Controls of Bridge Construction

HORIZONTAL CONTROLS

To ensure the bridge lines up correctly with the approach roadways, the initial survey and layout establishes one or more centerlines to guide the
construction of the bridge. The important centerlines to check include:

1) The centerline of construction (sometimes referred to as baseline of construction or survey line)
2) The centerline of structure
3) The centerline of roadway
4) The centerline of bearing (may also be called centerline of pier)

Depending on the contract, the centerlines of construction, structure, and roadway may be the same line or three different lines. For example, a two-lane bridge with no shoulders or with shoulders of equal widths would probably have one line for all three references. In most cases, however, one or more centerlines is different from the other centerlines.

Centerlines of bearing are transverse lines that bisect the bridge seats or bearing areas on abutments and piers and intersect the longitudinalcenterlines. Generally, if the centerlines of bearing intersect the longitudinal centerlines at an oblique angle (an angle other than a right angle), the bridge is said to be skewed or built on a skew. If the centerlines of bearing intersect the longitudinal centerlines at right angles, there is noskew. Degrees of skew, if any, are noted on the General Plan sheet and elsewhere on the plans.

VERTICAL CONTROLS

To maintain the proper grade of a bridge and the elevation of the various bridge components, all construction is required to be referenced to benchmarks. Benchmarks guide all elevation measurements from structure excavation and pile driving to pouring the bridge deck.

Benchmarks for bridges are established during the bridge layout and their locations are usually noted on the layout sheet. At least one benchmark on each side of the bridge is required to be checked for accuracy before construction begins. If a benchmark is on a structure that is to be removed, a temporary benchmark is established and protected at a site convenient to the new bridge. As soon as a footing or other permanent part of the new structure is poured, the temporary benchmark is transferred to the new structure.