Annual Average Daily Traffic (AADT) The total volume passing a point or segment of a highway facility in both directions for one year, divided by the number of days in the year.
Abutment
A retaining wall supporting the ends of a bridge or viaduct.
American Association of State Highway and Transportation Officials (AASHTO)
AASHTO is a nonprofit, nonpartisan association representing highway and transportation departments. Its guides and specifications are used to describe loading requirements for highway (vehicular) bridges.
Anchor Span
Located at the outermost end, it counterbalances the arm of a span extending in the opposite direction from a major point of support. Often attached to an abutment.
AnchorageBlock
Located at the outermost ends, the part of a suspension bridge to which the cables are attached. Similar in location to an abutment of a beam bridge.
Approach
The part of the bridge that carries traffic from the land to the main parts of the bridge.
Approach Span
The span or spans connecting the abutment with the main span or spans.
Aqueduct
A pipe or channel, open or enclosed, that carries water. May also be used as part of a canal to carry boats. Sometimes carried by a bridge.
Arch
A typically curved structural member spanning an opening and serving as a support.
Arch Bridge
A bridge whose main support structure is an arch. Additionally, the bridge may be termed a through arch, which is simply one where the roadway appears to go through the arch.
Arch Barrel
The inner surface of an arch extending the full width of the structure.
Awards Projects authorized to proceed with construction after bids have been received and accepted by the authorizing transportation agency.
Bascule Bridge
From the French word for "see-saw," a bascule bridge features a movable span (leaf) that rotates on a horizontal hinged axis (trunnion) to raise one end vertically. A large counterweight is used to offset the weight of the raised leaf. May have a single raising leaf or two that meet in the center when closed.
Beam
A horizontal structure member supporting vertical loads by resisting bending. A girder is a larger beam, especially when made of multiple plates. Deeper, longer members are created by using trusses.
Beam Bridge
A bridge built of beams, either classified as a short-span or long-span beam bridge, which is supported on beams whose ends rest on piers or abutments.
Bearing
A device at the ends of beams that is placed on top of a pier or abutment. The ends of the beam rest on the bearing.
Bedrock
The solid rock layer beneath sand or silt.
Bed Timbers
Timber components typically located between the top of an abutment or pier and the underside of the truss bottom chord. Intended to serve as sacrificial components they are easily replaced when deteriorated from rot, thus protecting truss components from similar deterioration.
Bent
Part of a bridge substructure. A rigid frame commonly made of reinforced concrete or steel that supports a vertical load and is placed transerse to the length of a structure. Bents are commonly used to support beams and girders. An end bent is the supporting frame forming part of an abutment.
Each vertical member of a bent may be called a column, pier or pile. The horizontal member resting on top of the columns is a bent cap. The columns stand on top of some type of foundation or footer that is usually hidden below grade.
A bent commonly has at least two or more vertical supports. Another term used to describe a bent is capped pile pier. A support having a single column with bent cap is sometimes called a "hammerhead" pier.
Bid Opening
The opening of construction project bids from contractors conducted by the transportation agency.
Bowstring Truss
A truss having a curved top chord and straight bottom chord meeting at each end.
Box Girder Bridge A box girder bridge is a bridge where the main beams comprise girders in the shape of a hollow box. The box girder normally comprises either prestressed concrete, structural steel, or a composite of steel and reinforced concrete. The box is typically rectangular or trapezoidal in cross-section. Box girder bridges are commonly used for highway flyovers and for modern elevated structures of light rail transport. Although normally the box girder bridge is a form of beam bridge, box girders may also be used on cable-stayed bridges and other forms.
Brace
A structural support or to strengthen and stiffen a structure to resist loads. Brace-Ribbed Arch (Trussed Arch)
An arch with parallel chords connected by open webbing.
Bridge Condition Ratings
Through periodic safety inspections, data is collected on the condition of the primary components of a structure. Condition ratings, based on a scale of 0-9, are collected for the following components of a bridge. A condition rating of 4 or less on one of the following item classifies a bridge as structurally deficient.
The bridge deck, including the wearing surface:
The superstructure, including all primary load-carrying members and connections
The substructure, considering the abutments and all piers
To be eligible for federal aid the following is necessary (a local match is required):
Replacement: bridge must have a sufficiency rating of less than 50 and be either functionally obsolete or structurally deficient.
Repair: bridge must have a sufficiency rating of less than 80 and the jurisdiction is prevented from using any additional federal aid for 10 years.
Cable
Part of a suspension bridge extending from an anchorage over the tops of the towers and down to the opposite anchorage. Suspenders or hangers are attached along its length to support the deck.
Cable-Stayed Bridge
A variation of suspension bridge in which the tension members extend from one or more towers at varying angles to carry the deck. Allowing much more freedom in design form, this type does not use cables draped over towers, nor the anchorages at each end, as in a traditional suspension bridge.
Caisson
"Caisson" is the French word for "box." A caisson is a huge box made of steel-reinforced and waterproof concrete with an open central core. At the base of the caisson is its "cutting edge" of plate steel. In a suspension bridge the caisson becomes the foundation, the pier, supporting for the bridge's towers.
Camber
A positive, upward curve built into a beam that compensates for some of the vertical load and anticipated deflection.
Camelback Truss
A truss having a curved top chord and straight bottom chord meeting at each end, especially when there are more than one used end to end.
Cantilever
A structural member that projects beyond a supporting column or wall and is counterbalanced and/or supported at only one end.
Cast-in-Place
Concrete poured within formwork on site to create a structural element in its final position.
Castellated Girder
A steel beam fabricated by making a zig zag cut along its web, then welding the two sides together at their peaks. This creates a beam that has increased depth and, therefore, greater strength, but is not increased in weight.
Catenary
Curve formed by a rope or chain hanging freely between two supports. The curved cables or chains used to support suspension bridges may be referred to as catenaries.
Catwalks
Temporary foot bridges, used by bridge workers to spin the main cables (several feet above each catwalk), and to attach the suspender cables that connect the main cables to the deck.
Centering
Temporary structure or falsework supporting an arch during construction.
Chord
Either of the two principal members of a truss extending from end to end, connected by web members.
Closed Spandrel Deck Arch
One of the oldest and strongest types of bridges, the arch has been used in bridge building since the Roman era. An arch supports weight by compression on an axis or keystone. The axis or keystone transfers the stress of gravity (the traffic load) outward toward the base of the arch. Supports called abutments absorb the pressure and stop the ends of the bridge from spreading apart. Most arch bridges have vertical supports called spandrels that distribute the weight on the deck to the arch below.
The first arch bridges were built of stone but now are constructed of concrete or steel for greater strength and durability. Arch bridges are strong because the entire structure is under compression. Therefore, they must be built of materials that are strong under compression. Putting more weight on the arch actually compresses the material and makes the structure more rigid. Most arch bridges span between 200-800 feet.
Column
A vertical, structural element, strong in compression.
Column Cross Brace
Transverse brace between two main longitudinal members.
Compression Member
An engineering term that describes a timber or other truss member that is subjected to squeezing or pushing. Also see tension member.
Condition Ratings
According to the National Bridge Inspection Standards (NBIS), condition ratings are used to describe an existing bridge or culvert compared with its condition if it were new. The ratings are based on the materials, physical condition of the deck (riding surface), the superstructure (supports immediately beneath the driving surface), and the substructures (foundation and supporting posts and piers). General condition ratings range from 0 (failed condition) to 9 (excellent).
Continuous SpanBeam Bridge
A simple bridge made by linking one beam bridge to another; some of the longest bridges in the world are continuous span beam bridges.
Crown
On road surfaces, where the center is the highest point and the surface slopes downward in opposite directions, assisting in drainage. Also a point at the top of an arch.
Culvert
A drain, pipe or channel that allows water to pass under a road, railroad or embankment.
Damping
The action of reducing the vibration of an object. This tends to return the vibrating object to its original position.
Dead Load
The static load imposed by the weight of materials that make up the bridge structure itself.
Debarrment
Legally preventing a company or person from participation in bid lettings or construction proceedings.
Deck
The roadway portion of a bridge, including shoulders. Most bridge decks are constructed as reinforced concrete slabs, but timber decks are still seen in rural areas and open-grid steel decks are used in some movable bridge designs.
Deck Bridge
A bridge in which the supporting members are all beneath the roadway.
Deck Plate Girder A plate girder bridge is a bridge supported by two or more plate girders. The plate girders are typically I-beams made up from separate structural steel plates (rather than rolled as a single cross-section), which are welded (or occasionally bolted or riveted) together to form the vertical web and horizontal flanges of the beam. In some cases, the plate girders may be formed in a
Z-shape rather than I-shape.
Plate girder bridges are suitable for short to medium spans and may support railroads, highways or other traffic.
In the deck-type bridge, a steel or reinforced concrete bridge deck is supported on top of two or more plate girders, and may act compositely with them. Additional beams may span across between the main girders, for example in the form of bridge known as ladder-deck construction. Also, further elements may be attached to provide cross-bracing and prevent the girders from buckling.
Deck Truss
A bridge whose roadway is supported from beneath by a truss.
Deck Truss Cantilever Bridge
A cantilever bridge is a bridge built using cantilevers: structures that project horizontally into space, supported on only one end. For small footbridges, the cantilevers may be simple beams; however, large cantilever bridges designed to handle road or rail traffic use trusses built from structural steel, or box girders built from prestressed concrete. A simple cantilever span is formed by two cantilever arms extending from opposite sides of the obstacle to be crossed, meeting at the center. In a common variant, the suspended span, the cantilever arms do not meet in the center; instead, they support a central truss bridge which rests on the ends of the cantilever arms.
Deflection
The displacement of a structural member or system under load.
Diaphragm
Bracing that spans between the main beams or girders of a bridge or viaduct and assists in the distribution of loads.
Diagonal
A sloping structural member of a truss or bracing system.
Diversion Channel
A bypass created to divert water around a structure so that construction can take place.
Downstream Face
The side of the bridge that is not against the water.
End Post
The outwardmost vertical or angled compression member of a truss.
Environmental Impact Statement (EIS)
A comprehensive study of potential social, economic and environmental impacts related to a federally-assisted project. Projects for which an EIS is required are defined in the National Environmental Policy Act of 1969, as amended.
Expansion Joint
A meeting point between two parts of a structure that is designed to allow for movement of the parts due to thermal or moisture factors while protecting the parts from damage. Commonly visible on a bridge deck as a hinged or movable connection.
Extrados
The outer exposed curve of an arch; defines the lower arc of a spandrel.
Eye Bar
A structural member having a long body and an enlarged head at each end. Each head has a hole though which a pin is inserted to connect to other members.
Fatigue
Cause of structural deficiencies, usually due to repetitive loading over time.
Federal Fiscal Year October 1 through September 30 of the following year.
Federal Highway Administration (FHWA) The mission of FHWA is to administer the Federal-Aid Highway Program to create the best transportation system in the world for the American people through proactive leadership, innovation, and excellence in service. The FHWA is a part of the United States Department of Transportation and is headquartered in Washington, D.C., with field offices located across the United States.
Fill
Earth, stone or other material used to raise the ground level, form an embankment or fill the inside of an abutment, pier or closed spandrel.
Fixed Arch
A structure anchored in its position. Compare to hinged arch.
Fixed-span Bridge
A bridge without a movable, or draw, span.
Floor Beam
Horizontal members that are placed transversely to the major beams, girders or trusses; used to support the deck.
Flutter
Self-induced harmonic motion. A self-excited aerodynamic instability that can grow to very large amplitudes of vibrations.
Flyover
A bridge formed when one roadway crosses over another at a higher level; an overpass.
Footing
The enlarged lower portion of the substructure or foundation that rests directly on the soil, bedrock or piles; usually below grade and not visible.
Force
Any action that tends to maintain or alter the position of a structure.
Forms
Temporary structures or molds made of wood, metal, or plastic used when placing concrete to ensure that it is shaped to its desired final form.
Formwork
A total system of support for freshly placed concrete, including the mold and all supporting members, hardware, and necessary bracing. Formwork must be strong enough to support the considerable weight and pressure of wet concrete without bending or breaking.
Fracture-Critical
A fracture-critical bridge is one that does not contain redundant supporting elements. This means that if those key supports fail, the bridge would be in danger of collapse. This does not mean the bridge is inherently unsafe, only that there is a lack of redundancy in its design.
Full-Depth Replacement of Concrete Deck
A technique used to restore the structural integrity and rideability of distressed concrete pavement. It involves removing the deteriorated concrete down to the base, repairing the base, and refilling the excavated area with new concrete. Full-depth replacement is a particularly effective technique for pavement repairs near joints and cracks. By removing and replacing isolated areas of deterioration, pavement can be restored close to its original condition.
Functionally Obsolete
A functionally obsolete bridge is one that was built to standards that are not used today. These bridges are not automatically rated as structurally deficient, nor are they inherently unsafe. Functionally obsolete bridges are those that do not have adequate lane widths, shoulder widths, or vertical clearances to serve current traffic demand, or those that may be occasionally flooded.
A functionally obsolete bridge is similar to an older house. A house built in 1950 might be perfectly acceptable to live in, but it does not meet all of today’s building codes. Yet, when it comes time to consider upgrading that house or making improvements, the owner must look at ways to bring the structure up to current standards.
Gabion
A galvanized wire box filled with stones used to form an abutment or retaining wall.
Girder
A horizontal structure member supporting vertical loads by resisting bending. A girder is a larger beam, especially when made of multiple metal plates. The plates are usually riveted or welded together.
Girder Bridge
A girder bridge is perhaps the most common and most basic bridge. The cross section of the girder takes the shape of the capital letter “I”. The vertical plate in the middle is known as the web, and the top and bottom plates are referred to as flanges. A box girder is much the same as an I-beam girder except it takes the shape of a box. The typical box girder has two webs and two flanges. However, in some cases there are more than two webs, creating a multiple chamber box girder. Other examples of simple girders include pi girders, named for their likeness to the mathematical symbol for pi, and T shaped girders.
Glu-lam Freespan
Glu-lam bridges can freespan over 100 feet. This makes a Glu-lam freespan the ideal choice for road overpasses, bridging steep ravines and creating a crossing where pile foundations would be prohibited.
Gross Vehicle Weight (GVW)
Refers to the total curb weight of the vehicle and payload. Expresses the maximum continuous load for vehicles traversing a bridge.
Gusset Plate
A metal plate used to unite multiple structural members of a truss.
Hanger
A tension member serving to suspend an attached member.
Haunch
The enlarged part of a beam near its supported ends that results in increased strength; visible as the curved or angled bottom edge of a beam.
Haunched Girder Typical slab-on-beam bridges have space between the bottom of the slab and the top of the top flanges of beams. This space, referred to as the fillet or haunch, typically consists of unreinforced concrete that increases the dead load of the section but is not normally considered to add strength.
Headwall
The device placed at the end of a bridge that comprises a large portion of the abutment. Headwalls are used to retain the road formation soil around and above the abutments and prevent erosion at the abutment.
Hinged Arch
A two-hinged arch is supported by a pinned connection at each end. A three-hinged arch also includes a third pinned connection at the crown of the arch near the middle of a span. Compare to fixed arch.
Humpback
A description of the sideview of a bridge having relatively steep approach embankments leading to the bridge deck.
