Saturday, 10 December 2016

Avoiding Common Errors In Setting-Out Operation -Engr. Osaz' Enobakhare

The whole idea behind setting out is to ensure that the building or structure is positioned on the ground or the surface it is to be built upon in accordance to the original site plan/drawings. It naturally involves four processes which include; the planning aspect; largely involving the building drawings (i.e. site plan and floor plans accompanied by short technical and general notes). 

This activity precedes the actual marking out of the positions of the building elements on the ground as represented in the plan/drawings using various tools and equipment e.g. the compass, theodolite, measuring tapes, lines, spirit level plum, square, etc. 

While marking out, control of level is simultaneously carried out. Taking levels across the entire building area is quite tasking because of the often non-uniform nature of land surfaces. If the levels are not properly taken, it would affect the integrity of the horizontal bearing of the entire structure and may further impede on the ground dimensions and sometimes its verticality. 

Control of verticality is the last step in this all important activity. The verticality of the base structure is very important because it will invariably determine the straightness of the vertical stretch of the building or structure. 

If the foundation is not erected or constructed properly in relation to horizontal and vertical bearings, the super-structure may be poorly projected upward –making it appear slant in an awkward way when viewed from outside and when you tour the interiors of affected buildings, the dimensional inconsistencies become even more obvious.

Buildings affected by these fundamental errors are not rare, hence understanding the causes of such errors and making conscious efforts towards eliminating them during construction is very imperative. The causes of these errors are; The false assumption that our eyes are always straight hence will give us straight horizontal or vertical projections and then relying heavily on it to mark or set out straight lines or points on the ground.  

Although the eye is the most useful sense organ for carrying out setting out operation, there is no existing proof that the human eye can always give a correctly straight projection at all times. Hence it is not very reliable for such functions; therefore avoid the temptation to use it in isolation to your instruments.

Poor reading of instruments used is another error often made during setting out operations. If you read the markings of the measuring tapes wrongly at some point, you will end up transferring that error to the other parts of the building area and if you make multiple errors in this regard, you would have incurred a multiplicity of errors, thereby making a mess of the whole process. Although there are acceptable margins of error, usually between ±0.01 to ±0.25mm for perimeter length and width, deviation from the stated margin could well affect the ground dimensions of the building. Likewise the inability to properly read or take horizontal levels as well as ascertain verticality using the instruments designed for such purposes could lead to terrible departure from the original design.

L-R: Engr. Osaz' & David(Site Worker) paying attention to details on 
a construction site in Ijebu-Ode, Ogun during setting-out operation, 2016

Disregard for building regulations stipulating the minimum standards for offsets, sizes and spatial arrangement of spaces within the building and its exteriors are common errors made. For instance, to develop a residential building in most parts of Nigeria, the existing regulations states that a minimum offset/airspace of 3 metres must be allowed between the face of the building and the plot perimeter on all sides and that a minimum offset of 6 metres or more must be allowed in relation to the adjoining public infrastructure especially an access road. So if such regulations were ignored during the site planning, it may attract demolition leading to wastage.

Improper squaring is another common error. In order to avoid this error, always check for squareness of the area that is being set-out by first projecting it as if it were a rectangle and then taking diagonal measurements which should always tally with what obtains on the plan. Check for consistencies as you move from one point to another along the horizontal plane. Once you observe any inconsistency, you can easily trace the source of the error and correct it before moving on.
Poor arrangement and positioning of pegs, lines and instruments used can also promote error in the setting out process. If the distances between pegs arranged in a straight line are beyond 6m, there is likelihood that some of the pegs may either fall out of place or the lines attached to them may sag off the mark. Also, if lines are connected to pegs in an irregular manner, it may lead to avoidable errors and if adequate clearance is not provided during the operation to accommodate legroom in the case of a manual excavation, it may also give rise to post setting-out errors.

No comments:

Post a Comment