How Bridge 9 holds it together

When I moved to the Minneapolis riverfront, I wanted to learn about the bridges. This stretch of the Mississippi River has so many of them. A week and a half after we unloaded the moving truck, the 35W bridge — a few blocks from our new home — collapsed. My desire to study bridges collapsed along with it.

But four years have passed. The memorial to the victims is finally under construction. I think I’m ready to dabble in bridges.

No better place to start than Northern Pacific Bridge No. 9, my favorite Minneapolis bridge, and the bridge featured at the top of this page. The first thing I learned was that the former railroad bridge near the University of Minnesota was called a deck truss bridge. A deck what?

Second word first. Truss. To oversimplify it, a bridge truss is just a pattern of interconnected triangles. It’s built of diagonal, vertical and horizontal elements (all performing different structural roles, but no physics here), and in the case of Bridge 9, the elements are made of metal, probably steel.

The deck is laid on top of the truss. As I walk, bicycle or dawdle across the bridge, I can’t see the truss as it does all the hard work of supporting the deck, defying gravity and keeping me out of the river.

The geometric maze of metal beneath the deck of Bridge 9 is actually two mazes. I had read this, and I had been told this by various people including Denis Gardner, a Minnesota bridge historian. Still, I looked at the bridge and I saw…a maze.

The best way to distinguish one maze, or web, from another is to look at how the metal pieces are connected. Some of the pieces, or members, are connected with pins.

Pinned connection on Bridge 9

Here’s another look at the same thing:

The pin-connected truss was built almost 125 years ago. Pinned connections were the height of bridge engineering at the time.

Other metal members of Bridge 9 are riveted together.

Riveted connections were stronger and more rigid than pinned connections, which allowed some movement. The Northern Pacific Railroad added the stronger web to Bridge 9 about ninety years ago so that the bridge could carry heavier trains.

The plate with all the rivets is called a gusset plate, and that term should ring a bell. The old 35W bridge was also a deck truss bridge. Its wimpy gusset plates sat like a 300-pound gorilla in the middle of the investigation following the 35W bridge collapse.

Bridge 9 carries pedestrians and bicyclists now. Its pins, rivets, and plates are probably strong enough to support such a light load, at least, I hope so.

Here’s a look at both types of connections. The light and spidery webs (with pins) are on the outside, the chunkier truss (with rivets and plates) is between them.

Bridge 9 holds itself together in more than one way, and that’s probably a good model for the rest of us.

If you’re interested in bridges, especially old bridges, I recommend Denis Gardner’s book, Wood, Concrete, Stone, and Steel: Minnesota’s Historic Bridges. It’s highly readable and full of photos of charming old bridges.

I thank Denis for taking the time to answer so many of my bridge questions.

I’ll leave you with a three-bridge photo. The view is looking downstream on the Mississippi River — first a curved pier of the new 35W bridge, then a concrete arch of the 10th Avenue Bridge, and finally the rusty webs of Bridge 9.

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4 Responses to How Bridge 9 holds it together

  1. LisaPeters says:

    Steve,
    Can you send me your 2007 article? The Strib website isn’t coughing it up for me. I had planned to write about the Short Line bridge, too. Amazing that trains still run across it. In his book, Denis Gardner says the Short Line bridge (he calls it the old Milwaukee Road bridge) downstream from the Franklin Avenue bridge was built in 1880, which makes it 131 years old.

  2. Joyce Sidman says:

    You explain things so well, Lisa, explicating the intricate things we overlook.

  3. Steve Brandt says:

    See for example my 2007 story re the Short Line Bridge

    Another concern for the county is the rail bridge’s construction.
    The bridge trusses are constructed with girders that are connected
    with large pins. Once those pins corrode, they can develop cracks
    under stress, but the extent of those cracks can’t fully be
    determined without sophisticated tests, according to a consultant’s
    report for the county.

    According to consultant URS, the Minnesota Department of
    Transportation has been replacing these pin-connected bridges, such
    as St. Paul’s old High Bridge.

    One concern for engineers is that unlike modern bridges, which
    are built to stand if one girder fails, one failed pin connection
    in a truss bridge of this style can lead to the entire bridge
    collapsing.

    “The fact that the structure carries railroad traffic today does
    not reduce the growing risk of catastrophic failure over the long
    term that has been experienced with bridges of this type,” the
    report said.

    The report gives the bridge’s age as 125 years; other reference
    sources say it is only 105 years old.

  4. Steve Brandt says:

    Pinned trusses can however corrode and lock up, leaving an inflexible truss.