A low, reinforced concrete wall wider at the base, tapering vertically to near mid-height, then continuing straight up to its top. The shape is designed to direct automotive traffic back toward its own lane of travel and prevent crossing of a median or leaving the roadway. Commonly used on new and reconstructed bridges in place of decorative ballustrades, railings or parapets.
A device connecting two or more adjacent parts of a structure. A roller joint allows adjacent parts to move controllably past one another. A rigid joint prevents adjacent parts from moving or rotating past one another.
Crosspieces used to connect the ribs in centering.
Members used to stabilize a structure by introducing diagonal connections.
An assembly of smaller pieces arranged in a gridlike pattern; sometimes used a decorative element or to form a truss of primarily diagonal members.
The date bids are scheduled to be opened by the transportation agency.
The part of a movable-span (or drawbridge) that opens to allow river traffic. Also called the draw-span.
Vehicular traffic, wind, water, and/or earthquakes.
Weight distribution throughout a structure; loads caused by wind, earthquakes and gravity affect how weight is distributed throughout a structure.
Load Posted Any bridge or structure restricted to carrying loads less than the legal load limit. Load posting a bridge is required by National Bridge Inspection Standards when a bridge is not capable of safely carrying a legal load.
The bottom horizontal member of a truss.
A beam supporting the spans and bearing directly onto a column or wall.
The longest span in a multi-span bridge and located between the bridge's main piers or towers (supports). Bridges typically compared using main-span lengths, which do not account for the length of the entire bridge or its approaches.
An individual angle, beam plate or built piece intended to become an integral part of an assembled frame or structure.
A bridge in which the deck moves to clear a navigation channel; a swing bridge has a deck that rotates around a center point; a drawbridge has a deck that can be raised and lowered; a bascule bridge deck is raised with counterweights like a drawbridge; and the deck of a lift bridge is raised vertically like a massive elevator.
National Environmental Policy Act of 1969 (NEPA)
Legislation requiring that any project using federal funding or requiring federal approval (including transportation projects) examine the effects of alternative choices on the environment before a decision is made.
Obligation Authority The maximum amount of federal formula funds that can be obligated or authorized in a federal fiscal year. The use of obligation authority does not affect the apportionment or allocation of federal funds a state receives. It only controls the rate of expenditure.
Open Spandrell Deck Arch
An arch bridge is based on the ancient concept of spanning an opening with a curved structural member. The arch transmits the load from the bridge deck to the abutments on both sides of the span and thus to the ground below. Early arch bridges were built of stone blocks wedged together to form the arch. Short modern arch bridges may use wood or concrete, while longer arch spans are built of steel. Since the arch requires no central support, it can be used to bridge long open spans. The arch can be either above or below the bridge deck. The arch pushes downward and outward against its abutments, which must be heavy to resist the thrust. Since the abutments transfer both horizontal and vertical forces from the bridge deck, arch bridges can only be used where the ground or foundation is solid and stable. The curved arch structure offers a high resistance to bending forces.
Arch bridges can be constructed with the deck above the arch (a deck arch bridge), or the deck can be hung from a segment of the arch which rises above the deck (a through arch or tied arch bridge). In a deck arch bridge, the space between the bottom of the arch and the deck can be solid (a closed spandrel deck arch) or open with supporting vertical members (an open spandrel deck arch).
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 by 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 could fail.