Portland’s Steel Bridge
Portland, Oregon, a.k.a. “Bridgetown,” boasts twelve feats of engineering that span the Willamette River, some more majestic than others (sorry, Marquam Bridge). As part of our studio-wide fascination with these bridges, I undertook a 3d study of Portland’s most unique bridge: the Steel.
The Steel Bridge was completed in 1912 as part of a massive effort to replace a bridge that was no longer sufficient for “modern” levels of rail and boat traffic. The original Steel Bridge, completed in 1888 and pictured below, was a double-decker swing bridge made of steel at a time when most bridges were made of wrought iron, thus the name.
The entire middle section swung about a central support in the shipping channel that gave two distinct paths with no height restrictions. However, this method of operation soon proved too slow for Portland’s increasingly frequent boat, vehicle, and rail traffic. The new bridge, designed by Kansas-based engineering firm Waddell & Harrington, kept its predecessor’s name. It featured a vertical lift mechanism while retaining the double-deck design for both rail and vehicle traffic. The new vertical lift was much faster, lasting about 45 seconds for maximum clearance.
Unique in its design, Portland’s Steel Bridge is currently the world’s only operating telescoping vertical lift bridge. (The nearby Hawthorne Bridge, designed by the same firm, is also a vertical lift bridge). The lower deck support columns nest inside the main support columns above when raised, and two sets of counterweights control vertical clearance of the two decks independently. The set of smaller counterweights are responsible only for the lower deck. When engaged, vehicle and light rail traffic can continue uninterrupted above.
When larger ships come down the channel, the larger counterweights engage, and both the lower and upper decks are pulled up.
More Fun Facts:
- The combined load of counterweights and lift spans totals to well over 9 million pounds.
- Over the years, the entire counterweight system has grown heavier to account for Trimet rail lines and additional reinforcement of the bridge decks.
- The bridge is also known as a “friction bridge”. Greasers work tirelessly to ensure that the pulleys and cables are sufficiently lubricated 24 hours a day. The bridge must be operated periodically, approaching boats or no, in order to keep the entire system properly lubricated.
- A series of reduction gears run alongside moving cables to make sure the lift mechanism doesn’t go too quickly–otherwise the bridge could shake itself apart!
- The bridge is owned by Union Pacific; Trimet and ODOT lease portions of the bridge from them. Trimet is currently planning an $11 million renovation project to replace the 30 year old MAX tracks currently in place on the upper deck of the bridge.
- You can still see remnants of the Old Steel Bridge today! There are still trolley tracks embedded in the east bank of the river.
The Steel Bridge is also uncommon in that it carries the most (5) modes of transport of any bridge of Portland: cars, light rail, pedestrians, and bicycles on the upper deck, heavy rail on the lower deck, and pedestrians and bicycles on the Eastbank Esplanade that runs alongside the lower deck. Historically, the bridge also carried Trolley buses and horse-and-buggies.
Modeling the bridge in 3d allowed me to gain a deeper understanding of the mechanism that makes this bridge significant. A happy coincidence allowed us to reach out to Joseph Boquiren, who worked to document the Steel Bridge for the Historic American Engineering Record (HAER) and assemble the drawings we used for reference. Joseph is a fellow SketchUp fan and happened to be in our SketchUp Meetup group here in Portland, so we were able to have him over to the studio for lunch to talk shop. It was a great example for how 3d visualization can be used as a tool for inquiry as well as presentation. There is no better way of understanding how parts of an assembly work together than to see them represented in 3d.