Movable bridges are used to provide both waterway for vessels and ships to pass though the bridge and provide traffic way for cars, trains, and other forms of transportation.
There are different types of movable bridges which can be constructed based on conditions of the project site and bridge related factors. Certainly, each type of movable bridge has various advantages and disadvantages.
Benefits and drawbacks of three distinct type of movable bridges including bascule, vertical lifting bridge and swing bridges are discussed.
Advantages and Disadvantages of Bascule Bridge
Advantages of Bascule Bridge
- It opens the water way for ships and vessels with considerable speed and it permits the passage of small size boats to pass through even if the passage is not opened completely.
- It is reported that, the passage of small boat through partially opened bascule bridge is safer compare with partial opening of vertical lift bridge and swing bridge, especially if the bascule bridge is double leaf.
- Whether fully or partially opened, most of bascule superstructure bridge is out of vessel reach during collision, so it would not suffer considerable damage.
- The time required to pass vessels through bascule bridge is smaller than that of vertical lifting bridge and swing bridge. This is because vessels may come closer to the partially opened bascule than partially opened swing or vertical lifting bridge.
- Both single leaf and double leaf bascule bridge provide obstacles for the cars, but single leaf provides barrier at one side of the road.
- Double leaf bascule bridge offers the broadest spaces for vessels compared with other types of movable bridges.
- The depth of the span that extended from the pier to the center of the bridge can be decreased.
Disadvantages of Bascule Bridge
- Bascule bridge is subjected to considerable wind load especially when it is opened. So, this should be accounted for during the design of bascule bridge.
- The machinery used to control bascule bridge should be crucially strong and robust compared to the case where wind load is not present.
- In the double leaf bascule bridge where the behavior of each leaf is similar to that of cantilever in carrying live loads, shear locks are provided at the location where the end of each leaf meet which normally at the center of the channel. These shear locks are likely to suffer from wearing. This is because road dirt and other detrimental material would pollute lubricant material, and heavy traffics will impose serious shocks on the locks. The shocks will be greater as the locks are getting weaker and consequently the locks would not be suitable to perform their tasks.
- The stability of double leaf bascule bridge 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. Any damages of these components due to differential temperature or wearing on the aligning components will lead to improper seating of bridge, and consequently the leaves will jump up and down under traffics which is not desirable.
Vertical Lifting Bridge Advantages and Disadvantages
Advantages of Vertical Lifting Bridge
- 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.
- The design and construction of vertical lifting bridge is easier compared with swing and bascule bridges.
- It is suitable to support heavy load structures like railroad bridge since vertical lifting bridge spans are approximately fixed.
- There is no restriction on the width and the number of trusses or main girders of vertical lifting bridge.
- Since vertical lifting bridge does not turn around in relation with railroads, double deck vertical lifting bridge is possible to construct and upper and lower decks can be moved up or down disregard of each other. Therefore, traffic of cars or trains need not to be blocked completely during the passage of small vessels that only lower deck required to be lifted.
Disadvantages of Vertical Lifting Bridge
- The most outstanding disadvantage of vertical lifting bridge is the restricted vertical space offered for the vessel passage. However, this has rarely caused undesired events.
- The entire width navigation channel cannot be used due to hinders posed by vertical lifting bridges even when the bridge is completely opened.
- The construction of vertical lifting towers is expensive. This is because towers should be at least 18m taller than required vertical space due to machineries and rope connection and as the height of towers increase the influence of wind load increases and consequently the construction cost rockets disproportionately.
- Vertical lifting bridges does not demonstrate satisfactory aesthetic appearance.
Advantages and Disadvantages of Swing Bridges
Advantage of Swing Bridges
- Wind load on swing bridge is minimum compared with other types of movable bridges.
- Since swing span moves horizontally during opening of the bridge, the moment generated by wind force is smaller compared with other movable bridge types (bascule and vertical lifting bridge).
- Two movable spans in one moving structure can be achieved in symmetrical swing bridge. This would be greatly advantageous to manage busy water way properly.
- It is a desirable option for locations where aesthetic play significant role in the construction of movable bridge because swing bridge does not move up to open so aesthetic of the bridge would not be affected as the bridge is opened for ships to pass through.
- Sizable piers are not required to support swing bridge because it neither lift during opening nor need counterweight as it is the case in bascule and vertical lift bridge.
- It is possible to construct double deck swing bridge because it does not lift into air to open.
Disadvantages of Swing Bridge
- It requires considerable maintenance because of large number of moving elements. So, this factor would make this type of bridge undesirable option when there is shortage of labor force.
- It needs longer times to operate compared to other types of moving bridge because it has larger number of main mechanical functions to undergo during opening and closing. Therefore, longer times will be needed to open and close the water way.
- Generally, it is assumed that wind load does not affect swing bridge considerably, but this statement is not entirely true and wind load may impose noticeable effect on the bridge for example it may exert shocking load on machinery of the bridge and as result machineries might fail to operate.
- Swing bridge needs more machinery to open and close the water way compared to bascule and vertical lifting bridge.
- The construction of supporting piers at the center of the channel provide two-way marine traffic, but this makes the bridge vulnerable to bridge collision and the size of vessel that can travel though the bridge may be decreased.
- Tools or devices used to detach swing railroad bridges is considerably expensive and fragile. That is why numerous undesired events occurred due to malfunction of such devices.