Under Development - Updated: 12/1/2002
Old bridges, designed before computers and based on limited experimental research, often provide the clearest examples of the simple trusses one can analyze in an intro-level course. The "Old North Park Bridge" (1904-1988) is a good example. Contrast the simplicity of the design below with a modern truss used in the (2003-??) convention center in Grand Rapids, Michigan. You might, however, notice some common elements in the two trusses if you look closely at them.
The Restored Bridge Span:
The original bridge had five spans set on piers in the river. One of those five spans has been restored and put to use in a park. It still has the original dimensions of 116 feet (35.3 m) in length divided into 8 truss sections of 14.5 feet (4.42 m). The truss is about 20.83 feet (6.35 m) high. [Thanks for doing the field check on this, Mom and Dad.]
This is a Pratt truss, characterized by the outward direction of the diagonals, with the addition of cross-bracing in the middle of the span. You may be able to see some of these details better from a slightly different angle:
You might also note how much heavier the upper (compression) members are compared to the lower (tension) members, which are almost invisible. [See closeup at bottom of this page.]
The bridge was built in 1903-4, opened to traffic in 1904 and closed to traffic in 1988.
See a separate page for details on its history.
Computer model of the bridge:
I built a model of this bridge using the West Point Bridge Design contest program. (See links at the bottom of this page for info on it.) I want to give special thanks to Col. Ressler for his assistance in getting this specific simulation to work in the program, although all of the design choices were mine.
Note: The West Point Bridge Design contest program fixes the spacing between deck supports at 4.00 m and also limits the location of joints to 0.50 m increments. As a result, the best that can be done when modeling the bridge with its 6.35 m tall by 4.42 m wide elements is to try to preserve the aspect ratio of those truss elements. Even then one must round off the resulting ratio of about 5.75/4.00; I chose to use 5.50/4.00. Thus the model truss is 5.50 m high and is a total of 32 m long. [FYI: the first two models used a 4x4 aspect ratio and another based on some published dimensions that proved to be quite wrong.]
The model bridge truss was designed so it just barely would carry the test load (the single truck shown below) built into the contest program. This action maximizes the contrast between the tension (blue) and compression (red) members in the program's test animation.
[Users of the program will realize that I modified the background of both the test animation and the drafting plan of the bridge so it would be more compact and still display cleanly here on the web.]
Insight from the model study:
The first thing one learns when applying the structural modeling program (or in physics class studying basic statics) is that the upper members are in compression and the lower members are in tension. The second thing you learn is that the members under tension can be much smaller because steel is very strong under tension. You can see this reflected in this century-old bridge:
Notice that the bottom part of the structure consists of only what you see here: a pair of "eye" bars bolted to the verticals. The bridge deck rests on beams attached to the vertical members at this same point, just as in the model. You might also notice that these two outer members are much thinner than the inner ones, one of which is just visible at the far right. This also reflects something seen in the model study.
This work is still in progress. Since this page is already too long, my plan is to include other pages that expand on these observations about the design and how it was realized in the actual 1904 bridge.
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if you have any questions.
My TCC home page.