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Glossary of Bridge Terminology



A-Truss: A four-panel truss having extended batter posts intersecting over the centre resembling somewhat the letter A. See Fig. 22dd.

Fig. 22dd

Abacus: The upper member of the capital of a column.

Abscissa: A term in rectangular coordinates referring to the horizontal distance of any point from the vertical axis.

Abutment: That part of a pier from which an arch springs. A structure sustaining one end of a bridge span and at the same time supporting the embankment which carries the track or roadway.

Abutment Line: The closing line of an equilibrium polygon.

Abutment Wall: A wall in an abutment, or a wall serving the purpose of an abutment.

Abutting Joint: A square joint confined to a single plane where the parts meet. In contra-distinction to a lap-joint where the splice is shingled.

Acceleration: The increase, in velocity which takes place in a unit of time.

Acid Bessemer Steel: A metal produced by the decarburization of crude pig iron in a converter where finely divided air currents are blown through the molten mass. The lining of the converter is of a silicious material that will have no effect on the phosphorus, hence that element is not eliminated.

Acid Open-hearth Furnace: A furnace used in the manufacture of Acid Open-hearth Steel.

Acid Open-hearth Process: That process of producing steel from pig and scrap iron, in which the first step is to remove most of the silicon, manganese, and carbon from the molten mass. Just before tapping, spiegeleisen or an artificial ferromanganese is added to the charge in order to destroy the oxide slag and prevent red shortness. The furnace is lined with a silicious material.

Acid Open-hearth Steel: A metal formed of pig iron, cast iron, and wrought iron or steel scrap, which is converted into steel by the direct action of an oxidizing flame in a regenerative gas furnace. The furnace is lined with a silicious material that has no effect on the phosphorus content.

Acid Steel: Steel made without the use of lime.

Activity of Cement: The time required for a cement to pass from its initial set to its final or hard set as determined by the Vicat Needle.

Actual Horsepower or Brake Horsepower: The actual horsepower of an engine as measured at the flywheel by a friction-brake or a dynamometer.

Adhesion: The force which holds together two bodies placed in close contact with each other.

Adiabatic Curve: A curve exhibiting the relation between the pressure and volume of a fluid upon the assumption that there is no transmission of heat during expansion or contraction.

Adjustable Eye-bar: An eye-bar that, can be lengthened or shortened after erection by means of a sleeve-nut, turn-buckle, or clevis.

Adjustable Key: Same as "Adjusting Key."

Adjustable Member: A member of a bridge, the length of which can be increased or diminished at will.

Adjusting Key: A wrench in which the jaws are made adjustable.

Administration: The direction or oversight of any office, service, or construction; or the management of public affairs.

Adulterant: A substance substituted partially for another without acknowledgment.

Adulteration: The partial substitution of one substance for another without acknowledgment.

Advancing Load Stress: A stress in a member induced by a load advancing on the structure.

Adze: A hand tool, having a curved cutting edge perpendicular to the handle, used for dressing the surfaces of timbers or stones.

Aeration Jet: A jet of water through which air travels.

Aggregate: The inert material such as sand, broken stone, etc., with which the cement or other adhesive material is mixed to form a concrete or mortar.

Air-blast: An air current forced upon a fire to stimulate combustion.

Air Brake: A system of braking mechanism operated by compressed air.

Air Chamber: An enclosed space containing air. In bridge work it usually refers to the working chamber in a pneumatic caisson.

Air Compressor: A machine by which air is compressed into a receiver so that its expansion may be utilized as a source of power.

Air Current: The moving of air through space or through a conduit.

Air Cushion: A buffer using air to absorb impact of a moving mass and gradually to bring it to rest.

Air Cylinder: A nearly air-tight hollow cylinder having a piston moving in it.

Air Dolly: A dolly operated by compressed air. Used between two beams.

Air Gauge: A dial on an air machine which records at all times the air pressure, usually in pounds per square inch.

Air Gun: A pneumatic riveting hammer.

Air Hammer: A machine hammer driven by compressed air, as an air riveting hammer.

Air Hoist: A hoisting device, usually consisting of a cylinder, piston, and piston-rod, operated by compressed air.

Air Hose: A hose for conveying air.

Air-lift: A hoisting apparatus that operates by means of compressed air.

Air Line: The shortest distance between two points on the earth's surface.

Air-lock: An air-tight, double-door antechamber of a caisson used for passing workmen or materials into or out of the caisson and to regulate the air pressure during such passage.

Air Piston: The piston that works in the air cylinder of an air compressor.

Air Pressure: The pressure exerted by the air due to its weight or to its being compressed and confined in a reservoir.

Air Pump: A pump for condensing and forcing air through an aperture or pipe.

Air Reamer: A reaming machine operated by compressed air.

Air-receiver: A reservoir in which air is received and stored.

Air Riveter: A riveting machine which is operated by compressed air.

Air-setting: Hardening by exposure to air. Usually applied to cement.

Air Shaft: A tube, pipe, conduit, or passageway for conveying air.

Air Slaked Lime: Lime which has absorbed moisture from the air.

Air Slaking: Decomposition of any material exposed to the air, such as lime.

Air Valve: A valve controlling the passage of air. Also a valve admitting air to a steam boiler, preventing the formation of a partial vacuum when the steam condenses.

Air Working Chamber: A chamber in a caisson into which compressed air is forced to expel the water so that laborers can work at excavating.

Algebraic Curve: A curve in which the equations in linear coordinates contain only the algebraic functions of the coordinates.

Alidade: The horizontal plate in a transit which carries the verniers, the level bulbs, and the standards, and which revolves about the graduated limb: an attachment on many instruments for measuring angles. A straight edge, having a telescope mounted thereon, used in plane table surveying.

Alignment: The state of being in line; the ground plan of a railway or other road in contradistinction to the grades or profile.

Alligator Riveter: A jaw riveter worked by the action of a cam, used in shopwork.

Alligator Wrench: A wrench with fixed spreading jaws, having an inside roughened surface, suggestive of the open mouth of an alligator.

Allowable Bearing Pressure: The maximum intensity of pressure on a support allowed by the specifications.

Allowable Unit Stress: The allowable stress per unit of area given in the specifications.

Alloy: A substance consisting of two or more metals mixed together, or non-metallic bodies mixed with metals, in intimate solution or combination with one another, forming, when melted, a homogeneous fluid.

Alloy Steel: A steel carrying a certain portion of some other metal, such as nickel or vanadium.

Alternate, or Alternative Layout: One of two or more different layouts, or schemes, for the same project.

Alternating Current: An electric current which flows alternately in opposite directions without interruption.

Altitude: height; the degree or amount of elevation above the foundation or ground.

Aluminum: A white metal with high tensile strength and low specific gravity. Used for purifying steel.

Aluminum Bronze: An alloy of copper containing about ten per cent of aluminum.

American Locomotive: A passenger locomotive having four pilot, four driving, and no trailer wheels.

Ammeter: An instrument for measuring or estimating in amperes the strength of electric currents. An ampere-meter.

Amorphous: Without regard for definite form; uncrystallized, structureless.

Amortization: A method for liquidating a debt by making annual payments to a sinking fund which in a given time with the accumulated interest becomes equal to the debt.

Amount: The sum of the principal plus accrued interest for a given time. In the case of a sinking fund involving periodic deposits of money, the amount of such fund is the sum of the "amounts" of the deposits.

Amplitude of Vibration: The maximum movement or displacement of any particle that vibrates.

Anchor: An apparatus which holds a floating object to the bottom,' or any device for holding an object to the ground or to other fixed objects.

Anchor Arm: The end portion of a cantilever bridge extending from one of the main piers to an anchor pier.

Anchor Bar: An eye-bar extending from the shoe of a span or tower into the concrete or masonry of the supporting pier or abutment for the purpose of holding down the span that rests thereon in case that it be subjected to uplift.

Anchor Bolt: A round, steel bolt embedded in concrete or masonry to hold down machinery, castings, shoes, spans, engine beds, etc.

Anchor Pier: A pier used in cantilever bridges to resist the uplift at the end of the anchor arm.

Anchor Pile: A pile used for the attachment of lines for anchorage purposes.

Anchor Plate: A square or rectangular plate, or washer, at the bottom of an anchor bolt.

Anchor Shackle: A bolt or clevis with two eyes and a screw bolt and key, used for securing a cable to the ring of an anchor; also employed for coupling chains.

Anchor Span: In a bridge consisting of a series of cantilevers, the span that separates two cantilever arms of other spans is termed an "anchor span."

Anchorage: A device for anchoring down any part subjected to uplift, such as the end of the anchor arm of a cantilever bridge.

Angle: The amount of divergence between two intersecting, straight lines. The term is also applied to an angle-iron section.

Angle Clip: Same as "Clip Angle."-- A short attaching angle that takes a portion of the stress of any main member.

Angle-iron: A rolled piece of steel having a cross-section shaped into a right angle.

Angle Joint: A joint in which two pieces meet at an angle.

Angle Lacing: A system of lacing in which angle-irons are used in place of bars.

Angle Lug: Same as "Clip Angle." -- A short attaching angle that takes a portion of the stress of any main member."

Angle of Friction: Same as "Angle of Repose."

Angle of Repose: The angle of inclination to the horizontal of an inclined plane on which a body will be just upon the verge of motion.

Angle of Rupture: The angle made with the transverse axis by the break in a test piece.

Angle of Torsion: The amount of rotation produced by a torque.

Angle of Twist: Same as "Angle of Torsion."

Angle Shears: A shearing machine especially adapted for cutting angles.

Angle Strut: A strut built up of angle irons.

Angular Fracture: A sharp-pointed or sharp-cornered fracture.

Angular Strain: Same as "Torsional Strain." -- A deformation in a member caused by a twisting moment.

Angular Velocity: The rate of angular motion.

Anneal: To reduce the brittleness and increase the ductility of metal by heating to, a certain temperature, then cooling slowly in air or oil.

Annealing Furnace: A furnace in which the process of annealing is carried on.

Annuity: A regular, yearly payment of a uniform sum of money.

Anvil: A heavy block of steel on which metals may be hammered, shaped, or forged.

Anvil Vise: A vise with an anvil on the fixed jaw.

Apex: The intersection of a web member with a chord or flange; also called a panel point.

Apex Load: The load at a panel point of a truss.

Apparent Stress: A term used to indicate that the stress has been determined by the principles of statics, and, therefore, ignoring the effect of the lateral deformation of the member or that of secondary -stresses.

Approach: The construction leading to the end of a bridge.

Apron: A device to protect a river bank or river bed against scour; a shield.

Aqueduct: An artificial canal for the conveyance of water, either above, on, or under the ground.

Arbitration Test Bar: A form of small test bar used for determining the quality of material going into a casting.

Arc: A portion of a curve. An arch.

Arch: Any bow-like curve, structure, or object, usually having the convex side upward, generally spanning an opening and producing horizontal as well as vertical reactions.

Arch Barrel: Same as "Arch Ring."

Arch Bridge: A curved structure which produces reactions inclined to the vertical.

Arch Buttress: A flying buttress; an arch springing from a buttress or pier.

Arch Centre: A temporary structure for supporting an arch while in the process of construction.

Arch Culvert: A culvert having an arch roof.

Arch Depth: The depth of the arch ring at any point at right angles to the axis.

Arch Rib: A rigid curved beam either solid or built up of members like a truss.

Arch Ring: That portion between the extrados and intrados of an arch, sometimes called an "Arch Barrel."

Arch Stone: Same as "Voussoir." -- A stone or block in the shape of a truncated wedge which forms part of an arch ring.

Arch Truss: A truss having an arched upper chord in compression and a straight bottom chord or tie rod with vertical hangers.

Arched Girder: A girder which is cut, bent, or built in the shape of an arch.

Architect's Rod: A very light and simple sliding level rod having two equal parts each seven-eighths of an inch square. When closed it is about five and a half feet long. It carries a target and is graduated into feet, inches, and fractions of inches.

Architects' Scale: A scale in which the units are divided duodecimally.

Area: The amount of surface included between certain closed boundary lines; any particular extent of surface, region, or tract.

Area Moment: Same as "Moment Area." -- The area enclosed by a moment curve.

Argillaceous: Containing a certain amount of clayey matter, such as shale.

Arithmetical Progression: A progression in which any term, other than the first, is derived from the preceding term by adding a fixed quantity.

Arris: The edge or ridge formed by the intersection of two surfaces.

Asbestos: A white, gray, or green-gray fibrous variety of hornblende; usually one containing but little aluminum, as tremolite or actinolite. At times it is called earth flax, mountain cork, and amiantus. It is considered fireproof.

Asbestos Packing: Packing made from asbestos fibre and put up in the form of wicking.

Asbestos Paper: A paper made from asbestos fibre.

Ashlar: Large squared blocks of stone. Also frequently used for cut-stone masonry.

Ashlar Masonry: Stone masonry composed of blocks cut to regular size, generally rectangular, laid in courses of uniform height.

Asphalt: A bituminous material employed for covering roofs, filling between paving blocks, forming surfaces of roads, etc.

Asphalt Furnace: A portable furnace in which asphalt cement is heated for use in roofing or paving.

Asphalt Rock: A limestone impregnated with bituminous material.

Asphalter: One who covers surfaces with asphalt.

Asphaltic Mastic: A mastic composed of refined asphalt and other constituents, melted together at a temperature between 275 and 400 F., and thoroughly agitated by suitable appliances until the materials are completely blended into a homogeneous mass; sometimes referred to as Asphaltic Cement.

Asphaltum: Same as "Asphalt."

Assay: A test of the composition, purity, weight, etc., of metals or metallic substances such as ores or alloys.

Assay Balance: A very sensitive, accurate balance used by assayers for weighing exceedingly small quantities of materials.

Assay Furnace: A small, simple form of furnace and muffler for heating metals in cupels.

Assembling Bolt: A threaded bolt for holding together temporarily the several parts of a structure during riveting.

Assembling Hoist: A hoist for lifting and assembling the component parts of trusses, spans, etc., in the shop or yard of a bridge plant.

Assistant Engine: A steam or hydraulic motor used to control the reversing gear of a marine engine, or to turn the shaft when the main engine is at rest.

Atlantic Locomotive: A passenger locomotive having four pilot, four driving, and two trailer wheels.

Auger: An instrument for boring holes larger than those made by a bit or gimlet; consisting of a helix with cutting prongs or edges.

Auger Bit: A small auger used with a brace or a bit-stock.

Automatic Gate: In bridgework, a steel, timber, or concrete gate that works automatically.

Automatic Switch: A switch that is worked automatically by the passage of a car, used principally by street railways; also in vertical lift bridges.

Average End-Area Formula: A formula for finding the approximate volume of a prismoid. Thus:

whereV = volume
A1 = area of one base
A2 = area of the other base
l = the perpendicular distance between bases

Average Haul: The mean distance that material is to be hauled. The distance from the centre of gravity of the cut to the centre of gravity of the fill in respect to all the material moved.

Awl: A sharp, pointed tool used for punching small holes in wood or leather without removing the material itself.

Ax or Axe: A hand tool used for hewing timber and chopping wood, also in some forms employed for surfacing stone.

Axe Hammer: A mason's hand tool consisting of a combined hammer and axe on a short handle.

Axed: A form of stone dressing. See "Axed Dressing."

Axed Ashlar: Ashlar blocks which have been finished or dressed with an axe.

Axed Dressing: A finish in stonework as left by the mason's axe in dressing the face to a plane surface.

Axed Stone: Stone roughly dressed with a heavy, axe-like tool.

Axial: Pertaining to or of the nature of an axis.

Axial Stress: A stress, either tension or compression, acting along and in the direction of the axis.

Axiom: A self evident principle or fact.

Axis: A line about which a figure or a body is symmetrically arranged, or about which such a figure or body rotates. A principal line through the centre of a figure or solid. A fixed line along which distances are measured or to which positions are referred.

Axis of Gravity: A line passing through the centres of gravity of successive elemental sections of a body.

Axis of Pressure: A line passing through the centroids of pressure of different successive sections of a body.

Axis of Resistance: A line connecting the centres of resistance of successive sections of a member.

Axis of Rotation: A line passing through the centre of rotation and perpendicular to the plane of rotation.

Axis of Symmetry: A line about which the parts of a figure or body are symmetrically disposed.

Axle: A pin or spindle about which any wheel or member revolves.

Axle Concentration: The load from one axle of a locomotive or vehicle concentrated on a structure, or twice a wheel load.

Axle Load: The load which comes on an axle of a wagon, car, or locomotive and is in turn transferred to the structure.

Azimuth: The angular position of an object referred to a meridian.