How To Make A Draw Bridge To Hold 25 Pounds

Describe Bridge

Background

A bridge over a navigable waterway must let boats and ships to cross its path, usually by beingness tall plenty to allow them to canvas underneath it. Sometimes it is impractical to build a bridge loftier enough; for instance, it may rise too steeply or block the view of an important landmark. In such cases, the bridge tin can be designed so information technology can be easily moved out of the way for vessels that are too large to sail under information technology.

The type of movable bridge that most people call up of as a draw span is similar to those that spanned medieval castle moats. Technically called "bascule bridges" from the French word for seesaw, they may open at ane terminate and lift to ane side (single leaf) or open in the eye and lift to both sides (double leaf). Some other common type of movable bridge is the vertical lift span, in which the movable section is supported at both ends and is raised vertically like an elevator. Retractable bridges are made and so the movable span slides back underneath an adjacent section of the bridge. Swing bridges are supported on vertical pivots, and the movable span rotates horizontally to open up the span.

Movable bridges are relatively rare considering they are more expensive to operate and maintain than stationary bridges. They also impede traffic—on the water when they are closed and on the roadway or track line when they are open. Of the 770 bridges for which the New York City Transportation Department is responsible, 25 are movable bridges, including at least one of each of the 4 types divers above.

History

A few ancient drawbridges were built, including one 4,000 years ago in Egypt and one 2,600 years ago in the Chaldean kingdom of the Middle E. But they were not commonly used until the European Center Ages. By the cease of the fifteenth century, Leonardo da Vinci was not only designing and building bascule bridges simply also cartoon plans and constructing scale models for a swing bridge and a retractable span.

The mod era of movable bridge structure began in the mid-nineteenth century following the development of processes for mass producing steel. Steel beams are low-cal and strong, steel bearings are durable, and steel engines and motors are powerful.

Many of the movable bridges currently in use in the United States were built in the early twentieth century. Equally they are being refurbished or replaced, two types of improvements can be made. First, more sophisticated design techniques and stronger, lighter materials allow new bridges to be congenital higher higher up the water. This means larger vessels can canvas under them; consequently, it is not necessary to open up them as frequently. Some modern replacements must be opened only one-fourth to one-third as often their predecessors. Second, some new bridges are operated hydraulically rather than being driven with gear mechanisms.

Raw Materials

Draw bridges are made primarily from concrete and steel. Seventy-five hundred short tons (6,804 metric tons) of structural steel and 150,000 short tons (13,6080 metric tons) of concrete were used in the Casco Bay Bridge

A typical draw bridge.

in Portland, Maine; information technology has a 360-pes (x-nm) tall opening and was completed in 1997.

Design

Each draw bridge is a unique structure designed for its particular location and traffic needs. There are at to the lowest degree one-half a dozen different pattern concepts, only the most mutual is the bascule blazon. In double-foliage or iv-foliage (a double-foliage bridge with separate leaves for each management of vehicular traffic) bascule bridges, each leaf can be raised and lowered independently.

The energy required to raise and lower the bascule leaves is greatly reduced past counterbalancing each leaf with a compact weight on the reverse side of the pivot axle (trunnion). In various bascule designs, this counterweight might be located above the roadway and allowed to pivot below the roadway as the span is raised, or information technology might be located below the roadway and allowed to descend into a basement level (often well beneath the waterline) equally the bridge opens. The counterweight is a massive concrete box containing chambers into which heavy, metal rods tin can be inserted to change the weight and its distribution. It might be located adjacent to the trunnion or, for greater leverage, exist set back a few yards (meters). As an case, each pair of 500-ton (450-metric-ton) leaves on the Casco Bay Bridge is balanced with an 800-ton (720-metric-ton) counterweight.

Likewise the leaves and the counterweights, the other primary elements of a bascule bridge are the trunnion and the elevator mechanism. A unmarried steel trunnion upwardly to 10 ft (3 m) in diameter and 65 ft (20 chiliad) or more in length may be used for one leaf of the movable span; or a divide, short trunnion may be used for each side of each leafage. The lift mechanism is commonly a rack-and-pinion gear arrangement driven by electrical motors.

The Manufacturing Process

Although each installation is different, the following is a generic description of the construction of a bascule bridge.

Piers

1 If the bascule back up piers will be located in the h2o, a cofferdam is built effectually the site for each pier. Steel panels are lowered into the h2o and driven into the riverbed to form a box. A clamshell digger

A. Bascule pit. B. Fender organisation. C. Bridge pier.

removes soil inside the cofferdam. Piles are inserted deep into the riverbed to support the great weight of the pier and the bascule leaves. Steel piles may be driven, or reinforced physical piles may exist poured, into drilled holes. The bottom of the cofferdam is sealed with a layer of concrete. The water is pumped out of the cofferdam to provide a dry area for constructing the pier.
ii Forms are congenital to shape the physical piers. Steel confined (rebar) are tied together to make a carefully designed reinforcing cage for the interior of the pier. The rebar muzzle is lowered into position within the forms. The forms are filled with physical. When the concrete has hardened, the forms are removed. Effectually the waterline, a protective layer of an erosion-resistant fabric, such as granite, may be attached to the pier. The cofferdam is removed.
iii A fender may exist built around the pier to protect it from existence hit by errant ships. For example, on the Casco Bridge, big concrete cylinders were erected upstream and downstream from each pier to support the ends of a steel fender. The fender was faced with slippery plastic to deflect small impacts. Under heavier impacts, the fender can deflect confronting safety bumpers and, if necessary, against crushable hollow concrete boxes that would keep the bear on from dissentious the pier itself.

Bascule leaves

4 I or more than trunnions are mounted on supports within the pier.
v A counterweight is synthetic and placed inside the pier.
six Gear drives and/or hydraulic elevator mechanisms are installed in the pier.
vii Two side girders are synthetic for the heel section of each leaf of the bridge. A trunnion bearing is mounted in an opening in each girder. The girder may exist equipped with gears that will mesh with the lift machinery, or it may exist fitted with paddles that hydraulic rams can push against.
8 The two side girders are lifted into the pier and eased over the ends of the trunnion. The heel section is completed with a crossbeam connecting the two side girders. The counterweight is attached to the heel department.
9 Boosted longitudinal girders may be hoisted into position between the side girders and fastened to the heel section. Steel braces are attached between the side girders and any other longitudinal girders. As pieces are added to the leaf, an appropriate amount of weight must too be added to the counterweight to maintain stability. This is particularly important if the span is being built in the airtight position and must be opened during structure to allow marine traffic to laissez passer.
10 The leaf is completed by attaching a tip section that connects the side girders (and any longitudinal girders) at the end opposite the heel. Devices chosen span locks are mounted on the leaf tips to connect opposite leaves when the bridge is down, so that vehicles driving on the span volition not make the leaves bounce. Additional locks can secure the leaves in their open position so wind does not force them back downward.

Finishing

11 Panels of steel-grate decking are installed atop the leaf. Sometimes a thin concrete surface is added.
12 Final balancing is accomplished by placing heavy iron, steel, or atomic number 82 rods in the correct counterweight compartments. When properly balanced, the leaf is slightly heavier than the counterweight so gravity gently lowers (closes) the bridge.

Ongoing Adjustments

Throughout the lifetime of the span, counterweight adjustments must be fabricated. Shortterm adjustments compensate for ice or snow accumulations, for instance. Longterm adjustments remainder leafage weight changes due to activities such as repaving or painting. When the 250-foot (75-m) long High Street Bridge in Alameda County, California, was refurbished in 1996, 25,000 pounds (11,000 kg) of paint and primer were removed from its two bascule leaves. The counterweights had to be adjusted earlier and later on repainting the span.

A dramatic example of the need to maintain proper weigh was shown by an accident on Chicago's Michigan Avenue Bridge on September 20, 1992. The 2-level, double-leaf bascule bridge was under-going repairs, and the concrete paving had been stripped off both the upper and lower decks. A big crane was parked behind the trunnion of one leaf, just to a higher place a counter-weight that had not been lightened to recoup for the paving removal. Condom locks may also have been improperly engaged or defective. The reverse side of the bridge was opened to allow a boat to laissez passer. When it closed and mated with the side that had remained downwards, the static half was jarred enough to release its unbalanced free energy. The leafage "sprang upwards without warning, like a gargantuan catapult, hurling equipment and debris hundreds of feet across Wacker Drive into buses, automobiles, and pedestrian traffic," according to an analysis in the Journal of the American Club of Mechanical Engineers. The article continued, "The rapid rotation of the span ripped information technology from its trunnion bearings and the entire span slammed to the bottom of the counterweight pit." 6 people were injured as they scrambled out of a double-decker struck by flying droppings, and the rear window of an occupied car was smashed by the wrecking ball attached to the crane equally it savage from the span.

The Future

There are two categories of movable bridge innovations. Refinements of traditional designs include minimizing the structure of big, submerged pits to receive counter-weights when the bridge is open. For case, the 17th Street Causeway Bridge in Fort Lauderdale, Florida, begun in 1998, allows compact counterweights to swing within V-shaped back up piers rather than down into basements below beefy piers. The South Eighth Street Span in Sheboygan, Wisconsin, completed in 1995, operates without whatsoever counterweight despite its comparatively heavy, reinforced concrete deck. Rather than being gear-driven, the 82-ft (25-ni) long single-leaf bascule is moved by a powerful hydraulic organization.

Other movable span innovations introduce entirely new concepts. For case, the Baltic Millennium Bridge in Gateshead, England (to be opened to the public in 2001), consists of two parabolic arches connected by a series of parallel cables. When the span is closed, one arch is horizontal and the other is vertical. The span opens by rotating vertically as a complete unit, raising the horizontal arch and lowering the vertical 1 until both rest approximately 45° and 164 ft (fifty thousand) above the water surface. The steel and aluminum structure is designed to carry pedestrian and wheel traffic across the 410-ft (125-m) wide River Tyne.

Where to Learn More than

Periodicals

"Biconvex Cable-Stayed Crossing Tilts Side-ways to Open up." Civil Applied science (May 1999): 17+.

Cassity, Patrick A., et al. "Rebound of the Bascule Span." Civil Engineering (August 1996): 48+.

Studney, Michael J. &lquo;When a Bridge Becomes a Catapult." Mechanical Engineering (December 1992): 51+.

Other

17th Street Causeway. http://www.dot.country.fl.u.s.a./structures/botm/17thstreet/17thstreet.htm . (May 2, 2000).

Watson, Sara Ruth, and John R. Wolfs. Bridges of Metropolitan Cleveland. (1998). http://web.ulib.csuohio.edu/SpecColl/bmc/index.html (May 3, 2000).

— Loretta Hall

Source: http://www.madehow.com/Volume-6/Draw-Bridge.html

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