Pros and Cons of a Drawbridge | Types of Drawbridge

What is Drawbridge?

A drawbridge is a structure across a body of water with moveable parts that can be lifted, rotated, or swung to allow traffic to pass through the waterway on which it sits.

Draw bridges are built so that a section of the bridge deck, the surface on which vehicles drive across, can move. Drawbridges are movable bridges that can be found in many locations throughout the world.

Purposes of Drawbridge

1. It is probably pretty obvious.

2. It makes a drawbridge unique when it moves, river traffic is not blocked and therefore can flow easily.

Pros and Cons of a Drawbridge

Pros of Drawbridge

  1. We  can have free passage of any height when the drawbridge is up regardless of the level the road is on.
  2. We  can always close a drawbridge (by cutting the chains used to draw the bridge up)
  3. It is relatively simple to construct and – given the gap is short enough – can be constructed from one side, i.e. a log across a stream is a sort of drawbridge, but without the chains.
  4. We don’t need a long lead-up to clear a given height as with suspension bridges.
  5. Opening the bridges effectively cuts the passage (except for a few movies) thereby isolating the bridged parts.
  6. Bridge can be balanced with counterweights making it easy to open and close

Cons of Drawbridge

  1. The weight of the bridge is proportional to the length, the torque quickly becomes problematic as the length extends, thus it is better to have 2 opposing drawbridges than a single twice as long. Thus there is a limit to the gap your drawbridge can reasonably span.
  2. If the bridge chains cut, then the bridge has to remain closed for a long time.
  3. There is obstruction of traffic besides the bridge at the time of opening and closing. It also make the gap of free passage narrower than actual.
  4. There is risk of accident due to sudden failure of the lever and anchor/fulcrum.
  5. Moving parts are always susceptible to malfunction.
  6. We cannot run wires or pipes thougth the length of bridge as the some parrts of bridge have to be seperated with each other.

Types of Drawbridge

  1. Swing Bridge
  2. Bascule bridge
  3. Vertical Lift Bridges

1) Swing Bridge

A swing bridge is a movable bridge that has as its primary structural support a vertical locating pin and support ring. Usually at or near to its center of gravity, about which the turning span can then pivot horizontally as shown in the animated illustration to the right.

Advantages of the swing bridge

  1. The complete weight is significantly reduced as compared to other moveable bridges.
  2. The central support is often mounted upon a beam along the axis of the watercourses, It helps to protect the bridge from watercraft collision while opening.

Disadvantages of the swing bridge

  1. In a symmetrical bridges, the central pier forms a hazard to navigation. Asymmetrical bridges may place the pivot near one side of the channel.
  2. Where a wide channel is not available , a large portion of the bridge may be over an area that would be easily spanned by other means.
  3. A wide channel will be reduced by the center pivot and foundation.
  4. It may rotate enough to cause accident to the traffic.

2) Bascule Bridges

A bascules bridge is also referred to as a drawbridge (moveable bridge) with a counterweight that continuously balances a span, or leaf, throughout its upward swing to provide clearance for boat traffic. It may be single or double leafed.

Advantages of the Bascule bridge

1. It opens the waterway for ships and vessels with considerable speed.

2. It permits the passage of small size boats to pass through even if the passage is not opened completely.

3. Whether fully or partially opened, most of the bascules superstructure bridge is out of vessel reach during a collision, so it would not suffers considerable damage.

4. Both single-leaf and double-leaf bascule bridges provide obstacles for the cars, but a single leaf provides a barrier at one side of the road.

5. Double leaf bascules bridge offers the broadest spaces for vessels compared with other types of movable bridges.

Disadvantages of the bascule bridge

1. Bascule bridges are subjected to considerable wind load especially when it is opened. So, this should be accounted for during the design of bascules bridges.

2. The machinery used to control bascules bridges should be crucially strong and robust compared to the case where wind load is not present.

3. The stability of double-leaf bascules bridges is based on the adequate seating of leaves on their live load shoe, alignment of the leaves, and fitting the end of each leaf by their locks.

4. Any damages of these components due to differential temperature or wearing on the alignment components will lead to improper seating of the bridge, and consequently, the leaves will jump up and down under traffic which is not desirable.

3) Vertical Lift Bridges

A vertical lift bridge or just lift bridge is a type of moveable bridge in which a span rises vertically while remaining parallel with the deck.

Advantage of vertical lift Bridge

1. Vertical lifting angle can be built approximately with any length that is required according to the project location and it is only restricted by ultimate simple span.

2. The design and construction of vertical lifting bridges are easier compared with swing and bascules bridges.

3.  It is suitable to support heavy load structures like railroad bridges since vertical lifting bridge spans are approximately fixed.

4.  There is no restriction on the width and the number of trusses or main girders of vertical lifting bridges.

Disadvantages of verticle lift bridge

Pros and Cons of a Drawbridge | Types of Drawbridge

1.  The most outstanding disadvantage of a vertical lifting bridge is the restricted vertical span offered for the vessel passage. However, this has rarely caused undesired events.

2. The entire width navigation channel cannot be used due to hinders posed by vertical lifting bridges even when the bridges are completely opened.

3. Vertical lifting bridges do not demonstrate a satisfactory aesthetic appearance.

Happy Learning

Civil Concept

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"Structural Engineer" with over 5 years of experience in estimation, structural design, and surveying. I am passionate about using my skills to create safe and sustainable structures. I am also a keen writer, and I enjoy sharing my knowledge and experiences with others.