Seventh Street Improvement Arches facts for kids
Quick facts for kids Seventh Street Improvement Arches |
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The Seventh Street Improvement Arches viewed from the south
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| Coordinates | 44°57′23″N 93°04′38″W / 44.95639°N 93.07722°W |
| Carries | Four lanes of  MN 5 (East Seventh Street) |
| Crosses | Bruce Vento Regional Trail (former trackage of St. Paul and Duluth Railroad) |
| Locale | Saint Paul, Minnesota |
| Characteristics | |
| Design | Skewed, helicoidal, double stone arch bridge |
| Width | 124 feet |
| Longest span | 41 feet (east arch), 30 feet (west arch) |
| History | |
| Opened | 1884 |
The Seventh Street Improvement Arches is a special bridge in Saint Paul, Minnesota. It's a double-arched stone bridge that used to go over railroad tracks. This bridge is very important because of its unique and tricky design. It's one of only a few bridges like it in the whole United States, and the only one known in Minnesota.
This amazing bridge was built between 1883 and 1884. Michael O'Brien and McArthur Brothers of Chicago built it. The clever person who designed it was William A. Truesdell. Because of its special history, the bridge was added to the National Register of Historic Places in 1989. It was also named an American Society of Civil Engineers Historic Civil Engineering Landmark in 2000.
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Why This Bridge Is Special
The bridge was planned in 1883 as part of a big project. This project aimed to improve Seventh Street in St. Paul. The goal was to connect downtown St. Paul with the Dayton's Bluff neighborhood more easily. Other parts of the project included a large iron bridge and a stone sewer.
Making the Street Less Steep
These improvements were needed because Seventh Street was very steep. It was hard for streetcars to travel between downtown and Dayton's Bluff. The city wanted to make the hill less steep so streetcars could go up and down easily.
A Tricky Design Problem
The Seventh Street Improvement Arches bridge had a big engineering challenge. The street crossed the St. Paul and Duluth railroad tracks at a sharp 63-degree angle. This made building a straight bridge impossible. The bridge also had to carry sewer and water pipes. Plus, it needed to match the new, less steep shape of the hill. This meant a lot of dirt and rock (called "fill") would be piled on top of the bridge.
Smart Engineering Ideas
The huge weight of the fill meant that a common bridge design wouldn't work. The engineer, William Truesdell, thought about using a classical French method for angled bridges. However, this method needed too much skilled stone cutting, which would be very expensive.
So, Truesdell decided to use a different method called the "helicoidal" or "spiral" method. This idea was first used by a British architect named Peter Nicholson in 1828. This method involved complex math, but Truesdell enjoyed math as a hobby, so he took on the challenge!
The special stones used in the arches, called voussoirs, were cut with curved surfaces. This made them form a series of parallel spirals. Calculating these curves was hard at first. But once the math was done, most of the voussoirs could be cut using the same pattern. This meant the stonecutters had to be very precise. Luckily, a skilled foreman helped organize the work.
The parts of the bridge that hold up the arches (called abutments, piers, and wing walls) were built with gray limestone. This stone came from local quarries in St. Paul. The voussoirs, ring stones, and other parts of the arches were made from a buff-colored limestone. This stone was quarried in Kasota, Minnesota.
How the Bridge Was Built
Construction on the bridge started in September 1883. Michael O'Brien of St. Paul was in charge of digging and building the foundations. McArthur Brothers of Chicago took over for the final construction of the bridge. The bridge officially opened for traffic on December 18, 1884.
Other engineers recognized Truesdell's amazing work. Even if the public didn't fully understand how special the bridge was, his colleagues did. When Truesdell passed away in 1909, the Association of Engineering Societies Journal called the bridge "the most important piece of masonry in the city."
What Happened Next?
The railroad tracks that the bridge used to cross are no longer there. But the Seventh Street Improvement Arches bridge still stands strong! Today, the Bruce Vento Regional Trail follows the old railroad line right under the bridge.
Images for kids
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North elevation
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Detail of the ring stones and the entrance to the east arch
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Detail of the ring stones at the top of one of the arches
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Interior view of the helicoidal construction. The gray stonework at the bottom is the horizontal pier on which the arches sit.
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The bridge's National Historic Civil Engineering Landmark plaque
