Tacoma Narrows Bridge history - Glossary


Abutment – A substructure element supporting each end of a single span or the extreme ends of a multi-span superstructure and, in general, retaining or supporting the approach embankment.

Anchor span – The span that counterbalances and holds in equilibrium the cantilevered portion of an adjacent span during construction.

Anchorage – Massive concrete structures, also called "cable anchorages" and "shore anchors." These are placed far enough back from the edge of the water to insure against sliding. They provide stability where the cable end is tied, withstanding the tremendous stress of the loaded cables.

Approach span – The span or spans connecting the abutment with the main span or spans.

Beam – A linear structural member designed to span from one support to another.

Bent – A substructure unit supporting each end of a bridge span; also called a pier; made up of two or more columns or column-like members connected at their top most ends by a cap, strut, or other member holding them in their correct positions.

Box girder –A support beam that is a hollow box; its cross-section is a rectangle or square.

Cable – The part of a suspension bridge or cable-stayed bridge that supports suspension support to the deck. The cable is made of many steel wires bound together into strands and anchored at each end.

Cable saddle – The cable saddles sit at the summit of each tower. They hold the main suspension cable where it crosses over each tower leg. As traffic, wind, and temperature changes affect the movement of the cables, the saddles absorb the load and shift it to the towers.

Cable spinning – The technique of pulling wires from the anchorage over towers and back to form the main cable. A "spinning wheel," or "traveler," carries the wires. At the anchorage the strands of wire are attached to the eye-bar. The wires are grouped into strands then bound tightly together to form strong suspension cables.

Cable-stayed bridge – A bridge in which the superstructure is directly supported by cables or stays, passing over or attached to a tower or towers located at the main pier(s).

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.

Cast-in-place – Concrete poured within form work on site to create a structural element in its final position.

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.

Chord – A horizontal member of a truss.

Cross brace – Transverse brace between two main longitudinal members.

Damping – The action of reducing the vibration of an object. This tends to return the vibrating object to its original position.

Dead load – A static load due to the weight of the structure itself.

Deck – The roadway portion of a bridge that directly supports vehicular and pedestrian traffic.

Deck bridge – A bridge in which the supporting members are all beneath the roadway.

Deck truss – A bridge whose roadway is supported from beneath by a truss.

Diagonal – A sloping structural member of a truss or bracing system.

Eye-bar – Steel bars that hold wire strands of the main cable, and that are attached to beams embedded in the concrete of an anchorage.

Expansion joint – A joint designed to provide means for expansion and contraction movements produced by temperature changes, load, or other forces.

Fatigue – Cause of structural deficiencies, usually due to repetitive loading over time.

Flutter – Self-induced harmonic motion. A self-excited aerodynamic instability that can grow to very large amplitudes of vibrations.

Footing – The enlarged, lower portion of a substructure that distributes the structure load either to the earth or to supporting piles; the most common footing is the concrete slab; "footer" is a colloquial term for footing.

Girder – A main support member for the structure that usually receives loads from floor beams and stringers; also, any large beam, especially if built up.

Hanger – A tension member serving to suspend an attached member.

Hinge – A point in a structure at which a member is free to rotate.

Joint – In stone masonry, the space between individual stone; in concrete, a division in continuity of the concrete; in a truss, the point at which members of a truss frame are joined.

Live load – Vehicular traffic.

Lower chord – The bottom horizontal member of a truss.

Main beam – A beam supporting the spans and bearing directly onto a column or wall.

Member – An individual angle, beam, plate, or built piece intended to become an integral part of an assembled frame or structure.

Oscillation – A periodic movement back and forth between two extreme limits. An example is the string of a guitar that has been plucked. Its vibration back and forth is one oscillation. A vibration is described its size (amplitude), its oscillation rate (frequency), and its timing (phase). In a suspension bridge, oscillation results from energy collected and stored by the bridge. If a part of the bridge has to store more energy than it is capable of storing, that part will probably fail.

Panel – The portion of a truss span between adjacent points of intersection of web and chord members.

Pier – A vertical support or substructure unit that supports the spans of a multi-span superstructure at an intermediate location between its abutments.

Pile – A shaft-like linear member that carries loads through weak layers of soil to those capable of supporting such loads.

Pile bent – A row of driven or placed piles with a pile cap to hold them in their correct positions; see Bent.

Plate girder – A large solid web plate with flange plates attached to the web plate by flange angles or fillet welds.

Portal – The clear, unobstructed space of a bridge forming the entrance to the structure.

Reinforced concrete – Concrete with steel reinforcing bars bonded within it to supply increased tensile strength and durability.

Resonance – The regular vibration of an object as it responds in step (at the same frequency) with an external force.

Rigid frame bridge – A bridge with moment-resistant connections between the superstructure and the substructure to produce an integral, elastic structure.

Riveted connection – A rigid connection of metal bridge members that is assembled with rivets. Riveted connections increase the strength of the structure.

Safety hangers – Back-up for original connections to provide redundancy; often added for seismic retrofit.

Span – The distance between piers, towers, or abutments.

Stable – Able to resist forces that can cause material deformation or structural collapse.

Steel – A very hard and strong alloy of iron and carbon.

Stay – Diagonal brace installed to minimize structural movement.

Strand shoe – The device at the end of an eye-bar that holds the wires of each strand for the main suspension cable. The strands loop around the shoes to form a continuous cable from anchorage to anchorage.

Stringer – A longitudinal beam supporting the bridge deck.

Strut – A piece or member acting to resist compressive stress.

Substructure – The parts of a suspension bridge that are below water or land; the piers, or tower foundations, and anchorages.

Superstructure – The parts of a suspension bridge that are above water or land; the towers, main cables, suspender cables, and related parts.

Suspension bridge – A bridge in which the floor system is supported by two cables that are supported upon towers and are anchored at their ends.

Suspended span – A simple span supported from the free ends of cantilevers.

Suspender cables –The vertical wires hung at regular intervals connecting the main suspension cables to the deck. The connection is made using a cable band.

Tension – A force that pulls or stretches.

Tie – A member carrying tension.

Torsion – A twisting force or action.

Tower – The vertical structure in a suspension bridge (or cable-stayed bridge) that holds up the suspension cables.

Trestle – A bridge structure consisting of spans supported upon frame bents.

Truss – A rigid, jointed structure made up of individual straight pieces arranged and connected, usually in a triangular pattern, so as to support longer spans.

Truss bridge – A bridge having a pair of trusses for the superstructure.

Tunnel – An underground passage open to daylight at both ends.

Upper chord – The top longitudinal member of a truss.

Viaduct – A series of spans carried on piers at short intervals.

Vortex – A rotary, swirling, circular motion of a fluid, like wind or water. The vortex forms a vacuum at its center that draws objects toward it. Examples of vortex motion are tornadoes and whirlpools, including the small whirlpool formed by water draining from a sink.

Warren truss – A triangular truss with sloping members (and no vertical members) between the top and bottom chords, forming the letter "W."

Web – The portion of a beam located between and connected to the flanges.

Web members – The intermediate members of a truss, not including the end posts, usually vertically or inclined.

Welded joint – A joint in which the assembled elements and members are united through fusion of metal.