Movable bridges are designed and constructed to change its position and occasionally its shapes to permit the passage of vessels and boats in the waterway. This type of bridge is generally cost effective since the utilization of long approaches and high piers are not required.
When the waterway is opened to vessels and ships, traffics over the bridge would be stopped and vice-versa.
There are different types of movable bridges and many of them are not construct nowadays while some of them are still considered as a movable bridge.
Different types of movable bridges which are still desirable and applicable are discussed.
Fig.1: One Type of Movable Bridge in London, UK
Fig.2: Movable Bridge in Chicago, USA
Types of Movable Bridges
Various types of movable bridge are available, but three of them are significantly desirable and practical are discussed in the following sections and number of special types of movable bridge will be discussed as well:
- Bascule bridge
- Swing bridge
- Vertical lifting bridge
- Special types of movable bridge
Bascule bridge, which is also called as drawbridge, is fixed and supported on an axis which is perpendicular to the bridge longitudinal centerline axis.
The horizontal line on which the bridge is pivoted is commonly located at the center of gravity of the bridge to create a balance between the weight of the bridge on either side of the horizontal pivotal axis.
It should be known that the weight balance is not accurate and slight inaccuracies are provided based on the future utilization of the bridge, for example, if the tendency is to open the waterway, then the weight is distributed to help opening the bridge and this weight distribution is termed as counterweight heavy, but if the trend is to employ the weight for closing the bridge, then it is called heavy span.
There are two major types of bascule bridges including single leaf and double leaf bascule bridge. In addition to tripe and quadrable types which are occasionally constructed. The term leaf is used for the part of the bridge that moves and opens the waterway consequently.
Fig.3: Single Leaf Bascule Bridge
Fig.4: Double Leaf Bascule Bridge
Bascule bridge is a suitable type of movable bridge for most situations. Not only does it structurally sound and reliable but also both construction and operation can be carried out economically.
Vertical Lifting Bridge
It is one of the most widely constructed and used type of movable bridge. It is composed of a span commonly truss type span which is supported by towers at the end of the span or at each corner of the span.
Counterweight is usually used to balance the weight of the span. Ropes, which travels over counterweight rotating sheaves fixed on towers, are utilized to connect the end of the span to the counter weight. Added to that, waterway is opened by moving the span up exactly in vertical direction.
Types of vertical lifting bridge includes double, triple and quadrable and the last two types are suitable for crowded areas such as in front of terminals.
If the machinery used to open and close the water way is fixed on the span, then the bridge is called span drive vertical lift bridge, whereas the bridge is termed as tower drive vertical lift bridge if machineries applied to lift and down the lift span is fixed on towers, in addition to many other variations of vertical lift bridges.
This type of bridge is considerably suitable for locations or cases where long spans are required because vertical lift bridges are substantially stable.
Fig.5: Vertical Lifting Bridge
Fig.6: Details of Lifting Bridges
Swing bridge is fixed on horizontal plane that turns around vertical axis to provide ways for vessels and ships to travel through the bridge. The horizontal plane is on a bearing installed on a pier which is termed as pivotal pier.
When the swing bridge is closed, the end of the span should be supported by resting piers or abutments if the total length of the bridge span is not very long. Machineries used to open and close swing bridge is more complicated compared to other types of movable bridges. The end of its span should be free during opening and closing that is why retractable rollers, wedges, shoe or jacks are introduced to lift the end of swing span.
Therefore, swing bridge moves horizontally around vertical axis to provide water way and vertical movement is not involved whereas other types of movable bridges need to move vertically to provide passage spaces for vessels.
From this discussion, one can conclude that the influence of wind force in swing bridge is less than that of bascule and vertical lifting bridge. However, the effect of wind force cannot be neglected since it can increase overturning moment significantly especially if the bridge is fixed on a bearing at its center.
The swing bridge should be supported both horizontally and vertically to carry traffics and prevent overstressing. This is because bridge stabilization due to gravity is not available as in the case of bascule and vertical lifting bridges
The span of swing bridge can be either truss or plate girder which is developed in latter times. The latter is more desired to be used since it is cost effective.
Fig.7: Swing Bridge
Fig.8: Swing Bridge
Special Types of Movable Bridge
Bridges that fall into this category are rarely constructed and uncommon. The decline in the application of such movable bridges is due to some factors such as the increase of applied loads, newly developed materials and safety precautions and concerns.
Retractile, pontoon retractile, pontoon swing, shear pole swing, Folding, Curling, removable spans, Submersible bridge, Tilt bridge, Transporter bridge, Jet bridge etc. are the special types of movable bridges.
Fig.9: Folding Bridge
Fig.10: Transporter Bridge
Fig.11: Pontoon Bridge