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Scratchbuilding a Whipple Truss Bridge in HO scale

From two Central Valley 150 Pratt Truss Bridge kits

By Craig Bisgeier

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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. 

Modified Central Valley truss 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.

Whipple Truss bridge floor and top chords

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.

A pair of girders spliced with a riveted splice plate. The kit's corner brace trimmed for the 50 degree Whipple truss portal angle.

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.

Starting to attach the C-C girders to the top chord

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.

Cutting the C-C girders to deck height

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.

Starting to apply the strap braces to the Whipple truss sides A pair of 'Tweezers' made to fit into the crowded corner of the top chord. Note the two strap braces eminating from the corner of the top chord. A close-up of the strap bracing on the Whipple truss bridge side.

Fast forward 12 months to February 2011...

Despite my better judgment I tried using the diagonal cross-bracing included in the kit that gets installed under the floor girders. They were designed to fit between the stock floor beams spaced about twice as far apart as my kitbashed beams, but I figured if I doubled them up it might look OK. Well, it didn't so I removed it and used styrene strips and sheet to rebuild the bracing from one beam to the next. Really, it didn't take much longer than the first attempt considering all the special cutting and fitting I had to do on the kit parts, and in the end my custom work looked a lot better and more realistic. My braces do not have the rivet detail, but that whole underside area will be hard to see and in shadow, so I can live with that. I doubt anyone will ever notice it, and since the new bracing looks 'right' it won't call attention to itself.

And finally I could go no farther without eye bars. This turned out to be a major headache, trying to figure out how to turn out large numbers of identical small, thin parts. I first tried cutting them from cardstock with knives and scissors, and that didn't work at all. Using the same methods on styrene sheet was no better, the process was too time consuming and it was nearly impossible to get consistent results. Then I tried to make a mold of the best, most accurate styrene part I had and cast the eye bars in resin. That did work to a point, but the right thickness was very hard to maintain. And in such thin cross-sections polyurethane resins have difficulty generating enough heat to set up properly.

Finally my good friend Bernie Kempinski of Alkem Scale Models took pity on me and laser-cut the eye bars for me based on CAD drawings I provided to him. It was really the only way to make it work from a repeatable quality and time-management perspective, and I'm very grateful to him for the help. He also provided me with replacement diagonal strap braces, since my original approach using the plastic strips supplied in the CV kit didn't work out so well. They were brittle, and I had no good way of attaching an eye to them. Unfortunately soon after Bernie did me this favor I got distracted and moved away from this project. But I did do some noodling on it for a while to figure out how to make all the truss connections work out. That groundwork served me well later on.

Looking over the detail photos of several pin-connected Whipple truss bridges I have, I determined how the different elements of the bridge came together at the pinned joints along the bottom chord. From the diagram I created, I formulated a plan to assemble the truss elements in a logical series of steps that would ensure proper alignment and order without having to try and get it all to fit at once. Leaving out the connections to the floor beams (to be done last), the plan called for the vertical floor hangers to be at the center of it all so I started there. These vertical beams would have the pin connections drilled right through them at a set distance from the top of the truss, so I carefully marked the hole locations with a caliper and drilled them out with a .036" drill, which matched the holes in the eye bars. I placed temporary pins in each hole made from hookup wire to help align and space the parts to come next.

I decided to build the back side of the truss first in case I made any mistakes I wanted to hide, so I temporarily installed a few of the eye bars on the front side over the pins to space the vertical floor hangers properly and then set the truss face-down on the workbench. I separated a few of the diagonal strap braces from the laser-cut sheet and carefully started to arrange them on the truss by slipping the eyes over the pins and marking / cutting the long ends to length.

Once the diagonal braces were in place the next step was to build up the lower chord eye bar chain. I started in the center and wiped a small dab of yellow glue over the eyes where they would be in contact with the diagonal braces and slipped the bar over the pins, pressing the parts together firmly for a good bond. I then worked out to either side from the center, overlapping the eye bars and gluing them to each other and the diagonals where they were located. I left the final connections to the truss ends until later. Finally I capped all the eye bars with a large hext nut Bernie had thoughtfully included in the laser cutting. I waited for the glue to set up, then flipped over the truss and built up the front as I had the back. When the first truss was done, I did the same with the second one.

So now the trusses were complete except for the diagonal bracing rods and the bridge floor. The diagonal rods were easy to do. I just cut lengths of music wire and twisted eye loops into one end, just large enough to fit around a pin connector, and left the other end a bit long. This was easy to do with a simple wire bending jig made from a block of wood and a couple of nails. I easily turned out all 20 rods in less than an hour.

Most prototype Whipple truss bridge's floors are hung from the trusses on heavy inverted U-bolts that loop around the center of the truss pins and get bolted to the bottom of the floor beam. I would use music wire to makes these parts, but for stability I planned to make them long loops and disguise the bottoms with a flat plastic plate. Before making wire loops I had to trim off the plastic posts at the bottom of the floor beams (how the stock bridge joins the trusses to the floor), since the lower chord would now run on top of the beams instead of below them. I also had to remove the small triangular gussets on the tops of the floor beams to make room for the lower chord which would sit just above the beam. To make the floor hanger loops I modified the wire bending jig with a second nail so I could create consistently sized loops of wire, like long chain links. These were sized to fit around the end of the floor beams and stick up just high enough to be slipped into the center of the vertical floor hangers, and then pinned in place. I tested a finished wire loop to make sure it fit properly and made all 20 I would need, but I had to complete a few more steps before I would join the trusses and floor together.

Since the next major step would be to actually mate the bridge floor with the trusses, I worked on getting the rest of the minor steps completed. This turned out to be a bit of a problem because the spacing for the Whipple truss has to be narrower than the Pratt design. crossbeams space the trusses a little farther apart than I needed them to be because the lower chord now sits closer to the center than it did in the Pratt design. was pretty straightforward to do, I glued the crossbeams into place (taking care to keep the joints square) and I used phosphor bronze wire to make the diagonal bracing between the crossbeams in the top between the two truss halves. Then I set the truss assembly to the side in a safe place until it was time for final assembly.

Central Valley provides 'bridge track' with the kit, but some modification was required. There is no rail, just tie sections about 4" long that fit between the cross beams in the Pratt truss design. Since I had twice as many cross beams in this bridge I had to cut the tie sections in half to make them fit, and fortunately in this case there were no major modifications, just some trimming and fitting. Once the ties are in place you set your own rail onto it and mash over the spike heads to hold it down. I prepainted the ties a dark gray / brown color and weathered them a bit for appearance. I would not install them until after the bridge was painted, but just before final assembly.
 

Stay tuned for more on this project!

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