Timber Bridge Construction Terminology

AASHTO – The American Association of State Highway and Transportation Officials develops widely used standards and specifications for the design and loading of highway bridges and other transportation infrastructure.

Abutment – The supporting structure at the end of a bridge that transfers bridge loads to the foundation and retains the roadway approach. Timber abutments may include headwalls and wing walls as part of the overall system.

ACQ (Alkaline Copper Quaternary) – A water-based wood preservative containing copper and quaternary compounds that protects wood against decay and insect attack. ACQ is one of several preservative treatment systems used for structural wood applications.

Bent – A bridge support assembly consisting of two or more piles connected by a cap beam. Bents transfer loads from the bridge superstructure through the piles to the supporting soil.

Bridge – A structure constructed to provide vehicular or pedestrian passage over an obstacle such as a waterway, ravine, wetland, roadway, or other obstruction.

Caisson – A deep-foundation element or casing installed below grade to provide structural support. Caissons may be used where site conditions require foundations extending to suitable bearing material.

Camber – An intentional upward curvature incorporated into a structural member or bridge system to compensate for anticipated deflection or achieve the desired finished profile.

CCA (Chromated Copper Arsenate) – A water-based wood preservative containing chromium, copper, and arsenic compounds. CCA-treated timber is commonly used in permitted industrial, agricultural, marine, and structural applications requiring protection against decay and wood-destroying organisms.

Combination Design – A bridge configuration combining pile-supported sections with one or more free-span sections to accommodate site conditions, waterways, access requirements, or other design considerations.

Culvert – A pipe, box, or similar structure that conveys water beneath a roadway, embankment, driveway, trail, or other surface.

CuNap (Copper Naphthenate) – An oil-borne wood preservative used to protect structural timber from decay, fungi, and wood-destroying insects.

Dead Load – The permanent load imposed by the weight of the bridge itself, including its structural components and other permanently attached materials.

Deck – The horizontal surface of a bridge that carries vehicular or pedestrian traffic. Timber vehicular bridges may incorporate a structural deck with an additional replaceable wear deck.

Deflection – The movement or displacement of a structural member from its original position when subjected to a load.

Diaphragm – A transverse structural member installed between primary bridge members to provide lateral stability, maintain spacing, and assist in distributing loads.

Elevation Difference – The vertical difference in height between two points, such as the elevations at opposite ends of a bridge.

Elevation View – A side or profile view of a bridge showing vertical dimensions, elevations, structural components, and other design information.

Filter Fabric – A geotextile material used to allow water to pass while limiting the migration of soil. It is commonly incorporated around abutments, retaining structures, and drainage areas to help control erosion.

Free Span – A bridge span supported at its ends without intermediate pile supports. Free-span construction may be used where intermediate foundations are impractical or undesirable due to waterways, roadways, environmental restrictions, steep terrain, or other site conditions.

Glulam (Glue-Laminated Timber) – An engineered structural wood product manufactured by bonding individual wood laminations together. Glulam members can provide greater structural capacity and longer spans than many conventional sawn-timber members.

GVW (Gross Vehicle Weight) – The total operating weight of a vehicle, including the vehicle, occupants, equipment, cargo, and payload. GVW may be used when establishing appropriate bridge loading requirements for maintenance and service vehicles.

Guide Rail – A barrier installed along the edge of a bridge or approach to help guide vehicles and reduce the potential for vehicles to leave the traveled surface.

Guide Rail Cap – The upper horizontal timber member installed along the top of a timber guide rail system.

Headwall – A structural component located at the end of a bridge or crossing that helps retain approach fill and stabilize the area surrounding the bridge opening.

HS20-44 – A legacy AASHTO highway bridge design loading designation based on a standardized tractor-semitrailer configuration. HS20-44 remains a commonly recognized reference when discussing bridge loading, although current bridge design may be governed by newer AASHTO loading criteria and applicable project requirements.

KDAT (Kiln Dried After Treatment) – Pressure-treated lumber that is kiln-dried after the treatment process to reduce moisture content. KDAT lumber generally provides improved dimensional stability and reduced shrinkage, checking, cupping, and warping compared with lumber installed at higher moisture content.

Live Load – A temporary or moving load applied to a bridge, including vehicles, pedestrians, equipment, and other loads that are not permanently part of the structure.

Penta (Pentachlorophenol) – An oil-borne wood preservative historically used for utility poles and other industrial wood products. Its use is subject to current regulatory requirements and approved applications.

Pile – A long structural foundation member, typically timber, steel, or concrete, driven or otherwise installed into the ground to transfer structural loads to suitable supporting soils.

Pile Cap (Cap Beam) – A horizontal structural member installed across the tops of piles to form a bent and support the bridge stringers or other superstructure components.

Pile-Supported Bridge – A bridge in which driven piles and pile bents provide the primary intermediate or end support for the structure and transfer bridge loads into the ground.

Plan View – A top-down view of a bridge showing its layout, dimensions, structural components, and relationship to surrounding features.

Refusal – A pile-driving condition in which further pile penetration becomes minimal under the specified driving method or equipment. Refusal criteria depend on the pile type, equipment, soil conditions, and project requirements.

Retrofitting – The modification or upgrading of an existing bridge or structure to improve performance, accommodate changed requirements, extend service life, or incorporate components not included in the original construction.

Riprap – A layer of durable stone or rock placed along embankments, shorelines, abutments, or drainage areas to help protect soil from erosion and scour.

Span – The horizontal distance between adjacent bridge supports, such as between bents or between an abutment and a bent.

Stringer – A longitudinal structural member that supports the bridge deck and transfers loads to the bridge bents, abutments, or other supporting members.

Vehicular Guardrail – A barrier system installed along the edge of a bridge to help contain and redirect vehicles and reduce the risk of vehicles leaving the traveled surface.

Vibratory Hammer – Pile-driving equipment that uses high-frequency vibration to reduce soil resistance and facilitate the installation or extraction of piles.

Wear Deck – A replaceable upper layer of decking installed over a structural bridge deck to provide the primary wearing surface and protect the structural decking below.

Wing Wall – A retaining wall extending from a bridge abutment or headwall that helps contain approach fill, stabilize the roadway transition, and control erosion.

X-Bracing – Diagonal structural members installed between piles or other bridge components in opposing directions to form an “X,” providing additional lateral stability and stiffness.