A definition for setting out is that it is the reverse surveying. Setting out begins with the plan and ends with the various elements of a particular engineering project correctly positioned in the area.
Setting out is the establishment of the marks and lines to define the position and level of the elements for the construction work so that work may proceed with reference to them.
There are now a numbers of national and international standards specifically dealing with the accuracy requirements for the setting out and the techniques.
Should be employed in order to minimise errors and ensure that the construction process proceeds smoothly.
It is essential that the setting out records to monitor the progress, accuracy, and any changes from the original design are kept by both Engineer and the Contractor as the scheme proceeds.
Vertical control
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 datum must be established on the site.
To established points of known reduced level near to and on the elements of the proposed scheme. Known as transferred bench marks.
TBM are often located in a new position on the scheme. The vertical control points are used to define reference planes in space, parallel to and usually offset from the selected planes of the proposed construction.
It is essential that the principle of working from whole to the part is adopted. In practice it is means ensuring that all vertical design points are set out either from MBM (master bench mark) or from the nearby TBM, and not from another vertical design point which has been established earlier.
This prevent an error in the reduced level of one design point being carried forward into that another.
Vertical alignment control
The principle behind verticality control is very straight forward, if the horizontal control on the groung floor slab can be accurately transferred to each higher floor as construction proceeds, then verticality will be maintained.
Depending on the heights involved, there are several different ways of achieving verticality. For single-storey structures, long spirit levels can be used quite effectively. For multi-storey structures, however, one of the techniques are preferable:
1. 1- Plumb-bob methods
2. 2- Theodolite methods
3. 3- Optical plumbing methods
4. 4- Laser methods
PLUMB-BOB METHOD
The traditional method of controlling verticality is to use plum-bobs, suspended on piano wire or nylon. A range of weight is available. From 3kg to 20kg and two plumb bobs are needed in order to provide a reference line from which the upper floors may be controlled.
In an ideal situation. The bob is suspended from an upper floor and moved until it hangs over a datum reference mark on the ground floor. If it is impossible or inconvenient to hang the plumb-bob down the outside of the structure then holes and openings must be provided in the floors to allow the plumb bob to hang through and some form of cantering frame will be necessary to cover the opening to enable the exact point to be fixed. Service ducts can be used but often these are not conveniently placed to provide a suitable baseline for control measurement. Its is also not always possible to use a plumb bob over the full height of a building owing to the need to ‘finish’ each floor as work progresses.
TTHE PROBLEM OF PLUMBOB METHOD
The problem of wind currents in the structure usually causes the bob to oscillate and the technique can be time consuming if great accuracy is required.
TOVERCOME THE PROBLEM OF THE PLUMBOB METHOD
To overcome, two theodolites, set up on lines at right angles to each other. Could be used to check the position of the wire and to estimate the mean oscillation position.
One partial solution to dampening the oscillation is to suspend the bob in a transparent drum of oil or water. However, this tends to obscure the ground control mark being used and if this occurs it become necessary either to reference the plumb line to some form of staging built around or above the drum or to measure offsets to the suspended line.
A freshly concreted wall is being checked for verticality. The plum bob is suspended from a piece is being checked for verticality. The plum bob is suspended from a piece of timber nailed to the top of the formwork and immersed in a tank of oil or water. Offsets form the back of the formwork a measured at top and bottom with due allow acne. For any steps or tapers in the wall. Any necessary adjustments are making with a push pull prop. Increasing the weight of the bob reduces its susceptibility to oscillations but these are rarely eliminated completely.
THE ADVANTAGES
· Plumb bob are relatively inexpensive and straightforward in use. There are particularly useful for monitoring verticality over short distance.
· To ensure that it is created in a vertical position, the frame is simply pivoted about its base until the pended plumb bob touches the stretched wire
THEODOLITE METHOD
There are many cases that we can look from the theodolite methods in the controlling verticality. For more accurate vertical alignment a theodolite should be used. In this case for one or two floors the theodolite could be set up over a hole in the floor and its optical plummet used to set up over a station on the ground floor. However, the quality of the telescope on the optical plummet is inadequate to perform this operation accurately from any great height. A typical working method is to set up the theodolite on the ground floor over the control station and then sight the telescope vertically upwards. The theodolite telescope can point vertically to the near zenith for upward alignment, provided the collimation circle is small. The vertical circle of the theodolite is used for leveling to about 0.5’ accuracy. A diagonal eyepiece must be fitted to the telescope because the normal eyepiece is obscured by the body of the horizontal circle, and such an attachment is available for most makes of theodolite. For accuracy the instruments must be used in both the face right and face left positions.
The vertical sighting should be through a space in the building for example a lift or service shaft, sighting onto a temporary target at the appropriate level. Alternatively the sight can be up the appropriate floor. An alternative approach is to use a single special prism, a pentagonal prism attached at the objective end of the telescope of a theodolite. Some old instruments were capable of pointing vertically downwards through a hollow primary axis; others were flitted with an auxiliary side telescope for this purpose. Besides, now these are produced by manufacturers and facilities the transfer of a given bearing to different levels, as in connection of surface and underground lines, as well as upward and downward plumbing. Provided the telescope’s line of sight is truly horizontal, then a vertical sighting line is obtained so that a point can be suitably located at a required level on that line. Turning the instrument through 180º in azimuth gives a second positions and the mean of two positions gives the plumb position with respect to the theodolite axis. This set of observations can be repeated for rotations of 90º and 270º in azimuth to the original bearing and also after changing face.
OPTICAL PLUMBING METHOD
Optical plumbing method can be undertaken in several ways. Either the optical plummet of a theodolite can be used or the theodolite can be fitted with a diagonal eyepieces or any optical plumbing device especially manufacted for the purpose can be employed.
When carrying out optical plumbing, holes and openings must be provided in the floors and a centering frame must be used to establish the exact position
USING THE OPTICAL PLUMMET OF A THEODOLITE
The optical plummet of a theodolite provides a vertical line of sight in a down wards direction which enables the instrument to be centered over a ground mark.
Optical plummet are usually incorporate into all modern theodolite but there are also special attachments which fit into a standard tribrach and enable high accuracy centering to be obtained not only to reference marks below the instrument but also to control point above the instrument.
Optical plummet are usually in corporate visible laser plummets instead, which serve exactly the same purpose but do not require the operator to look into a telescope. These are optical roof and ground point plummets which enable centering to be achieved to 0.3 mm over a distance of 1.5m.on some instruments, a switch-over knob permits a selection between ground and roof point plumbing.
USING A DIAGNAL EYESIS
Diagonal eyepiece attachment is available for most theodolites. These are interchanged with the conventional eyepiece and enable the operator to through the telescope while it is inclined at very high angles of operation.
They can be used to transfer control point upwards to special targets either up the outside of the building or through opening left in the floors the procedure is as follows:
1. The theodolite with the diagonal eyepiece attached is centered and leveled over the point to be transferred as normal using its built in optical plummet.
2. The theodolite is rotated horizontally until the horizontal circle read 0°
3. The telescope is tilted until it is pointing vertically upwards. if an electronic reading instrument is being used the display will indicate when the telescope is vertical. in the case of an optical reading instrument, an addition diagonal eyepiece must be fitted to the optical reading telescope to enable it to be read
4. A Perspex target is placed over the hole on the upper floor and an assistant is directed by the theodolite operator to mark a line on the Perspex which coincides with the image of the horizontal cross hair in the telescope.
5. The theodolite is rotated horizontally until the horizontal circle read 180°
6. The telescope is tilted to give a vertical line of sight once again.
7. This Perspex target is again viewed.
The whole procedure is now repeated with the horizontal circle of the theodolite reading first 90° and then 270°.the mean of the two lines obtained from these values will be the correct one. The transferred point lies at the intersection of this mean line and that obtained in the 00° and 180° position.
LASER METHOD
The laser instruments used in surveying and setting out have beams which are either visible or invisible beam. The safety classification system used to categories individual laser surveying instruments. There generally two different types of laser surveying instruments, either alignment lasers or rotating lasers, each of which have been developed for different types of applications. The alignment can be used to control verticality, pipelaying and tunneling. The rotating lasers are used in generally site leveling and setting out internal fittings.
ALIGNMENT LASER AND THEIR APPLICATIONS
This alignment laser produces a single visible beam which, when used for alignment purpose, has the important advantage of producing a constantly present reference line which can be used without interrupting the construction works. A number of different instruments are manufactured for alignment purposes; examples include laser theodolites, precision laser plummets, pipe lasers and tunnel lasers.
LASER THEODOLITE
Lasers theodolites are a combination of a laser and a theodolite, that are purpose built instruments which have the laser tube permanently attached as shown in figure 11.72 for Sokkia’s LDT50 laser digital theodolite. The laser beam generated by a laser eyepiece attachment or a laser theodolite coincides exactly with the line of collimation and is focused using the telescope focusing screw to appear as a red dot in the centre of the cross hairs as shown in figure 14.60.On looking into the eyepiece, the observer sees a reflection of the beam which is perfectly safe. The beam can be intercepted with the aid of suitable targets over daylight ranges of 200 – 300m and night of 400 – 600m.
Laser theodolites can be used in place of conventional theodolites in almost any alignment or intersection technique and, once set up; the theodolite can be left unattended. However, since the instrument could be accidentally knocked or vibration of nearby machinery could deflect the beam, it is essential that regular checks are taken to ensure that the beam is in its intended position.
One application of laser theodolites is for controlling verticality. In using this laser to control verticality, it should be set up on the ground floor slab directly over the ground point to be transferred. The beam is then projected vertically either up the outside of the building or through special openings in the floors. The beam is intercepted as it passes the floor to be referenced by the use plastic targets fitted in the openings or attached to the edge of the slab. The point at which the beam meets the targets is marked to provide the reference.
ANALYSIS
| METHOD | INSTRUMENT | ADVANTAGES | DISANVANTAGES |
| Plumb-bob methods | 1.plumb-bob 2.piano wire or nylon 3.water or oil | 1.relatively inexpensive 2.straight in use 3. Limited space or restricted lines may not allow any instrument to be set up, to overcome it, the plumb-bob method is the best. | 1. Wind current in the structure usually cause the bob to oscillate. 2. Time wasted on the technique if great accuracy is needed. |
| Theodolite methods | 1.theodolite 2.pegs 3.target instrument 4.tripod | 1.Able to check the position of wire 2. Can estimate the mean oscillation position 3.More accurate than the plum-bob methods | 1. Could not be performed in limited space or restricted lines of sight |
| Optical plumbing methods | 1.Optical plummets 2.Diagonal eyepiece 3.Tripod 4.Zenith and nadir plummet | 1.Ensures minimal errors by applying the principle working from whole to the part 2. High accuracy, with precision of +/-1 | 1. The devices only for short range work and do not provide a vertical line of sight of sufficient accuracy to control a high-rise structure |
| Laser methods | 1.Laser theodolite 2.Pipe laser 3.Tripod 4.Rotating laser sensor 5.Levelling staff | 1. Enable many surveying and setting out operations to be carried out by just one person 2. Considerable savings in the time required to complete the work 3. Significant improvements in the accuracy | 1. Laser hazard 2. Relatively are expensive |
REFERENCE
- Basic metric surveying, third edition :
W S Whyte
Principle lecture
School of land and building studies
Leicester Polytechnic
- Practical surveying and computation, revised second edition :
A.L.Allan, 1997
Laxton’s
- Surveying for construction :
W.H Irvine
Megraw Hill
- Surveying for engineers :
Third edition
- Surveying for engineers :
Fourth edition
J.Uren and W.F.Price
Palgrave Mcmillan
- Surveying :
Seventh edition (1998)
A.Bannister, S.Raymond and R.Baker
Longman.
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