Back to Basics: Transferring elevations up a structure.
Multi-story building construction or tall-column construction on a bridge project requires that ground elevations be transferred vertically as the structure progresses upward to maintain the design grades. Occasionally, this can be accomplished by normal differential leveling procedures of backsights and foresights when there is an adjacent structure or high ground. However, the most common method of transferring elevation up into a structure is to use a good calibrated chain and measure vertically along a column, the elevator/stair core or any other part of the structure that is solid and progressing vertically. A simple process, but one that requires following exact procedures to obtain accurate results.
Plan for the efficient and accurate establishment of the elevations. Check with the project superintendent and the foremen to determine what floor the elevations are needed on. Determine if the elevation is needed on just one side of the structure, on two sides, or all the way around it. Locate two nearby bench marks, one to start from and one to check into after a temporary bench mark has been established on the structure. Be sure to make the check prior to measuring up along the side of the building in case one of the bench marks has been disturbed.
Using the basic techniques of differential leveling, run a level loop to the structure and establish a temporary bench mark (TBM) at the bottom of the structure (see Figure 1). Typically, TBMs are simply a mark on the wall with the elevation written there. Remember to describe and identify the point chosen in your field notes. Complete the level loop by returning to a second bench mark. Check your results. If your work is acceptable (i.e., it meets accuracy standards for the project), proceed with the transfer up the side of the structure. If your work is unacceptable, re-run your level loop.
Using a chain, transfer the elevation up the side of the structure to the floor on which the elevation is needed (see Figure 2). Have one person accurately hold the zero end of the chain on the TBM just established at the bottom and have another person carefully establish a new TBM up on the floor of the structure. (For buildings, this is typically 4 feet above finish floor.) No extra tension needs to be applied to the chain in this instance. Its own weight should keep it straight and taut. Measure the appropriate vertical distance needed to establish that 4-foot mark. Care should be taken to ensure the TBM is located where it will be visible from the instrument when it is set up on the floor.
To use the TBM up on the structure, set up a leveling instrument and establish your HI by backsighting a rod held on the TBM (see Figure 3). Perform leveling tasks as if you were on the ground to establish needed elevations on the floor of the structure.
Repeat this process as needed for other floors of the building. If the elevations are needed on a bridge column, the process is the same. However, the elevations may be needed inside the form. In that case, the elevation may be transferred on the outside of the form up to the top of the form, and then measured down on the inside of the form to establish the top of concrete elevation.
Note: Often, points that need elevations up on structures may be above the line of sight of the instrument. If this is the case, those on the ceiling or on the bottom of a beam will require the level rod to be used upside down (see Figure 4). This will result in a positive foresight that must be added to the HI rather than subtracted! Be careful in your notekeeping.
Differential leveling calculations:
HI = BS + Elevation
Elevation = HI - FS
If the bottom of the rod is above the line of sight (inverted), then: Elevation = HI + FS.