How to Calculate Estimation for Building Works?
The following three methods can be used to estimate the quantities of various items such as earthwork in excavation, foundation concrete, brickwork in foundation and plinth, brickwork in the superstructure etc.:
• Centre Line Method
• Long and short wall method
• Crossing Method
1. Centre Line Method
The total center line length of walls in a building is first calculated in this method of estimation. The center line length is then multiplied with the breadth and depth of the item in question so as to get the total quantity at a time.
The center-line length for each type will be worked out separately for different sections of the walls in the building. In case of verandah walls or partition joining with the main wall, there is a reduction of the center-line length by half of the breadth of the layer of the main wall that joins with the partition or verandah wall at the same level.
Using this method, you can estimate more quickly, this method is also as accurate as the rest of the methods.
This method has no advantage over the other methods only in the case of unsymmetrical wall where the center-line length varies at every layer.
In case of estimating circular, hexagonal, octagonal, etc. shaped buildings, this method can be adopted.
2. Long and Short Wall Method
The longer walls in a building, usually in one direction, are considered as long walls and are measured from out to out in this method. For the short or partition walls in a perpendicular direction of the long walls are considered as short walls. It is measured from in-to-in for a chosen layer of work.
The length of long and short walls is multiplied individually by the breadth and height of the corresponding layer and it is added to get the quantity.
In every layer of footing the lengths of long and short walls varies. It is important to determine center to center lengths individually from the plan to calculate the lengths of long and short walls.
The length of long wall, out-to-out must be calculated by adding half breadth of a wall at each end with the center to center length.
Therefore, the length of the short wall can be found out by measuring in-to-in after subtracting half breadth at each end from the center to center length.
There is a decrease in the length of long-wall from earthwork to brickwork in the superstructure but in case of a short wall the length increases.
In some cases, a wall is considered short at one end and long at the other end, the reason why such case happens is because a long wall is joined with another long wall and later the joining end of the wall which was considered as long wall is treated as short end and therefore called long-short-wall.
3. Crossing Method
In this method the overall perimeter of the building is calculated and then subtracted from the four times thickness of the wall to obtain the center-line length.
This method is not used widely now.
First calculate the total length of the center-line which remains constant for a varied width of works, then multiple this constant length of the center-line with the breadth and height to estimate the quantities.
Therefore, the total length of center-line = 2 [ (3m + (2 x (30 cm/2)) + (2.5m + (2 x (30 cm/2))]
Which results to the total length of center-line = 12.2 m
Example for the Long and Short wall Method
Center to Center length of long walls = 3 + (2 x 0.115) = 3.3 m
Center to Center length of long walls = 2.5 + (2 x 0.115) = 2.80 m
In every layer of footing the length of the long walls out-to-out and the short walls in-to-in vary.