Issue 41 use this 10/4/06 12:50 Page 1 In the picture The definitive publication for bridge professionals worldwide | Issue No. 41 | Fourth Quarter 2005 | www.bridgeweb.com Issue 41 use this 10/4/06 12:51 Page 2 movable bridges A moving story The sheer scope and variety of movable bridges that have been built in the last decade demonstrate just a few of the exciting possibilities that are available to designers and architects. Charles Birnstiel, Jeffrey Routson, and Paul Skelton explain movable bridge is generally a bridge across a navigable waterway ropes that pass over sheaves at the rest piers of the lift span; these sheaves that has at least one span which can be temporarily moved in order are usually on top of the towers. The counterweights minimise the energy to increase vertical clearance for vessels sailing through the channel. required to move the span. Vertical lift bridges can be categorised by the drive Few of the thousands of existing movable highway and railway machinery location; the main types of balanced vertical lift bridges are: the Abridges are alike, considering their architectural, structural, and span drive, the tower drive, the connected tower drive, and the pit drive, or mechanical features. Yet movable bridges can be classified on the basis of the table lift. motion of their movable spans; in terms of the displacement and axes of The movable span of a swing bridge rotates about a vertical, or pivot axis. displacement (see table). If this is at the centre of the span, it is said be symmetrical or to have equal The movable deck, or leaf, of a simple trunnion bascule rotates about length arms; otherwise it is unsymmetrical or bobtailed. The self-weight of a a horizontal axis that is fixed in location. The shafts which define the axis of swing span is usually balanced about the pivot, hence bobtailed spans require rotation are usually physically fixed to the bascule leaf and rotate in bearings counterweights at the ends of the shorter arms. There are three sub-types; the mounted on the bascule pier. However, bascules have been built which have centre-bearing, the rim-bearing, and the combined-bearing. trunnions that do not rotate and the leaf rotates about them in bearings that On centre-bearing swing bridges, the movable span self-weight is are part of the bascule girders. In both cases, the leaf rotates about the fixed balanced on the pivot thrust bearing. To prevent the open draw from horizontal axis in a see-saw motion. tipping under unbalanced loads, such as wind, balance wheels are provided Simple trunnion bascules are often selected where unlimited vertical that roll on a large-diameter circular track concentric with the pivot clearance is required for navigation, and usually each leaf is nominally bearing. balanced by a counterweight fastened to the bascule girder. The counter- When the span is balanced these wheels normally clear the track by weight reduces the size of the mechanical system needed to operate the about 5mm; the intent being that the centre bearing support all the dead span, and provides a measure of safety if the mechanism or controls fail. load when the draw is open. Most centre bearings are plain mechanical There are many subtypes of trunnion bascules, distinguished by the location thrust bearings, consisting of a bronze disc sliding on a hardened steel disc. of the counterweight. However, anti-friction thrust bearings with a spherical rolling element are In 1893 William Scherzer received a US patent for a rolling bascule, being specified more frequently for new bridges. Hydraulic bearings that which became known as a rolling lift bridge. The leaf rotates about a support the entire dead load have also been used for centre-bearing swing transverse horizontal axis, which simultaneously translates in the longitudinal bridges. direction in a motion rather like a rocking chair. Almost all rolling bascules are On rim-bearing swing bridges, all the dead load of the superstructure nominally balanced by counterweights fixed to the bascule girder. Three is supported by a large-diameter rolling element bearing – a slewing common types of rolling lift bascules are; the deck type, the half-through plate bearing – or tapered rollers running on a large-diameter track. The tapered girder or truss (pony), and the through-truss. rollers run on a circular track whose diameter is usually about the same as Rolling lift bascule bridges are characterised by the cylindrically-curved the transverse spacing of the outer swing span trusses. When the bridge is end of the bascule girders or trusses, which roll on tracks. As the curved parts closed, the rim bearing supports both dead load and live load. Rim bearings of the bascule girders roll along the tracks, the bascule leaf rotates open or are used for wide or heavily-loaded swing bridges. The tandem bobtailed closed. Slippage between the girder treads and the tracks on which they roll swing bridge at El Ferdan, Egypt, that spans some 300m between pivot is prevented by lugs or teeth on the treads, which mechanically engage sockets piers, is rim-bearing. in the track, or vice versa. For a given channel width, the combined motion of Some swing bridges have structural framing systems which distribute the a rolling lift bridge allows for the use of a shorter bascule leaf when compared superstructure self-weight to the pivot bearing and the rim bearing in a to a simple trunnion bascule bridge. Medium and long-span simple trunnion bascules need bascule piers with deep pits if the roadway profile is close to the high-water level of the channel, in order that the counterweights remain dry during leaf rotation. To avoid this, Displacement Movable bridge type the counterweights may be located remote from the bascule girder, usually overhead. Rotation about a fixed transverse Trunnion Bascule One example of this ancient concept is the balance beam or ‘Dutch-style’ horizontal axis bascule bridge. To maintain the balance of the system for all angles of opening, the geometric figure defined by the trunnion at the rear of the leaf, Rotation about a transverse horizontal axis that Rolling Bascule the counterweight trunnion at the top of the tower, the hinge at the end of the simultaneously translates balance beam, and the hinge near midlength of the deck, must form a parallelogram. Rotation about a fixed vertical axis Swing US patents for bascules with remote counterweights were granted to Translation along a fixed vertical axis Vertical Lift Joseph Strauss around the year 1900 and bridges with his specific counter- weight arrangements are often called Strauss bascules. Three common Translation along a fixed horizontal axis Retractile & Transporter versions are the under-deck counterweight bascule, the heel trunnion with a vertical overhead counterweight, and the heel trunnion with a swinging Rotation about a longitudinal axis Gyratory counterweight. The distinguishing feature of all Strauss bascules is the parallelogram connecting the counterweight to the movable leaf. The leaf Rotation about multiple transverse Folding rotates about a horizontal axis. horizontal axes The movable span of a vertical lift bridge is raised along a fixed vertical The transverse axes mentioned in the table are usually oriented at 90˚ to the axis in order to provide additional vertical clearance for the passage of longitudinal axis of the bridge. vessels. It is usually nominally balanced by counterweights connected to wire Issue 41 use this 10/4/06 12:52 Page 3 movable bridges Woodrow Wilson Bridge CSX rolling lift bridge A simple trunnion bascule bridge is currently under construction at Washington, DC in The CSX rolling lift bridge over the East Pascagoula River at Pascagoula in Mississippi the USA. It has eight leaves providing for 12 lanes of Capital Beltway vehicular traffic carries a single-track line of the CSX railway; it is a through-truss Scherzer type with an and capacity for a future light rail line across the Potomac River. The total width of the overhead counterweight. The span is 51.4m and the leaf width is 6.7m, centre to centre leaves is 71.4m and the span between the trunnion bearings is 82.4m. One of the Warren trusses. The radius of roll is 8.2m and the bascule can rotate to an architectural feature is the open inverted delta bascule piers which permit the trunnions opening angle of 71 degrees. The span drive machinery consists of two low-speed to be located outboard of the pier, thereby reducing the cantilever spans of the bascule high-torque radial piston hydraulic motors driving the pinions through reduction leaves. The floor system consists of the bascule girders (two per leaf), floorbeams, gearing The machinery system was designed to hold the leaf at an opening angle of 71 stringers, and a reinforced concrete deck which is made composite with the stringers degrees under a sustained wind load of 2.4kPa; in the event of a hurricane, the span is by means of stud shear connectors. Midspan locks were designed to provide moment closed and tied down. The consulting engineer is Hazelet & Erdal, the owner is CSX transfer in addition to vertical shear transfer, for live loads. The leaves are fully counter- Transportation. weighted and the bascule span drive machinery is of a conventional, mechanical design that has been thoroughly tried and tested. Alternating current induction motors (flux CSX bridge over vector controlled) rotate the leaves through reduction gearing. the East Hardesty & Hanover is the engineer for the bascule portion of the bridge; Parsons Pascagoula River Transportation Group is the prime design consultant and the general engineering consultant (Hazelet & Erdal) is a joint effort of Parsons Brinckerhoff, URS Greiner, and Rummel, Klepper & Kahl.