2.9.1 Introduction
Movable bridges are bridges that allow marine traffic to go under or through the bridge opening by altering the position of the superstructure during passage of traffic beneath the bridge. The most com- mon use for movable bridges involves allowing for passage of boat or marine traffic through the bridge opening. The principal advantage of this type of structure is that a very flat grade can be maintained on the roadway as vertical clearance for boats or ships is provided by moving or opening the bridge to marine traffic. The main disadvantages to this type of structure result from the disruption of highway or rail traffic when the bridge is open, the impediment to marine traffic when the bridge is closed, and the expense of having personnel on duty to maintain bridge operations.
There are many types of movable bridges and the diversity of these structures is a tribute to engineers and their various designs. Movement is usually provided by a mechanical system that is powered by electric motors using operating winches, gearing, or hydraulic pistons.
Most of the movable bridges can be grouped into three main types: vertical lift bridges, swing span bridges, and bascule or draw bridges. Specific examples of these types of bridges with a brief description of the movements are discussed in the following sections.
TABLE 2.7 Key Facts of the Three Sisters
Sixth Street Bridge Seventh Street Bridge Ninth Street Bridge Official Name Roberto Clemente Andy Warhol Rachel Carson Total Length Main Span Side Spans 884 ft (269.4 m) 430 ft (131.1 m) 215 ft (65.5 m) each 884 ft (269.4 m) 442 ft (134.7 m) 221 ft (67.4 m) each 840 ft (256.0 m) 410 ft (125.0 m) 215 ft (65.5 m) each Roadway Width Vehicular Lanes Sidewalks 38 ft (11.6 m) 2 2 38 ft (11.6 m) 2 2 38 ft (11.6 m) 2 2 Navigation Clearance 40 ft (12.2 m) 40 ft (12.2 m) 40 ft (12.2 m) Opened to Traffic October 19, 1928 June 17, 1926 November 26, 1926
2.9.2 Sacramento Tower Bridge
The Tower Bridge (Figure 2.63) in Sacramento, California is an example of a vertical lift bridge. This is a type of movable bridge where the span rises vertically while remaining parallel with the bridge deck. The span is raised on towers by cables and counterweights located at the ends of the movable span. The bridge carries State Route 275 over the Sacramento River and was one of the first vertical lift bridges on the California Highway System. The bridge carries four lanes of vehicular traffic over the river. The overall bridge length is 737 ft (225 m) while the lift span portion of the bridge is 209 ft (63.7 m) and pro- vides 100 ft (30.5 m) of vertical clearance above high water when raised. Two towers are supported by two concrete piers at a depth of 50 ft (15.2 m) below water.
The bridge was designed and is owned by the California DOT. The bridge was constructed by Sir William Arrol and Company and George Pollock and Company. The bridge was opened to traffic in December 1935 at a cost of approximately $666,000.
2.9.3 George P. Coleman Memorial Bridge
The George P. Coleman Memorial Bridge (Figure 2.64) in coastal Virginia is an example of a swing span bridge. This is a type of movable bridge where the superstructure turns or pivots horizontally about a support pier to allow marine traffic to pass through the bridge opening. The bridge is a double swing span bridge and carries US-17 over the York River between Yorktown and Gloucester Point, Virginia. The bridge carries four lanes of vehicular traffic over the river. The overall bridge length is 3750 ft (1140 m) while the turn span portion of the bridge is 450 ft (150 m) and provides 60 ft (18 m) vertical clearance above high water in the closed position.
The bridge was originally built in 1952 but was reconstructed and widened in 1995. For the 1995 reconstruction, the new swing spans were floated into position in a finished state to minimize disrup- tion to traffic on the bridge. The 1995 reconstruction was designed by Parsons Brinkerhoff and the construction was performed by Tidewater Construction Corporation. The reconstructed bridge was reopened to traffic in 1995 at a cost of approximately $76.8 million.
2.9.4 Woodrow Wilson Memorial Bridge
The Woodrow Wilson Memorial Bridge (Figure 2.65) located near Washington, DC, is an example of a bascule or draw bridge. This is a type of movable bridge where the bridge opens by rotating about a horizontal axis parallel to the waterway. The Woodrow Wilson Memorial Bridge crosses the Potomac
River between Alexandria, Virginia, and Oxon Hill, Maryland. The bridge carries I-95 and I-495 over the Potomac River and is one of only a handful of movable bridges on the U.S. interstate system.
The Woodrow Wilson Memorial Bridge consists of two parallel double leaf bascules that carry twelve lanes of vehicular traffic over the Potomac River. The overall bridge length is 6736 ft (2053 m) while the Bascule portion of the bridge spans 269 ft (82 m) and provides 70 ft (21 m) of vertical clearance in the closed position. At the present time, the bridge opens approximately 65 times a year to allow ship traffic to pass the bridge.
The movable portions of the bridge were designed by Hardesty & Hanover, LLP with Finley McNary Engineers designing the unique precast, post-tensioned V-piers. The bridge was constructed by the American Bridge Company and Edward Kraemer & Sons, with PDM Bridge, LLC doing the steel construc- tion for the bascule span. The bridge was completed in May 2008 at a cost of approximately $826 million.