03.05 Surveying

Categories: Buildability

Setting-out
The process of setting out on any building site may be described as the determination and establishment of a well-defined system of lines, distances and planes to provide a stable network for the determination of the accurate position and level of buildings and building elements.

Every measurement technique is subject to error.  The accuracy achieved in setting-out depends on the accuracy of instrumentation and the competence and care applied to take linear measurements, establish and transfer horizontal levels and verticals and delineate angles and straight lines (BRE Digest 234).  Errors result in inaccuracy of the prescribed grid used for locating components.
 

Accuracy of instruments
Where deviations are fully compensated by the adjustability of the cladding fixings, the precision and operation of measuring instruments in setting out and erecting the cladding dictate the theoretical upper-bound accuracy of the erected facade.

The table below gives the accuracy that can be achieved with different commonly used surveying instrments.
 

Line and level can be set out by means of a taut piano wire.  The accuracy of this method will depend on the tension in the wire pulling it straight.  However, it will be as accurate or more so than the methods shown in the table for measurement of level.
 

Accuracy of position
The system of measurement also affects the positioning of components when setting out as a result of the accumulation of errors.  Ideally all measurement should be related to a common building grid established by a single contractor (presumed to be the lead contractor).  If each specialist contractor working on a site marks their own grid from which to work then there is risk of confusion and a compounding of errors.  However accurately the separate grids are measured and marked there will be a difference between the two.

When setting out the mullions of a stick system the position of each should be measured from a common point on the edge of the floor slab.  Measuring the position of each mullion from the position of its neighbouring mullion means that many measurements will be made in the positioning of the later mullions.
 

Dimensional surveys of the ‘as-built’ frame
It may be thought that inaccuracies in the construction of the primary structural frame can be accounted for by surveying the frame once it is built.  Unfortunately there is seldom time between completion of the frame and start of manufacture of the brackets to manufacture on the basis of any information gained.  Indeed the brackets for the cladding may be manufactured before the frame is completed.  However, measurement of the completed frame will show whether problems are likely to be encountered during fixing of the cladding.

Structural frames move as a result of temperature and moisture changes and because of applied loads.  Any survey of the frame will only describe its position under the climatic conditions and load at the time of survey.  Section 04.03 covers structural movement due to load and Section 01.09 covers movement due to moisture and temperature.

BS 5964 describes procedures that acknowledge the measurement errors due to the survey. This should overcome discrepancies where agreement cannot be reached as the requirements are beyond the capabilities of the survey process.  A building will cycle through a range of positions during the day and will “shakedown” progressively over its first few months by more than 10mm (Pope, 1997).
 

Dimensional surveys of the finished cladding
Cladding moves as a result of temperature and moisture changes and because of movement of the supporting structure.  Any survey of the cladding will only describe its position under the climatic conditions and load at the time of survey.  Section 01.10 covers movement due to moisture and temperature.