The Midtown Greenway Coalition recently commisioned a feasibility study on an extension of the Greenway over the Short Line Bridge into St. Paul. I’m very interested in the project (I even did a little fundraising for the feasibility study), so I decided to read the report. As a software engineer, the bridge-building jargon was foreign to me but I found the document very straightforward and readable with a few outside references. Here’s a few interesting things I learned along with some stray thoughts about the project.
CP Rail (who owns the bridge today) has been totally non-cooperative in the project so far, so the engineering team flew a drone with cameras and LIDAR up to the bridge to get detailed measurements and photographs. That’s pretty cool.
The Short Line bridge is an example of a Baltimore Truss Bridge, which is based on the Pratt Truss design with vertical and diagonal pieces forming triangle-shaped supports for the bridge structure.
The bridge uses Eyebars, long metal rods with formed holes connected by pins, as the main connection between sections. Exhibit B8 on page 26 is a good photo of an eyebar
Engineers refer to the layers of primer, paint, and sealant on a structure as a Paint System. The paint on the Short Line Bridge is all gone and the girders have begun to corrode. The report refers to this as a “full paint system failure”.
Corrosionpedia is a thing that exists, by the way.
In a bridge deck, the large beams that go down the length of the bridge are called floor beams and the smaller pieces going longitudanally are called stringers
Scour is the name for erosion of a river bed caused by the flow of the river, particularly around a bridge pier. The engineers did not find significant scour around the two piers of the short line bridge, which is good news.
Trestles are the criss-cross structures that hold up the approach sections on both sides of the bridge. Wooden rollercoasters are built up the same way.
The report notes that “Parameter changes will have a direct impact on project costs”, which is delightful engineer-speak for “if you want a fancier bridge, you’re gonna have to pay more money”.
The engineers determined that the Short Line Bridge is currently a Fracture Critical bridge, because if any of a few dozen specific steel pieces (“Fracture Critical Members”) fails, the bridge would totally or partially collapse. There are an estimated 18,000 fracture critical bridges in the US - that website uses the example of the 35-W Bridge collapse in Minneapolis to illustrate the concept.
There are four different alternatives discussed to acheive a pedestrian and bike trail across the bridge: (1) If the railroad abandons the bridge, a bike trail could be built in it’s place at relatively low cost. Trains and bikes could operate side-by-side if the bridge is either (2) rehabilitated or (3) reconstructed anew on the existing piers. The bike trail could also be built atop the rail bridge (4) in a double-decker configuration.
I find it charming that one of the bullet points in the analysis is the views that cyclists/pedestrians are likely to experience on the bridge. If the trail and trains share the same grade, there will need to be a big safety wall, but on a double-decker bridge or a trail-only bridge, “Users can experience unimpeded views to the north and south above the safety railing.”
The double-decker bridge would require 23.5 feet of vertical clearance over the railway, so that design would require ramps up and down to the higher bridge. This is not quite as high as the climb up to the existing Martin Olav Sabo Bridge over Hennepin Ave.
I wonder if any of the graffiti artists featured in Exhibit B10 know that their work was included in an engineering study.