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GENERAL INSTRUCTIONS
AND ASSEMBLY METHODS
(The following instructions are aimed primarily at the HO scale modeler. Modelers in other scales should adjust dimensions appropriately.)
Removing Pieces from Cardstock Sheet
Basic Bridge Concepts
As any railroad crosses the countryside, it must cross hundreds, if not thousands of rivers, streams, creeks and drainage ditches. Just how each of those is crossed is a basic economic decision. Although model railroaders love big bridges, 12 inches to the foot railroaders hate big bridges. Basically, they build the smallest and cheapest bridge needed to do the job. If a drainage pipe with some dirt over it will work, that is what they build.
But, of course, we want to talk about real bridges here, not drainage culverts. Up to about 15 to 20 feet, most bridges will be I-beam bridges. They look somewhat like a plate girder bridge, but actually they are made using I-beams that have been rolled into the final shape in a steel mill while the metal is still white hot.
Past about 25 feet and up to about 100 feet, the plate-girder bridge is very common. These bridges are made from steel plates and angles that are riveted or welded together to form the girder. These are found in either deck or through versions. The deck version has the train crossing the bridge on top of the girders, whereas the through version has the train passing between the girders. The deck version is less costly (because the floor system is much simpler) so it will always be used when the vertical clearance under the bridge is high enough to permit whatever passes under the bridge to do so safely.
The economic limit for a plate girder bridge is about 80 to 100 feet. Beyond that, the truss is the most common type. The concept of the truss is really quite simple. The strongest shape in nature is the triangle. You cannot change the shape of a triangle without changing the length of one of the sides. So, if you take the basic bridge shape needed, and subdivide it to create lots of straight sided triangles, you can make a very strong truss. For the bridge designer, the goal becomes to make the lightest bridge possible that will carry the required load. Why the lightest bridge? Because the lightest bridge will use the least material, and therefore be lower in cost.
As you might guess, there are as many truss designs as there are bridge designers. However, a few have really worked well and withstood the test of time. They are the Warren, Pratt, and Castleton (which is a modification of the Pratt truss). They can be built as a deck or through type, and the top chord can be either straight or curved.
The concept of the curved top cord is to reduce weight and cost. The longer a bridge is the taller the truss must be to be stiff enough to safely carry the loads. The stresses in the bridge are highest in the middle and least at the ends. Therefore, the bridge doesn’t need to be as tall near the end as it does at the middle. The result is a low arch on the top of the bridge, which is very pleasing to the eye. Be aware that although the top has something of an arch shape, it does not function as a true arch.
Truss Bridge Terminology
If you look at a truss bridge from the side, the top-most member is called the TOP CHORD. The top chord is always in compression, which is to say that the loads and stresses try to make the top cord shorter. The bottom-most member is called the BOTTOM CHORD, which is always in tension. Tension forces tend to stretch the bottom chord and make it longer.
Although steel is equally strong in tension and compression, compression forces will cause the top to fail by buckling long before it will fail because the material strength is too low. To combat this, bridge compression members use heavier plate materials and have larger cross-sectional shapes to prevent buckling. The end result is that the top chord is almost always the heaviest member of the bridge. Because the bottom chord is always in tension, and buckling is not possible, it is often the smallest member of the bridge.
The rest of the truss is made up of VERTICAL and DIAGONAL members. These transmit forces between the top and bottom chords, and keep the entire bridge from collapsing. These members are sometimes in tension and sometimes in compression. The size and cross-sectional shape of these members are determined by the compression loads they must withstand. They are typically way over designed for the tension loads.
The truss is subdivided into panels. A panel is formed by the top chord, bottom chord, and two verticals. Panels usually have one diagonal, although there may be two. There may also be sub-vertical, sub-diagonals, and even sub-horizontals. This is way beyond what we want to cover here, except to point out that these details are usually what give different truss designs their name. For example, some bridge designer built a bridge at Castleton-on-Hudson. It was a basic Pratt truss with lots of sub-verticals, sub-diagonals and sub-horizontals added. Thus we have the Castleton truss.
The end panels of a truss may be very different than the other panels. They are often triangular shaped and are the place where the top and bottom chords are connected together.
If you want to know more we strongly suggest the NMRA Data Sheets D6b.1 through D6c.522. These are about 32 pages of wonderful detail about bridges of all types.
Our Basic Method
Prototype steel bridges are built using plate material that is cut to the required length and width. Angle material is used at the corners to provide a means to rivet or weld the plate pieces together. The plate material is typically either one or two inches thick, and the angle is typically 6 or 8 inches on a side.
Our basic method replaces the steel with cardstock and the angles with balsa strip wood. The cardstock is .011 thick, almost exactly 1 scale inch. The primary purpose for the wood is to provide a surface to glue the lacing to, but also the bridge members made of both cardstock and wood are much stronger against the compressive loads the bridge will see when in use.
Although the balsa wood members are square (or rectangular) instead of the true angles they should be, from a few feet away, most people will not be able to tell the difference. The assembled and painted bridge is very convincing.
Also, our bridge doesn’t have any rivet heads. Very few people will notice they are missing, but you may add them if you wish. A fairly small bridge would have several hundred thousand rivets. A large one may easily have over a million rivets. The cardstock we use can easily be embossed using a rivet tool and typical rivet dimensions and patterns can be found in the NMRA Data Sheets mentioned above.
Gluing pieces together seems to be messy, regardless of the glue used. We prefer glues that can be cleaned up with water because they do not emit toxic fumes and clean up is easy. We recommend two products to use, but if you have a favorite and want to use something else, feel free to do so.
The amount of glue to apply is important. Applying too much glue increases both warpage and cleanup problems, so we want to use as little glue as possible. But you can also use too little glue, as one of our test assemblers discovered. If you use too little glue the bridge tends to fall apart with subsequent handling. While trying to repair these problems, more pieces became loose. When painted, the moisture in the paint caused the paper to swell and more glue joints popped free. The moral of this story is-- use as little as possible, but be sure the surfaces being glued together are covered with glue, especially on the main joints between the floor and the sides.
The glue products we recommend are Elmers Glue All and Avery Glue sticks. When covering large areas, the glue stick is preferred because it has less water in it and warps the wood and paper less. Everywhere else we use the white glue.
The water in these glues is an important consideration. Both the cardstock and wood absorb the water and swell up, distorting the pieces. Therefore, it is important to not work too fast, and be vigilant about keeping the parts flat while the glue dries. A good case in point are the main truss inside and outside surfaces. The cardstock pieces have wood strips glued to them to provide strength and provide and an edge to glue the lacing onto. Within a few minutes after the glue is applied and the wood and paper are glued together, the bare wood side takes on a major concave shape. As the glue dries, the warpage decreases, but the part will never return to the flat state it started in. If you place a few weights on the piece while the glue is still wet, and force it flat, the assembly will remain flat when the glue has dried.
The weights do not need to be heavy, or expensive. We use machine nuts that you can buy at any hardware store. A bag of 20 costs only a few dollars. We have three sizes that we use, ¾, ½, and ¼ inch nuts.
Our general procedure for gluing the pieces together starts with clearing away a flat space to work on. We like to use the assembly jig mentioned elsewhere. First, we remove a few pieces from the precut cardstock sheets. These are glued onto the assembly and any excess glue cleaned up. We then place the piece flat on the jig and place weights all over the assembled piece. Next we prepare a few more pieces while the glue sets up and begins drying. We remove the weights, glue on the new pieces, and return the assembly to the jig and replace the weights. This is repeated until the assembly is complete. Occasionally we get some popcorn or something to drink, and let the assembly sit for awhile with the weights keeping everything flat. It takes a few dozen weights to make this work, but the result is well worth it. Also keep the weights on the final assembly until the glue is completely dry--at least overnight.
Applying the glue is also worth mentioning. The glue sick is fairly obvious. Just swipe the sticky end of the stick on either the wood or the cardstock and then assemble the pieces. The white glue is also fairly obvious, but you need to be able to control the volume and rate at which the glue is delivered. We have tried various methods over the years, but we recently discovered Elmers glue pens. They have a fine tip that provides excellent control.
Removing Pieces from Cardstock Sheet
The precut cardstock pieces are laser cut. The laser has been set up to cut out the pieces, but leave them attached to the cardstock sheet. This was done to reduce the chance for pieces to become lost, bent, torn, or otherwise damaged while they are being handled in our factory, as well as at your workbench while building the model. If any part becomes lost or damaged, we will happily replace it.
If you hold the cardstock sheet with a bright light behind it, you can clearly see the pieces have been cut all the way through. A closer look will reveal that hundreds of tiny fibers still hold the piece in place. You need to break these fibers or cut them with a sharp knife to release the pieces from the cardstock sheet. Most pieces can be removed using your fingers. Gentle pushing near the cut line, or working the cut line a little by bending it back and forth, will usually break the remaining fibers.
Smaller pieces often need a slightly different method. Place the cardstock sheet on a flat surface and place a pointed object (the point of a hobby knife works well) right in a corner where two cut lines join. While holding the piece down with the knife, lift the cardstock sheet gently and the piece will tend to peel out of the sheet. Once you’ve got it started in this way, it usually fairly easy to remove the whole piece.
On a few occasions you may have to use a sharp knife to go over some, or even all, of the existing laser cuts to release the piece from the sheet. This should be rare, but it is better to do this than destroy the part.
Sometimes, removing the individual pieces can be a bit tricky, but with a little practice and experience, you will be doing it like a champ.
Separating Wood Pieces
Most of the strip wood needed for the bridge has been laser cut from sheet material. To make it easier to handle the many pieces, and to eliminate the problem of losing parts, we decided to keep the pieces connected together. You must cut them apart. Use a sharp hobby knife or a razor blade to cut through the wood "bridges" that hold the pieces together.
Wood Breakage
All the wood pieces used are fragile and break easily. If you break a piece, we recommend you glue is back together using white glue. With a little care, you can make a repair that is undetectable in the finished bridge.
If a piece is totally destroyed, we will be happy to replace it.
Assembly jig
The base of the assembly jig needs to have a surface that the glue won’t tend to adhere to, and which can easily be cleaned up. We use pieces of shelving material that can be purchased at any discount building materials store. It is basically dense chipboard that has been covered with glossy paper. White glue doesn’t tend to stick to the glossy paper covering. You can enhance the effect by waxing the surface with a heavy paste wax. Both the white glue and the glue stick we recommend can be easily cleaned up using a damp rag.
The alignment pins are made from steel wire that is .065 inches in diameter. We have included the pins needed with the kit.
The following instructions are a little generic because they are not specific to any one kit. Find the jig pattern pieces that have been printed on regular weight paper. The pattern has been printed full size, and for most of our kits it takes more than one piece of paper, so be sure to locate all the pieces. DON’T CUT ANYTHING. Since the pattern spans more than one page, you must reassemble them into one continuous sheet. Each pattern piece has a line printed on it with a dimension given in inches. When correctly assembled they will form one line that will be straight, and it will be exactly the length stated. We will call this the "Gage Line".
Start by loosely assembling the pieces so you can see how they fit together. Notice there is considerable overlap, and the printing does not extend all the way to the edge of the piece of paper. Trim away some of the excess paper by cutting along the "CUT" line. Again, loosely assemble the pieces, arranging them so the printing is continuous across the pages.
The next step is easier to do than it is to explain. Use a yardstick, or similar tool, to re-establish the straight line and measure the required length for the gage line. Shift the middle pieces a little from side to side until all the structure lines are lined up and straight, and the gage line is straight and the correct length. If you can’t get everything precisely lined up, balance everything as best possible, but don’t cheat on the gage line at all. Use any type of sticky tape to carefully tape the pieces together without disturbing the alignment of the jig. Don’t cover up the drilling marks with a tape you cannot see though. The pattern is now ready.
Take the piece of shelving, or whatever you are using, and tape the pattern onto one side. Use a center punch or a sharp nail to make a staring hole for the drill at each of the drilling marks. Use a #51 drill to drill a 0.067" diameter hole about 3/8 to ½ inch deep at each of the punch marks. Remove the drilling pattern from the base piece, which is now ready to use.
To use the jig, push the alignment pins supplied with the kit into the holes. They should insert easily by hand. The pins can be removed using your fingers, or pliers if the pin sticks a little.
Tweezers
Many of the parts you will be working with are small and fragile. A better tool than fingers are needed to handle these parts. Enter the often maligned tweezers. The cause of the frustration many people have with tweezers is that the tweezers are damaged, or in some other way not ready for use. Unlike most knives and razor blades, tweezers do not usually come ready to use. In a minute we will tell you how to fix the common tweezer problems, but first, let’s look at what the common tweezer problems are.
Too Small—A good pair of tweezers should be 4 inches to 6 inches long. They should fit comfortably in your hand. If they are too short (say like the tweezers used to pluck eyebrows) you will end up with your hand feeling cramped and you become too concerned about the tweezers to be paying attention to the parts you are working with.
Too Stiff—If the tweezers are too stiff, you will lose the delicate sense of touch needed to be able to know you have grasped the part you are trying to pick up.
Too Blunt—The tweezers must have a sharp point. Most commercially available tweezers are blunt so no one will be injured by the points. The problem is that you can hardly pick anything up from a flat surface with them. If the points are sharp, you can slip the point under the part and it is easy to pick up.
Damaged Points—Too often we have seen tweezers that look like they were last used to open a paint can. The points are mangled and the owner complains that they don’t work well! Let’s think about it for a minute. If the points don’t touch each other because they are crossed, what are you planning to pick up with them? If the points don’t touch each other because the tweezers come together ¼ inch back from the points, or because one point is longer than the other, what are you planning to pick up with them? The points must be smooth, straight, needle sharp, and touch each other only at the very tips.
How to Repair and Sharpen Tweezers
If tweezers are in really bad shape, the first thing to do is get the general shape of the points correct. Use pliers to bend them in place. If needed, hammer on the tips to get them back in order. Manipulate them until they look right, without pronounced bowing inward or outward, and the points just touch at the tip. If you cannot do this, the tweezers are beyond repair.
To sharpen the tweezers, use a good grind stone with multiple grades of coarseness. Please do not use a powered grinding wheel. It is too fast, generates heat that can damage the tweezers, and you just can’t control what you are doing.
Step1
Using the coarsest stone, start by holding the tweezers vertical to the stone, with the points touching the stone. Hold the tweezers near the points and grind back and forth until the points are flat. You don’t need a large flat area, but the idea here is to be sure the points are exactly the same length and that we touch right at the end of the points.
Repeat using the finer coarseness stone. Stop when the coarse grind marks have all been removed and the finer grind marks are uniform.
Step 2
Hold the tweezers as shown in the figure below. Hold them so the space between the leaves is vertical, the angle between the tweezers and grind stone is about 15 degrees, and the index finger is almost directly over the points. The angle is not really important. But it needs to be shallow so the points will be sharp. Also, don’t squeeze the points together so hard that they may become deformed as they get sharper and more delicate.
Grind with a side-to-side motion. Occasionally turn the turn the tweezers over 180 degrees and grind the other side of the points. The idea is to carefully grind the points to a knife-edge.
Step 3
Turn the tweezers 90 degrees and begin grinding the knife-edge into a point. Basically repeat step 2, but with the tweezers in the new orientation. If you grind too far you will end up with offset points, so watch for that.
Step 4
Repeat steps 2 and 3, but this time use the finer grind stone. When you finish, you will have very sharp, smooth points.
Painting
We strongly recommend you paint the completed bridge using an airbrush. Just about any paint that can be airbrushed can be used. Hand brushing will not give good results because you just can’t get to all the interior surfaces. Many modelers have airbrushes so if you don’t have one, you may be able to borrow one from a friend. You could also use spray paint from a can, but we don’t recommend it because they are too difficult to control and get the paint right where you want it.
When the airbrush is properly adjusted, the paint is almost dry when it hits the surface being painted. Covering with several light coats of paint is better than one heavy coat.
Common colors for bridges are black, silver, and rust. However, almost anything goes. We have seen highway bridges that were red, orange, blue, green and white. For most railroad bridges, painting is a low priority item. Many bridges were painted when they were new, and never painted again. You can apply all the weathering techniques used with locomotives, rolling stock and structures.
A special note needs to be made about the bridge floor. We suggest you paint the floor of the bridge before you lay track on it. And, we also recommend you lay the track before you assemble the floor into the final bridge assembly. It is virtually impossible to lay the track inside a completely assembled, through type bridge. We suggest you assemble the floor, paint the floor, lay track, and then paint and weather the track as you desire. Cover the rails and ties with masking tape just before assembling the floor into the bridge. The tape will protect your work while you paint the rest of the bridge structure.
Track
We do not include any track, rail, ties, or related items. Individual modelers have such a wide variety of preferences that we feel it is impossible to satisfy everyone. Following is information to help you design and build a realistic track system for your bridge. WHATEVER YOU BUILD, THE MAXIMUM DEPTH OF THE TIES, RAILS AND ANYTHING ELSE YOU USE IS ¼ INCH. This is enough to use 1/8-inch ties and code 100 rail.
There are two common track systems used—open deck and ballasted deck. The open deck type is pretty much as it sounds. There is open space between the ties and you can see through the floor. Ballasted deck bridges look like a continuation of the typical ballasted track roadbed. Each has advantages and disadvantages. Either type is appropriate for any bridge.
Frequently, train crews must walk the length of their train, especially if some emergency has stopped their train and the crew must inspect it before they can proceed. If the train is on a bridge there may be no way for the train crew to cross the bridge. For this purpose, walkways are installed over the floor beams to provide a footpath. Sometimes these walkways are fairly elaborate, and sometimes incredibly simple. In the modern era, the tendency is toward more elaborate walkways, which are usually at the same height as the rails and even include handrails.
Open Deck Floor
Ties for open deck floors are 8" x 8" and 10 feet long. In HO scale that works out to be 3/32 x 3/32 x 1 3/8 inches. On bridges the ties are normally placed on 12-inch centers (9/64 inch in HO scale) and bolted directly to the stringers. The ties are further stabilized by bolting additional bridge ties to the ends of the ties already attached to the stringers.
A simple way to add a walkway to this type of floor is to make every third tie longer so it extends five scale feet (11/16 inch in HO scale) beyond the standard bridge tie. Cover the extended ties with 2-inch thick (0.023 inch in HO scale) planking. The walkway may be on either/or both sides of the track.
If the walkway is too narrow, or positioned in such a way that a moving train cannot safely pass someone on the walkway, then safety escapes need to be provided. Safety escapes are normally positioned every 100 feet or so. Bridges under 200 feet typically do not have any safety escapes.
For modeling purposes, there are several manufacturers who make bridge sections of flex-track, Campbell Scale Model, Micro Engineering and Walthers to name three. Walthers also has "V" transition sections of track to bring the guardrails together. If you prefer to hand-lay track there are several suppliers of wood ties that can be used.
Ballasted Deck Floor
The ballasted deck is basically formed by creating a closed floor and then laying ballasted track using the same track and methods used for standard track. The closed floor is usually a trough constructed of steel or concrete. The trough is filled with ballast and the track laid on it. The ties and tie spacing used are the same as for normal track. Guardrails are usually placed inside the rails. Needs for a walkway are same as with an open-deck bridge.
For modeling purposes, the trough should be about 13 to 16 ft wide. Our double track bridges are designed using a two inch centerline-to-centerline spacing. If you are building the double-track bridge, you may want to consider building one large trough that carries both tracks, rather than two narrower troughs, running side-by-side. In HO Scale, the trough for the single track bridge should be about 1.75 to 2.0 inches wide, and about 3.75 to 4 inches wide for the double track bridge.
We suggest you make the floor using heavy card stock, similar to that used for manila folders. Cut the material into long strips that are the width of the finished floor. Along the long outside edge, glue strip wood, about .063 square, so as to make a trough shape to hold the ballast. Glue this to the bridge floor and lay track using your usual method. Throughout all this work, be sure to keep the floor flat so the track will also run flat and not undulate.
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