Opening (Swing, Bascule & Lift)
The vast majority of bridges that people are aware of are fixed in their position and are so well integrated into the road structure that they become part of the scenery. Fixed span bridges are normally the preferred choice as there is no interference to the flow of traffic, no maintenance of machinery and a reduced risk of collision from ships.
The alternative to a fixed span is to change one of the spans so that it can be opened to allow shipping to pass upstream and downstream. For a fixed span bridge to allow ocean going ships to pass beneath, a clearance of 70 metres is normally required. The initial opening span bridges probably formed part of floating bridges on wide rivers where it was not known how to construct piers for a fixed bridge. In addition opening spans not only permitted ships to pass but also formed a line of defence similar to a drawbridge over a castle moat.
Opening spans are generally of three types: horizontal swing, bascule lift and vertical lift bridge.
The classical swing bridge is that of two symmetrical rotating spans on a pier located in the centre of the shipping channel. The structure acts as a two span continuous beam with the bridge closed and a double cantilever when open. The machinery is normally housed in the centre pier along with the control room. On the older bridges the bridge rotated on a toothed track located on the outer edge of the centre pier. In some of the modern day bridges the centre shaft lifts the bridge clear of the pier and then rotates it. Normally the bridge rotates 90˚.
In some cases the supporting pier is not in the centre of the two spans so in order to get a balanced system the weight of the shorter span is increased so that it is equal to that of the longer span.
In the 1800’s there were a small number of horizontal swing bridges constructed where the overall bridge was a single span but each half rotated about a support on each river bank.
One of the longest swing bridges built was in New York, USA, separating Staten Island from New Jersey. This was the Arthur Kill Bridge with two symmetrical spans of 76m built in 1890 and replaced in 1959 by a double span vertical lift.
The bascule lift bridge may be a single or a double leaf bridge. In the latter case both lifting leaves meet each other at the centre in the closed position. Some of the earliest bridges were constructed in timber but nowadays they are generally constructed of steel and many variations are in existence. One such variation is the Scherzer Bridge where the opening span is effectively built on the road deck. There are two pairs of Scherzer bridges in Dublin city in the Docklands. One pair sits at the mouth of George’s Dock and Custom House Dock, and the other is beside the Convention Centre, crossing the mouth of the Royal Canal. They are no longer in active use. Scherzer bridges did not require cofferdams and did not deteriorate from being submerged in water. The disadvantage however is that they are considered unsightly as the counterweight is overhead and the rotation space has to be kept clear of debris.
The basic principal of a bascule bridge is that it needs sufficient power to lift the open span in a vertical direction. This is achieved by ensuring that the bridge is balanced. That means a weight (called counterweight) equivalent to the weight of the opening span is fixed to the bridge deck but at the opposite side of the rotating arm. This ensures that the minimum of power is needed to lift the bridge. These counterweights are very often incorporated in structures so that as the bridge is lifted the counterweight travels downwards inside the structure and does not become submerged in water.
The bridge may be operated by mechanical means or by hydraulics. This is done from a control room with a clear view of the river. This type of bridge would generally open and close quicker than the other type of bridges.
The use of counterweights overhead and cables fixed to the deck are nowadays mainly used for short and medium size bridges. In the Netherlands this type of bridge is symbolic of the landscape being low lying, having a high water table and a multiplicity of canals.
A vertical lift bridge is one where the whole bridge deck, while remaining horizontal, is lifted vertically by pulling on the four corners using two or four towers at either end of the open span. While the maximum height it can be raised is determined by the height of the towers, this system allows the bridge to be raised to the minimum height for a ship to pass beneath it, thus minimising the opening time and delays to traffic on the bridge.
The operation of the bridge is by means of cables fixed to the open span. These cables pass over the top of the towers and are fixed to counterweights. The machinery ensures that the bridge travels upwards and the counterweights downwards. This ensures that everything is balanced during the vertical movement. The presence of the counterweights minimise the amount of energy needed to raise the bridge.
The vertical lift bridge has been very popular in the United States and many may be seen in New York. Where long spans are needed this type of bridge has proved itself to be more suitable and more economical than other types of opening bridges. The largest bridge in the USA is Arthur Kill Bridge, spanning from Staten Island to New Jersey, with a span of 170m and a height clearance of 41m. It was built in 1959 and replaced a horizontal swing bridge with two 76m spans.
After World War II vertical lift bridges developed significantly, particularly in Europe. The design of the towers, both in concrete and steel, improved both structurally and aesthetically using the newly acquired knowledge of materials and detailing the external finishes to high standards.
In some of the more modern short span bridges hydraulic jacks have been used to raise the bridge. The most modern vertical lift bridge is the Jacques Chaban-Delmas Bridge located in Bordeaux, which was designed by Michel Virlogeux. It has a total of 433m and consists of five spans; the central lifting span is 117m long, and it is flanked by four approach spans, ranging in length from 62m to 81m.