How To Series ... Building A Brass Bridge (Page #1)
The Floorbeam and Post Assembly
The bridge I will be using is the PRR 380ft Whitford Bridge. The same techniques and principles can be found in almost any bridge models. Almost any bridge will have the same structural members. The Track is supported directly on the Stringers which are under the rails. The Stringers are supported by the Floorbeams. The Floorbeams are supported by the Posts and the Posts are supported by the Chord that gives the bridge it's familiar shape. There are a lot of other parts and we will get to them. The ones mentioned are the main ones for now. Let's get building.
Get the components together to build the Floorbeam. The Main Web, Upper Capstrips, Lower Capstrips and the Detail Overlays that add the metal layers to the Web. Cut the parts at the ties that hold them together and file off the tie remnants.

Begin with the Main Web and the Bottom Capstrip. Notice that the Web has tabs that fit into the notches that are in the Bottom Capstrip.

Secure the Bottom Capstrip to the Web ensuring that the Cap is perpendicular to the Web. Attach the second Bottom Capstrip over the first.

Set aside when done.



Get the components together to build the Posts. They consist of the Post Frame, X-Bracing and Detail Overlays (not shown).

Before bending the Posts, attach the X-Bracing to the Post Frame. This will keep the Frame from distorting during the bend process. Be sure to attach both X-Bracings.

The Detail Overlays are shown in this image. They will be added after the Post has been bent.



The posts actually have five sides. That is, the post has one small side that will fold under another side to allow the sides to be bonded together. More references for bending brass can be found in our Bending Brass Tutorial. I use the hand seamer for bending.

The big secret in bending brass columns is: do not try to bend all the sides to 90 degrees. It just won't work! Instead, you bend the first and the last bends to 90 degrees and all the rest to 45. Once the bends have been started, it easy to finish them. Bending the intermediate bends allows to get the bender in and get the required bends to 90 degrees.

Start by bending the small "fifth" side 90 degrees to the solid side of the Post. If using the seamer, bend one section about 1/4 of the way, then the next section and so on.



Holding the Post, begin bending the next side to 45 degrees. Attaching the X-Bracing first now makes the open Post Frame quite ridged.

Once the second bend is made, bend the third side to 45 degrees as done with the second.

Finally, bend the last side to a full 90 degrees.




At the end of the initial bending process, the Post will look something like the image. It is now quite easy to complete the intermediate bends to bring the first and last sides together for attachment.

The small side will go under the side with X-Bracing.



Starting and one end and working in sections, secure the first and last sides together using either CA glue or solder. Ensure the end of the side with X-Bracing is aligned with the corner.

Actual bridge Posts are constructed with two solid sides, angles secured to the edges of the solid sides and X-Bracing between the angles. To duplicate that construction, we need to apply a Detail Overlay for the solid side.

Since there are slots and holes in the solid sides, they must be accurately aligned. I use some tubing of the same diameter to help line things up. The inside of the Post has the large slot and a smaller slot towards the top. Be sure to use the right Overlay and orientate it correctly.

 Put some thick slow CA glue one one end and use a toothpick to evenly distribute the glue. Use the tubing to line up the holes. Working from one end to the other, push the overlay onto the side. Use some TP to soak up excess glue.

Once one end is secure, lift up the other end and apply/distribute glue on the other end. Use the tubing to align the holes and work the Overlay on to the side from the center out. Here is the look that we wanted to achieve.




After both Overlays are added to the Posts, we are ready to add the Floorbeam. Notice the slot in the Post. Notice the big tab on the ends of the Floorbeam.

The Floorbeam tab is inserted into the Post slot. You may have to use your knife point to open the slot if you got glue in the slot.

Use a square and then secure the Floorbeam tab to the Post from the inside of the Post. Glue or solder the tab to both sides of the Post.

Install the second Post to the Floorbeam using a square.

Actual bridge Floorbeams are constructed from a solid steel sheet referred to as the Web. Other sheets and angles are then attached to the web to strengthen and stiffen the Floorbeam as a whole.

To duplicate that construction, we need to apply a Detail Overlay over the web between the two Posts. It is very important that all tie remnants are removed before attaching to the assembly.


Start with one Overlay and test fit both corners one at a time. As long as all the tie remnants have been removed, it should fit perfectly. Apply some thick SLOW CA glue to the Overlay JUST around the end that will go into the corner.

Install the Overlay ensuring that it is snug against BOTH the Post and the bottom Capstrips of the Floorbeam. Don't worry about the rest of the Overlay as long as the bottom of the Overlay is snug against the Capstrip. Use TP to remove excess glue. Wait five min before doing the next corner.

Before doing the next corner, some advice. The Overlay must, and has, tight tolerances. It is likely that in the next step that the center of the Overlay may bulge slightly off the Floorbeam. It will depend on how snug you got the Overlay into the first corner, so expect this. Some filing of the unattached end could totally eliminate it. If slight, don't sweat it and it will come out later.

Gently bend the Overlay upwards so you can place glue on the other end. Remember, toothpicks are great for spreading glue around and keeping your fingers far from the glue. With the glue on the Overlay, get the it into the corner trying to eliminate any bulge.



Battle of the Bulge ... We are going to secure the bottom of the Overlay to the Floorbeam and remove any/all bulging. It will be assimilated! Use a toothpick and get some glue between the Web and the Overlay at the bottom. Use TP to remove excess glue. Use the Hand Seamer to compress the entire bottom of the Overlay onto the Floorbeam. Hold with heavy pressure for two min.

Repeat for the other bottom.


The top of the Overlay is secured to the Floorbeam Web using process. Place glue on one half of the Floorbeam top. Use TP to remove excess glue. Use the Hand Seamer to compress the entire top of the Overlay onto the Floorbeam. Hold with heavy pressure for two min.

Repeat for the other top half.


The Mid Sway Brace is a structure on the bridge to keep the Posts verticle. Forces tend to cause the Posts to lean in one or the other direction. When this happens, the structure looks like a parallelogram instead of a rectangle. The Mid Sway Brace, and the cross braces to the Top Chords, hold the Posts verticle.

Sway Braces are almost always built-up from two angle bars secured together to form a "T" bar with X-Bracing in between them. The "T" bars will be at both top and bottom of the Sway Brace.

To duplicate this structure, get the items shown from the kit sprue. The main body of the Sway Brace, the CapStrips with an etched line to simulate the angles and the Rivet Overlay for the angle sides.

Begin by securing the top and bottom CapStrips to the main body. The main body has tabs and the CapStrips have slots to align and hold the body in the center of the strips. Secure the Capstrips.

Now add the Rivet Overlay down the center of the Sway Brace. This adds all of the detail and metal layers to complete the duplication of the prototype fabrication.






Add the completed Mid Sway Brace to the Posts and secure the tabs of the Brace from inside the Post.

 

Sway Brace Detail - Left Side

 

 

 

 

 

 

Sway Brace Detail - Right Side

 

 

 

 

 

Sway Brace Detail - Outside of Post



 
The final step in the construction of the Floorbeam and Post assembly is to cut through the Floorbeam Detail Overlay with a cutoff wheel so the Stringers can slide into place.

 

This concludes Chapter #1. Next chapter ... The Stringers


 

Note: This document is preliminary. Items contained within may change without notice.
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