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Glossary of Bridge Terminology



I-Beam: A rolled structural shape having a cross-section resembling the letter "I."

I-Beam Bridge: A small bridge consisting of a floor supported on I-beams.

I-Beam Dog: A special pair of dogs for lifting and moving I-beams.

I-Beam Girder: A girder composed of an I-Beam.

I-Beam Hook: Same as "I-Beam Dog."

Ice Apron: An ice breaker, or starling, placed on the up-stream end of a bridge pier to protect it from the moving ice.

Ice-break, or Ice-breaker: A structure of masonry or timber (as a pier or a cluster of piles) for the protection of bridge piers against moving ice.

Ice Guard: A fender placed at the up-stream end of a bridge pier to divert the ice or else to break up the large cakes into small pieces.

Idle Gear: An internmediate gear wheel running loosely on its own axle, used to convey motion from one wheel to another, all three being upon different axles.

Idle Pulley: Same as "Loose Pulley." -- A pulley which turns loosely on its shaft.

Idle Wheel: A wheel which runs loosely on its shaft.

Idler: Same as Idle Wheel.

Ignition: Firing; setting on fire; provision for firing.

Impact: The act of striking. The forcible momentary contact of a moving body with another either moving or at rest.

Impact-Allowance Load: A percentage allowance for impact applied to the equivalent uniform live load. See also "Coefficient of Impact."

Impact Load: A load due to "Impact."

Impact-load Stress: Same as "Impact Stress."

Impact Stress, or Impact Load Stress: Any stress caused by the sudden application of a load over and above that which the load at rest would produce.

Impervious: Not susceptible of being passed through, or penetrated. Generally refers to the percolation of water.

Impost: The point where an arch rests on a wall or column, or the upper part of a pier from which an arch springs.

Impulse: The effect of a blow or thrust.

Impulsive Force: A force which produces a finite change of motion in an indefinitely brief time.

In the Clear: Out of the way of moving objects.

Inch-pound: A unit of energy or work. The work done in raising a pound vertically through an inch. A unit of moment equal to a force of one pound acting with a lever-arm of one inch.

Inch Stress: A stress distribution on a square inch of area; the common unit of stress in metals.

Inch Ton: A unit of work equal to that involved in raising one ton one inch high.

Incise: To cut into; to engrave. To form by cutting. To carve.

Inclined End Post: An inclined compression member at the end of the truss. Also called "Batter Post" and "Batter Brace."

Inclined Plane: A plane which makes an angle less than ninety degrees with the horizontal.

Inclined Strut: A compression member placed in an inclined position.

Incrustation: The act of covering or lining with any foreign substance; also the lining itself.

Indentation Roller: A hand tool for roughening concrete surfaces, consisting of a roller with teeth mounted in a frame attached to a handle.

Indented: Notched by a small hollow or depression.

Indented Finish: A finish made on cement work by running an indentation roller over it while soft.

Indeterminate Stress: A stress which cannot be determined by the principles of statics.

Indicated Horsepower: The power developed in the cylinder of a steam engine as determined from an indicator diagram. It is equal to the mean effective pressure in pounds per square inch, multiplied by the area of the piston in square inches, by the piston speed in feet per minute, and divided by thirty-three thousand (33,000).

Indicator: A marker. The pointer on a steam gauge or any recording instrument. An instrument for measuring the steam pressure, at various positions of the piston, in an engine cylinder.

Indicator Diagram: The diagram showing the relation between pressure and piston travel in an engine cylinder, as traced by indicator.

Indifferent Equilibrium: That condition of a body when a slight displacement of it is retained without further motion.

Indirect Stress: A stress induced by another stress.

Indirect Wind Load: A transferred wind load.

Indirect Wind Stress: A stress due to a transferred wind load.

Induced Stress: Same as "Indirect Stress."

Indurated Fibre: A hard, thick, dense, fibrous material used for insulation in electrical apparatus.

Inelastic: Not elastic; rigid; unyielding.

Inertia: That property of matter by virtue of which it persists in a state of rest or of uniform motion in a straight line unless some force changes that state. The state or quality of being inert. Indisposition to move or to act. Inertness.

Inflection: A change of curvature from concavity to convexity, or vice versa.

Inflection Point: The point where reversal of curvature occurs. Same as point of contraflexure. See also "Contraflexure."

Influence Line: A line which represents the variation of moment, shear, stress, deflection, or similar function at a particular point in the structure, due to a load of unity moving across it.

Ingot: A large mass of metal cast in a mould.

Ingot Iron: Soft steel cast in ingots, sometimes with about three per cent of copper added.

Ingot Mould: A flask in which metal is cast into a large block or ingot.

Ingot Steel: Steel run from the furnace into rectangular moulds to be subsequently rolled or forged.

Ingredient: A component part or element of a compound or mixture.

Initial Set: The beginning of the hardening process of cement mortar as determined by the Vicat needle.

Initial Set of Cement: Same as "Initial Set."

Initial Stress: Stress put on a member before the regular loads are applied. This is accomplished by making the member a trifle shorter or longer than the required normal length and then forcing it into place in the structure, or by operating a turnbuckle after erection.

Initial Tension: Tension applied to a member before it is subjected to the principal load.

Injection Condenser, or Jet Condenser, or Siphon Condenser: A form of condenser in which the injected water comes in contact with the steam.

Injector: An apparatus for forcing water into a steam boiler by means of an enclosed jet or nozzle, through which the steam issues at a high velocity, drawing water through a suction pipe and carrying it along to the boiler in a feed pipe, where, because of its high velocity and force of impact, it is able to overcome the back pressure and enter the boiler.

Inlay: That which is inserted or laid in something else. To do such insertion. To decorate by insertion.

Inner Guard-rails: Guard-rails placed between the gauge lines of a car track.

Inner Hip: The intersection of the inner inclined end post with the top chord in the arm of a swing span.

Inner Lock Tender: Same as "Inside Lock Tender."

Inside Caliper: A caliper for measuring any inside diameter.

Inside Lock Tender: The man inside the air-lock who manipulates the pressure valve and the opening of the lock doors.

Inspection: Critical examination. Close or careful survey of work, materials, etc.

Inspection Bureau: An organization or firm composed of a number of inspectors who have determined upon the best and most economic method of inspection and who make a business of doing inspection work on a large scale.

Inspector: One whose duty is to secure by careful supervision the proper performance of work of any kind, and the proper kind of materials going into the work, all according to the plans and specifications.

Instrument-man: In engineering work, the person who runs a level or a transit.

Insulation: That state in which the transfer of electricity or heat from a certain body to other bodies is prevented by the interposition of a non-conductor. The non-conductor itself.

Insulator: A device, fixture, or material which insulates one body from another.

Intake: The construction work at the head of a pipe or canal for regulating the admission of water to said pipe or canal.

Intensity of Stress: The stress per unit of area. Also called "Unit Stress."

Interlaced: Interwoven; intercrossed.

Interlocking: The action of linking into each other, or the joining fast together by mutual or reciprocal action.

Interlocking Device: Any mechanism for interlocking.

Interlocking System: A system of railroad switches and signals which by a locking mechanism insures the setting of a signal on the movement of a switch, and prevents the movement of more than one switch at a time.

Intermediate Bent: Any bent between the end bents.

Intermediate Deck: A deck between two other decks, or at some distance vertically from either chord.

Intermediate Floor-beam: Any floor-beam between the end floor beams.

Intermediate Girder: Any girder between the two outside girders.

Intermediate Post: A post between the two outside posts in a timber bent.

Intermediate Sill: A horizontal member in the plane of a timber trestle bent between the elevations of cap and sill, to which the posts are framed.

Intermediate Span: Any one of the spans between the end spans of a bridge.

Intermediate Stiffener: Any one of the stiffeners on a plate girder between the end stiffeners.

Intermediate Strut: An overhead strut in high bridges attached to the posts of opposite trusses and lying between the upper lateral strut and the floor. In deck bridges, if used at all, it would be between the upper and the lower lateral struts.

Intermediate Truss: The centre truss of a three-truss span.

Internal Combustion Engine: An engine in which the fuel, such as gas, vapor, gasoline, or oil is burned direct in the cylinder, generating a high temperature and high pressure in the gases of combustion, which expand behind a piston and drive it forward.

Internal Force: Same as "Internal Stress."

Internal Stress: Any stress in a member.

Intersection: A place of crossing; cancellation. A point common to two lines or a line and a surface.

Intrados: The concave curve of an arch. The lower surface of the voussoirs (when in position) of a masonry arch.

Invert: To turn upside down; to turn end for end. The bottom or inverted arch of a sewer or tunnel.

Inverted Arch: An arch having its intrados below the axis or springing line.

Inverted Catenary: A curve formed by reversing the position of an ordinary catenary so as to make it convex upward.

Inverted Catenary Curve: A curve formed by revolving the ordinary catenary one hundred and eighty degrees.

Invoice: A bill from the seller for goods shipped to the buyer, with information concerning the size, character, weight, etc., of the shipment, given in more or less detail. This bill may or may not have the prices of the goods shown on it.

Involute: A curve described by the end of a string as it unwinds from a cylinder while remaining taut.

Involute Curve: Same as "Involute."

Involute Tooth: A form of gear tooth in which the faces conform to an arc of an involute and the flanks to radial planes.

Iron: A common but important and abundant metal having a specific gravity of about eight. The pure metal has a white, lustrous appearance, does not harden appreciably on quenching, and is strongly attracted by a magnet, although it cannot be made magnetic except when containing carbon, or while an electric current is passed around it. The term is often applied to a tool or utensil made of iron. Also applied to various structural shapes.

Iron-bound: Bound together by bands of iron.

Iron-founder: One who makes iron castings.

Iron Foundry: The place where iron castings are made.

Iron Furnace: A general term for any iron working furnace, such as a blast furnace, puddling furnace, etc.

Iron-gray: A gray hue.

Iron-master: A manufacturer of iron.

Iron-oxide: An intimate combination of oxygen and iron, such as rust. Also see " Ochre."

Iron-red: A red of somewhat orange tint as produced by iron rust.

Iron Rust: The oxide of iron.

Iron Sand: Sand containing considerable quantities of small particles of iron ore.

Iron Saw: Same as "Hot Saw." -- A circular saw for hot steel or iron shapes.

Iron Scab: A scab or scab-plate made of iron.

Iron Scale: A loose coating of oxide which forms on heated iron during the process of forging.

Iron-smith: A worker in iron.

Iron-stain: A stain made by iron rust on some object.

Iron Stone: An oxide of iron rendered impure through the admixture of silica and clay.

Iron Work: Any construction using iron members.

Iron-works: The plant or place where iron structures are fabricated and assembled.

Iron-worker: A bridgeman or man who helps erect iron or steel.

Ironwork: See "Iron Work."

Irregular Course: A course in which the thicknesses of the stones vary at intervals.

Irregular Curve: A draftsman's tool for drawing curved lines of varying radii.

Irregular Fracture: An extremely uneven fracture.

Isodomon: One of the varieties of masonry in Greek architecture in which the blocks forming the courses were of equal thickness and of equal length, and so disposed that the vertical joints of the upper course came over the middle of the blocks in the course immediately below, all blocks being joined by horizontal dowels.

Isometric Projection: A mode of geometrical drawing in which three planes are projected at equal angles upon a single plane, and all the measurements are upon the same scale; used at times to show machinery, buildings, etc.

Isosceles: Having two legs or sides equal, as in a triangle.

Isotropic: Having the same physical properties in every direction.