The diagram below shows my drawing of the basic outer frame of the car. It shows the top view and the side view. The prototype's overall length from end sill to end sill is 32 feet and 3 inches. The Evergreen styrene C-channel that I used for the end sill, which, by the way are 12 inches tall, measure a scale 4 inches deep. Therefore I cut the 2"x10" scale styrene strip to a length of 31 feet and 7 inches, to get the prototype's overall length. The C-channels were cut to a width of 9 feet and 4-1/2 inches. The prototype's width appears to be 9 feet and 6-1/2 inches. However, I want to add the distinctive side plates (which in the prototype are probably extensions of the side sills), which will be made out of styrene strip that is one scale inch thick. So, if you add two of those, one on each side of the end sill, you get the prototype's overall width. Note that the C-channel sticks out below the side sill, which accounts for the two-inch height difference.
This represents the official start of scratchbuilding three of these cars at the same time. The photo was taken on March 21, 2014, so that represents the official start (although I had spent some time doing research beforehand). Since I want to build three of these cars at the same time, I gathered six pieces of Evergreen strip styrene that are a scale 10 inches wide and 2 inches thick. I marked the length as accurately as possible on one of the strips. I then used this elaborate set-up to mark a line across all six of the strips. This makes sure that they are all the exact same length, otherwise you wind up with crooked bodies. The plastic T-square is where the line is drawn.
Next, I carefully lined them all up in the bench vise, and filed the ends to make sure that they were indeed all the same length.
These are the humble beginnings of the cars. I am building them on a piece of glass, hence the layout being reflected in the photo.
I recently bought these Rite-Way clamps. This is my first project where I'm using them. Love 'em! They are perfect for this type of work, where you want to be sure to glue two pieces of styrene together at a perfect 90-degree angle. I fiddle around with the end sill and side sill parts until they line up perfectly. Only then do I apply the glue to bond them. The magnets of the clamps hold the two parts at the correct angle, and the flat surface of the glass ensures that they are both on the same plane. You get two in the box, so while the one is drying, I can be working on the other frame parts. Note that because there is a height difference between the end sills (12") and the side sills (10"), I am building them upside-down.
And here are the three basic frames put together.
The diagram below shows the side of the end sill. I built mine up by putting a 10-inch deep strip of styrene on top of the end sills. I added to that a 1x4 strip vertically. These are close to the prototype's dimensions, although the prototype probably used 1/2" thick steel. The sides, which in the prototype are probably extensions of the side sills, are a scale one-inch thick sheet of 12"x11".
I wanted to show how I installed the top 10-inch wide plate on top of the end sill. I wanted to make sure that it lined up with the end of the end sill. I used two metal squares, against which I placed the end sill. I then used a scrap piece of plastic that forced the end sill against the squares, using another metal weight. This kept the frame from moving on me while I tried to place the strip of styrene and applied the glue.
Fairly quickly after applying the glue, I moved the squares away from the frame so as to avoid having the glue attach the styrene to the metal. Note that I used a scale 1"x10" styrene strip, and that I cut it wider than I needed.
Next, it was a simple matter of gluing a 1"x4" strip vertically against the plate. This vertical strip will provide the connection point for the bracing that is used to hold the ends of the hopper body in place.
This photo shows the sides of the end sills completed. I used a small round file to file the fillet shape of the plate.
Studying the PRR H21a drawings that I have, I concluded that it looks like the core of the center sill consists of two C-channels that are 10 inches tall. They have a "lip" that appears to be about 2-3/4 inches wide. From outside edge to outside edge, the drawings seem to indicate a total distance of 18-3/8 inches.
I don't have any scale 10-inch C-channel material (Evergreen makes it, but I just didn't have any in stock), so I used a 2"x10" strip, to which I glued two strips of 1"x3" styrene (as shown in the drawings above). I could have used 1"x10" and 1"x2" strips to get a closer-to-prototype C-channel, but the ones I used make for a stronger model, and they are easier to work with. The center sill is mostly hidden, and also it will be filled with lead shot later on, so it needs to be fairly stout. The important thing is that the outside edges are 18-3/8 inches apart. To position the two C-channels, preserving the prototype's 18-3/8 inches of outside edge separation, I calculated that the outside edges need to be 4 feet from the outside edge of the frame, as indicated in the diagram below.
Once fabricated, I cut and filed the pieces to length and glued them to the underside of the plate that covers the top of the end sills. To position them correctly, I marked the spots on the end sill with pencil, and then glued them accordingly. The important thing to note here is that the center sill pieces need to be the same length as the side sill pieces, otherwise the frame will not be square. A note I want to make for myself here is that I probably should have cut the side and center sill pieces at the same time, so that I could make sure that they indeed were the same length. Update: later prototype photos I received and studied revealed that the center sill was taller in its middle, and not a straight section like I have done here. This will need to be revisited the next time I build more of these hoppers.
Studying the scale drawings of the H21a, I concluded that there is about 11 inches of space between the bottom of the center sill and the bottom of the bolters. I made a styrene block of 20 scale inches square that is 8 inches tall, which I glued to the bottom of the center sill. Note that before I did this, I had glued a block of styrene in between the center sill C-channels (you can see it in the photo after this one). The purpose of all of this styrene is to allow the drilling of a hole into which the truck's screw is mounted. The center of the block is 5 scale feet from the end sill.
Next, I made these strips of styrene that measure 20 inches in width and are 6 feet and 9 inches long. I made it out of one scale inch thick styrene. These are the bottom braces of the bolster, and they have a chamfered corner. The flat ends are 10 inches wide, to match the next piece to be attached.
But before I go much further, I completed all the styrene that is needed for installing the truck mounting screw. I took the models to the drill press, and as best as I could, drilled holes in them for the screws I intend to use for mounting the trucks. These holes must be perpendicular to the body, otherwise the car will lean to one side.
I then cut some 10-inch wide, 1-inch thick styrene and glued those to the tops of the 20-inch wide pieces I had installed. I tried to make sure that their center line was five feet away from the end sill.
Next, I glued the strips to the underside of the side sill, which is what the scale drawings of the H21a seem to indicate. The excess will be trimmed off.
I received this photo by Bill Lane of PRR #494739 which clearly shows that the center sill framing members were taller than the side sill framing members. The next time I scratchbuild more of these cars, I will have to take that into account. I used 10-inch center sill members, but it looks like from the photo that they used closer to 12 or 14" channels.