Glossary of Timber Bridge Terminology

Additional Information

AASHTO- American Association of State Highway and Transportation Officials (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. 


Abutment- The supporting structure at the end of the bridge which retains the roadway approach to the bridge. ATS Builders, timber bridges include a timber abutment and wing wall as standard features. 


ACQ (Alkaline Copper Quarternary)- A pressure treatment no longer used by the masses.  Other safe treatments such as Copper Azol (CA) replaced this treatment which is commonly used by ATS to protect wood against decay and insects.  CA contains copper and Azol (fungicides that attack decay organisms) that work together to protect the lumber and is safe for hardware.


Bent- A supporting unit of a bridge comprised of two or more piles connected by a cap or another member. This connecting member distributes superimposed loads on the bent. When combined with a system of diagonal or horizontal bracing attached to the piles, the entire construction distributes its loads onto the foundation. 


Bridge - A structure built over a ravine, waterway, highway, or wetlands area for the purpose of allowing vehicles or pedestrian access. 


Caisson– The commonly used name for a cylindrical casing which becomes the structural support for a piling. The piling is inserted and concrete is usually poured into the casing. This is used in areas where it may be impossible or impractical to drive piling alone without this extra support. 


Camber- The arch of a deck, sloping downward from the center toward the sides or, the arching of the deck upward measured at the centerline in inches per foot beam. The camber could be for either aesthetic reasons or structural integrity. 


CCA (Chromate Copper Arsenate)- A water-based wood preservative containing arsenic, chromium and copper which is used to protect against decay, insects and increase the life of the timber.  It is a common pressure treatment which ATS uses to increase the overall longevity of your bridge.   


Combination Design- A bridge design whereby pile supported sections are supplemented by free span section(s). 


Culvert - A pipe or concrete box structure that conveys flow from open channels, swales, or ditches under a roadway, driveway, fill soil, or surface structure. The pipe can be made of galvanized corrugated metal, plastic, aluminum or concrete. 


CuNap(Copper Napthenate)- CuNapis an oil-based pressure treatment for wood which seals and protects against water damage, rotting, insects, and many other dangers. 


Dead load- The static load imposed by the weight of the materials that make up a given structure. 


Deck - The horizontal riding surface of a bridge. All ATS Timber Vehicular 

Bridges include a structuraldeck and timber weardeck.


Deflection- The displacement of a structural member or system under load. 


Diaphragm- The structural supporting member that is placed perpendicular between stringers for added support and stability.  


Elevation Difference- The elevation difference is the height from one end of the bridge to the other end of the bridge. ATS can easily incorporate this feature.


Elevation View- The side view of a bridge, usually seen on shop drawings.


Filter fabric- A form of erosion control, whereby fabric is "wrapped around" the timber abutment in order to prevent soil loss.  This fabric allows for water to freely pass through while not allowing soil loss. This is used on all ATS timber abutments.


Free span- A bridge that is supported only on both ends of the embankment and not in the center section with a pile. A free span bridge is ideal for crossing large areas where pile foundations would be prohibited, such as road overpasses, extremely environmentally restricted areas and extremely steep ravines.


Glue-lam- An engineered timber product manufactured by gluing together individual pieces of dimensional lumber. Glue-lam beams can span a much greater distance than with timber and, pound for pound, glue-lam beams are much stronger than sawn timber.  It is also a more attractive way to span large distances compared with concrete and steel.


GVW- Gross Vehicle Weight refers to the total curb weight of the vehicle and payload. GVW expresses the maximum continuous load for vehicles traversing a bridge. ATS's Light Maintenance Capacity bridges are 5-ton GVW loading; Heavy Maintenance Capacity is rated for 10-ton GVW. 


Guide rail Cap- The timber piece at the top of a guide rail that finishes, or "caps" the railing.  


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. 


HS20-44- A designation set forth by AASHTO. “HS” refers to the type of vehicles a bridge or highway can accommodate; “20” refers to the loading specification of the bridge; “44” indicates the year the specification was adopted. HS20-44 capacity means that the bridge or highway is able to safely accommodate 3-4 axle vehicles, such as a large semi-truck and trailer.  All ATS Timber Vehicular Bridges meets or exceeds these AASHTO requirements. ATS can also design HS25-44 Timber Vehicular Bridges, which are able to accommodate a much higher continuous loading capacity. 


KDAT (Kiln Dried After Treatment)- KDAT is wood that is dried in a kiln after pressure treatment.  When wood is pressure treated, moisture in the form of the chosen treatment is introduced into the wood.   As it dries, lumber often shrinks, cups, and warps.  Kiln-drying the lumber before installation greatly prevents substantial shrinkage, cupping, or warping.  It also has superior strength and stiffness. 


Live Load– Is the variable load on a structure (e.g. a bridge) such as moving traffic or pedestrians. It is usually expressed in terms of pounds per square foot.  Standard loading for ATS timber bridges are 85 lbs./square foot for pedestrian bridges and 100 lbs./square foot for boardwalk complexes. Both can be increased according to client needs.

Pedestrian guide rail- A barrier or protection attached to the sides of a bridge to be a guide for moving pedestrian traffic. This can be decorative or customized to some extent according to client's needs.  


Penta(Pentachlorophenol)- The most commonly used oil based preservative used to pressure treat wood and guard against decay and insect infestation.


Pile- A long timber, concrete or steel structural element that is driven or otherwise embedded into the ground for the purpose of supporting a structure.  


Pile-supported– A type of bridge design whereby evenly spaced piling driven into the ground are used as the support system for the bridge. 


Piling Cap – Is the main ingredient in the support structure for weight displacement on a timber bridge.  This timber is placed on top of the piling at each bent and underneath the stringers that connect the bents.  


Plan View- View of a bridge from above the structure, usually seen on shop drawings.


Retrofit- Modification of an existing structure to incorporate changes not available at time of original construction. 


Refusal- This is a term used in pile driving.  When piling is driven into the ground to the point that the piling is "refusing" to penetrate the ground further with the weight of the heavy equipment used in driving bearing on the piling.  At this point it would be noted that "the piling was driven to the point of refusal." 


Rip rap- A layer of large stones or broken rock placed on an embankment as erosion control and protection.  


Span- The span is the distance, horizontally, between individual piling or the abutment and piling on a bridge (between the "bents"). 


Specifications- A document that explains all material and construction requirements of the bridge structure to be constructed, usually used by engineers or architects in the planning stages of construction.


Stringer- A supporting member that runs lengthwise on a bridge, also known as a joist.  


Vehicular guardrail- A fence-like barrier or protection built within the roadway shoulder area to be a guide for moving vehicular traffic and to hinder the accidental passage of such traffic beyond the roadway. This also can be decorative or customized to some extent according to client's needs.  


Vibratory hammer– A specialized piece of equipment to reduce or eliminate the inner stabilizing forces of soils by means of vibration and changing soil deformations. Only low force is then required to drive in the massive piles. The vibratory’sown weight and pre-tension are sufficient to drive the piles to the point of refusal quickly and efficiently.  


Wear deck- The timber wear-surface on the deck of a bridge. This deck, consisting of a second layer of wood planks, is built on top of the surface structure of the deck, in a perpendicular fashion. The deck can be repaired or replaced if needed and will not compromise the integrity of the structure if done. It is standard on all ATS Timber Vehicular Bridges and an option for light or heavy maintenance bridges.   


Wing wall- A wall used to retain the soil behind the abutment.  Wing walls help from losing valuable soil from the approaches on the bridge ends.


X-bracing- A form of additional supports for the piling of a bridge. The timbers are placed in a “criss-cross” pattern which appear as a “X”, joining the supporting piling. 

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