Post 2: The Trunnion Bascule Bridge

The Trunnion Bascule Bridge, which exists in both single and dual-leaf forms, is probably the most generic and well known type of movable bridge. 'Bascule' comes from the French for 'Balance' and indeed that is what this design is all about. A large piece of concrete or some other counterweight is used to offset the weight of the span. To open, the span rotates on a trunnion, and as the counterweight goes down, the bridge goes up.

Let's see if we can't have a closer look at the parts of the bridge and how this works:

 

Sorry, I just love playing in AutoCAD. Anyway, we can see the center of rotation is the Trunnion, which in real life varies in diameter but can approach 2 feet, upon which the entire rotation is based. The trunnion is precisely machined (often of bronze) and is effectively a very large bearing. The large diameter is required because during rotation, the entire weight of the span and counterweight are more or less concentrated on it. The Counterweight, which is shown here as sort of a polygonal shape with diagonal stripes, is made of concrete and depending on density requirements can often include metal inserts for added weight. The last features of note here are the Live Load Shoes, which, when the bridge is closed, serve to carry the live loads of vehicle traffic instead of having them concentrate on the trunnion. 

For simplicity's sake, I drew a typical single-leaf bridge. In bascule bridge terminology, a 'leaf' is simply the bridge deck, i.e. the moving part. There can be single- or dual-leaf designs. In the case of dual-leaf, instead of a live-load shoe at the 'toe' of the bridge leaf, there is a another bridge leaf. In order to limit relative vertical deflection between the bridge leafs at the joint, Span Locks are used. The most common span lock system is a linear prong which extends from one side of the joint to the other and slides into a receiver there.  (A similar arrangement also secures a single-leaf bridge to its footing.) On the Lake Washington Ship Canal's four trunnion bascule bridges, these span locks are audible from the shore, and on the Montlake bridge especially, somewhat visible, particularly the flywheel of the span lock motor. The next time you walk on one of these bridges, look down through the joint on either side and try to find the span locks. They should be the only thing that bridges the gap from one side to the other. 

Below are a couple of photos which should clarify the live load shoes.  The bridge is the Fremont Bridge on the LWSC in Seattle, Washington, and is one of the busiest movable bridges in the United States.

Overview from near-elevation at about 1/3 full open

Here we see the live load shoe and contact plate in both the open and near-closed position. As is plainly seen, the bridge rotates down and settles into position in the saddle shown. Keep in mind the massive scale of the components here, the railing is 3ft high or so, and as such the shoe and plate are probably around 18 inches square.

I know we covered a lot in this post and even still have not mentioned how these bridges are pulled open and shut. Next time we will look to Alameda, California and the Park Street Bridge which illustrates the gear system quite well. I may also take some pictures of the drive system on display at the South Park Bridge in Seattle, to exemplify the motors and gearing involved. Until then, see you soon!

Welcome/Post number 1

Hello drawbridge enthusiasts and/or those with curiosity about drawbridges! 

Welcome to a blog which I hope to craft into a decent mode of passing on the things I have learned about my fascination with drawbridges!

First, a bit about me. I am a 20 year old Civil Engineering student at the University of Washington in beautiful Seattle, Washington, where I have the pleasure of exploring the 9-½ drawbridges located here, in addition to some a little farther out. (What is 'Half' a drawbridge? We'll get to that.) Anyway my current goal in Engineering School is to go on to become a drawbridge engineer. My fantasy goal is to become the preeminent movable bridge engineer in the world, who becomes the international authority on the topic and whom municipalities all over the world come to for advice and consulting. It's a big fish idea, but thankfully, movable bridges are a small pond in the world. It is a very niche market, but I hope to make my mark.

Anyway the first couple of posts are going to give a brief overview of drawbridges and the history, types and functions of the drawbridges in the world, and then continue on to photos of specific examples. Enjoy!

Post 1: The Movable Bridge

 

Salmon Bay Bridge in Seattle, Wa.

A movable bridge, as implied by name, is a bridge which can be moved, nowadays usually with the primary goal of affording more channel space to vessels than could be provided with a fixed span bridge. When we hear the term drawbridge, we often think of castles and moats, and indeed early movable bridges were of that type, deployed for defense over moats. It was not until industrialization, however, that movable bridges began to take the forms we know today. Certainly they existed in wooden forms. Vincent Van Gogh was familiar with them as his paintings show, but it wasn't until the advent of steel as a building material that large spans could be crossed (However, large wooden swing spans did exist).

Anyway the movable bridge evolved into three main types, each with various subtypes to go along, as well as minor types. The three basic bridges are:

The Swing Bridge: Rotate the bridge in a horizontal plane to clear the channel
The Vertical-Lift Bridge: Lift the bridge straight up out of the way of the vessel
The Bascule Bridge: Use a large weight to see-saw the bridge out of the way

With the wonders of autoCAD, I will now attempt to illustrate what is going on.

And here are physical examples of each:

Alright that does it for post 1, over the next couple of posts we will delve into the evolution of each type and the particulars. See you soon!