How To Draw A Realistic Bridge

Draw Bridge

Background

A bridge over a navigable waterway must allow boats and ships to cross its path, usually past being alpine enough to allow them to sail underneath information technology. Sometimes it is impractical to build a span high plenty; for example, information technology may rise too steeply or block the view of an important landmark. In such cases, the bridge can be designed so it can exist easily moved out of the fashion for vessels that are too large to sail nether it.

The type of movable span that most people think of every bit a depict bridge is similar to those that spanned medieval castle moats. Technically called "bascule bridges" from the French word for seesaw, they may open up at one end and elevator to 1 side (single leaf) or open in the heart and elevator to both sides (double foliage). Another 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 so the movable span slides back underneath an side by side section of the span. Swing bridges are supported on vertical pivots, and the movable span rotates horizontally to open the bridge.

Movable bridges are relatively rare because 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 Metropolis Transportation Department is responsible, 25 are movable bridges, including at least one of each of the 4 types defined higher up.

History

A few ancient drawbridges were congenital, including 1 4,000 years ago in Egypt and ane 2,600 years ago in the Chaldean kingdom of the Heart Due east. Merely they were not commonly used until the European Centre Ages. By the end of the fifteenth century, Leonardo da Vinci was non only designing and building bascule bridges but also cartoon plans and constructing scale models for a swing bridge and a retractable bridge.

The modern era of movable span structure began in the mid-nineteenth century following the development of processes for mass producing steel. Steel beams are light and potent, steel bearings are durable, and steel engines and motors are powerful.

Many of the movable bridges currently in use in the The states were built in the early on twentieth century. Equally they are beingness refurbished or replaced, two types of improvements can be made. First, more sophisticated blueprint techniques and stronger, lighter materials allow new bridges to exist built college in a higher place the water. This ways larger vessels can sail under them; consequently, it is not necessary to open them as frequently. Some modern replacements must exist opened only one-quaternary to ane-3rd every bit often their predecessors. 2d, some new bridges are operated hydraulically rather than being driven with gear mechanisms.

Raw Materials

Describe bridges are made primarily from concrete and steel. Seventy-five hundred brusk tons (six,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-foot (ten-nm) alpine opening and was completed in 1997.

Design

Each depict bridge is a unique construction designed for its particular location and traffic needs. There are at least half a dozen different design concepts, only the virtually common is the bascule type. In double-leaf or four-foliage (a double-leaf bridge with carve up leaves for each direction of vehicular traffic) bascule bridges, each leaf can be raised and lowered independently.

The energy required to enhance and lower the bascule leaves is greatly reduced past counterbalancing each foliage with a compact weight on the opposite side of the pivot axle (trunnion). In diverse bascule designs, this counterweight might be located above the roadway and allowed to pin below the roadway equally the bridge is raised, or it might be located below the roadway and allowed to descend into a basement level (often well below the waterline) as the span opens. The counterweight is a massive concrete box containing chambers into which heavy, metal rods can exist inserted to change the weight and its distribution. It might be located adjacent to the trunnion or, for greater leverage, be set dorsum a few yards (meters). Every bit an example, each pair of 500-ton (450-metric-ton) leaves on the Casco Bay Span is counterbalanced with an 800-ton (720-metric-ton) counterweight.

Besides the leaves and the counterweights, the other principal elements of a bascule bridge are the trunnion and the lift mechanism. A single steel trunnion upwardly to 10 ft (3 yard) in diameter and 65 ft (20 thousand) or more in length may be used for one foliage of the movable span; or a dissever, brusque trunnion may be used for each side of each leaf. The lift mechanism is commonly a rack-and-pinion gear arrangement driven by electric motors.

The Manufacturing Procedure

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

Piers

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

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

removes soil inside the cofferdam. Piles are inserted deep into the riverbed to support the corking weight of the pier and the bascule leaves. Steel piles may be driven, or reinforced concrete piles may be 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 surface area for constructing the pier.
2 Forms are built to shape the concrete piers. Steel bars (rebar) are tied together to make a advisedly designed reinforcing cage for the interior of the pier. The rebar cage is lowered into position inside the forms. The forms are filled with physical. When the physical has hardened, the forms are removed. Around the waterline, a protective layer of an erosion-resistant cloth, such every bit granite, may exist attached to the pier. The cofferdam is removed.
three A fender may be built effectually the pier to protect it from being hit by errant ships. For example, on the Casco Span, 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 minor impacts. Under heavier impacts, the fender can deflect against rubber bumpers and, if necessary, against crushable hollow physical boxes that would keep the affect from damaging the pier itself.

Bascule leaves

4 One or more than trunnions are mounted on supports within the pier.
5 A counterweight is synthetic and placed inside the pier.
half dozen Gear drives and/or hydraulic elevator mechanisms are installed in the pier.
7 Two side girders are constructed for the heel section of each leaf of the bridge. A trunnion bearing is mounted in an opening in each girder. The girder may be equipped with gears that will mesh with the lift machinery, or information technology may be 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 2 side girders. The counterweight is attached to the heel section.
nine Boosted longitudinal girders may be hoisted into position betwixt the side girders and attached to the heel department. Steel braces are fastened between the side girders and any other longitudinal girders. As pieces are added to the leaf, an appropriate amount of weight must as well be added to the counterweight to maintain stability. This is specially of import if the span is being built in the airtight position and must be opened during construction to allow marine traffic to pass.
10 The foliage is completed by attaching a tip section that connects the side girders (and any longitudinal girders) at the end opposite the heel. Devices called bridge locks are mounted on the leaf tips to connect opposite leaves when the bridge is down, so that vehicles driving on the bridge will non brand the leaves bounciness. Additional locks can secure the leaves in their open position so current of air does not strength them dorsum downwardly.

Finishing

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

Ongoing Adjustments

Throughout the lifetime of the bridge, counterweight adjustments must be fabricated. Shortterm adjustments recoup for ice or snow accumulations, for example. Longterm adjustments residual leaf weight changes due to activities such as repaving or painting. When the 250-foot (75-chiliad) 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 adapted before and after repainting the span.

A dramatic instance of the need to maintain proper counterbalance was shown by an blow on Chicago's Michigan Artery Span on September 20, 1992. The ii-level, double-foliage bascule bridge was nether-going repairs, and the physical paving had been stripped off both the upper and lower decks. A large crane was parked backside the trunnion of one leaf, only above a counter-weight that had not been lightened to compensate for the paving removal. Rubber locks may besides have been improperly engaged or defective. The reverse side of the bridge was opened to permit a boat to pass. When information technology airtight and mated with the side that had remained down, the static half was jarred plenty to release its unbalanced energy. The leaf "sprang up 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 Society of Mechanical Engineers. The article connected, "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 bus struck by flight droppings, and the rear window of an occupied car was smashed past the wrecking ball attached to the crane as information technology vicious from the span.

The Future

There are two categories of movable bridge innovations. Refinements of traditional designs include minimizing the construction of large, 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 meaty counterweights to swing within Five-shaped support piers rather than down into basements beneath bulky piers. The South Eighth Street Bridge 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 unmarried-leaf bascule is moved by a powerful hydraulic system.

Other movable bridge innovations introduce entirely new concepts. For instance, the Baltic Millennium Bridge in Gateshead, England (to be opened to the public in 2001), consists of 2 parabolic arches connected by a serial of parallel cables. When the bridge is closed, i arch is horizontal and the other is vertical. The span opens past rotating vertically equally a complete unit of measurement, raising the horizontal arch and lowering the vertical one until both rest approximately 45° and 164 ft (l m) above the water surface. The steel and aluminum structure is designed to carry pedestrian and bicycle traffic across the 410-ft (125-k) wide River Tyne.

Where to Acquire More than

Periodicals

"Biconvex Cablevision-Stayed Crossing Tilts Side-means to Open." Ceremonious Engineering (May 1999): 17+.

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

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

Other

17th Street Causeway. http://world wide web.dot.state.fl.us/structures/botm/17thstreet/17thstreet.htm . (May 2, 2000).

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

— Loretta Hall

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