Page 216 - Urban Construction Project Management

Layouts and Surveying 171
Exhibit 7-9
Surveying by
trigonometry.

Elevation of floor

x
Transit q
3'

30'
x = tan q × 30

The subcontractor’s surveyor again establishes the elevations of each floor of the
building. Thus, for a steel structure the steel subcontractor will have their surveyor
determine the elevation of the top of steel at each floor. This is usually done by
trigonometry as noted in Exhibit 7-9. When the concrete has to be poured over the
metal deck of a steel structure, the concrete subcontractor’s surveyor establishes the
top slab elevation. In most cases, the elevator sill height is maintained as the bench-
mark for the floor.

The use of trigonometry for determining the elevation can be used for all floors.
As the surveyor starts determining the elevations of higher floors, the calculations
may be affected by temperature differential and the curvature of the Earth. In addi-
tion, the compression of the structure has to be taken into account. The owner’s
design team should determine potential compression for a steel or concrete
structure.

Elevations at a site are measured from established datum. Most datum use the average
(mean) sea level as “0” datum. The mean sea level (0 datum) is determined by measur-
ing tide gauges (high and low tides) at the seacoast city you are using for a period of
19 years. For inland areas, the mean sea level is “brought” to the interior land area that
is being considered. Most cities have monuments established by the National Geodetic
Survey (NGS), which is part of the U.S. Department of Commerce, National Oceanic
and Atmospheric Administration. Information on NGS can be obtained from their
Website www.ngs.noaa.gov.

STEEL STRUCTURES

The structural frames for steel buildings are erected at a fast pace. Thus, the align-
ment of the structure must be “true” if the building is not to lean like the tower of
Pisa. To avoid these problems, the surveyor checks the alignment of the columns with