Definition of Setting Out
- Setting out is the process of extracting information from the construction drawings, and pegs, profiles or other marks are then set to control the construction works and to ensure that each features in the drawings are constructed in the right position and to the correct level.
- According to ISO_7078: 1985 Building Construction - Procedures for Setting out, Measurement & Surveying - Vocabulary & Guidance Notes: Setting out is defined as the establishment of marks and lines to define the position and level of elements of the construction work so that works may proceed with reference to them.
- Often used definition: Setting out is the reverse of "surveying", i.e. surveying is a
process of producing a plan or a map of a particular area, while, setting out begins with
the plan and ends with the various elements of an engineering project correctly
positioned in the area (Uren, J. et al., 2006).
- This process is contrasted with the purpose of "surveying" which is to determine the positions of existing features on site by measurement.
- Good work practices and techniques in setting out is essential to minimize errors and to ensure the construction process proceeds smoothly.
- Good knowledge is vital, as the setting out phase is one of the most important stages in any civil engineering construction project.
- Mistakes in setting out can be costly and should be borne by the contractor.
Even though the "surveying" and "setting out" are opposite processes to each other, but the techniques and instruments used for both processes are identical.
Among all parties who are involved in construction works, the "setting out" is the responsibility of the contractor.
Main Principles of Setting Out Operations
Three (3) main principles of setting out operations:
Horizontal control technique
Vertical control technique
Aims of Setting Out
Two (2) main aims when undertaking setting out operation:
- Various elements of the scheme in the construction drawings must be correct in all three dimensions, both relatively and absolutely, that is each must be in its correct size, plan position and correct reduced level.
2.Once setting out begins, it must proceed quickly with little or no delay in order that the works can proceed smoothly and the cost can be minimized.
Stages of Setting Out
Overall, there are three (3) stages of setting out need to be carried out in construction works:
Initial setting out - limits of work and site
Stage 1: Setting out of foundation and sewer pipeline
Stage 2: Setting out the design points
BS 5964-1:1990 has highlighted three-stage order of reference systems commonly adopted for large and complex building projects.
Position points - the location of the details of the building
Setting Out Planning
A reconnaissance of the site and planning of the setting out are essential.
Based on the construction drawing, a suitable reference system should be selected and established.
A suitable reference system selected will depend on:
The shape and size of the site
The positions of any existing buildings or obstructions
The positions of the proposed building and ancillary works
The sequence of excavation and construction woks
The chosen reference system should be such that redundant observations are possible and that the measuring points can be referred to during construction.
The position of the main ground station should be chosen and protected such that they are at a minimum risk to damage or movement and unobstructed lines of sight can be maintained.
• Location grids are used to assist the planning authorities and designers in plotting the location of boundaries, buildings, roads, underground utilities or other features.
• If the building to be set out, it is important to mark on the plan of the building site the approximate position of the structural grid or site grid.
• Site grids can be considered as the transfer of the location grid from the plan or drawing to the site by setting out.
##STAGES IN SETTING OUT
- Initial Setting Out
• The marking of site clearance and excavation areas.
• Methods of setting out:
a. Polar setting out method
b. Intersection method
c. Offset method
d. Method of free station points
• Establish permanent point
• Position of permanent point
• Establish benchmark
2. Stage 1 Setting Out
• In practice, 1st stage setting out involves the use of many horizontal and vertical control techniques.
• The purpose of this stage is to locate the boundaries of the works in their correct position on the ground surface and to define major elements.
• In order to do this, horizontal and vertical control points must be established on or near the site.
3. Stage 2 Setting Out
• Second stage setting out continues from the 1st stage setting out.
Example: Beginning at the ground floor slab, or road sub-base level, etc.
• Up to this point, all the control points will be outside the main construction.
Example: The pegs defining building corners, center lines and so on will be knocked out during excavation works and only the original control points will be undisturbed.
PRINCIPLE OF SETTING OUT
Horizontal Control Technique
- Establish horizontal control points in the East (E) and North (N) coordinates points on the site so that the design points for each of the elements of the scheme in the construction drawings can be correctly fixed in position.
- Two (2) factors to consider in establishing horizontal control points:
a. The control points should be located through out the site in order all the design points can be fixed from at least two of them so that the work can be independently checked.
b. The design points must be set out to the accuracy stated in the specifications.
- The construction and protection of control points is very important.
- Wooden pegs are usually used for non-permanent marking (control points).
- Horizontal control points can be:
A baseline is a line running between two points of a known position.
Any baseline required to set out a project should be specified on the setting out plan/drawing by the designer and included in the contract.
Baseline can take many forms.
Two specified points joined;
i. Run between two buildings.
ii. Mark the boundary with an existing building/ development.structure
iii. Mark the center line for a new road.
b. Reference Grid
- A control grid enables points to be set over a large area.
Several different grids can be used in setting out:
(i) Grid Survey
(ii) Structural Grid
(iii) Grid Site
(iv) Grid Secondary - is established inside the structure from the structural grid when it is no longer possible to use the structural grid to establish internal features of the building.
c. Offset Pegs
Whether used in the form of a baseline or a grid, the horizontal control points are used to establish design points on the proposed structure.
Once excavations for the foundations begin, the corner pegs will be lost. To avoid this extra pegs, offset pegs are used.
PRINCIPLE OF SETTING OUT
Vertical Control Technique
• In order the design points can be positioned at their correct levels, vertical control points of known elevation relative to some specified vertical datum are established. Some vertical control techniques:
The positions of TBMs should be fixed during the initial reconnaissance so that their construction can be completed in good time and they can be allowed to settle before levelling them in. In practice, 20mm diameter steel bolts and 100mm long, driven into existing steps, ledges, footpaths etc. are ideal.
b. Sight rail
These consist of a horizontal timber cross piece nailed to a single upright or a pair of uprights driven into the ground. The upper edge of the cross piece is set to a convenient height above the required plane of the structure, usually to the nearest 100mm, and should be a height above ground to ensure convenient alignment by eye with the upper edge.
c. Travellers and Boning Rods
• A traveller is similar in appearance to a sight rail on a single support and is portable. The length of the upper edge to its base should be a convenient dimension to the nearest half meter.
• Travellers are used in conjunction with sight rails. The sight rails are set some convenient value above the required plane and the travellers are constructed so that their length is equal to this value.
• As excavation works proceeds, the traveller is sighted in between the sight rails and used to monitor the cutting and filling.
d. Slope rails or batter boards
• For controlling side slopes on embankments and cuttings slope rails are used.
• For an embankment the slope rails usually define a plane parallel to the slope of the embankment offset by a convenient distance.
Controls For Setting Out
- The setting out of
work required the
following two controls
- Horizontal control
- Vertical control
▪ It consist of establishing reference marks of known plan positions from which horizontal distances are measured for setting out
▪ The control points are generally used to establish a base line near one face of the structure
▪ The setting out is done by taking measurements from the base line
▪ Horizontal control can be achieved by establishing reference grid
▪ The grid which is used for actual setting out of the salient points of the structure
• The primary control points may be the triangular stations
• The secondary control are referred to these primary control stations
- It consist of establishment of reference marks of known elevation relative to some specified datum
- All levels at the site are normally reduced to a nearby bench mark, usually known as Master Bench Mark(MBM)
This Master Bench Mark(MBM) is used to establish a number of Temporary Bench Mark(TBM)
SETTING OUT FOR EXCAVATION OF BUILDING FOUNDATION
• Before the excavation for the proposed foundation is commenced, the site shall be cleared of vegetation, brushwood, stumps of trees, debris, etc.
• Next is to set out a baseline for the work.
• For setting out the foundations of small buildings, the center line of the longest outer wall of the building is first marked on the ground by stretching a string between wooden or mild steel pegs driven at the ends.
• For accurate work, nails can be fixed at the center of the pegs.
• Two pegs, one on either side of the central peg, are driven at each end of the line. Each peg is equidistant from the central peg, and the distance between the outer pegs corresponds to the width of foundation trench to be excavated.
• When string is stretched joining the corresponding pegs at the two extremities of the line, the boundary of the trench to be excavated can be marked on the ground with dry lime powder.
• The center lines of other walls, which are perpendicular to the long wall, are then marked by setting out right angles.
A right angle can be set out by forming a triangle with 3, 4 and 5 units long.
• These dimensions should be measured with the help of a steel tape. Alternatively, a theodolite or prismatic compass may be used for setting out right angles.
• Similarly, outer lines of the foundation trench of each cross-wall can be set out, as shown in the following figure below.