HISTORY OF SURVEYING

TYPES OF SURVEY & SURVEYING TECHNIQUES

SURVEYING UNITS, TERMS & SLANG  HISTORY OF SURVEYING BASIC'S OF SURVEYING SURVEYING TECHNIQUES SURVEYING GLOSSARY, ABBREVIATIONS, WATER TERMS & TREES

HISTORY
     Surveying is the science and art of measuring distances and angles on or near the surface of the earth. It is an orderly process of acquiring data relating to the physical characteristics of the earth and in particular the relative position of points and the magnitude of areas. Evidence of surveying and recorded information exists from as long ago as five thousand years in places such as China, India, Babylon and Egypt.
Ancient Egyptian surveyors were called harpedonapata (rope-stretcher). They used ropes and knots, tied at pre-determined intervals, to measure distances. The 3-4-5 triangle (later formalized by Pythagoras) was discovered to give a right angle easily by using a rope knotted at distances of 3,4 and 5 units (as below) and shaped (stretched) to form a triangle with a knot at each corner (vertex).

An early instrument for leveling is shown below. It was made from three pieces of wood in the form of an isosceles triangle. A plumb bob suspended from the apex of the frame aligned itself with a notch at the midpoint of the base only when the base was level.

Early Egyptian level

The great Pyramids were presumably laid out using knotted ropes, simple levels and water trough levels for the foundations. The Egyptians used these techniques in the field whilst the Greeks (Pythagoras, Archimedes, Eratosthenes) proved the geometric reasoning behind the principles and demonstrated a clear relationship between mathematics and surveying.
Archimedes (by 250 B.C.) recorded that the circumference of the earth is 30 myriads of stadia (300 000 stadia). Stadia is an ancient Greek measure of a distance of 202 yards, or approximately 185 metres. Eratosthenes supported this notion and by some complicated reasoning and calculations using the summer solstice, the sun, angles of shadows and the known position of two towns Syene and Alexandria (Syene was known to be 5 000 stadia due south of Alexandria), found the circumference of the earth to be 25 000 miles. It is in fact 24 881 miles - not bad!

For leveling the Greeks used a chorobate. Wooden poles were often used for linear measurements.

Roman level (Chorobate)

Navigation skills were needed for exploration. Lodestone (a naturally magnetized rock - magnetite) was first used to locate magnetic north and in time the magnetic compass developed for navigation on land and water. An Englishman, Thomas Digges, used the word theodolite to describe an instrument, graduated in 360 degrees, used to measure angles in the mid-1500's. By 1590 the plane table, credited as the invention of Jean Praetorius, was in use. It remained in similar form until the early 1900's. A telescope attached to a quadrant for measuring angles permitted the development of the surveying procedures of triangulation in the 1660's. Sextants, which are precision instruments made from brass or aluminum, became very useful for ocean navigation where celestial observations were taken to plot a ship's position. Sextants accurately measure angles between celestial objects such as the stars, moon and sun, and the horizon from which calculations of position can be made. They are a sophisticated refinement of earlier instruments (cross-staff, quadrant, octant to name a few). The sextant remains today as a valuable tool for ocean navigation. The first dumpy levels appeared in the first half of the 1700's combining a telescope with a bubble level. In 1831, in Philadelphia, W. J. Young invented the transit which exhibited marvelous improvements in accuracy of surveying methods. It allowed the telescope to revolve (transit) on its axis. This meant that both forward and backward sightings could be accurately taken and by repeating the process errors were minimized.
The military requirements of two world wars provided the motivation for vast improvements in the design of surveying equipment and execution of surveying operations. In the 1950's Electronic Distance Measurement (EDM) largely replaced triangulation methods for distance measurements. Electronic data collection brought many changes to surveying procedures. The transfer of data collected electronically to computer plotting and drawing systems has reduced time and relieved tedious manual drawing work. The scope of surveying has extended beyond land measurement to include environmental concern for such things as water resources, energy requirements, marine exploration, demarcation of boundaries and protection of the environment.
The use of GPS (Global Positioning System) in surveying procedures is the most recent and revolutionary change to impact land measurement. GPS is very accurate, quick and reliable. However, in conversation with practicing surveyors, I discovered that there is trend to keep older mechanical dumpy levels. This is because they can always be relied on, and trusted, when electronic equipment is faulty or fails, to complete the job in hand.

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STANDARD SURVEYING UNITS

TYPES OF SURVEY & SURVEYING TECHNIQUES SURVEYING UNITS, TERMS & SLANG  HISTORY OF SURVEYING BASIC'S OF SURVEYING SURVEYING TECHNIQUES SURVEYING GLOSSARY, ABBREVIATIONS, WATER TERMS & TREES     STANDARD SURVEYING UNITS

Acre - The (English) acre is a unit of area equal to 43,560 square feet, or 10 square chains, or 160 square poles. It derives from a plowing area that is 4 poles wide and a furlong (40 poles) long. A square mile is 640 acres. The Scottish acre is 1.27 English acres. The Irish acre is 1.6 English acres. Are -Non S>I Metric = 100 sq. metres Arpent - Unit of length and area used in France, Louisiana, and Canada. As a unit of length, approximately 191.8 feet (180 old French 'pied', or foot). The (square) arpent is a unit of area, approximately .845 acres, or 36,802 square feet. Cable - Imperial = 100 Fathoms. From the cable (Several ropes twisted together -very strong) used to secure ships at sea. Chain - Imperial = 4 poles = 22 yards.  Attempt by Edmund Gunter (U.K. 1620's) to make land measure, a decimal system. His 66 foot chain had 100 links. , but possibly variant by locale. See also Rathbone's chain. The name comes from the heavy metal chain of 100 links that was used by surveyors to measure property bounds. Circle - One revolution = 360° Colpa - Old Irish measure of land equal to that which can support a horse or cow for a year. Approximately an Irish acre of good land. Degree -  60 nautical miles (latitude) Fathom - Imperial = 6 feet. From old english faethm. Foot - Imperial = 12 inches. Based on the length of the human foot. Engineer's Chain - A 100 foot chain containing 100 links of one foot apiece. Furlong - Unit of length equal to 40 poles (220 yards). Its name derives from "furrow long", the length of a furrow that oxen can plow before they are rested and turned. See Gunter's chain. Gunter's Chain - Unit of length equal to 66 feet, or 4 poles. Developed by English polymath Edmund Gunter early in the 1600's, the standard measuring chain revolutionized surveying. Gunter's chain was 22 yards long, one tenth of a furlong, a common unit of length in the old days. An area one chain wide by ten chains long was exactly an acre. In 1695 Queen Elizabeth I had the mile redefined from the old Roman value of 5000 feet to 5280 feet in order for it to be an even number of furlongs. A mile is 80 chains. Hectare - Metric unit of area equal to 10,000 square meters, or 2.471 acres, or 107,639 square feet. Hide - A very old English unit of area, a hide was of variable size depending on locale and the quality of the land. It was the amount of land to support a family, and ranged from 60 to 180 acres. After the Norman conquest in 1066 it became standardized at around 120 acres. Inch - Imperial. From old english (ynce) meaning one twelfth. Labor - The labor is a unit of area used in Mexico and Texas. In Texas it equals 177.14 acres (or 1 million square varas). League (legua) - Unit of area used in the southwest U.S., equal to 25 labors, or 4428 acres (Texas), or 4439 acres (California). Also, a unit of length-- approximately three miles. Link - Unit of length equal to 1/100 chain (7.92 inches). Mile - Imperial = 1760 yards = 8 furlongs From mille passum, a term used by the Roman armies of Caesar 'Mile' and 'pace'. Minute (1/60 of 1°) - 1 nautical mile (latitude) Morgen - Unit of area equal to about .6309 acres. It was used in Germany, Holland and South Africa, and was derived from the German word Morgen ("morning"). It represented the amount of land that could be plowed in a morning. Myriad  - Historical (Greek) 1 myriad = 10 000 stadia Nautical mile - Used in marine and aeronautical navigation as 1nm = 1° (latitude) Octant (45°) - Historical. Meaning one eighth Out - An 'out' was ten chains. When counting out long lines, the chain carriers would put a stake at the end of a chain, move the chain and put a stake at the end, and so on until they ran "out" of ten stakes. Perch - See pole . Point - A point of the compass. There are four cardinal points (North, South, East, West), and 28 others yielding 32 points of 11.25 degrees each. A survey line's direction could be described as a compass point, as in "NNE" (north northeast). To improve precision, the points would be further subdivided into halves or quarters as necessary, for example, "NE by North, one quarter point North". In some areas, "and by" meant one half point, as in "NE and by North". Pole - Unit of length and area. Also known as a perch or rod. As a unit of length, equal to 16.5 feet. A mile is 320 poles. As a unit of area, equal to a square with sides one pole long. An acre is 160 square poles. It was common to see an area referred to as "87 acres, 112 poles", meaning 87 and 112/160 acres. Pueblo - A Spanish grant of less than 1000 acres. Quadrant (90°) - Historical. Meaning one quarter. Rancho - A Spanish grant of more than 1000 acres. Rathbone's Chain - A measuring chain two poles, or 33 feet, in length. Rod - See pole Rood - Unit of area usually equal to 1/4 acre. Sea league - Imperial = 3 nautical miles Sextant (60°) - Imperial = 3 nautical miles Stadia (Stade) - Historical (Greek) = 202 yards Vara - Unit of length (the "Spanish yard") used in the U.S. southwest. The vara is used throughout the Spanish speaking world and has values around 33 inches, depending on locale. The legal value in Texas was set to 33 1/3 inches early in the 1900's. Virgate - An old English unit of area, equal to one quarter of a hide. The amount of land needed to support a person. Yard - Imperial = 3 feet. From odl English gerd, or gierd (rod)

STANDARD SURVEYING TERMS

Aliquot - The description of fractional section ownership used in the U.S. public land states. A parcel is generally identified by its section, township, and range. The aliquot specifies its precise location within the section, for example, the northwest quarter of the southeast quarter. Azimuth - The number of degrees from north (or other reference direction) that a line runs, measured clockwise. Benchmark - A survey mark made on a monument having a known location and elevation, serving as a reference point for surveying. Call - Any feature, landmark, or measurement called out in a survey. For example, "two white oaks next to the creek" is a call. Chain carrier - An assistant to the surveyor, the chain carriers moved the surveying chain from one location to another under the direction of the surveyor. This was a position of some responsibility, and the chain carriers took an oath as "sworn chain carriers" that they would do their job properly. Condition - See Conditional line. Conditional line - An agreed line between neighbors that has not been surveyed, or which has been surveyed but not granted. Corner - The beginning or end point of any survey line. The term corner does not imply the property was in any way square. Declination - The difference between magnetic north and geographic (true) north. Surveyors used a compass to determine the direction of survey lines. Compasses point to magnetic north, rather than true north. This declination error is measured in degrees, and can range from a few degrees to ten degrees or more. Surveyors may have been instructed to correct their surveys by a particular declination value. The value of declination at any point on the earth is constantly changing because the location of magnetic north is drifting. First station - See Point of Beginning Gore - A thin triangular piece of land, the boundaries of which are defined by surveys of adjacent properties. Loosely, an overlap or gap between properties. See also strip. Landmark - A survey mark made on a 'permanent' feature of the land such as a tree, pile of stones, etc. Line Tree - Any tree that is on a property line, specifically one that is also a corner to another property. Mete - A limit point or mark. To butt up against. Metes and Bounds - An ancient surveying system that describes a parcel of land in terms of its relationship to natural features and adjacent parcels. Merestone - A stone that marks a boundary. See monument. Monument - A permanently placed survey marker such as a stone shaft sunk into the ground. Point of Beginning - The starting point of the survey Plat - A drawing of a parcel of land. Range - In the U.S. public land surveying system, a north-south column of townships, identified as being east or west of a reference longitudinal meridian, for example, Range 3 West. See township. Searles Spiral - A surveying technique used by railroad surveyors in the the late 1800s and early 1900s whereby they approximate a spiral by use of multiple curved segments. Section - In the U.S. public land surveying system, an area one mile square. See aliquot. Strip - A rectangular piece of land adjoining a parcel, created when a resurvey turns up a tiny bit larger than the original survey. The difference is accounted for by temperature or other effects on measuring chains. See also gore. Tie line - A survey line that connects a point to other surveyed lines. Township - In the U.S. public land surveying system, an area six miles square, containing 36 sections. The townships are organized in rows and are identified with respect to a reference latitudinal baseline, for example, Township 13 North. See range. Traverse - 1) any line surveyed across a parcel, 2) a series of such lines connecting a number of points, often used as a base for triangulation. Witness Tree - Generally used in the U.S. public land states, this refers to the trees close to a section corner. The surveyor blazed them and noted their position relative to the corner in his notebook. Witness trees are used as evidence for the corner location.

SURVEYING SLANG

Surveying, like any profession, has its special terms and slang. Some are just humorous, some help distinguish similar sounds (e.g. eleven and seven), and some are just plain strange! Balls - Slang for numeric .00, as in 4-balls (4.00) Beep - Verb. To use a magnetic detector to look for iron pipe, etc. Boot - To raise the levels rod some number of inches so as to be visible to the instrument man, e.g. "Boot 6!" means "raise it 6 inches." Bullseye - Zero degrees of inclination. Burn - See shoot Burn one - Measure from the one foot mark on the tape rather than from the end of the tape in order to increase the accuracy of the measurement. Cut line - To clear vegetation for a line of sight between two survey control points. Double nickel - Slang for .55, as in 6-double nickel (6.55) Dummy or dummy-end - The base or zero end of a tape or chain, as in "hold dummy at the face of the curb." EDM - Electromagnetic Distance Measurement device, the instrument used by modern surveyors that replaces the use of measurement chains. It determines distance by measuring the time it takes for laser light to reflect off a prism on top of a rod at the target location. Ginney - A wooden dowel 6-9 inches in length with a sharpened end. Set in the ground to mark survey points. Glass - The EDM prism. Gun - Originally, a transit, but potentially any measurement instrument in use, e.g. theodolite, EDM, or Total Station. Hours - Degrees Hub and Tack - A 2" by 2" stake that is set in the ground and that contains a nail ("tack") that precisely marks the point being set. Legs - Tripod Pogo - Prism pole Punk - See railroad. Railroad - Slang for eleven, as in 42-railroad (42.11) Rodman - The person holding the rod with the EDM prism. This person is the modern version of a chain carrier or chain man. Shoot - Measure distance with an EDM Spike - Usually a 60 penny nail used to mark survey points in hard ground. Tie - To locate something with the transit or other measuring device. Top - Slang for eleven. See railroad. Trip - Slang for triple digits, as in trip5 means 555, and 43trip7 means 43.777 Turn - The rodman is told to stay in place while the gun or level is moved to a new location. Zero - Zero degrees, minutes, and seconds. A perfect zero.

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SURVEYING TECHNIQUES

TYPES OF SURVEY & SURVEYING TECHNIQUES

SURVEYING UNITS, TERMS & SLANG  HISTORY OF SURVEYING BASIC'S OF SURVEYING SURVEYING TECHNIQUES SURVEYING GLOSSARY, ABBREVIATIONS, WATER TERMS & TREES

Techniques The techniques used for surveying and land measurement are dependent upon the type of survey being conducted. In this section there are two choices:  Types of survey Techniques Types of Survey A simple classification is: 1.  Land surveys, which fix property lines, calculate land areas and assist with the transfer of real property from one owner to another. 2.  Engineering surveys, which collect the data needed to plan and design engineering projects. The information ensures the necessary position and dimension control on the site so that the structure is built in the proper place and as designed. 3.  Informational surveys obtain data concerning topography, drainage and man-made features of a large area. This data is portrayed as maps and charts. Another way to make a simple classification is: 1.  Geodetic surveys are precise and over large areas require the curvature of the earth to be considered. Distances and angle measurements must be very, very accurate. A wide variety of techniques are used including triangulation, traversing, trilateration, levelling and astronomical direction fixing. 2.  Plane surveys, which consider the surface of the earth to be a plane. Curvature is ignored and calculations are performed using the formulas of plane trigonometry and the properties of plane geometry. These may be considered accurate for limited areas. Sub-categories of the major classes provide more insight into the various fields of surveying as follows: ·   Property surveys determine boundary lines, property corners, rights-of-way provide data necessary for the preparation of land sub-divisions. ·    Cadastral surveys are executed by the Federal Government in connection with the disposal of vast areas of land known as the public domain. ·    Route surveys are necessary for the design and construction of various engineering projects such as roads, railways, pipelines, canals and powerlines. ·    Industrial surveys, or optical metrology, are used in the aircraft and other industries where very accurate dimensional layouts are required. ·   Topographic surveys are performed to gather data necessary to prepare topographic maps. These are multicolour contour maps portraying the terrain; and rivers; highways, railways, bridges and other man-made features. ·    Hydrographic surveys map the shorelines of bodies of water; chart the bottom of streams, lakes, harbours and coastal waters; measure the flow of rivers; and assess other factors affecting navigation and water resources. The sounding of depths by radar is involved in this type of survey. ·   Mine surveys determine the position of underground works such as tunnels and shafts, the position of surface structures and the surface boundaries. ·    Aerial surveys use photogrammetry to produce a mosaic of matched vertical photographs, oblique views of landscape and topographic maps drawn from the photographs. ·    Construction surveys fix elevations, horizontal positions and dimensions for construction projects. ·    Control surveys provide basic horizontal and vertical position data. These are called datum. For most surveying work the vertical position of points in terms of height above a curved reference surface is mean sea level. Techniques 1. Triangulation 2. Trilateration 3. Traverse 4. Leveling 5. Radiation Triangulation Triangulation consists of a series of connected triangles which adjoin or overlap each other, angles being measured from determined fixed stations. Triangulation reduces the number of measures that need to be taped and for this reason is often a preferred method of survey. A known base-line measurement is required. Three examples of triangulation systems are shown below. A single chain of triangles is a rapid and economical system for covering a narrow strip of land. A chain of quadrilaterals is more accurate with checks being made by various combinations of angles and sides as the survey proceeds. Larger areas use a central point arrangement. A point to note is that all angles should be more than 20°. Angles less than 20° are not considered valid for fixing position. They introduce inaccuracies. This is much the same in navigation where a fix by two bearings requires an angle of intersection of approximately 90°, and for three bearings approximately 60°. Angles less than 30° are not acceptable. ^ Back to techniques Trilateration Trilateration uses electronic distance measuring equipment to directly measure the lengths of the sides of triangles from which the angles can be calculated. It is a very useful method for rough terrain where positions can be accurately carried forward and is seen as an alternative method to triangulation. ^ Back to techniques Traverse A traverse consists of a series of lines, whose lengths and directions are measured, connecting points whose positions are to be determined. The route of the traverse line can be adjusted for obstacles such as rough or timbered terrain, swampy land, buildings and areas of heavy traffic. A traverse may be either open or closed as shown below. An open traverse begins at a point of known position and ends at a station whose relative position is unknown. This type of traverse is frequently used for preliminary surveys for highways. A closed traverse begins and ends at the same point whose position is known. An example of this type of traverse is a perimeter survey of a tract of land. ^ Back to techniques Leveling Leveling is the operation of determining differences of elevation by measuring vertical distances directly on a graduated rod with the use of a leveling instrument such as a dumpy level, transit or Theodolites. This method is called direct leveling or differential leveling. Indirect leveling can be done using the principle that differences in elevation are proportional to the differences in atmospheric pressure. The difference in elevation between two points can also be determined trigonometrically using vertical angles and horizontal or inclined distances. Differential leveling Benchmarks are very important in leveling. They are permanent objects of known elevation located where there is least likelihood of disturbance. They may be a metal or concrete post set close to the ground, a notch in a tree root or a peg or spike in a tree. ^ Back to techniques Radiation Radiation is a surveying technique often used in conjunction with a plane table. From a fixed position directly above the corresponding location on the ground bearings are taken to various points at the boundary of the survey area. The bearing lines are drawn on the paper on the table. Distances to the points are measured and then converted to the required scale on the survey sheet. Radiation surveying can be performed in a similar way directly on the ground taking bearings and distances from a fixed position and recording them for later work. Similarly plane table surveying may be used with other techniques such as a traverse or triangulation. ^ Back to techniques

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Hold Your Ground

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Build the Bridge

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Forces acting on suspension bridges

Three kinds of forces operate on any bridge: the dead load, the live load, and the dynamic load. Dead load refers to the weight of the bridge itself. Like any other structure, a bridge has a tendency to collapse simply because of the gravitational forces acting on the materials of which the bridge is made. Live load refers to traffic that moves across the bridge as well as normal environmental factors such as changes in temperature, precipitation, and winds. Dynamic load refers to environmental factors that go beyond normal weather conditions, factors such as sudden gusts of wind and earthquakes. All three factors must be taken into consideration when building a bridge.

Use other than road and rail

Cable-suspended footbridge at Dallas Fort Worth Airport Terminal D

The principles of suspension used on the large scale may also appear in contexts less dramatic than road or rail bridges. Light cable suspension may prove less expensive and seem more elegant for a footbridge than strong girder supports. Where such a bridge spans a gap between two buildings, there is no need to construct special towers, as the buildings can anchor the cables. Cable suspension may also be augmented by the inherent stiffness of a structure that has much in common with a tubular bridge.

Construction sequence (wire strand cable type)

New Little Belt suspension bridge, 1970 Denmark

One of the main cable saddles atop the Golden Gate Bridge.

Manhattan Bridge in New York City with deck under construction from the towers outward.

Suspender cables and suspender cable band on the Golden Gate Bridge in San Francisco. Main cable diameter is 36 inches, and suspender cable diameter is 3¹⁄₂ inches.

Lions' Gate Bridge with deck under construction from the span's center

Typical suspension bridges are constructed using a sequence generally described as follows. Depending on length and size, construction may take anywhere between a year and a half (construction on the original Tacoma Narrows Bridge took only 19 months) to as many as a decade (the Akashi-Kaikyō Bridge's construction began in May 1986 and was opened in May, 1998 - a total of twelve years).

1. Where the towers are founded on underwater piers, caissons are sunk and any soft bottom is excavated for a foundation. If the bedrock is too deep to be exposed by excavation or the sinking of a caisson, pilings are driven to the bedrock or into overlying hard soil, or a large concrete pad to distribute the weight over less resistant soil may be constructed, first preparing the surface with a bed of compacted gravel. (Such a pad footing can also accommodate the movements of an active fault, and this has been implemented on the foundations of the cable-stayed Rio-Antirio bridge. The piers are then extended above water level, where they are capped with pedestal bases for the towers.
2. Where the towers are founded on dry land, deep foundation excavation or pilings are used.
3. From the tower foundation, towers of single or multiple columns are erected using high-strength reinforced concrete, stonework, or steel. Concrete is used most frequently in modern suspension bridge construction due to the high cost of steel.
4. Large devices called saddles, which will carry the main suspension cables, are positioned atop the towers. Typically of cast steel, they can also be manufactured using riveted forms, and are equipped with rollers to allow the main cables to shift under construction and normal loads.
5. Anchorages are constructed, usually in tandem with the towers, to resist the tension of the cables and form as the main anchor system for the entire structure. These are usually anchored in good quality rock, but may consist of massive reinforced concrete deadweights within an excavation. The anchorage structure will have multiple protruding open eyebolts enclosed within a secure space.
6. Temporary suspended walkways, called catwalks, are then erected using a set of guide wires hoisted into place via winches positioned atop the towers. These catwalks follow the curve set by bridge designers for the main cables, in a path mathematically described as a catenary arc. Typical catwalks are usually between eight and ten feet wide, and are constructed using wire grate and wood slats.
7. Gantries are placed upon the catwalks, which will support the main cable spinning reels. Then, cables attached to winches are installed, and in turn, the main cable spinning devices are installed.
8. High strength wire (typically 4 or 6 gauge galvanized steel wire), is pulled in a loop by pulleys on the traveler, with one end affixed at an anchorage. When the traveler reaches the opposite anchorage the loop is placed over an open anchor eyebar. Along the catwalk, workers also pull the cable wires to their desired tension. This continues until a bundle, called a "cable strand" is completed, and temporarily bundled using stainless steel wire. This process is repeated until the final cable strand is completed. Workers then remove the individual wraps on the cable strands (during the spinning process, the shape of the main cable closely resembles a hexagon), and then the entire cable is then compressed by a traveling hydraulic press into a closely packed cylinder and tightly wrapped with additional wire to form the final circular cross section. The wire used in suspension bridge construction is a galvanized steel wire that has been coated with corrosion inhibitors.
9. At specific points along the main cable (each being the exact distance horizontally in relation to the next) devices called "cable bands" are installed to carry steel wire ropes called Suspender cables. Each suspender cable is engineered and cut to precise lengths, and are looped over the cable bands. In some bridges, where the towers are close to or on the shore, the suspender cables may be applied only to the central span. Early suspender cables were fitted with zinc jewels and a set of steel washers, which formed the support for the deck. Modern suspender cables carry a shackle-type fitting.
10. Special lifting hoists attached to the suspenders or from the main cables are used to lift prefabricated sections of bridge deck to the proper level, provided that the local conditions allow the sections to be carried below the bridge by barge or other means. Otherwise, a traveling cantilever derrick may be used to extend the deck one section at a time starting from the towers and working outward. If the addition of the deck structure extends from the towers the finished portions of the deck will pitch upward rather sharply, as there is no downward force in the center of the span. Upon completion of the deck the added load will pull the main cables into an arc mathematically described as a parabola, while the arc of the deck will be as the designer intended — usually a gentle upward arc for added clearance if over a shipping channel, or flat in other cases such as a span over a canyon. Arched suspension spans also give the structure more rigidity and strength.
11. With completion of the primary structure various details such as lighting, handrails, finish painting and paving are installed or completed.

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Variations

Underspanned suspension bridge

Micklewood Bridge as illustrated by Charles Drewry, 1832

Eyebar chain cables of Clifton Suspension Bridge

Wire strand cables of Golden Gate Bridge

The Yichang Bridge, a plate deck suspension bridge, over the Yangtze River in China

In an underspanned suspension bridge, the main cables hang entirely below the bridge deck, but are still anchored into the ground in a similar way to the conventional type. Very few bridges of this nature have been built, as the deck is inherently less stable than when suspended below the cables. Examples include the Pont des Bergues of 1834 designed by Guillaume Henri Dufour; James Smith’s Micklewood Bridge; and a proposal by Robert Stevenson for a bridge over the River Almond near Edinburgh.
Roebling's Delaware Aqueduct (begun 1847) consists of three sections supported by cables. The timber structure essentially hides the cables; and from a quick view, it is not immediately apparent that it is even a suspension bridge.

Suspension cable types

The main suspension cable in older bridges was often made from chain or linked bars, but modern bridge cables are made from multiple strands of wire. This contributes greater redundancy; a few flawed strands in the hundreds used pose very little threat, whereas a single bad link or eyebar can cause failure of the entire bridge. (The failure of a single eyebar was found to be the cause of the collapse of the Silver Bridge over the Ohio River). Another reason is that as spans increased, engineers were unable to lift larger chains into position, whereas wire strand cables can be largely prepared in mid-air from a temporary walkway.

Deck structure types

Most suspension bridges have open truss structures to support the roadbed, particularly owing to the unfavorable effects of using plate girders, discovered from the Tacoma Narrows Bridge (1940) bridge collapse. Recent developments in bridge aerodynamics have allowed the re-introduction of plate structures. In the picture of the Yichang Bridge, note the very sharp entry edge and sloping undergirders in the suspension bridge shown. This enables this type of construction to be used without the danger of vortex shedding and consequent aeroelastic effects, such as those that destroyed the original Tacoma Narrows bridge.

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