How Surveying Guides Bridge Construction
Accurate measuring and mapping is essential in locating and designing new spans across land and water.
If and when Congress passes a bill to fund more infrastructure construction, companies that provide surveying services for bridge construction are likely to become very busy. According to a January 2017 report from the American Road & Transportation Builders Association, there are “85 million daily crossings on nearly 56,000 structurally deficient U.S. bridges.” While those bridges are not unsafe, they do require repairs and in some cases replacement.
That’s good news for surveyors, who have several different responsibilities in bridge construction projects. To start, they usually assist the project engineers in making pre-construction or reconnaissance surveys of an area to help determine the best spot for the bridge.
Once a location has been determined, surveyors will make a more detailed topographic map. There are two surveying methods they can employ. Plane surveying, which doesn’t take the curvature of the earth into consideration in its measurements, is best suited for small, flat areas; it might work for a short bridge span. Geodetic surveys, on the other hand, do build the curvature of the earth into their calculations; they are considered more accurate than plane surveys and are important in the construction of longer bridge spans. Many construction surveyors today use some combination of the two methods.
Both types of survey establish horizontal and vertical control points (stations) and other references points on the site (construction stakeouts) that will guide contractors as they build the bridge.
Some surveying companies will also monitor the bridge’s structure during and after construction. By attaching sensors to various parts of the structure, they can keep an eye on any changes that occur as loads are added to the bridge. These changes are known as dynamic deformation.
Construction surveyors’ instruments include various types of levels for measuring elevation, angles and slopes, as well as theodolites (essentially, rotating telescopes mounted on a base) for measuring angles in the horizontal and vertical planes. Recent innovations include electronic distance measurement (EDM) tools that use lasers, infrared rays and microwaves for measurement; total station instruments that combine the capabilities of EDM and theodolites; automatic levels; and GPS systems that can provide very precise measurements in the X, Y and Z planes.
Precision is essential in bridge construction surveying, as officials in Cleveland, Ohio, can attest. In 2012, contractors erected one of the large piers for the city’s Inner Belt Bridge almost three feet north of the spot where it should have met the structure’s steel girders. The error was apparently due to an inaccurate survey. Fortunately, the pier was not structurally significant and the contractor was able to do some retrofitting to make it work.