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Contents

Section 1 Introduction

Section 2 Setting out

Section 3 Profiles and Sight Rails

Section 4 Horizontal Curves
                         



 

Description:  This unit introduces the methods that can be used in order to set out a site or a building.

Author:  Gates MacBain Associates


Section 1 Introduction

This Module is designed to complement your lectures and the practical work within your course.  It does not replace the lectures or experience that you will need to get by carrying out the practical tasks. You may find it useful to reinforce or clarify the tuition you receive on your course.  




Section 2 Setting-out 



Aims and Objectives


At the end of this section you should:
  • Understand how control points and co-ordinates are used to set out the position of buildings etc.

This unit explores methods used to set out buildings and establish site profiles which are used to guide the construction process. Figure 1 below comprises three pairs of semi detached houses. This site is to be used as we look at how the positions of the actual structures are fixed and site profiles to guide the excavation and pouring concrete into the foundations.    



Figure 1: Section of Development Site    

At the beginning of a contract it is normal to set a number of control points around the site which are either pegs or studs hammered into a road surface. These control points are established using a closed traverse survey as described in unit 3. The contract drawings will give co-ordinate details for all the structures and personnel inspection chambers (manholes) to be built along with data on invert levels of drains. The setting out data which we will use for this unit is shown in Figure 2 below.   


Fig_2_SOP

Figure 2: Control Points & Drainage System     

The data for the control points and Personnel Inspection Chambers (MH 26 & 27) is as follows:   

Reference Point Eastings Northings Elevation 

Control Point 1 125.350 113.050 106.215 

Control Point 2 123.430 133.800 106.705 
MH 26 126.000 104.400 105.900 (Base) 
MH 27 120.950 133.500 105.500 (Base)  

The co-ordinates for each corner of the central structure are as shown in Figure 3. Note: The finished floor level (FFL) is set at 107.200m      


Fig_3_SOP

Figure 3: Setting Out Data for Plots 71 & 72   


Setting Out Procedure
  • A theodolite or total station is set up over a control point which has known co-ordinates.
  • The instrument is then pointed at another control point in order to orientate the instrument to north. If a conventional theodolite is being used it normal to turn the instrument to north and reset the horizontal angle to zero.
  • The distances and bearings from that control point to those points which require setting out are now located and fixed with pegs.
  • A control point which is located near to the structure to be set out will ensure maximum accuracy.
  • If a total station is used the co-ordinates are entered into the software and the prism target will allow the bearing and distance to be located for each point.
  • If a conventional theodolite is used the co-ordinates of the control point are compared with those of each point to be set out. This will require whole circle bearings and distances to be calculated for each point.
  • The procedure is then to simply turn to the required bearing and measure the distance ensuring that the tape is held as near horizontal as possible

Whole Circle Bearings (WCB) 

We will look at the calculations for Points B1, B2, MH26 & MH27 from Control Point 2. 

The data shown in black in Table 1 was established when the control points were set up. The remaining data is taken from contract documents.        

Point Eastings Northings WCB Distance
Control 1 125.350 113.050 185 17 12 20.839
Control 2 123.430 133.800 00 00 00 0.000
B1 117.100 125.020  
B2 119.110 115.080  
MH 26 126.000 104.400  
MH 27 120.950 133.500  
 
Table 1: Setting Out Data               

Orientation to North is achieved by comparing Control Points 1 & 2 which are 20.839m apart       

Fig_4_SOP



















Figure 4: Orientation to North           

The horizontal bearing on the theodolite would be a random value when the instrument is set up and sighted on to Control Point 1.     

Let us assume that the horizontal bearing is 214° 39' 40'' when looking at Control Point 1. The orientation value (174° 42' 48'') is added to the displayed bearing (174° 42' 48'' + 214° 39' 40'') giving a total of 389° 22' 28''. Because this is greater than 360 we turn the theodolite to 29° 22' 28'' (389° 22' 28'' 360). In practice it will be turned to 29° 22' 20'' as most basic instruments are accurate to the nearest 20''.   At this point the horizontal bearing is set to zero to enable setting out using WCBs.   


Calculation of WCBs and Distances     

Using the co-ordinates in the above table we will explore how to calculate WCBs and distances to the   point being set out. First we need to understand which quadrant of the whole circle is being used from our control point.     


Fig_5_SOP
Figure 5: WCB Calculation             


Clearly it can be seen in Figure 5 that bearing B1 (blue line) is in the third quadrant in which case clockwise bearings will be between 180° and 270°.   

The WCB for B1 is calculated by comparing co-ordinates between the occupied control point and the point to be set out    


Point Eastings Northings
Control 2 123.430 133.800
B1 117.100 125.020   

Table 2: B1 Co-ordinates       

To navigate from Control Point 2 to point B1 we go WEST 6.330m and SOUTH 8.780m   
Remember: If an easting increases we are moving EAST but if it decreases we are moving WEST and   

If a northing increases we are moving NORTH but when it decreases we are moving SOUTH   

Distance from Control Point 2 to B1 = √ 8.7802 + 6.3302 = 10.824m   

This produces a triangle which looks like the one shown in Fig 6 below:      



Fig_6_SOP
Figure 6: B1 Calculations         

To locate the centre point of MH27 from Control Point 2 we move 2.480 m WEST and 0.300m SOUTH   

WCB for MH27 = 180 + Tan è (2.480 / 0.300) = 263° 6' 9''   

Distance from Control Point 2 to MH27 = √ 2.4802 + 0.3002 = 2.498 m      


Point Eastings Northings WCB Distance
Control 1 125.350 113.050 185 17 12 20.839
Control 2 123.430 133.800       00  00  00 0.000
B1 117.100 125.020 215 47 24 10.824
B2 119.110 115.080  
MH 26 126.000 104.400  
MH 27 120.950 133.500        263  6  9 2.498  

Table 3  


Publications

  • Irvine, W, (1995) Surveying for Construction, McGraw-Hill: Berkshire (Chapter 13)


Self-Assessment Task

  • Complete the details in the Table 3 for B2 and MH26 using the procedures described.




Section 3  Profiles and Sight Rails


Aims and Objectives


At the end of this section you should: 
  • Setting up profiles to guide excavation for foundations in buildings using profiles to guide the positioning of walls.
  • Setting profiles to guide the line and level of new drainage systems.


Profiles and sight rails are erected by setting out engineers to guide the construction of buildings and drainage systems as the contract progresses. 

These are used on site to guide: 

  • Excavation of trenches for concrete foundations and pipelaying
  • The levelling of concrete
  • Setting a pipe to the correct invert level
They comprise two vertical posts and a horizontal rail as shown in Figure 7 below:    


Fig_7_SOP
Figure 7: Profiles and Sight Rails  

Profile positioning is 2m beyond each corner location. The dotted red line (see Figure 8) is the centre line of the trench (half wall thickness behind the wall face)     


Fig_8_SOP
Figure 8: Profiles for Trench Excavation & Footings   


Setting Up Site Profiles 

The procedure used is as follows: 
  • Identify the corner pegs which have been position using optical instruments
  • Extend the line of the wall by say 2m (on some sites 3m might be used)
  • Measure back towards the building a distance equal to half the wall thickness at each end of the extended line at fix a peg (one at each end)
  • Assuming a 600mm bucket is to be used for excavation measure 500mm each side of the peg and fix in place the profile posts.
  • Using an optical level determine the level of one of these post tops (remember to relate this to site or ordnance datum by back sighting on to a known level)
  • Look at the drawing for the finished floor level (FFL) in our case it is 107.200.
  • Nail the rail at the correct level by measuring down the required amount from the post top
  • When both profiles are in place suitable travelling rods can be made to guide excavation and concreting.
  

Sight Rails for Drainage 

The procedure used is similar to the above except that: 
  • Pegs set out are at the centre of Personnel Inspection Chambers (Manholes)
  • The chambers are usually constructed first and then pipes laid between them
  • Supporting posts tend to be taller and are typically 1.5 to 2m apart but again centred on the drain line
  • The size of travelling rods will be larger often 2 3m
  • The invert levels are shown on the contract drawings for each chamber
  • Drains are laid to a gradient whilst FFLs are level
  • Processes used to fix the rail levels are just the same except we are setting out gradients.
  • Sight rails need to be set at a comfortable useable height of 1 1.5m above the ground
 
Fig_9_SOP

  



Figure 9: Longitudinal section between MH27 & MH26 




Publications

  • Irvine, W, (1995) Surveying for Construction, McGraw-Hill: Berkshire (Chapter 12)


Self-Assessment Task

  • Set out the four corners of a building and then using a datum given by your tutor set up a profile to guide excavation for a footing on the assumption that the wall thickness is 280mm.




Section 4 Horizontal Curves



Aims and Objectives

At the end of this section you should:

  • Setting out curves using optical instruments or measured offsets 

It is often necessary to set out horizontal curves on site when laying out the road system for a new development. This is usually achieved using optical instruments although small lengths of up to around 60m can be set out using chords and offsets which are measured. 


Fig_10_SOP

Figure 10: Curves Basics 

IA = Intersection Angle
IP = Intersection Point
TP = Tangent Point
T = Tangent Length (Distance between TP and IP)

Symbols used in Calculations   

è = Angle at the Centre
ä = Deflection Angle 

Useful relationships    

IA = è = 1802
ä = è
IA = 2x 


Setting Out : Curves

The most accurate method is to use a theodolite or total station at TP1 and range the curve using deflection angles and measured distances. A total station would set out using co-ordinates and Electromagnetic Distance Measurement (EDM) whilst the theodolite would find the bearing then the distance would be measured with a tape.

It is possible to set out curves using tapes only but this can be inaccurate over distances of more than about 75m. We will explore a method of setting out using tapes at the end of this unit.

Fig_11_SOP



















Figure 11: TP1, Chord and Deflection Angles


We are assuming that a 900m Radius is to be set out. The intersection point chainage from the beginning of the alignment is 366.295m and the intersection angle is 175°.

Calculations
Let Radius (R) = 900m
Let IA = 175°
T = R cot (x) where x = IA/2 = 39.295m
Let IP chainage be 366.295
TP1 = IP T = 366.295 39.295 = 327m
LC (Length of Curve) = 2πR (Φ/360) where Φ = 180 IA
LC = 2 x π x 900 x (5/360) = 78.540m
TP2 = TP1 + LC = 327 = 78.540 = 405.540m

We can now produce Table 4 showing chainages at key points on the alignment with deflections to set out the curve at 10m intervals with the calculations explained below   
Chainage Sub Chord Deflection
DEG MIN SEC
327 0 0 0 0
330 3 0 5 4.78
340 13 0 24  49.71
350 23 0 43 55.65
360 33 1 03 1.58
370 43 1 22 7.51
380 53 1 41 13.45
390 63 2 00 19.38
400 73 2 19 25.31
405.540 78.54 2 30 00

Table 4: Deflection Data Deflection Calculations


Formula Deflection (in minutes) = (1718.9 C) / R

C = Sub chord Length          R = Radius

Example : At chainage 370 C = 43m R = 900

Deflection (Mins)  =  (1718.9 x 43) / 900  = 82.125 mins 
Deflection (Deg)    mins /60 = 1° 22' 7.51''


To set out a curve and convert a curved alignment into co-ordinates we must have the following data:
Chainages & Deflections (Table 4)
Straight line distance from TP1 to a point on the curve (EDM Distance)

We now consider how these are determined.


Co-ordinates and EDM Distance

The EDM column is used to show the straight line distance between TP1 and the point being set out. This distance is calculated using Sine of the deflection times 2R. 

All these values are shown in Table 5 ( you might verify the calculation)

So at TP2 EDM    = Sin 2.5 x 1800 = 78.515 (Distance along the chord from TP1 to TP2)    


Table_5_SOP
Table 5: EDM & Co-ordinates

If we assume that TP1 is at 100 East and 100 North and that the tangent runs due north we can calculate the co-ordinates. In practice the tangent may run either east or west of north by a given angle. In this example we keep this example simple by saying the tangent runs due north.

Co-ordinates are calculated for each point. We will look at a value mid table at chainage 370.

Using the EDM which is the hypotenuse of the triangle along with the deflection angle 1 22 7.51 we can determine the distance north and the distance east from the tangent point.

Distance north = Cos 1 22 7.51 x 42.996 = 42.984 + 100 = 142.984
Distance East = Sin 1 22 7.51 x 42.996 = 1.027 + 100 = 101.027


Setting Out Curves: Measurement

A common method used is halving and quartering. We can think of a curve as having a chord and a major offset thus:


Fig_12_SOP






Figure 12: Chord & Major Offset

Let us call the half chord (L) and the major offset (D)

To set out a curve using these parameters we use the following method:
  • Establish the chord at say 50m and the major offset at 1m at the centre (L = 25 D= 1)
  • Extend the lines as shown in blue and find the centre of each one (halving) (Fig 13)
  • At each centre point project D/4 (quartering) as shown offset is now 250 mm
  • Continue halving the sub chord an quartering the previous offset until the required curve is produced. 


Fig_13_SOP
Figure 13     

Formulae      

R = (L2 + D2 ) / 2D
D = R - √R2 - L2

So in the case above the Radius was (252 + 12 ) / 2 = 313m 


Publications

  • Irvine, W, (1995) Surveying for Construction, McGraw-Hill: Berkshire (Chapter 12)


Self-Assessment Task

  • Complete the missing values in Table 5.







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