An important first step in understanding the principles and processes of bridge construction is learning basic bridge terminology. Although bridges vary widely in material and design, there are many components that are common to all bridges. In general, these components may be classified either as parts of a bridge superstructure or as parts of a bridge substructure.
The superstructure consists of the components that actually span the obstacle the bridge is intended to cross and includes the following:
- Bridge deck
- Structural members
- Parapets (bridge railings), handrails, sidewalk, lighting and some drainage features
The deck is the roadway portion of a bridge, including shoulders. Most bridge decks are constructed as reinforced concrete slabs, but timber decks are occasionally used in rural areas and open-grid steel decks are used in some movable bridge designs (bascule bridge). As polymers and fiber technologies improve, Fiber Reinforced Polymer (FRP) decks may be used. Bridge decks are required to conform to the grade of the approach roadway so that there is no bump or dip as a vehicle crosses onto or off of the bridge. The most common causes of premature deck failure are:
- Insufficient concrete strength from an improper mix design, too much water, improper amounts of air entraining admixtures, segregation, or improper curing
- Improper concrete placement, such as failure to consolidate the mix as the concrete is placed, pouring the concrete so slowly that the concrete begins the initial set, or not maintaining a placement rate.
- Insufficient concrete cover due to improper screed settings or incorrect installation of the deck forms and/or reinforcement
A bridge deck is usually supported by structural members. The most common types are:
- Steel I-beams and girders
- Precast, prestressed, reinforced concrete bulb T beams
- Precast, prestressed, reinforced concrete I beams
- Precast, prestressed, concrete box beams
- Reinforced concrete slabs
Secondary members called diaphragms are used as cross-braces between the main structural members and are also part of the superstructure. Parapets (bridge railings), handrails, sidewalks, lighting, and drainage features have little to do with the structural strength of a bridge, but are important aesthetic and safety items. The materials and workmanship that go into the construction of these features require the same inspection effort as any other phase of the work.
The substructure consists of all of the parts that support the superstructure. The main components are abutments or end-bents, piers or interior bents, footings, and piling. Abutments support the extreme ends of the bridge and confine the approach embankment, allowing the embankment to be built up to grade with the planned bridge deck.
When a bridge is too long to be supported by abutments alone, piers or interior bents are built to provide intermediate support. Although the terms may be used interchangeably, a pier generally is built as a solid wall, while bents are usually built with columns.
The top part of abutments, piers, and bents is called the cap. The structural members rest on raised, pedestal-like areas on top of the cap called the bridge seats. The devices that are used to connect the structural members to the bridge seats are called shoes or bearings. Abutments, bents, and piers are typically built on spread footings. Spread footings are large blocks of reinforced concrete that provide a solid base for the substructure and anchor the substructure against lateral movements.
Footings also serve to transmit loads borne by the substructure to the underlying foundation material. When the soils beneath a footing are not capable of supporting the weight of the structure above the soil, bearing failure occurs. The foundation shifts or sinks under the load, causing structure movement and damage.
In areas where bearing failure is likely, footings are built on foundation piling . These load-bearing members are driven deep into the ground at footing locations to stabilize the footing foundation. Piling transmits loads from the substructure units down to underlying layers of soil or rock.