of the arch – something you can see by looking at the pictures of the bridge. SteelConstruction.info () states that:
Thrust arches rely on horizontal restraint from the foundations…. The vertical and horizontal reactions resolve into a force along the arch members – the horizontal component is of significant magnitude.
The tied-arch offers a solution when it can be arranged that the deck is at such a level that it can carry the horizontal force as a tie member….
By taking the arch thrust through the tie member, the primary requirement for the substructure reduces to only carrying vertical loads. It can be seen that one end will still require a longitudinal restraint to carry wind, braking, acceleration and skidding forces, and that the other end is permitted to move longitudinally.
Now, I’m clearly not a mechanical engineer. But, reading the rest of the article, I believe this means that the arch does all the work of supporting the weight. Since arches get stronger as they compress, the load on the bridge (the “deck” described in the quote above” actually helps strengthen the bridge as a whole. As long as the deck itself can support the stresses, obviously.
So, how do you build one of these things? Luckily, the American Institute of Steel Construction has the text of Design of Steel Tied Arch Bridges: An Alternative online, which describes the process generally. It states:
In the proposed scheme, the arch ribs are erected first. They may be erected using a high-line or each half of the arch rib may be rotated into place from it bearing. During the erection, thrust must be taken by the abutments or by a temporary cable between the ends of the ribs.
After the arch ribs are erected, permanent cables are placed between the ems of arch ribs to carry dead load thrust. The cables must be supported by the hangers to prevent sagging and reduction of the effective modulus.
The deck and tie beam are precast concrete units. Each unit of the deck exerts full width of the bridge. ‘The deck is cast integrally with the tie beams. Each unit is equal in length to the hanger spacing. The deck is supported on composite steel transverse floor beams which frame into the tie beams. The units are floated under the bridge and lifted into place by hoists connected to the arch ribs.
When the units are in place and cast-in-place concrete completes the closure in the center, the units are post tensioned. Finally, the ends of the deck are cast-in-place and the deck is post tensioned to the ends of the arch ribs.
Sadly, I couldn’t find any specifics about how the Big Mac Bridge was built. But, since it’s a tied arch bridge, it should have been something along these lines. I’m not done, though, because my son also wanted to know how the legs (or piers as they are apparently properly known) got put in the water. For the answer to that, I’ll turn to an article on Quora with the useful title How are bridges built over water? Apparently, there are three methods:
a) foundation may be sunk inside the bed from the top
b) Rigs may be employed to cast / drive piles on which a cap is then cast to support the pier
c) A cofferdam (a wall enclosing an area inside a water body) is first prepared, inside which water is constantly pumped out and dry working conditions are maintained. The foundation is then constructed inside the cofferdam.
Again, I wasn’t able to locate any specifics on how the Big Mac Bridge was built, so I don’t know which method was used.
Oh, and here’s a lovely fact. The bridge opened in 1976, with southbound traffic being able to use it in January of 1976 and northbound traffic being able to use it in December of the same year. Two years later, the Federal Highway Administration released a Technical Advisory designed to “acquaint the Federal Highway Administration and States with problems recently associated with tied arch bridges and to emphasize the need for a thorough evaluation of alternate designs which provide more redundancy”. It hen goes on to provide five bullet points on the way these bridges can fail, and why some other bridge type should be used. What a lovely thing to know about a bridge that I probably drive on eight times a month.