By Craig Bisgeier
February 2010 - It's a week after the big Springfield MA train show and I'm home sick, recovering from a bout of Pneumonia I picked up at the show. After feeling very poorly all week, I'm finally able to manage to do something. But I can't work in the basement because it's too cold, and I can't do anything where I might exert myself for fear my Pneumonia might come roaring back. I had a few options like decaling passenger cars and such, but I thought it would be fun to start a new project I've been planning for years - building a single-track Whipple through truss bridge to cross the Still River between Bethel and Danbury on the Housatonic layout.
The Whipple truss is an older and more complicated iron bridge design from around the 1870's than the Pratt design which became very popular for bridges built of both wood and steel/iron. The Whipple truss had a few distinguishing features that set it apart from the simpler Pratt design:
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The most obvious difference is the Whipple has nearly twice the number of vertical members than a Pratt design, creating a higher number of tall rectangular panels. The Pratt truss has larger, nearly square panels along the side of the bridge. |
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An obvious consequence of the higher number of vertical members is a like increase in the frequency of cross-members along the roadbed that support the bridge deck. Generally the Whipple has almost twice as many cross-members as the Pratt. |
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The Whipple has 45 degree diagonal strap braces that overlap two (sometimes three) of the side panels. Pratt trusses also have diagonal braces but they only cross over one panel, and often are arranged like a zig-zag, from up to down and then down to up on the next panel. The strap braces on the Whipple always start high and dive down towards the center of the bridge. An additional brace that sits at about 60 degrees runs from the end of the top chord to the second cross member, and does not overlap an additional panel. |
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Interestingly, there is no vertical post descending from the end of the upper chord of the truss. Instead the Whipple has a pair of heavy rods that hang from the corner of the truss and tie into the first cross member. The Pratt truss has a regular vertical member in this position. |
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The main girders that make up the portal on each end are more vertical on the Whipple truss than the Pratt truss, about 60 degrees compared to the Pratt which is around 45 degrees. This gives the two bridges a very different look. |
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Unlike the Pratt design which most often uses a box girder as the lower chord, most Whipple truss bridges have a pin-connected series of eye bars that make up the lower chord. |
I gathered my materials, which included a pair of Central Valley 150' Pratt Through Truss Bridge kits. The first thing I did was to read the CV instructions on how to build the Pratt truss bridge. From there, I got out the pattern supplied with the kit and modified it to reflect the bridge I wanted to build instead. Using several reference photos I have found over time on the internet, I reworked the plan until it showed most of the elements I wanted to include. To keep things simple I tried to keep the proportions about the same, and added additional vertical girders into the plan between those already printed on the pattern.
It was obvious I was going to need more cross-members than came in one kit. Well, I'd planned for that a long time ago and bought two kits, always figuring I'd have a ready source of extra parts. It was also obvious that the girders which ran the length of the bridge were going to be too long, but I would be able to cut them in half with a razor saw and they would space out nearly perfectly. So that was where I started, and spent about a half-hour making long girders short. When that was dome I filed them all to the same length so the gaps would be equal. It worked out to just about 1-3/4" from cross member to cross member. I built the bridge deck as per the kit instructions, just adding a few extra cross members from the second kit where needed. I used the CV kit's bracing to help hold the structure square and straight while building, but at some point the stock parts have to come off and be replaced by custom-cut gussets and strip styrene braces.
When the floor was more or less done I put it aside to dry and started working on the side trusses. CV recommends that you take a 24" wood board and attach a straight edge to it to build the truss on and it was a good idea. It really helps keep everything straight and aligned. In fact I ended up copying the most essential lines and measurements from the pattern onto the working board to help save time. Definitely do this if you build this kit stock or otherwise. Anyway, I got busy building up all the girders I was going to need, 12 B-B lattice box girders and 14 C-C box girders. I was able to get all the B-B girders pot of one kit, but needed to pull 4 more out of the second kit to get the number I'd need. Took a while to get them all assembled too.
Once I had the B-B girders all together, I took four of them and cut a 30 degree angle on one end of each, these became the ends of the top chord of the truss. The stock girders were long enough to span 3 of the tall panels, so I cut them where they fell across the third joint and took a third girder for each side and cut it to length to fit in between the other two. I glued them together and later followed them up with a few riveted bracing panels from the kit to reinforce the joints. Then I took the remaining four B-B girders and cut a 30 degree angle on an end of them too (well, close probably, I cut them to match the angle on the top chord), and then glued them to the top chord, being careful to maintain the 60 degree angle from the plan. These also got followed up with angled riveted corner braces that had to be trimmed to match the new angle of the portal. I used the cutoffs to fill in the open area left by the old angle cut underneath.
After giving the truss side a little time to set up, I followed the CV instructions and cut away the lattice underneath the B-B beams in the top chord, leaving room to insert the ends of the C-C beams into it from below. Having the locations of the vertical members already drawn on the working board really helped with this step. In very short order I had gotten all the C-C vertical beams inserted, squared up and glue in place. Again after a little drying time, I took out the strap braces from the kit and started to apply them carefully to the side following the Whipple bracing pattern I'd already copied onto the paper pattern. Here and there I had to go back under and trim away a bit more of the lattice from under the top chord to get the strap braces to fit properly. But again, in short order this was done too.
About this time I started to give some more thought to the bottom chord of the bridge. I went back to look at my reference photos and noticed that the line of eye-bars running the length of the bridge did not lie at the lower edge of the cross members the way the full box beam of the Pratt truss does. Instead, it sits at about bridge deck height or just below that. I had previously scribed the height of the deck and the height of the lower crossbeams onto my working board, so I simply went and cut the vertical members right to the deck line. I left the end portals long enough to reach the same height as the original bridge, figuring I can always cut off the excess if I need to.
The strap bracing on the ends that lie at 45 degrees was interesting to figure out. I'd already applied the straps that sit at 60 degrees and link up to the first vertical member, but this othe set starts in the corner also and reaches across two panels like most of the other braces. The plastic straps are thicker than those on the prototype, and I was concerned about trying to fit them into the gap inside the top chord with two braces already in there. What I ended up doing was taking two straps and gluing them together at one end, making what looked like a 'tweezers' with a narrow end. I inserted the closed end into the corner of the top chord after weaving the open end around the first vertical member, and glued it in place. This worked out really well and there's even a little bit of room left in there.
While leaving everything to set up and harden, I went back to look at the floor again. I noted where I'm going to have to make some changes. Like the pins that stick out of the quarter circle corners on the cross members - not in the right place, they will have to go. And like I said before the entire bracing scheme underneath needs to be reworked. Very soon though, it will be time to set up the ties and track that come with the bridge kit. A few molded in ties will need to be removed to fit the additional cross members, but that should be easy. The hardest part of the project ahead is going to be fabricating the eye bars - I'll need about 40 of them, pretty much identical. I can have them laser-cut which will make them perfect, but that's expensive. I could make a couple myself and then cast them in resin, but that will take too long. Got to decide what to do here...