Ultimate ‘Scrap-Building’: Bascule Bridge – Introduction

Ultimate ‘Scrap-Building’: Bascule Bridge – Introduction

Those of you who are stuck with me for a while obviously know about my N Scale Layout, Wrightsville Port. Set in the backdrop of the 1960’s East Coast of the United States, this fictional layout is about the rather less eventful life of a small, suburban port town. You would see the unimpressive appearance of almost everything, from the ships, to the boats, the trains, and even the buildings and streetlights, deliberately modeled that way. The structures are built and decorated to be old, ordinary, and heavily weathered by the sea air. However, there are a few things that are ‘slightly more extraordinary’ than the rest.  An operating Bascule Bridge is one of them. This bridge is the entry point to the main quay, the grain and oil facilities, and is located in a very prominent place in the layout.  It is bound to attract the attention from anyone viewing the layout for the first time. Moreover, the fact that it the ‘leaf’ (deck) actually lifts is very exciting when in the middle of an operating session.  It gives a very realistic sense of operating your railroad in a port setting. If you are insane about animation, you can arrange for a small boat or ship to cross under the open bridge.  I can then guarantee you that it will be very difficult for you to keep your visitors interested in anything else on your layout! Here is a quick video of an operating session where you see how this little bridge can make a huge difference in appearance and can even play a role in the operating session.

I have noticed that more people are taking an interest in waterside railroad modeling these days. I believe the most common reason for it is that you can bring in much more variety by adding the element of water than you can achieve with anything else. Whether it’s increasing the scenic and aesthetic value of the build, thinking beyond stereotype operation, or bringing more options into a confined space, waterside layouts are gaining popularity amongst serious railroad modelers. However, building a waterside layout, particularly a port, is still considered to be a niche in the model railroad community for multiple reasons; a very important one being the requirement for heavy duty scratchbuilding and/or kit bashing. Some suitable waterside models (Boats, port/quay detail parts, ships etc.) are commercially available now, but the cost, even as kits, is quite steep. In addition, even if you are ready for heavy-duty scratchbuilding and kit bashing, getting the right reference materials is often problematic.  In most cases you will find only a handful of people who have built something that you plan to build, or suitable drawings. The only way (in my humble opinion) to overcome this Catch 22 situation is to spend time researching your subject.  Finding/getting the right documents/drawings and talking to the right people (these people may be far from the railroad modeling community, but much closer to the real thing, in the real world) may be a problem, but Google and different online forums might help you find a few. At least that’s how I progressed. Since I have already ‘invented the wheel’ regarding modeling this bridge, I would love to share how I found effective solutions to some of the most critical challenges that I faced while building Wrightsville Port, and in a very cost-effective way. In most cases, other than the time and labor, the material cost is virtually zero. I am a big advocate of recycling and used absolute scrap materials from other models, throwaway household stuff and waste materials to build my models, hence the term ‘Scrap-building.’

What is a ‘Bascule Bridge?

In the real world, a Bascule Bridge is one of the cleverest inventions, using the principles of physics in a most effective manner. The word Bascule (French origin) means ‘balancing.’ The principle of this type of bridge is that it uses the weight of the span (deck, also known as the ‘leaf’) and its concrete counterweight to continuously balance each other, and thus, can be moved with relatively little effort. In fact, the design is so clever that only the initial ‘push’ matters, the weights balance themselves in such a unique way that the bridge can then be drawn (or closed) with a minimum expense of energy. There are many variations of a Bascule Bridge (also known as a draw bridge). The one that particularly stands out, The Strauss Bascule Bridge was perfected and patented by Joseph Strauss of Golden Gate Bridge fame. From a modeler’s perspective, this design is the most desirable, not only because of its famous origin, but also because of its unique (and in my opinion beautiful, in a very mechanical way) appearance that is bound to draw attention from someone completely unknowledgeable of bridges.

Generally, this is a complicated structure.  Some distinctive examples are the St. Charles Air Line Bridge in Chicago, Salmon Bay Bridge in Seattle, Johnson Street Bridge in Victoria Harbor, British Columbia, Canada and the Pacific Electric’s Bascule Bridge in San Pedro, California. There are plenty more, but these are my favorites and should be a good starting point for the subject. A quick Google image search will show you that some of these structures are so complicated that it is a difficult task to replicate the intricacies of these prototypes even in a larger scale, therefore, for N scale, the structure needs to be simplified. Unless you are truly a fine scale modeler, and you’re planning to build an exact replica of one of the complicated prototypes, the simplified truss design will give you joy for a long time. If you are planning to build a fine scale model, and you want to build the intricate truss designs of these prototypes, then I believe this article is an excellent starting point. Your path will, most probably, lead you to use high quality styrene or metal truss components and other professional model supplies.

Principles of a Bascule Bridge

In Sketch 1, I have illustrated the basic principle of a bascule bridge. As mentioned earlier, the weight of the leaf and the concrete counterweight are equal, hence a relatively small force is required to provide the initial thrust. Once the bridge starts moving, it requires much less external force, just that required to keep control in the operator’s hand. There are 4 primary parts to this type of Bascule Bridge (also known as a Strauss-Trunnion Bascule Bridge. The leaf of the bridge rotates around a heavy-duty pivot that is known as a Trunnion, hence the name):

  1. The Leaf/Bridge Span: For a small/medium span (65-120 scale feet/20-50 scale meters), this can be a simple Pratt or Warren truss design. For a longer span (over 120 scale feet/50 scale meters), a modified half Pennsylvania, Camelback or Parker design may be used. This is to achieve a decreasing weight ratio from the trunnion to the far end.  This ensures more stability over a longer span. This is the exact concept that is used in building a prototype Bascule.
  2. The Tower: This is the only fixed structure in the whole assembly. The purpose of the tower is to provide a frame of movement for the moving parts, the leaf, and the rocking truss. This structure also houses the trunnions for both the leaf and the rocking truss.
  3. Rocking Truss: This is a moving component that houses the concrete counterweight that is fixed to this truss.
  4. Counterweight Link: This is the link between the leaf and the counterweight through the rocking truss. This is a simple link pivoted at the top of the end posts of the span (facing the tower), and the rocking truss.

The most important design consideration is to maintain the proper length ratio so that, when connected, the Counterweight link, and the end posts of the leaf, the tower and the rocking truss essentially form a parallelogram. Notice that when the bridge moves the interior angles of the parallelogram constantly change, but it remains a parallelogram. This is crucial for proper alignment of the tracks and the bridge, as a whole, both in the real world and in the scale model world.

In the next post, I will talk about developing a reference drawing. Make sure to subscribe to the blog to receive the article streight in your inbox.

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