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Layout of a Building
Layout
of a building or a structure shows the plan of its foundation on the ground
surface according to its drawings, so that excavation can be carried out
exactly where required and position and orientation of the building is exactly
specified. It is set out according to foundation plan drawings and specifications
provided by the engineer or an architect.In order to understand layout or
setting out of a building we must understand some of the technical terms
related to this job which are described below.
BASELINE
A
baseline is a straight reference line with respect to which corners of the
building are located on the ground. It may be outer boundary of a road or curb
or boundary of the area or simply a line joining any two points.
HORIZONTAL CONTROLS
Horizontal
controls are the points that have known co-ordinates with respect to a specific
point. These points are then used to locate other points such as corners of a
layout using various techniques. There should be plenty of control points so
that each point of foundation plan can be located precisely on the ground.
VERTICAL CONTROLS
In order that
design points on the works can be positioned at their correct levels, vertical
control points of known elevation relative to some specified vertical datum are
established. In practice, 20mm diameter steel bolts and 100mmlong,
with known reduce levels driven into existing steps, ledges, footpaths etc. may
serve as vertical controls.
BATTERBOARDS AND OFFSET PEGS
Once
points specifying the layout are located on ground pegs are driven in the
ground at that spot. Once excavations for foundations begin, the corner pegs
will be lost. To avoid these extra pegs called offset pegs are used. Batter
boards are normally erected near each offset peg and are used to relocate the
points after the excavation has been done.
LAYING OUT A RECTANGULAR BUILDING SITE
Starting
from a baseline (line AB in Figure 4-1) that is parallel to construction,
establish the maximum outer borders (AB, CD, AC, BD) of the building area.
Suppose
we know the co-ordinates(x,y) of the points X with respect to point A then we
can locate it by measuring their x distance along line AB and y distance along
line AC and BD respectively to locate them. These two points can be joined to
make line XX. To locate point G and H, straight line are set out using 3-4-5
triangle rule and distance XG and XH which is known is marked on those lines.
After the four corners (X, X, G. and H) have been located, drive stakes at each
corner. Dimensions are determined accurately during each step.
LAYING OUT AN IRREGULAR BUILDING SITE
Where
the outline of the building is other than a rectangle, the procedure in
establishing each point is the same as defined for laying out a simple
rectangle. However, more points have to be positioned, and the final proving of
the work is more likely to disclose a small error. When the building is an
irregular shape, it is sensible to first lay out a large rectangle which will
includes the entire building or the greater part of it. This is shown in Figure
4-2 as HOPQ When this is established, the remaining portion of the layout will
consist of small rectangles, each of which can be laid out and shown
separately. These rectangles are shown as LMNP ABCQ, DEFG, and IJKO in Figure
EXTENDING LINES
Since
the corner pegs of the building are to be removed during excavation these
points are transferred outside that periphery by extending lines and driving
pegs in the ground. The following procedure applies to a simple layout as shown
in Figure 4-4, page 4-4, and must be amended to apply to different or
more
complex layout problems:
Step
1: After
locating and dipping stakes A and B. erect batter boards
1,
2, 3, and 4. Extend a chalk line (X) from batter board 1 to batter
board
3, over stakes A and B.
Step
2:
After locating and dipping stake C, erect batter boards 5 and
- Extend chalk line Y from batter
board 2 over stakes A and C to
batter
board 6.
Step
3:
After locating and dipping stake D, erect batter boards 7 and
- Extend chalk line Z from batter
board 5 to batter board 7, over
stakes
C and D.
Step
4:
Extend line O from batter board 8 to batter board 4, over stakes D and B.
Where
foundation walls are wide at the bottom and extend beyond the outside
dimensions of the building, the excavation must be larger than the laid-out
size. To lay out dimensions of this excavation, measure out as far as required
from the building line on each batter board and stretch lines between these points,
out
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