Friday, November 18, 2022

Overview of Strauss Movable Bridge Designs

Joseph Strauss graduated from the University of Cincinnati in 1892. His career soon took him to Chicago. After working on the project by the Sanitary Disctrict to reverse the flow of the Chicago River, he "became a Principal Assistant Engineer in charge of the Chicago office of Modjeski & Angier." He was charged with studying the use of bascule bridges because the War Department declared the swing bridges in the Chicago River had to be replaced. Modjeski did not like his plan so he resigned and joined Rall Bascule Bridge Co. as the Chief Engineer. After a year there, in 1902 he started his own Strauss Bascule & Concrete Bridge Co. He built his first bridge for the Wheeling & Lake Erie Railroad in 1904-05. It was the first bascule bridge to use concrete instead of cast iron or even more expensive materials for the counterweight. But concrete was not as dense, thus it required a bigger counterweight. Rather than make the watertight tail pit bigger, which would be more expensive, he mounted the counterweight high above the tracks to completely eliminate the expense of the tail pit. [StructureMag]
Figure via TheStraussBasculeBridgeCo via BridgeHunter, cropped via Dennis DeBruler
See Dennis DeBruler for a patent diagram

He quickly simplified the design to mounting the counterweight vertially behind a support tower to form a parallelogram. And the machinery room is beneath the bridge instead of on top of a tower. A curved rack under the counterweight end of the span is used instead of an operational strut.
Otis Ellis Hovey, Movable Bridges, p.117 via HAER-data, p16 via Dennis DeBruler

Here is another diagram. I presume that R is the rack and P is the pinion. Note that the main trunnion, G, is high above the foundation compared to other bascule designs. In this document, G is also used to denote the center of gravity.
Google eBook via Bridge Hunter

By 1908, he had scaled up his design to serve a double-track railroad bridge for the C&NW.
Street View

HAER IL-142, p. 18, initial design, Kinzie (Wells)

But that design put a limit on the size of the counterweight. If the weight was too big, it would hit the tracks. So he developed his heel trunnion design to raise the counterweights even higher. And the use of "elephant ear" counterweights allowed them to be even larger because they could go below the track surface.
20150513 1427
The St. Charles Air Line Bridge is in the foreground, and the B&OCT Bridge is in the background.
The St. Charles Air Line Bridge was built in 1919 over the original South Branch river bed, and it was 260' long. Today it spans the straignted river bed and is "just" 220' long.

20150502 9639, digitally zoomed to camera resolution

HAER IL-157, p. 19, climax design, St. Charles Airline

I wondered why I could not see an elephant ear on this side of the B&OCT bridge in the above photo. The photo below explains why. Strauss chose to use a monolith counterweight for this bridge that was never longer than 220'.
Marty Bernard posted via Dennis DeBruler

Street View, Jul 2019
 If you look at a satellite map, this is now a bridge to nowhere because it was used for passenger service. Specifically, the tracks east of the bridge and west of Canal Street have been removed. 

In this diagram, The member T-C-E is the right side of a fixed triangle setting on the piers. E is the trunnion of the movable span and T is the counterweight trunnion. Points F, T, H and E all pivot and they form the parallelogram that changes shape was the span raises. This diagram has to be wrong because it shows the operating pinion on the movable span rather than on the fixed triangle. At the beginning of the 20th Century, they did not have remote control so the machinery house was also the control room. Unlike a lift bridge where the span remains horizontal, a control room on a bascule span would be a very interesting ride. Is this an example were the arthur includes a subtle error to catch copyright violations? (I remember a teacher explaining that not all of the answers in the back of a textbook are correct because it is an easy way to document copyright violations.)
Google: Waddell's 1916 Bridge Engineering eBook via Bridge Hunter

I finally came across a diagram of the underneath version. Unfortunately, this diagram doesn't make sense. The whole purpose of the counterweight is to move the center of gravity of the span close to the main trunnion. The center of gravity should not be almost halfway across the channel as indicated by this diagram. How the links are are arranged under the bridge no longer matters because they changed their design from a parallelogram of links to the curved rack of the Chicago Style Trunnion Bridge.
HAER-data, p10 via Dennis DeBruler

SUC bridges for which I have notes:

Back to the overhead counterweight design.

I used to think the overhead counterweight design was rare because I came across a lot of heel trunnion bridges before I saw any SOC designs. The first SOC design I saw was in Jul 18, 2020, the Walnut Street Bridge in Green Bay, WI. Then I saw my first still-standing bridge in Jul 23, 2020 in Zanesville, OH. I have learned that Straus used the OC design for small bridges and the heel trunnion design for large bridges. Small bridges tend to be used over small canals. Many small canals are no longer used. Since those bridges tended to be for roads, the states would replace them with fixed bridges since they get 80% federal funds for new bridges and 0% for fix an existing bridge. (Note that the Zanesville bridge is a railroad bridge. They generally don't get federal funds to build a new bridge.)  Here is the Bridge Hunter list for SOC bridges.

B&O on Staten Island has a horizontal link in down position and an article that brags about
Notice the claim about "exactly balanced." That would be true in this case because the top and bottom cords of the parallelogram are horizontal when the span is down. The claims about "positively connected to the foundations" and "the loads are transmitted to the same point" are arguments in favor to trunnion designs and against his competitors' (Scherzer and Rall) designs.
Google: 1908 Railroad Gazette eBook via Bridge Hunter via Dennis DeBruler

I think it is significant that the B&O bridge is the only one I have found where the short links are horizontal when the span is down. Normally those links are tilted a little bit above horizontal as depicted in both of the above diagrams. In the first diagram, note the distances X+Y (closed) and X'+Y' (partially open). I check a bascule bridge design to see how Y' shrinks as X' shrinks to keep the center of gravity over its pivot point. Likewise, as the span closes, X' and Y' should proportionally grow longer. The problem with a SOC design that has an angled short link is that Y' quits growing and actually shrinks as the link goes past 90-degrees. But X' continues to grow. Granted, the sin at 90-degrees changes slowly (sin 90 = 1 and sin of 95 = .996), but Y' is changing in the wrong direction. I don't see how the bridge can remain "exactly balanced." Note that in his heel trunion design, the Y' does keep growing as X' grows.

SOC bridges for which I have notes:

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