A-B Comparisons of the Concrete and Steel Kind

There's something so, well, concrete about the real world. While Google can run millions of A-B comparisons to compare how well people respond to a new feature, it's not that often that you get to see an A-B comparison with 400 vertical feet of concrete and two suspension bridges spanning the same body of water, just feet away from each other. That A-B comparison is available for anyone to see at the Penobscot Narrows Bridge and Observatory in Bucksport, Maine.  (See Google Map here.)

The original suspension bridge, called the Waldo-Hancock Bridge, opened in 1931, with a tower height of 235 feet. It uses the tensile strength of steel in two giant cables that carry the load of the roadway. Vertical cables attach the roadway to the suspension cables. The whole bridge looks delicate and light. The Penobscot Narrows Bridge and Observatory (where this sign was photographed) has a pylon height almost double, at 447 feet. The two bridges just couldn't be more different, even though they perform the same purpose, in the same place: to carry traffic across the Penobscot Narrows.

I love the process of design, and especially the downstream effects of a few design choices. Here, the first key choice is material. The original bridge was steel, and the new one is concrete. Steel is great in tension. Concrete loves compression. The older bridge is the classic suspension bridge where a few large suspension cables carry the load, and the bridge deck is hung from the cable. It all looks rather delicate, and the steel is mostly in tension, except for the two support towers which act as pillars to hold up the main cables (not the roadway itself).

Meanwhile, the cable-stayed bridge is the perfect solution for a concrete structure. Look at how these cables are pulling the roadway towards the pillars. The cables are not only holding the roadway up, but they are also compressing the roadway with great force. If you made the roadway structure from steel, you'd have to design it to take a heavy compression load, which would be very difficult. For concrete, compression is a walk in the park. 

The towers of a cable-stay bridge have to be very high to get the cables to be at the correct angle to the roadway. If the towers are too low, the cable will provide too much compression and not enough roadway lift. But seldom do you see the results of design decisions this starkly presented side by side. Look at how incredibly massive the new tower is. 

This photo shows the new bridge under construction. The cable-stay design has a cool feature: each side is self-sufficient! The two halves can be built incrementally until they meet. And it appears you can even build the tower and the bridge together as you go. 

The two designs also played out very differently when something goes wrong. Seventy years of salt air had done damage to the main suspension cables of the Waldo-Handcock Bridge. Maine DOT tried to rehabilitate the cables, but along the way realized they were unsound. An emergency contract was put out to add new cables to keep the bridge safe, and at the same time the load limit was cut dramatically. A replacement bridge would need to be designed and built in record time.

What happens inside a cable-stay bundle is quite different than on a normal suspension bridge. Here, each individual strand runs in parallel. And each can be replaced individually. And each is coated in epoxy to resist corrosion. 

They're even testing new, carbon fiber cables that will altogether eliminate the issue of corrosion. Here, we see the steel cable on the left, and the equivalent carbon fiber cable on the right. They've actually removed a few steel cables and replaced them with carbon fiber to test how they do. They can even track individual cables and assess how well they are holding up.

If you click on the picture, it should get big enough to read the sign.

Here's the view from the tower. It's a beautiful vista. The paper plant may not be scenic, but it provides lots of local jobs.

The old bridge lives on, perversely saved by budget cuts. The state lacks funds needed for demolition. So go see them both while the bad economy allows it!

And next time you're doing design, keep in mind that the decisions you make now will have impacts as great as you see in these two bridges. Even in software, the downstream nature of the software is often tightly tied to the key decisions you made in the very, very beginning of the project. So give those basic decisions lots of thought, and learn how they play out before you ...well...cast them in concrete.