Bridge layout and staking is normally done by the Contractor, or Subcontractor, as Construction Engineering. Layout involves establishing construction control points that are used to maintain the horizontal and vertical alignment of the work that follows. After performing the layout, the Surveyor furnishes the Contractor with the information required to complete the layout and to conduct the work. Technicians who have little or no survey work experience are required to participate in the layout operation to acquaint themselves with the locations of important construction control points and the methods used to establish those points.
The first step in bridge layout is to locate previously established control points on each end of the bridge site. The control points were established during the preliminary survey to represent the baseline of construction or the survey line. This line is typically designated as Line “A” on the plans. Control points for the centerline of the structure and/or roadway are also located if they are different from the survey line. All points are checked for alignment and referenced with offset stakes. The station of one of the control points is determined for use in locating the abutments and piers.
The next step is to locate each unit of the substructure at points along the survey line. Reference stakes for these points are set to the left and right of the centerline by turning the skew angle. To insure accuracy, the survey crew double and triple checks the skew angle. The accuracy of the skew angle may be checked by measuring the distance between reference points on the left and right sides. If the distances between the points are equal on both sides, the skew angle is correct.
Enough reference points are set to insure easy replacement of the centerline control. The reference points are protected and identified by guard stakes. Once the reference points are set, the crew double-checks the elevations of the bench marks. Again, temporary benchmarks are required to be established when a benchmark on an existing structure is to be replaced.
If the bridge deck is to match an existing roadway, the edges and centerline of the roadway are required to be profiled and checked against the elevation of the new structure. The last step in the bridge layout is staking the footings and taking cross-sections of the footing areas. The cross-sections are used to determine how much material the Contractor is required to remove during structure excavation.