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It’s been nearly three months ago that we published a front page story about the ongoing plans to dismantle the old iron bridge at Vera Cruz and include it in an expansion of the Rivergreenway Trail. Part of the story was a reference to the bridge as perhaps being the only “Double Whipple Truss bridge” remaining in Indiana.
Heard more than one request to learn just what a “Double Whipple Truss” is and what makes it so special. Promised more than one of those people that I’d get around to looking into it and report back.
Seems that a fellow named Squire Whipple came up with the “Whipple Truss” form of bridge architecture as a stronger version of the Pratt truss. Patented in 1847, it was also known as the “Double-intersection Pratt” truss because the diagonal tension members cross two panels, while those on the Pratt truss only crossed one.
The railroads - principle builders of bridges in the 1800’s - took an immediate liking to the Whipple plan as it was stronger, more rigid and downright handy for spans that had been too great for the Pratt truss to handle.
There are plenty of pictures on the internet of “Whipple truss” bridges, but only scattered references to any “Double Whipple Truss” bridges, one of which is a picture of our own Vera Cruz bridge pirated right out of the News-Banner. (Well, “pirated” may be a bit too strong, since they give us credit and we’re much more flattered than we are offended.)
From what I can see in the pictures, it appears the difference is that Whipple Truss bridges have the diagonal truss going only one way, while the Vera Cruz bridge has trusses going both ways. Thus the Double Whipple. (Not to be confused with Double-Charmins, double-squeezed by Mr. Whipple, rumored to be the great great grandson of Squire.)
One source stated that the Indiana Historical Bureau notes one bridge as being a “Triple Whipple,” but does not indicate where that rival might be.
And another website has the proof: a formula by which truss bridges are built which proves the worthiness, so they say, of the Whipple Truss: m = 2xj-3.
Write that down (m is the total number of truss members and j is the total number of joints). It represents the minimum number of trusses to carry normal loads. Whipple Truss bridges were said to always exceed that formula, so you can only imagine how strong that makes a Double Whipple.
Which brings us back to the original inquiries as to how something
that sounds so wimpy as “Double Whipple” can be so strong. One of the
mysteries of life, we can only surmise.
by MARK MILLER
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